Using post-settlement demography to estimate larval survivorship: a coral reef fish example.

Many species have multi-stage life cycles in which the youngest stages (e.g., larvae) are small, dispersive, and abundant, whereas later stages are sessile or sedentary. Quantifying survival throughout such early stages is critical for understanding dispersal, population dynamics, and life history evolution. However, dispersive stages can be very difficult to sample in situ, and estimates of survival through the entire duration of these stages are typically poor. Here we describe how demographic information from juveniles and adults can be used to estimate survival throughout a dispersive larval stage that was not sampled directly. Using field measurements of demography, we show that detailed information on post-settlement growth, survival, and reproduction can be used to estimate average larval survivorship under the assumption that a typical individual replaces itself over its lifetime. Applying this approach to a common coral reef fish (bicolor damselfish, Stegastes partitus), we estimated average larval survivorship to be 0.108% (95% CI 0.025-0.484). We next compared this demography-based estimate to an expected value derived from published estimates of larval mortality rates. Our estimate of larval survivorship for bicolor damselfish was approximately two orders of magnitude greater than what would be expected if larval mortality of this species followed the average, size-dependent pattern of mortality inferred from a published sample of marine fishes. Our results highlight the importance of understanding mortality during the earliest phases of larval life, which are typically not sampled, as well as the need to understand the details of how larval mortality scales with body size.

Sexual and lifetime selection on body size in a marine fish: the importance of life-history trade-offs.

Many field measurements of viability and sexual selection on body size indicate that large size is favoured. However, life-history theory predicts that body size may be optimized and that patterns of selection may often be stabilizing rather than directional. One reason for this discrepancy may be that field estimates of selection tend to focus on limited components of fitness and may not fully measure life-history trade-offs. We use an 8-year, demographic field study to examine both sexual selection and lifetime selection on body size of a coral reef fish (the bicolour damselfish, Stegastes partitus). Selection via reproductive success of adults was very strong (standardized selection differential=1.04). However, this effect was balanced by trade-offs between large adult size and reduced cumulative survival during the juvenile phase. When we measured lifetime fitness (net reproductive rate), selection was strongly stabilizing and only weakly directional, consistent with predictions from life-history theory.

Ontogenetic and spatial variation in size-selective mortality of a marine fish.

Although body size can affect individual fitness, ontogenetic and spatial variation in the ecology of an organism may determine the relative advantages of size and growth. During an 8-year field study in the Bahamas, we examined selective mortality on size and growth throughout the entire reef-associated life phase of a common coral-reef fish, Stegastes partitus (the bicolour damselfish). On average, faster-growing juveniles experienced greater mortality, though as adults, larger individuals had higher survival. Comparing patterns of selection observed at four separate populations revealed that greater population density was associated with stronger selection for larger adult size. Large adults may be favoured because they are superior competitors and less susceptible to gape-limited predators. Laboratory experiments suggested that selective mortality of fast-growing juveniles was likely because of risk-prone foraging behaviour. These patterns suggest that variation in ecological interactions may lead to complex patterns of lifetime selection on body size.

Weight gain and adjustment of feeding territory size in migrant hummingbirds.

Rufous hummingbirds periodically establish and defend territories along their summer southward migration route. Using artificial perches attached to spring or electronic balances in the field, we were able to measure daily weight changes in undistributed, individually marked birds. The territory size (number of flowers) of individual birds varied from day to day. Four of five intensively studied birds adjusted their territories to that size which was associated with the fastest sustained rate of weight gain attained at any stable territory size. The one exception was explicable on the basis of its unusually high weight. These results are consistent with the assumption of optimization theory that animals are capable of assessing when their behavior (e.g., territory size) is suboptimal and then making adjustments toward an optimum. The results also suggest, although not conclusively, that these birds are selected to maximize their rate of weight gain on each stopover prior to resuming migration.

Damselfish as keystone species in reverse: intermediate disturbance and diversity of reef algae.

Substrates located within the defended territories of Hawaiian damselfish for 1 year were subjected to intermediate grazing intensity and, as a result, showed greater diversity of algae than substrates either protected within fish-exclusion cages or exposed to intense fish grazing outside territories. Thus, this damselfish enhances local diversity on reefs through "intermediate-disturbance" effects, and is a keystone species that decreases rather than increases overall predation intensity relative to areas where it is absent.

Night-Shocker: Predatory Behavior of the Pacific Electric Ray (Torpedo californica).

Diving observations off Santa Barbara, California, indicate that Pacific electric rays (Torpedo californica) enter inshore reefs at night and actively prey on fish with the aid of powerful electric discharges.