Elsevier

Biological Conservation

Volume 119, Issue 2, September 2004, Pages 191-206
Biological Conservation

Do regional gradients in land-use influence richness, composition and turnover of bird assemblages in small woods?

https://doi.org/10.1016/j.biocon.2003.11.003Get rights and content

Abstract

Small patches of natural or semi-natural habitat have an important role in the conservation of biodiversity in human-dominated environments. The values of such areas are determined by attributes of the patch as well as its context in the surrounding land mosaic. There is a need for better understanding of the ways in which assemblages are influenced by patch context and the scale over which this occurs. Here we examine the influence of regional environmental gradients on the richness, annual turnover and composition of breeding bird species in small woods in south-eastern England. Regional gradients were defined independently of woods by an ordination of attributes for 5 km × 5 km landscape units across a 2100 km2 region. Patch-level attributes, particularly area, were the most important predictors for most bird variables. For woodland migrants and woodland-dependent species, variables representing the context of each wood, either at a local or regional scale, explained significant additional variance in species richness after accounting for wood area, but did not do so for species turnover. Significant context effects for woodland-dependent species related to the extent of hedges and woodland cover in the local vicinity (<1 km radius), whereas for woodland seasonal migrants the best predictors of richness after patch area were two regional environmental gradients. The initial cue to settlement for migrants may be at a coarse regional scale, with selection for suitable landscapes that have a greater extent of woodland cover. Edge species showed different responses: they were influenced by the diversity of structural features in woods, and were a more-dominant component of the avifauna in isolated woods in open fenland environments of the region. Significant relationships between coarse regional gradients (25 km2 units) and bird assemblages in small woods (0.5–30 ha) suggest that population and community processes in the avifauna operate across a broader scale than local patch neighbourhoods. They also highlight the importance of adopting a landscape or regional perspective on potential changes to land-use in rural environments, and on the conservation management of small reserves.

Introduction

Throughout the world, small areas of natural or semi-natural habitat have an important role in maintaining biodiversity in human-dominated environments (Saunders et al., 1987; Forman, 1995; Laurance and Bierregard, 1997). Consequently, much attention has been given to identifying factors that influence the conservation value of such areas. Many studies have described relationships between the number of animal species, or occurrence of particular species, and attributes of habitat patches such as their size, shape, habitat diversity and management history (Freemark and Merriam, 1986; Bennett, 1987; Loyn, 1987; Bolger et al., 1997; Deacon and Mac Nally, 1998). The location of patches in relation to other suitable habitat has also been identified as a significant influence. Thus, features such as the distance to potential source populations (Opdam et al., 1985; Newmark, 1991; Thomas and Jones, 1993), the amount of nearby similar habitat (Askins et al., 1987; Jansson and Angelstam, 1999; Graham and Blake, 2001), and the presence of linking habitats (Bright et al., 1994; Haas, 1995) have been shown to complement patch-level characteristics as influences on the distribution of animal species.

The growing recognition of the importance of landscape context (e.g. Wiens, 2002; Wiens et al., 2002) has led to closer scrutiny of the structure of landscapes within which habitat patches are located. It is not only similar areas of habitat that may be important influences, but also the composition and spatial arrangement of other land-uses of differing quality for the biota (e.g. urban areas, roads, wetlands, arable land). Increasing attention is being given to the extent to which the overall structure of the landscape may influence the status of populations and communities (Pearson, 1993; Bennett and Ford, 1997; Fuller et al., 1997; Baillie et al., 2000; Pope et al., 2000; Soderstrom and Part, 2000; Bellamy et al., 2003). Indeed, it has been suggested (Diamond et al., 1987; Saunders et al., 1991; Hobbs, 1993) that processes operating in the wider landscape or regional land mosaic may ultimately have as great, or greater, influence on the composition and status of the biota in small patches than within-patch processes. First, surrounding land-uses can modify the dynamics of metapopulations by influencing the capacity of species to disperse through the landscape. Different forms and intensities of land-use offer different levels of ‘resistance’ to the movements of species (Laurance, 1991; Gascon et al., 1999; Lindenmayer et al., 2002). Second, particular land-uses may offer additional food, shelter or other resources that complement those available to species within habitat patches (Pope et al., 2000). Third, other plant and animal species or disturbance processes associated with surrounding land-uses can have a marked impact on habitat quality in isolated patches (Saunders et al., 1991). There may be invasion or degradation of natural habitats (Hobbs, 1993; Scougall et al., 1993; Abensperg-Traun et al., 1996), or the imposition of new predators and competitors that result in population declines or change in the structure of assemblages (Andrén, 1992; Grey et al., 1998).

Therefore, one expects that habitat patches of similar size or vegetation type will show differences in their biotic communities depending on the context of the patch in relation to patterns of land-use and the physical environment. Of course, different species will respond to landscape context in different ways and at different scales. A number of studies have detected context effects by testing the influence of habitat availability within a defined radius of a patch, on the occurrence or richness of species within that resource patch (Brennan et al., 2002). The scale for such studies has often been ⩽1 km (e.g. Pearson, 1993; Jansson and Angelstam, 1999; Soderstrom and Part, 2000; Graham and Blake, 2001), a relatively small distance when studying a mobile taxon such as birds. Context effects have seldom been tested over a larger landscape scale (e.g. 1–10 km) or in relation to environmental variation and land use across the entire region in which study sites are located.

In this study we examine the potential effect of regional context on assemblages of breeding birds in small woods in south-eastern England. Previous work in this study area showed that the richness of breeding bird species (Bellamy et al., 1996a) and the distribution of individual species (Hinsley et al., 1995a) are influenced both by the characteristics of woods in which birds nest and habitat attributes in the surrounding landscape. Distances of up to 1 km radius around woods were used to assess features such as woodland cover and length of hedgerows. The various types of responses to landscapes in the region suggested that species are making different sets of choices between large-scale habitat factors and fine-scale selection of sites for breeding (Hinsley et al., 1995b). Choices at larger scales appear to be important for some species. For example, analysis of the distribution of the nuthatch Sitta europaea (Bellamy et al., 1998) indicated the importance of variation in woodland cover between regions in southern England. Consequently, in this paper we use data pertaining to land-use and the physical environment to identify regional environmental gradients across a 2100 km2 region, independently of the locations of woods where birds were censused. We then examine three aspects of breeding assemblages in woods – species richness, relative species turnover and composition – to explore the extent to which the context of a wood in relation to these regional gradients influences the status of its bird assemblage. The implications of regional context effects for the conservation of biota are then discussed.

Section snippets

Study area

The study was carried out in south-east England in a region of 2100 km2 (70 km × 30 km) in the counties of Cambridgeshire and Lincolnshire (Fig. 1). Two main landform types occur in the region: flat expanses of drained fenland in the east and more-undulating countryside of heavy clays in the south-west. Land-use throughout the region is dominated by intensive arable farming, with the main crops being autumn-sown cereals (barley, wheat), oilseed rape, sugar beet and potatoes. Towns and numerous

Variation in breeding bird assemblages among woods

Data on species richness, species turnover and the composition of breeding bird assemblages were calculated for 88 woods ranging from 0.5 to 29.9 ha in area. Most woods were small; the mean size was 2.8 ha and only 16% (14/88) were larger than 5 ha. A total of 49 species was recorded breeding in these woods during the study (Table 1). The 3-year means for the number of species per wood ranged from 5 to 24, with an overall mean of 12.6 species.

Six species of migrants primarily depend on woodland

Discussion

A fundamental principle in landscape ecology is that ‘landscape context matters’. This is not only relevant to conceptual development in the discipline (e.g. Forman, 1995; Wiens, 1995) but is of great practical importance for the conservation of biodiversity in human-dominated environments. Many species now occupy patches of habitat within heavily modified land mosaics in which the composition and configuration of habitats differ markedly from the natural state. Likewise, many small

Acknowledgements

This research was initiated while AFB was on study leave at Centre for Ecology and Hydrology, Monks Wood. He thanks Dr. Barry Wyatt for the use of facilities at Monks Wood, the School of Ecology and Environment, Deakin University for study leave, and Land and Water Australia for a Travelling Fellowship that assisted with travel costs. We thank Gary Luck and Jim Radford for their comments on the manuscript. Thanks also to all the landowners, managers and keepers who allowed access to their land.

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