VI.2 Biodiversity, Ecosystem Functioning, and Ecosystem Services
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VI.2 Biodiversity, Ecosystem Functioning, and Ecosystem Services Shahid Naeem OUTLINE 1. Biodiversity and the determinants of ecological fate 2. Biodiversity and ecosystem function 3. The implications for ecosystem services 4. Conclusions The biological activities of plants, animals, and microorganisms influence the chemical and physical processes of their surroundings, and if one were to modify the distribution and abundance of these organisms, ecosystem functioning, or biogeochemical activity, would change. For example, trees in a forest sequester atmospheric carbon dioxide and locally enhance evaporation; invertebrates in a marine ecosystem mix and aerate sediments; and microorganisms in an aquatic ecosystem decompose organic matter. Reduce the number or mass of these organisms, and ecosystem functions, such as primary production in the forest, the rates of sediment aerationinthe marine ecosystem, and ratesofdecomposition inthe aquatic ecosystem,are likelytobealtered.If ecosystem functions are altered, then it stands to reason that ecosystem services,whichare ecosystemfunctionsthatbenefit humans, are also likely to be altered. Suppose, however, rather than reducing the number or mass of organisms, we reduced only their diversity—would ecosystem functioning and services be affected? The answer is ‘‘yes.’’ To see why, this chapter considers the fundamental relationship between biological processes and ecosystem functioning, the evidence for and mechanisms by which biodiversity influences this relationship , and how ecosystem services are likely to be affected by changes in biodiversity. GLOSSARY biodiversity. The genetic, taxonomic, and functional diversity of life on Earth including temporal and spatial variability. biogeochemistry. Geochemical processes influenced by biological processes. complementarity. Two or more species using the same resources in different ways. Earth system. Global-scale biogeochemical processes. ecosystem function, functioning, or process. Biogeochemical activities of ecosystems. The most common metric of ecosystem functioning is primary production , but other metrics include decomposition, nutrient mineralization, community or ecosystem respiration , or other measures of energy flow and nutrient cycling. Note that ‘‘function’’ refers to activity, not purpose. Compare with ecosystem property or ecosystem service. ecosystem property. A measure of the state (e.g., species richness or standing biomass) or dynamic properties (e.g., resilience, resistance, robustness, reliability, predictability, or susceptibility to invasion) of an ecosystem. Compare with ecosystem function. sampling effect. See selection effects. selection effects. When the relationship between biodiversity and ecosystem functioning is significantly above or below zero, there are several possible reasons for such an effect. The simplest is that increasingly diverse ecosystems have greater probabilities of including species that have disproportionately positive or negative effects on ecosystem functioning. The former causes a positive relationship between biodiversity and ecosystem functioning and is known as a positive selection or sampling effect. The latter causes a negative relationship between biodiversity and ecosystem functioning and is known as a negative selection effect. A positive relationship can also be attributable to increasing amounts of complementarity or facilitation in a community as diversity increases. Increasing complementarity and facilitation can increase the efficiency of resource use in an ecosystem and therefore increase the amount of ecosystem functioning that occurs for a given unit of resource (e.g., light, water, or space). Loureau and Hector (2001) have developed analytical means for separating these co-occuring effects in studies of the relationship between biodiversity and ecosystem function. 1. BIODIVERSITY AND THE DETERMINANTS OF ECOLOGICAL FATE Ecosystem in Microcosm If one takes a bottle, fills it halfway with water, adds a rich variety of inorganic nutrients and trace metals, then seals it, sterilizes it, places it in sunlight, and then watches closely . . . for an eternity, almost nothing would happen. If, however, one were to take an identical bottle and add a single photosynthetic, nitrogen-fixing cyanobacterium , like a single cell of the heterocyst blue-green algae one finds growing in rice paddies, the bottle’s environment will be utterly transformed in just a few days. With the addition of just one species, the previously clear water would become a cloudy organic soup, the bottle would warm as the dark liquid absorbed light, and the air in the bottle, or its headspace, would be completely altered in its chemical composition. There are three possible fates for this ecosystem in microcosm. First, it could attain a life-sustaining equilibrium, and the cyanobacteria would persist inde finitely. Second, it could oscillate (predictably or chaotically) between harsh and equitable conditions. For example, the cyanobaceria could go through population swings between high and low densities, and the chemical states of the bottle could fluctuate between low and high levels of acidity. Third, the microcosm could collapse to sterility. For example, if acidity...