Abstract:
The threespot damselfish, Stegastes planifrons,
maintains individual territories that are clustered on
coral patch reefs. My objective was to understand the
effects of territory clustering on behavior and fitness.
Fish with territories in the center of a cluster had
(relative to edge fish): higher mating success (number of
eggs), higher aggressive chase rates with conspecifics,
lower chase rates to heterospecifics, lower overall chase
rates, lower grazing rates by intruders, and smaller
territories. Feeding rate, survivorship, and age at
maturity did not vary with territory position. Therefore,
central fish appeared to have higher fitness, which was
probably related to the lower energetic costs of territory
defense there.
Center and edge territories differed in habitat
complexity, and the density of potential algal
competitors, egg predators, and various food and
invertebrate species. These microhabitat features could
provide different quality shelter, nest or feeding sites
and thus might explain the positional differences in
fitness. An experiment in which I changed the position of
treatment fish from the center to the edge of a cluster,
without altering microhabitat, showed that position per
Se, and not microhabitat variation, caused the center-edge
differences.
Vacated space in the center of a cluster was fought
over more vigorously and reoccuppied sooner than similar
space on the edge. Settlement to one of two depopulated
clusters was preferentially to the cluster center. These
data indicated that threespots compete for the more
desirable central positions. Therefore, these populations
can be considered simultaneously recruitment limited (in
terms of local population size) and resource limited (in
terms of local reproductive output and perhaps global
population size).
Aggressive chases with conspecifics were lower on the
cluster edge than at any distance toward the center, while
chases to heterospecifics had the opposite pattern. The
results of chases with conspecifics did not fit the
predictions of the model by Stamps et al. (1987) . This
discrepancy may be a result of habituation between
territorial neighbors.
