Contrasting taxonomic and phylogenetic diversity responses to forest modifications: comparisons of taxa and successive plant life stages in South African scarp forest.

Ingo Grass,Roland Brandl,Alexandra Botzat,Nina Farwig,Eike Lena Neuschulz,M Alex Smith

doi:10.1371/journal.pone.0118722

Abstract

The degradation of natural forests to modified forests threatens subtropical and tropical biodiversity worldwide. Yet, species responses to forest modification vary considerably. Furthermore, effects of forest modification can differ, whether with respect to diversity components (taxonomic or phylogenetic) or to local (α-diversity) and regional (β-diversity) spatial scales. This real-world complexity has so far hampered our understanding of subtropical and tropical biodiversity patterns in human-modified forest landscapes. In a subtropical South African forest landscape, we studied the responses of three successive plant life stages (adult trees, saplings, seedlings) and of birds to five different types of forest modification distinguished by the degree of within-forest disturbance and forest loss. Responses of the two taxa differed markedly. Thus, the taxonomic α-diversity of birds was negatively correlated with the diversity of all plant life stages and, contrary to plant diversity, increased with forest disturbance. Conversely, forest disturbance reduced the phylogenetic α-diversity of all plant life stages but not that of birds. Forest loss neither affected taxonomic nor phylogenetic diversity of any taxon. On the regional scale, taxonomic but not phylogenetic β-diversity of both taxa was well predicted by variation in forest disturbance and forest loss. In contrast to adult trees, the phylogenetic diversity of saplings and seedlings showed signs of contemporary environmental filtering. In conclusion, forest modification in this subtropical landscape strongly shaped both local and regional biodiversity but with contrasting outcomes. Phylogenetic diversity of plants may be more threatened than that of mobile species such as birds. The reduced phylogenetic diversity of saplings and seedlings suggests losses in biodiversity that are not visible in adult trees, potentially indicating time-lags and contemporary shifts in forest regeneration. The different responses of taxonomic and phylogenetic diversity to forest modifications imply that biodiversity conservation in this subtropical landscape requires the preservation of natural and modified forests.

Highlights

Subtropical and tropical forests are hotspots of global biodiversity [1]
There were no differences in α-TD among the successive plant life stages, i.e., adult trees, saplings, and seedlings
Increasing forest disturbance led to a significant loss in α-phylogenetic diversity (PD) across all plant life stages but had no effect on the α-PD of birds (Table 1b, Fig. 3)

Summary

Introduction

Subtropical and tropical forests are hotspots of global biodiversity [1]. At the same time, they are among the most threatened ecosystems worldwide, with the ongoing intensification of human land-use posing a high risk [2]. Subtropical and tropical forests are increasingly found only as human-modified forest landscapes [3,4]. This conversion from primary into modified forest has profound effects on the composition of forest communities [4,5]. The assumption that some species are robust towards forest modification becomes problematic when negative effects are masked by a time-lag between habitat modification and species extinction, resulting in an ‘extinction debt’ [9,10]. Negative effects of forest modification are typically more readily visible in mobile species such as birds than in sessile species such as plants [11,13]

Methods

Results

Conclusion