B) Average difference in the body masses of resource and consumer species
at the bottom two and top two trophic levels for each system type.
Toxin resistance confers significant advantages to consumer species and has been observed in a wide range of herbivores and predators intimately involved with chemically defended prey (Brodie and Brodie, 1990; Mauricio et al.
Toxins have evolved as highly effective signal molecules for select consumer species possessing chemoreceptive ability.
In addition to our target species, other common consumer species
include: short-tailed shrews (Sorex cinereus), native mice (Peromyscusspp.
4, meaning that for every unit of algae consumed by each consumer species
, grazers fix 3.
In these circumstances (which pertain to many taxa), the effect of trophic rank on the species - area relationship should be magnified, because an obligate generalist consumer species
will be absent unless each of its required resource types is present.
These two possibilities will be examined in a later study; both allow two consumer species
to coexist in the absence of a predator, and addition of the predator may actually prevent coexistence.
We constructed microcosms that contained autotrophs, decomposers, and dead organic and inorganic nutrients and varied the number of consumer species
stocked in each microcosm.
For example, to analyze the effects of the interaction among Colpoda, Bodo, and Wyeomyia, a factorial design having these three consumer species
as factors was extracted, and only the three-way Colpoda x Bodo x Wyeomyia interaction was interpreted; lower order interaction terms and main effects were ignored.
Two consumer species
that share prey may also interact via effects on the functional and numerical responses of shared predators, and these ecological interactions may be positive or negative (Abrams 1987c, Holt and Kotler 1987).
To illustrate how invasion resistance changes with time, consider 500 assembly sequences drawn from a species pool with 5 basal and 15 consumer species
[ILLUSTRATION FOR FIGURE 3 OMITTED].
To complete the trophic chain, we now consider a consumer species
i + 1 feeding on species i.