Biology Reference
In-Depth Information
regional fauna, suggesting that many reef fishes could have persistent self-
sustaining local populations.
All studies of reef fish have been conducted at the level of local
populations. Sources of density dependence may be fecundity, mortality
of eggs and larvae before settlement, mortality during transition from
pelagic to demersal stage, or mortality of juveniles/adults after settlement
(Hixon and Webster
2002
). According to these authors, various studies
have confirmed that growth of reef fish and hence fecundity are density
dependent. Mortality during settlement can be considerable, and recruit-
ment to local populations is per capita density dependent, i.e., a decreasing
proportion of the total population is recruited with increasing size of the
local population. On the other hand, rates of loss of eggs and larvae during
presettlement mortality seem to be density-independent but are little
known. Therefore, according to the authors, density dependence must
act at a later stage in order to guarantee persistence of the metapopulation.
Mortality at postsettlement is better documented, although few studies
deal with mortality due to interspecific competition. Predation, on the
other hand, has been shown to cause density-dependent mortality, which
supports the recruitment limitation hypothesis. The authors surveyed the
literature and found a broad range of results concerning recruitment
patterns both in observational and experimental studies. Recruitment in
reef fishes appears to be highly variable, but after conversion to per capita
rates, recruitment in all cases seemed to be ''pseudo-density dependent.''
A survey of postsettlement mortality permitted calculation of the instant-
aneous per capita mortality rate as a function of local population density.
Fourteen observational and experimental studies from 24 separate data
sets on 15 fish species were evaluated. The evaluation showed that
postsettlement density-dependent mortality is indeed widespread
among reef fishes. Combined consideration of recruitment and mortality
led to the conclusion that local population fluctuations are bounded as the
result of density-dependent processes. Figure
7.1
shows this combined
effect for two species of reef fish, suggesting regulation of local populations.
Equilibrium is reached where recruitment and mortality lines cross,
provided there is no emigration or immigration. Concerning the
mechanisms involved in density-dependent regulation, data for recruit-
ment are lacking but may be intraspecific competition and/or cannibalism.
For predation, the most important source of mortality in reef fishes,
density-dependent effects have been demonstrated in several studies.
For example, predators aggregate at sites where new recruits are common,
whereas sites with few recruits are ignored.
