BIOCENTRIC (Naturally sustainable cycles) Ecosystem Processes

The traits and diversity or organisms and their interactions in communities strongly affect ecosystem processes. This chapter describes the patterns of community effects on ecosystem processes. Species traits interact with the physical environment to govern ecosystem processes

Predators, such as planktivorous fish, that feed on the same prey also constitute a functional type or feeding guild. However, no two species or individuals within a functional type are ecologically identical, so, as our understanding improves or our questions become more refined, we expect to recognize situations in which species diversity within functional types or genetic diversity within species has detectable ecosystem consequences.
Natural ecosystems are currently experiencing major changes in species diversity and the traits of dominant species. Earth is currently in the midst of the sixth major extinction event in the history of life (Pimm ci al. 1995). Although the causes of earlier extinction events (e.g., the extinction of dinosaurs) are uncertain, they probably resulted from sudden changes in physical environment caused by factors such as asteroid impacts or pulses of volcanism. Current extinction rates are 100- to 1000-fold higher than prehuman extinction rates and could rise to 10,000-fold, if species that are currently threatened become extinct

The current extinction event is unique in the history of life in that it is biotically driven, specifically by the effect of the human species on land use, species invasions, and atmospheric and environmental change.

Although human impacts affect many processes at global scales (Vitousek 1994a), the loss of species diversity is of particular concern because it is irreversible. For this reason, it is critical to understand the functional consequences of the current large losses in species diversity (Chapm et al. 2000b). Although global extinction of a species is a major conservation concern, localized extinctions or large changes in abundances happen more frequently and have the largest effects on functioning of ecosystems.
A second biotic change of global proportions is the frequent introduction of exotic species into ecosystems. People have intentionally and inadvertently moved thousands of species around the globe, leading toward a homogenization of the global biota. Exotic species often change the physical and biotic environment enough to alter the dominance or eliminate native species from an ecosystem. Although extinction and immigration of species are natural ecological processes, the dramatic increase in frequency of these events in recent decades is rapidly changing the types and numbers of species throughout the globe. There are many ethical, aesthetic, and economic reasons for concern about changes in biodiversity. The changes are occurring so quickly, however, that we must assess their functional consequences for population, community, and ecosystem processes. Changes in species composition often have a greater effect on ecosystem processes than do the direct impacts of global changes in atmospheric composition and climate. Understanding the nature of biotic impacts on ecosystem processes is therefore critical to predictions of future changes in ecosystems. In this chapter we focus on the ecosystem consequences of changes in communities and the associated changes in species traits.
The number, relative abundance, identity, and interactions of species all affect ecosystem processes. Species in a given trophic level almost always differ in some traits that affect ecosystem processes. Sun and shade species, for example, differ in the conditions under which they contribute to carbon inputs; presence of both types of species therefore increases the efficiency with which light is converted to net primary production (NPP) Nitrogen-fixing cyanobacteria differ from non- fixing phytoplankton in their impacts on nitrogen cycling; presence of both types of species therefore influences the response of nitrogen cycling to variations in phosphorus inputs to lakes. No single species can perform all of the functional roles that are exhibited by a trophic level. A diversity of species is functionally important because it increases the range of organismic traits that are represented in an ecosystem and therefore the range of conditions under which ecosystem properties can be sustained. From an ecosystem perspective, species diversity is simply a summary variable that describes the total range of biological attributes of all the species in the ecosystem. The functional consequences of a change in diversity depend on the number of species present (species richness), their relative abundances (species evenness), the identity of species that are present (species composition), the interactions among species, and the temporal and spatial variation in these properties. Each of these components of diversity affects the functioning of ecosystems.