Tropical Forest Community Ecology

302 Tad C. Theimer and Catherine A. Gehring

0

5

10

15

20

25(a)

(b)

Years since exclosure

Mean species richness

− 8

− 6

− 4

− 2

0

2

4

6

8

Years since exclosure

Change in species richness

Figure 17.4 (a) The mean (±standard deviation, SD) number of seedling species recorded on open and exclosure
plots over the 3 years after exclosures were erected. Pairedt-tests showed significantly higher (P<0.005) species
richness on exclosure plots in all but the first year after exclosure. (b) These observed rates of species accumulation are
hypothesized to result from increased mortality and subsequent higher species extinction rates on open plots (open
triangles, solid line) versus exclosure plots (closed diamonds, solid line), with little to no increase in the species pool
available to colonize open plots (open squares, solid line) compared with exclosure plots (closed squares, solid line)
and therefore an overall lower net rate of species accumulation for open plots (open circles, dotted line) versus
exclosure plots (closed circles, dotted line).

relative abundance of community members with
differing attributes and the time over which those
communities are monitored. We describe below
what we believe are four major implications of this
view for studies of vertebrate impacts on tropical
rainforest community dynamics.

First, the relative proportion of species in a com-

munity for which vertebrate dispersal significantly

increases the seed/spore shadow determines the

magnitude of the difference in species pools avail-

able in the presence or absence of vertebrates,

and thereby the rate of species accumulation.