Habitat fragmentation and extinction thresholds on fractal landscapes

Habitat fragmentation is a potentially critical factor in determining population persistence. In this paper, we explore the effect of fragmentation when the fragmentation follows a fractal pattern. The habitat is divided into patches, each of which is suitable or unsuitable. Suitable patches are either occupied or unoccupied, and change state depending on rates of colonization and local extinction. We compare the behaviour of two models: a spatially implicit patch-occupancy (PO) model and a spatially explicit cellular automaton (CA) model. The PO model has two fixed points: extinction, and a stable equilibrium with a fixed proportion of occupied patches. Global extinction results when habitat destruction reduces the proportion of suitable patches below a critical threshold. The PO model successfully recreates the extinction patterns found in other models. We translated the PO model into a stochastic cellular automaton. Fractal arrangements of suitable and unsuitable patches were used to simulate habitat fragmentation. We found that: (i) a population on a fractal landscape can tolerate more habitat destruction than predicted by the patch-occupancy model, and (ii) the extinction threshold decreases as the fractal dimension of the landscape decreases. These effects cannot be seen in spatially implicit models. Landscape struc-ture plays a vital role in mediating the effects of habitat fragmentation on persistence.     

Habitat specialization predicts genetic response to fragmentation in tropical birds

Habitat fragmentation is one of the most severe threats to biodiversity as it may lead to changes in population genetic structure, with ultimate modifications of species evolutionary potential and local extinctions. Nonetheless, fragmentation does not equally affect all species and identifying which ecological traits are related to species sensitivity to habitat fragmentation could help prioritization of conservation efforts. Despite the theoretical link between species ecology and extinction proneness, comparative studies explicitly testing the hypothesis that particular ecological traits underlies species‐specific population structure are rare. Here, we used a comparative approach on eight bird species, co‐occurring across the same fragmented landscape. For each species, we quantified relative levels of forest specialization and genetic differentiation among populations. To test the link between forest specialization and susceptibility to forest fragmentation, we assessed species responses to fragmentation by comparing levels of genetic differentiation between continuous and fragmented forest landscapes. Our results revealed a significant and substantial population structure at a very small spatial scale for mobile organisms such as birds. More importantly, we found that specialist species are more affected by forest fragmentation than generalist ones. Finally, our results suggest that even a simple habitat specialization index can be a satisfying predictor of genetic and demographic consequences of habitat fragmentation, providing a reliable practical and quantitative tool for conservation biology.     

Habitat fragmentation and extinction rates within freshwater fish communities: a faunal relaxation approach

Aim To estimate population extinction rates within freshwater fish communities since the fragmentation of palaeo‐rivers due to sea level rise at the end of the Pleistocene; to combine this information with rates estimated by other approaches (population surveys, fossil records); and to build an empirical extinction–area relationship. Location Temperate rivers from the Northern Hemisphere, with a special focus on rivers discharging into the English Channel, in north‐western France. Methods (1) French rivers. We used a faunal relaxation approach to estimate extinction rates in coastal rivers after they became isolated by the sea level rise. Tributaries within the Seine were used to build a species–area relationship for a non‐fragmented river system to predict species richness in coastal rivers before their fragmentation. (2) Other rivers. Extinction rates obtained for four other Holarctic river systems fragmented at the end of the Pleistocene, the fragmented populations of one salmonid species (Japan) and the fossil records from the Mississippi Basin were included in the study. Results (1) French rivers. Within strictly freshwater fish species, rare and/or habitat specialist species were the most affected by fragmentation. In contrast, euryhaline species were not affected. A negative relationship between extinction rate and river basin size was observed. (2) Other rivers. Our study established a common scaling relationship for freshwater fish population extinction rates that spans seven orders of magnitude in river basin size. Main conclusions This study strongly suggests that extinctions of fish populations occurred within French coastal rivers after they became isolated 8000 years ago. The patterns observed at regional and inter‐continental scales are consistent with the expectation that large populations are less prone to extinction than small ones, resulting in a strong extinction–area relationship coherent over a large spatio‐temporal scale. Our study is the first multi‐scale quantitative assessment of background extinction patterns for freshwater fishes.     

栖息地破碎化与鸟类生存

栖息地破碎化给野生动物带来的不良后果是全球生态学家和保护生物学家共同关心的问题.自从提出栖息地破碎化是导致生物多样性丧失的关键因素之一的论点后,近20年来,栖息地破碎化研究一直是生态学和保护生物学最活跃的前沿研究领域之一.栖息地破碎化是一动态过程,可在多尺度上发生并蕴涵着复杂的空间模式变化.栖息地破碎化对鸟类的生态学效应主要体现在面积效应、隔离效应和边缘效应等.这些效应影响着鸟类的分布、基因交流、种群动态、扩散行为、种间关系和生活史特征等,最终影响着鸟类的生存.介绍和总结了栖息地破碎化过程、研究的理论依据及栖息地破碎化对鸟类生存产生的诸多影响.     

Habitat fragmentation and species richness

In a recent article in this journal, Fahrig (2013, Journal of Biogeography, 40 , 1649–1663) concludes that variation in species richness among sampling sites can be explained by the amount of habitat in the ‘local landscape’ around the sites, while the spatial configuration of habitat within the landscape makes little difference. This conclusion may be valid for small spatial scales and when the total amount of habitat is large, but modelling and empirical studies demonstrate adverse demographic consequences of fragmentation when there is little habitat across large areas. Fragmentation effects are best tested with studies on individual species rather than on communities, as the latter typically consist of species with dissimilar habitat requirements. The total amount of habitat and the degree of fragmentation tend to be correlated, which poses another challenge for empirical studies. I conclude that fragmentation poses an extra threat to biodiversity, in addition to the threat posed by loss of habitat area.     

Habitat fragmentation versus fragmented habitats

Habitats often show similar present structuring, but contrasting histories: habitats occur naturally fragmented due to abiotic or biotic factors over long time periods, but may also have become fragmented only recently through transformation from interconnected to highly fragmented habitats within short time periods. Species and populations being faced with such contrasting habitat scenarios also show contrasting responses at species and intraspecific level. Organisms and populations from naturally fragmented habitats may show a reduction in their genetic load (purging) due to purifying selection in isolation. In contrast, sudden habitat transformations from interconnected to highly fragmented structures and the resulting transition from gene flow or panmixia to strong population differentiation often have negative effects on biota; while species occur in interconnected population networks (maintaining a high proportion of genetic diversity), a sudden breakdown of gene flow may lead to a severe loss of genetic diversity and the manifestation of weakly deleterious alleles. In consequence, fragmented habitats need not have a negative impact on species per se, but the history of habitat structures, particularly fast transformation processes, may severely affect the persistence and fitness of species.     

Assessing habitat loss and fragmentation and their effects on population viability of forest specialist birds: Linking biogeographical and population approaches

Aim

Biogeographic approaches usually have been developed apart from population ecology, resulting in predictive models without key parameters needed to account for reproductive and behavioural limitations on dispersal. Our aim was to incorporate fully spatially explicit population traits into a classic species distribution model (SDM) using Geographic Information Systems (GIS), aiming at conservation purposes.

Location

Southern South America.

Methods

Our analysis incorporates the effects of habitat loss and fragmentation on population viability and therefore provides insights into how much spatially explicit population traits can improve the SDM prediction of habitable habitat. We utilized a well‐studied focal endemic bird of South American temperate rainforests (Scelorchilus rubecula). First, at a large scale, we assessed the historical extent habitat based on climate envelopes in an SDM. Second, we used a land cover change analysis at a regional scale to account for recent habitat loss and fragmentation. Third, we used empirically derived criteria to predict population responses to fragmented forest landscapes to identify actual losses of habitat and population. Then we selected three sites of high conservation value in southern Chile and applied our population model. Finally, we discuss the degree to which spatially explicit population traits can improve the SDM output without intervening in the modelling process itself.

Results

We found a historical habitat loss of 39.12% and an additional forest cover loss of 3.03% during 2000–2014; the latter occurred with a high degree of fragmentation, reducing the overall estimation of (1) carrying capacity by ?82.4%, ?33.1% and ?45.1% and (2) estimated number of pairs on viable populations by ?84.1%, ?33.0% and ?54.6% on the three selected sites.

Main conclusion

We conclude that our approach sharpened the SDM prediction on environmental suitability by 54.4%, adjusting the habitable area by adding population parameters through GIS, and allowing to incorporate other phenomena as fragmentation and habitat loss.

     

Habitat fragmentation as a result of biotic and abiotic factors controls pathogen transmission throughout a host population

1. The hypothesis that habitat fragmentation (biotic or abiotic) alters the transmission of disease within a population is explored using field data from a well-studied amphibian-pathogen system. 2. We used the Ambystoma tigrinum-A. tigrinum virus (ATV) model system to show how habitat fragmentation as a result of emergent vegetation and habitat management affects disease transmission dynamics in ponds across a landscape. 3. We quantified variation in ATV infection over time and across the landscape. ATV infection was significantly higher in ponds modified for livestock use (P = 0.032). Disease incidence decreased with increased amounts of emergent vegetation (P < 0.001). These factors appear to control disease transmission by altering the host contact rate and with it disease transmission. 4. A field experiment to test the effect of emergent vegetation on the distribution of larvae in ponds demonstrated a behavioural change in larvae found in sparsely vegetated ponds. Microhabitat choices resulted in larvae being concentrated at the pond edge resulting in a 'halo effect' in sparsely vegetated ponds, whereas larvae in heavily vegetated ponds were distributed more evenly throughout. Microhabitat choice affects the effective density that larvae experience. This 'halo effect' increases contact rates in the shallows of sparsely vegetated ponds and increases the transmission of a directly transmitted pathogen. 5. Despite recurrent epidemics of a lethal Ranavirus in tiger salamanders on the Kaibab Plateau, Arizona, USA, these populations persist. We discuss the implications of our results in the context of density-dependent transmission and homogeneous mixing, two increases key assumptions of epidemiological theory.     

Habitat fragmentation: A long and tangled tale

In this essay: I provide a brief history of habitat fragmentation research; I describe why its “non‐questions” (‘Is habitat fragmentation a big problem for wildlife species?” and, “Are the effects of habitat fragmentation generally negative or positive?”) are important to conservation; I outline my role in tackling these questions; I discuss reasons why the culture of habitat fragmentation research is largely incapable of accepting the answers; and I speculate on the future of habitat fragmentation research.     

Habitat fragmentation and species diversity in competitive communities

Habitat loss is one of the key drivers of the ongoing decline of biodiversity. However, ecologists still argue about how fragmentation of habitat (independent of habitat loss) affects species richness. The recently proposed habitat amount hypothesis posits that species richness only depends on the total amount of habitat in a local landscape. In contrast, empirical studies report contrasting patterns: some find positive and others negative effects of fragmentation per se on species richness. To explain this apparent disparity, we devise a stochastic, spatially explicit model of competitive species communities in heterogeneous habitats. The model shows that habitat loss and fragmentation have complex effects on species diversity in competitive communities. When the total amount of habitat is large, fragmentation per se tends to increase species diversity, but if the total amount of habitat is small, the situation is reversed: fragmentation per se decreases species diversity.     

Habitat associations of British breeding farmland birds

Capsule Territory distribution for ten species was most strongly positively influenced by the presence of hedges and woodland edge.

Aims To describe and rank the importance of different habitat predictors on the distribution of bird territories.

Methods We derived territory maps for ten bird species across 25 sites on English lowland farmland in 2002. We related habitat predictors to the distribution of these species using information theoretic methods.

Results Habitat predictors were ranked as follows (numbers in parentheses indicate the number of species with a strong effect): hedge presence (8), boundary height (7), woodland edge (6), tree presence in boundary (4), brassica (mainly oil seed rape) (3), within‐field vegetation height (3), boundary strip (3), boundary width (3), tilled fields (3), winter set‐aside (2), ditch (1), winter stubble fields (1).

Conclusions Non‐cropped habitats had the most consistent positive effects across all ten species, with crop types and their margins exerting smaller effects.     

Habitat destruction and extinction in competitive and mutualistic metacommunities

Because habitat loss is a leading cause of extinction, it is important to identify what kind of species is most vulnerable. Here, I use algebraic and graphical techniques to study metacommunity models of weak competition or locally facultative mutualism in which species may coexist within patches. Because a competition–colonization trade‐off is not required for regional coexistence of competitors, poor competitors are often regionally rare and most prone to extinction, in contrast to results from previous models of strongly competitive metapopulations. Metacommunities of mutualists can suffer the abrupt extinction of both species as habitat destruction is increased. These highlight the importance of identifying the mechanisms by which species coexist to predict their response to habitat loss.     

Sexual selection and the risk of extinction in birds

The relationship between sexual selection and extinction risk has rarely been investigated. This is unfortunate because extinction plays a key role in determining the patterns of species richness seen in extant clades, which form the basis of comparative studies into the role that sexual selection may play in promoting speciation. We investigate the extent to which the perceived risk of extinction relates to four different estimates of sexual selection in 1030 species of birds. We find no evidence that the number of threatened species is distributed unevenly according to a social mating system, and neither of our two measures of pre-mating sexual selection (sexual dimorphism and dichromatism) was related to extinction risk, after controlling for phylogenetic inertia. However, threatened species apparently experience more intense post-mating sexual selection, measured as testis size, than non-threatened species. These results persisted after including body size as a covariate in the analysis, and became even stronger after controlling for clutch size (two known correlates of extinction risk). Sexual selection may therefore be a double-edged process-promoting speciation on one hand but promoting extinction on the other. Furthermore, we suggest that it is post-mating sexual selection, in particular, that is responsible for the negative effect of sexual selection on clade size. Why this might be is unclear, but the mean population fitness of species with high intensities of post-mating sexual selection may be especially low if costs associated with multiple mating are high or if the selection load imposed by post-mating selection is higher relative to that of pre-mating sexual selection.     

Habitat fragmentation resulting in overgrazing by herbivores

Habitat fragmentation sometimes results in outbreaks of herbivorous insect and causes an enormous loss of primary production. It is hypothesized that the driving force behind such herbivore outbreaks is disruption of natural enemy attack that releases herbivores from top-down control. To test this hypothesis I studied how trophic community structure changes along a gradient of habitat fragmentation level using spatially implicit and explicit models of a tri-trophic (plant, herbivore and natural enemy) food chain. While in spatially implicit model number of trophic levels gradually decreases with increasing fragmentation, in spatially explicit model a relatively low level of habitat fragmentation leads to overgrazing by herbivore to result in extinction of the plant population followed by a total system collapse. This provides a theoretical support to the hypothesis that habitat fragmentation can lead to overgrazing by herbivores and suggests a central role of spatial structure in the influence of habitat fragmentation on trophic communities. Further, the spatially explicit model shows (i) that the total system collapse by the overgrazing can occur only if herbivore colonization rate is high; (ii) that with increasing natural enemy colonization rate, the fragmentation level that leads to the system collapse becomes higher, and the frequency of the collapse is lowered.     

Habitat destruction, environmental catastrophes, and metapopulation extinction

The extinction process of fragmented populations, characterized by a small number of conspecifics inhabiting each patch, is heavily affected by natural and human disturbance. To evaluate the risk of extinction we consider a network of identical patches connected by passive or active dispersal and hosting a finite, discrete number of individuals. We discuss three types of disturbance affecting the metapopulation: permanent loss of habitat patches, erosion of existing patches, and random catastrophes that wipe out the entire population of a patch. Starting from an infinite-dimensional Markov model that fully accounts for demographic stochasticity, we reduce it to finite dimension via moment closure with negative-binomial approximation. The compact models obtained in this way account for the dynamics of the fraction of empty patches, the average number of individuals in occupied patches, and the variance of their distribution. After comparing the performance of these compact models with that of the infinite-dimensional model in the case of no disturbances, we then proceed to computing persistence-extinction boundaries as bifurcation lines of the compact models in the space of demographic and disturbance parameters. We consider bifurcations with respect to demographic and environmental parameters and contrast our results with those of previous theories. We find out that environmental catastrophes increase the risk of extinction for both frequent and infrequent dispersers, while the random loss of patches has a much larger influence on frequent dispersers. This influence can be counterbalanced by active dispersal. Local erosion of habitat fragments has a larger influence on infrequent than on frequent dispersers. We finally discuss the important synergistic effects of disturbances acting simultaneously.     

Ecological correlates of extinction risk in Chinese birds

China is one of the countries with the richest bird biodiversity in the world. Among the 1372 Chinese birds, 146 species are considered threatened and three species are regionally extinct according to the officially released China Biodiversity Red List in 2015. Here, we conducted the first extensive analysis to systematically investigate the patterns and processes of extinction and threat in Chinese birds. We addressed the following four questions. First, is extinction risk randomly distributed among avian families in Chinese birds? Second, which families contain more threatened species than would be expected by chance? Third, which species traits are important in determining the extinction risk in Chinese birds using a multivariate phylogenetic comparative approach? Finally, is the form of the relationship between traits additive or nonadditive (synergistic)? We found that the extinction risk of Chinese birds was not randomly distributed among taxonomic families. The families that contained significantly more threatened species than expected were the hornbills, cranes, pittas, pheasants and hawks and eagles. We obtained eleven species traits that are commonly hypothesized to influence extinction risk from the literature: body size, clutch size, trophic level, mobility, habitat specificity, geographical range size, nest type, nest site, flocking tendency, migrant status and hunting vulnerability. After phylogenetic correction, model selection based on Akaike's information criterion identified the synergistic interaction between body size and hunting vulnerability as the single best correlate of extinction risk in Chinese birds. Our results suggest that, in order to be effective, priority management efforts should be given both to certain extinction‐prone families, particularly the hornbills, pelicans, cranes, pittas, pheasants and hawks and eagles, and to bird species with large body size and high hunting vulnerability.     

Habitat fragmentation and the functional efficiency of temperate dung beetles

To understand how current patterns of habitat loss and fragmentation will ultimately affect ecosystem functioning, we need to match experimental manipulations of community structure with real changes occurring in the landscapes of today. In this study, we examine the consequences of habitat fragmentation on a key function: the decomposition of dung by invertebrates. In a microcosm experiment, we use previous observations of dung beetle assemblage structure in fragmented and intact landscapes to create realistic differences in assemblages of small, dung‐dwelling species in the genus Aphodius. We ask whether such differences will affect ecosystem functioning, and how their effects compare to those of removing full functional groups: dung‐dwelling Aphodius, tunnelling Geotrupes stercorarius, and/or earthworms. As measured by changes in dung fresh weight, we observe an overriding impact of removing G. stercorarius, with the amount of dung remaining at any one time doubling if the species is excluded. Compared to this major effect, there seem to be less effects of removing Aphodius, ambiguous effects of excluding earthworms, and no detectable effects of relatively minor changes in Aphodius assemblages as induced by current levels of fragmentation. Overall, our results support the general notion that different species contribute highly unevenly to overall ecosystem functioning. Most importantly though, our findings suggest that the functional consequences of habitat loss will depend on taxon‐specific responses to landscape modification. Only by addressing these responses may we predict the actual consequences of habitat loss.     

Habitat and landscape effects on abundance of Missouri's grassland birds

Of 6 million ha of prairie that once covered northern and western Missouri, <36,500 ha remain, with planted, managed, and restored grasslands comprising most contemporary grasslands. Most grasslands are used as pasture or hayfields. Native grasses largely have been replaced by fescue (Festuca spp.) on most private lands (almost 7 million ha). Previously cropped fields set aside under the Conservation Reserve Program (CRP) varied from a mix of cool-season grasses and forbs, or mix of native warm-season grasses and forbs, to simple tall-grass monocultures. We used generalized linear mixed models and distance sampling to assess abundance of 8 species of breeding grassland birds on 6 grassland types commonly associated with farm practices in Missouri and located in landscapes managed for grassland-bird conservation. We selected Bird Conservation Areas (BCAs) for their high percentage of grasslands and grassland-bird species, and for <5% forest cover. We used an information-theoretic approach to assess the relationship between bird abundance and 6 grassland types, 3 measures of vegetative structure, and 2 landscape variables (% grassland and edge density within a 1-km radius). We found support for all 3 levels of model parameters, although there was less support for landscape than vegetation structure effects likely because we studied high-percentage-grassland landscapes (BCAs). Henslow's sparrow (Ammodramus henslowii) counts increased with greater percentage of grassland, vegetation height-density, litter depth, and shrub cover and lower edge density. Henslow's sparrow counts were greatest in hayed native prairie. Dickcissel (Spiza americana) counts increased with greater vegetation height-density and were greatest in planted CRP grasslands. Grasshopper sparrow (A. savannarum) counts increased with lower vegetation height, litter depth, and shrub cover. Based on distance modeling, breeding densities of Henslow's sparrow, dickcissel, and grasshopper sparrow in the 6 grassland types ranged 0.9–2.6, 1.4–3.2, and 0.1–1.5 birds/ha, respectively. We suggest different grassland types and structures (vegetation height, litter depth, shrub cover) are needed to support priority grassland-bird species in Missouri. © 2011 The Wildlife Society.     

Habitat fragmentation may not matter to species diversity

Conservation biologists worry that fragmenting a bloc of natural habitat might reduce its species diversity. However, they also recognize the difficulty and importance of isolating the effect of fragmentation from that of simple loss of area. Using two different methods (species-area curve and Fisher's alpha index of diversity) to analyse the species diversities of plants, tenebrionid beetles and carabid beetles in a highly fragmented Mediterranean scrub landscape, we decoupled the effect of degree of fragmentation from that of area loss. In this system, fragmentation by itself seems not to have influenced the number of species. Our results, obtained at the scale of hectares, agree with similar results at island and continent scales.