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Forest loss and fragmentation effects on woody plant species richness in Great Britain
... The fragmentation of natural habitats is defined as a landscape-scale process involving forest loss, where the changing configuration or arrangement of forest cover (Fahrig, 2003;Long et al., 2010) is considered one of the main causes of biodiversity loss in habitats (Benítez-Malvido et al., 2016;de Albuquerque and Rueda, 2010;Gibson et al., 2013;Kupfer et al., 2006;Rodríguez-Loinaz et al., 2012). Disordered land use and occupation, current economic models, and population growth (Ribeiro et al., 2009;Tabarelli et al., 2010;Tabarelli and Gascon, 2005) drive the fragmentation process and change the floristic and structural patterns of forest communities (Augusto et al., 2000;Carvalho, et al., 2016;Sousa et al., 2017). ...
... The transformation of forested areas into patches can cause changes in the ecosystem interaction, especially in the dispersal of populations and the genetic material flow, leading to demographic stochasticity (ALBUQUERQUE; RUEDA, 2010;CEREZO et al., 2010;HEINKEN;WEBER, 2013). Attanasio et al. (2012) highlighted the influence of forest patches in the maintenance of water resources, thinking in the interception process of rain by the trees. ...
Urbanization process transforms original landscapes into an anthropic mosaic, causing impacts on hydrologic cycles and on landscape structure and functions. Aiming at the maintenance of the water resources and biodiversity, in an urbanized watershed, the objective of this study was the definition of priority areas for forest restoration. We used a Multicriteria Evaluation (MCE) method, following the steps: criteria definition, identification of the criteria importance, and criteria aggregation through Weighted Linear Combination (WLC). According to the experts, consulted in the context of the Participatory Technique, only two criteria represented the studied landscape: proximity to drainage network and proximity to forest patches. The first criterion was considered twice more important than the second, and through the pairwise comparison matrix, it was obtained respectively the criterion weights of 0.83 and 0.17. The priority map was obtained through the criteria aggregation, using WLC, that considered the criterion weights. The result was a priority map, indicating 5.06% of the study area with very-high priority for forest restoration; 5.22% with high priority; 5.76% with medium priority; 5,42% with low and; 78.53% with very-low priority. We can say that the framework predefined for the study proposed a scenario for priority areas that allowed driving the actions in order to obtain a landscape restoration, beginning through a forest corridor in the riparian zone. Thus, we concluded that the definition of priority areas for forest restoration is possible in an urbanized landscape, using the traditional WLC Multicriteria method.
... . In human dominated landscapes, habitat loss changes landscape configuration, altering the size and isolation of forested fragments which strongly determines patterns of species richness and abundance (Albuquerque & Rueda 2010, Fahrig 2013, Villard & Metzger 2014. Non-linear relationships between species richness and forest cover at the landscape scale are known to occur (Radford et al. 2005, Pardini et al. 2010, with abrupt species losses occurring after even small changes in forest cover after a certain threshold (Fahrig 2002, Bailey et al. 2010. ...
Habitat loss is the main driver of the current high rate of species extinction, particularly in tropical forests. Understanding the factors associated with biodiversity loss, such as the extinction of species interactions and ecological functions, is an urgent priority. Here, our aim was to evaluate how landscape-scale forest cover influences fruit biomass comparing different tree functional groups. We sampled 20 forest fragments located within landscapes with forest cover ranging from 2 to 93 percent in the Atlantic forest of southern Bahia, Brazil. In each fragment, we established five plots of 25 9 4 m and carried out phenological observations on fleshy fruit throughout 1 year on all trees ≥5 cm dbh. We estimated fruit availability by direct counting of all fruits and derived fruit biomass from this count. We used spatial mixed linear models to evaluate the effects of forest cover on species richness, abundance, and fruit biomass. Our results indicated that forest cover was the main explanatory variable and negatively influenced the total richness and abundance of zoo-choric and shade-tolerant but not shade-intolerant species. A linear model best explained variations in richness and abundance of total and shade-tolerant species. We also found that forest cover was positively correlated with the fruit biomass produced by all species and by the shade-tolerant assemblages, with linear models best explaining both relationships. The loss of shade-tolerant species and the lower fruit production in fragments with lower landscape-scale forest cover may have implications for the maintenance of frugivore, seed dispersal service, and plant recruitment. Abstract in Portuguese is available with online material.
... This is a serious gap in knowledge, given that management practices are often carried out at intermediate scales. Moreover, forest fragmentation is a widespread process (Wulder et al. 2009;Albuquerque & Rueda 2010), and consequently species diversity analysis at the forest patch level is of crucial importance. Although conserving plant biodiversity has become a key element of Central European forest management policy, only a limited number of studies have examined the influence of coppice abandonment on biodiversity (Kopecky et al. 2013), and very few of them used large data sets (Battles et al. 2001). ...
Question: We compared active and abandoned beech coppices in terms of: (i) structural features (ii) total, understory and overstory plant diversity across spatial scales (iii) species richness of beech forest specialist species.
Location: Central Apennines, Italy.
Methods: We applied a multiscale approach, working at fine scale (management units) and at a coarser scale (forest patch). Two forest patches were selected according to management regimes (active coppicing, 1-40 years; abandoned coppicing, over 40 years) in each of the two study areas (Bolognola and Ussita). Within each forest patch, 20 stands were sampled by 20 m x 20 m plots, for a total of 80 sampling units.
Results: We found that the height of standards and the number of dominant trees were negatively correlated with the total number of species. At the plot scale, the number of beech specialist species was significantly higher in abandoned plots. At the forest patch scale, the number of species in Bolognola was markedly higher in the actively coppiced forest than in the abandoned one, while the opposite result was found at Ussita. Regarding the beech specialist species richness, the abandoned forest at Ussita showed greater richness than the coppiced forest, while in Bolognola we found the contrary. The managed forest hosted more overstory species than the abandoned one in both areas. Surprisingly, the total beech specialist species richness was higher in the coppiced plots than in the abandoned ones. The species assemblages were more similar between patches having different management regimes within the same area, than between patches having the same management regime across different areas.
Conclusions: This material provides a novel contribution to the study of species diversity patterns in this forest system, suggesting the importance of a multiple scale approach in forest diversity studies. The beech specialist species can largely persist in a heterogeneous coppice landscape, where abandoned stands are mixed with stands under regular coppicing. The results can link current knowledge about beech coppice diversity on the landscape scale with that on the plot scale, and thus help guide new conservation planning.
... On the one hand, human factors, such as human activities (Araújo et al., 2002;Cincotta et al., 2000;Clergeau et al., 2006;Donald et al., 2001) and, in particular, the alteration of habitats (Kiesecker et al., 2001;Peres et al., 2010) are major causes of biodiversity loss (Brooks et al., 2002;Cardillo et al., 2004;Gaston, 2006;McKee et al., 2003;McKinney, 2001;Singh, 2002;Van Rensburg et al., 2004). On the other hand, several studies have even shown a positive relationship between human density and biodiversity, indicating that species-rich areas and human settlements often co-occur (Albuquerque & Rueda, 2010;Luck, 2007;Maffi, 2005;Sutherland 2003). However, this might be a purely correlative effect in many instances, particularly for species that are associated with farming and human habitation such as aphids (Pautasso & Powel, 2009) or ants (Schlick-Steiner et al., 2008) that may behave as invasive pests causing an absolute loss of diversity by displacing other species. ...
The 'EU Council conclusions on biodiversity post-2010' re-enforced Europe's commitment to halt biodiversity loss by 2020. Identifying areas of high-value for biodiversity conservation is an important issue to meet this target. We investigated the geographic pattern of terrestrial vertebrate diversity status in Europe by assessing the species richness, rarity, vulnerability (according to IUCN criteria), and a combined index of the three former for the amphibians, reptiles, bird and mammals of this region. We also correlated the value of all indices with climate and human influence variables. Overall, clear geographic gradients of species diversity were found. The combined biodiversity index indicated that high-value biodiversity areas were mostly located in the Mediterranean basin and the highest vulnerability was found in the Iberian peninsula for most taxa. Across all indexes, the proportion of variance explained by climate and human influence factors was moderate to low. The results obtained in this study have the potential to provide valuable support for nature conservation policies in Europe and, consequently, might contribute to mitigate biodiversity decline in this region.
... This is a significant gap because management practices are often carried out at intermediate scales and this is likely to have an important effect on species diversity. Moreover, forest fragmentation represents a widespread process (Wulder et al. 2009, Albuquerque & Rueda, 2010, and consequently species diversity analysis at the forest patch level is of crucial importance. Although plant biodiversity has become a key element of Central European forest management, there are still limited number of studies which examine the influence of coppice abandonment on biodiversity (Kopeky et al., 2013) and very few of them used large dataset (Battles et al. 2001;Chiarucci and Bonini, 2005). ...
PUBLISHED RESULTS:
Cervellini M, Fiorini S, Cavicchi A, Campetella G, Simonetti E, Chelli S, Canullo R, Gimona A (2017) Relationships between understory specialist species and local management practices
in coppiced forests–Evidence from the Italian Apennines. Forest Ecology and Management
385: 35–45. https://doi.org/10.1016/j.foreco.2016.11.027
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PHD THESIS - ABSTRACT
Social Ecological Systems (SESs), also called human–environment systems sustainable over time, are non-linear in nature, cross-scale in time and in space, and have an evolutionary character. They are one system, with critical feedbacks across temporal and spatial scales.
Studying and understanding dynamics of coupled human and natural systems using a multidisciplinary approach is becoming an important way for achieving integrated knowledge of complex interactions between elements and processes affecting our support systems.
The PhD research work here presented was able to fulfill gaps in knowledge and contribute to the integrated characterization of the Social Ecological System of coppice forest in the Central Italian Apennine. Building on the previous research conducted on the ecological dimension of the coppicing system in the March region, we developed a social-research survey approach able to characterize the socio-ecological factors affecting the forest system in the area.
Particularly this research provided: i) an additional more deep ecological understanding of the variables influencing understory vascular plant distribution of the beech coppice system at different forest spatial and temporal scales; ìì) which are the human actors, characterizing this ecological system, how they behave and differences existing between cutters’ groups; iii) which are the direct and indirect interactions between cutters actions and ecological variables characterizing the coppice forests and finally iv) develop a causal functioning scheme of the studied social-ecological system.
Results suggests a convergence of factors affecting decision making, supporting the importance of the ecological dimension, instead of just formal rules. Furthermore insights on the relationships coupling the social and the ecological system suggest possible resilience mechanism of understory functional diversity related to diffuse ecological knowledge and management practices.
... However, the landscape metrics analysis indicated that revegetation and reforestation combined were not able to restore the mean patch size of the landscape to its pre-cultivation status: despite the natural regrowth and large scale reforestation projects of the last few decades, the mean patch size in 2004 was only about 24% of that in the pre-cultivation era (Table 6). Because it has previously been reported that the landscape fragmentation represent a threat to biodiversity (Albuquerque and Rueda, 2010;Olff and Ritchie, 2002;Sirami et al., 2010), the implications in terms of biodiversity loss may be substantial (e.g., habitat loss). Marked increases in fragmentation occurred especially in the Rapid cultivation stage and, to some extent, in the abandonment stage, whereas increases in interspersion featured prominently in the Rapid cultivation and plantation stages. ...
The aim of this study is to detect and quantify the dominant land cover changes in a human dominated forest site in Northern Japan. Vegetation maps prepared from aerial photos and socioeconomic information were used to define three land cover change trajectories: the rapid cultivation stage (1947–1968), the abandonment stage (1968–1978) and the plantation/reforestation stage (1978–2004). The analysis revealed that in the rapid cultivation stage, the degradation from broadleaved forest to dwarf bamboo brush occurring in more than 3% of the landscape was the only dominant signal of change. In the abandonment stage, the pasture land-dwarf bamboo brush, dwarf bamboo brush-broadleaved forest, and broadleaved forest-conifer-broadleaved forest transitions covering about 18% of the landscape were the dominant change processes. In the plantation stage where the dominant signals of change affected about 27% of the landscape, these three transitions were also observed in addition to pasture and dwarf bamboo brush-conifer plantation transitions. Patch density (PD) increased in the rapid cultivation stage. In spite of natural revegetation and the large-scale reforestation project between 1978 and 2004, the mean patch size of the landscape in 2004 was only 24% of the pre-cultivation era. Mean proximity index (MPI) and interspersion juxtapostition index (IJI) showed contrasting trends, but the latter exhibited high values at extreme values of mean patch size (MPS). The relative ability of other pattern metrics to measure fragmentation of the landscape is highlighted. Prompt mitigation of adverse land change requires close monitoring by land use planners.
The planet’s biodiversity is increasingly threatened by the effects of rapid environmental change. Biodiversity loss led by expanding anthropogenic activity, combined with the effects of changing climatic regimes, pose a threat to natural communities and the ecosystem services that they underpin. Understanding how communities of species vary across space and the factors that drive such variation is essential to predicting the impacts of human and climate-induced changes on biodiversity and ecosystem services. This knowledge can be used to plan conservation areas that effectively represent the underlying ecological processes that maintain biodiversity and ensure the provision of ecosystem services crucial to human wellbeing.
Beta diversity has been variably defined since the concept was first introduced as a measure of variation in species assemblages. This has given rise to the development of different measures of beta diversity, and to varying conclusions regarding the variation of species composition across space. In this thesis, I aimed to investigate the spatial structuring of tree species turnover patterns (beta diversity) and the potential drivers of these patterns, within the highly environmentally heterogeneous Mesoamerican region. In addition, the effect of habitat loss on tree diversity up to the year 2000 was estimated, and the representativeness of the protected area system was assessed in an effort to highlight potential areas of conservation concern across the region. The efficacy of ecoregions in representing turnover patterns was also assessed.
The spatial structuring of areas of low similarity (high beta diversity) was clustered in the north and south of the study region, as well as within the Central American mountainous regions. Areas of low similarity mostly fell within dry, pine-oak, and montane ecoregions. Moist forests were indicated as having high similarity. The congruence of the modelled pattern of beta diversity in trees with patterns of high beta diversity in other taxa provides insights into areas of potential conservation efforts. Habitat loss in the past decade was high in the southern areas of the isthmus indicating that countries like Nicaragua, Costa Rica, and Panama had lost high proportions of their original habitat and species. However, habitat loss was intensive throughout the region, and areas of high beta diversity were found to be among the least represented in the current protected areas system. Classification based on beta diversity broadly resembled the WWF ecoregional classification for the region. This suggests that models based on broad climatic variables can help elucidate beta diversity patterns at broad spatial scales, although conservation planning also requires robust information at finer scales.
This account presents information on all aspects of the biology of Sorbus torminalis (L.) Crantz (Wild Service-tree) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history and conservation. Sorbus torminalis is an uncommon, mostly small tree (but can reach 33 m) native to lowland England and Wales, and temperate and Mediterranean regions of mainland Europe. It is the most shade-tolerant member of the genus in the British Isles, and as a result, it is more closely associated with woodland than any other British species. Like other British Sorbus species, however, it grows best where competition for space and sunlight is limited. Seedlings are shade tolerant but adults are only moderately so. This, combined with its low competitive ability, restricts the best growth to open areas. In shade, saplings and young adults form a sapling bank, showing reproduction and extensive growth only when released. Sorbus torminalis tolerates a wide range of soil reaction (pH 3·5-8·0) but grows best on calcareous clays and thin soils over limestone. Sorbus torminalis is a sexual, diploid, non-apomictic species that has hybridised with a number of other Sorbus species to form microspecies. The hermaphrodite flowers are primarily insect pollinated. Seed production is reliable only in warm years, especially at the edge of its range, although even then seed viability is low. The fruits are primarily dispersed by carnivorous mammals. Seeds display embryo dormancy but most will germinate the first spring after falling. This tree is very tolerant of short droughts but only moderately tolerant of frost, hence its southerly and lowland distribution. It faces no particular individual threats although the small size of most populations makes it susceptible to habitat loss and fragmentation, particularly through the loss of open coppiced areas. As a consequence, it appears to be declining throughout Britain and Europe despite its wide range of historical uses and the high value of its timber. The extent to which these losses will be offset by increases due to climate change is unknown.
Context
Forest loss and fragmentation negatively affect biodiversity. However, disturbances in forest canopy resulting from repeated deforestation and reforestation are also likely important drivers of biodiversity, but are overlooked when forest cover change is assessed using a single time interval.
Objectives
We investigated two questions at the nexus of plant diversity and forest cover change dynamics: (1) Do multitemporal forest cover change trajectories explain patterns of plant diversity better than a simple measure of overall forest change? (2) Are specific types of forest cover change trajectories associated with significantly higher or lower levels of diversity?
Methods
We sampled plant biodiversity in forests spanning the Charlotte, NC, region. We derived forest cover change trajectories occurring within nested spatial extents per sample site using a time series of aerial photos from 1938 to 2009, then classified trajectories by spatio-temporal patterns of change. While accounting for landscape and environmental covariates, we assessed the effects of the trajectory classes as compared to net forest cover change on native plant diversity.
Results
Our results indicated that forest stand diversity is best explained by forest change trajectories, while the herb layer is better explained by net forest cover change. Three distinct forest change trajectory classes were found to influence the forest stand and herb layer.
Conclusions
The influence of forest dynamics on biodiversity can be overlooked in analyses that use only net forest cover change. Our results illustrate the utility of assessing how specific trajectories of past land cover change influence biodiversity patterns in the present.
Background: Landscape fragmentation constitutes one of the most severe causes of global biodiversity loss. Aims: We studied Fagus sylvatica forests with different levels of fragmentation to address the following question: do fragmented and non-fragmented forests present a similar floristic composition and richness, structural parameters and ecological features? Methods: Vascular plant species were randomly sampled based on a beech forest map classified into three fragmentation levels. We compared overall native and diagnostic species richness patterns of the different fragmentation levels using rarefaction curves and the ratio between diagnostic and all species curves. We also contrasted different fragmentation levels of beech forests, focusing on floristic information, structural parameters, standard ecological features and the distribution of edge and clearing species. Results: Rarefaction analysis showed two opposite trends: the diversity of diagnostic species decreased in fragmented forests as the overall diversity increased. In highly fragmented forests, we found significantly higher values for therophyte and phanerophyte frequencies, light Ellenberg indicator values and edge and clearing species diversity. Conclusions: The integration of floristic analysis, particularly of certain diagnostic groups, with structural and ecological studies is more sensitive and significant than species richness alone, and could offer useful information for forest conservation and management.
Habitat fragmentation may adversely affect the ability of natural enemies to control pest outbreaks in agricultural landscapes by interfering with their search behavior and ability to aggregate in response to prey. We determined how landscape structure affected
the ability of two ladybird beetles (Coleoptera: Coccinellidae) to track aphid populations in experimental landscapes that differed in the abundance and degree of fragmentation of red clover (Trifolium pratense). One coccinellid was a native species (Coleomegilla maculata Pallas) and the other (Harmonia axiridis Timberlake) was introduced specifically for the biological control of crop pests such as pea aphids (Acyrrlzosiphon pisurn Harris; Homoptera: Aphididae).
Landscape structure exhibited a threshold in lacunarity (a measure of interpatch distances) below 20% habitat. at which point clover patches became significantly more isolated. This threshold in landscape structure was mirrored by a similar threshold in the distribution of pea aphid populations. The distribution of the biocontrol agent, H. axyridis, tracked this threshold in aphid distribution, but the native coccinellid, C. maculata, was unable to do
so in fragmented clover landscapes. Although C. maculata was a more active forager within clover cells, overall it was less mobile and moved significantly less among clover cells and between landscapes than H. axyridis, which may have contributed to its inability to track
aphid populations in fragmented landscapes. The two coccinellids did not differ in their search success within fragmented landscapes. however. and it was only in clumped landscapes that H. axyridis maximized search success and foraged within clover cells that had
2.5-3 times more aphids than those in which C. maculata occurred.
Thus, the potential of predators to control pest populations in fragmented landscapes may ultimately reflect the extent to which thresholds in landscape structure interfere with the aggregative response of predators. In this system, the aggregative response of coccinellids was more closely tied to thresholds in the distribution of clover than aphids. With its greater mobility, H. axyridis was more effective than the indigenous C. maculata at tracking aphids when they occurred at low patch occupancy (below the threshold in landscape structure), which is a requisite for successful biocontrol. If native insect predators are generally more sensitive to habitat fragmentation, greater reliance may be placed on the introduction of exotic species for biocontrol, which is not without economic cost and
potential ecological impacts to native insect communities. Our study demonstrates that, in addition to economic thresholds, there are also ecological thresholds that must be surmounted if biocontrol measures are to be successful. In addition to enhancing vegetational diversity within agroecosystems, conservation biological control should also strive to mitigate fragmentation effects on natural enemies, especially if thresholds in landscape structure disrupt predator-prey interactions and compromise the efficacy of biocontrol programs.
Critical thresholds are transition ranges across which small changes in spatial pattern produce abrupt shifts in ecological responses. Habitat fragmentation provides a familiar example of a critical threshold. As the landscape becomes dissected into smaller parcels of habitat, landscape connectivity-the functional linkage among habitat patches-may suddenly become disrupted, which may have important consequences for the distribution and persistence of populations. Landscape connectivity depends not only on the abundance and spatial patterning of habitat, but also on the habitat specificity and dispersal abilities of species. Habitat specialists with limited dispersal capabilities presumably have a much lower threshold to habitat fragmentation than highly vagile species, which may perceive the landscape as functionally connected across a greater range of fragmentation severity. To determine where threshold effects in species' responses to landscape structure are likely to occur, we developed a simulation
Habitat fragmentation has been shown to influence the abundance, movements, and persistence of many species. We asked the following questions: (1) Do species respond mainly to habitat loss or to the changes in habitat configuration resulting from this loss? (2) Do species exhibit sharp thresholds in their response to forest cover or configuration? We compared the relative influence of forest cover and configuration on 15 bird species in 33 landscapes (6.25 km²) in eastern Ontario, Canada. Forest cover in these landscapes varied between 3.4% and 66.8%. The metrics we used to quantify forest configuration were correlated to forest cover, so we regressed these configuration metrics against cover and used the residuals in logistic regression models. Of the 15 forest bird species included in the analyses, the presence of only 3 (Downy Woodpecker [ Picoides pubescens], Brown Creeper [Certhia americana], and White-breasted Nuthatch [ Sitta carolinensis]) was not significantly related to either cover or configuration of woodland. Forest cover and configuration each were significant predictors of the presence of 6 species in landscapes occupied in both years, and 3 species responded both to cover and configuration. Models based on single years showed variability in the landscape characteristics that were significant predictors of the presence of each species. These results indicate that (1) landscape structure was an important predictor of bird distribution, (2) both forest cover and configuration were important predictors of species presence, and (3) responses were species-specific. Effects of forest cover and configuration on species presence generally were not characterized by sharp thresholds, preventing the application of simple management rules. Although forest cover is an important feature of landscape structure, our results indicate that woodland configuration is a far from negligible component that should also be incorporated in conservation strategies.
HABITAT destruction is the major cause of species extinctions13. Dominant species often are considered to be free of this threat because they are abundant in the undisturbed fragments that remain after destruction. Here we describe a model that explains multispecies coexistence in patchy habitats4 and which predicts that their abundance may be fleeting. Even moderate habitat destruction is predicted to cause time-delayed but deterministic extinction of the dominant competitor in remnant patches. Further species are predicted to become extinct, in order from the best to the poorest competitors, as habitat destruction increases. More-over, the more fragmented a habitat already is, the greater is the number of extinctions caused by added destruction. Because such extinctions occur generations after fragmentation, they represent a debta future ecological cost of current habitat destruction.
Landscape ecologists have been eager to make their research applicable to forest management. We examine how landscape ecology has contributed to shaping the way forest management is currently practiced. Landscape ecology research in forested ecosystems can be divided into two general areas: (1) the study of fragmentation issues, which focuses on the effects of forest fragmentation on species conservation; and (2) the development of landscape projection models, which focuses on patch dynamics and the effects of spatial arrangement of patches on ecosystem processes. Fragmentation issues have become priorities in the minds of forest managers, but research to date has over-emphasized the effects of landscape structure on species conservation. We suggest that the research focus should move toward the study of threshold effects of landscape change on the relative influence of habitat loss and habitat configuration on species conservation in forest-dominated landscapes. Landscape projection models are rapidly becoming important tools in forest management planning, and they hold great promise as a means to bring landscape ecologists and forest managers together. The ability to produce future landscapes under different management scenarios and to compare these to landscapes produced by natural disturbance regimes will help to focus both managers and scientists on understanding the key interactions among human activities, landscape features, and ecological processes.
Habitat thresholds are usually defined as “points of abrupt change” in the species–habitat relationships. Habitat thresholds can be a key tool for understanding species requirements, and provide an objective definition of conservation targets, by identifying when habitat loss leads to a rapid loss of species, and the minimum amount of habitat necessary for species persistence. However, a large variety of statistical methods have been used to analyse them. In this context, we reviewed these methods and, using simulated data sets, we tested the main models to compare their performance on the identification of thresholds. We show that researchers use very different analytical tools, corresponding to different operational definitions of habitat thresholds, which can considerably affect their detection. Piecewise regression and generalized additive models allow both the distinction between linear and nonlinear dynamics, and the correct identification of break point position. In contrast, other methods such as logistic regression fail because they may incorrectly detect thresholds in gradual patterns, or they may over or underestimate the threshold position. In conservation or habitat modelling, it is important to focus efforts efficiently and the inappropriate choice of statistical methods may have detrimental consequences.
We hypothesized that landscape structure affects movement of individuals through the landscape, which affects the rate and pattern of disease transmission. Based on this hypothesis, we predicted a relationship between landscape structure and disease incidence in spatially structured populations. We tested this prediction for hantavirus incidence in deer mice (Penomysens moniculatus), using a novel index of habitat fragmentation for transect data. A series of four stepwise logistic regression analyses were conducted on serological and ecological data from 2837 mice from 101 sites across Canada. The significant variables, ranked in decreasing order of size of their effect on virus incidence were: human buildings, landscape composition (amount of deer mouse habitat in the 1-km radius landscape surrounding each site), landscape configuration (fragmentation of deer mouse habitat in the 1-km radius landscape surrounding each site), mean annual temperature, and seasonal variation. Our results suggest that epidemiological models should consider not only the demographic structure of the host population, but its spatial structure as well, as inferred from landscape structure. Landscape structure can have a greater effect on the pattern of distribution of a virus in its host population than other ecological variables such as climate and seasonal change. The usefulness of landscape data in epidemiological models depends on the use of the appropriate spatial scale, which can be determined empirically. Epidemiological models with a spatially structured host population can benefit from the explicit consideration of landscape structure.
Previous studies on various scales and for various European regions and North America have shown that cities harbour more plant species than the surrounding landscape. It has been argued that the greater number of plant species is usually caused by a high number of alien plants promoted by human influence. We analysed native and naturalized vascular plant species distribution data from a comprehensive German database comparing city and non-city grid cells of 10 minutes latitude × 6 minutes longitude (c. 130 km 2 ). The number of city grid cells (n = 68) and non-city grid cells (n = 1856) differed by two orders of magnitude and species richness was highly autocorrelated. We therefore used resampling techniques. We resampled the species richness of 68 randomly selected grid cells 9999 times. This showed that not only naturalized alien but also native plant species richness was significantly higher in city grid cells. To relate environmental variables to species richness, we used 10,000 analyses of covariance of 68 city grid cells and 68 randomly selected non-city grid cells. We demonstrated that a large proportion of the higher native plant species richness could be explained by the number of geological types per grid cell (i.e. a measure of natural geological diversity). Additionally, we showed by resampling the number of geological types per grid cell that cities are not randomly distributed but are in fact in areas of high geological diversity. Hence, we conclude that city areas are preferentially located in pre-existing biodiversity hotspots and argue that they are species rich not because of but in spite of urbanization.
Habitat fragmentation may adversely affect the ability of natural enemies to control pest outbreaks in agricultural landscapes by interfering with their search behavior and ability to aggregate in response to prey. We determined how landscape structure affected the ability of two ladybird beetles (Coleoptera: Coccinellidae) to track aphid populations in experimental landscapes that differed in the abundance and degree of fragmentation of red clover (Trifolium pratense). One coccinellid was a native species (Coleomegilla maculata Pallas) and the other (Harmonia axyridis Timberlake) was introduced specifically for the biological control of crop pests such as pea aphids (Acyrthosiphon pisum Harris Homoptera: Aphididae). Landscape structure exhibited a threshold in lacunarity (a measure of interpatch distances) below 20% habitat, at which point clover patches became significantly more isolated. This threshold in landscape structure was mirrored by a similar threshold in the distribution of pea aphid populations. The distribution of the biocontrol agent, H. axyridis, tracked this threshold in aphid distribution, but the native coccinellid, C. maculata, was unable to do so in fragmented clover landscapes. Although C. maculata was a more active forager within clover cells, overall it was less mobile and moved significantly less among clover cells and between landscapes than H. axyridis, which may have contributed to its inability to track aphid populations in fragmented landscapes. The two coccinellids did not differ in their search success within fragmented landscapes, however, and it was only in clumped landscapes that H. axyridis maximized search success and foraged within clover cells that had 2.5-3 times more aphids than those in which C. maculata occurred. Thus, the potential of predators to control pest populations in fragmented landscapes may ultimately reflect the extent to which thresholds in landscape structure interfere with the aggregative response of predators. In this system, the aggregative response of coccinellids was more closely tied to thresholds in the distribution of clover than aphids. With its greater mobility, H. axyridis was more effective than the indigenous C. maculata at tracking aphids when they occurred at low patch occupancy (below the threshold in landscape structure), which is a requisite for successful biocontrol. If native insect predators are generally more sensitive to habitat fragmentation, greater reliance may be placed on the introduction of exotic species for biocontrol, which is not without economic cost and potential ecological impacts to native insect communities. Our study demonstrates that, in addition to economic thresholds, there are also ecological thresholds that must be surmounted if biocontrol measures are to be successful. In addition to enhancing vegetational diversity within agroecosystems, conservation biological control should also strive to mitigate fragmentation effects on natural enemies, especially if thresholds in landscape structure disrupt predator-prey interactions and compromise the efficacy of biocontrol programs.
The literature on effects of habitat fragmentation on biodiversity is huge. It is also very diverse, with different authors measuring fragmentation in different ways and, as a consequence, drawing different conclusions regarding both the magnitude and direction of its effects. Habitat fragmentation is usually defined as a landscape-scale process involving both habitat loss and the breaking apart of habitat. Results of empirical studies of habitat fragmentation are often difficult to interpret because (a) many researchers measure fragmentation at the patch scale, not the landscape scale and (b) most researchers measure fragmentation in ways that do not distinguish between habitat loss and habitat fragmentation per se, i.e., the breaking apart of habitat after controlling for habitat loss. Empirical studies to date suggest that habitat loss has large, consistently negative effects on biodiversity. Habitat fragmentation per se has much weaker effects on biodiversity that are at least as likely to be positive as negative. Therefore, to correctly interpret the influence of habitat fragmentation on biodiversity, the effects of these two components of fragmentation must be measured independently. More studies of the independent effects of habitat loss and fragmentation per se are needed to determine the factors that lead to positive versus negative effects of fragmentation per se. I suggest that the term "fragmentation" should be reserved for the breaking apart of habitat, independent of habitat loss.
Little is known of the separate effects of habitat fragmentation and degradation, which are typically confounded by habitat loss. We examined the separate effects of habitat availability, fragmentation, and degradation on the species richness and abundances of commensal species occupying coral colonies on Australia's Great Barrier Reef. Species richness, total abundance, and the abundances of the two most common commensals, a crab (Trapezia cymodoce) and a shrimp (Palaemonella sp.), increased with coral colony volume. However, relationships between coral colony volume and abundances of these two species differed. The minimum size of coral colonies inhabited by the shrimps was considerably smaller than that inhabited by the crabs. Furthermore, while abundances of these shrimps increased with colony volume, the crabs tended to occupy their hosts as male: female pairs. We manipulated two aspects of the habitat provided by these corals to their commensals. We fragmented habitats while controlling for habitat loss, and we degraded habitats by killing corals without affecting the physical structure of the habitat. Habitat degradation caused rapid declines in species richness, total abundance, and the abundances of the crab and shrimp species. While these declines were rapid they were not instantanceous, suggesting that some secondary degradation of these habitats caused these declines. Habitat fragmentation, on the other hand, had no effect on species richness, total abundance, or the abundance of the shrimp species. In contrast, abundances of trapezid crabs increased in response to fragmentation, most likely as a result of the liberation of space from territory holders that was then colonized by additional individuals. The patch sizes used in this study were considerably smaller than those used in many terrestrial fragmentation studies. Also, marine populations tend to be more open than terrestrial ones and composed of species with greater dispersal capabilities. Therefore, the application of these results to practical conservation problems will require extreme caution. Nevertheless, disentangling the effects of habitat fragmentation and degradation from habitat loss made effects of fragmentation per se visible. Because these results are novel, such effects of habitat fragmentation could be more widespread than is currently appreciated, and relevant to the development of new management tools for biological conservation.
Critical thresholds are transition ranges across which small changes in spatial pattern produce abrupt shifts in ecological responses. Habitat fragmentation provides a familiar example of a critical threshold. As the landscape becomes dissected into smaller parcels of habitat, landscape connectivity-the functional linkage among habitat patches-may suddenly become disrupted, which may have important consequences for the distribution and persistence of populations. Landscape connectivity depends not only on the abundance and spatial patterning of habitat, but also on the habitat specificity and dispersal abilities of species. Habitat specialists with limited dispersal capabilities presumably have a much lower threshold to habitat fragmentation than highly vagile species, which may perceive the landscape as functionally connected across a greater range of fragmentation severity. To determine where threshold effects in species' responses to landscape structure are likely to occur, we developed a simulation
A discrete reaction-diffusion model was used to estimate long-term equilibrium populations of a hypothetical species inhabiting patchy landscapes to examine the relative importance of habitat amount and arrangement in explaining population size. When examined over a broad range of habitat amounts and arrangements, population size was largely determined by a pure amount effect (proportion of habitat in the landscape accounted for >96% of the total variation compared to <1% for the arrangement main effect). However, population response deviated from a pure amount effect as coverage was reduced below 30%-50%. That deviation coincided with a persistence threshold as indicated by a rapid decline in the probability of landscapes supporting viable populations. When we partitioned experimental landscapes into sets of "above" and "below" persistence threshold, habitat arrangement became an important factor in explaining population size below threshold conditions. Regression analysis on below-threshold landscapes using explicit measures of landscape structure (after removing the covariation with habitat amount) indicated that arrangement variables accounted for 33%-39% of the variation in population size, compared to 27%-49% for habitat amount. Thus, habitat arrangement effects became important when species persistence became uncertain due to dispersal mortality.
How large will be the loss of species through human activities? And over what time period might that loss unfold? Habitat destruction is the leading cause of species extinction. Generally, many of the species found across large areas of a given habitat are represented in smaller areas of it. So habitat loss initially causes few extinctions, then many only as the last remnants of habitat are destroyed. Thus, at current rates of habitat destruction, the peak of extinctions might not occur for decades. But we should not be complacent. On page 853 of this issue, Myers et al.1 document an uneven, highly clumped, distribution of vulnerable species over the world's land surface. Within these 'biodiversity hotspots', habitats are already disproportionately reduced.
Land use has generally been considered a local environmental issue, but it is becoming a force of global importance. Worldwide
changes to forests, farmlands, waterways, and air are being driven by the need to provide food, fiber, water, and shelter
to more than six billion people. Global croplands, pastures, plantations, and urban areas have expanded in recent decades,
accompanied by large increases in energy, water, and fertilizer consumption, along with considerable losses of biodiversity.
Such changes in land use have enabled humans to appropriate an increasing share of the planet's resources, but they also potentially
undermine the capacity of ecosystems to sustain food production, maintain freshwater and forest resources, regulate climate
and air quality, and ameliorate infectious diseases. We face the challenge of managing trade-offs between immediate human
needs and maintaining the capacity of the biosphere to provide goods and services in the long term.
Human settlements are expanding in species-rich regions and pose a serious threat to biodiversity conservation. We quantify the degree to which this threat manifests itself in two contrasting continents, Australia and North America, and suggest how it can be substantially alleviated. Human population density has a strong positive correlation with species richness in Australia for birds, mammals, amphibians, and butterflies (but not reptiles) and in North America for all five taxa. Nevertheless, conservation investments could secure locations that harbor almost all species while greatly reducing overlap with densely populated regions. We compared two conservation-planning scenarios that each aimed to represent all species at least once in a minimum set of sampling sites. The first scenario assigned equal cost to each site (ignoring differences in human population density); the second assigned a cost proportional to the site's human population density. Under the equal-cost scenario, 13-40% of selected sites occurred where population density values were highest (in the top decile). However, this overlap was reduced to as low as 0%, and in almost all cases to <10%, under the population-cost scenario, when sites of high population density were avoided where possible. Moreover, this reduction of overlap was achieved with only small increases in the total amount of area requiring protection. As densely populated regions continue to expand rapidly and drive up land values, the strategic conservation investments of the kind highlighted in our analysis are best made now.
The first edition of this book has established itself as one of the leading references on generalized additive models (GAMs), and the only book on the topic to be introductory in nature with a wealth of practical examples and software implementation. It is self-contained, providing the necessary background in linear models, linear mixed models, and generalized linear models (GLMs), before presenting a balanced treatment of the theory and applications of GAMs and related models. The author bases his approach on a framework of penalized regression splines, and while firmly focused on the practical aspects of GAMs, discussions include fairly full explanations of the theory underlying the methods. Use of R software helps explain the theory and illustrates the practical application of the methodology. Each chapter contains an extensive set of exercises, with solutions in an appendix or in the book’s R data package gamair, to enable use as a course text or for self-study.
This book had its origin when, about five years ago, an ecologist (MacArthur) and a taxonomist and zoogeographer (Wilson) began a dialogue about common interests in biogeography. The ideas and the language of the two specialties seemed initially so different as to cast doubt on the usefulness of the endeavor. But we had faith in the ultimate unity of population biology, and this book is the result. Now we both call ourselves biogeographers and are unable to see any real distinction between biogeography and ecology.
This chapter uses variants of the standard metapopulation model to examine several interlinked questions in community ecology. Classical metapopulation theory assumes that landscapes are comprised of a large number of patches available for colonization. This chapter examines how landscape heterogeneity influences the composition of local communities. Straightforward extensions of standard metapopulation models have been used in this chapter to examine the raised questions. The focus remains on theory development and the articulation of hypotheses which warrant empirical scrutiny. The chapter uses variants of the Levins metapopulation model to examine some potential consequences for community structure of habitat heterogeneity. The theoretical results suggest that sparse habitats in a heterogeneous landscape are likely to sustain a biased array of species, including habitat specialists with unusually high colonization or low extinction rates, and habitat generalists sustained via spillover from more abundant habitats.
The chapter introduces the idea that the relationships between natural conditions and the outcome of an observation may be deterministic, random, strategic or chaotic, and that numerical ecology addresses the second type of data; it describes the role of numerical ecology among the various phases of an ecological research. The chapter includes discussion of the following topics: spatial structure, spatial dependence, and spatial correlation (independent observations, independent descriptors, linear independence, independent variable of a model, independent samples, origin of spatial structures, tests of significance in the presence of spatial correlation, and classical sampling and spatial structure), statistical testing by permutation (classical tests of significance, permutation tests, alternative types of permutation tests), computer programs and packages, ecological descriptors (i.e. variables: mathematical types of descriptors, and intensive, extensive, additive, and non-additive descriptors), descriptor coding (linear transformation, nonlinear transformations, combining descriptors, ranging and standardization, implicit transformation in association coefficients, normalization, dummy variable coding, and treatment of missing data (delete rows or columns, accommodate algorithms to missing data, estimate missing values). The chapter ends on a description of relevant software implemented in the R language.
I reviewed and reconciled predictions of four models oil the effect of habitat fragmentation on the population extinction threshold, and I compared these predictions to results, from empirical studies. All four models predict that habitat fragmentation call, under some conditions, increase the extinction threshold such that, in more fragmented landscapes, more habitat is required for population persistence. However, empirical studies have shown both positive and negative effects of habitat fragmentation on population abundance and distribution with about equal frequency, suggesting that the models lack some important process(es). The two colonization-extinction (CE) models predict that fragmentation call increase the extinction threshold by up to 60-80%; i.e., the amount of habitat required for persistence can shift from <5% of the landscape to >80% of the landscape, with a shift from completely clumped to completely fragmented habitat. The other two models (birth-immigration-death-emigration, or BIDE models) predict much smaller potential effects or fragmentation oil the extinction threshold, of no more than a 10-20% shift in the amount of habitat required for persistence. This difference has important implications, for conservation. If fragmentation can have a large effect on the extinction threshold, then alteration of habitat pattern (independent of habitat amount) call be all effective tool for conservation. On the other hand, if the effects of fragmentation on the extinction threshold are small, then this is a limited option, I suggest that the difference in model predictions results from differences in the mechanisms by which the models produce the extinction threshold. In the CE models, the threshold occurs by an assumed reduction in colonization rate with decreasing habitat amount. In the BIDE models, loss of habitat is assumed to increase the proportion of the population that spends time in the matrix, where reproduction is not possible and the mortality rate is assumed to be higher (than in breeding habitat). Habitat loss therefore decreases the overall reproduction rate and increases the overall mortality rate on the landscape. I hypothesize that this imposes a constraint on the potential for habitat fragmentation to mitigate effects of habitat loss in BIDE models. To date, empirical studies of the independent effects of habitat loss and fragmentation suggest that habitat has a much larger effect than habitat fragmentation on the distribution and abundance of birds, supporting the BIDE model prediction, at least for this taxon.
We monitored the short term behavioral and demographic responses of gray-tailed voles (Microtus canicaudus) to the reduction and fragmentation of their habitat. Our objectives were (1) to test whether animals perished or moved into remaining fragments after 70% of their habitat was removed; and (2) to test the null hypothesis that the social structure and demography of animals would not differ between habitats consisting of one large continuous fragment (625 m2), a mosaic of 25 small fragments (each 25 m2) separated by 4 m of bare ground, and control, unmanipulated habitats (1850 m2). We conducted the experiment in 12, 0.2-ha enclosures planted with alfalfa with four replicates for each of two manipulated treatments and a control. A 70% reduction in habitat did not adversely affect adult survival, reproductive rate, juvenile recruitment, or population size. However, an influx of unrelated females into habitat fragments resulted in decreased juvenile recruitment in those fragments. Voles from cleared habitat moved into the remaining habitat and did not measurably affect the resident population. Similarly, the demography of voles did not differ significantly among the large-fragment, small-fragment, and control enclosures. Peak density estimates based on the amount of habitat in each enclosure were 545 animals per hectare in control, 1056 in large-fragment, and 2880 in small-fragment enclosures. Reduced movement of animals among the small fragments was the most obvious effect of habitat fragmentation. Six percent of females and 15% of males moved among small fragments within a week compared to approximately 60% moving comparable distances in large-fragment and control enclosures. Rates of juvenile dispersal and sexual maturation declined throughout the summer on all treatments, were associated with season and density, and were only marginally associated with habitat loss and fragmentation. We conclude that at the time of habitat removal and fragmentation, populations were small enough to accommodate a 70% reduction in habitat and still continue to increase in numbers. The social system of gray-tailed voles was sufficiently flexible to accommodate an influx of animals to withstand densities> 1000 voles per ha. The behavioral and demographic features of gray-tailed voles are similar to those reported for other small mammals, thus confirming the use of voles for ecological model systems in habitat fragmentation studies.
Monitoreamos las respuestas demográficas y conductuales de corto plazo a la reducción y la fragmentación del hábitat de ratones de campo de cola gris (Microtus canicaudus). Nuestro objetivos fueron (1) probar si los animales morían o se movían a los fragmentos después de que el 70% de su hábitat fue removido y (2) probar la hipótesis nula de que la estructura social y la demografía de los animales no difería entre hábitats consistentes de un fragmento grande y continuo (625 m2), un mosaico de 25 fragmentos pequeños (25 m2 cada uno) separados por 4 m de suelo limpio y habitats-control no manipulados (1850 m2). Desarrollamos el experimento en 12 encierros de 0.2 ha sembrados con alfalfa y con cuatro réplicas de cada uno de los tratamientos manipulados y un control. La reducción de 70% del hábitat no afectó la supervivencia de adultos, la tasa reproductiva, el reclutamiento de juveniles ni al tamaño poblacional. Sin embargo, la llegada a los fragmentos de hábitat de hembras no relacionadas resultó en un decremento del reclutamiento juvenil en esos fragmentos. Los ratones de hábitat removido se movieron al hábitat disponible y su efecto sobre la población residente no fue medible. De manera similar, la demografía de los ratones no difirió significativamente entre los encierros de fragmento grande, fragmentos pequeños y control. Las estimaciones de densidad máxima, basadas en la cantidad de hábitat en cada encierro, fue de 545 animales/ha en el control, 1056 en el fragmento grande y 2880 en los encierros de fragmento pequeño. El efecto mas obvio de la fragmentación del hábitat fue la reducción de movimiento de animales entre los fragmentos pequeños. El 6% de hembras y 15% de machos se movieron entre los fragmentos pequeños en una semana mientras que aproximadamente el 60% se movió distancias comparables en los encierros de fragmento grande y control. Las tasas de dispersión juvenil y de maduración sexual declinaron durante el verano en todos los tratamientos, estuvieron asociadas con la estación y la densidad, y se asociaron solo marginalmente con la pérdida y fragmentación del hábitat. Concluimos que al momento de la remoción y fragmentación del hábitat las poblaciones eran lo suficientemente pequeñas para acomodar a 70% de reducción de hábitat y aun continuar el incremento en sus números. El sistema social de los ratones de campo de cola gris fue suficientemente flexible para acomodar el arribo de animales hasta sostener densidades de> 2000 ratones/ha. Las características conductuales y demográficas de los ratones de campo de cola gris son similares a las reportadas para otros mamíferos pequeños, confirmando así el uso de ratones de campo para modelos ecológicos en estudios de fragmentación.
Gray wolves (Canis lupus) in upper Michigan, USA, have been monitored since 1991 when breeding activity in mainland Michigan was documented for the first time since 1954. Based on winter track counts, the mean annual rate of increase in abundance was 19% from 1995 to 2002, with the population reaching an estimated 278 animals in 2002. Our objectives were to (1) increase the efficiency of wolf management in Michigan by evaluating alternative and less extensive sampling approaches for population estimation, and (2) evaluate habitat for wolves based on occupancy after a decade of recovery. For the first analysis, we created 22 discrete sampling units that cover upper Michigan, and we evaluated abundance estimates based on various sampling plans using known distribution and populations from the 2000–2002 winter track surveys. We evaluated each plan based on the precision, bias, and confidence interval coverage. A random sampling plan with regression estimator returned the most precise estimates, but a stratified sampling plan, using low, medium, and high wolf density strata had the greatest precision at lowest effort. For the habitat evaluation, we compared white-tailed (Odocoileus virginianus) deer density and road density between wolf pack locations from 1995 to 2001 to random locations outside of the current wolf range. We estimated white-tailed deer density by a spatial interpolation of pellet group counts. Our resource selection function indicated that probability of wolf occupation of an area was positively correlated with deer density, and it was relatively constant for road densities <0.4 km/km2 but declined sharply at higher road densities. For areas habitable by wolves in upper Michigan, we predict a road density threshold of 0.7 km/km2 and a deer density threshold of approximately 2.3–5.8 deer/km2. We believe that these results will aid managers who need to estimate wolf abundance and predict wolf distribution.
Determine the type of habitat that owls like.
Approaches to determining the number of components to interpret from principal components analysis were compared. Heuristic procedures included: retaining components with eigenvalues (Xs) > 1 (i.e., Kaiser-Guttman criterion); components with bootstrapped Xs > 1 (bootstrapped Kaiser-Guttman); the scree plot; the broken-stick model; and components with Xs totalling to a fixed amount of the total variance. Statistical ap- proaches included: Bartlett's test of sphericity; Bartlett's test of homogeneity of the cor- relation matrix, Lawley's test of the second X; bootstrapped confidence limits on successive Xs (i.e., significant differences between Xs); and bootstrapped confidence limits on eigen- vector coefficients (i.e., coefficients that differ significantly from zero). All methods were compared using simulated data matrices of uniform correlation structure, patterned ma- trices of varying correlation structure and data sets of lake morphometry, water chemistry, and benthic invertebrate abundance. The most consistent results were obtained from the broken-stick model and a combined measure using bootstrapped Xs and associated eigen- vector coefficients. The traditional and bootstrapped Kaiser-Guttman approaches over- estimated the number of nontrivial dimensions as did the fixed-amount-of-variance model. The scree plot consistently estimated one dimension more than the number of simulated dimensions. Bartlett's test of sphericity showed inconsistent results. Both Bartlett's test of homogeneity of the correlation matrix and Lawley's test are limited to testing for only one and two dimensions, respectively.
Habitat destruction and fragmentation are the root causes of many conservation problems. We conducted a literature survey and canvassed the ecological community to identify experimental studies of terrestrial habitat fragmentation and to determine whether consistent themes were emerging from these studies. Our survey revealed 20 fragmentation experiments worldwide. Most studies focused on effects of fragmentation on species richness or on the abundance(s) of particular species. Other important themes were the effect of fragmentation in interspecific interactions, the role of corridors and landscape connectivity in individual movements and species richness, and the influences of edge effects on ecosystem services. Our comparisons showed a remarkable lack of consistency in results across studies, especially with regard to species richness and abundance relative to fragment size. Experiments with arthropods showed the best fit with theoretical expectations of greater species richness on larger fragments. Highly mobile taxa such as birds and mammals, early-successional plant species, long-lived species, and generalist predators did not respond in the “expected” manner. Reasons for these discrepancies included edge effects, competitive release in the habitat fragments, and the spatial scale of the experiments. One of the more consistently supported hypotheses was that movement and species richness are positively affected by corridors and connectivity, respectively. Transient effects dominated many systems; for example, crowding of individuals on fragments commonly was observed after fragmentation, followed by a relaxation toward lower abundance in subsequent years. The three long-term studies (14 years) revealed strong patterns that would have been missed in short-term investigations. Our results emphasize the wide range of species-specific responses to fragmentation, the need for elucidation of behavioral mechanisms affecting these responses, and the potential for changing responses to fragmentation over time.
Resumen: La destrucción y la fragmentación del hábitat son las causas fundamentales de muchos problemas de conservación. Realizamos un sondeo de la literatura y examinamos de cerca la comunidad ecológica para identificar estudios experimentales sobre la fragmentación de hábitats terrestres y para determinar si emergen temas homogéneos de estos estudios. Nuestro sondeo revela que existen 20 estudios experimentales de fragmentación en el ámbito mundial. La mayoría de los estudios enfocan en los efectos de la fragmentación sobre la riqueza de especies, o en la(s) abundancia(s) de ciertas especies en particular. Otros temas importantes fueron el efecto de la fragmentación sobre las interacciones interespecíficas, el papel de los corredores y la conectividad del paisaje en los movimientos individuales y la riqueza de especies y la influencia de los efectos de bordes sobre los servicios proporcionados por el ecosistema. Nuestras comparaciones muestran una carencia notable de homogeneidad en los resultados de los estudios, especialmente en lo referente a la riqueza y a la abundancia de especies, y su relación con el tamaño de los fragmentos. Experimentos con artrópodos demostraron que existía un mejor ajuste entre los valores teóricos esperados y los valores reales de aumentos en la riqueza de especies en fragmentos grandes. Los taxones altamente móviles ( por ejemplo, aves y mamíferos), las especies de plantas en sucesión temprana, las especies de gran longevidad y los depredadores generalistas no respondieron de la manera “esperada”. Entre las razones que explican estas divergencias se incluyen los efectos de bordes, la liberación competitiva en los fragmentos de hábitat y la escala espacial del experimento. Una de las hipótesis más aceptadas establece que el movimiento y la riqueza de especies son afectadas positivamente por los corredores y la conectividad, respectivamente. Algunos efectos pasajeros dominaron muchos sistemas; por ejemplo, el hacinamiento de individuos en fragmentos se observó a menudo después de la fragmentación, seguido de un disminución de la abundancia en los años posteriores. Los tres estudios a largo plazo (=14 años) revelaron fuertes patrones que hubieran sido ignorados en investigaciones a corto plazo. Nuestros resultados señalan el amplio rango de respuestas especie-específicas, la necesidad de elucidar mecanismos de comportamiento que afectan las respuestas a la fragmentación y el potencial de respuestas cambiantes a la fragmentación a lo largo del tiempo.
The pattern of woody species richness decline with a decrease in woody vegetation cover was studied within a tallgrass prairie. The decline in species richness is highly non-linear, with a well-defined threshold below which species richness collapses. This relationship can be understood after considering information on how landscape structure changes with woody vegetation cover, and how species richness is related to landscape structure.
In a given area, human activities usually cause the extinction of native species and the establishment of non-native species. A key conservation issue is whether non-native establishment tends to outpace native species extinction to produce a net gain in species richness. To determine this, empirical data must be accumulated at various scales. I show that, within the United States, the number of established non-native plant species per state does tend to outpace the number of extinct and threatened species per state. The net gain in plant species is strongly and positively correlated with human population density. Continuation of this trend predicts substantial gains in net plant species richness for all states in the United States as human population grows. This contrasts with freshwater fishes, where most states show a net loss of species diversity as extinct and threatened species exceed established non-native species. Changes in fish diversity do not correlate strongly with human population or non-native species but are largely driven by the decline of native fish species.
Aim To predict French Scarabaeidae dung beetle species richness distribution, and to determine the possible underlying causal factors.
Location The entire French territory has been studied by dividing it into 301 grid cells of 0.72 × 0.36 degrees.
Method Species richness distribution was predicted using generalized linear models to relate the number of species with spatial, topographic and climate variables in grid squares previously identified as well sampled (n = 66). The predictive function includes the curvilinear relationship between variables, interaction terms and the significant third-degree polynomial terms of latitude and longitude. The final model was validated by a jack-knife procedure. The underlying causal factors were investigated by partial regression analysis, decomposing the variation in species richness among spatial, topographic and climate type variables.
Results The final model accounts for 86.2% of total deviance, with a mean jack-knife predictive error of 17.7%. The species richness map obtained highlights the Mediterranean as the region richest in species, and the less well-explored south-western region as also being species-rich. The largest fraction of variability (38%) in the number of species is accounted for by the combined effect of the three groups of explanatory variables. The spatially structured climate component explains 21% of variation, while the pure climate and pure spatial components explain 14% and 11%, respectively. The effect of topography was negligible.
Conclusions Delimiting the adequately inventoried areas and elaborating forecasting models using simple environmental variables can rapidly produce an estimate of the species richness distribution. Scarabaeidae species richness distribution seems to be mainly influenced by temperature. Minimum mean temperature is the most influential variable on a local scale, while maximum and mean temperature are the most important spatially structured variables. We suggest that species richness variation is mainly conditioned by the failure of many species to go beyond determined temperature range limits.
This study examined the effects of habitat fragmentation on meadow vole (Microtus pennsylvanicus) population dynamics in experimental landscape patches. The study was conducted from May–November 1993 at the Miami University
Ecology Research Center. Eight 0.1-ha small mammal enclosures were used. Four enclosures contained a 160 m2 nonfragmented patch and four enclosures contained four 40 m2 fragmented patches. Thus, each treatment was replicated 4 times in a systematic research design. The patches in both treatments
contained high-quality habitat surrounded by low-quality matrix. Six pairs of adult meadow voles were released in each enclosure
on 27 May 1993. Populations were monitored by live-trapping and radio-telemetry methods. Significantly greater densities of
female voles were found during October in the fragmented treatment compared to the nonfragmented treatment. Also, significantly
more females than males were found in the fragmented treatment compared to the nonfragmented treatment for the total study
period. Significantly more subadult and juvenile males were found in the matrix versus the patch of the nonfragmented treatment
compared to the fragmented treatment. Males in the fragmented treatment had significantly greater mean home range size than
males or females in the nonfragmented treatment. There appears to exist a relationship between patch fragmentation and the
social structure of meadow vole populations; this relationship appears to function as a population regulatory mechanism.
Anthropogenic forest fragmentation and other kinds of human disturbance, such as selective logging, can reduce the diversity of plant and animal species. To evaluate the impact of fragmentation and small-scale disturbance on forest regeneration, we assessed species richness and total abundance of adult trees in comparison with seedlings in the heavily fragmented and disturbed Kakamega Forest, western Kenya. In nine differently disturbed 1-ha study blocks distributed across the main forest and fragments, we mapped all trees >10 cm in diameter at breast height. Additionally, we established ninety 1-m2 seedling plots within these 1-ha study blocks which were monitored over 2.5 years. We recorded altogether 74 species of adult trees (30-43 per block) and 64 seedling species (24-41 per block). Neither fragmentation nor small-scale disturbance had an impact on adult tree species richness or total tree abundance. Yet, fragmentation and especially small-scale disturbance significantly reduced seedling species richness, particularly of late-successional species. While human impact did not affect diversity of adults, the impoverished species richness of seedlings suggests a reduced potential for regeneration and a loss of tree diversity in the long-term.
Landscape fragmentation has often been seen as an only ecological problem. However, fragmentation also has a societal perspective, namely, in how humans perceive landscape fragmentation and in how landscape fragmentation potentially influences human well-being. These latter aspects have rarely been addressed so far. The inter-relationship of ecological and human dimensions of landscape fragmentation becomes especially evident when looking at the landscape where most people in industrial countries live, namely in suburban and urban areas. In these areas, landscape planners and environmental managers are confronted with the problem that landscapes should fullfil various functions, often with conflicting goals, e.g. nature reserves to enhance species richness vs. recreational areas for city-dwellers. We reviewed the ecological and sociological literature relevant for fragmentation in suburban and urban landscapes. In an interdisciplinary approach, we evaluated whether there are similarities and dissimilarities between the ecological and the human aspects of landscape fragmentation. We found important similarities. An example is that for both, humans and biodiversity, the loss of semi-natural areas has more drastic effects than the fragmentation of these areas per se. However, there are also relevant differences. We concluded that in densely populated landscapes a shift from responsive planning to an intentional design of environments is therefore needed.
Tropical montane forests have been recognised as having global conservation importance. However, they are being rapidly destroyed in many regions of the world. Our study focuses on the rate of loss and patterns of fragmentation in tropical montane forests in the Highlands of Chiapas, Mexico, during a 25-year period. Data from Landsat satellite imagery from 1975 (MSS), 1990 (TM) and 2000 (ETM+) were used to ascertain annual deforestation rates of 1.3 and 4.8% for the 1975–1990 and 1990–2000 periods, respectively. Spatial patterns of forest fragmentation were identified using selected landscape indices. Increases in the number of forest fragments (3520–10,542), patch density (1.0–3.2 patches/100 ha), and total edge length (24,781–38,400 km) were associated with decreases in the mean patch size (65.0–8.7 ha), largest patch index (60.7–4.0%), total core area (99,422–9,611 ha), and mean proximity index (101,369–1405). The observed trends indicate increasing deforestation and fragmentation, particularly during the 1990–2000 period. Circa 50% of the forest cover in the Highlands has been lost in 25 years, and a proportion of the remaining forests have been degraded as a result of human use. Increasing human population and a more demanding use of soils for agriculture and timber arise as the major causes of deforestation in the study area. We suggest that conservation efforts should be focused on the management of the natural system and the management of the external influences on it, particularly the detection of hotspots, passive and active restoration and sustainable forest exploitation by the local indigenous communities.
The goal of this study was to delineate the circumstances in which fragmentation of breeding habitat affects population survival. Fragmentation is defined (literally) as the breaking apart of habitat; note fragmentation does not imply loss of habitat. I developed a spatially explicit simulation model in which I varied the spatial pattern of breeding habitat in the landscape from contagious to fragmented, while also varying a disturbance regime, breeding habitat permanence, and the life history and movement attributes of organisms living in the landscape. The simulation results suggest that fragmentation of breeding habitat affects population survival only under the following relatively narrow set of conditions: (1) the average between-generation movement distance of the organism is about 1–3 times the expected nearest distance between breeding sites; (2) the breeding habitat of the organism covers less than 20% of the landscape; (3) the habitat is not ephemeral; (4) the organism has high breeding site fidelity; and (5) the mortality rate in the non-breeding habitat areas is much higher than the mortality rate in breeding habitat areas. Note that all of these conditions must hold for there to be an effect of breeding habitat fragmentation on population survival. These results suggest that spatially explicit simulation modelling of population dynamics is only necessary under a relatively narrow range of conditions.
Because most macroecological and biodiversity data are spatially autocorrelated, special tools for describing spatial structures and dealing with hypothesis testing are usually required. Unfortunately, most of these methods have not been available in a single statistical package. Consequently, using these tools is still a challenge for most ecologists and biogeographers. In this paper, we present SAM (Spatial Analysis in Macroecology), a new, easy-to-use, freeware package for spatial analysis in macroecology and biogeography. Through an intuitive, fully graphical interface, this package allows the user to describe spatial patterns in variables and provides an explicit spatial framework for standard techniques of regression and correlation. Moran's I autocorrelation coefficient can be calculated based on a range of matrices describing spatial relationships, for original variables as well as for residuals of regression models, which can also include filtering components (obtained by standard trend surface analysis or by principal coordinates of neighbour matrices). SAM also offers tools for correcting the number of degrees of freedom when calculating the significance of correlation coefficients. Explicit spatial modelling using several forms of autoregression and generalized least-squares models are also available. We believe this new tool will provide researchers with the basic statistical tools to resolve autocorrelation problems and, simultaneously, to explore spatial components in macroecological and biogeographical data. Although the program was designed primarily for the applications in macroecology and biogeography, most of SAM's statistical tools will be useful for all kinds of surface pattern spatial analysis. The program is freely available at www.ecoevol.ufg.br/sam (permanent URL at http://purl.oclc.org/sam/).
Two major international initiatives - the Convention on Biological Diversity's target to reduce the rate of biodiversity loss by 2010, and the Millennium Ecosystem Assessment - raise the profile of ecological data on the changing state of nature and its implications for human well-being. This paper is intended to provide a broad overview of current knowledge of these issues. Information on changes in the status of species, size of populations, and extent and condition of habitats is patchy, with little data available for many of the taxa, regions and habitats of greatest importance to the delivery of ecosystem services. However, what we do know strongly suggests that, while exceptions exist, the changes currently underway are for the most part negative, anthropogenic in origin, ominously large and accelerating. The impacts of these changes on human society are idiosyncratic and patchily understood, but for the most part also appear to be negative and substantial. Forecasting future changes is limited by our poor understanding of the cascading impacts of change within communities, of threshold effects, of interactions between the drivers of change, and of linkages between the state of nature and human well-being. In assessing future science needs, we not only see a strong role for ecological data and theory, but also believe that much closer collaboration with social and earth system scientists is essential if ecology is to have a strong bearing on policy makers.
In a variety of research settings, investigators may wish to detect and estimate a threshold in the association between continuous variables. A threshold model implies a non-linear relationship, with the slope changing at an unknown location. Generalized additive models (GAMs) (Hastie and Tibshirani, 1990) estimate the shape of the non-linear relationship directly from the data and, thus, may be useful in this endeavour.
We propose a method based on GAMs to detect and estimate thresholds in the association between a continuous covariate and a continuous dependent variable. Using simulations, we compare it with the maximum likelihood estimation procedure proposed by Hudson (1966).
We search for potential thresholds in a neighbourhood of points whose mean numerical second derivative (a measure of local curvature) of the estimated GAM curve was more than one standard deviation away from 0 across the entire range of the predictor values. A threshold association is declared if an F-test indicates that the threshold model fit significantly better than the linear model.
For each method, type I error for testing the existence of a threshold against the null hypothesis of a linear association was estimated. We also investigated the impact of the position of the true threshold on power, and precision and bias of the estimated threshold.
Finally, we illustrate the methods by considering whether a threshold exists in the association between systolic blood pressure (SBP) and body mass index (BMI) in two data sets.
Thesis (Ph. D.)--Harvard University, 1995. Includes bibliographical references (leaves 215-240). Photocopy. s
The single large or several small (SLOSS) problem has been addressed in a large number of empirical and theoretical studies, but no coherent conclusion has yet been reached. Here I study the SLOSS problem in the context of metapopulation dynamics. I assume that there is a fixed total amount A(0) of habitat available, and I derive formulas for the optimal number n and area A of habitat patches, where n=A(0)/A. I consider optimality in two ways. First, I attempt to maximize the time to metapopulation extinction, which is a relevant measure for metapopulation viability for rare and threatened species. Second, I attempt to maximize the metapopulation capacity of the habitat patch network, which corresponds both with maximizing the distance to the deterministic extinction threshold and with maximizing the fraction of occupied patches. I show that in the typical case, a small number of large patches maximizes the metapopulation capacity, while an intermediate number of habitat patches maximizes the time to extinction. The main conclusion stemming from the analysis is that the optimal number of patches is largely affected by the relationship between habitat patch area and rates of immigration, emigration and local extinction. Here this relationship is summarized by a single factor zeta, termed the patch area scaling factor.