In this chapter we have explored the relationships between biological diversity and ecosystem functioning for moist forest, highlighting the shortage of information that is needed to assess biodiversity-ecosystem functional relationships. The shortage is even more dramatic for other types of tropical forests, which have not received the attention that has been directed toward wet forests. Dry forests, montane forests and wetlands are important types of tropical ecosystems. Because of their extension, the magnitude of anthropogenic alteration they are experiencing, and their crucial contribution to tropical biological diversity, losses of biodiversity in them may influence the functional properties of all types of tropical ecosystems. These ecosystem types require consideration if we are to document the range of variation of the relationships between biological diversity and ecosystem functioning. Interestingly, changes in forest structure and functioning are similar along gradients of temperature, fertility and moisture in spite of the dramatically different changes associated with each one. Thus, decreases in productivity, standing biomass, decomposition rates, life form diversity and species richness, and increases in below-ground allocation of plant resources accompany all three gradients. Determining the reasons for similar responses to different environmental conditions is a challenge for future tropical forest research.
Ecologists traditionally estimate primary production as the total amount of carbon fixed per unit area. Although such aggregated estimates are useful for some purposes, these data have a number of significant limitations. First, lumped values seriously underestimate above-ground primary production by ignoring carbon allocated to nectar, flowers and fruits - key resources for a range of influential consumer groups - and allocation downward into mycorrhizae. Second, lumping primary production tells us nothing about how carbon is allocated among different plant parts (roots, wood, leaves, nectar, flowers and fruits), and essentially treats primary producers as a single functional group. If we want to assess the contribution of different species, functional groups and life forms to ecosystem-level processes, we need more disaggregated estimates of primary production. Plant materials are packaged in fundamentally different ways, and these differences determine the identity of consumer groups and the rates of consumption and energy flow in ecosystems.
One of the most important areas of research on relationships between bio diversity and ecosystem functioning is determining the influence of the species composition of leaf litter on rates of decomposition and subsequent mineralization. Limited data suggest that decomposition rates can be mediated by litter diversity, with rates being higher for litter from richer species assemblages than from poorer species assemblages (Burghouts el al. 1994). Further testing of this relationship in tropical systems will shed light on the relationship between species richness and pivotal ecosystem processes such as decomposition.
Remarkably little is known about below-ground plant and microbial processes. Remedying this situation needs to be a major priority if we are to assess rigorously the links between biodiversity and ecosystem-level processes. Three areas are in particular need of attention. First, are there predictable structural patterns in the root systems of multispecies tropical assemblages? Second, how species-rich is the soil microbiota and into how many functional groups do those species fall? Third, when tropical forests are perturbed such that species or groups of species are deleted, do compensatory responses by other taxa result in reoccupation of the space?
Manipulative experiments need to be supplemented by research efforts focussed on comparative studies that contrast (a) naturally occurring monospecific or low-diversity forests with neighboring high-diversity forests, (b) human-damaged systems with neighboring undisturbed systems, and (c) forest ecosystems along gradients of precipitation, soil fertility and elevation, Only with such studies can a comprehensive picture of the importance of biodiversity for ecosystem functioning be developed.
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