Michael S. Webster

Population diversity and the portfolio effect in an exploited species

One of the most pervasive themes in ecology is that biological diversity stabilizes ecosystem processes and the services they provide to society, a concept that has become a common argument for biodiversity conservation. Species-rich communities are thought to produce more temporally stable ecosystem services because of the complementary or independent dynamics among species that perform similar ecosystem functions. Such variance dampening within communities is referred to as a portfolio effect and is analogous to the effects of asset diversity on the stability of financial portfolios. In ecology, these arguments have focused on the effects of species diversity on ecosystem stability but have not considered the importance of biologically relevant diversity within individual species. Current rates of population extirpation are probably at least three orders of magnitude higher than species extinction rates, so there is a pressing need to clarify how population and life history diversity affect the performance of individual species in providing important ecosystem services. Here we use five decades of data from Oncorhynchus nerka (sockeye salmon) in Bristol Bay, Alaska, to provide the first quantification of portfolio effects that derive from population and life history diversity in an important and heavily exploited species. Variability in annual Bristol Bay salmon returns is 2.2 times lower than it would be if the system consisted of a single homogenous population rather than the several hundred discrete populations it currently consists of. Furthermore, if it were a single homogeneous population, such increased variability would lead to ten times more frequent fisheries closures. Portfolio effects are also evident in watershed food webs, where they stabilize and extend predator access to salmon resources. Our results demonstrate the critical importance of maintaining population diversity for stabilizing ecosystem services and securing the economies and livelihoods that depend on them. The reliability of ecosystem services will erode faster than indicated by species loss alone.

Coastal oceanography sets the pace of rocky intertidal community dynamics

The structure of ecological communities reflects a tension among forces that alter populations. Marine ecologists previously emphasized control by locally operating forces (predation, competition, and disturbance), but newer studies suggest that inputs from large-scale oceanographically modulated subsidies (nutrients, particulates, and propagules) can strongly influence community structure and dynamics. On New Zealand rocky shores, the magnitude of such subsidies differs profoundly between contrasting oceanographic regimes. Community structure, and particularly the pace of community dynamics, differ dramatically between intermittent upwelling regimes compared with relatively persistent down-welling regimes. We suggest that subsidy rates are a key determinant of the intensity of species interactions, and thus of structure in marine systems, and perhaps also nonmarine communities.

CHAPTER 14 – Density Dependence in Reef Fish Populations

This chapter suggests two major patterns related to the density dependence in coral reef fish population. Demographic density dependence is generally defined as an effect of the present and/or past population sizes on the per-capita population growth rate. Coral reef fishes support major fisheries and are the object of conservation efforts in their own right. They are ideal subjects for both observational and experimental population studies. The total local recruitment rate varies in a broad variety of patterns as a function of local population size. Although recruitment is not a consistent source of demographic density dependence in every study, conversion from total to per-capita recruitment results in apparent or pseudo-density dependence, which can provide a nonmechanistic kind of local population equilibrium unrelated to the regulation of the entire metapopulation. The data on 20 species from six families shows that postsettlement mortality is often density-dependent, especially shortly after settlement, which is caused largely by predation. Overall, 17 of 20 species experienced density-dependent per-capita mortality at some time and place. In most studies that reported density-independent mortality, the study of older juvenile fishes suggested that regulation via mortality is an early postsettlement phenomenon. The prevalence of density-dependent mortality has two effects. First, local populations may be an important source of regulation for the entire metapopulation, suggesting the importance of conserving the mechanisms causing local density dependence. Second, the more restrictive version of the recruitment limitation hypothesis that predicts that postsettlement mortality may be density-independent has been falsified in most cases.

Climate Change, Ecosystem Impacts, and Management for Pacific Salmon

Abstract As climate change intensifies, there is increasing interest in developing models that reduce uncertainties in projections of global climate and refine these projections to finer spatial scales. Forecasts of climate impacts on ecosystems are far more challenging and their uncertainties even larger because of a limited understanding of physical controls on biological systems. Management and conservation plans that explicitly account for changing climate are rare and even those generally rely on retrospective analyses rather than future scenarios of climatic conditions and associated responses of specific ecosystems. Using past biophysical relationships as a guide to predicting the impacts of future climate change assumes that the observed relationships will remain constant. However, this assumption involves a long chain of uncertainty about future greenhouse gas emissions, climate sensitivity to changes in greenhouse gases, and the ecological consequences of climate change. These uncertainties in f...

DENSITY DEPENDENCE VIA INTERCOHORT COMPETITION IN A CORAL-REEF FISH

Demographic density dependence is essential for regulating population dynamics. For populations with multiple age or size cohorts, interactions between cohorts may be an important source of density dependence. Between-cohort interactions may be especially intense for many marine fish populations because of the potential disparity in competitive ability between small new recruits and relatively large resident conspecifics. Indeed, for marine fishes, density dependence in local populations appears to be widespread, particularly during a crucial time period following settlement of larvae into juvenile and adult habitats. I tested whether the demography of newly settled juveniles of the planktivorous fairy basslet (Gramma loreto) was affected by the density of adult conspecifics. Using spatially isolated local populations on coral reefs in the Bahamas, I experimentally transplanted a cohort of juveniles to local populations with three treatments of adult density: adults absent, average adult density, and high adult density (twice average). By following the fate of the transplanted juveniles, I determined that juvenile mortality increased as a function of adult density. Juveniles did not experience significantly higher rates of aggression or lower growth at higher densities of adults. However, in the presence of adults, juveniles generally fed at the back of aggregations, where feeding rates tend to be lower. Automated time-lapse video revealed that predators were found most frequently toward the back of aggregations. Thus competition with adults for prime feeding positions may increase predation risk for juveniles by forcing them to overlap spatially with predators. Consequently, interactions between juveniles and adults could influence the demography of subsequent generations in ways that promote local population regulation.

ODD SPECIES OUT AS PREDATORS REDUCE DIVERSITY OF CORAL-REEF FISHES

When predators differentiate among prey species, they commonly select the most abundant species. Surprisingly, on coral reefs in Australia and the Bahamas, we found evidence to the contrary: using null models, we observed that generalist predators had a greater impact on less-abundant species, thereby reducing local species richness. Dispro- portionate effects on these rare species were evident during a narrow window of time between settlement and 1-2 days later, highlighting how interactions during or immediately following settlement may influence subsequent community structure. Differences in species richness between treatments that were established during this narrow window of time eventually disappeared from reefs in Australia but persisted on reefs in the Bahamas. Our results highlight an unexpected and largely unexplored ecological interaction whereby pred- ators reduce prey diversity.

TEMPORAL DENSITY DEPENDENCE AND POPULATION REGULATION IN A MARINE FISH

Population regulation requires between-generation temporal density dependence, which has proved difficult to document unequivocally in nature, especially in marine fishes. Field experiments and observations in the Bahamas demonstrated that such population regulation occurs in the coral-reef fish Gramma loreto. I performed manipulative field experiments that identified mortality as the primary source of demographic density dependence among local populations. The causative mechanism involved a response in the behavior of predators, which both aggregated and spent more time hunting where prey fish densities were high. Spatially density-dependent mortality (i.e., density dependence among populations) led to between-generation temporal density dependence (i.e., density dependence within populations over time) indicative of population regulation. Over three years—roughly two generations—local populations followed regular annual cycles of abundance that stemmed from seasonal patterns of recruitment. Despite cons...