Evidence that specialists are special

Abstract

Habitat fragmentation is recognized as a major threat to biological diversity. One of the most simple, but ingenious, ways of applying ecological theory to conservation problems linked to fragmentation is the use of species-area curves. The classic species-area relation is S5cAz, where S is the number of species in a spatial area A, and c and z are constants. The constant z indicates how sensitive species richness is to marginal changes in habitat area, and is useful for scale-independent comparisons of species-area curves. Alternatively, one may be interested in assessing conservation status by how population densities change with habitat area, so-called ‘individual-area’ relationships. The idea here is that populations of only a small number of target species are quantified, enabling species-specific conservation measures. Steffan-Dewenter and Tscharntke 1xButterfly community structure in fragmented habitats. Steffan-Dewenter, I. and Tscharntke, T. Ecol. Lett. 2000; 3: 449–456CrossrefSee all References1 have now added a new dimension to this problem, showing that life-history traits may be important mediators in developing conservation priorities based on species- and individual-area relationships.The authors studied 61 butterfly species on 33 calcareous grasslands ranging from 300 to 76 000 m2. They recorded numerous habitat, population and species variables, which permitted them to see how butterfly feeding niches were associated with individual-area and species-area curves. Each butterfly species was categorized into one of four feeding niches: polyphagous, oligophagous, strongly oligophagous, and monophagous. Steffan-Dewenter and Tscharntke found that although total butterfly density decreases with habitat area, the density of a group of four monophagous species significantly increases. This would appear to be the first study to show that opposing responses of density to habitat area may be linked to resource specialization.Steffan-Dewenter and Tscharntke additionally discovered that when z-values were calculated for each level of food plant specialization, there was an increasing relationship between the rank level of specialization and z-values. What this means is that monophagous species richness is likely to be more sensitive to habitat fragmentation than are more generalist species in this community. Moreover, in accord with theory, these authors show that specialists on higher trophic levels (i.e. monophagous and strongly oliphagous butterflies) are more sensitive to fragmentation than are species at lower trophic levels (the plant community).This study should inspire both community ecologist and conservation biologist alike. Both the species number and population density approaches indicate that resource specialists in this butterfly community are more sensitive to decreases in habitat area than are generalists. Because specialists and not simply ‘total diversity’ can often be the target of conservation, the study by Steffan-Dewenter and Tscharntke is unique in showing how species biology can be used to narrow-down conservation options.