Ecological applications of niche theory - Ecological Applications: Toward a Sustainable World - page 44

2.2.2 Are we
modeling
fundamental or
realized niches?

The models of Peterson et al. (2003) make acceptable predictions about invasion
success despite being based on a restricted number of niche dimensions: they only
include an incomplete set of conditions and have no dimensions at all explicitly
related to resource use (nutrient availability, etc.) or the presence of enemies.
Whether these are actually models of fundamental or realized niches is debatable.
For example, if the same critical enemies (competitors, predators, parasites) are
present in both native and invaded ranges it could be claimed that the models rep-
resent realized rather than fundamental niches. However, when the same set of
species is not present in both locations (often the case for invaders, for example,
and one of the reasons some of them do so well), heavy reliance on climate matching
models may be misplaced.
Large parts of the globe now experience much higher nutrient concentrations,
particularly nitrogen (because of fertilizer application and atmospheric deposition
resulting from the burning of fossil fuels). While some plant species are able to use
the additional nitrogen for increased biomass production (such as the crops sown
by farmers), other meadow species, including herbs, sedges and bryophytes, cannot
effi ciently use higher nitrogen inputs (Zechmeister et al., 2003). The fundamental
niches of these meadow species are not usually limited by high nutrient concentra-
tions, but their realized niches are. This is because at higher nutrient levels they
become subject to interspecifi c competition and are excluded by more competitive
species.
Furthermore, the availability of nutrients, particularly phosphorus, can be
enhanced by mutualistic relationships with mycorrhizal fungi (Box 2.1). In a con-
trolled laboratory experiment involving the shrub Ardisia crenata , an invader of the
southeastern United States, inoculation with mycorrhizal fungi from the roots of an
established shrub increased the growth rate of seedlings. In addition, the outcome
of competition between seedlings of the invader and those of a native tree, Prunus
caroliniana , depends on mycorrhizal fungi (Bray et al., 2003). The presence of
mycorrhizae allowed Prunus seedlings to invest more to leaf area, enabling them to
compete more effectively with the invading Ardisia .
At least in some cases then, climate matching may not provide an adequate
description of likely patterns of invader establishment. Managers may need to take
other characteristics into account, such as relative competitive ability, which can
itself depend on the details of nutrient availability (sometimes modifi ed by fungal
mutualism). Grazing intensity, too, can modify the likelihood of invader success
because plant species vary in their vulnerability to grazing (this is dealt with in
detail in Chapter 3, Section 3.4.3).

2.2.3 When humans
disrupt ecosystems
and make it easy for
invaders

Shea and Chesson (2002) use the phrase niche opportunity to describe the potential
provided in a given region for invaders to succeed - in terms of a high availability
of resources and appropriate physicochemical conditions, coupled with a lack or
scarcity of natural enemies. They note that human activities often disrupt conditions
in a way that provides niche opportunities for invaders - river regulation, dealt with
next, is a case in point.
I noted in Section 2.1 the diffi culty of visualizing the multidimensional niche of
a species when more than three dimensions are involved. However, a mathematical
technique called ordination allows us to simultaneously display species and envi-
ronmental variables on the same graph, the two dimensions of which combine the