Principles of terrestrial ecosystem ecology.pdf

12
Community Effects on
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.
Introduction
Species traits interact with the physical environment
to govern ecosystem processes. Up to
this point, we have emphasized only the most
general properties of organisms. We discussed
primary producers, for example, as if they were
a homogeneous group of organisms in an
ecosystem and indicated that primary production
can be broadly predicted from climate and
parent material. Species differences in traits
such as photosynthesis, root allocation, and
litter quality, however, strongly affect the functioning
of terrestrial ecosystems. Similarly, the
phosphorus requirements and prey size preferences
of zooplankton govern patterns of nutrient
cycling in lakes. Under what circumstances
must we know the traits of individual organisms
within a trophic group to understand ecosystem
processes? In this chapter, we explore this question
through discussion of functional types—
that is, groups of species that are ecologically
similar in their effects on ecosystem processes
(Chapin 1993a, Smith et al. 1997). Termites,
homeotherm herbivores, nitrifying bacteria,
and evergreen shrubs are examples of functional
types that have predictable general
effects on 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 et 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 (Fig. 12.1).
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
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