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Abstract

1. Soil disturbance by animals affects the availability of water, nutrients, sediment and seeds, which are critical for the maintenance of functional ecosystems. We examined long-lived faunal structures across six vegetation communities in the northern Chihuahuan desert of New Mexico, USA, testing the proposition that disturbances in undesertified grassland differ in magnitude and effect from those in desertified grassland. 2. Vertebrate and invertebrate disturbances totalled 18.9 structures ha⁻¹ across 18 sites. The most common were pits and mounds of American badgers (Taxidea taxus, 32%), nests of the ant Aphaenogaster cockerelli (18.8%) and mounds of kangaroo rats (Dipodomys spectabilis, 31%). 3. Desertification was associated with a doubling of the density of structures, but no effects on cover or volume. The greatest density was in desertified mesquite and creosote bush shrublands, and the lowest density in undesertified grass swales. Badger and wood rat (Neotoma sp.) mounds were significant indicators of desertified communities. 4. Desertification did not affect the density of kangaroo rat mounds (6.7 ha⁻¹ in black grama grasslands and creosote bush shrublands). However, mounds in creosote bush shrubland were smaller and had more and larger shrubs than adjacent inter-mound hummocks. Desertification was associated with increases in the density of Aphaenogaster cockerelli and Trachymyrmex smithii nests, and declines in Pogonomyrmex rugosus nests. Substantial increases in soil nitrate and electrical conductivity on Myrmecocystus nests were associated with desertification. 5. Synthesis. Desertification shaped this desert environment in two main ways. First, while kangaroo rat mound density changed little with desertification, mounds in shrubland continued to enhance shrub persistence long after abandonment, reinforcing desertification processes. Second, marked changes in the density of nests of the key ant species altered the spatial distribution of soil nitrate and electrical conductivity, likely affecting soil fertility and the distribution of desert plants. Our results highlight the importance of animal activity in shaping desert plant communities, and in maintaining or reinforcing desertification processes.

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... A literature search of published papers using as keywords ÔshrublandÕ and ÔdesertificationÕ together in the abstract revealed a substantial number (46%) of entries (143 papers) from the Chihuahuan Desert of southwestern United States (e.g. Huenneke et al. 2002;Jackson et al. 2002;Bestelmeyer 2005;Duval & Whitford 2008;Eldridge et al. 2009) andSouth Africa (e.g. Adeel 2008;Sankaran & Anderson 2009). ...
... The combined effect of these processes is a strengthening of the Ôfertile islandÕ effect around shrubs, making shrublands extremely resistant to change and enhancing the persistence and development of shrublands at the expense of grasslands (Schlesinger et al. 1996;Whitford 2002). Losses of grassland biota with encroachment have been shown to reinforce the shrub-dominant state (Eldridge et al. 2009). ...
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Ecology Letters (2011) 14: 709–722 Encroachment of woody plants into grasslands has generated considerable interest among ecologists. Syntheses of encroachment effects on ecosystem processes have been limited in extent and confined largely to pastoral land uses or particular geographical regions. We used univariate analyses, meta-analysis and structural equation modelling to test the propositions that (1) shrub encroachment does not necessarily lead to declines in ecosystem functions and (2) shrub traits influence the functional outcome of encroachment. Analyses of 43 ecosystem attributes from 244 case studies worldwide showed that some attributes consistently increased with encroachment (e.g. soil C, N), and others declined (e.g. grass cover, pH), but most exhibited variable responses. Traits of shrubs were associated with significant, though weak, structural and functional outcomes of encroachment. Our review revealed that encroachment had mixed effects on ecosystem structure and functioning at global scales, and that shrub traits influence the functional outcome of encroachment. Thus, a simple designation of encroachment as a process leading to functionally, structurally or contextually degraded ecosystems is not supported by a critical analysis of existing literature. Our results highlight that the commonly established link between shrub encroachment and degradation is not universal.
... Non-native species invasions are also facilitated by fine-scale disturbances that alter the physical characteristics of soil surfaces and create microhabitats conducive to seed entrapment that can lead to seedling survival and plant establishment (Chambers 2000). The physical activity of animals also alters soil nutrients, with patches of high soil N and P occurring near animal dwellings (Wagner et al. 2004;Eldridge et al. 2009). These areas provide microsites with increased available resources that non-native species may exploit. ...
... At the patch scale, invasion by Schismus could be facilitated by soil disturbance associated with small fossorial mammals (Schiffman 1994) and soil turnover created by harvester ants (Wagner et al. 2004). The activities of rodents and ants in desert ecosystems are important in the creation of spatiotemporal resource patches (Wagner et al. 2004;Eldridge et al. 2009). At the landscape scale, invasion by Schismus could be facilitated by increased human activity. ...
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Habitat modification (i.e., disturbance) and resource availability have been identified as possible mechanisms that may influence the invasibility of plant communities. In the Mojave Desert, habitat disturbance has increased dramatically over the last 50years due to increased human activities. Additionally, water availability is considered to be a main limiting resource for plant production. To elucidate the effects of soil disturbance and water availability on plant invasions, we created experimental patches where we varied the levels of soil disturbance and water availability in a fully crossed factorial experiment at five replicated field sites, and documented responses of native and non-native winter annuals. The treatments did not significantly affect the density (seedlingsm−2) of the non-native forb, Brassica tournefortii. However, the relationship between silique production and plant height differed among treatments, with greater silique production in disturbed plots. In contrast to Brassica, density of the non-native Schismus spp. increased in soil disturbed and watered plots, and was greatest in disturbed plots during 2009 (the second year of the study). Species composition of the native annual community was not affected by treatments in 2008 but was influenced by treatments in 2009. The native forb Eriophyllum sp. was most dense on water-addition plots, while density of Chaenactis freemontii was highest in disturbed plots. Results illustrate that habitat invasibility in arid systems can be influenced by dynamics in disturbance regimes and water availability, and suggest that invasiveness can differ between non-native annual species and among native annuals in habitats undergoing changing disturbance and precipitation regimes. Understanding the mechanistic relationships between water availability and non-native plant responses will be important for understanding the effects of shifting precipitation and vegetation patterns under predicted climate change in arid ecosystems. Keywords Brassica tournefortii –Invasibility–Invasiveness–Nitrogen–Precipitation–Resource pulse– Schismus
... Such local disturbances often enable the accumulation of detritus [81], seeds [82], and runoff water [69,83], and alter the microclimatic conditions within the pits [84] (Figure 2). The accumulation of resources and the more favorable climatic conditions promote primary production and plant recruitment [85]. For example, ant nests in the Chihuahuan desert enhance soil moisture and nitrogen content and serve as hot spots for annual plants [85]. ...
... The accumulation of resources and the more favorable climatic conditions promote primary production and plant recruitment [85]. For example, ant nests in the Chihuahuan desert enhance soil moisture and nitrogen content and serve as hot spots for annual plants [85]. By altering the microtopography of the desert surface, animals may promote plant recruitment and reduce resource leakage enhancing the ecosystem productivity [86]. ...
Article
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Nutrient dynamics in most terrestrial ecosystems are regulated by moisture-dependent processes. In drylands, nutrient dynamics are often weakly associated with annual precipitation, suggesting that other factors are involved. In recent years, the majority of research on this topic focused on abiotic factors. We provide an arthropod-centric framework that aims to refocus research attention back on the fundamental role that macro-arthropods may play in regulating dryland nutrient dynamics. Macro-arthropods are prevalent in drylands and include many detritivores and burrowing taxa that remain active during long dry periods. Macro-arthropods consume and process large quantities of plant detritus and transport these nutrients to the decomposer haven within their climatically buffered and nutritionally enriched burrows. Consequently, arthropods may accelerate mineralization rates and generate a vertical nutrient recycling loop (VRL) that may assist in explaining the dryland decomposition conundrum, and how desert plants receive their nutrients when the shallow soil is dry. The burrowing activity of arthropods and the transportation of subterranean soil to the surface may alter the desert microtopography and promote desalinization, reducing resource leakage and enhancing productivity and species diversity. We conclude that these fundamental roles and the arthropods’ contribution to nutrient transportation and nitrogen fixation makes them key regulators of nutrient dynamics in drylands.
... In both, fossorial animals dig holes that turn soil, alter water percolation and provide refuges. On land, the holes also engineer temperature-buffered environments and alter water flow of surface runoff (Eldridge et al., 2009). In aquatic environments, burrows stabilize and aerate sediments, and their construction can increase the suspension (bioturbation) and thus bioavailability of deposited organic particles and nutrients (Volkenborn et al., 2009;Woodin, 1976). ...
Article
Ecosystem engineers (EEs) strongly influence ecosystems by affecting the abiotic properties of a system to which many biota respond. EEs can thus be pivotal species in restoration by helping to move systems toward desired states much faster and more efficiently than direct human intervention on the abiotic state. For EEs to play a central, purposeful role in restoration, it is important to identify guiding principles about how the EEs may best be selected and incorporated. I discuss three important aspects to determine 1) where the utility for EEs is high; 2) where EEs can most easily establish, are easy to handle, and scaling‐up their use is possible; and 3) how to recognize and value multiple, coupled, and trait‐dependent engineering functions of EEs. Understanding these aspects of EEs should help guide purposeful and efficient choices in our approach to restoration.
... Digging by mycophagous animals manipulates the substrate and creates a variety of disturbances that have multiple benefits to the overall ecosystem ( Eldridge et al. 2009). It may also directly alter plant community composition through affecting seed dispersal and seedling recruitment (Fleming at al. 2014). ...
... In arid regions, animals that create soil disturbances have been identified as important ecosystems engineers. Soil modifications by animals include pits which serve as collection areas for seeds and organic debris (Whitford, 1998), soil tunnels and chambers that affect water infiltration (James et al., 2008), and burrows and mounds that affect water infiltration, soil nutrients, abundance, and diversity of plants and soil biota (Eldridge et al., 2009; Ginzburg et al., 2008; Ukabi et al., 2009; Whitford and Kay, 1999; Whitford et al., 2008). Some ecosystem engineers, such as beavers building dams on streams, have pervasive, cascading effects on other organisms and physical/chemical processes that have earned those species designation as keystone ecosystem engineers (Brown, 1995). ...
Article
The potential role of two species of pack rats (Neotoma albigula and Neotoma micropus) as keystone ecological engineers was examined by estimating the species diversity of invertebrates living in the nest middens, and nitrogen mineralization rates in soils associated with the middens. Although pack-rat middens in tarbush (Flourensia cernua) shrublands were smaller than those in creostebush (Larrea tridentata) shrublands, they housed a higher abundance and diversity of arthropods. The Neotoma spp. middens were an important microhabitat for crickets (Gryllus sp.), wolf spiders (Lycosa spp.), and lycid beetle larvae (Lycidae) in all of the shrub habitats. There were five arthropod taxa that occupied all middens in the creosote-bush shrubland, and 12 arthropod taxa that occupied all middens in the tarbush shrubland. Soils associated with pack-rat middens had significantly higher soil organic-matter content than reference soils. Nitrogen mineralization was significantly higher in soils associated with pack-rat middens than in reference soils. Neotoma spp. create habitats with moderate microclimates that are essential for several invertebrates, thus contributing to maintenance of biodiversity. The effects of middens on soil organic matter and nitrogen mineralization create nutrient-rich patches. Neotoma spp. affect biodiversity and critical ecosystem processes, thus supporting the designation of keystone ecological engineers.
... Birds and small mammals often play different ecological roles. An association between birds and small mammals in habitats altered by small mammal activity could result in a coupling between different ecosystem processes at a landscape scale [2,8,14,15]. Granivorous birds tend to show different foraging site preferences and efficiencies, and different seed preferences and efficiencies, compared to small mammals [16][17][18][19][20]. Birds and mammals may also be complementary in that birds frequently act as seed dispersers [4,21,22], and small mammal disturbances to the soil can create bare or sheltered soil patches which enhance seedling establishment and recruitment [23,24]. ...
Article
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Birds are important mobile link species that contribute to landscape-scale patterns by means of pollination, seed dispersal, and predation. Birds are often associated with habitats modified by small mammal ecosystem engineers. We investigated whether birds prefer to forage on degu (Octodon degus) runways by comparing their foraging effort across sites with a range of runway densities, including sites without runways. We measured granivory by granivorous and omnivorous birds at Rinconada de Maipu, central Chile. As a measure of potential bird foraging on insects, we sampled invertebrate prey richness and abundance across the same sites. We then quantified an index of plot-scale functional diversity due to avian foraging at the patch scale. We recorded that birds found food sources sooner and ate more at sites with higher densities of degu runways, cururo mounds, trees, and fewer shrubs. These sites also had higher invertebrate prey richness but lower invertebrate prey abundance. This implies that omnivorous birds, and possibly insectivorous birds, forage for invertebrates in the same plots with high degu runway densities where granivory takes place. In an exploratory analysis we also found that plot-scale functional diversity for four avian ecosystem functions were moderately to weakly correllated to expected ecosystem function outcomes at the plot scale. Degu ecosystem engineering affects the behavior of avian mobile link species and is thus correlated with ecosystem functioning at relatively small spatial scales.
... According to Gibbens et al. (2005) and Viglizzo (2011), overgrazing may trigger a positive feedback that accelerates the loss of grasses and the growth of shrubs, causing a parallel and progressive decline of the livestock carrying capacity of the ecosystem. Furthermore, the loss of grassland biota due to encroachment may reinforce the shrub-dominant state (Eldridge et al., 2009). ...
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A vast body of literature demonstrated that anthropogenic disturbances such as overgrazing and fire are key drivers of abrupt transition between vegetation types in ecosystems. In this study, we propose that the hydrological context (described in terms of rainfall, evapotranspiration and water yield) is a first-order, primordial determinant of the propensity of ecosystems to undergo transition. This implies that the anthropogenic disturbance is a second-order determinant that is strongly conditioned by the first one. Through the meta-analysis of existing studies, a collection of 685 geo-referenced study cases was organized to study the hydrological characteristics of three climatic regions and three ecosystems that vary in their relation between woody and grassy plants. Thus, humid, sub-humid and dry climatic regions, respectively, receiving >1000, 500–1000 and <500 mm year−1, were studied, and possible transition mechanisms among grasslands/savannas, shrublands and forests were analysed. The results showed that the ecohydrological context determines the probabilities of ecosystems transitions in different climatic regions and the prevalence of alternative transition mechanisms. We showed that transition of forests into other ecosystems is highly improbable in high-precipitation regions, more probable and likely subject to a bi-stable and reversible regime in sub-humid regions, and highly probable and irreversible in dry regions. Factors such as runoff, deep-water drainage, fire, flammable/nonflammable biomass and overgrazing were considered as hypothetical transition mechanisms. As a novel finding, we demonstrate that ecohydrology, as a determinant of transition, is a factor that operates at a hierarchical level higher than that of the human-driven disturbance. A synthetic graphical model was proposed to characterize resilience (the capacity of ecosystems to withstand transition) in the three study climatic regions. Copyright
... Despite the explicit recognition of long-term grazing as the trigger for changes in resource distribution, many authors have invoked the Schlesinger et al. (1990) model to support the notion that shrub encroachment equates with, or leads universally to, desertification, degradation or ecosystem dysfunction (e.g. Ludwig and Tongway 1995;Huenneke et al. 2002;Goslee et al. 2003;Peters et al. 2006;Blaum et al. 2007;Eldridge et al. 2009, to name a few). ...
Article
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Since European settlement, there has been a dramatic increase in the density, cover and distribution of woody plants in former grassland and open woodland. There is a widespread belief that shrub encroachment is synonymous with declines in ecosystem functions, and often it is associated with landscape degradation or desertification. Indeed, this decline in ecosystem functioning is considered to be driven largely by the presence of the shrubs themselves. This prevailing paradigm has been the basis for an extensive program of shrub removal, based on the view that it is necessary to reinstate the original open woodland or grassland structure from which shrublands are thought to have been derived. We review existing scientific evidence, particularly focussed on eastern Australia, to question the notion that shrub encroachment leads to declines in ecosystem functions. We then summarise this scientific evidence into two conceptual models aimed at optimising landscape management to maximise the services provided by shrub-encroached areas. The first model seeks to reconcile the apparent conflicts between the patch-and landscape-level effects of shrubs. The second model identifies the ecosystem services derived from different stages of shrub encroachment. We also examined six ecosystem services provided by shrublands (biodiversity, soil C, hydrology, nutrient provision, grass growth and soil fertility) by using published and unpublished data. We demonstrated the following: (1) shrub effects on ecosystems are strongly scale-, species-and environment-dependent and, therefore, no standardised management should be applied to every case; (2) overgrazing dampens the generally positive effect of shrubs, leading to the misleading relationship between encroachment and degradation; (3) woody encroachment per se does not hinder any of the functions or services described above, rather it enhances many of them; (4) no single shrub-encroachment state (including grasslands without shrubs) will maximise all services; rather, the provision of ecosystem goods and services by shrublands requires a mixture of different states; and (5) there has been little rigorous assessment of the long-term effectiveness of removal and no evidence that this improves land condition in most cases. Our review provides the basis for an improved, scientifically based understanding and management of shrublands, so as to balance the competing goals of providing functional habitats, maintaining soil processes and sustaining pastoral livelihoods.
... In this respect, many studies have demonstrated the importance of burrowing and digging for nutrient cycling, seed dispersal, vegetation community dynamics, intra-and inter-species interactions, biodiversity, ecosystem resilience to environmental change and habitat restoration and management (e.g. Byers et al., 2006;Eldridge, 2011;Eldridge et al., 2012;Eldridge, Whitford, & Duval, 2009;Fleming et al., 2014;Hansell, 1993;James, Eldridge, & Moseby, 2010;Kinlaw & Grasmueck, 2012;Maestre et al., 2012;Martin, 2003;Meadows & Meadows, 1991;Zaitlin & Hayashi, 2012). ...
Article
The zoogeomorphological impact of burrowing animals varies in time and space as a result of the particular life history traits of the organisms involved, the patchy distribution of habitat resources, and fluctuations in population size. Such ecological complexity presents a major challenge for biogeomorphologists wishing to upscale from individuals to populations. Using a unique ecological data set for Eurasian badgers (Meles meles L.) in Wytham Woods, Oxfordshire, UK, we show that direct zoogeomorphological impact (soil displacement during sett excavation) is constrained by fluctuations in overall population size. Modeled digging rates for individual badgers (0.19–4.51 m3 yr−1) varied depending on the ecological function of the sett they are associated with, and we estimate that the whole population has displaced 304–601 ± 72 m3 of soil during the construction of 64 setts. This represents an overall excavation rate of 6.7–19.4 m3 (6.0–17.5 t) yr−1 in sett areas or 1.42–4.12 g m−2 yr−1 when averaged over the whole 424 ha woodland. As well as direct soil displacement, badger digging exposes material that is initially susceptible to erosion by water relative to undisturbed and litter-covered soils. Over time, setts become stabilized, representing unique landforms that persist in the landscape for decades to centuries.
... Such species are therefore likely to have a significant effect on the resilience and ability of ecosystems to respond and recover from disturbances (e.g. Eldridge et al., 2009;Davidson et al., 2012;Schickmann et al., 2012). They can also have an important role in ecosystem restoration of modified or degraded landscapes and/or ecosystems under stress due to other environmental changes, such as anthropogenic disturbances including logging, fragmentation, grazing from introduced animals, and climate change (e.g. ...
... 例如全球升温改变资源的空间异质性, 干旱地区 因为有限的资源使竞争者自我约束而不会过度生长繁 殖, 这会增加旱地群落的非传递性并减少干旱对物种 丰富度的负面影响 [52] . [54] , 非传递性竞争也更有可 能出现在异质环境中 [41] . [56] . ...
Article
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Maintaining biodiversity requires direct and indirect interspecific interactions. The study of direct interactions, including competition, predation, mutualism, commensalism, and parasitism, has greatly promoted the development of species coexistence theory. However, as a simplified method to explain community assembly, direct interactions alone cannot adequately explain the coexistence of related species in the absence of indirect interactions. Indirect interactions can be defined as the impact of one species on another that is mediated or transmitted by a third, and they are considered to be a more complex and less understood type of interspecific interaction. During the past two decades, scientists have gradually realized that a type of density-mediated indirect interaction, known as intransitive competition, is ubiquitous in nature. Intransitive competition can be described as a game of “rockpaper-scissors” in which three species A, B and C have competitive ranks A>B>C>A. In this competitive mode, competitive exclusion between relative species is counteracted by mutual restrictiveness, and there are no “invincible” species in the community. Therefore, intransitive competition is an important factor in promoting species coexistence, which affects species distribution patterns, community structure and ecosystem functions. To summarize the theory of intransitive competition, this paper discusses the definition, characteristics, and detection methods of intransitive competition and the main factors affecting intransitive competition as well. There are five formation modes of intransitive competition. Resource utilization mode occurs when species have different competitive abilities for resources. Life history mode arises if species cannot maintain an absolute competitive advantage throughout their life stages. Behavioral trade-off mode follows when species have different ways of acquiring resources. Competition trade-off mode and allelopathy mode require species to develop both aggressive and defensive survival strategies. Our paper also explores three characteristics of intransitive competition. First, the populations of species participating in intransitive competition often exhibit periodic fluctuations in their frequency. which are regulated by dynamic equilibrium. Second, intransitive competition systems involving an odd number of species can maintain community stability, whereas systems involving an even number of species will theoretically collapse. Finally, multiple intransitive competition loops may appear in the community, and intransitive competition may also be nested in other coexistence patterns. This nested feature makes intransitive competition difficult to detect and measure. Fortunately, current research methods such as competition matrix, transfer matrix and invasion growth rate model can be used to infer the existence of intransitive competition and quantify its prevalence and importance. Competitive rank has been considered as one of the most important factors affecting intransitive competition. The higher the coefficient of variation of competitive rank among species, the more negative impact on intransitive competition. At the same time, global-scale environmental changes could also be critical factors: drought increases the intransitive competition in the community, but eutrophication caused by land-use intensification reduces the intransitive competition. Functional traits are non-negligible because species traits adapted to high-productivity environments often impede the formation of intransitive competition. Finally, to promote the understanding of indirect interaction and multi-species coexistence strategies, we propose that future research on intransitive competition should focus on long-term controlled field experiments, coupled with theoretical considerations of high-order interactions, complex networks, and ecosystem function.
... 最 后,土壤 微 生 物和土壤动物 在 灌丛下的活 动较为强烈,进 一步 促 进了灌 丛下养分循环. 而草本植物生物量的降低,进一步加强了灌 木植物的定居及稳定发展 [68] . 这些因素共同作用,形成正反 馈,促使灌丛向不断稳定和加强的方向发展,最终形成沃岛 (Fertile Island). ...
Article
Shrub encroachment can change the ecosystem structure and function, even lead to the degradation or desertification of arid or semiarid grassland. This review comprehensively analyzed the impacts of precipitation, air temperature, soil character, biotic factor and human activities on shrub encroachment, and summarized the influences of shrub encroachment on ecosystem as well as the effective control measures against shrub encroachment. The increasing amount and distribution pattern change of precipitation generally facilitate the shrub encroachment. The global warming can alleviate the freeze injury to the shrub, and then decrease its mortality rate, which is favorable to the expansion of the shrub. Due to high adaption ability of shrub species, the land degradation and over-grazing can also facilitate shrub encroachment. Appropriate frequency of fire can suppress the shrub encroachment. Shrub encroachment is often associated with lower pasture value of the grassland, as well as changes of soil moisture and nutrient pattern, but has different impacts on grassland biodiversity and biomass. There are some effective control measures on shrub encroachment including reducing anthropogenic disturbance, applying adequate fire and artificially removing shrub. To improve the effectiveness of shrub encroachment control, a combination of several measures should be applied according to the specif.ic environment. Presently, the study results of shrub encroachment were mostly based on observation in field study without supporting data from big-scale and long-term field experiments. Therefore, further study on the process of shrub encroachment is needed. Besides, there are no consistent conclusions on whether the shrub encroachment process can be reversed as well as how it can be reversed. Although research on shrub encroachment just started in China, the phenomenon has happened long time ago in the grassland of this country. Thus, comparative studies between China and other regions can help clarifying the formation mechanisms and process of shrub encroachment.
... On the other hand, the nest making by Messor spp. with the dominant plant of Artemisia siberi in the area and killing of it caused the absence of this in the nests although it could reduce the competition and increase the resources for other plant species in the nests in a manner that they are few or rare in the desired region but because of different life forms and differences in their structure and function, different effects can be observed on ecosystem processes (Bestelmeyer et al., 2006); also, the key role of this shrub plant which is always important for multiple sources in dry environment by extensive root system concerning its effects on the soil water balance, climate cycle as compared to grass and forbs (Hooper & Vitousek, 1997;Eldridge et al., 2009) can be harmful to the stability and dynamics of the region. A.siberi damaging by Messor spp. in the center of nests can have some reasons given that there can be more food sources, better soil conditions and thermal balance that can be a good place to start nests. ...
Article
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Harvester ants are known as one of the most renowned bio-disturbances in the arid and semi-arid ecosystems that affecte vegetation by collecting seeds and harvesting plants. It seems that physiognomy of plant association in steppe shrub land of Roodshoor, Saveh Iran has been highly changed by harvester ants‟ activities that caused to conduct this research. The study was carried out on active and inactive nests and control site from June to August 2012. Diversity indices and functional groups such as vegetative form, longevity, and photosynthesis type were analyzed as compared to criteria in three sites. Results showed that richness, diversity, and vegetation cover in the ant colonies through increasing the annual forbs and rare species (Campanula stricta and Lepidium vesicarium) were more than the control site. For evenness index, however, there was no significant difference between the control and nest sites. In contrast, the function of plant community in the active nests due to the decrease of dominant shrub frequency of the area that is Artimisia siberi by Messor spp. was less than the control site. Low diversity, richness, plant function, and high vegetation cover in inactive nests were also observed as the results of the presence and activities of the ants in the active nests. Hence, the ant activities in the active and inactive nests can bring out micro sites with different plant associations so that regarding high density and quantity of the ant nests in all the area, it can decrease the key plants and change their functions. It therefore will debilitate the stability and function of this rangeland ecosystem.
... Digging by mycophagous animals manipulates the sub- strate and creates a variety of disturbances that have mul- tiple benefits to the overall ecosystem ( Eldridge et al. 2009). It may also directly alter plant community composition through affecting seed dispersal and seedling recruitment (Fleming at al. 2014). ...
Chapter
INTRODUCTION Fungi play a pivotal role in the forest food web, are central to its functioning and essential to maintaining a healthy balance. They interact with the food chain at many levels. Mycorrhizal fungi live together with “the producers” in a mutualistic association on the roots of plants. saprobic fungi are “decomposers” or “recyclers” breaking down dead plant and animal material releasing nutrients into the ecosystem for recycling, and the pathogenic fungi are parasites responsible for 70% of all known plant diseases but nevertheless can be considered consumers of their living plant or animal hosts. Fungi also represent an important source of food for numerous animals which are the most important forest consumers – the mycophagists. These animals include invertebrates which predominantly browse on fungal hyphae, spores, ectomycorrhizal root tips and reproductive structures (fruiting bodies), and vertebrates, particularly mammals and less frequently birds, which eat fruiting bodies. Many mycophagous small mammals are in turn prey for raptors, mammalian carnivores and martens and thus form important links in the trophic structure of forest ecosystems (Trappe et al. 2009). An example is the threatened northern spotted owl (Strix occidentalis caurina) that feeds primarily on northern flying squirrels (Glaucomys sabrinus) (Hallett et al. 2003), which in turn feeds predominantly on hypogeous fungi (truffles) (Weigl, 2007; Hallett et al. 2003). Consequently factors that reduce truffle production have a detrimental effect on the population of northern flying squirrels and threaten the survival of the northern spotted owl. In this chapter, we outline the characteristics of the fruiting bodies eaten by vertebrate mycophagous animals, list the most important groups of these, and describe their ecological roles in fungal spore dispersal. The possible exploitation of mycophagous animals in truffle cultivation is also discussed.
... An important body of work now shows that, through direct pedturbation and herbivory, burrowing mammals create mosaics of unique habitat across the landscape that cascade into community and ecosystem level processes in grasslands and woodlands around the world (North America: Huntly and Inouye 1988;Weltzin et al. 1997;Whitford and Kay 1999;Davidson and Lightfoot 2006;Eldridge et al. 2009;Ceballos et al. 2010;Davidson et al. 2010;South America: Branch et al. 1996, 1999, Machicote et al. 2004Villarreal et al. 2008;Asia: Zhang et al. 2003;van Staalduinen and Werger 2007). Our study adds to this literature by demonstrating that burrowing herbivores also alter vegetation structure and increase heterogeneity in the landscape indirectly by modifying fire regimes. ...
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Although native herbivores can alter fire regimes by consuming herbaceous vegetation that serves as fine fuel and, less commonly, accumulating fuel as nest material and other structures, simultaneous considerations of contrasting effects of herbivores on fire have scarcely been addressed. We proposed that a colonial rodent, vizcacha (Lagostomus maximus), reduces and increases fire intensity at different stages in its population cycle in the semiarid scrub of Argentina. Specifically, we hypothesized that, when colonies are active, vizcachas create natural fire-breaks through intense grazing, generating over time patches of large unburned shrubs in grazed zones. In contrast, when colonies are abandoned, recovery of fine fuels and previous accumulation of coarse wood on colonies during territorial displays increases fire intensity, creating patches of high shrub mortality. To test these hypotheses, we estimated stem age of the dominant shrub (Larrea divaricata) and measured aboveground biomass in zones actively grazed by vizcachas and in ungrazed zones, and compared densities of live and dead shrubs on abandoned colonies and adjacent zones following fire. In active colonies, age and biomass of shrubs were much greater in grazed than ungrazed zones. In abandoned colonies that had been burnt, density of dead, burned shrubs was higher and density of live shrubs was lower than in adjacent zones. These results support our hypotheses and reveal a new interaction between native herbivores and fire, in which herbivores augment fire intensity by gathering fuel. Our findings indicate that, through opposing effects on fire, native herbivores enhance the heterogeneity of vegetation in woody-dominated ecosystems.
... Due to their activity and abundance in agroecosystems and natural systems, some species may impact deeply ecological processes (Folgarait, 1998), but our understanding of the principal ecosystemic services supplied by ants is still very limited (Del Toro et al., 2012). Two of these ecosystem services are particularly important: bioturbation resulting in soil turnover, both at the superficial and subsurface levels, where ants act as ecosystem engineers (Lobry de Bruyn and Conacher, 1994;Lavelle et al., 2006;Tschinkel, 2015;Tschinkel and Seal, 2016), and changes in soil physical properties, which, in turn, may provide benefits for neighbouring plants (Lobry de Bruyn and Conacher, 1990;Eldridge et al., 2009). ...
... Decomposition of plant and animal detritus is necessary for release of nutrients that become available for new plant growth(Wood, Lawrence, Clark & Chazdon 2009). The infusion of organic matter into soil improves soil texture and water-holding capacity(Eldridge, Whitford & Duval 2009;Brody, Palmer, Fox-Dobbs & Doak 2010). Termites and dung beetles, in particular, provide a major agricultural service by removing and burying livestock dung, thereby preventing fouling of pasture forage by dung accumulation, increasing carbon and water storage in soil, reducing nitrogen loss via erosion and volatilization and reducing livestock losses due to hematophagous flies (Doube in this issue). ...
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Natural ecosystems provide a variety of services on which humans, and other organisms, depend for survival and well-being. These ecosystem services can be categorized as provisioning (production of food, fiber, water and other resources), cultural (non-material benefits, such as recreation, spiritual and other aesthetic values), supporting (primary production, pollination, decomposition and soil formation necessary for resource production) and regulating (biological control and other feedback mechanisms that maintain relatively consistent delivery of services). Ecosystems also have been viewed as producing “disservices”, such as pests, litter, biological hazards such as diseases, animal attacks, allergenic and poisonous organisms, and geophysical hazards such as floods and storms. Many of these disservices are induced by management practices such as deforestation and concentration of agricultural crops. Insects affect ecosystem services in a variety of ways, positively and negatively. The papers in this special issue are focused on managing insects and ecosystems, and their interactions, in ways that ensure sustainability of ecosystem services and that minimize induction of disservices.
... The combined effect of these processes is a strengthening of the "fertile island" effect around shrubs, making shrublands extremely resistant to change and enhancing the persistence and development of shrublands at the expense of grasslands (Schlesinger et al., 1996;Whitford, 2002). Losses of grassland biota with encroachment have been shown to reinforce the shrub-dominant state (Eldridge et al., 2009)." ...
Thesis
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This thesis explores the role of plant diversity in simulated climate–vegetation interaction towards the end of the “African Humid Period” (AHP) in models of different complexity, from the conceptual model by Claussen et al. (2013) to the Dynamic Global Vegetation Model JSBACH, the land component of the Max Planck Earth System Model MPIESM1. In the light of recently published pollen data and the current state of ecological literature, the conceptual model by Claussen et al. (2013) reproduces the main features of different plant types interacting together with climate, but it does not capture the reconstructed AHP plant diversity. With a new model version adjusted to AHP vegetation, I can simulate a diverse mosaic-like environment as reconstructed from pollen, and I observe a stabilizing effect of high plant diversity on vegetation cover and precipitation. Plant composition ultimately determines the stability of the climate–vegetation system. The assessment of plant diversity in JSBACH illustrates that the “Plant Functional Type” (PFT) concept is not capable to capture AHP plant diversity and cannot depict mosaic-like environments as reconstructed from pollen. However, offline simulations with different PFT compositions confirm that high PFT diversity can smooth the vegetation response to a – here prescribed linear – precipitation decline. Eventually, the steepness of vegetation decline depends on the composition rather than on the number of PFTs. In coupled ECHAM6/JSBACH simulations, PFT diversity significantly affects land surface parameters, water cycling, surface energy budget, and atmospheric circulation patterns during the AHP as well as the rate and timing of the transition from a wet “green” state to a dry “desert” state. Higher precipitation is not necessarily associated with a higher vegetation cover fraction and a higher stability of the climate–vegetation system, but determined by the properties of prevailing PFTs, thus PFT composition. Despite different underlying assumptions, all considered levels of model complexity lead to the same conclusions: high plant diversity could stabilize a climate–vegetation system, but plant composition is the decisive factor for the climate–vegetation feedback strength and consequently for the system response to changes in orbital forcing. This highlights that the choice of plant types/PFTs and their representation in models significantly affect simulated climate–vegetation interaction during the AHP, the extent of the “green” Sahara, and the timing and rate of transition to the “desert” state. From this I conclude that accounting for plant diversity in future studies – not only on palaeoclimates – could significantly improve the understanding of climate–vegetation interaction and the simulation of the vegetation response to changing climate.
... Digging can also alter plant dynamics by reducing plant competition through the creation of gaps allowing plant recruitment (Sandom, Hughes & Macdonald, 2013). These processes can affect plant composition and patterns (Boeken et al., 1995;Davidson & Lightfoot, 2008;Eldridge, Whitford & Duval, 2009). ...
Article
The eastern bettong (Bettongia gaimardi), a medium-sized digging marsupial, was reintroduced to a predator-free reserve after 100 years of absence from the Australian mainland. The bettong may have the potential to restore temperate woodlands degraded by a history of livestock grazing, by creating numerous small disturbances by digging. We investigated the digging capacity of the bettong and compared this to extant fauna, to answer the first key question of whether this species could be considered an ecosystem engineer, and ultimately if it has the capacity to restore lost ecological processes. We found that eastern bettongs were frequent diggers and, at a density of 0.3-0.4 animals ha-1, accounted for over half the total foraging pits observed (55%), with echidnas (Tachyglossus aculeatus), birds and feral rabbits (Oryctolagus cuniculus) accounting for the rest. We estimated that the population of bettongs present dug 985 kg of soil per ha per year in our study area. Bettongs dug more where available phosphorus was higher, where there was greater basal area of Acacia spp. and where kangaroo grazing was less. There was no effect on digging of eucalypt stem density or volume of logs on the ground. While bettong digging activity was more frequent under trees, digging also occurred in open grassland, and bettongs were the only species observed to dig in scalds (areas where topsoil has eroded to the B Horizon). These results highlight the potential for bettongs to enhance soil processes in a way not demonstrated by the existing fauna (native birds and echidna), and introduced rabbit.
... Rostagno and Delvalle 1988), accumulation of windblown(Ravi et al 2007) or raindrop-splashed sediment(Parsons et al 1992), bioturbation by burrowing animals(Eldridge et al 2009), or a combination of these processes(Bochet et al 2000, Parizek et al 2002, Buis et al 2010. ...
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Semiarid karst landscapes are often the source areas for regionally important groundwater supplies. Like savannas across the globe, these landscapes are experiencing an increase in woody plant cover-often referred to as woody plant encroachment. Although this phenomenon is commonly viewed as leading to increased transpiration and reduced groundwater recharge, this may not be true of all ecosystems. For example, in the Edwards Plateau region of central Texas-where the underlying geology is karst-dramatic increases in baseflows have occurred concurrently with the expansion of woody plants. It has been suggested that in this context woody plants, especially juniper (Juniperus spp.), are partially responsible for boosting recharge by improving soil infiltrability, but this hypothesis has not been systematically evaluated. Our study examined the effects of an important encroaching shrub (Redberry juniper) on soil infiltrability in the Edwards Plateau. We carried out a large number of infiltration tests to determine soil infiltrability and used a dye tracer followed by soil profile excavation to estimate the potential for deep percolation. Tests were performed at increasing distances under juniper shrubs of five size classes, ranging from young seedlings to mature shrubs. We found that in soils underlying shrubs, infiltrability was quintupled and percolation depth almost tripled compared with soils in intercanopy zones. Surprisingly, shrub size was not a significant factor. Even the soils beneath the smallest shrubs had much higher infiltrability than intercanopy soils, showing that these woody plants modify soil properties at very early stages. We also found that both infiltrability and percolation depth gradually increased with proximity to the trunk and showed a strong correlation with litter thickness. Our results provide support for the hypothesis that in semiarid karst landscapes, woody plant encroachment-especially the invasion of juniper-can play an important role in enhancing groundwater recharge by improving the soil infiltrability.
Chapter
The chapter deals with the interactions of animal populations and their habitats in a more general view. The many factors controlling these interactions are considered in the following sections.
Chapter
Ecosystems provide a number of free services, especially provisioning services, on which humans depend for survival. All food comes from ecosystems; often ecosystems manipulated for crop or timber production. Fresh water also is a product of ecosystems, along with numerous medical and industrial products. In addition, ecosystems provide cultural services, such as recreational, religious, and educational uses. Supporting and regulatory services include processes, such as pollination, decomposition, and biological control, that contribute to production and delivery of harvestable resources. Insects often have been vilified as threats to human health and food production but also are important sources of food, water, medical and industrial products, cultural values, and inspiration for technological advances. An important challenge for resource and pest management professionals is how to manage pests without threatening the sustainability of ecosystem services.
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Methods to detect and quantify shifts in the state of ecosystems are increasingly important as global change drivers push more systems toward thresholds of change. Temporal relationships between precipitation and aboveground net primary production (ANPP) have been studied extensively in arid and semiarid ecosystems, but rarely has spatial variation in these relationships been investigated at a landscape scale, and rarely has such information been viewed as a resource for mapping the distribution of different ecological states. We examined the broad-scale effects of a shift from grassland to shrubland states on spatiotemporal patterns of remotely sensed ANPP proxies in the northern Chihuahuan Desert. We found that the normalized difference vegetation index (NDVI), when averaged across an eight-year period, did not vary significantly between these states, despite changes in ecosystem attributes likely to influence water availability to plants. In contrast, temporal relationships between precipitation and time-integrated NDVI (NDVI-I) modeled on a per-pixel basis were sensitive to spatial variation in shrub canopy cover, a key attribute differentiating ecological states in the region. The slope of the relationship between annual NDVI-I and 2-year cumulative precipitation was negatively related to, and accounted for 71% of variation in, shrub canopy cover estimated at validation sites using high spatial resolution satellite imagery. These results suggest that remote sensing studies of temporal precipitation–NDVI relationships may be useful for deriving shrub canopy cover estimates in the region, as well as for mapping other ecological state changes characterized by shifts in long-term ANPP, plant functional type dominance, or both. Keywordsaboveground net primary production–normalized difference vegetation index–precipitation–remote sensing–Chihuahuan Desert–state change–shrub encroachment–grassland–shrubland
Article
Biotic modifiers are species that strongly alter environmental conditions, and that often shape local communities. When more than one biotic modifier co-occurs, the potential for interactive effects exists, where the effects of one species may negate or enhance the impacts of a second biotic modifier. Here we test for an interactive effect of aardvark (Orycteropus afer) burrows and tree presence on soil (soil temperature, moisture and compaction) and understorey vegetation properties (plant cover, species richness and composition) in a semi-arid savannah. Aardvark burrowing altered abiotic conditions, with soil temperature and compaction being lower, and soil moisture higher, in burrow entrances than at the burrow mounds and control microsites. In addition, the impact of aardvark burrowing on soil moisture and soil compaction was mediated by the occurrence of trees. In contrast, vegetation cover and species richness were independently affected by burrowing and trees, while vegetation composition was only affected by the occurrence of trees, but not by burrowing. This research suggests that co-occurring biotic modifiers can have both independent and interactive effects, and disentangling their impacts will help understand the mechanisms underlying dryland plant communities.
Article
An emerging area of interest in geomorphology over the past two decades has been the effects of biota on ecosystem processes. We examined the roles of a range of vertebrates on soil disturbance in two markedly different environments, the semi-arid woodland of eastern Australia and a Chihuahuan Desert grassland–shrubland in the south-western United States. Foraging pits of soil-disturbing vertebrates varied markedly from small scratchings of heteromyid (mainly Dipodomys spp.) rodents (1.8×10−4m3) to deep (1.0×10−2m3) excavations of the burrowing bettong (Bettongia leuseur) and greater bilby (Macrotis lagotis). Vertebrates moved substantial volumes of soil in both environments, and activity was highly temporally and spatially variable. At large spatial scales, soil disturbance by echidnas (Tachyglossus aculeatus) and Gould's sand goannas (Varanus gouldii) was substantially greater in communities dominated by shrubs, and where domestic livestock had been excluded. Heteromyid rodents tended to excavate more foraging pits in coarse-textured vegetation communities (both grasslands and shrublands). In both environments, foraging was concentrated close to perennial plants such as grass tussocks and tree canopies rather than in the interspaces. Foraging pits of Chihuahuan desert animals tended to be higher in labile carbon and support greater levels of infiltration, though this was plant community-dependent. Overall our results indicate that animal foraging is an important geomorphic mechanism capable of mobilizing substantial volumes of soil in arid and semi-arid environments and with potential effects on soil function.
Article
Resources in semi-arid landscapes are concentrated around woody plants (trees and shrubs), and therefore attract soil–disturbing fauna. Globally the trend has been to remove encroaching shrubs from semi-arid shrublands to increase their value for pastoralism, potentially affecting shrub–resident biota. We examined the distribution of animal disturbances created by a range of organisms (e.g. ants, scorpion, cicada, reptiles, small mammals) under two ploughing treatments, with and without grazing, in a semi-arid shrubland. We hypothesized that 1) animal structures would be clustered around resource-rich shrub patches, and 2) the density and composition of animal structures would differ between undisturbed (ungrazed and unploughed) and disturbed (ploughed and/or grazed) plots. Overall, we found more animal disturbance within shrub patches than in the interspaces irrespective of grazing and/or ploughing treatments, and well-defined increases in density of animal disturbances with increasing shrub cover. Our study highlights the importance of shrub hummocks as habitat patches for animals, and the potential negative feedbacks processes arising from shrub removal.
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Despite once being described as common, digging mammal species have been lost from the Australian landscape over the last 200 years. Around half of digging mammal species are now extinct or under conservation threat, and the majority of extant species have undergone marked range contractions.Our aim is to identify the role of digging mammals in ecosystem processes throughout Australia. We highlight how the actions of digging mammals are vital for maintaining ecosystem functioning and how their extirpation has led to loss of ecosystem functions.A review of the literature indicates that many aspects of the influence of bioturbation on ecosystem functioning have been studied. The role of digging mammals in arid and semi-arid zones has been previously established. We collate and review a broader scope of studies, including those carried out in the mesic woodlands and forests of Australia. We identify roles of digging mammals in the context of ecosystem functioning and conservation management.Bioturbation significantly alters soil processes, increasing soil turnover and altering the chemical and structural properties of soil. Greater water infiltration and decreased surface run-off and erosion alter soil hydrophobicity and increase soil moisture. Diggings capture organic matter, provide habitat for a diversity of microscopic and macroscopic organisms, and increase nutrient cycling. Mycophagous mammals disperse fungi (e.g. mycorrhizae), while all diggings can create suitable sites for fungal growth. Diggings also capture plant seeds, increasing seedling germination, recruitment and plant growth. The overall effect of mammal diggings is therefore increased plant vigour and resilience, increased biodiversity and consequently improved ecosystem functioning.We propose that the loss of digging mammals has contributed to the deterioration of ecosystems in Australia. Recognising the roles of digging mammals will inform potential management options such as species translocations or reintroductions.
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Background and Aims Soil texture is an important determinant of ecosystem structure and productivity in drylands, and may influence animal foraging and, indirectly, plant community composition. Methods We measured the density and composition of surface disturbances (foraging pits) of small, soil-foraging desert vertebrates in shrubland and grasslands, both with coarse- and fine-textured soils. We predicted that the density and functional significance of disturbances would be related more to differences in texture than shrub encroachment. Results Soil texture had a stronger influence on animal foraging sites than shrub encroachment. There were more disturbances, greater richness and abundance of trapped seed, and greater richness of germinating plants on coarse- than fine-textured soils. Pits in coarse soils trapped 50 % more litter than those in finer soils. Apart from slightly more soil removal and greater litter capture in shrubland pits, there were no effects of encroachment. Conclusions Although the process of woody encroachment has been shown to have marked effects on some ecosystem properties, it is likely to have a more subordinate effect on surface disturbances and therefore their effects on desert plant communities than soil texture. Our results highlight the importance of animal activity in shaping desert plant communities, and potentially, in maintaining or reinforcing shrub dominant processes.
Article
In harsh, resource-limited rangelands, plants and other structures, such as animal burrows, can ameliorate extreme microclimate conditions and increase resource availability for other species. The aim of this study was to determine whether relic bilby (Macrotis lagotis) refuge burrows had the potential to contribute to a rangeland restoration program by moderating microclimate and accumulating resources. During the day, the burrows maintained significantly higher relative humidity than control microsites. At night, temperature was significantly higher and humidity was significantly lower in the burrows than the control microsites. Both temperature and humidity were also significantly less variable in the burrows. There was some overlap between burrows and control microsites in soil chemistry, but burrows were significantly higher in soil moisture, pH(CaCl2), exchangeable magnesium, exchangeable potassium, mineral nitrogen, and total cation exchange capacity, and significantly lower in bio-available aluminium. Soils in burrows contained three times more mineral nitrogen, which has been previously shown to increase plant diversity and abundance in the presence of additional moisture. These results suggest the relic bilby burrows could potentially provide more suitable habitats for the establishment and productivity of other species by moderating microclimates, accumulating nutrients and soil moisture, and ameliorating the potentially detrimental effects of bio-available aluminium.
Article
Mammals that forage for food by biopedturbation can alter the biotic and abiotic characteristics of their habitat, influencing ecosystem structure and function. Bandicoots, bilbies, bettongs and potoroos are the primary digging marsupials in Australia, although most of these species have declined throughout their range. This study used a snapshot approach to estimate the soil turnover capacity of the southern brown bandicoot (Isoodon obesulus, Shaw 1797), a persisting digging Australian marsupial, at Yalgorup National Park, Western Australia. The number of southern brown bandicoots was estimated using mark-recapture techniques. To provide an index of digging activity per animal, we quantified the number of new foraging pits and bandicoot nose pokes across 18 plots within the same area. The amount of soil displaced and physical structure of foraging pits were examined from moulds of 47 fresh foraging pits. We estimated that an individual southern brown bandicoot could create similar to 45 foraging pits per day, displacing similar to 10.74 kg of soil, which extrapolates to similar to 3.9 tonnes of soil each year. The digging activities of the southern brown bandicoots are likely to be a critical component of soil ecosystem processes.
Article
Plateau pikas (Ochotona curzoniae) bioturbation affects the soil nutrient concentrations of the alpine meadow in the Qinghai-Tibetan Plateau (QTP) by the arrangement of bare land. This study investigated the effect of the disturbance produced by plateau pika bioturbation on primary soil nutrient concentrations of the Kobresia pygmaea meadow in view of overall and site scale and further analyzed the effect of the disturbance intensity of plateau pika on the soil nutrient concentrations of vegetated land and bare land. Our results showed that the disturbance by plateau pika bioturbation significantly decreased the soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP), and increased the available phosphorus (AP), NO3⁻-N and NH4⁺-N of bare land. Moderate disturbance intensities increased the SOC, TN and TP of both vegetated land and bare land and reduced the NO3⁻-N, NH4⁺-N and AP of vegetated land while increasing those of bare land. Disturbance within the threshold disturbance intensities of plateau pika is beneficial to the soil C and N, accumulating higher available nutrient concentrations in the topsoil, which would provide good habitats for certain rare plants and supply nutrients for the nitrophilous graminoid plants to grow well in the QTP. These results suggest that the understanding of the responses of the soil nutrient concentrations to plateau pika bioturbation should consider the bare land under disturbance intensities of plateau pika and alpine meadow type.
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Theoretical models suggest that scale-dependent feedbacks between plant reproductive success and plant patch size govern transitions from highly to sparsely vegetated states in drylands, yet there is scant empirical evidence for these mechanisms. Scale-dependent feedback models suggest that an optimal patch size exists for growth and reproduction of plants and that a threshold patch organization exists below which positive feedbacks between vegetation and resources can break down, leading to critical transitions. We examined the relationship between patch size and plant reproduction using an experiment in a Chihuahuan Desert grassland. We tested the hypothesis that reproductive effort and success of a dominant grass (Bouteloua eriopoda) would vary predictably with patch size. We found that focal plants in medium-sized patches featured higher rates of grass reproductive success than when plants occupied either large patch interiors or small patches. These patterns support the existence of scale-dependent feedbacks in Chihuahuan Desert grasslands and indicate an optimal patch size for reproductive effort and success in B. eriopoda. We discuss the implications of these results for detecting ecological thresholds in desert grasslands.
Article
The burrowing bettong (Bettongia lesueur) is a rat-kangaroo that digs while foraging and is the only marsupial to construct warrens and live communally in them. It previously occurred over a large part of Australia, but was extinct on the mainland by the early 1960s, before its role in ecosystem function could be understood. Bettongs, sourced from island and captive populations, were reintroduced as part of a rangeland restoration program at Matuwa (Lorna Glen) in central Western Australia. This study compared soil properties and plant productivity on the active and relic parts of a single warren to determine whether the presence of the bettongs was likely to be of benefit at the patch scale. Soil rock content, hydraulic conductivity, mineral nitrogen, phosphorus, potassium and sulfur were significantly higher for soils on the active part than on the relic part of the warren. Cotton bush (Ptilotus obovatus) shrubs were similar in size for the areas compared, but those growing on the active part of the warren had significantly more living tissue, greater leaf biomass and larger leaves, with higher moisture and nutrient content. These responses are indicative of access to additional mineral nutrients, probably derived from the urine and faeces of the bettongs. Only one warren was investigated in the present study and thus, this was a 'patch' level, unreplicated study. A broader, landscape-scale study is recommended, to compare relic warrens and active warrens with undisturbed sites, to confirm the hypotheses raised as a result of this study.
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Species extinctions alter ecosystem services, and the magnitude of this impact is likely to change across environmental gradients. In Australia, soil‐disturbing mammals that are now considered ecologically extinct are thought to be important ecosystem engineers. Previous studies have demonstrated microsite‐level impacts of reintroduced soil‐disturbing mammals on soil functions, but effects are yet to be tested across larger scales. Further, it is unclear how impacts vary across environmental gradients and if the restoration potential of reintroductions changes with climate. We examined the effects of soil‐disturbing mammal reintroductions across a large rainfall gradient in Australia to test the hypothesis that ecosystem engineering effects on soil function depend on climate. We compared soil labile carbon, available nitrogen and the activity of four enzymes associated with nutrient cycling in three microsite types with and without soil‐disturbing mammals in five sites along a large rainfall gradient (166‐870 mm). Soil enzyme activity was greatest in the presence of soil‐disturbing mammals and increased with rainfall, but soil available carbon and nitrogen varied across the gradient and among microsites. Microsite effects were often stronger than any effects of soil‐disturbing mammals, with soil beneath vegetated patches (shrubs and trees) having greater enzyme activity, carbon and nitrogen than bare soils. However, soil‐disturbing mammals homogenised nutrient distributions across microsites. The impacts of soil‐disturbing mammals on soil function previously detected at micro‐scales was detected at a landscape‐scale. However, the overall effects of soil‐disturbing mammals on soil functions varied with productivity (rainfall). The context of soil‐disturbing mammal reintroductions is thus likely to be critical in determining their effectiveness in restoring soil function. This article is protected by copyright. All rights reserved.
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Foraging by herbivores alters individual plants and vegetation communities directly, and engineering behaviors such as bioturbation (digging and turning soil) and biodeposition (deposition of feces and urine) can affect soils and physical properties that indirectly influence vegetation and other organisms. Patchy environments often concentrate the activities of animals, potentially increasing the magnitude of their impacts on the vegetative community over time. To evaluate the potential for herbivorous engineers to enhance existing heterogeneity, we quantified the direct and indirect effects of a burrowing herbivore, the pygmy rabbit (Brachylagus idahoensis), on soil and vegetation in the sagebrush steppe ecosystem of the western United States, and we evaluated whether the effects were related to duration of occupancy by rabbits. Mounded microtopography (i.e., mima mounds) creates distinct resource islands with relatively tall and dense sagebrush shrubs where pygmy rabbits concentrate burrowing and foraging activities. We quantified soil and vegetation characteristics on mima mounds occupied by rabbits for 1-12 yr and on unoccupied mounds. We expected that browsing would negatively influence slow growing sagebrush shrubs, but that digging and biodeposition would enhance soil nutrients and water infiltration. In addition, we hypothesized that the net effect on sagebrush reproduction would be positive because indirect effects on soil would enhance seed production by mature sagebrush and seedling growth, and because bioturbation would increase seed retention and germination. Pygmy rabbit occupancy had significant cumulative effects on both soil and vegetation properties on occupied mima mounds. Over time, browsing reduced sagebrush canopy cover and percent of individual shrubs that were alive. Soil properties were less influenced by the duration of occupancy of mima mounds than by the localized spatial influence of burrowing; elevated nitrogen levels were associated with burrow entrances. Two measures of sagebrush reproduction (seedling recruitment and inflorescence biomass) increased with duration of burrow occupancy, suggesting that over longer time frames pygmy rabbits enhanced reproduction and recruitment of sagebrush shrubs. Our data demonstrate multiple pathways by which an herbivorous engineer can influence habitat heterogeneity, and they suggest that although pygmy rabbits are inconspicuous on the landscape, the species might play an important role in maintaining and augmenting heterogeneity in the sagebrush steppe.
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Desert ecosystems are one of the fastest urbanizing areas on the planet. This rapid shift has the potential to alter the abundances and species richness of herbivore and plant communities. Herbivores, for example, are expected to be more abundant within urban desert remnant parks located within cities due to anthropogenic activities that concentrate food resources and reduce native predator populations. Despite this assumption, previous research conducted around Phoenix, AZ, USA has shown that top-down herbivory led to equally reduced plant biomass in both urban and outlying locations. It is unclear if this insignificant difference in herbivory at urban and outlying sites is due to unaltered desert herbivore populations or altered activity levels that counteract abundance differences. Small rodent herbivore/granivore populations were surveyed at four sites inside and four sites outside of the core of Phoenix during fall 2014 and spring 2015 in order to determine whether abundances and richness differ significantly between urban and rural sites. In order to survey species composition and abundance at these sites, 100 Sherman traps and eight larger wire traps that are designed to attract and capture small vertebrates such as mice, rats, and squirrels were set at each site for two consecutive trap nights. Results suggest that the commonly assumed effect of urbanization on herbivore abundances does not apply to small rodent populations in a desert city, as overall small rodent abundances were statistically similar regardless of location. Though a significant difference was not found for species richness, a significant difference between small rodent genus richness at these sites was observed, with altered community composition. The compositional differences likely reflect the altered vegetative community and may impact ecological interactions at these sites.
Article
Increases in the cover or density of woody plants (encroachment) and overgrazing by European livestock are two major drivers of ecosystem structure and function in drylands and are often associated with land degradation. Although the effects of encroachment and overgrazing on vascular plants are relatively well known, little is known about their effects on arthropods such as ants. We examined ant community composition at sites ranging in shrub cover in a wooded dryland in eastern Australia, testing the notion that increasing shrub cover and grazing intensity would alter ant assemblages and functional group composition. We used ants because they are abundant, diverse, and respond to small-scale environmental changes. Increasing shrub cover had no effect on ant richness, diversity or evenness, but increased the abundance of Subordinate Camponotini. Larger shrubs tended to have greater ant richness, and abundance of Cold Climate Specialists, but fewer Generalized Myrmicinae and Hot Climate Specialists. More intense grazing was associated with greater ant richness, diversity and evenness, a greater abundance of Hot Climate Specialists and Dominant Dolichoridae, and lower abundant of Subordinate Camponotini. Our study indicates that changes in grazing intensity had stronger effects on ant richness than any increase in shrub cover. The effects of grazing likely resulted from changes in soil surface complexity such as more bare ground associated with grazing-induced degradation.
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The thermal environment of both natural and artificial burrows of Dipodomys spectabilis was moderate as compared to surface ambient conditions but was related to them. Burrow air temperatures were similar to, but not identical with, soil temperatures. Burrow humidity was generally near saturation and was related to both burrow temperature and surface ambient humidity. CO<sub>2</sub> concentration in kangaroo rat burrows was variable but was always higher than ambient. Burrow CO<sub>2</sub> concentration resulted from a complex of factors that included soil moisture and temperature and proximity of an animal. Surface wind velocity probably plays an important role in burrow ventilation. Burrow structure and soil characteristics influence environmental conditions of burrows.
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The biochemistry of the weathering, landscape movements, and chemical transformations of phosphorus and its availability of plants were examined in a chronosequence of soils developed from quartz monzonite alluvium in southern New Mexico. Total P in the soil profile decreased with increasing soil age and was removed from the ecosystem as readily as the most leachable base cations. Although Ca-bound forms of P decreased with increasing soil age, Ca-P remained the singlee largest fraction of total P in all soils. In contrast, Fe- and Al-bound P was a very small percent of total P in all soils. There was little evidence for the stabilization of P by soil organic matter within this ecosystem; both soil organic P and microbial P represented very small pools of total soil P. Phosphorus availability, measured by in situ resin bags, was not well correlated with soil age or total soil P, and P concentrations in shrub tissues did not reflect changes in forms or total amounts of soil P. The biogeochemical cycle of P in this system differs sharply from that in a more mesic, forested system, where fixation by iron and aluminium oxides and biologic activity play more dominant roles in the conservation of P within the ecosystem.
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Species composition and cover of spring annual plant communities on banner tailed kangaroo rat Dipodomys spectabilis mounds in a N Chihuahuan Desert grassland differed from those on intermound areas. After seasons of adequate precipitation, cover of annual plants was greater on mounds than on adjacent areas; dominant species on mounds were those known to increase with N fertilization. Soil N content was consistently higher in mound soils than in intermound soils, but plant cover on mounds was not different from intermound areas following periods of limited precipitation despite differences in N levels. -from Authors
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We examined the spatial distribution of soil nutrients in desert ecosystems of the southwestern United States to test the hypothesis that the invasion of semiarid grasslands by desert shrubs is associated with the development of ''islands of fertility'' under shrubs. In grasslands of the Chihuahuan Desert of New Mexico, 35-76% of the variation in soil N was found at distances <20 cm, which may be due to local accumulations of soil N under Bouteloua eriopoda, a perennial bunchgrass. The remaining variance is found over distances extending to 7 m, which is unlikely to be related to nutrient cycling by grasses. In adjacent shrublands, in which Larrea tridentata has replaced these grasses over the last century, soil N is more concentrated under shrubs and autocorrelated over distances extending 1.0-3.0 m, similar to mean shrub size and reflecting local nutrient cycling by shrubs. A similar pattern was seen in the shrublands of the Mojave Desert of California. Soil PO4, Cl, SO4, and K also accumulate under desert shrubs, whereas Rb, Na, Li, Ca, Mg, and Sr are usually more concentrated in the intershrub spaces. Changes in the distribution of soil properties may be a useful index of desertification in arid and semiarid grasslands worldwide.
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Graminivory by kangaroo rats (Dipodomys spp.) was investigated as a potential mechanism for the keystone role of these rodents in the dynamics of desert grasslands. Experiments confirmed that Ord's kangaroo rats (Dipodomys ordii) cut and consumed a large proportion of the tillers of three Chihuahuan Desert tussock-forming grass species. Field observations indicated that the characteristically cut grass tillers were absent from all-rodent and medium-sized kangaroo rat exclosures, but were frequent in large-sized kangaroo rat and rabbit exclosures, indicating that the medium-sized kangaroo rats (D. ordii, D. merriami) were responsible for grass cutting. Tiller waste as a percentage of peak standing crop ranged from 7% in grassland habitats to 0.7% in Flourensia cernua shrubland. Of the 13 species of perennial, tussock-forming grasses measured, only one, Muhlenbergia porteri, had no tillers cut by kangaroo rats. This study demonstrates that the keystone role of kangaroo rats in Chihuahuan Desert grassland ecosystems is probably the result of their graminivory.
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The influence of seed harvester ant (Pogonomyrmex rugosus) colonies on soil properties and soil surface and moisture characteristics was investigated through comparison of adjacent, nonnest (reference, 4 in beyond ant colony) areas in Las Vegas, Nevada. Effects of ant colonies on both terrace and slope sites were investigated. Soil moisture content and soil bulk density in a creosote bush (Larrea tridentata)-dominated shrubland were significantly lower, while soil temperature, soil organic matter, and percent pore space were significantly higher in soils with ant nests relative to adjacent reference soils. Soil pH and texture did not differ significantly between nest and reference soils. Among soil surface characteristics, percent bare soil and rock (gravel, cobble, and boulder) cover were not significantly different between nest and reference soils. In evaluating soil moisture characteristics, soils with ant nests had a significantly higher water infiltrability and greater depth of water penetration, but a significantly lower area of water spread (surface-water runoff) at both terrace and slope sites. Between the 2 geomorphic surfaces, water infiltrability and depth of water penetration were significantly greater at the terrace than at the slope. Water-borne soil movement (fluvial erosion) was significantly greater at the slope than terrace but did not differ significantly between nest and reference soils. The presence of active P. rugosus colonies in the L. tridentata-dominated shrubland altered certain soil properties and appeared to have a protective influence on the soil by fostering more infiltration and less runoff of surface water in southern Nevada.
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We examined the spatial distribution of soil nutrients in desert ecosystems of the southwestern United States to test the hypothesis that the invasion of semiarid grasslands by desert shrubs is associated with the development of "islands of fertility" under shrubs. In grasslands of the Chihuahuan Desert of New Mexico, 35-76% of the variation in soil N was found at distances <20 cm, which may be due to local accumulations of soil N under Bouteloua eriopoda, a perennial bunchgrass. The remaining variance is found over distances extending to 7 m, which is unlikely to be related to nutrient cycling by grasses. In adjacent shrublands, in which Larrea tridentata has replaced these grasses over the last century, soil N is more concentrated under shrubs and autocorrelated over distances extending 1.0-3.0 m, similar to mean shrub size and reflecting local nutrient cycling by shrubs. A similar pattern was seen in the shrublands of the Mojave Desert of California. Soil PO 4, Cl, SO 4, and K also accumulate under desert shrubs, whereas Rb, Na, Li, Ca, Mg, and Sr are usually more concentrated in the intershrub spaces. Changes in the distribution of soil properties may be a useful index of desertification in arid and semiarid grasslands worldwide.
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As a result of a long-term study of a site in SE Arizona, the authors observed a positive effect of the mounds of the banner-tailed kangaroo rat on the growth, flowering and fruiting, and survival of creosotebush, the dominant woody perennial of the community. This effect became obvious at this site long after the kangaroo rats disappeared from the system, probably as a result of invasion of creosotebushes and other shrubby vegetation into what was previously a desert grassland system. Whereas creosotebushes had a negative effect on the kangaroo rat population, the mounds created by these rodents have had a residual positive efffect on the creosotebushes. -from Authors
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The presence of denuded areas caused by harvester ants did not lower the production of a Nuttall saltbush community in the Big Horn Basin of Wyoming. Increased saltbush production around the perimeter of the denuded areas compensated for the absence of saltbush within the denuded area.
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From 1981 to 1993 the density of colonies of Myrmecocystus depilis and Aphaenogaster cockerelli decreased after being stable from 1958-1981. The density of Myrmecocystus mexicanus declined after consistently increasing from 1958-1981. The declines after 1981 were principally due to decreases in rates of founding of new colonies. Aphaenogaster cockerelli became locally extinct by 1993, possibly because of thermal stress or dampness within its shallow nests. Both Myrmecocystus species may have been affected by shading by an increasing vegetation cover. Myrmecocystus mexicanus nests that were experimentally shaded either died or moved entrances away from the shade. The only naturally shaded colonies of M. depilis that survived from 1981 to 1993 had moved their entrances beyond shade. Estimates of average and maximum life spans of colonies were: M. depilis 8.6 yr and 35 yr, M. mexicanus 8.9 yr and 23 yr, A. cockerelli 5.2 yr and 23 yr.
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Pogonomyrmex rugosus, a group forager, foraged preferentially on seeds of plant species with shed large quantities of seeds at 1 time. Pogonomyrmex desertorum, an individual forager took seeds in relation to their availability but concentrated on grass seeds late in the growing season. Pogonomyrmex californicus selected seed species which allowed it to avoid contact with its congeners. During a dry year, P. rugosus exhibited little selectivity, supporting the idea that selectivity should be decreased under conditions of low food abundance. Pogonomyrmex spp. removed a significant fraction of the seed production of only 1 species, Boutleoua barbata. It is suggested that Pogonomyrmex foraging may affect the relative abundance of plant species.
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Losses of dissolved nutrients (N, P, K, Ca, Mg, Na, Cl, and SO_4) in runoff were measured on grassland and shrubland plots in the Chihuahuan desert of southern New Mexico. Runoff began at a lower threshold of rainfall in shrublands than in grasslands, and the runoff coefficient averaged 18.6%in shrubland plots over a 7-year period. In contrast, grassland plots lost 5.0 to 6.3% of incident precipitation in runoff during a 5.5-year period. Nutrient losses from shrubland plots were greater than from grassland plots, with nitrogen losses averaging 0.33 kg ha^−1 yr^−1 vs 0.15 kg ha^−1 yr^−1, respectively, during a 3-year period. The greater nutrient losses in shrublands were due to higher runoff, rather than higher nutrient concentrations in runoff. In spite of these nutrient losses in runoff, all plots showed net accumulations of most elements due to inputs from atmospheric deposition. Therefore, loss of soil nutrients by hillslope runoff cannot, by itself, account for the depletion of soil fertility associated with desertification in the Chihuahuan desert.
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Rapid, nondestructive methods are needed to quantify plant biomass dynamics. Methods known,as dimension,analysis can be used to establish regression relationships between,plant biomass,and easily obtained plant measurements.,Regression analyses were used to estimate the dry weight of foliage, living and dead stems and roots from canopy area and volume,for eight desert shrubs. The results show that volume,and canopy,area are generally suitable estimators. Regression equations developed for Larrea tridentata compare,favorably with equa- tions in similar studies in Arizona and Nevada, suggesting that our results might be applicable in other desert regions, at least for shrubs with well-defined growth forms. Other considerations when,using these techniques are discussed.
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We examined the relationship between a Chihuahuan Desert grassland keystone species (banner tailed kangaroo rat, Dipodomys spectabilis) and several vegetation and soil indicators of rangeland health in order to define a threshold indicator value for irreversible change in ecosystem structure and function. The abundance of occupied and/or abandoned D. spectabilis burrow-mounds was assessed at 117 sites in south-central New Mexico where previous studies had reported vegetation cover and composition. The most robust indicator for presence/absence of D. spectabilis was shrub cover. D. spectabilis did not occur at sites with shrub cover >20%. It was concluded that a threshold value of 20% shrub cover could be applied to assessment and monitoring of Chihuahuan Desert rangelands because higher shrub cover results in the local extinction of this keystone species. The combination of data on the presence/absence of a keystone species with vegetation and soil indicators provides a method for identifying thresholds of degradation that may be irreversible.
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Harvester ants, Pogonomyrmex maricopa, construct cemented caps on the sand mound nests in a fine sand dune area. The caps are approximately 60% calcium carbonate that is transported from the underlying calcium carbonate layers. The caps protect the nest structure from being eroded away during high-wind periods. Partial erosion of the cemented caps adds calcium carbonate to the sand dune soils.
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On Dipodomys spectabilis mounds, the spring annual flora was dominated by Descuraina pinnata and Eschscholtzia mexicana; intermound areas were dominated by Eriastrum diffusum. Peak aboveground standing biomass of spring annuals on mounds and intermounds respectively was 133.3 ± 5.9 g m⁻² and 25.6 ± 6.3 g m⁻². In summer, the mounds had a high density and biomass of Tidestromia lanuginosa. Intermound areas were dominated by Haplopappus gracilis and Eriogonum abertianum. Peak aboveground standing biomass of summer annuals on mounds and intermounds respectively, was 172.7 ± 7.3 g m⁻² and 98.4 ± 2.3 g m⁻². Bulk density was lower on mounds; nitrate and total N contents of the mound soils were higher than those of intermound soils. Mound soils dried faster than intermound soils. Soil water potentials on mounds and intermounds at depths of 20 and 30 cm decreased continuously from January until mid-May, then fluctuated in response to individual rainstorms. -from Authors
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In arid areas of North America, nests of the seed-harvesting ant Pogonomyrmex rugosus tend to be elevated in mineral nitrogen and other soil nutrients relative to other microhabitats. We investigated the roles of decomposition, N mineralization, and plant nutrient uptake in maintaining high standing stocks of nutrients in P. rugosus ant nests. Decomposition rates of standard cellulose substrates placed on the surface of ant nests and other desert microhabitats suggest that conditions found in ant nests and bare areas are conducive to higher rates of decomposition than conditions under shrubs. In laboratory incubations of moist soil, net N mineralization rates were significantly higher in soil from ant nests than from bare areas and under two of three plant species. Net N mineralization rates measured in situ were much lower than those measured in laboratory incubations, but ant nest soil still exhibited higher rates at one of two sites. Litter collected from ant mounds, composed chiefly of seed chaff, was similar in N content to litter collected from underneath the dominant plant species, but had a significantly higher mean δ15N. Using this distinctive isotope signature as a tracer, we found no evidence that large perennial shrubs tap ant nests as a source of N. An invasive, annual grass species was significantly enriched in 15N, had higher leaf %N, and produced more seeds when growing on the mound than when growing several meters away; however P. rugosus nest surfaces are typically free of such annuals. We conclude that both high rates of nutrient cycling relative to other Mojave Desert microhabitats and low N utilization by the surrounding vegetation contribute to high standing stocks of mineral N in P. rugosus nests.
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Deserts are environmentally similar, leading to hypotheses of convergence in the properties of desert biotic communities as well as the components of these communities. One area that has received considerable attention has been granivory by desert rodents. Specific tests for convergence in the impact of rodents as granivores, by means of bait-removal experiments, however, have shown that the high levels of seed removal by rodents in the North American deserts differs from that of rodents in the South American, Australian and South African deserts. The only studies to measure the impact of rodents on desert seed fluxes confirm these patterns, with rodents consuming up to 86% of seed production in North American deserts, but less than 1% of seed production in South African deserts. A variety of hypotheses have attempted to explain these variations in desert rodent granivory. These include recent extinctions of granivores, that seed burial, low soil nutrients and/or limiting seed production prevented the radiation of granivorous small mammals, and that particular deserts are too young or too recently colonised by rodents for granivorous rodents to have evolved. Alternative hypotheses suggesting that climate variability may have precluded the development of specialised granivores need to be tested. -from Authors
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Organic matter in contact with soil is quickly colonised by fungi. This study compared communities of moulds found in the soil of burrows of the banner-tailed kangaroo rat,Dipodomys spectabilisMerriam, to those found in surrounding grassland. Soil samples (n= 138) were collected between 1989 and 1991. Fungal colonies were five times more abundant in burrow soils than away from burrows. Community composition also differed substantially, with many fungi more abundant in burrows than away from burrows. A diverse, but relatively predictable, community of moulds inhabits the burrow soils and is available to colonise seeds hidden within burrows. The results suggest that mutualistic interactions between the moulds and the rodent are possible, but there are as yet no data evaluating this. Burrows and dens of many animals, especially in arid environments, are likely to be important sites of microbial activity and decomposition.
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Arbuscular mycorrhizal (AM) fungal infection was assessed for dominant Chihuahuan Desert annual and perennial plant species. Annuals were determined to have significantly less AM infection than their perennial counterparts. Additionally, annuals were shown to have significantly smaller fine root diameters than perennials. Within 15 of the 17 species examined, the presence of AM hyphae in a root segment was not a significant determinant of root diameter, indicating than in most plant species mycorrhizal infection is not directly promoting root diameter increases. The results of this survey show that the dominant plant species of the Chihuahuan Desert group into two categories: (i) short-lived annuals with thin roots and low AM dependency, (ii) longer-lived perennials with thick mycorrhizal roots. r 2003 Published by Elsevier Science Ltd.
Article
The activities of Pogonomyrmex occidentalis (Cresson) result in alterations to the shortgrass plains ecosystem of northeastern Colorado. During nest construction and maintenance, large amounts of soil (2.8 kg/colony) are transferred to the surface and certain soil characteristics modified. Bulk density is lower beneath the mound, and sand content of the nest area higher in comparison to that of the surrounding area. The plant-clearing habits of these ants exert only a minor influence on total plant cover. The maximum area cleared (27.3 m2/ha) occurred in an ungrazed area, which represents 0.3% of the total area. There was an increase in vegetation standing crop values around the cleared disc which partially compensates for the clearings. Higher P and NO3 concentrations were detected in the vicinity of the mound. Mineral accumulation associated with western harvester ant colonies represents a nutrient pool in a localized area—an important consideration when evaluating mineral cycling in the shortgrass ecosystem.
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We compared the density and diversity of ant colonies on areas where the vegetation had been altered by treatment with herbicides and on untreated areas. Highest densities of ant colonies were on the untreated mesquite dune habitat and the grassland habitat. The treated habitats were characterized by differences in shrub cover and composition and cover of forbs and grasses. Conomyrma bicolor and Solenopsis xyloni were significantly more numerous on the mesquite dunes and grassland, respectively. The fungus-culturing ant, Trachymyrmex smithi, was abundant on the treated dune habitats and absent in the grassland habitat. Ant communities were most similar on the two sites subjected to vegetation modification, and most different comparing those dune habitats with the untreated mesquite dunes.
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Soils of Messor capensis nest-mounds had a higher infiltrability, but were drier than inter-mound soils after a rain event of 26 mm at the end of the hot, dry summer and after 59·5 mm of frontal rains in winter. Soils of M. capensis nest-mounds contain significantly more organic matter, phosphorus, potassium and nitrogen than soils of inter-mound spaces. Plants growing on mounds generally contained more leaf nitrogen than plants in inter-mound spaces. A bioassay, using radishes grown in soils from ant nest-mounds showed fewer symptoms of nutrient deficiencies, grew faster, had lower root:shoot ratios and were significantly heavier at harvest than those grown in inter-mound soils.
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Compensatory population dynamics among species stabilise aggregate community variables. Inter-specific competition is thought to be stabilising as it promotes asynchrony among populations. However, we know little about other inter-specific interactions, such as facilitation and granivory. Such interactions are also likely to influence population synchrony and community stability, especially in harsh environments where they are thought to have relatively strong effects in plant communities. We use a manipulative experiment to test the effects of granivores (harvester ants) and nurse plants (dwarf shrubs) on annual plant community dynamics in the Negev desert, Israel. We present evidence for weak and inconsistent effects of harvester ants on plant abundance and on population and community stability. By contrast, we show that annual communities under shrubs were more species rich, had higher plant density and were temporally less variable than communities in the inter-shrub matrix. Species richness and plant abundance were also more resistant to drought in the shrub under-storey compared with the inter-shrub matrix, although population dynamics in both patch types were synchronised. Hence, we show that inter-specific interactions other than competition affect community stability, and that hypothesised mechanisms linking compensatory dynamics and community stability may not operate to the same extent in arid plant communities.
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Fertile patches are created and maintained by a combination of physical and biologically-mediated processes including soil disturbance by animals. We examined the creation of fertile patches by 4 vertebrates, the greater bilby Macrotis lagotis, burrowing bettong Bettongia lesueur, European rabbit Oryctolagus cuniculus, and Gould's sand goanna Varanus gouldii within dunes, ecotones, and swales in a dunefield in arid South Australia. These animals all create pits when foraging for subterranean food resources. We hypothesized that 1) the effect of pits on litter capture would vary among landscapes and animal species, 2) larger pits would trap more litter and seed, 3) pits would contain more viable seed than the surrounding matrix, and 4) the effect of pits on soil chemistry would vary among animal species, and be greater in landscapes with more finely textured soils. We found that litter was restricted almost exclusively to the pits, and was greater in pits with larger openings. Litter capture was greater in ecotones and dunes than in swales. A total of 1307 seedlings from 46 genera germinated from litter samples taken from the pits, but no seedlings emerged from samples taken from soil surrounding the pits. Foraging pits contained significantly higher levels of total C and N than surrounding soil, and total C and N concentrations were greatest in swales and lowest in dunes. Pits contained ca 55% more mineralisable N that surface soils, and pits constructed by bilbies and bettongs contained half the concentration of mineralisable N as those of rabbits and goannas. Concentrations of mineral N and mineralisable N were also greatest in the swales. Our results demonstrate the importance of animal-created pits as nutrient sinks and sites for seedling establishment, and suggest that changes in the composition of arid zone vertebrates may have resulted in profound changes to nutrient and soil dynamics in arid Australia.
Article
Question: Does shrub invasion at ecotones indirectly limit grass establishment by increasing mammalian seedling herbivory?Location: Chihuahuan Desert, New Mexico, USA.Methods: We tested the hypothesis that herbivore-related mortality of seedlings of the dominant perennial grass Bouteloua eriopoda would be highest in shrub-dominated portions of grassland-shrubland ecotones. We tested the hypothesis in two Chihuahuan Desert sites featuring similar shrub encroachment patterns but different shrub species, grass cover, and different abundances of small mammals. Within each site we transplanted B. eriopoda seedlings to grass-dominated, middle, and shrub-dominated positions of replicate ecotones during the time of year (mid-summer) when they would naturally appear and monitored seedling fates. We estimated population size/activity of putative small mammal herbivores.Results: Seedlings were killed by mammals in greater numbers in shrubland than in grassland or middle ecotone positions at the site with large herbivore numbers. At the site with low herbivore numbers, most seedlings were killed in middle ecotone positions. The abundance patterns of herbivores did not parallel patterns of seedling herbivory across the ecotones or between sites.Conclusions: Seedling herbivory is an important process and is related to vegetation composition, but the mechanisms underlying the relationship are not clear. We speculate that variation in small mammal foraging behavior may contribute to seedling herbivory patterns. Restoration strategies in the Chihuahuan Desert need to account for the abundance and/or behavior of native herbivores.
Article
Losses of dissolved nutrients (N, P, K, Ca, Mg, Na,Cl, and SO4) in runoff were measured on grasslandand shrubland plots in the Chihuahuan desert ofsouthern New Mexico. Runoff began at a lowerthreshold of rainfall in shrublands than ingrasslands, and the runoff coefficient averaged 18.6%in shrubland plots over a 7-year period. In contrast,grassland plots lost 5.0 to 6.3% of incidentprecipitation in runoff during a 5.5-year period. Nutrient losses from shrubland plots were greater thanfrom grassland plots, with nitrogen losses averaging0.33 kg ha–1 yr–1 vs0.15 kg ha–1 yr–1, respectively, during a 3-year period. Thegreater nutrient losses in shrublands were due tohigher runoff, rather than higher nutrientconcentrations in runoff. In spite of these nutrientlosses in runoff, all plots showed net accumulationsof most elements due to inputs from atmosphericdeposition. Therefore, loss of soil nutrients byhillslope runoff cannot, by itself, account for thedepletion of soil fertility associated withdesertification in the Chihuahuan desert.
Article
We estimated the density of subterranean termites Gnathamitermes tubiformans at 800,000 · ha-1 for a standing crop biomass of 2 kg · ha-1 Predation losses were estimated to be 5,73 kg · ha-1 · yr-1 representing the major release of nutrients from termites to surficial soil layers. Nutrient fluxes from termites to predators amounted to 410g N·ha-1·yr-1, 33 g S · ha-1 · yr-1 and 19 g P · ha-1 · yr-1. These fluxes account for 8% of the litter N, 1.5% of the litter P and 2.9% of the litter S. The termites fixed an estimated 66 g · ha-1 · yr-1 atmospheric N and returned an estimated 100 g · ha-1 · yr-1 in the surface gallery carton. Since losses of elements from subterannean termites were greater than standing crops, we estimated an annual turnover of N at 3.5 times per year, P of 2.5 times per year, and S of 2.5 per times per year. Since surface foraging, predation and alate flights are pulse regulated by rainfall, nutrient flows through subterranean termites are episodic and releases of nutrients accumulated in termite biomass preceeds or is coincident with productivity “pulses” of some shallow rooted plants. We propose that subterranean termites are important as regulators in desert nutrient cycles.
Article
The effects of harvester ant (Pogonomyrmex rugosus) nests on the density and cover of spring annual plants and on soil characteristics were measured at three locations characterized by different soils and dominant vegetation on a desert watershed. There were few differences in vegetation and soils associated with harvester ant nests at locations at the base of the watershed where brief periods of flooding and sediment deposition occur at periodic intervals. At mid-slope locations, there were significant increases in total nitrogen, inorganic phosphorus, and cover (biomass) of four species of spring annuals at the edges of nest disks when compared with reference sites. The spring annuals that exhibited increased cover were species that increase biomass as a function of available nitrogen. At a clay-loam, Scleropogon-Hilaria, grassland site, there were significant reductions in the concentrations of Ca2+ and Mg2+, significant increases in nitrate and total nitrogen, but a significant increase in cover in only one species of annual plant. The data demonstrate that the effects of ants on soil properties and vegetation vary with site location and soil type.