Principles of terrestrial ecosystem ecology.pdf

268 12. Community Effects on Ecosystem Processes
the loss of one of a group of ecologically similar
species because, in the latter case, the remaining
species could continue to perform the relevant
ecological functions of that functional
type. The more species there are in a functional
type, the less likely it is that a gain or loss of a
single species from that functional type will
have large ecosystem effects. Our challenge, as
ecologists, is to identify the traits of organisms
that have strong effects on ecosystems; species
with these traits are likely to be strong interactors
in ecosystems (Paine 2000).
Species interactions govern the traits that are
expressed most clearly in ecosystems. The
impact of a species on ecosystem processes
depends on its interactions with other species.
The impact of deer on terrestrial vegetation or
the impact of Daphnia on algal biomass of
lakes, for example, depends on the density of
their predators. The mechanisms by which
species diversity influences ecosystem processes
often depend on species interactions
such as competition, facilitation, and predation.
Species Effects on
Ecosystem Processes
Species are most likely to have strong ecosystem
effects when they alter interactive controls,
which are the general factors that directly
regulate ecosystem processes. These controls
include the supply of resources that are essential
for primary production, climate, functional
N inputs (g m -2 yr -1 )
2.4
1.2
0
Myrica
absent
Myrica
present
Net N mineralization rate
(g N m -2 yr -1 )
6
3
0
Myrica
absent
types of organisms, disturbance regime, and
human activities (see Chapter 1).
Species Effects on Resources
Resource Supply
Species traits that influence the supply of limiting
resources have major impacts. The supply
of resources required for growth of primary
producers is one of the interactive controls to
which ecosystem processes are most sensitive
(see Chapter 1). These resources include light,
nutrients, and, on land, water. For this reason,
species traits that alter the supply of limiting
resources will substantially alter ecosystem
processes.
The introduction of a strong nitrogen fixer
into a community that lacks such species can
substantially alter nitrogen availability and
cycling. Invasion by the exotic nitrogen-fixing
tree Myrica faya in Hawaii, for example,
increased nitrogen inputs, litter nitrogen concentration,
and nitrogen availability (Vitousek
et al. 1987) (Fig. 12.3).A nitrogen-fixing invader
is most likely to be successful in ecosystems that
are nitrogen limited; have no strong nitrogen
fixers; and have adequate phosphorus, micronutrients,
and light (Vitousek and Howarth 1991).
Thus we expect large ecosystem changes from
invasion of nitrogen-fixing species primarily in
combinations of the following circumstances:
(1) low nitrogen supply (early primary succession
in the temperate zone and in other
low-nitrogen environments), (2) distant from
Myrica
present
Litter N concentration (%)
1.4
0.7
0
Myrica
absent
Myrica
present
Figure 12.3. Impact of the nitrogen-fixing tree Myrica faya on nitrogen inputs, litter nitrogen concentration,
and nitrogen mineralization rate in a Hawaiian montane forest (Vitousek et al. 1987).