Ecological applications of niche theory - Ecological Applications: Toward a Sustainable World - page 40

in the basic ecological factors that determine a species' success, and therefore account for its
distribution in time and space. Niche modeling, on the other hand, uses native geographical distri-
bution to determine some important dimensions of the niche (usually abiotic conditions to do with
altitude, temperature, humidity, etc.; sometimes called 'climate matching' - Peterson, 2003) that
can then be extrapolated to determine two-dimensional geographical areas elsewhere that lay
within the abiotic requirements of the species. Such climate envelope models perform best when
they are based on a large number of data points for the focal species (50-75 or more), spread over
the whole climatic range that it occupies (Kadmon et al., 2003). The models are inevitably based
on incomplete pictures, but they often seem to be adequate for successful prediction.
In fact, the complete n -dimensional niche has not been determined for any species - it remains
very much an idea! However, the niche dimensions of some species are more completely known
than others, with information coming from laboratory studies, where conditions are experimentally
varied and biological performance is measured, as well as fi eld-based descriptive approaches such
as climate matching.
The realized niche - enemy action and mutual aid
Provided that a location has conditions within acceptable limits for a given species, and provided
also that it contains all necessary resources, then the species can, potentially, occur and persist
there. Whether or not it does so depends on a further factor - its occurrence may be precluded by
the action of individuals of other species that compete with, prey upon, or parasitize it. Usually, a
species has a larger ecological niche in the absence of enemies than it has in their presence. In
other words, there are certain combinations of conditions and resources that can allow a species
to maintain a viable population, but only if it is not being adversely affected by other species. This
led Hutchinson to distinguish between the fundamental and the realized niche. The former describes
the overall potentialities of a species; the latter describes the more limited spectrum of conditions
and resources that allow it to persist, even in the presence of competitors, predators and
parasites.
As an example of the role of competition in reducing the size of the fundamental niche, consider
Connell's (1961) classic study in Scotland of two species of barnacle, Chthamalus stellatus and
Balanus balanoides . These coexist on the same shore but their distributions, when looked at on a
fi n e r s c a l e , o v e r l a p very little - Balanus outcompetes (for space from which to fi lter food) and
excludes Chthamalus from the lower zones of rocky shores where, if Balanus is absent (experimen-
tally removed by Connell), Chthamalus can maintain a population. In similar vein, the native fi sh
Galaxias depressiceps has a fundamental niche that includes small streams and larger rivers in
southern New Zealand; but predation by exotic brown trout ( Salmo trutta ) now restricts its range
to headwaters above the waterfalls that prevent upstream trout migration (Townsend, 2003).
Finally, the extinction of nearly 50% of the endemic birds of Hawaii, which were broadly spread
across the islands, and the restricted distributions of those that remain, have been attributed in
part to introduced bird parasites (malaria and bird pox; van Riper et al., 1986).
Just as negative species' interactions can play a role in determining distributions, so can the
positive effects of mutualists. Take, for example, the tropical anemone fi sh Amphiprion percula ,
which retreats amongst the tentacles of the sea anemone Heteractis magnifi ca when danger threat-
ens, but also provides protection to the anemone from grazers (Elliott & Mariscal, 2001). Either
species may tolerate the abiotic conditions at a particular location, but the success of each also
depends on the presence of its mutualist. Similarly, most higher plants have intimate mutualisms
between their roots and fungi (mycorrhiza) that capture nutrients from the soil, which they trans-
port to the plants in exchange for plant photosynthetic products. Many plant species can live
without their mycorrhizal fungi in soils where nutrients and water are in good supply, but in the
harsh competitive world of natural plant communities the presence of its fungus is often necessary
if a plant is to prosper (Buscot et al., 2000).
The interspecifi c complications of distribution patterns (expressed in realized as opposed to
fundamental niches) are not incorporated in the climate matching models described in the previous
section, but in some cases we ignore them at our peril.