Processes of community assembly in an environmentally heterogeneous, high biodiversity region

Background/Question/Methods

Applying functional trait analysis approaches to multi-year multi-site biodiversity inventory data can provide deeper insights into the processes affecting community assembly. We examined associations between community composition, species functional traits, and environmental conditions for plant communities in the Baviaanskloof Mega-reserve, a protected area within South Africa’s Cape Floristic Region (CFR). Our study area is ~2500 km² and has extreme gradients in climatic conditions, is heterogeneous for other environmental conditions (e.g., edaphic features, topographic variation, fire history, etc.), and includes vegetation from three major biome types within the CFR, resulting in both high alpha and beta diversity. Given these characteristics, the Baviaanskloof was an ideal location to examine trait by environment relationships and to assess the role of environmental filtering on plant community composition. We carried out vegetation surveys in this area in 1992 and 2011. We also measured functional traits for greater then 350 species in 2011, and integrated these data with the vegetatin survey data, using community aggregated trait – redundancy analysis and non-parametric randomization methods to examine trait by environment relationships, and determine if environmental filtering plays a major role in structuring these communities.  

Results/Conclusions

Examining each year’s observations separately, we found functional trait values were weakly associated with environmental gradients with the exception of time since last fire, suggesting that environmental filtering may play a limited role in structuring community composition. However, we also found that a cluster analysis of functional trait dissimilarity among plots adequately discriminated plots into their major vegetation types, whereas species composition differences did not. This suggests that the mechanisms causing biome boundaries may also lead to filtering of functional traits. Further, examination of changes in community aggregated functional traits through time shows that communities are functionally more similar than predicted by null models, demonstrating that some filtering processes are in effect; though species composition appears to be governed by lottery-type dynamics within major vegetation types. Ultimately, while community-structuring processes appear to be weak within vegetation types, there is evidence for clear functional distinctions between vegetation types, most likely driven by fire dynamics.