Environmental Engineering Reference
In-Depth Information
PREDATOR CONTROL OF SPECIES LOWER IN THE FOOD CHAIN
A classic indirect effect occurs in the three-trophic-level food chain (fig-
ure 2.2a) in which the top carnivore species (C) feeds on the herbivore
species (H), which in turn feeds on a plant species (P). Because the carni-
vore reduces the abundance of herbivores that inflict damage to the plants,
the carnivore provides an indirect benefit to the plants.That is, from the
plant's perspective, carnivores fulfill the adage “the enemy of my enemy is
my friend” (Holt 2001). Ecologists call this indirect effect a trophic cascade
because the effects of manipulating the top trophic level of the system cas-
cades down the food web to the lowest trophic level.This is the kind of
indirect effect that is imagined whenever managers introduce predator
species to control an outbreak of insect pests that are devastating to eco-
nomically important crops.
EXPLOITATIVE COMPETITION—INGENIOUS WAYS OF DIVIDING-UP RESOURCES
A fundamental tenet of ecology is that no two competing species can co-
exist by exploiting exactly the same resource—called the competitive ex-
clusion principle.The species that wins in an exploitative competitive system
(figure 2.1c) is the one that consumes the resource the quickest or draws it
down to levels where individuals of the other species can no longer meet
their food requirements and sustain themselves. But many ecological sys-
tems, especially ones that have species-rich plant assemblages, seem to con-
tradict this fundamental tenet. Indeed, globally the approximately three
hundred thousand species of terrestrial plants have only about twenty dif-
ferent limiting resources (light, water, CO
2
, and minerals), and ecological
science has shown that at most only three or four resources are limiting in
any single location (McKane et al. 2002).
Resource limitation of plants is especially acute in arctic tundra systems
in which plant growth is limited almost singularly by soil nitrogen availabil-
ity. At the same time, many species of grasses, deciduous and evergreen
shrubs, and forbs co-occur within areas of 0.1 square meter—a “paradox
of diversity” (McKane et al. 2002). But, plants have evolved ingenious ways
of dividing up the resources making the paradox only apparent.
Tundra plant species differ in rooting depth, timing of life-cycle devel-
opment, and uptake preferences for different chemical forms of nitrogen
(ammonium, nitrate, and amino acids).A sedge species (
Carex bigelowii
) uses
mainly nitrate, which differs from a cottongrass species (
Eriophorum vagina-
tum
) and low-bush cranberry (
Vaccinium vitis-idaea
), which draw mainly from
