Tropical Forest Community Ecology

Response of Seedlings and AMF Spores to Vertebrates 299

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Change in species number

Lost via extinction Added via colonization

Figure 17.3 Hypothetical rate of species accumulation or loss on plots open to terrestrial vertebrates (open
symbols) or protected from them (solid symbols). In this example, the species pool available to plots open to
vertebrates would be higher than that available to plots where vertebrates are excluded because vertebrates carry
seeds/spores to plots where they would otherwise not arrive. As a result, the rate of species addition would be greater
on open plots (open squares) than on exclosure plots (solid squares). If terrestrial vertebrates increased overall
mortality rates and that mortality was density independent, species loss on open plots should be more rapid (open
triangles) than on exclosure plots (solid diamonds). However, if mortality due to terrestrial vertebrates was more
strongly density dependent than on exclosure plots, rate of species loss would be lower on open plots than on
protected plots (open diamonds) because more rare species would survive in the presence of vertebrates.

of terrestrial vertebrates. In the following para-
graphs, we develop a simple conceptual model
based on this analogy to explain how terrestrial
vertebrates could potentially alter species rich-
ness, and discuss how that model could explain
why tree seedling and AMF spore communities
differed in their response to vertebrate exclosure.
Species colonizing our plots could be drawn
from one of two species pools, a local pool of
those species with propagules that would arrive
at the site without dispersal by terrestrial verte-
brates, and an enhanced pool that would include
the addition of species with propagules that were
dispersed by terrestrial vertebrates but that would
not arrive by other means. Our exclosure plots
would be limited to the local species pool while
the open plots would be limited by the enhanced
species pool, and in both cases the probability

that a new species would be added as species

accumulated would decline to zero as the total

species pool available was approached. The dif-

ference between exclosure and open plots in the

number and rate of plant or AMF species accu-

mulated would depend on the relative abundance

of plant or fungal species with propagules dis-

persed by terrestrial vertebrates beyond their seed

or spore shadow (Figure 17.3).

The probability of species extinction on any plot

would depend on both the overall level of mortal-

ity and how that mortality was distributed among

species. In the case of seedlings, if the overall level

of seedling mortality was increased in the pres-

ence of terrestrial vertebrates, and that mortality

fell on seedlings of different species without regard

to their relative abundance (density-independent

mortality), then the overall extinction rate on