Chapter 17

VARIATION IN TREE

SEEDLING AND ARBUSCULAR

MYCORRHIZAL FUNGAL

SPORE RESPONSES TO

THE EXCLUSION OF

TERRESTRIAL VERTEBRATES:

Implications for How

Vertebrates Structure

Tropical Communities

TadC.TheimerandCatherineA.Gehring

OVERVIEW

Vertebrates can impact species-rich, tropical plant communities by acting both as dispersers of seeds and spores and
as agents of seed and seedling mortality. When we excluded terrestrial vertebrates from 14 small (6 m×7.5 m)
plots of Australian tropical rainforest, we found significantly higher seedling recruitment and survival, resulting in
higher seedling species richness and diversity, on exclosure plots. These results contrasted with those for arbuscular
mycorrhizal fungal (AMF) spores, in which vertebrate exclusion led to decreased abundance, richness, and diversity.
In this chapter, we develop a conceptual model that explains this difference between AMF spore and tree seedling
responses to vertebrate exclusion. We hypothesize that when vertebrates act primarily as agents of seed or spore
dispersal, as they do for AMF spores in our system, they increase local species richness by increasing the rate of local
colonization. When vertebrates act primarily as agents of random mortality, as they do for seeds and seedlings in
our system, they increase the rate of local extinction and depress local species richness. In the latter case, vertebrates
increase recruitment limitation and could potentially maintain diversity on larger spatial scales, although this was
not the case at the scales we measured. Overall, our study suggests that (1) the relative proportion of tree or fungal
species in a community for which vertebrate dispersal significantly increases the seed/spore shadow determines the
magnitude of the difference in species pools available in the presence or absence of vertebrates, and thereby the rate of
species accumulation and potential for terrestrial vertebrates to alter species diversity; (2) if seedling or spore mortality
due to vertebrates is high enough, and not overall more strongly density dependent than in their absence, the net effect
of terrestrial vertebrates will be to reduce local species richness due to increased extinction rates and thereby increase
local dispersal limitation by reducing the probability that species arriving at a site will successfully establish there;
and (3) indirect effects of terrestrial vertebrates, like that of altering AMF spore species richness, could alter seedling
community dynamics, but these effects may take considerable time to be expressed.