Biodiversity and Ecosystem Functioning - Ecological Society of ...

Issues in Ecology Number 4 Fall 1999
A third category of value, ecosystem services, is the
focus of this report. The organisms that live, grow,
reproduce, and interact within ecosystems help to mediate
local and regional flows of energy and materials (Figure
4). Energy flow refers to the capture of light energy by
green plant or algal photosynthesis and its dispersal as
chemical energy throughout the food web to plant- or
algal-feeding animals, predators, and eventually
decomposers. The flow of materials involves the recycling
of carbon, nitrogen, phosphorus and other elements
between living organisms and the air, water, and soil.
These biologically mediated energy and materials flows
contribute to many ecological or life support services that
benefit human welfare such as greenhouse gas regulation,
water treatment, erosion control, soil quality control, and
plant growth. Ecosystem services can also include cultural
benefits, such as religious, aesthetic, recreational, or
inspirational values that humans derive from ecosystems.
Determining whether biodiversity per se is important
to ecosystem functioning has been difficult, partly because
many of the factors such as habitat conversion that reduce
local biodiversity also directly affect many ecological
processes, masking the more subtle impacts of species
loss on functioning. Recent studies, however, have begun
4
Figure 2 - Biodiversity loss and the
earth’s changing biomes. The top map
shows the earth’s major biomes, such
as graslands in orange and forests in
blue, prior to the introduction of agriculture.
The bottom map shows the
spread of agronomic and other managed
ecosystems where red represents
a region that is greater than 50% managed
ecosystems. (After Sisk et al.
1994, BioScience)
to shed considerable light on the issue. These studies have
shown that ecosystems are indeed sensitive to changes in
the numbers and kinds of species found in their communities.
In this report, we provide an overview of ecosystem
functioning, review the distinction between taxonomic
biodiversity (i.e., species numbers) and functional biodiversity,
and evaluate the current status of research concerning
ecosystem responses to changes in biodiversity.
ECOSYSTEM FUNCTIONING
Ecosystem functioning reflects the collective life
activities of plants, animals, and microbes and the effects
these activities — feeding, growing, moving, excreting
waste, etc. — have on the physical and chemical conditions
of their environment. (Note that “functioning” means
“showing activity” and does not imply that organisms
perform purposeful roles in ecosystem-level processes.)
A functioning ecosystem is one that exhibits biological
and chemical activities characteristic for its type. A
functioning forest ecosystem, for example, exhibits rates
of plant production, carbon storage, and nutrient cycling
that are characteristic of most forests. If the forest is
converted to an agroecosystem, its functioning changes.