Assessing the broad-scale impact of agriculturally transformed and protected area landscapes on avian taxonomic and functional richness
Introduction
Croplands and pastures have become one of the largest terrestrial biomes on the planet, occupying around 40% of the land surface (Foley et al., 2005). These landscapes are becoming increasingly important for biodiversity conservation as pressure from anthropogenic activities compromises the effectiveness of reserve networks (DeFries et al., 2007). Reserves alone will not be able to maintain landscape level biodiversity or resulting ecosystem services if they are isolated from the surrounding matrix (Gaston et al., 2002, DeFries et al., 2005). Agricultural landscapes, although unable to support many habitat specialist species, can be important matrix ecosystems that contribute to species persistence by providing areas of phase differentiation, resource consistency and enemy-free areas (Poiani et al., 2000, Robinson et al., 2001, Okes et al., 2008). Thus, landscapes containing both croplands and patches of natural vegetation embedded within the agricultural matrix may interact to foster functional ecosystems. Agricultural landscapes therefore play a potentially important regional role in landscape function and their individual characteristics will ultimately affect the resilience and adaptive capacity of embedded and adjacent ecosystems (Cumming et al., 2005).
Species–area curves and biodiversity ‘hotspots’ have been used extensively in spatial conservation planning (Zimmerman and Bierregaard, 1986, Pearson and Carroll, 1998, Myers et al., 2000, Gotelli and Colwell, 2001). These approaches focus on species representation rather than ecological processes (Gaston et al., 2001, Wiersma and Urban, 2005). Species richness, however, does not fully describe the functional diversity of a community (Doak and Mills, 1994, Diaz and Cabido, 2001, Petchey and Gaston, 2002, Naeem and Wright, 2003) or the resilience of important ecosystem functions (e.g., nitrogen fixation or pest control) to species loss. Functional diversity arises from the relative functional trait richness that species possess (Tilman et al., 1997, Petchey and Gaston, 2006), and the distribution of functions within and across scales provides ecosystems with resilience (Peterson et al., 1998, Allen et al., 2005). Both conservation plans and current global biodiversity scenarios typically lack process-oriented metrics of ecosystems (Cumming, 2007), such as those based on asymmetrically distributed species traits that represent multiple ecological processes. Thus, planning for ecosystem services at landscape levels is constrained by a simple lack of knowledge as to whether or not ecosystems are robust to species loss, how functional group richness varies across space, and how this variation relates to ecosystem resilience (Bengtsson et al., 2003). Although species richness is strongly and positively correlated with functional richness, there is considerable spatial variation in this relationship for South African birds (Cumming and Child, in press). Studies detailing the causes and consequences of functional variation in space and time have the potential to link functional and taxonomic perspectives and will be vital for effective conservation management, especially if conservationists are to develop reserve networks that exhibit functional resilience (Cowling et al., 1999, Allen et al., 2005).
Elucidating the patterns of functional richness in space is a crucial first step in being able to determine the resilience of service delivery, especially since the spatial overlap between ecosystem services and species richness remains ambiguous (Egoh et al., 2009). In this paper we explore how avian functional richness is distributed across either predominantly protected or predominantly agricultural landscapes and discuss the potential mechanisms for these distributions. We use the functional group classification of Sekercioglu (2006) because of its explicit focus on birds as multifaceted actors in ecosystem dynamics. Birds constitute an extremely well-studied taxon and their importance as mobile links in the dynamics of both natural and human-transformed landscapes is extensively documented (reviewed by Sekercioglu, 2006). Declines in avian functional richness are predicted to have severe consequences for the maintenance and provision of a diverse array of ecosystem services (Sekercioglu et al., 2004).
Agricultural practices may provide novel resources that some species are better at exploiting than others, thereby simplifying community and trophic structure while maintaining overall abundance (La Sorte, 2006). For example, generalist waterfowl have successfully spread across agricultural landscapes in South Africa (Okes et al., 2008), with negative effects on local avian diversity (Curtis et al., 2007). If agricultural transformation reorganises avian communities at local scales, it is important to establish whether these effects translate into emergent landscape patterns that can later be used to assess regional functional resilience. Here we provide a first step towards this goal by quantifying and mapping avian functional richness patterns in selected pairs of sites across South Africa.
Section snippets
Matched pair setup and data sources
We used a matched pair sampling design to control for potentially confounding variables. A matched pair format is appropriate because we compared the relative differences in functional richness between similar landscapes that are predominantly covered by agriculture with those that contain protected areas. The pairs themselves were carefully matched in terms of land-use, environmental variables and sampling effort. Furthermore, we used matched pair t-tests to avoid the confounding effects of
Matched pair analyses
117 matched pairs across South Africa were suitable for analysis (n = 234 grid cells, Fig. 1). Agricultural land-use covered an average of 39.9 ± 18.6% of unprotected cells while nature reserves covered an average of 31.4 ± 27.8% of protected cells (both of which are significantly high average values relative to parameter values; see supplementary Figs. 4 and 5). Matched pairs contained similar numbers of checklists per cell (an average of 85 ± 75 and 88 ± 85 checklists per cell for unprotected and
Discussion and conclusions
These results demonstrate that protected landscapes capture facets of avian functional richness that differ from those captured by landscapes that contain agricultural land-cover. Most importantly, scavengers and raptors are negatively affected by agricultural transformation whereas nutrient dispersers, grazers and granivores appear not to be affected. Most landscape studies have tended to focus on spatial sampling extents that are completely covered by a specific land-use type and then compare
Acknowledgments
We are grateful to Doug Harebottle and Michael Brooks of the Avian Demography Unit (UCT) for supplying the SABAP dataset as well as helpful advice on its idiosyncrasies. Thanks also to Rob Simmons and Mark Anderson for useful information on vultures and diclofenac in South Africa. This research was supported by the University of Cape Town and a DST/Centre of Excellence bursary from the Percy FitzPatrick Institute of African Ornithology. We would also like to thank two anonymous referees who
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