Impact of Low-Head Dam Removal on River Morphology and Habitat Suitability in Mountainous Rivers

Dam removal is considered an effective measure to solve the adverse ecological effects caused by dam construction and has started to be considered in China. The sediment migration and habitat restoration of river ecosystems after dam removal have been extensively studied abroad but are still in the exploratory stage in China. However, there are few studies on the ecological response of fishes at different growth stages. Considering the different habitat preferences of Schizothorax prenanti (S. prenanti) in the spawning and juvenile periods, this study coupled field survey data and a two-dimensional hydrodynamic model to explore the changes in river morphology at different scales and the impact of changes in hydrodynamic conditions on fish habitat suitability in the short term. The results show that after the dam is removed, in the upstream of the dam, the riverbed is eroded and cut down and the riverbed material coarsens. With the increase in flow velocity and the decrease in flow area, the weighted usable area (WUA) in the spawning and juvenile periods decreases by 5.52% and 16.36%, respectively. In the downstream of the dam, the riverbed is markedly silted and the bottom material becomes fine. With the increase in water depth and flow velocity, the WUA increases by 79.91% in the spawning period and decreases by 67.90% in the juvenile period, which is conducive to adult fish spawning but not to juvenile fish growth. The changes in physical habitat structure over a short time period caused by dam removal have different effects on different fish development periods, which are not all positive. The restoration of stream continuity increases adult fish spawning potential while limiting juvenile growth. Thus, although fish can spawn successfully, self-recruitment of fish stocks can still be affected if juvenile fish do not grow successfully. This study provides a research basis for habitat assessment after dam removal and a new perspective for the subsequent adaptive management strategy of the project.

Quantifying the nature and strength of intraspecific density dependence in Arctic mosquitoes

Processes that change with density are inherent in all populations, yet quantifying density dependence with empirical data remains a challenge. This is especially true for animals recruiting in patchy landscapes because heterogeneity in habitat quality in combination with habitat choice can obscure patterns expected from density dependence. Mosquitoes (Diptera: Culicidae) typically experience strong density dependence when larvae compete for food, however, effects vary across species and contexts. If populations experience intense intraspecific density-dependent mortality then overcompensation can occur, where the number of survivors declines at high densities producing complex endogenous dynamics. To seek generalizations about density dependence in a widespread species of Arctic mosquito, Aedes nigripes, we combined a laboratory experiment, field observations, and modeling approaches. We evaluated alternative formulations of discrete population models and compared best-performing models from our lab study to larval densities from ponds in western Greenland. Survivorship curves from the lab were the best fit by a Hassell model with compensating density dependence (equivalent to a Beverton-Holt model) where peak recruitment ranged from 8 to 80 mosquitoes per liter depending on resource supply. In contrast, our field data did not show a signal of strong density dependence, suggesting that other processes such as predation may lower realized densities in nature, and that expected patterns may be obscured because larval abundance covaries with resources (cryptic density dependence). Our study emphasizes the importance of covariation between the environment, habitat choice, and density dependence in understanding population dynamics across landscapes, and demonstrates the value of pairing lab and field studies.

Lunar rhythms in growth of larval fish

Growth and survival of larval fishes is highly variable and unpredictable. Our limited understanding of this variation constrains our ability to forecast population dynamics and effectively manage fisheries. Here we show that daily growth rates of a coral reef fish (the sixbar wrasse, Thalassoma hardwicke) are strongly lunar-periodic and predicted by the timing of nocturnal brightness: growth was maximized when the first half of the night was dark and the second half of the night was bright. Cloud cover that obscured moonlight facilitated a 'natural experiment', and confirmed the effect of moonlight on growth. We suggest that lunar-periodic growth may be attributable to light-mediated suppression of diel vertical migrations of predators and prey. Accounting for such effects will improve our capacity to predict the future dynamics of marine populations, especially in response to climate-driven changes in nocturnal cloud cover and intensification of artificial light, which could lead to population declines by reducing larval survival and growth.

Reproductive phenology across the lunar cycle: parental decisions, offspring responses, and consequences for reef fish

Most organisms reproduce in a dynamic environment, and life-history theory predicts that this can favor the evolution of strategies that capitalize on good times and avoid bad times. When offspring experience these environmental changes, fitness can depend strongly upon environmental conditions at birth and at later life stages. Consequently, fitness will be influenced by the reproductive decisions of parents (i.e., birth date effects) and developmental decisions (e.g., adaptive plasticity) of their offspring. We explored the consequences of these decisions using a highly iteroparous coral reef fish (the sixbar wrasse, Thalassoma hardwicke) and in a system where both parental and offspring environments vary with the lunar cycle. We tested the hypotheses that (1) reproductive patterns and offspring survival vary across the lunar cycle and (2) offspring exhibit adaptive plasticity in development time. We evaluated temporal variation in egg production from February to June 2017, and corresponding larval developmental histories (inferred from otolith microstructure) of successful settlers and surviving juveniles that were spawned during that same period. We documented lunar-cyclic variation in egg production (most eggs were spawned at the new moon). This pattern was at odds with the distribution of birth dates of settlers and surviving juveniles-most individuals that successfully survived to settlement and older stages were born during the full moon. Consequently, the probability of survival across the larval stage was greatest for offspring born close to the full moon, when egg production was at its lowest. Offspring also exhibited plasticity in developmental duration, adjusting their age at settlement to settle during darker portions of the lunar cycle than expected given their birth date. Offspring born near the new moon tended to be older and larger at settlement, and these traits conveyed a strong fitness advantage (i.e., a carryover effect) through to adulthood. We speculate that these effects (1) are shaped by a dynamic landscape of risk and reward determined by moonlight, which differentially influences adults and offspring, and (2) can explain the evolution of extreme iteroparity in sixbars.

Integrating population dynamics models and distance sampling data: a spatial hierarchical state-space approach

Stochastic versions of Gompertz, Ricker, and various other dynamics models play a fundamental role in quantifying strength of density dependence and studying long-term dynamics of wildlife populations. These models are frequently estimated using time series of abundance estimates that are inevitably subject to observation error and missing data. This issue can be addressed with a state-space modeling framework that jointly estimates the observed data model and the underlying stochastic population dynamics (SPD) model. In cases where abundance data are from multiple locations with a smaller spatial resolution (e.g., from mark-recapture and distance sampling studies), models are conventionally fitted to spatially pooled estimates of yearly abundances. Here, we demonstrate that a spatial version of SPD models can be directly estimated from short time series of spatially referenced distance sampling data in a unified hierarchical state-space modeling framework that also allows for spatial variance (covariance) in population growth. We also show that a full range of likelihood based inference, including estimability diagnostics and model selection, is feasible in this class of models using a data cloning algorithm. We further show through simulation experiments that the hierarchical state-space framework introduced herein efficiently captures the underlying dynamical parameters and spatial abundance distribution. We apply our methodology by analyzing a time series of line-transect distance sampling data for fin whales (Balaenoptera physalus) off the U.S. west coast. Although there were only seven surveys conducted during the study time frame, 1991-2014, our analysis detected presence of strong density regulation and provided reliable estimates of fin whale densities. In summary, we show that the integrative framework developed herein allows ecologists to better infer key population characteristics such as presence of density regulation and spatial variability in a population's intrinsic growth potential.

Born at the right time? A conceptual framework linking reproduction, development, and settlement in reef fish

Parents are expected to make decisions about reproductive timing and investment that maximize their own fitness, even if this does not maximize the fitness of each individual offspring. When offspring survival is uncertain, selection typically favors iteroparity, which means that offspring born at some times can be disadvantaged, while others get lucky. The eventual fate of offspring may be further modified by their own decisions. Are fates of offspring set by birthdates (i.e., determined by parents), or can offspring improve upon the cards they've been dealt? If so, do we see adaptive plasticity in the developmental timing of offspring? We evaluate these questions for a coral reef fish (the sixbar wrasse, Thalassoma hardwicke) that is characterized by extreme iteroparity and flexible larval development. Specifically, we monitored larval settlement to 192 small reefs over 11 lunar months and found that most fish settled during new moons of a lunar cycle (consistent with preferential settlement on dark nights). Settlement was significantly lower than expected by chance during the full moon and last quarter of the lunar cycle (consistent with avoidance of bright nights). Survival after settlement was greatest for fish that settled during times of decreasing lunar illumination (from last quarter to new moon). Fish that settled on the last quarter of the lunar cycle were ~10% larger than fish that settled during other periods, suggesting larvae delay settlement to avoid the full moon. These results are consistent with a numerical model that predicts plasticity in larval development time that enables avoidance of settlement during bright periods. Collectively, our results suggest that fish with inauspicious birthdates may alter their developmental trajectories to settle at better times. We speculate that such interactions between parent and offspring strategies may reinforce the evolution of extreme iteroparity and drive population dynamics, by increasing the survival of offspring born at the "wrong" time by allowing them to avoid the riskiest times of settlement.

The nose knows: linking sensory cue use, settlement decisions, and post-settlement survival in a temperate reef fish

Habitat selection by animals that migrate or disperse ultimately determines the biotic and abiotic environment they will experience in subsequent life stages. Intuitively, for habitat selection to be adaptive, animals should respond positively to cues produced by habitat characteristics that will enhance their fitness in the new environment. However, there are many examples of dispersing animals where individuals are attracted to cues produced by factors that reduce their fitness after arrival. In this study, we use a temperate reef fish to examine the relative importance of habitat-associated cues in habitat selection decisions, and assess whether use of these cues is adaptive across early life stages. We used a series of laboratory- and field-based manipulative experiments to test: (1) what habitat-associated cues are likely used to locate suitable habitat; (2) whether in situ settlement patterns reflect the cue response tested in the laboratory; and (3) whether the aspects of the habitat that stimulate settlement are the same as those that maximize survival. We observed a positive response to multiple habitat-associated cues, with conspecific cues eliciting the strongest behavioral response in laboratory choice experiments, and a strong inverse density-dependent relationship at settlement. Macroalgal cues also elicited a positive response at settlement, but were associated with higher mortality after settlement, suggesting that habitat selection decisions are not always adaptive. We argue that this non-intuitive behavior may still be adaptive if it improves fitness at an earlier life stage, as habitat selection behavior is the result of tradeoffs in fitness costs across multiple stages.

Interactive effects of shelter and conspecific density shape mortality, growth, and condition in juvenile reef fish

How landscape context influences density-dependent processes is important, as environmental heterogeneity can confound estimates of density dependence in demographic parameters. Here we evaluate 19 populations in a shoaling temperate reef fish (Trachinops caudimaculatus) metapopulation within a heterogeneous seascape (Port Phillip Bay, Australia) to show empirically that shelter availability and population density interact to influence juvenile mortality, growth and condition. Although heterogeneity in shelter availability obscured the underlying patterns of density dependence in different ways, the combination of habitat and its interaction with density were two to six times more important than density alone in explaining variation in demographic parameters for juveniles. These findings contradict many small-scale studies and highlight the need for landscape-scale observations of how density dependence interacts with resource availability and competition to better understand how demographic parameters influence the dynamics of metapopulations in heterogeneous environments.

The Prevalence and Importance of Competition Among Coral Reef Fishes

Although competition is recognized as a core ecological process, its prevalence and importance in coral reef fish communities have been debated. Here we compile and synthesize the results of 173 experimental tests of competition from 72 publications. We show that evidence for competition is pervasive both within and between species, with 72% of intraspecific tests and 56% of interspecific tests demonstrating a demographically significant consequence of competition (e.g., a decrease in recruitment, survival, growth, or fecundity). We highlight several factors that can interact with the effects of competition and make it more difficult to detect in field experiments. In light of this evidence, we discuss the role of competition in shaping coral reef fish communities and competition's status as one of several processes that contribute to species coexistence. Finally, we consider some of the complex ways that climate change may influence competition, and we provide suggestions for future research.

Characteristics of settling coral reef fish are related to recruitment timing and success

Many marine populations exhibit high variability in the recruitment of young into the population. While environmental cycles and oceanography explain some patterns of replenishment, the role of other growth-related processes in influencing settlement and recruitment is less clear. Examination of a 65-mo. time series of recruitment of a common coral reef fish, Stegastes partitus, to the reefs of the upper Florida Keys revealed that during peak recruitment months, settlement stage larvae arriving during dark lunar phases grew faster as larvae and were larger at settlement compared to those settling during the light lunar phases. However, the strength and direction of early trait-mediated selective mortality also varied by settlement lunar phase such that the early life history traits of 2–4 week old recruit survivors that settled across the lunar cycle converged to more similar values. Similarly, within peak settlement periods, early life history traits of settling larvae and selective mortality of recruits varied by the magnitude of the settlement event: larvae settling in larger events had longer PLDs and consequently were larger at settlement than those settling in smaller pulses. Traits also varied by recruitment habitat: recruits surviving in live coral habitat (vs rubble) or areas with higher densities of adult conspecifics were those that were larger at settlement. Reef habitats, especially those with high densities of territorial conspecifics, are more challenging habitats for young fish to occupy and small settlers (due to lower larval growth and/or shorter PLDs) to these habitats have a lower chance of survival than they do in rubble habitats. Settling reef fish are not all equal and the time and location of settlement influences the likelihood that individuals will survive to contribute to the population.

Two's company, three's a crowd: food and shelter limitation outweigh the benefits of group living in a shoaling fish

Identifying how density and number-dependent processes regulate populations is important for predicting population response to environmental change. Species that live in groups, such as shoaling fish, can experience both direct density-dependent mortality through resource limitation and inverse number-dependent mortality via increased feeding rates and predator evasion in larger groups. To investigate the role of these processes in a temperate reef fish population, we manipulated the density and group size of the shoaling species Trachinops caudimaculatus on artificial patch reefs at two locations with different predator fields in Port Phillip Bay, Australia. We compared mortality over four weeks to estimates of predator abundance and per capita availability of refuge and food to identify mechanisms for density or number dependence. Mortality was strongly directly density dependent throughout the experiment, regardless of the dominant predator group; however, the limiting resource driving this effect changed over time. In the first two weeks when densities were highest, density-dependent mortality was best explained by refuge competition and the abundance of benthic predators. During the second two weeks, food competition best explained the pattern of mortality. We detected no effect of group size at either location, even where pelagic-predator abundance was high. Overall, direct density effects were much stronger than those of group size, suggesting little survival advantage to shoaling on isolated patch reefs where resource competition is high. This study is the first to observe a temporal shift in density-dependent mechanisms in reef fish, and the first to observe food limitation on short temporal scales. Food competition may therefore be an important regulator of postsettlement reef fish cohorts after the initial intense effects of refuge limitation and predation.

High population density enhances recruitment and survival of a harvested coral reef fish

A negative relationship between population growth and population density (direct density dependence) is necessary for population regulation and is assumed in most models of harvested populations. Experimental tests for density dependence are lacking for large-bodied, harvested fish because of the difficulty of manipulating population density over large areas. We studied a harvested coral reef fish, Lutjanus apodus (schoolmaster snapper), using eight large, isolated natural reefs (0.4-1.6 ha) in the Bahamas as replicates. An initial observational test for density dependence was followed by a manipulation of population density. The manipulation weakened an association between density and shelter-providing habitat features and revealed a positive effect of population density on recruitment and survival (inverse density dependence), but no effect of density on somatic growth. The snappers on an individual reef were organized into a few shoals, and we hypothesize that large shoals on high-density reefs were less vulnerable to large piscivores (groupers and barracudas) than the small shoals on low-density reefs. Reductions in predation risk for individuals in large social groups are well documented, but because snapper shoals occupied reefs the size of small marine reserves, these ecological interactions may influence the outcome of management actions.

Lethal effects of habitat degradation on fishes through changing competitive advantage

Coral bleaching has caused catastrophic changes to coral reef ecosystems around the world with profound ecological, social and economic repercussions. While its occurrence is predicted to increase in the future, we have little understanding of mechanisms that underlie changes in the fish community associated with coral degradation. The present study uses a field-based experiment to examine how the intensity of interference competition between juveniles of two species of damselfish changes as healthy corals degrade through thermal bleaching. The mortality of a damselfish that is a live coral specialist (Pomacentrus moluccensis) increased on bleached and dead coral in the presence of the habitat generalist (Pomacentrus amboinensis). Increased mortality of the specialist was indirectly owing to enhanced aggression by the generalist forcing the specialist higher up and further away from shelter on bleached and dead coral. Evidence from this study stresses the importance of changing interspecific interactions to community dynamics as habitats change.

Benefits for plants in ant-plant protective mutualisms: a meta-analysis

Costs and benefits for partners in mutualistic interactions can vary greatly, but surprisingly little is known about the factors that drive this variation across systems. We conducted a meta-analysis of ant-plant protective mutualisms to quantify the effects of ant defenders on plant reproductive output, to evaluate if reproductive effects were predicted from reductions in herbivory and to identify characteristics of the plants, ants and environment that explained variation in ant protection. We also compared our approach with two other recent meta-analyses on ant-plant mutualisms, emphasizing differences in our methodology (using a weighted linear mixed effects model) and our focus on plant reproduction rather than herbivore damage. Based on 59 ant and plant species pairs, ant presence increased plant reproductive output by 49% and reduced herbivory by 62%. The effects on herbivory and reproduction within systems were positively correlated, but the slope of this relationship (0.75) indicated that tolerance to foliar herbivory may be a common plant response to absence of ant guards. Furthermore, the relationship between foliar damage and reproduction varied substantially among systems, suggesting that herbivore damage is not a reliable surrogate for fitness consequences of ant protection. Studies that experimentally excluded ants reported a smaller effect of ant protection on plant reproduction than studies that relied upon natural variation in ant presence, suggesting that study methods can affect results in these systems. Of the ecological variables included in our analysis, only plant life history (i.e., annual or perennial) explained variation in the protective benefit of mutualistic ants: presence of ants benefitted reproduction of perennials significantly more than that of annuals. These results contrast with other quantitative reviews of these relationships that did not include plant life history as an explanatory factor and raise several questions to guide future research on ant-plant protection mutualisms.

High-quality habitat and facilitation ameliorate competitive effects of prior residents on new settlers

Many species disperse during their lifetime. Two factors that can affect the performance of individuals following dispersal are the presence of conspecifics and intrinsic habitat quality at the settlement site. Detecting the influence of these factors can be difficult for at least two reasons: (1) the outcomes of interactions with conspecifics are often variable including both competition and facilitation, and (2) selection of high quality habitats often leads to positive covariance between habitat quality and density. In this study, I investigate positive and negative effects of resident blue streak cleaner wrasse (Labroides dimidiatus) on the growth and survival of recently settled conspecifics while accounting for habitat quality. Juvenile L. dimidiatus settle near adult conspecifics, but likely have to compete with resident adults for access to food. However, field experiments indicate that settlers have access to more resources at occupied sites, and as a result, grow faster despite evidence for competition with residents. This result is a direct consequence of two factors: (1) resident conspecifics facilitate settlers by attracting client fish, and (2) resident conspecifics are strongly associated with high quality habitat. These results highlight the need to simultaneously consider habitat quality and competitive and facilitative interactions between conspecifics when making inferences about ecological processes from spatial patterns of individual performance.

Synthesizing mechanisms of density dependence in reef fishes: behavior, habitat configuration, and observational scale

Coral and rocky reef fish populations are widely used as model systems for the experimental exploration of density-dependent vital rates, but patterns of density-dependent mortality in these systems are not yet fully understood. In particular, the paradigm for strong, directly density-dependent (DDD) postsettlement mortality stands in contrast to recent evidence for inversely density-dependent (IDD) mortality. We review the processes responsible for DDD and IDD per capita mortality in reef fishes, noting that the pattern observed depends on predator and prey behavior, the spatial configuration of the reef habitat, and the spatial and temporal scales of observation. Specifically, predators tend to produce DDD prey mortality at their characteristic spatial scale of foraging, but prey mortality is IDD at smaller spatial scales due to attack-abatement effects (e.g., risk dilution). As a result, DDD mortality may be more common than IDD mortality on patch reefs, which tend to constrain predator foraging to the same scale as prey aggregation, eliminating attack-abatement effects. Additionally, adjacent groups of prey on continuous reefs may share a subset of refuges, increasing per capita refuge availability and relaxing DDD mortality relative to prey on patch reefs, where the patch edge could prevent such refuge sharing. These hypotheses lead to a synthetic framework to predict expected mortality patterns for a variety of scenarios. For nonsocial, nonaggregating species and species that aggregate in order to take advantage of spatially clumped refuges, IDD mortality is possible but likely superseded by DDD refuge competition, especially on patch reefs. By contrast, for species that aggregate socially, mortality should be IDD at the scale of individual aggregations but DDD at larger scales. The results of nearly all prior reef fish studies fit within this framework, although additional work is needed to test many of the predicted outcomes. This synthesis reconciles some apparent contradictions in the recent reef fish literature and suggests the importance of accounting for the scale-sensitive details of predator and prey behavior in any study system.

Ecological, behavioral, and genetic factors influencing the recombinant control of invasive pests

Invasive species are a major threat to biodiversity, cost the world economy billions of dollars annually, and are often difficult, if not impossible, to control using current approaches. Recombinant technologies could revolutionize management of such pests but would be subject to a range of genetic, behavioral, and ecological factors that could limit their efficacy or applicability. We use a realistically parameterized combined population dynamics/genetics model to assess the potential of, and constraints on, a suite of recombinant approaches that have been suggested for pest control. We show that, of the options suggested to date, a genetic construct that distorts operational sex ratios by sterilizing, killing, or sex-changing one gender and being inherited through the other, is not only potentially the most effective means of pest control, but also one that remains effective over the widest range of ecological and behavioral conditions. All methods, however, are sensitive in particular to the degree of density dependence in the pest population and to operational issues such as maximum copy number and stocking levels, which affect introgression rates. Optimal investment strategies for an integrated pest management program that includes the nonlinear interactions of recombinant strategies and complementary management options can be assessed through the sensitivity analyses. The subtle effects of even minor variability in some parameters, such as extra mortality due to the presence of the construct, further suggest that genetic techniques be applied in an active adaptive management framework, so that strategies can be regularly optimized as the impacts of a release program are assessed.

Spatial distribution of limited resources and local density regulation in juvenile Atlantic salmon

1. Spatial heterogeneity of resources may influence competition among individuals and thus have a fundamental role in shaping population dynamics and carrying capacity. In the present study, we identify shelter opportunities as a limiting resource for juvenile Atlantic salmon (Salmo salar L.). Experimental and field studies are combined in order to demonstrate how the spatial distribution of shelters may influence population dynamics on both within and among population scales. 2. In closed experimental streams, fish performance scaled negatively with decreasing shelter availability and increasing densities. In contrast, the fish in open stream channels dispersed according to shelter availability and performance of fish remaining in the streams did not depend on initial density or shelters. 3. The field study confirmed that spatial variation in densities of 1-year-old juveniles was governed both by initial recruit density and shelter availability. Strength of density-dependent population regulation, measured as carrying capacity, increased with decreasing number of shelters. 4. Nine rivers were surveyed for spatial variation in shelter availability and increased shelter heterogeneity tended to decrease maximum observed population size (measured using catch statistics of adult salmon as a proxy). 5. Our studies highlight the importance of small-scale within-population spatial structure in population dynamics and demonstrate that not only the absolute amount of limiting resources but also their spatial arrangement can be an important factor influencing population carrying capacity.

Influence of Palythoa caribaeorum (Anthozoa, Cnidaria) zonation on site-attached reef fishes

This work aimed to test the influence of Palythoa caribeorum, a widely distributed zoanthid in the Atlantic, on site-attached reef fish in a subtropical rocky shore. Density, richness and vertical distribution of reef fish inside (ID) and outside (OD) previously chosen P. caribaeorum dominance patches were compared through stationary visual censuses along three different periods. Fishes were grouped in different trophic guilds to evidence differences in resources uses in both treatments. A complexity index was estimated by the chain link method and percentage covering of benthic organisms was obtained analyzing random points from replicated photo-quadrats. We observed thirty-eight species of fishes, belonging to twenty-five families. Reef fish communities between studied patches were similar,both in terms of species composition and vertical distribution. Considering only the most site-attached fishes, which were the most frequent and abundant species, data showed that ID sustains higher diversity and abundance than OD. Results showed that benthic composition differ significantly among patches whereas complexity remained without differences. Otherwise, results indicated that these areas might play an important role in space limitation, structuring neighboring benthic community and consequently reef fish assemblages.

Spatially correlated recruitment of a marine predator and its prey shapes the large-scale pattern of density-dependent prey mortality

Patterns of predator dispersal can be critical to the dynamics of prey metapopulations. In marine systems, oceanic currents may shape the dispersal of planktonic larvae of both predators and prey, producing spatial correlations in the recruitment of both species and distinctive geographic patterns of prey mortality. I examined the potential for this phenomenon in two fishes, a wrasse and its grouper predator, at a Caribbean island where the near-shore oceanographic regime produces a temporally consistent spatial pattern of fish recruitment. I found that recruitment and adult abundance of groupers were spatially correlated with recruitment of wrasse prey. Furthermore, the local abundance of predators strongly affected the nature of density-dependent prey mortality. At sites with few predators, wrasse mortality was inversely density-dependent, while mortality was positively density-dependent at sites with higher predator densities. This phenomenon could be important to the dynamics of any metacommunity in which physical forces produce correlated dispersal.

HABITAT COMPLEXITY MODIFIES POST-SETTLEMENT MORTALITY AND RECRUITMENT DYNAMICS OF A MARINE FISH

For species that have an open population structure, local population size may be strongly influenced by a combination of propagule supply and post-settlement survival. While it is widely recognized that supply of larvae (or recruits) is variable and that variable recruitment may affect the relative contribution of pre- and post-settlement factors, less effort has been made to quantify how variation in the strength of post-settlement mortality (particularly density-dependent mortality) will affect the importance of processes that determine population size. In this study, I examined the effects of habitat complexity on mortality of blue rockfish (Sebastes mystinus) within nearshore reefs off central California. I first tested whether variation in habitat complexity (measured as three-dimensional complexity of rocky substrate) affected the magnitude of both density-independent and density-dependent mortality. I then used limitation analysis to quantify how variation in habitat complexity alters the relative influence of recruitment, density-independent mortality, and density-dependent mortality in determining local population size. Increased habitat complexity was associated with a reduction in both density-independent and density-dependent mortality. At low levels of habitat complexity, limitation analysis revealed that mortality was strong and recruitment had relatively little influence on population size. However, as habitat complexity increased, recruitment became more important. At the highest levels of habitat complexity, limitation by recruitment was substantial, although density-dependent mortality was ultimately the largest constraint on population size. In high-complexity habitats, population dynamics may strongly reflect variation in recruitment even though fluctuations may be dampened by density-dependent mortality. By affecting both density-independent and density-dependent mortality, variation in habitat complexity may result in qualitative changes in the dynamics of populations. These findings suggest that the relative importance of pre- vs. post-settlement factors may be determined by quantifiable habitat features, rather than ambient recruitment level alone. Because the magnitude of recruitment fluctuations can affect species coexistence and the persistence of populations, habitat-driven changes in population dynamics may have important consequences for both community structure and population viability.

Plants as reef fish: fitting the functional form of seedling recruitment

The life histories of many species depend first on dispersal to local sites and then on establishment. After dispersal, density-independent and density-dependent mortalities modify propagule supply, determining the number of individuals that establish. Because multiple factors influence recruitment, the dichotomy of propagule versus establishment limitation is best viewed as a continuum along which the strength of propagule or establishment limitation changes with propagule input. To evaluate the relative importance of seed and establishment limitation for plants, we (1) describe the shape of the recruitment function and (2) use limitation and elasticity analyses to quantify the sensitivity of recruitment to perturbations in seed limitation and density-independent and density-dependent mortality. Using 36 seed augmentation studies for 18 species, we tested four recruitment functions against one another. Although the linear model (accounting for seed limitation and density-independent mortality) fitted the largest number of studies, the nonlinear Beverton-Holt model (accounting for density-dependent mortality) performed better at high densities of seed augmentation. For the 18 species, seed limitation constrained population size more than other sources of limitation at ambient conditions. Seedling density reached saturation with increasing seed density in many studies, but at such high densities that seedling density was primarily limited by seed availability rather than microsite availability or density dependence.

THE SCALE AND CAUSE OF SPATIAL HETEROGENEITY IN STRENGTH OF TEMPORAL DENSITY DEPENDENCE

The importance of density dependence in natural communities continues to spark much debate because it is fundamental to population regulation. We used temporal manipulations of density to explore potentially stabilizing density dependence in early survivorship among six local populations of a tropical damselfish (Dascyllus flavicaudus). Specifically, we tested the premise that spatial heterogeneity in the strength of temporal density dependence would reflect variation in density of predators, the agent of mortality. Our field manipulations revealed that mortality among successive cohorts of young fishes was density dependent at each reef, but that its strength varied by approximately 1.5 orders of magnitude. This spatial heterogeneity was well predicted by variation among the six reefs in the density of predatory fishes that consume juvenile damselfishes. Because density dependence arose from competition for enemy-free space within a shelter coral, the mortality consequence of the competition depended on the neighborhood density of predators. Thus, the scale of heterogeneity in the density dependence largely reflected attributes of the environment that shaped the local abundance of predators. These results have important implications for how ecologists explore regulatory processes in nature. Failure to account for spatial variation could frequently yield misleading conclusions regarding density dependence as a stabilizing process, obscure underlying mechanisms influencing its strength, and provide no insight into the spatial scale of the heterogeneity. Further, models of population dynamics will be improved when experimental approaches better estimate the magnitude and causes of variation in strength of stabilizing density dependence.

Complex interplay between intrinsic and extrinsic drivers of long-term survival trends in southern elephant seals

Background

Determining the relative contribution of intrinsic and extrinsic factors to fluctuations in population size, trends and demographic composition is analytically complex. It is often only possible to examine the combined effects of these factors through measurements made over long periods, spanning an array of population densities or levels of food availability. Using age-structured mark-recapture models and datasets spanning five decades (1950–1999), and two periods of differing relative population density, we estimated age-specific probabilities of survival and examined the combined effects of population density and environmental conditions on juvenile survival of southern elephant seals at Macquarie Island.

Results

First-year survival decreased with density during the period of highest population size, and survival increased during years when the Southern Oscillation Index (SOI) anomaly (deviation from a 50-year mean) during the mother's previous foraging trip to sea was positive (i.e., El Niño). However, when environmental stochasticity and density were considered together, the effect of density on first-year survival effectively disappeared. Ignoring density effects also leads to models placing too much emphasis on the environmental conditions prevailing during the naïve pup's first year at sea.

Conclusion

Our analyses revealed that both the state of the environment and population density combine to modify juvenile survival, but that the degree to which these processes contributed to the variation observed was interactive and complex. This underlines the importance of evaluating the relative contribution of both the intrinsic and extrinsic factors that regulate animal populations because false conclusions regarding the importance of population regulation may be reached if they are examined in isolation.

Variation in and correlation between intrinsic rate of increase and carrying capacity

Intrinsic population growth rate and density dependence are fundamental components of population dynamics. Theory suggests that variation in and correlations between these parameters among patches within a population can influence overall population size, but data on the degree of variation and correlation are rare. Replicate populations of a specialist aphid (Chaetosiphon fragaefolii) were followed on 11 genotypes of host plant (Fragaria chiloensis) in the greenhouse. Population models fit to these census data provide estimates of intrinsic growth rate and carrying capacity for aphid populations on each plant genotype. Growth rate and carrying capacity varied substantially among plant genotypes, and these two parameters were not significantly correlated. These results support the existence of spatial variation in population dynamic parameters; data on frequency distributions and correlations of these parameters in natural populations are needed for evaluation of the importance of variation in growth rate and density dependence for population dynamics in the field.

Density dependence in marine fish populations revealed at small and large spatial scales

Experimental manipulation of population density has frequently been used to demonstrate demographic density dependence. However, such studies are usually small scale and typically provide evidence of spatial (within-generation) density dependence. It is often unclear whether small-scale, experimental tests of spatial density dependence will accurately describe temporal (between-generation) density dependence required for population regulation. Understanding the mechanisms generating density dependence may provide a link between spatial experiments and temporal regulation of populations. In this study, I manipulated the density of recently settled kelp rockfish (Sebastes atrovirens) in both the presence and absence of predators to test for density-dependent mortality and whether predation was the mechanism responsible. I also examined mortality of rockfish cohorts within kelp beds throughout central California to evaluate temporal (between-generation) density dependence in mortality. Experiments suggested that short-term behavioral responses of predators and/or a shortage of prey refuges caused spatial density dependence. Temporal density dependence in mortality was also detected at larger spatial scales for several species of rockfish. It is likely that short-term responses of predators generated both spatial and temporal density dependence in mortality. Spatial experiments that describe the causal mechanisms generating density dependence may therefore be valuable in describing temporal density dependence and population regulation.

Allee effects and conspecific cueing jointly lead to conspecific attraction

Conspecific attraction is the preferential settlement into habitat patches with conspecifics. To be a good proximate strategy, fitness gains from settling with conspecifics must outweigh the costs of higher conspecific densities, such as intraspecific competition. Two types of benefits have been proposed to explain conspecific attraction: Allee effects (i.e., positive density dependence) and conspecific cueing (using conspecifics as an indicator of habitat quality). I present empirical evidence for conspecific attraction in the settlement of the porcelain crab, Petrolisthes cinctipes Randall (Anomura: Porcellanidae). Previous work demonstrated that P. cinctipes experiences strong intraspecific competition and that both Allee effects and conspecific cueing are present in P. cinctipes life-history. I developed an empirically-based fitness model of the costs and benefits of settling with conspecifics. Based on this model, I simulated optimal settlement to habitat patches that varied in conspecific density and habitat quality, where the correlation between density and habitat quality determined the level of conspecific cueing. I tested whether Allee effects alone, conspecific cueing alone, or Allee effects and conspecific cueing together could provide an ultimate explanation for the proximate settlement behavior of P. cinctipes. The settlement simulation was consistent with empirical settlement only when Allee effects and conspecific cueing were both included. Three life-history features are critical to this conclusion: (1) fitness is maximized at intermediate density, (2) fitness depends on the decisions of previous settlers, and (3) conspecific density provides good information about habitat quality. The quality of information garnered from conspecifics determines whether conspecific attraction is a good proximate strategy for settlement. I present a graphical illustration demonstrating how Allee effects and conspecific cueing work together to influence fitness, providing a conceptual framework for other systems.

PREDATION, HABITAT COMPLEXITY, AND VARIATION IN DENSITY-DEPENDENT MORTALITY OF TEMPERATE REEF FISHES

Density dependence in demographic rates can strongly affect the dynamics of populations. However, the mechanisms generating density dependence (e.g., predation) are also dynamic processes and may be influenced by local conditions. Understanding the manner in which local habitat features affect the occurrence and/or strength of density dependence will increase our understanding of population dynamics in heterogeneous environments. In this study I conducted two separate field experiments to investigate how local predator density and habitat complexity affect the occurrence and form of density-dependent mortality of juvenile rockfishes (Sebastes spp.). I also used yearly censuses of rockfish populations on nearshore reefs throughout central California to evaluate mortality of juvenile rockfish at large spatial scales. Manipulations of predators (juvenile bocaccio, S. paucispinus) and prey (kelp, gopher, and black-and-yellow [KGB] rockfish, Sebastes spp.) demonstrated that increasing the density of predators altered their functional response and thus altered patterns of density dependence in mortality of their prey. At low densities of predators, the number of prey consumed per predator was a decelerating function, and mortality of prey was inversely density dependent. However, at high densities of predators, the number of prey killed per predator became an accelerating response, and prey mortality was directly density dependent. Results of field experiments and large-scale surveys both indicated that the strength of density-dependent mortality may also be affected by the structural complexity of the habitat. In small-scale field experiments, increased habitat complexity increased the strength of density-dependent mortality. However, at large scales, increasing complexity resulted in a decrease in the strength of density dependence. I suggest that these differences resulted from scale-dependent changes in the predatory response that generated mortality. Whether increased habitat complexity leads to an increase or a decrease in the strength of density-dependent mortality may depend on how specific predatory responses (e.g., functional or aggregative) are altered by habitat complexity. Overall, the findings of this study suggest that rates of demographic density dependence and the resulting dynamics of local populations may largely depend upon attributes of the local habitat.

Density-dependent habitat selection and performance by a large mobile reef fish

Many exploited reef fish are vulnerable to overfishing because they concentrate over hard-bottom patchy habitats. How mobile reef fish use patchy habitat, and the potential consequences on demographic parameters, must be known for spatially explicit population dynamics modeling, for discriminating essential fish habitat (EFH), and for effectively planning conservation measures (e.g., marine protected areas, stock enhancement, and artificial reefs). Gag, Mycteroperca microlepis, is an ecologically and economically important warm-temperate grouper in the southeastern United States, with behavioral and life history traits conducive to large-scale field experiments. The Suwannee Regional Reef System (SRRS) was built of standard habitat units (SHUs) in 1991-1993 to manipulate and control habitat patchiness and intrinsic habitat quality, and thereby test predictions from habitat selection theory. Colonization of the SRRS by gag over the first six years showed significant interactions of SHU size, spacing, and reef age; with trajectories modeled using a quadratic function for closely spaced SHUs (25 m) and a linear model for widely spaced SHUs (225 m), with larger SHUs (16 standardized cubes) accumulating significantly more gag faster than smaller 4-cube SHUs (mean = 72.5 gag/16-cube SHU at 225-m spacing by year 6, compared to 24.2 gag/4-cube SHU for same spacing and reef age). Residency times (mean = 9.8 mo), indicative of choice and measured by ultrasonic telemetry (1995-1998), showed significant interaction of SHU size and spacing consistent with colonization trajectories. Average relative weight (W(r)) and incremental growth were greater on smaller than larger SHUs (mean W(r) = 104.2 vs. 97.7; incremental growth differed by 15%), contrary to patterns of abundance and residency. Experimental manipulation of shelter on a subset of SRRS sites (2000-2001) confirmed our hypothesis that shelter limits local densities of gag, which, in turn, regulates their growth and condition. Density-dependent habitat selection for shelter and individual growth dynamics were therefore interdependent ecological processes that help to explain how patchy reef habitat sustains gag production. Moreover, gag selected shelter at the expense of maximizing their growth. Thus, mobile reef fishes could experience density-dependent effects on growth, survival, and/or reproduction (i.e., demographic parameters) despite reduced stock sizes as a consequence of fishing.