Abstract:
Life-history theory predicts that populations experiencing different levels of
extrinsic mortality will evolve divergent reproductive strategies. Previous work in the livebearing
fish Brachyrhaphis rhabdophora shows that individuals from populations that occur
with piscivorous fish mature earlier and at smaller sizes and have more and smaller offspring
than fish from populations without predators. However, until now, there have been no data to
demonstrate that differences in mortality rates actually exist between predator and predatorfree
sites. Here we present the results of a serial mark–recapture field study designed to
estimate mortality rates in natural populations of B. rhabodophora from Costa Rica. We found
that fish from predator environments experience higher overall mortality rates and
proportionally higher adult mortality rates than fish from predator-free environments. We
then ask what impact differences in mortality rates have on B. rhabdophora population
dynamics. Using a population matrix modeling approach, we found that B. rhabdophora that
co-occur with predators have population growth rates similar to those without predators and
both have confidence intervals that span k¼1.0. However, elasticity analysis revealed that the
most important life-history stages for population growth in predator environments are found
early in life and include growth through early ontogenetic stages and survival as small adults;
in contrast, the most important life-history stages for population growth in predator-free
environments occur late in life, including survival once large juvenile and adult stages are
reached. Hence, we demonstrate two important links between predation and population
demography, one ecological due to the direct impacts of predator-induced mortality and the
other expressed through predator-mediated reproductive adaptation.