How does spatial heterogeneity influence resilience to climatic changes? Ecological dynamics in southeast Madagascar

Abstract

Conceptual models suggest a link between spatial heterogeneity, diversity, and resilience, but few empirical studies exist to demonstrate such an ecological relationship. In this study, we investigated the nature of spatial heterogeneity and resilience of two forest fragments from Madagascar's highly endangered littoral forest, and two nearby sites in the surrounding ericoid grassland. This ecosystem has been subjected to a number of large environmental disturbances over the last 6000 years, including a late Holocene sea-level rise of 1-3 m above the present level, pronounced drought events, and natural and anthropogenic fires. The aims were to determine the driving mechanisms for heterogeneity and to compare the impact of large environmental disturbances among the four sites.Overall, our results indicate that, contrary to previous assumptions of continuous forest cover, the ecosystem was already spatially heterogeneous prior to the arrival of humans. Differences in groundwater, nutrients, and fires maintained a natural mosaic of forest and open vegetation giving rise to high floristic diversity. The four sites differed significantly in their resilience to climatic disturbances, as measured by compositional turnover and by the timing and extent of biotic recovery. The highly diverse littoral forest reverted more quickly to forest than the surrounding vegetation, which was composed of open Uapaca woodland (a noanalogue community) prior to the formation of the ericoid grassland. Further, species and populations from sites with low nutrients and nitrogen-fixing species as dominants were able to persist despite climatic perturbations, whereas there was higher turnover in the other sites. Overall, minimal extinction occurred in the ecosystem despite widespread landscape transformations. It is suggested that spatial heterogeneity and diversity in southeastern Madagascar may have enhanced species survival during climatic perturbations. Fossil extinction among abundant taxa .1% of total sum (date last observed in fossil record) Fabaceae type (950 cal yr BP), Solanaceae type 2 (850 cal yr BP) Uncaria (1150 cal yr BP), Solanaceae type 2 (1026 cal yr BP), Solanaceae type 3 (976 cal yr BP), Symphonia (950 cal yr BP) Note: SD is standard deviation. Fabaceae type (966 cal yr BP), Euphorbiaceae type 110 (635 cal yr BP), Solanaceae type 2 (403 cal yr BP) none 5 hhttp://bcal.shef.ac.uk/i

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