Virah-Sawmy Et Al. 2009. Madagascar CC & Spatial Heterogeneity.Pdf
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Ecological Monographs, 79(4), 2009, pp. 557–574 Ó 2009 by the Ecological Society of America How does spatial heterogeneity influence resilience to climatic changes? Ecological dynamics in southeast Madagascar 1,4 2 1,3 MALIKA VIRAH-SAWMY, LINDSEY GILLSON, AND KATHERINE JANE WILLIS 1Oxford Long-term Ecology Laboratory, Oxford University Centre for the Environment, South Parks Road, Oxford OX1 3QY United Kingdom 2Plant Conservation Unit, University of Cape Town, Rondebosch 7701, South Africa 3Department of Biology, University of Bergen, N-5007, Bergen, Norway 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 no- analogue 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. Key words: alternative stable states; drought; edaphic; extinction; heterogeneity; Madagascar; no- analogue; nutrient; recovery; refugia; turnover; water. INTRODUCTION heterogeneous landscapes are less likely to go regionally A number of ecological models have suggested that a extinct. diverse and heterogeneous ecosystem is more ecologi- Diversity may also contribute to resilience. It is cally resilient to environmental stress as it has an predicted that the large regional species pool in increased ability to absorb disturbance without dramatic heterogeneous sites may maintain the assembly and change to ecosystem properties and processes. Also, functioning of an ecosystem by increasing the probabil- such ecosystems may recover more quickly (Holling ity of recruiting key species, which will maintain 1973, Gunderson 2000). For example, it is predicted that ecological processes during periods of environmental spatial heterogeneity will enhance species survival stresses (Tilman 1999, Loreau et al. 2001). This during rapid climatic changes (Thuiller et al. 2005). ‘‘insurance’’ hypothesis also postulates that even if high One explanatory model suggests that heterogeneous diversity is not critical for maintaining ecosystem landscapes, where species are generally patchily distrib- processes under benign environmental conditions, nev- uted with sub- or metapopulations, are more likely to ertheless, during periods of environmental fluctuations, experience different microenvironmental conditions to high diversity provides insurance or a buffer because the the same climatic perturbations and therefore fluctuate large number of species present responds differentially asynchronously (Yachi and Loreau 1999, Loreau et al. (Yachi and Loreau 1999, Loreau et al. 2001, 2003). 2003). As a consequence, species with subpopulations in Both spatial heterogeneity and diversity, then, have the potential to increase ecological and engineering Manuscript received 14 July 2008; revised 26 November resilience. These theoretical models have, however, 2008; accepted 17 February 2009. Corresponding Editor: C. C. Labandeira. rarely been tested in real ecosystems. One of the 4 E-mail: [email protected] difficulties of studying heterogeneity and diversity is 557 558 MALIKA VIRAH-SAWMY ET AL. Ecological Monographs Vol. 79, No. 4 that they are scale dependent; therefore, data are needed Previous work on the littoral forest has hypothesized both in time and space. Paleoecological studies in that the fragmented nature of this ecosystem is as a multiple sites provide the means of testing ideas about result of previous anthropogenic activity (Ganzhorn et heterogeneity by being both spatially constrained and al. 2001, de Gouvenain and Silander 2003, Consiglio et temporally explicit. Further, resilience of ecosystems to al. 2006). It also is hypothesized that the littoral forest perturbations can also be measured in the fossil records would have been contiguous with the lowland evergreen by measuring the time required for a system to return to forest that occupies the adjacent eastern mountain an equilibrium following a disturbance (defined as slopes (de Gouvenain and Silander 2003) and would engineering resilience; e.g., Holling 1973, Gunderson have been continuous along the whole of the eastern 2000). Compositional turnover in the paleoecological coast, of which 90% has been lost and transformed into record can indicate the amount of disturbance that a an ericoid grassland (Consiglio et al. 2006). system can absorb without a drastic change in its Alternative scenarios for change are also possible. internal properties (defined as ecological resilience; e.g., Recently, it has been shown that the littoral forest of Holling 1973, Gunderson 2000). Mandena has been subjected to intense environmental In this study, we investigated the nature of spatial fluctuation, which included late Holocene highest sea- heterogeneity and resilience in Madagascar’s smallest level rise of 2–3 m above the present level between 1800 and most endangered ecosystem, the littoral forest and 800 cal yr BP (calibrated years before present; (Dumetz 1999, Ganzhorn et al. 2001, Cadotte et al. Battistini et al. 1976, Camoin et al. 1997, 2004), four 2002, de Gouvenain and Silander 2003). The fragmented pronounced drought events which occurred around littoral forest, interspersed by sharp boundaries of 5800, 4500, 3200, and 950 cal yr BP, and episodes of ericoid grassland, provided an ideal landscape system local and regional fires (Virah-Sawmy et al. 2009). to test ideas of the relation between resilience, hetero- In this study, we focused on the littoral forest of geneity, and diversity. Biotic responses to environmental Mandena and Ste-Luce along the southeast coast. The two littoral forest fragments share many endemic plant stresses that included climatic desiccation, high sea-level species and are dominated by Asteropeia (Asteropeia- rise, and fire episodes were contrasted among the four ceae), Bembicia and Homalium (Flacourtiaceae), Cyno- sites. The vegetation in the four sites is: a short-canopy metra (Fabaceae), Syzygium (Myrtaceae), Pandanus littoral forest (M15), tall-canopy littoral forest (S9), and (Pandanaceae), Tripselium (Moraceae), and Uapaca their matrix vegetation (defined as the vegetation types (Euphorbiaceae), but there are important differences in that embed the forest fragments), and ericoid grassland terms of structure and composition. For example, the (MM and SM) (Fig. 1; Table 1). Specifically, we tall canopy littoral forest (S9) has different emergent addressed the following questions: (1) How do the four trees from the short-canopy littoral forest (M15). They sites compare in functional groups and vegetation also differ in the range and types of local endemics composition through time and what factors (edaphic, present. These floristic differences are attributed to climatic, fire, human impacts) contribute to spatial and physiographic differences, including a rainfall gradient biotic heterogeneity? (2) How did the four sites respond running from north to south, a marine influence to environmental fluctuations (sea-level rise, aridity, and attributable to littoral forest located at varying distances fire) and what are the interactions and feedback loops from the coast, and to edaphic variations in nutrients among biotic and abiotic components? (3) How does (Dumetz 1999). resilience (using compositional turnover and time The ericoid grassland of Mandena and Ste-Luce also required to return to former diversity and composition are similar to each other and composed of Erica as determinants) differ at the local scale? (4) Is there a (Ericaceae), Myrica (Morella) (Myricaceae), Helichry- link between heterogeneity, resilience, and persistence? sum spp. and Veronica nudicaulis (Asteraceae), and the dominant grass, Aristidia similis. Again, there are some STUDY SITE important floristic differences between the two sites. The Madagascar’s littoral forest has been identified ericoid grassland in Mandena is homogenously