Species richness, hotspots, and the scale dependence of range maps in ecology and conservation

Abstract

Most studies examining continental-to-global patterns of species richness rely on the overlaying of extent-of-occurrence range maps. Because a species does not occur at all locations within its geographic range, range-map-derived data represent actual distributional patterns only at some relatively coarse and undefined resolution. With the increasing availability of high-resolution climate and land-cover data, broad-scale studies are increasingly likely to estimate richness at high resolutions. Because of the scale dependence of most ecological phenomena, a significant mismatch between the presumed and actual scale of ecological data may arise. This may affect conclusions regarding basic drivers of diversity and may lead to errors in the identification of diversity hotspots. Here, we examine avian range maps of 834 bird species in conjunction with geographically extensive survey data sets on two continents to determine the spatial resolutions at which range-map data actually characterize species occurrences and patterns of species richness. At resolutions less than 2°(Ϸ200 km), range maps overestimate the area of occupancy of individual species and mischaracterize spatial patterns of species richness, resulting in up to two-thirds of biodiversity hotspots being misidentified. The scale dependence of range-map accuracy poses clear limitations on broad-scale ecological analyses and conservation assessments. We suggest that range-map data contain less information than is generally assumed and provide guidance about the appropriate scale of their use.biodiversity ͉ birds ͉ geographic range ͉ range occupancy ͉ species richness pattern A growing number of studies is concerned with continentalto-global scale patterns of biodiversity (1-4) or summary patterns of species' ecological traits such as body size (5) and range size (6, 7). At similarly broad scales, many conservation studies are examining the richness of rare or threatened taxa (8-11) and their distribution in relation to existing reserve networks (12)(13)(14). Over such broad geographical extents, expertdrawn range maps (extent of occurrence maps) are typically the primary source of data on species distributions, and thus taxon richness is estimated by counting the number of species ranges that overlap each cell of an underlying grid system. Of course, it has been appreciated for decades that species do not occur at all locations throughout their range (7), and recent work (15)(16)(17) has shown that discrepancies may exist between richness patterns inferred from range maps and those inferred from survey data. This mismatch has been attributed to an inherent difference in the spatial scale at which range maps and surveys capture distributional information (15). However, although the spatial scale of surveys is typically explicit in the sampling methodology (e.g., 0.1-hectare Gentry forest plots, 40-km North American Breeding Bird Survey routes, and 0.25°ϫ 0.25°South African Bird Atlas cells), the actual scale at which range maps characterize the distribution ...

A global analysis of the impacts of urbanization on bird and plant diversity reveals key anthropogenic drivers

“…Range maps were rasterized using a cylindrical equal-area projection and a cell area of 3091 km 2 . Because range maps provide representations of species' extent of occurrence at coarse resolutions with little or no consideration of changes in occupancy owing to land-use change [26], they are ideally suited to estimate nonurban density of bird species within larger areas such as cities. Non-urban density of plant species was estimated using the cokriging plant richness model from Kreft & Jetz [25].…”

When do conservation planning methods deliver? Quantifying the consequences of uncertainty

“…This was intended to represent both erroneous overestimation (Hurlbert and Jetz, 2007) and the case where it is easier and cheaper to identify potential habitat (for example by modelling and/or remote sensing) than habitat which is actually occupied. This is particularly important for species that are poor dispersers and in situations where there are large barriers to dispersal, such as roads in peri-urban settings.…”

“…The second assumed that the Apparent map overestimated the number of patches in the Correct map by 50%. The value of 50% was chosen to produce a demonstrable effect, although the overestimation of potential habitat may be larger in some situations (see for example, Hurlbert and Jetz (2007)). The third model assumed the carrying capacity of each patch was 50% lower than shown on the Apparent maps.…”

Global meta-analysis reveals low consistency of biodiversity congruence relationships

“…Yet information on the number and spatial distribution of species is lacking for the majority of taxa on earth 4 . Although broad-scale patterns of biodiversity are well documented 5 , accurate descriptions of the distribution of biodiversity break down at fine spatial, temporal or taxonomic scales, even for well-described groups such as vascular plants 6 or vertebrates 7 . Further, ongoing efforts to resolve the biases and deficiencies of taxonomic data sets are unlikely to occur quickly enough to allow improved conservation decision making 8 .…”