Abstract
Despite an increasing amount of data on the effects of tropical land use on continental forest fauna and flora, it is debatable whether the choice of the indicator variables allows for a proper evaluation of the role of modified habitats in mitigating the global biodiversity crisis. While many single-taxon studies have highlighted that species with narrow geographic ranges especially suffer from habitat modification, there is no multi-taxa study available which consistently focuses on geographic range composition of the studied indicator groups. We compiled geographic range data for 180 bird, 119 butterfly, 204 tree and 219 understorey plant species sampled along a gradient of habitat modification ranging from near-primary forest through young secondary forest and agroforestry systems to annual crops in the southwestern lowlands of Cameroon. We found very similar patterns of declining species richness with increasing habitat modification between taxon-specific groups of similar geographic range categories. At the 8 km2 spatial level, estimated richness of endemic species declined in all groups by 21% (birds) to 91% (trees) from forests to annual crops, while estimated richness of widespread species increased by +101% (trees) to +275% (understorey plants), or remained stable (- 2%, butterflies). Even traditional agroforestry systems lost estimated endemic species richness by - 18% (birds) to - 90% (understorey plants). Endemic species richness of one taxon explained between 37% and 57% of others (positive correlations) and taxon-specific richness in widespread species explained up to 76% of variation in richness of endemic species (negative correlations). The key implication of this study is that the range size aspect is fundamental in assessments of conservation value via species inventory data from modified habitats. The study also suggests that even ecologically friendly agricultural matrices may be of much lower value for tropical conservation than indicated by mere biodiversity value.
Highlights
Since the seminal paper by Lawton et al [1], numerous studies have dealt with biodiversity patterns of tropical land use gradients and analysed indicator properties of different taxa [2,3,4,5]
Point level estimates of widespread bird species richness showed an opposite trend in having lowest species richness in near-primary forest (NF) (19.764.2) and secondary forest (SF) (15.761.8), but increasing by 88% to annual crops (AC) (37.167.5), and having intermediate richness in agroforestry systems (AF) (27.965.3)(F (3;20) = 20.48; p = 0.000)
Estimates of widespread bird species richness did not differ significantly between NF and SF (Tukey’s Honest Significant Difference test, p.0.05). This pattern was similar at habitat level, where the increase from NF (30.763.8) to AC (66.565.6) amounted to 117%, and 48% to AF (45.362.8) (Figure 1b)
Summary
Since the seminal paper by Lawton et al [1], numerous studies have dealt with biodiversity patterns of tropical land use gradients and analysed indicator properties of different taxa [2,3,4,5]. Looking more closely into these studies, it appears that patterns of alpha (point) diversity can be highly taxon-specific but that a general pattern of high beta turnover across habitats is visible in all taxa: most altered habitats usually contain most altered biotic communities which may - or may not - be as diverse as primary forest but in any case composed of different species [6] Such inventory-based studies are being published and referred to in conservation journals, they more often allow conclusions rather on the ‘biodiversity value’ of modified landscapes rather than on their ‘conservation value’ [7]. The taxa were chosen because they are frequently used indicators given the relatively moderate skills and sampling efforts needed for assessment [1,2,3,4], and because their geographic ranges are relatively well established and accessible [19–25 and Appendix S1]
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