Biodiversity and Ecosystem Processes in Tropical Ecosystems

Gordon H. Orians 1 

 

Received 19-I-2000. Accepted 24-I-2000.




Living organisms on Earth are being subjected to major disruptions in the form of massive exchanges of species -pathogens, predators, and competitors- among biogeographic regions, overharvesting, habitat destruction, pollution, and climate change. Rates of habitat conversions, especially of forests, are higher in tropical regions than elsewhere on Earth (FAO 1993, Skole and Tucker 1993, Whitmore 1997). Until recently, biologists have directed most of their efforts toward estimating the extent and causes of species loss and developing ways to reduce the rate of extinctions. However, during the past decade much attention been paid to the consequences of the loss of species for the functioning of ecological processes (Chapin et al. 1998, Loreau 1998, Naeem 1996, 1997, Schulze and Mooney 1993, Tilman, 1997, Tilman and Downing 1994, Yachi & Loreau 1999). Most of this research has been conducted in temperate ecosystems or in the laboratory.

To assess the state of knowledge of the role of biodiversity in tropical forests in maintaining ecosystem processes, such as primary and secondary productivity, materials processing, provision and maintenance of structure, resistance to invasions and recovery from disturbances, a workshop was held at Oaxtepec, Morelos, Mexico, December 4-7,1993 (Orians, Dirzo, and Cushman 1996).  Here I summarize research conducted since the workshop and suggest some areas where research is urgently needed.

Tropical ecosystems are characterized by both high richness of species in many taxa and complex biotic interactions among component species. Most tropical plants are animal pollinated, are fed on by a wide variety of animals, and depend on animals for dispersal of their seeds. Tropical ecosystems exist in climates that vary in mean annual temperature, total annual rainfall and the length and severity of dry seasons, but have relatively minor seasonal changes in temperature. Seasonality of rainfall exerts a strong influence on temporal patterns of primary and secondary production and decomposition. Species richness in most taxa of macroorganisms is positively correlated with mean annual rainfall (Clinebell et al. 1995, Gentry 1992, Primack 1993) and inversely correlated with the length of the dry season. Both variables are strongly correlated in most tropical regions.

In this overview I focus on three central questions. Is productivity of tropical ecosystems positively correlated with biodiversity?  Are more species-rich ecosystems more resistant to and better able to recover from disturbances?  Does biodiversity enhance its own maintenance?

Biodiversity and Ecosytem Productivity: Four non-mutually exclusive hypotheses have been proposed to account for a positive relationship between biodiversity and productivity. The "sampling effect hypothesis" suggests that species-rich plant communities are more productive because they have a higher probability of containing and being dominated by one or a few highly productive species. The "niche complementarity hypothesis" proposes that ecological differences among species result in more complete utilization of resources, and, hence, higher productivity, in more species-rich communities. The "positive mutualist interactions hypothesis" suggests that richer communities have more positive mutualistic interactions among component species than more depauperate communities. The "statistical effects" hypothesis suggests that stability and productivity increase with species richness because of the statistical averaging of fluctuations in the abundances of individual species (Doak et al. 1998).where 


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