Biology Reference
In-Depth Information
Astrocaryum murumuru at a site in southeastern Peru. In the absence of the
white-lipped peccary, a seed predator, the density of the palm increased by
a factor of 1.7. Re-introduction of the predator reduced the density of the
palm to a level found prior to the disappearance of the predator, and the
distribution of seedlings with respect to safe sites reverted to that of 21 years
earlier. Predators are also involved in regulating densities of the coral reef
fish Gramma loreto. Webster (
2003
) performed manipulative field experi-
ments demonstrating that predator-induced mortality is the primary
source of density dependence. Over a three-year period (about two
generations), local populations underwent regular annual cycles of abun-
dance, the result of seasonal recruitment patterns, but adult density
remained almost constant. The fish live in single species aggregations of
about 10 to more than 100 individuals. Movement between adjacent reefs
is very rare (about 0.7% of tagged residents emigrated per month).
Unmanipulated populations were compared with populations in which
recruitment was experimentally increased. All fish in each population
were captured and tagged, and demographic rates were monitored.
Subsequent recruitment was checked by looking for untagged fish.
Emigration was checked by looking for tagged fish within about 10 m
of each experimental population. Net immigration and net mortality
were measured by recapturing all fish at the end of the experiment,
and by comparison with the original census taken at the beginning of
the experiment. To check for predator responses to density variations,
nine additional pairs of populations were filmed. Population size in the
experimental populations was measured over three years. Results were
as follows.
Population size decreased in experimentally increased populations
due to density-dependent changes in demographic rates. Both emigration
rates and per capita mortality were density-dependent. However, differ-
ences in mortality between unmanipulated and manipulated populations
were five times greater than those in emigration. In contrast, both natural
recruitment and immigration were density-independent. The experi-
ments thus showed that density-dependent changes were primarily due
to mortality. Cause of mortality was neither interspecific competition
(other species were not present) nor intraspecific competition (aggression
did not increase, and growth rates did not decrease at higher densities),
it was predation. Predatory fish spent significantly more time in popula-
tions with experimentally increased densities. The overall result was that
control and manipulated populations converged completely, becoming
indistinguishable.
