Degradation of tropical forests through selective logging and shifting agriculture is both widespread and continuing yet, despite several decades of research into this problem, the impacts of such disturbance on the diversity of fauna within tropical forests are only poorly understood. Severe disturbance (e.g. clearfelling and conversion of forest to grassland) usually reduces diversity (Holloway, Kirk-Spriggs & Chey 1992) but impacts of moderate habitat disturbance, such as commercial selective logging where some semblance of forest remains, are unclear. For example, in birds, one of the best studied taxa in tropical regions, both increased and decreased diversity following disturbance have been reported. In addition, responses by one taxon appear to bear little relationship to responses by other taxa, and so the search for indicator taxa of disturbance effects has met with little success (Lawton et al. 1998; Perfecto et al. 2003). Given the rapid loss of tropical forest habitats, ecologists and conservationists urgently need to understand the factors contributing to this lack of consensus. Such understanding could in turn lead to important new insights into the general patterns of species’ responses to habitat disturbance, and so enable reliable predictions to be made of future impacts of disturbance on biodiversity.
In temperate regions, there is some consensus on methods of sampling animal taxa to allow comparison between sites and studies (e.g. standardized transect sampling for butterflies; Pollard 1977). In contrast, there is little consensus in tropical regions, and a wide range of methodologies has been used. This lack of agreement makes it difficult to deduce any general patterns of species’ responses to habitat disturbance among different tropical studies. For instance, the biases involved in different methods for censusing forest birds are well known (Bibby, Burgess & Hill 1992; Sutherland 1996): point counts are generally more efficient in mature forest, whereas mist nets are more efficient in young forest (Whitman, Hagan & Brokaw 1997; Blake & Loiselle 2001; Pagan, Thompson & Burhans 2002). Such biases may affect reported responses of birds to disturbance, but this has not been studied. In addition, there has been little consideration of how other factors such as number and position of sampling points may influence results obtained.
Describing and explaining patterns of species diversity and understanding the consequences of diversity loss are central themes in ecology (Huston 1994). One commonly observed pattern is the relationship between species richness and size of area sampled (species–area relationship; Gaston & Blackburn 2000). Thus, spatial scale is important in estimating diversity and there is increasing evidence that the perceived importance of other ecological variables and processes also depends crucially on the spatial scale at which variables are measured (Bellehumeur & Legrandre 1998; He & Gaston 2000; Rahbek & Graves 2000; Robinson, Brawn & Robinson 2000; Lennon et al. 2001). Not only are measures of diversity per se scale-dependent, but the perceived pattern of diversity change following rainforest modification may also be dependent on the spatial scale at which studies are carried out. It has been assumed in previous studies that sampling similar sized areas in different habitats is sufficient to control for any area effects when investigating impacts of disturbance, but this may be a false assumption if habitat modification affects the relationship between area sampled and number of species recorded. Previously, we suggested for Lepidoptera that studies carried out at small spatial scales were more likely to report increased diversity following forest disturbance, whereas larger-scale studies reported decreased diversity (Hamer & Hill 2000). These effects suggest that the spatial scale chosen for sampling may influence the results obtained, but it is not clear whether these scale effects are widespread or also affect other taxa. Birds are the best studied animal taxon in tropical regions, and we analytically reviewed the published literature to investigate whether or not the spatial scale at which studies were carried out has affected the recorded responses of birds to disturbance. In addition, there have been a number of new Lepidoptera studies published recently and we investigated how many of these new studies are in agreement with predictions from our previous findings. We also compared the effects of spatial scale in studies of birds and Lepidoptera.