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The establishment and naturalization of non-native Russian olive (Elaeagnus angustifolia) in southwestern US riparian habitats is hypothesized to have negative implications for native flora and fauna. Despite the potential for Russian olive establishment in new riparian habitats, much of its ecology remains unclear. Arid river systems are important stopover sites and breeding grounds for birds, including some endangered species, and understanding how birds use Russian olive habitats has important implications for effective non-native species management. We compared native bird use of sites that varied in the amount of Russian olive and mixed native/non-native vegetation along the San Juan River, UT, USA. From presence/absence surveys conducted in 2016during the breeding season, we found 1) fewer bird species and functional groups used Russian olive habitats and 2) the composition of species within Russian olive habitats was different from the composition of species in mixed native/non-native habitats. Our results suggest Russian olive may support different bird compositions during the breeding season and as Russian olive continues to naturalize, bird communities may change. Finally, we highlight the paucity of research surrounding Russian olive ecology and stress the need for rigorous studies to improve our understanding of Russian olive ecology.
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... The majority of these studies focused on birds and found that within Russian olive stands, cavity nesting birds decreased (Fischer et al. 2012) and generally fewer bird species were found compared to native or mixed nonnative stands (Mahoney et al. 2019). Others have documented numerous birds using the berries as a food source and the branches as preferred nesting sites (Leatherman 2011). ...
Russian olive (Elaeagnus angustifolia) is an invasive tree that has spread throughout much of the western United States. The mode of seed dispersal occurs by hydrochory and possibly by birds. Seed dispersal by frugivorous mammals has not been investigated. Between 15 October and 4 November 2020, we walked through Russian olive windbreaks in western North Dakota, USA, and surveyed for mammal scat, and found 10 coyote (Canis latrans) and 54 porcupine (Erethizon dorsatum) scats that contained intact Russian olive seeds. We subsequently evaluated the viability, germination frequency, and time to germination of seeds ingested by coyote and porcupine relative to un‐ingested control seeds harvested from trees at sites where we collected scat. Overall, Russian olive seeds that passed through mammal intestinal tracts had similar viability and equivalent (porcupine) or higher (coyote) germination frequency compared to controls. Additionally, coyote‐ingested seeds germinated earlier (time to germination was low) than controls, but porcupine‐ingested seeds were similar to controls. Thus, our data supports the idea that mammals may be agents of regional Russian olive seed dispersal. Russian olive seeds can be dispersed via endozoochory by mammals. Thus, mammals could play a role in spreading Russian olive into areas where other modes of transport (e.g., hydrochory) cannot.
... Thus, these three studies conducted over nearly 750 km or 7 • of latitude agree that selective foraging of beavers promote highly invasive species. This switch from native to exotic species is also associated with changes in the arthropod and avian communities  and beaver preference for native cottonwoods (Figure 2) facilitates the invasion of exotic species such as tamarisk and Russian olive (see also [48,49]). To avoid further conversion to exotic species, managers should remove exotics and protect native cottonwoods from beavers through fencing or other non-destructive means. ...
The North American beaver (Castor canadensis Kuhl) and cottonwoods (Populus spp.) are foundation species, the interactions of which define a much larger community and affect a threatened riparian habitat type. Few studies have tested the effect of these interactions on plant chemistry and a diverse arthropod community. We experimentally examined the impact of beaver foraging on riparian communities by first investigating beaver food preferences for one cottonwood species, Fremont cottonwood (P. fremontii S. Watson), compared to other locally available woody species. We next examined the impact of beaver foraging on twig chemistry and arthropod communities in paired samples of felled and unfelled cottonwood species in northern Arizona (P. fremontii) and southwestern Colorado (narrowleaf cottonwood, P. angustifolia James, and Eastern cottonwood, P. deltoides W. Bartram ex Marshall). Four major patterns emerged: (1) In a cafeteria experiment, beavers chose P. fremontii six times more often than other woody native and exotic species. (2) With two cottonwood species, we found that the nitrogen and salicortin concentrations were up to 45% greater and lignin concentration 14% lower in the juvenile resprout growth of felled trees than the juvenile growth on unfelled trees (six of seven analyses were significant for P. fremontii and four of six were significant for P. angustifolia). (3) With two cottonwood species, arthropod community composition on juvenile branches differed significantly between felled and unfelled trees, with up to 38% greater species richness, 114% greater relative abundance and 1282% greater species diversity on felled trees (six of seven analyses with P. fremontii and four of six analyses with P. angustifolia were significant). The above findings indicate that the highest arthropod diversity is achieved in the heterogenous stands of mixed felled and unfelled trees than in stands of cottonwoods, where beavers are not present. These results also indicate that beaver herbivory changes the chemical composition in 10 out of 13 chemical traits in the juvenile growth of two of the three cottonwood species to potentially allow better defense against future beaver herbivory. (4) With P. deltoides, only one of five analyses in chemistry was significant, and none of the four arthropod community analyses were significant, suggesting that this species and its arthropod community responds differently to beaver. Potential reasons for these differences are unknown. Overall, our findings suggest that in addition to their impact on riparian vegetation, other mammals, birds, and aquatic organisms, beavers also may define the arthropod communities of two of three foundation tree species in these riparian ecosystems.
... Of course, our functional designations were simplistic with only four groups, as the functional roles of ground-dwelling arthropods are less-defined than other communities (e.g., riparian arthropods Fig. 8. Dissimilarity analysis (β-diversity) of ground-dwelling arthropod functional groups (rows) partitioned into incidence-based turnover (β jtu ) and nestedness (β jne ) in the first column, and abundance-based turnover (β BC.BAL ) and nestedness (β BC.GRA ) of elevational site pairs (i.e., each site and its immediately adjacent uphill site) in the second column. (Kennedy et al. 2016), fish (Villéger et al. 2013), or birds (Mahoney et al. 2019)). Further partitioning arthropods into more defined functional groups may yield new insights into their elevational patterns. ...
Patterns of biodiversity along elevational gradients elucidate how climate shapes biological communities and help predict ecosystem responses to environmental change. Arid elevational gradients are particularly interesting because temperature limitations at high elevations and precipitation limitations at low elevations cause mid-elevation peaks in diversity. Ground-dwelling arthropods form highly diverse communities but few studies document elevational patterns of their full diversity. Here we investigate the elevational patterns of ground-dwelling arthropods in northern Arizona on the Colorado Plateau, an arid and understudied region in the United States. We sampled seven sites along an elevation gradient from 1,566 to 2,688 m corresponding to a difference of 6.5°C average annual temperature and 620 mm average annual precipitation. We captured 16,942 specimens comprising 169 species, mostly ants and beetles, and discovered a new ant species. First- and second-order elevation terms significantly correlated to multiple measures of arthropod α and β diversity. Arthropod abundance, richness, and Shannon-Wiener diversity index peaked at mid-elevations, with functional groups (i.e., omnivores, predators, detritivores, and herbivores) showing similar patterns. Community composition varied significantly across the gradient, correlated with changes in elevation, and was driven by shifts of ants dominating low- to mid-elevations, to beetles dominating high-elevations. Dissimilarity among sites was driven by high species turnover with 59% of species exclusive to a single site, whereas nestedness among sites was low except at the lowest elevation site. High rates of turnover and elevation-dependent communities suggest that ground-dwelling arthropods are highly vulnerable to environmental change, particularly at lower elevations in arid regions.
... It may also increase the availability of soil nitrogen resources and microbial diversity in the rhizosphere (Yildiz et al. 2017). Moreover, ecological studies conducted on this woody species have generally dealt with revegetation (Espeland et al. 2017), afforestation restoration (Yildiz et al. 2017), potential distribution and limiting climatic factors (Zhang et al. 2018), and its status as a bird habitat (Mahoney et al. 2019). However, no studies to date have been conducted on ROS formation, ROS detoxification, or enzymes and antioxidant capacity under the combined effects of biostimulants such as AMF and humic substances in Russian olive (Elaeagnus angustifolia L). ...
Arbuscular mycorrhizal fungi (AMF) and potassium humate (KH) are separately known as significant biostimulants, but their combined effect on plants remains elusive. This study investigated the single and combined roles of AMF and KH on the antioxidant defense system in Russian olive (Elaeagnus angustifolia L.) leaves. Soil below the seeds was inoculated with indigenous AMF spores (Funneliformis, Claroideoglomus; 500 spores per seed). The KH (1.5 g/ per 1 kg of seed) was applied during sowing. Growth, leaf-water ratio, chlorophyll fluorescence, lipid peroxidation, H2O2 content, antioxidant enzymes, and antioxidant capacity were analyzed in treated and untreated plants. Combined AMF and KH applications had a greater recovery effect on vegetative organ growth than separate treatments. With combined treatment, plants maintained leaf water status and chlorophyll fluorescence, while peroxidation of lipid membranes and H2O2 content was reduced. Moreover, increases in superoxide dismutase and glutathione reductase activity prevented cellular damage from reactive oxygen species. Total phenolic content and antioxidant capacity values were remarkably higher in plants grown under the combined treatment. As a result, compared with their separate applications, a combination of AMF and KH enhanced the antioxidant defense system by increasing antioxidant enzymes and antioxidant capacity and, thus, could be used to enhance plant growth.
Native species can have a range of responses to nonnative introductions, from negative to positive, and understanding how and why native species respond differently to nonnatives remains an important management challenge. Based on differences and similarities in ecology and behavior, we predicted how abundance and diet of two native warblers, Lucy’s warbler (Oreothlypis luciae) and yellow warbler (Setophaga petechia), would differ in habitats with different amounts of nonnative tamarisk trees and the three nonnative insects obligately dependent on tamarisk (Tamarix spp.). Specifically, we predicted that Lucy’s warblers would have similar densities across sites, yellow warbler densities would be inversely related to tamarisk cover, and both warblers, being generalist insectivores, would incorporate tamarisk biocontrol insects in their diet. Based on point counts and fecal samples at six sites along the Virgin River in the southwestern United States, we found that yellow warblers decreased in abundance with increasing tamarisk cover, while Lucy’s warbler abundance did not and that diet of the two warblers did not differ, with both species exhibiting strong selection for the nonnative tamarisk weevil (Coniatus splendidulus) and weak to no selection for the nonnative tamarisk leafhopper (Opsius stactogalus). Both warblers showed negative selection for the tamarisk beetle (Diorhabda carinulata) and its larvae, even when those insects were 10–100 times more abundant during outbreaks. Although both warblers exploited the novel food resources offered by tamarisk, with those insects contributing half or more of total prey biomass, Lucy’s warblers were better able to maintain densities in tamarisk habitats. We hypothesize this was due to the Lucy’s warbler’s ability to exploit a broader array of habitats surrounding tamarisk sites and its cavity nesting habit that buffers its nests from the higher temperatures and lower humidity of tamarisk-dominated habitat. Our results suggest that predictions based on detailed knowledge of the form and function of native and nonnative species can be used to predict native bird response to nonnatives.
Plant functional types (or guilds) increasingly are being used to predict vegetation response to global changes. Continued human population growth coupled with projected warmer and drier climate will alter the hydrologic regimes of many arid-zone rivers, including intermittent rivers. We aimed to identify (i) woody plant guilds associated with distinct stream types of an arid region and (ii) plant traits indicating adaptation to the selective pressures of water availability and fluvial disturbance.We used hierarchical clustering to identify 11 plant guilds from floodplains, terraces and uplands of eight Arizona rivers that vary in surface flow permanence, depth to ground water and intensity of fluvial disturbance.Six guilds were riparian pioneers with small, wind-dispersed seeds, three guilds were late-seral, shade-tolerant riparian taxa with large animal-dispersed seeds, and two guilds were composed of desert xerophytes. Within the riparian pioneer and seral groups, guilds varied in water acquisition and productivity traits including wood density and rooting depth.The community-weighted traits varied or covaried with water availability and fluvial disturbance. Root: shoot ratio, canopy height and leaf area were influenced strongly by water availability, with the latter two showing a nonlinear response to changes in water table depth. Leaf length increased, and wood density decreased, as sites become wetter and more fluvially disturbed. Community-weighted seed mass, seed dispersal and spinesence varied most strongly with elevation above thalweg (an indicator of decreasing fluvial disturbance).These analyses will enable prediction of changes in the relative abundance of plant types and plant traits in response to changes in stream flow regimes, such as shifts towards greater intermittency. The distribution patterns of guilds among riparian habitat types emphasise the importance of focusing conservation efforts not only on the limited number of perennial rivers remaining in arid regions, but also on intermittent and ephemeral rivers with shallow water tables.
Riparian corridors possess an unusually diverse array of species and environmental processes. This @'ecological@' diversity is related to variable flood regimes, geomorphic channel processes, altitudinal climate shifts, and upland influences on the fluvial corridor. This dynamic environment results in a variety of life history strategies, and a diversity of biogeochemical cycles and rates, as organisms adapt to disturbance regimes over broad spatio-temporal scales. These facts suggest that effective riparian management could ameliorate many ecological issues related to land use and environmental quality. We contend that riparian corridors should play an essential role in water and landscape planning, in the restoration of aquatic systems, and in catalyzing institutional and societal cooperation for these efforts.
The exotic tree, Russian olive (Elaeagnus angustifolia), has invaded riparian zones throughout much of the western Unites States. Al-though promoted as a useful species for wildlife be-cause of its abundant edible fruit, evidence for its value to breeding birds remains sparse. We compared rela-tive rates of usage, nest success, and cowbird parasit-ism of birds breeding in Russian olive versus native tree species at a site where Russian olive is a minor component. Some species, such as the Mourning Dove (Zenaida macroura) and Yellow-breasted Chat (Icteria virens), preferentially placed their nests in Russian ol-ive. Nest success was similar for nests in Russian olive and native species. During 1997, nests of the Willow Flycatcher (Empidonax traillii) were significantly more likely to be parasitized by Brown-headed Cow-birds (Molothrus ater) when placed in Russian olive than in native species, although nest success was not The impacts of invasive exotic species on native biota and ecosystems have become a major concern among conservation biologists and land managers. Almost half of the threat-ened and endangered species in the United States are imperiled by alien species (Wilcove et al. 1998). In the Southwest, anthropogenic alterations of flood regimes and extensive clearing of riparian woodlands have promoted the invasion and proliferation of several exotic woody plants, particularly saltcedar (Tamarix ramosissima) and Russian olive (Elaeagnus angustifolia). Much is known about saltcedar and its ecological impacts (Lovich and de Gouvenain 1998), but Russian olive remains relatively poorly known.
Nonnative plant invasions are a management concern, particularly in riparian forests, but little is known about mechanisms through which they influence vertebrate communities. In the American Southwest, native trees such as cottonwood (Populus spp.) are thought to provide better habitat for breeding birds than nonnative plants, which are more tolerant of human-altered conditions. To evaluate effects of riparian forest composition on riparian-nesting birds, we examined nest plant use along two rivers in New Mexico that differed in abundance of nonnative vegetation. Of the nests we observed, 49% along the Middle Rio Grande were constructed in nonnative plants, compared with 4% along the Gila River. Birds in the canopy and cavity-nesting guilds constructed less than 5% of their nests in nonnative plants along either river. At the Middle Rio Grande, birds in the subcanopy/shrub guild constructed 67% of their nests in nonnative plants. Despite the relatively low availability of cottonwoods, they were used by greater numbers of species than any other woody plant at either river. Riparian obligates and species of conservation concern in the canopy and cavity guilds were especially dependent on cottonwood and Arizona sycamore (Platanus wrightii). Our results show that, although nonnative trees and shrubs support large numbers of nests for certain birds, cottonwoods and other large native trees are disproportionately important to riparian bird communities.
Riparian systems in the western United States provide important habitat for bird communities during all times of the year. In recent decades, invasive plants, such as Russian olive (Elaeagnus angustifolia), have achieved broad distribution and local dominance in many western riparian areas, raising concerns over the loss of ecological function within these systems. In 2005 and 2006 we conducted avian point counts and surveyed vegetation cover at 95 points along the Snake and Columbia Rivers in southeastern Washington to investigate the effects of total woody vegetation cover and the relative proportion of Russian olive cover on breeding and wintering riparian bird communities. Our results indicated that riparian habitats dominated by Russian olive can support diverse and abundant bird communities, though cavity nesting species were noticeably sparse. Bird density and species richness were best explained by a quadratic relationship to total woody vegetation cover in both seasons, as was breeding bird community composition, with greatest density and richness in intermediate cover levels. We found no indication that the proportion of the woody vegetation comprised of Russian olive strongly influenced any of these bird community metrics. Given that Russian olive comprised 81.6% of the riparian vegetation in our study area, it is unclear from our results how Russian olive would affect bird communities in regions where native vegetation is more abundant. Regardless, complete eradication of Russian olive from riparian systems where the plant is a major component will reduce the overall habitat value for birds by eliminating significant structural complexity.
Riparian ecosystems include relatively mesic vegetative communities and associated faunas occurring between aquatic and more xeric upland sites. In eastern North America, these ecosystems often occur in broad zones and are referred to as floodplains or bottomlands. In the west, they are conspicuous as narrow belts of vegetation along ephemeral, inter- mittent, and perennial streams and rivers and are most obvious in steppe, shrubsteppe, and desert regions. Vegetation associated with streams has been referred to as the "aorta of an ecosystem" (Wilson 197932) because of its significance to the perpetuation of water, fish, wildlife, rangeland, and forest resources. Historically, riparian ecosystems have been subjected to both subtle and dramatic perturbations from water management practices (Carothers and Johnson 1975, Curtis and Ripley 1975) (Fig. l), agricultural conver- sions (Best et al. 1979, Conine et al. 1979), grazing (Cope 1979, Knopf and Cannon 1982), channelization (Barclay 1979, McCall and Knox 1979), and recreational development (Aitchison 1977, Schmidly and Ditton 1979, Johnson and Carothers 1982). Riparian systems represent areas of max- imum potential conflict between users of timber, grazing, recreational, water, and wildlife resources (Thomas et al. 1979). Additionally, exotic woody species such as salt cedar (Tumavix pentandra) and Russian olive (Elaeagnus angustifolia) have naturalized extensively within western ri- parian ecosystems (Robinson 1965, Horton 1977, Olson and Knopf 1986b), displacing native woody species that provide valuable avian habitats but also providing additional habitats for selected species of wildlife (Knopf and Olson 1984, Hunter et al. 1985). Riparian ecosystems have recently attracted much attention, especially relative to the management of public lands in the west. Within the last
Current methods of evaluating wildlife habitat for management,purposes can be arranged in a hierarchy of increasing generality. The most general level is evaluation of wildlife habitat for entire com- munities on the basis of inferences drawn from vegetational structure. At the base of the hierarchy the high resolution studies, upon which accuracy at the higher hierarchical levels depends, usually assume that habitat quality for a species is positively correlated with the density of the species. If habitat quality for a wildlife species is a measure of the importance of habitat type in maintaining a particular species, habitat quality should be defined in terms of the survival and production characteristics, as well as the density, of the species occupying that habitat. Situations in which habitat quality thus defined is not expected to be positively
A pesar de los esfuerzos muy difundidos de evitar los fuegos silvestres mediante la reductión de la densidad de la vegetatión inflamable, poco se conoce sobre los efectos de esta práctica sobre la biología reproductiva de las aves de bosque. Examinamos la selectión de los sitios de anidación y la supervivencia de los nidos del picaflor Archilochus alexandri en los bosques ribereños de Nuevo México, con y sin reducción de combustible. En las parcelas sin tratamiento, los picaflores anidaron frecuentemente en árboles exóticos como Tamarix spp. y Eleagnus angustifolia. Después de la reducción de combustible, se incrementó el uso de Populus deltoides ssp. wislizenii como sustrato para los nidos y las aves anidaron a mayores alturas. Aunque la reducción del combustible influenció la selección de hábitat y del sitio de anidación, no afectó inmediatamente la supervivencia de los nidos. Un modelo de expositión logistica que incluyó los efectos del año y de la interactión de la altura del nido con el sustrato brindó la mejor explicatión de la supervivencia de los nidos. Las tasas de supervivencia diaria de los nidos estimadas con este modelo variaron entre años desde 0.970 (95% IC: 0.949–0.982) a 0.992 (95% IC: 0.983–0.996), correspondiendo a tasas de supervivencia durante el período de 31% (95% IC: 13.7%–50.1%) a 73% (95% IC: 52.1%–85.9%). Adicionalmente, en todos los substratos excepto Tamarix, la supervivencia de los nidos disminuyó con la altura del nido. Nuestros estimados de supervivencia de los nidos fueron elevados, lo que sugiere que el bosque ribereño a lo largo de la parte media del Río Grande brinda hábitat de anidación de alta calidad para esta especie. La reducción del combustible, sin embargo, disminuye la disponibilidad de los sitios de anidación y puede bajar la supervivencia del nido mediante la remoción de sitios potenciales de anidación en el sotobosque, forzando a los picaflores a anidar a mayores alturas donde el riesgo de depredatión es mayor.
The exotic tree, Russian olive (Elaeagnus angustifolia), has invaded riparian zones throughout much of the western Unites States. Although promoted as a useful species for wildlife because of its abundant edible fruit, evidence for its value to breeding birds remains sparse. We compared relative rates of usage, nest success, and cowbird parasitism of birds breeding in Russian olive versus native tree species at a site where Russian olive is a minor component. Some species, such as the Mourning Dove (Zenaida macroura) and Yellow-breasted Chat (Icteria virens), preferentially placed their nests in Russian olive. Nest success was similar for nests in Russian olive and native species. During 1997, nests of the Willow Flycatcher (Empidonax traillii) were significantly more likely to be parasitized by Brown-headed Cowbirds (Molothrus ater) when placed in Russian olive than in native species, although nest success was not significantly different. Our results may not apply to areas where Russian olive is common.
Understanding why some introduced species become naturalized and invasive whereas others do not is a major focus of invasion ecology. Invasive species risk assessments address this same question, but are not typically based on the results from recent ecological studies. Applying results from the ecological literature to risk assessment is difficult, in part because there are no general explanations of invasion likelihood across taxa. Most ecological studies are also specific to a particular region and it is unclear whether outcomes in one region will necessarily apply to another. Here we show how a hierarchical Bayesian statistical framework can make better use of ecological studies for applied risk assessments. We focus on three key opportunities afforded by these models: (1) the ability to leverage information from one region to form prior expectations for other regions about which little is known, (2) the ability to quantify uncertainty of predictions, and (3) flexibility to incorporate within-group heterogeneities in probabilities of naturalization. We illustrate these principles using a case study where we predict the probability of plant taxa naturalizing in New Zealand and Australia, showing how prior information can be particularly valuable when data are limited. As more studies document invasion patterns around the world, a framework that can formally incorporate prior information will help link the accumulating data on species introductions to risk assessments.
Many of the world’s large river systems have been greatly altered in the past century due to river regulation, agriculture, and invasion of introduced Tamarix spp. (saltcedar, tamarisk). These riverine ecosystems are known to provide important habitat for avian communities, but information on responses of birds to differing levels of Tamarix is not known. Past research on birds along the Colorado River has shown that avian abundance in general is greater in native than in non-native habitat. In this article, we address habitat restoration on the lower Colorado River by comparing abundance and diversity of avian communities at a matrix of different amounts of native and non-native habitats at National Wildlife Refuges in Arizona. Two major patterns emerged from this study: (1) Not all bird species responded to Tamarix in a similar fashion, and for many bird species, abundance was highest at intermediate Tamarix levels (40–60%), suggesting a response threshold. (2) In Tamarix-dominated habitats, the greatest increase in bird abundance occurred when small amounts of native vegetation were present as a component of that habitat. In fact, Tamarix was the best vegetation predictor of avian abundance when compared to vegetation density and canopy cover. Our results suggest that to positively benefit avian abundance and diversity, one cost-effective way to rehabilitate larger monoculture Tamarix stands would be to add relatively low levels of native vegetation (∼20–40%) within homogenous Tamarix habitat. In addition, this could be much more cost effective and feasible than attempting to replace all Tamarix with native vegetation.
Exotic vegetation has become a major habitat component in many ecosystems around the world, sometimes dramatically changing the vegetation community structure and composition. In the southwestern United States, riparian ecosystems are undergoing major changes in part due to the establishment and spread of the exotic Tamarix (saltcedar, tamarisk). There are concerns about the suitability of Tamarix as habitat for birds. Although Tamarix habitats tend to support fewer species and individuals than native habitats, Arizona Breeding Bird Atlas data and Birds of North America accounts show that 49 species use Tamarix as breeding habitat. Importantly, the relative use of Tamarix and its quality as habitat vary substantially by geographic location and bird species. Few studies have examined how breeding in Tamarix actually affects bird survivorship and productivity; recent research on Southwestern Willow Flycatchers has found no negative effects from breeding in Tamarix habitats. Therefore, the ecological benefits and costs of Tamarix control are difficult to predict and are likely to be species specific and site specific. Given the likelihood that high-quality native riparian vegetation will not develop at all Tamarix control sites, restoration projects that remove Tamarix but do not assure replacement by high-quality native habitat have the potential to reduce the net riparian habitat value for some local or regional bird populations. Therefore, an assessment of potential negative impacts is important in deciding if exotic control should be conducted. In addition, measurable project objectives, appropriate control and restoration techniques, and robust monitoring are all critical to effective restoration planning and execution.
Elaeagnus angustifolia (Russian olive) is an alien tree that is increasingly common in riparian habitats of western North America. This paper reviews
the pertinent scientific literature in order to determine the status ofE. angustifolia as a riparian invader and to suggest ecological reasons for its success.Elaeagnus angustifolia meets the biogeographic, spread, and impact criteria for invasive species. Ecological characteristics likely enabling its
invasiveness include adaptation to the physical environmental conditions that characterize semi-arid riparian habitats, lack
of intense pressure from herbivores, and tolerance of the competitive effects of established vegetation. We believe that the
success of this species is at least partly due to its ability to take advantage of the reduced levels of physical disturbance
that characterize riparian habitats downstream from dams. Control ofE. angustifolia is likely to be most promising where natural river flow regimes remain relatively intact.
Concern about spread of non-native riparian trees in the western USA has led to Congressional proposals to accelerate control efforts. Debate over these proposals is frustrated by limited knowledge of non-native species distribution and abundance. We measured abundance of 44 riparian woody plants at 475 randomly selected stream gaging stations in 17 western states. Our sample indicates that Tamarix ramosissima and Elaeagnus angustifolia are already the third and fourth most frequently occurring woody riparian plants in the region. Although many species of Tamarix have been reported in the region, T. ramosissima (here including T. chinensis and hybrids) is by far the most abundant. The frequency of occurrence of T. ramosissima has a strong positive relation with the mean annual minimum temperature, which is consistent with hypothesized frost sensitivity. In contrast the frequency of occurrence of E. angustifolia decreases with increasing minimum temperatures. Based on mean normalized cover, T. ramosissima and E. angustifolia are the second and fifth most dominant woody riparian species in the western USA. The dominance of T. ramosissima has been suspected for decades; the regional ascendance of E. angustifolia, however, has not previously been reported.
Woodlands of the exotic saltcedar (Tamarix chinensis) have replaced forests of native Fremont cottonwood (Populus fremontii) and willow (Salixspp.) along many rivers of the American South-west. In the middle basin of the San Pedro River, saltcedar dominates only at the drier sites where the surface and ground-water conditions no longer support cottonwood–willow forests. At sites with perennial (or near-perennial) stream flow, saltcedar is co-dominant with Fremont cottonwood. However, saltcedar has been declining in importance at these sites, perhaps due to recent occurrence of conditions that favour cottonwood establishment (frequent winter flooding, high rates of stream flow during spring, exclusion of livestock). This shift provides evidence of capacity for self-repair in degraded Sonoran riparian ecosystems. In the upper basin, in contrast, saltcedar has increased in relative abundance at sites that show evidence of ground-water decline, signaling a need for vigilance in river management. Saltcedar is generally sparse in the upper basin, probably due to the combination of cool temperatures and persistence of perennial or near-perennial stream flows in most areas. Throughout the San Pedro River, saltcedar and cottonwood both have been influenced by changing flood patterns. Expansion of Fremont cottonwood populations and initial colonization by saltcedar both correlate with post-1960 increases in fall and winter flood frequency and decreases in summer flood size.
Understanding patterns of plant population mortality during extreme weather events is important to conservation planners because the frequency of such events is expected to increase, creating the need to integrate climatic uncertainty into management. Dominant plants provide habitat and ecosystem structure, so changes in their distribution can be expected to have cascading effects on entire communities. Observing areas that respond quickly to climate fluctuations provides foresight into future ecological changes and will help prioritize conservation efforts. We investigated patterns of mortality in six dominant plant species during a drought in the southwestern United States. We quantified population mortality for each species across its regional distribution and tested hypotheses to identify ecological stress gradients for each species. Our results revealed three major patterns: (1) dominant species from diverse habitat types (i.e., riparian, chaparral, and low- to high-elevation forests) exhibited significant mortality, indicating that the effects of drought were widespread; (2) average mortality differed among dominant species (one-seed juniper[Juniperus monosperma (Engelm.) Sarg.] 3.3%; manzanita[Arctostaphylos pungens Kunth], 14.6%; quaking aspen[Populus tremuloides Michx.], 15.4%; ponderosa pine[Pinus ponderosa P. & C. Lawson], 15.9%; Fremont cottonwood[Populus fremontii S. Wats.], 20.7%; and pinyon pine[Pinus edulis Engelm.], 41.4%); (3) all dominant species showed localized patterns of very high mortality (24–100%) consistent with water stress gradients. Land managers should plan for climatic uncertainty by promoting tree recruitment in rare habitat types, alleviating unnatural levels of competition on dominant plants, and conserving sites across water stress gradients. High-stress sites, such as those we examined, have conservation value as barometers of change and because they may harbor genotypes that are adapted to climatic extremes.
Resumen: El entendimiento de los patrones de mortalidad de poblaciones de plantas durante eventos climáticos extremos es importante para los planificadores de conservación porque se espera que la frecuencia de tales eventos aumente, creando la necesidad de integrar la incertidumbre climática a la gestión. Las plantas dominantes proporcionan hábitat y estructura al ecosistema, así que se puede esperar que cambios en su distribución tengan efectos de cascada en toda la comunidad. La observación de áreas que responden rápidamente a las fluctuaciones climáticas proporciona un panorama de futuros cambios ecológicos y ayudará a la definición de prioridades de esfuerzos de conservación. Investigamos los patrones de mortalidad en seis especies de plantas dominantes durante una sequía en el suroeste de Estados Unidos. Cuantificamos la mortalidad poblacional para cada especie en su área de distribución regional y probamos hipótesis para identificar los gradientes de estrés ecológico para cada especie. Nuestros resultados revelaron tres patrones mayores: (1) las especies dominantes en diversos tipos de hábitats (i.e., ribereño, chaparral y bosques de baja a alta elevación) presentaron mortalidad significativa, lo que indica que los efectos de la sequía fueron extendidos; (2) la mortalidad promedio fue diferente (Juniperus monosperma [Engelm.] Sarg.) 3.3%; Arctostaphylos pungens Kunth, 14.6%; Populus tremuloides Michx., 15.4%; Pinus ponderosa P. & C. Lawson, 15.9%; Populus fremontii S. Wats., 20.7%; y Pinus edulis Engelm., 41.4%); (3) todas las especies dominantes mostraron patrones localizados de mortalidad muy alta (24–100%) consistentes con gradientes de estrés hídrico. Los gestores de tierras deberían planificar para la incertidumbre climática mediante la promoción del reclutamiento de árboles en tipos de hábitat raros, lo que aligeraría los niveles no naturales de competencia sobre las plantas dominantes y conservaría sitios a lo largo de gradientes de estrés hídrico. Los sitios con estrés alto, como los que examinamos, tienen valor de conservación como barómetros de cambio y porque pueden albergar genotipos que están adaptados a cambios climáticos extremos.
1 The invasion of habitats by non-native plant and animal species is a global phenomenon with potentially grave consequences for ecological, economic, and social systems. Unfortunately, to date, the study of invasions has been primarily anecdotal and resistant to generalization.
2 Here, we use insights from experiments and from long-term monitoring studies of vegetation to propose a new theory in which fluctuation in resource availability is identified as the key factor controlling invasibility, the susceptibility of an environment to invasion by non-resident species. The theory is mechanistic and quantitative in nature leading to a variety of testable predictions.
3 We conclude that the elusive nature of the invasion process arises from the fact that it depends upon conditions of resource enrichment or release that have a variety of causes but which occur only intermittently and, to result in invasion, must coincide with availability of invading propagules.
Ecology Letters (2011) 14: 702–708
Biological invasions cause ecological and economic impacts across the globe. However, it is unclear whether there are strong patterns in terms of their major effects, how the vulnerability of different ecosystems varies and which ecosystem services are at greatest risk. We present a global meta-analysis of 199 articles reporting 1041 field studies that in total describe the impacts of 135 alien plant taxa on resident species, communities and ecosystems. Across studies, alien plants had a significant effect in 11 of 24 different types of impact assessed. The magnitude and direction of the impact varied both within and between different types of impact. On average, abundance and diversity of the resident species decreased in invaded sites, whereas primary production and several ecosystem processes were enhanced. While alien N-fixing species had greater impacts on N-cycling variables, they did not consistently affect other impact types. The magnitude of the impacts was not significantly different between island and mainland ecosystems. Overall, alien species impacts are heterogeneous and not unidirectional even within particular impact types. Our analysis also reveals that by the time changes in nutrient cycling are detected, major impacts on plant species and communities are likely to have already occurred.
How anthropogenic climate change will affect hydroclimate in the arid regions of southwestern North America has implications
for the allocation of water resources and the course of regional development. Here we show that there is a broad consensus
among climate models that this region will dry in the 21st century and that the transition to a more arid climate should already
be under way. If these models are correct, the levels of aridity of the recent multiyear drought or the Dust Bowl and the
1950s droughts will become the new climatology of the American Southwest within a time frame of years to decades.
Since its introduction before 1900, Elaeagnus angustifolia has become naturalized in many areas throughout the Rocky Mountains states. Avian and mammalian communities were inventoried at areas of monotypic stands in Colorado, Idaho and Utah. Russian-olive stands tended to support avian communities intermediate in species richness and alpha diversity to native riparian and upslope communities. Beta diversity comparisons indicated stronger relationships between Russian-olive and riparian communities than upslope communities. Small mammal communities contained low species richness, but followed the pattern of intermediate diversity between the native sites. Russian-olive is intermediate in height to the native communities, and frequently establishes along the interface of the two. The species appears to be competitively inferior to native overstory species. The continued expansion of Russian-olive will increase the width of lowland riparian zones at some locations, providing additional habitats for especially those avian species that are associated with tall-shrub vegetation.-from Authors
Russian-olive is a small tree or large multistemmed shrub that was introduced to Canada and the United States from Eurasia in the early 1900s. It was provisioned in large numbers during the last century to prairie farmers as a shelterbelt plant and remains a popular and widely available ornamental. Now invasive within some riparian ecosystems in the western United States, Russian-olive has been declared noxious in the states of Colorado and New Mexico. With traits including high shade tolerance and a symbiotic association with nitrogen-fixing bacteria, Russian-olive has the potential to dominate riparian vegetation and thus radically transform riparian ecosystems. Especially alarming is its capacity to influence nutrient dynamics within aquatic food webs. Our objective is to draw attention to Russian-olive as a potential threat to riparian ecosystems within Canada, especially in the southwest, where invasion is becoming commonplace. We review what is known about its biology and about the threats it poses to native organisms and ecosystems, and we summarize management and control efforts that are currently underway. We conclude by proposing a research agenda aimed at clarifying whether and how Russian-olive poses a threat to riparian ecosystems within western Canada. Nomenclature: Russian-olive; Elaeagnus angustifolia L. ELGAN.
We examined abundance and richness of wood warblers among vegetation types at the Bosque del Apache National Wildlife Refuge and the Rio Grande Nature Center in the Middle Rio Grande Valley of New Mexico. Rarefaction analysis indicated that there were marginally significant differences in species richness between sites. In spring, wood-warbler richness was greater at the Bosque del Apache than at the Nature Center. This pattern reflected the greater likelihood of catching uncommon migrants at the Bosque del Apache than at the Nature Center. The same seven species were most common at both sites although their order of abundance differed between sites. Capture rates of these species were greater at the Nature Center than at the Bosque del Apache in the fall, but not during the spring. In general, variation in capture rates among vegetation types was greater in the Bosque del Apache than in the Nature Center. In particular the capture rates in willow were greater than in other vegetative types.
Avian use of saltcedar (Tamarix chinensis) along the middle Pecos River was compared with similarly collected data along the lower Colorado River and Rio Grande. Use of saltcedar ranked high among all bird groups in all seasons on the middle Pecos River. In contrast, many species do not occur in saltcedar on the lower Colorado River, while few species winter in saltcedar on the lower Rio Grande. Occurrence of granivores and insectivores during winter in saltcedar on the Pecos River may be explained by seed-producing shrubs and annuals within or adjacent to these habitats. Most breeding birds on the Pecos River are summer visitors. These breeding species, though present, do not occur in saltcedar on the Colorado River despite abundant food resources and occur in intermediate abundances on the Rio Grande. Densities of several summer-visiting insectivores have declined markedly on the Colorado River since the proliferation of saltcedar, whereas they have remained relatively stable in other river valleys to the east. Biogeographical considerations, specifically elevational (climatic) gradients, are suggested reasons for this phenomenon.
Vegetation structure and the numbers of shrubs were measured at nest sites of 11 species of riparian birds in a tamarisk (Tamarix ramosissima) community to characterize breeding habitat by species. Discriminant function analysis indicated that riparian birds exhibited significant differences in their use of nesting habitat. Bell's vireo (Vireo bellii), yellow warbler (Dendroica petechia), and yellow-breasted chat (Icteria virens) were relative generalists in their use of nesting habitat, while common yellowthroat (Geothlypis trichas) and Bullock's oriole (Icterus galbula bullockii) were relative specialists. Bell's vireo and American coot (Fulica americana) used the most dissimilar habitats. Willow flycatcher (Empidonax traillii) and yellow warbler used the most similar habitats. Willow flycatcher and yellow warbler consistently used habitat most similar to that used by other species. The tamarisk community created by the operation of Glen Canyon Dam represents the ecological equivalent of native habitat for some riparian birds, and its presence has enhanced breeding habitat for these 11 species of birds.
Investigated the possible importance of indivdual tree species in avian habitat selection by comparing tree species contributions to contributions of horizontal and vertical patchiness and density of vegetation. Individual bird species responded with greater frequency to number of particular species of trees than to any variables depicting structure. Many of the commonly found correlations of bird community relationships to vegetation profiles may be the result of combining analyses of many different bird species with many different tree of plant species associations. Studies of avian habitat use include measures of tree species composition of the sites even if foliage profile or denisty measures alone provide statistically significant results.-from Authors
Exotic species can provide abundant food resources for native consumers, but predicting which native species will respond positively remains a challenge. We studied the foraging behavior of black-capped (Poecile atricapillus) and mountain (P. gambeli) chickadees in western Montana to compare the degree to which these congeric and syntopic consumers exploited larvae of Urophora, an exotic biological control insect living within the seedheads of the invasive forb, spotted knapweed (Centaurea stoebe). Chickadees typically forage within tree or shrub cover, whereas knapweed and hence Urophora larvae thrive in open grassland away from cover. We found that black-capped chickadees were much more likely than mountain chickadees to forage for Urophora. Black-capped chickadees strategically minimized time spent in open habitats by flying out from cover to retrieve knapweed seedheads and immediately returning to cover to extract the larvae. Black-capped chickadees also employed an atypical hovering technique nearly twice as often as their congeners did, particularly when foraging away from cover. Via this hovering technique, birds were able to gather knapweed seedheads from erect plants rather than searching for seedheads on the ground. These shifts in foraging behavior allowed black-capped chickadees to exploit Urophora larvae to a much greater degree than their congeners while minimizing exposure to a high-risk habitat, an outcome with potentially important community-wide consequences. Behavioral flexibility has been used to predict the success of invading species. We suggest that behavioral flexibility may also be used to predict how native species will respond to invasions, particularly the availability of exotic food resources.
Mechanical clearing and herbicide-burn treatments were compared to evaluate salteedar (Tamarix chinensis Lour.) control and recovery along the Rio Grande on the Bosque del Apache National Wildlife Refuge, Socorro, N.M. The herbicide-burn treatment included an aerial application of imazapyr (+/-)-2-[4,5dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3- pyridinecarboxylic acid] + glyphosate [N-(phosphonomethyl)glycine] (0.6 + 0.6 kg ai hat rate) followed 3 years later by a prescription broadcast fire that eliminated > 99% of the standing dead stems. Six years after initial herbicide application, saitcedar mortality was 93%. Mechanical saltcedar clearing entailed removing aerial (trunks and stems) growth by blading, stacking and burning debris, followed by removal of underground plant portions (root crowns) by plowing, raking, and burning stacked material. Saitcedar mortality 3 years after mechanical clearing averaged 70%, which was deemed unsatisfactory. Thus, root plowing, raking, and pile burning was repeated. Three years later, after the second mechanical clearing, saitcedar mortality was 97%. Costs for the herbicide-burn treatment averaged $283 ha(-1), whereas mechanical control costs were $884 ha(-1) for the first surface and root clearing and an additional $585 ha(-1) for the second root clearing. Riparian managers should consider environmental conditions and restoration strategies prior to selecting a saltcedar control approach. Although control costs were significantly lower for the herbicide-burn treatment compared to mechanical clearing in this study, the choice of methods should always consider alternative control strategies for saitcedar. Frequently, combinations of methods result in more efficient, cost-effective results.
Invasion by trees into grasslands is commonly associated with negative consequences to many species of breeding birds. We investigated relationships between abundance of Russian olive (Elaeagnus angustifolia), nesting black-billed magpies (Pica pica), and duck nest success on management areas in southeastern Idaho. Duck nest success tended to vary inversely with abundance of Russian olive at the regional scale, averaging 42.9% on management areas where Russian olive abundance was low, 19.8% where it was moderate, and 6.8% where it was high. Intensive studies during 1992-1993 and 1995-1996 at Sterling Wildlife Management Area (SWMA), an area with especially high abundance of Russian olive and density of nesting magpies, produced mixed evidence on the relationships between Russian olive, magpies, and duck nest success. Survival of artificial duck nests at SWMA increased slightly with increasing distance from Russian olive, but median distance to nearest active magpie nest and to nearest Russian olive did not differ between successful and depredated duck nests. However, median distance between duck nests and nearest active magpie nest was <200 m, and we found only one duck nest that was >400 m from an active magpie nest. Hence, few duck nests at SWMA may have been far enough from magpie nests to provide detectable increase in nest security. We predicted that density of nesting magpies would decrease and success of duck nests would increase after managers removed Russian olive from a 347-ha treatment area midway through our study. Magpies shifted nesting substrate, primarily to tall big sagebrush (Artemisia tridentata) plants following treatment, but neither magpie nest density nor duck nest success changed as predicted. We urge managers to carefully consider the risks of accepting (or introducing) trees in historically treeless areas.
The history of naturalization of Russian olive, Elaeagnus angustifolia L., in Utah is presented. During the first half of this century Russian olive became a common species in cultivation in Utah cities. By 1924 the tree was becoming established in nature in central Utah. Naturalization has been most evident during the last two decades. At present Russian olive is a conspicuous part of the vegetation in the valleys of central and northern Utah. The histories of naturalization of the plant in other western states are similar to that in Utah.
Aim To test the hypothesis that anthropogenic alteration of stream-flow regimes is a key driver of compositional shifts from native to introduced riparian plant species.
Location The arid south-western United States; 24 river reaches in the Gila and Lower Colorado drainage basins of Arizona.
Methods We compared the abundance of three dominant woody riparian taxa (native Populus fremontii and Salix gooddingii, and introduced Tamarix) between river reaches that varied in stream-flow permanence (perennial vs. intermittent), presence or absence of an upstream flow-regulating dam, and presence or absence of municipal effluent as a stream water source.
Results Populus and Salix were the dominant pioneer trees along the reaches with perennial flow and a natural flood regime. In contrast, Tamarix had high abundance (patch area and basal area) along reaches with intermittent stream flows (caused by natural and cultural factors), as well as those with dam-regulated flows.
Main conclusions Stream-flow regimes are strong determinants of riparian vegetation structure, and hydrological alterations can drive dominance shifts to introduced species that have an adaptive suite of traits. Deep alluvial groundwater on intermittent rivers favours the deep-rooted, stress-adapted Tamarix over the shallower-rooted and more competitive Populus and Salix. On flow-regulated rivers, shifts in flood timing favour the reproductively opportunistic Tamarix over Populus and Salix, both of which have narrow germination windows. The prevailing hydrological conditions thus favour a new dominant pioneer species in the riparian corridors of the American Southwest. These results reaffirm the importance of reinstating stream-flow regimes (inclusive of groundwater flows) for re-establishing the native pioneer trees as the dominant forest type.
Within the Mojave, Sonoran, and Chihuahuan Desert subdivisions of the North American Desert in the U.S., more than half of 143 total amphibian and rep- tilian species perform as riparian and/or wetland taxa. For the reptiles, but not the amphibians, there is a sig- nificant inverse relationship between riparianness (obli- gate through preferential and facultative to nonriparian) and desertness. In addition to the nondesert species (N=36) present, there are two evolutionary kinds of desert species in the herpetofauna: true desert species (N-20), and desert-included species (N=87); the former are obligate specialists, the latter are facultative gener- alists. Quantitative aspects of desertness, riparianness, species richness, nondesert taxa and others are exam- ined. A large part of the herpetofauna of North America is located extensively and abundantly in riparian habitats. No other terrestrial vertebrate group is a better indicator of the biological health of riparian ecosystems. Within the "warm deserts" of the Southwest United States more than half of the total amphibian and reptilian species perform as riparian and/or wetland taxa. Riparian taxa are obligate, preferential, or facultative components of riparian ecosystems. Thus including the nonriparian condition, four levels of riparianness (R), or riparian dependency, are recognized (Dick-Peddie and Hubbard 1977, Johnson, and others 1987). Moreover, for deserts, in addition to the distinction between desert species and nondesert species, there is a clear distinction between two evolutionary kinds of desert species: true desert species and desert-included species. True desert species are obligate specialists in the real sense that they have evolved within desert environments, while the desert- included species tend to be facultative generalists that include desert environments in their much wider and often widely extensive ecological and geographical distributions. Thus including the nondesert condition, three levels of desertness (D) are recognized (Lowe 1968; and others, 1986):
Riparian vegetation provides important habitat for migrating and breeding birds in the arid south-western United States. Surveys conducted in native cottonwood (Populus fremontii var. wislizeni) and introduced saltcedar (Tamarix chinensis) vegetation during spring, summer and fall were used to compare avian use of these two vegetation types along the Middle Rio Grande Valley in central New Mexico. Bird species richness in cottonwood and saltcedar did not differ during any season, but species composition varied. Overlap in species between the two habitats was lowest in spring and increased in fall, with more species unique to cottonwood than to saltcedar in all seasons. A number of obligate riparian species readily used saltcedar, while others were restricted to areas dominated by native vegetation. Neotropical migrants showed a slight preference for cottonwood in the spring, but some migrant species were detected in saltcedar as well, possibly reflecting the dense nature of this vegetation. However, although saltcedar was used by a number of species, maintenance of native vegetation will be necessary for the persistence of many bird species in the Middle Rio Grande Valley.
Initially introduced to western United States to provide ecosystem services such as erosion control, Tamarix by the mid-1900s had became vilified as a profligate waster of water. This large shrub continues, today, to be indicted for various presumed environmental and economic costs, and millions of dollars are expended on its eradication. In this review, we examine the role of scientists in driving changes in perceptions of Tamarix from valuable import to vilified invader and (in some instances) back to a productive member of riparian plant communities. Scientists over the years have sustained a negative perception of Tamarix by, among other things, (1) citing outmoded sources; (2) inferring causation from correlative studies; (3) applying conclusions beyond the scope (domain) of the studies; and (4) emphasizing findings that present the species as an extreme or unnatural agent of change. Recent research is challenging the prevailing dogma regarding Tamarix’s role in ecosystem function and habitat degradation and many scientists now recommend management shifts from “pest plant” eradication to systemic, process-based restoration. However, prejudice against this and other non-native species persists. To further close the gap between science and management, it is important for scientists to strive to (1) cite sources appropriately; (2) avoid reflexive antiexotic bias; (3) avoid war-based and pestilence-based terminology; (4) heed the levels of certainty and the environmental domain of studies; (5) maintain up-to-date information on educational Web sites; and (6) prior to undertaking restoration or management actions, conduct a thorough and critical review of the literature.
Russian-olive (Elaeagnus angustifolia) is a small Eurasian tree that has escaped from cultivation and become naturalized, primarily along watercourses throughout the western United States. We examined germination and establishment of Russian-olive and plains cottonwood (Populus deltoides), the principal native riparian tree of the Great Plains, under a range of experimental moisture and light conditions. The fewest seedings established under the driest conditions; seedling biomass was predictably lower in the shade; root-to-shoot ratios were higher for cottonwood, higher in the sun, and higher under drier conditions. Several interactions were also significant. The timing of germination and mortality varied between plains cottonwood and Russian-olive: cottonwood germinated in mid-June in all treatments in a single pulse with subsequent mortality; the timing and amount of Russian-olive germination differed substantially across treatments with little net mortality. Differences in life-history traits of these species, including seed size, viability, and dispersal, help explain treatment differences. Russian-olive will likely remain an important component of riparian communities along both unregulated and regulated western rivers because it succeeds under conditions optimal for cottonwood establishment and under many conditions unfavorable for cottonwood. Furthermore, many western states still encourage planting of Russian-olive, and control techniques tend to be labor-intensive and expensive.
Based on a hydrogeological survey and geochemical and isotopic technology, a case study of the Shiyang River Basin is presented to illustrate ground-water resources and geochemistry and their changes caused by the impact of human activity in the arid area of Northwest China. The aquifer is mainly recharged by surface water originating in mountain regions, and there is extensive transfer between rainfall, surface water and ground-water. The deep ground-water is old, approximately 40 ka, and was recharged in a colder and wetter climate environment. The shallower water is mainly palaeowater mixed with limited modern recharge. The sources of salinity are from weathering of rock in mountain areas and from higher evaporation leading to higher salinity along the line of ground-water flow. Human activity, in particular large-scale water resources development associated with dramatic population growth in the last 50 years, has led to tremendous changes in the ground-water regime. Recharge has been reduced by 50% and ground-water abstraction exceeds recharge by 0.41×109 m3 yr−1. Consequently, the ground-water level has fallen widely by between 3 and 5 m, with a maximum fall of 35 m in several towns. These hydrological changes have resulted in a serious degradation of the ecosystem. It is suggested that modernized irrigation technology and new regulation to cover water resources management and allocation within the river basin are urgently needed to achieve sustainable development.
Saltcedar invasion has many economic and environmental effects, including displacement of native riparian vegetation and associated wildlife. A biological control program led to the approval in 1994 of two insects for introduction but was delayed by the presence of the endangered southwestern willow flycatcher (SWWF) in saltcedar. In 2001, the saltcedar leaf beetle was released in six states but not where the SWWF was present. Delays circumvent the benefits that saltcedar suppression could have for other declining species, including many rare or absent in ecosystems dominated by saltcedar. Numerous birds forage within saltcedar vegetation but in lower numbers and diversity than in native stands that provide better habitat and insect resources. Successful establishment by saltcedar leaf beetle resulted in extensive saltcedar defoliation, and observations of wildlife feeding on the beetles in an otherwise depauperate system suggest that biocontrol may enhance habitat quality for many species, including the SWWF. Consideration of the multiple species affected by saltcedar would have allowed more effective invasive plant management in this case, but delays also reflect drawbacks in federal administrative structures related to invasive species management in 'natural areas' as much as problems with a narrow focus on a single species. A functionally integrated approach where research and management decisions are made cooperatively would allow more rational management of invasive species in wildland ecosystems.
Understanding patterns of plant population mortality during extreme weather events is important to conservation planners because the frequency of such events is expected to increase, creating the need to integrate climatic uncertainty into management. Dominant plants provide habitat and ecosystem structure, so changes in their distribution can be expected to have cascading effects on entire communities. Observing areas that respond quickly to climate fluctuations provides foresight into future ecological changes and will help prioritize conservation efforts. We investigated patterns of mortality in six dominant plant species during a drought in the southwestern United States. We quantified population mortality for each species across its regional distribution and tested hypotheses to identify ecological stress gradients for each species. Our results revealed three major patterns: (1) dominant species from diverse habitat types (i.e., riparian, chaparral, and low- to high-elevation forests) exhibited significant mortality, indicating that the effects of drought were widespread; (2) average mortality differed among dominant species (one-seed juniper[Juniperus monosperma (Engelm.) Sarg.] 3.3%; manzanita[Arctostaphylos pungens Kunth], 14.6%; quaking aspen[Populus tremuloides Michx.], 15.4%; ponderosa pine[Pinus ponderosa P. & C. Lawson], 15.9%; Fremont cottonwood[Populus fremontii S. Wats.], 20.7%; and pinyon pine[Pinus edulis Engelm.], 41.4%); (3) all dominant species showed localized patterns of very high mortality (24-100%) consistent with water stress gradients. Land managers should plan for climatic uncertainty by promoting tree recruitment in rare habitat types, alleviating unnatural levels of competition on dominant plants, and conserving sites across water stress gradients. High-stress sites, such as those we examined, have conservation value as barometers of change and because they may harbor genotypes that are adapted to climatic extremes.
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Factors controlling the establishment of Fremont cottonwood seedlings on the upper Green River, USA
Relationships between Russian olive and duck nest success in southeastern Idaho