Landscape-level thresholds of habitat cover for woodland-dependent birds

Abstract

Theory suggests that a disproportionate loss of species occurs when total habitat cover decreases to 10–30% of the landscape. To date, little empirical evidence has been collected to test for such thresholds in habitat cover, especially at the landscape scale. Here, we present empirical data on the species richness of woodland-dependent birds collected systematically from 24 landscapes (each 100 km²) that sample a gradient in habitat cover from <2% to 60%. To compare the relative effects of habitat cover and habitat configuration, landscapes with similar amounts of habitat but contrasting configuration (i.e., aggregated versus dispersed) were surveyed and the richness of woodland-dependent birds collated for each landscape. The relationship between species richness, habitat cover and habitat configuration was examined using analysis of co-variance (ANCOVA), multiple linear regression and univariate non-linear modelling. There was a significant effect of habitat cover (co-variate) in the ANCOVA, but the main treatment effect of configuration was not significant. However, comparison of non-linear models indicated that the shape of the response curve of species loss with decreasing habitat cover differed between aggregated and dispersed landscapes. Species richness was significantly related to habitat cover in all analyses, explaining between 55% and 60% of the variance in regression models. Mean patch shape complexity and the extent of habitat aggregation were also significant explanatory variables, but explained less than 10% of the variance in richness of woodland birds. Biogeographic variables (range in elevation and geographic location) explained up to 14% of the variance in species richness. There was strong evidence for a threshold response in species richness: non-linear models (broken-stick, exponential, inverse) exhibiting a sharp decline in species richness in landscapes with less than 10% habitat cover provided a better fit to the observed data than linear models. To our knowledge, this is the first empirical demonstration of landscape-level thresholds in species richness. We emphasise that thresholds in species richness denote multiple species’ extinction events, the end point of the process of species decline. For viable populations, habitat cover must be maintained well above the threshold level. Finally, thresholds of assemblage measures, such as species richness, potentially mask compositional changes in the avifauna community and may also conceal the loss of species with greater sensitivity to landscape change.

Introduction

Throughout the world, concern about the effects of habitat loss and degradation has stimulated much research into the status of faunal species and assemblages in fragmented landscapes. Much of this work has been carried out at the ‘patch-level’; that is, the units of study are spatially discrete remnants of habitat and their use by the fauna. This has provided new understanding of factors that influence the occurrence and status of species in remnant habitats (Bright et al., 1994, Margules et al., 1994, Redpath, 1995), and of the way in which the richness and composition of assemblages are related to attributes such as size and isolation of remnants, vegetation type, and land management practices (Klein, 1989, Hinsley et al., 1995, Bolger et al., 1997, Mac Nally et al., 2000a).

A recurring theme from many of these studies is that effective conservation of the biota requires a broader ‘landscape’ or ‘regional’ perspective on the dynamics of populations and the function of ecological processes. First, single patches of habitat are rarely large enough to support long-term, self-sustaining populations of most species of concern. Rather, persistence depends upon multiple populations and the capacity for interaction between them (Opdam, 1991, Fahrig and Merriam, 1994). Second, some species need to have access to different types of landscape elements to obtain required resources (Law and Dickman, 1998, Dunning et al., 1992, Manning et al., 2004) This may require regular movement for concurrent use of different parts of the landscape (e.g., for foraging and breeding) or sequential use of different habitats to track temporally varying resources (Mac Nally and Horrocks, 2000, Pope et al., 2000). Third, there is considerable evidence that landscape context has an important influence on species composition and on ecological processes within habitat patches (Hobbs, 1993, Lindenmayer et al., 2002, Bennett et al., 2004). Thus, land uses within the mosaic surrounding a habitat patch warrant consideration.

There also is wide recognition among land managers that planning for conservation must occur at broad spatial scales (Saunders et al., 1996). However, knowledge of landscape-level requirements for effective conservation of biodiversity in land mosaics is limited. Key issues include the amount (or percent cover) of habitat needed to achieve conservation goals, and the relative importance of habitat configuration (Fahrig, 2002). To date, these issues have been addressed mainly through computer simulation modelling (With and Crist, 1995, Fahrig, 1997, With and King, 1999), in which the conclusions are largely dependent on the modelling approach (Fahrig, 2002). The few studies in which empirical data have been collected at the landscape level concur on the importance of amount of habitat but differ in their assessment of configuration effects (Trzcinski et al., 1999, Villard et al., 1999, Cooper and Walters, 2002, Krauss et al., 2004).

An important outcome from modelling has been the recognition that the relationship between species occurrence and landscape pattern is often non-linear. Rather, there appear to be critical thresholds at which a small change in spatial pattern can produce an abrupt shift in ecological response (Andrén, 1994, With and Crist, 1995, Huggett, this issue). The effect of habitat spatial pattern on landscape connectivity appears to be particularly influential (With and Crist, 1995), a view supported by empirical studies that have demonstrated threshold responses to spatial isolation at the patch level (Jansson and Angelstam, 1999, Cooper and Walters, 2002, Radford and Bennett, 2004). Andrén’s (1994) review of modelling simulations and patch-level studies concluded that a major ecological change occurs when habitat cover declines to approximately 10–30% of the landscape. Above this level, population decline or species loss is likely to be linearly related to habitat loss, but below a critical threshold the effect of habitat loss is exacerbated by fragmentation effects and rapid population decline or species loss occurs.

These issues are significant for land management and conservation planning because they have implications for setting goals for habitat protection, and for the cost-effectiveness of restoration actions. Andrén’s (1994) ‘fragmentation threshold’ of between 10% and 30% of habitat cover has been embraced by land managers (Barrett, 2000, McIntyre et al., 2000, North Central Catchment Management Authority, 2003) despite a lack of empirical data that test the relationship between habitat extent, habitat configuration and critical thresholds at the landscape level (Harrison and Bruna, 1999, Fahrig, 2002). In this paper, we address this gap in knowledge by reporting on an empirical study of the response of woodland-dependent birds to the pattern of woodland habitat in landscapes in south-eastern Australia. We used landscapes of 100 km² as the unit of investigation, selected to represent a contrast in habitat configuration at different levels of habitat cover. Two key questions underpin this study: (i) what is the relative importance of habitat amount and habitat configuration in determining species richness of woodland-dependent birds at the landscape level; and, (ii) is there evidence for a critical threshold in amount of habitat for species richness of woodland-dependent birds?