Habitat loss and fragmentation affecting mammal and bird communities—The role of interspecific competition and individual space use
Highlights
► We model mammal and bird communities on the basis of individual home range formation. ► Small scale processes are important for community response to landscape changes. ► Complex effects on the size distributions can be explained by competitive release. ► Birds seem to be able to buffer fragmentation effects on abundance by space use.
Introduction
The ongoing destruction and fragmentation of habitat are considered the greatest contributors to recent and potential future extinctions (Ewers and Didham, 2006, Fahrig, 2003). While numerous studies have investigated the effects of landscape modifications on single species (Debinski and Holt, 2000) or functional types (Jeltsch et al., 2011, Körner and Jeltsch, 2008, Körner et al., 2010), the complex interplay of mechanisms affecting interacting animal communities at small scales (e.g. individual foraging behaviour or resource competition), is still poorly understood.
Even though differences in methodology and terminology in various studies cause difficulties to synthesise general conclusions (Fahrig, 2003), effects of reduced habitat area (habitat loss) on populations are relatively consistent among studies and rather well understood. A large number of studies report threshold behaviour (so called ‘extinction thresholds’) of animal populations with reduced amount of habitat, and several theoretical modelling studies have proposed a variety of mechanistic explanations for such non-linear dynamics, ranging from percolation theory and isolation effects to time lag and Allee effects (Bascompte and Sole, 1996, Fahrig, 2002, Harrison and Bruna, 1999, Swift and Hannon, 2010). In contrast, reported effects of fragmentation per se on populations are less clear and often even contradictory (compared to the effects of habitat loss). Fragmentation per se here means the spatial configuration of habitat only while total habitat area remains unchanged (this aspect was also intensively discussed in the ‘SLOSS debate’, e.g. Wilcox and Murphy, 1985 and the references therein). Different studies report positive or negative effects of fragmentation on species occurrence or abundance, but some also report no effect (Fahrig, 2003, Smith et al., 2011). The interplay of habitat fragmentation with habitat loss (for example how fragmentation affects the threshold behaviour with habitat loss, or how total habitat area controls the strength of the fragmentation effect) still poses a particularly difficult challenge to scientists and conservation managers.
Interspecific interactions have been shown to increase the complexity of system response to landscape modifications (Banks et al., 2007, Brown, 2007, Debinski and Holt, 2000, Nee and May, 1992). Empirical investigation of such complex systems is difficult, and most studies are therefore limited to either a focus on the abundance of single species (i.e. they miss the community context), or to species richness of communities (i.e. they miss information on the condition of the different populations) (Debinski and Holt, 2000). Various modelling approaches have been developed to disentangle mechanisms controlling how populations or communities respond to habitat loss and fragmentation. The vast majority of these models can be categorized in the family of metacommunity models (e.g. Hawkes, 2009, Leibold et al., 2004; for more detailed description and categorization of different models see for example Flather and Bevers, 2002, Kareiva et al., 1990, Swift and Hannon, 2010). Metacommunity models work at large spatial and temporal scales and focus on dispersal as the crucial spatial process affected and constrained by landscape configuration.
Processes at small scales, such as foraging behaviour, space use and local resource competition, however, play a crucial role in how individuals and species in interacting communities cope with heterogeneous resource distributions (Buchmann et al., 2012, Debinski and Holt, 2000, Gautestad and Mysterud, 2010, Hawkes, 2009, Morales et al., 2010, Nee and May, 1992, Pita et al., 2010, Ritchie, 1998, Smith et al., 2011). Different space use behaviours of individuals of different taxa—for example the higher mobility and larger home ranges of birds compared to mammals (Breitbach et al., 2010, Ottaviani et al., 2006)—can also affect the response of communities to changes in resource distributions. Nevertheless, theoretical studies investigating such small scale mechanisms (e.g. optimal foraging behaviour, Nonaka and Holme, 2007, Skorka et al., 2009, but also Gautestad and Mysterud, 2010), have generally not made the step to consider species interactions, implying they are not yet geared towards exploring community questions. The main reason might be that studies accounting for the importance of individual behaviour and space use are too complex (Nonaka and Holme, 2007, Van Moorter et al., 2009) and often designed for a specific single species (e.g. Bowers et al., 1996, Skorka et al., 2009). We have recently proposed a simple alternative, an individual-based spatially explicit model of individual home range formation of multiple mammal species parameterized by allometric relationships (Buchmann et al., 2011, Buchmann et al., 2012). This approach considers the important role of individual space use and resource competition on home range formation (e.g. Nee and May, 1992, Pita et al., 2010, Swihart et al., 1988), thereby enabling mechanistic investigation of the processes structuring animal communities.
In this study we use a modification of the model described in Buchmann et al. (2012) to elaborate the role of interspecific competition and individual space use for communities facing landscape changes. In addition to mammals, we also parameterize the model for the first time for birds and explore how not only habitat loss, but also fragmentation (and the combination of both), affect the body mass distribution of these communities.
Section snippets
Methods
Our modelling study aims to explore the response of the body mass distribution—namely the individual size distribution (ISD, after White et al., 2007)—of mammal and bird communities to habitat loss and fragmentation, emphasising the role of interspecific competition and individual space use. In the Methods section we first explain the generation of simulation landscapes (including loss and fragmentation of habitat) followed by a brief description of the allometric model of home range formation
Results
The distribution of individual body mass (ISD) showed a strong response to habitat loss (less suitable habitat area S) and fragmentation (spatial configuration of remaining habitat, controlled by the Hurst-factor H). However, the response was different for mammal and bird communities (Fig. 2a, b and c, d, respectively). For both taxa, habitat loss of up to 25% (100% to 75% suitable habitat area) does not significantly alter the ISD (indicated by the ISD exponent) or the size of the largest
Discussion
To our knowledge, this is the first explanation of how habitat loss and fragmentation affect the individual size distribution (ISD) of mammal and bird communities in terrestrial systems. In contrast to aquatic systems, the mechanisms that control body mass distributions, and how these mechanisms are affected by environmental change, are still largely unexplored in terrestrial communities (White et al., 2007). Our mechanistic model of individual home range formation in animal communities enables
Conclusions
Using allometric relationships to combine resource competition and space use, considering individual physiology and foraging behaviour, our model predicts both common and distinct patterns of community response to habitat loss and fragmentation for mammals and birds. Our results have direct implications for community conservation based on habitat management: Firstly, the findings emphasize that mammals are particularly sensitive to a combination of habitat loss and habitat fragmentation,
Acknowledgements
We thank N. Blaum and E. Rosmanith and various members from the research group Plant Ecology and Nature Conservation of the University of Potsdam for helpful suggestions and ideas, as well as D. Ottaviani for providing us with her data on bird body masses and home ranges for re-analysis. C. Buchmann would like to thank the Graduate Initiative on Ecological Modelling (‘UpGrade’) of the University of Potsdam for financial support. F. Jeltsch and F. Schurr acknowledge support from the European
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