The predation gauntlet: early post-settlement mortality in reef fishes

Most marine fishes have pelagic larvae that settle to benthic juvenile/adult habitats. Ecologists have argued that mortality rates are particularly high during the settlement transition, but relevant data have been sparse. Recently, researchers have used several novel techniques to estimate the magnitude of predation mortality during the settlement transition. We used meta-analysis to determine that for 24 taxonomically diverse species in geographically widespread locations, an estimated 55.7% (CI: 43.0–65.5%) of juveniles were consumed within 1–2 days of settlement. Such high mortality highlights this brief period as a key phase in the life history of fishes and supports the view that these communities are strongly influenced by predation. Additionally, we argue that because predators have such strong effects on juvenile survival, the population and community dynamics of reef fishes may be linked to human exploitation of reef predators.     

Population dynamics of coral-reef fishes: Controversial concepts and hypotheses

Abstract Knowledge of processes that drive the local population dynamics of coral-reef fishes is important for managing reef fisheries, and for using these species as models for understanding the ecology of demersal marine fishes in general. However, the reef-fish literature is replete with poorly defined concepts and vague hypotheses regarding the issue of population dynamics. Dichotomous arguments, such as whether or not recruitment drives population dynamics, are misdirected because they fail to incorporate several important concepts. First, changes in local population size are driven by four demographic rates (birth, death, immigration and emigration), all of which must be studied to understand population dynamics. Second, all populations that persist do so because at least one of these demographic rates operates in a density-dependent way that is both sufficiently strong and appropriately time-lagged. Therefore, identifying the source(s) of direct density dependence is critical for understanding the limits to variation in population size (i.e. population regulation). Third, regulation does not imply a simple point equilibrium in population size; density dependence in populations of reef fishes is bound to lie within a field of stochastic variation, and thus be difficult to detect. Since its formal origin in 1981, the ‘recruitment limitation’ hypothesis for explaining local population dynamics in reef fishes has undergone ambiguous changes in definition that threaten its usefulness. ‘Recruitment, ‘originally defined as the appearance of newly settled fish on a reef, more recently is often measured months after settlement, thus confounding pre- and post-settlement processes. ‘Limitation, ‘ which originally referred to recruitment being so low as to preclude local populations from reaching densities where resources were limiting, is more recently defined as an absence of any form of density dependence after settlement. The most effective means of testing whether post-settlement mortality is in fact density-independent is to examine patterns of mortality directly, rather than indirectly by interpreting the shape of the relationship between initial recruit density and subsequent adult density within a cohort (the recruit-adult function). Understanding the population dynamics of coral-reef fishes will require a more equitable focus on all four demographic rates, be they density dependent or not, as well as greater attention to identifying sources of density dependence. Such a pluralistic focus necessitates integrated studies of both pre- and post-settlement processes conducted at multiple spatial and temporal scales. For example, recent evidence suggests that density-dependent pre-dation on new recruits that have settled among reefs at different densities may prove to be an important source of local population regulation, especially via the aggregative response of transient piscivores.     

A key phase in the recruitment dynamics of coral reef fishes: post-settlement transition

Synopsis Recent studies of recruitment dynamics in demersal fishes have placed major emphasis on presettlement mortality, and little on events bridging late larval and early juvenile periods. Observations on 68 taxa of Caribbean coral reef fishes before and during settlement revealed the existence of a distinct post-settlement life phase called the transition juvenile, associated with the act of recruitment. Transition juveniles were found as solitary individuals, in conspecific groups, or in heterospecific groups. The groups were either uniform or heterogenous in appearance. The complexity of the transition phase and its apparently widespread occurrence in coral reef fishes suggests that important aspects of population structure may be determined between settlement and first appearance as a full-fledged juvenile.     

Population Dynamics and Spatial Distribution of Coral Reef Fishes: Comparison Between Continuous and Isolated Habitats

Recent studies have shown that there are high degrees of spatial and temporal stability in coral reef fish assemblage structures in a continuous habitat, in contrast to results of observations in isolated habitats. In order to determine the reason for the difference in temporal stability of fish assemblage structures in a continuous habitat site and an isolated habitat site, population dynamics and spatial distributions of coral reef fishes (six species of pomacentrids and two species of apogonids) in the two habitat site were investigated over a 2-year period in an Okinawan coral reef. The population densities of pomacentrid and apogonid species increased in juvenile settlement periods at both sites, but the magnitude of seasonal fluctuation in population density was significantly greater at the isolated habitat site, indicating that the rate of juvenile settlement and mortality rate in the isolated habitat were greater than those in the continuous habitat. The magnitude of aggregation of fishes, which affects density-dependent biological interactions that modify population density such as competition and predation, was also significantly greater at the isolated habitat site, especially in the juvenile settlement season. Most of the fishes at the isolated habitat site exhibited more generalized patterns of microhabitat selection because of less coral coverage and diversity. The seasonal stability in the species composition of fishes was greater at the continuous habitat site than that at the isolated habitat. Our findings suggest that the relative importance of various ecological factors responsible for regulation of the population density of coral reef fishes (e.g., competition, predation, microhabitat selection and post-settlement movement) in a continuous habitat site and the isolated habitat site are different.     

Risky Business: Temporal and Spatial Variation in Preindustrial Dryland Agriculture

Traditional dryland agriculture in the Pacific island was often labor-intensive and risky, yet settlement and farming in dry areas played an important role in the development of Polynesian societies. We investigate how temporal and spatial climatic fluctuations shape variation in agricultural production across dryland landscapes. We use a model that couples plant growth, climate, and soil organic matter dynamics, together with data from Kohala, Hawai'i, to understand how temperature, rainfall, nitrogen availability, and cropping activity interact to determine yield dynamics through time and space. Due to these interactions, the statistical characterization of rainfall alone is a poor characterization of agricultural yield. Using a simple linear model of human population dynamics, we show that the observed yield variation can affect long-term population growth substantially. Our approach to analyzing spatial and temporal fluctuations in food supply, and to interpreting the population consequences of these fluctuations, provides a quantitative evaluation of agricultural risk and human carrying capacity in dry regions.     

From fish to fashion: experimental and theoretical insights into the evolution of culture

Recent years have witnessed a re-evaluation of the cognitive capabilities of fishes, including with respect to social learning. Indeed, some of the best experimental evidence for animal traditions can be found in fishes. Laboratory experimental studies reveal that many fishes acquire dietary, food site and mating preferences, predator recognition and avoidance behaviour, and learn pathways, through copying other fishes. Concentrating on foraging behaviour, we will present the findings of laboratory experiments that reveal social learning, behavioural innovation, the diffusion of novel behaviour through populations and traditional use of food sites. Further studies reveal surprisingly complex social learning strategies deployed by sticklebacks. We will go on to place these observations of fish in a phylogenetic context, describing in which respects the learning and traditionality of fish are similar to, and differ from, that observed in other animals. We end by drawing on theoretical insights to suggest processes that may have played important roles in the evolution of the human cultural capability.     

The short‐lived neon damsel Pomacentrus coelestis: implications for population dynamics

Daily increments of Pomacentrus coelestis, an abundant and well‐studied fish, were validated for the life of the fish and depending on the location, age‐maxima were estimated to be 127–160 days on reefs separated by tens to hundreds of kilometres on the Great Barrier Reef. This contrasts with congeners and other damselfishes that live for 5 years or more. Otoliths of P. coelestis were thinner and had different patterns of banding when compared with relatively long‐lived congeners. It is suggested that banding patterns in P. coelestis may be related to patterns of maturation and spawning. The consequences of a short life would have a great influence on the population dynamics of this widespread species. Further, the demographics and habitat preferences of this species suggest rapid colonization and establishment of breeding populations that would quickly change the relative abundance of sympatric fishes.     

Depth distributions of coral reef fishes: the influence of microhabitat structure,settlement, and post-settlement processes

Many coral reef fishes have restricted depth ranges that are established at settlement or soon after, but the factors limiting these distributions are largely unknown. This study examines whether the availability of microhabitats (reef substrata) explains depth limits, and evaluates whether juvenile growth and survival are lower beyond these limits. Depth-stratified surveys of reef fishes at Kimbe Bay (Papua New Guinea) showed that the abundance of new settlers and the cover of coral substrata differed significantly among depths. A field experiment investigated whether settling coral reef fishes preferred particular depths, and whether these depth preferences were dependent on microhabitat. Small patch reefs composed of identical coral substrata were set up at five depths (3, 6, 10, 15 and 20 m), and settlement patterns were compared to those on unmanipulated reef habitat at the same five depths. For all species, settlement on patch reefs differed significantly among depths despite uniform substratum composition. For four of the six species tested, depth-related settlement patterns on unmanipulated habitat and on patch reefs did not differ, while for the other two, depth ranges were greater on the patch reefs than on unmanipulated habitat. A second experiment examined whether depth preferences reflected variation in growth and survival when microhabitat was similar. Newly settled individuals of Chrysiptera parasema and Dascyllus melanurus were placed, separately, on patch reefs at five depths (as above) and their survival and growth monitored. D. melanurus, which is restricted to shallow depths, had highest survival and growth at the shallowest depth. Depth did not affect either survival or growth of C. parasema, which has a broader depth range than D. melanurus (between 6 and 15 m). This suggests that the fitness costs potentially incurred by settling outside a preferred depth range may depend on the strength of the depth preference.     

Paleogenetic records of Daphnia pulicaria in two North American lakes reveal the impact of cultural eutrophication

Understanding the evolutionary consequences of the green revolution, particularly in wild populations, is an important frontier in contemporary biology. Because human impacts have occurred at varying magnitudes or time periods depending on the study ecosystem, evolutionary histories may vary considerably among populations. Paleogenetics in conjunction with paleolimnology enable us to associate microevolutionary dynamics with detailed information on environmental change. We used this approach to reconstruct changes in the temporal population genetic structure of the keystone zooplankton grazer, Daphnia pulicaria, using dormant eggs extracted from sediments in two Minnesota lakes (South Center, Hill). The extent of agriculture and human population density in the catchment of these lakes has differed markedly since European settlement in the late 19th century and is reflected in their environmental histories reconstructed here. The reconstructed environments of these two lakes differed strongly in terms of environmental stability and their associated patterns of Daphnia population structure. We detected long periods of stability in population structure and environmental conditions in South Center Lake that were followed by a dramatic temporal shift in population genetic structure after the onset of European settlement and industrialized agriculture in its watershed. In particular, we noted a 24.3‐fold increase in phosphorus (P) flux between pre‐European and modern sediment P accumulation rates (AR) in this lake. In contrast, no such shifts were detected in Hill Lake, where the watershed was not as impacted by European settlement and rates of change were less directional with a much smaller increase in sediment P AR (2.3‐fold). We identify direct and indirect effects of eutrophication proxies on genetic structure in these lake populations and demonstrate the power of using this approach in understanding the consequences of anthropogenic environmental change on natural populations throughout historic time periods.     

The significance of variable and density-independent post-recruitment mortality in local populations of reef fishes

Abstract In this paper I focus on how post-settlement mortality may modify initial patterns of settlement in reef fish. Infrequent recruitment surveys may underestimate the role of early post-settlement mortality as most mortality in reef fishes occurs shortly after settlement. Consequently, results from infrequent recruitment surveys shed little light on the mechanisms producing patterns of abundance because these surveys ignore early post-settlement mortality. Variation in density-independent mortality may be a common mechanism that can prevent a positive relationship between larval settlement and subsequent population abundance. Although density-dependent mortality is the most commonly recognized mechanism that can disrupt the correlation between settlement and adult abundance, density-independent mortality’ can also destroy this correlation if the variance associated with post-settlement mortality is greater than variance in settlement. This point is illustrated with a simulation model in which I modelled two populations: a piscivorous fish population that was recruitment-limited with constant mortality, and a prey population that had variable recruitment and mortality that was a function of the size of the predator population. The results of this model indicate that even when mortality of prey is density-independent, predation can determine prey abundance when variation in piscivore recruitment is high relative to prey recruitment. Thus, initial patterns of prey settlement can be modified by a recruitment-limited predator population.     

Can cytoplasmic incompatibility inducing Wolbachia promote the evolution of mate preferences?

The maternally inherited bacterium, Wolbachia pipientis, manipulates host reproduction by rendering uninfected females reproductively incompatible with infected males (cytoplasmic incompatibility, CI). Hosts may evolve mechanisms, such as mate preferences, to avoid fitness costs of Wolbachia infection. Despite the potential importance of mate choice for Wolbachia population dynamics, this possibility remains largely unexplored. Here we model the spread of an allele encoding female mate preference for uninfected males alongside the spread of CI inducing Wolbachia. Mate preferences can evolve but the spread of the preference allele depends on factors associated with both Wolbachia infection and the preference allele itself. Incomplete maternal transmission of Wolbachia, fitness costs and low CI, improve the spread of the preference allele and impact on the population dynamics of Wolbachia. In addition, mate preferences are found in infected individuals. These results have important consequences for the fate of Wolbachia and studies addressing mate preferences in infected populations.     

Seasonality in ecology: Progress and prospects in theory

Seasonality is an important feature of essentially all natural systems but the consequences of seasonality have been vastly underappreciated. Early work emphasized the role of seasonality in driving cyclic population dynamics, but the consequences of seasonality for ecological processes are far broader. Yet, seasonality is often not explicitly included in either empirical or theoretical studies. Many aspects of ecological dynamics can only be understood when seasonality is included, ranging from the oscillations in the incidence of childhood diseases to the coexistence of species. Through several case studies, we outline what is now known about seasonality in an ecological context and set the stage for future efforts. We discuss various approaches and tools for incorporating seasonality in mathematical models. We argue, however, that these tools are still limited in scope and more easily-accessible approaches need to be developed.     

Parasites of coral reef fish larvae: its role in the pelagic larval stage

The pelagic larval stage is a critical component of the life cycle of most coral reef fishes, but the adaptive significance of this stage remains controversial. One hypothesis is that migrating through the pelagic environment reduces the risk a larval fish has of being parasitised. Most organisms interact with parasites, often with significant, detrimental consequences for the hosts. However, little is known about the parasites that larval fish have upon settlement, and the factors that affect the levels of parasitism. At settlement, coral reef fishes vary greatly in size and age (pelagic larval duration), which may influence the degree of parasitism. We identified and quantified the parasites of pre-settlement larvae from 44 species of coral reef fishes from the Great Barrier Reef and explored their relationship with host size and age at settlement, and phylogeny. Overall, less than 50% of the larval fishes were infected with parasites, and over 99% of these were endoparasites. A Bayesian phylogenetic regression was used to analyse host-parasite (presence and intensity) associations. The analysis showed parasite presence was not significantly related to fish size, and parasite intensity was not significantly related to fish age. A phylogenetic signal was detected for both parasite presence and intensity, indicating that, overall, closely related fish species were likely to have more similar susceptibility to parasites and similar levels of parasitism when compared to more distantly related species. The low prevalence of infection with any parasite type and the striking rarity of ectoparasites is consistent with the ‘parasite avoidance hypothesis’, which proposes that the pelagic phase of coral reef fishes results in reduced levels of parasitism.

     

The non-linear relationship between body size and function in parrotfishes

Parrotfishes are a group of herbivores that play an important functional role in structuring benthic communities on coral reefs. Increasingly, these fish are being targeted by fishermen, and resultant declines in biomass and abundance may have severe consequences for the dynamics and regeneration of coral reefs. However, the impact of overfishing extends beyond declining fish stocks. It can also lead to demographic changes within species populations where mean body size is reduced. The effect of reduced mean body size on population dynamics is well described in literature but virtually no information exists on how this may influence important ecological functions. The study investigated how one important function, scraping (i.e., the capacity to remove algae and open up bare substratum for coral larval settlement), by three common species of parrotfishes (Scarus niger, Chlorurus sordidus, and Chlorurus strongylocephalus) on coral reefs at Zanzibar (Tanzania) was influenced by the size of individual fishes. There was a non-linear relationship between body size and scraping function for all species examined, and impact through scraping was also found to increase markedly when fish reached a size of 15–20 cm. Thus, coral reefs which have a high abundance and biomass of parrotfish may nonetheless be functionally impaired if dominated by small-sized individuals. Reductions in mean body size within parrotfish populations could, therefore, have functional impacts on coral reefs that previously have been overlooked.     

Habitat selection as a source of inter‐specific differences in recruitment of two diadromous fish species

• 1 For aquatic species with highly dispersive offspring, the addition of new individuals into an area (recruitment) is a key process in determining local population size so understanding the causes of recruitment variability is critical. While three general causative mechanisms have been identified (the supply of individuals, habitat selection and mortality), we have a limited understanding of how variation in each is generated, and the consequences this may have for the spatial and temporal distribution of recruits.
• 2 We examined whether active habitat selection during settlement could be the cause of variability in populations of two diadromous fish species using a field survey and laboratory‐based choice experiments. If larval behaviour is important, we predicted there would be inter‐specific differences in abundance between sites during the survey, and that larvae would prefer water collected from sites with higher conspecific abundances during the experiments.
• 3 During the field survey, significant differences were detected between two rivers (the Cumberland and Grey), with one species (Galaxias maculatus) found in higher abundances at one site (the Cumberland River) while comparable numbers of a closely related species (Galaxias brevipinnis) were caught at both sites. Laboratory choice experiments were conducted to determine whether larval preferences during settlement could be the cause of these differences. G. maculatus larvae showed a preference for freshwater over saltwater, indicating that the fish may be responding to reduced salinities around river mouths during settlement. The results of a second experiment were consistent with the notion that larval preferences could be the mechanism driving differences in the populations of the two rivers, with G. maculatus preferring water collected from the Cumberland River while G. brevipinnis did not prefer water from either river.
• 4 These results demonstrate that active habitat selection may be important in establishing spatial patterns of larvae at settlement, and that multiple cues are likely to be involved. This study also demonstrates that the behaviours exhibited by individuals can strongly influence the structure and dynamics of populations of aquatic species with complex life cycles.

     

Talkin' 'bout my generation: environmental variability and cohort effects

In variable environments, it is probable that environmental conditions in the past can influence demographic performance now. Cohort effects occur when these delayed life-history effects are synchronized among groups of individuals in a population. Here we show how plasticity in density-dependent demographic traits throughout the life cycle can lead to cohort effects and that there can be substantial population dynamic consequences of these effects. We show experimentally that density and food conditions early in development can influence subsequent juvenile life-history traits. We also show that conditions early in development can interact with conditions at maturity to shape future adult performance. In fact, conditions such as food availability and density at maturity, like conditions early in development, can generate cohort effects in mature stages. Based on these data, and on current theory about the effects of plasticity generated by historical environments, we make predictions about the consequences of such changes on density-dependent demography and on mite population dynamics. We use a stochastic cohort effects model to generate a range of population dynamics. In accordance with the theory, we find the predicted changes in the strength of density dependence and associated changes in population dynamics and population variability.     

Strong density-dependent survival and recruitment regulate the abundance of a coral reef fish

Debate on the control of population dynamics in reef fishes has centred on whether patterns in abundance are determined by the supply of planktonic recruits, or by post-recruitment processes. Recruitment limitation implies little or no regulation of the reef-associated population, and is supported by several experimental studies that failed to detect density dependence. Previous manipulations of population density have, however, focused on juveniles, and there have been no tests for density-dependent interactions among adult reef fishes. I tested for population regulation in Coryphopterus glaucofraenum, a small, short-lived goby that is common in the Caribbean. Adult density was manipulated on artificial reefs and adults were also monitored on reefs where they varied in density naturally. Survival of adult gobies showed a strong inverse relationship with their initial density across a realistic range of densities. Individually marked gobies, however, grew at similar rates across all densities, suggesting that density-dependent survival was not associated with depressed growth, and so may result from predation or parasitism rather than from food shortage. Like adult survival, the accumulation of new recruits on reefs was also much lower at high adult densities than at low densities. Suppression of recruitment by adults may occur because adults cause either reduced larval settlement or reduced early post-settlement survival. In summary, this study has documented a previously unrecorded regulatory mechanism for reef fish populations (density-dependent adult mortality) and provided a particularly strong example of a well-established mechanism (density-dependent recruitment). In combination, these two compensatory mechanisms have the potential to strongly regulate the abundance of this species, and rule out the control of abundance by the supply of recruits.     

Floater survival affects population persistence. The role of prey availability and environmental stochasticity

We develop two individual-based models using a large and detailed data set (information gathered over more than a century) on a population of a longlived and territorial predator, the Spanish imperial eagle. We investigated the relationship between survival and predator pressure, prey behaviour and patch availability (i.e. settlement areas). Survival of dispersing individuals was highly dependent on the number of available settlement areas, mediated by prey availability. Changes in prey behaviour due to predation pressure (e.g. shifting from diurnal to nocturnal activity) can decrease their availability for predators even if the density significantly exceeds the predator needs. Environmental stochasticity had a strong influence on population viability when it occurred in a synchroneous way between breeding and settlement areas, and an increase in floater mortality negatively influenced stability and dynamics of the breeding segment of populations in reproductive areas. Our simulations demonstrated the link between the dynamics in settlement and breeding areas: factors affecting floater survival also influence whole population dynamics. Moreover, model outputs provided insights into the relationship between environmental stochasticity and population dynamics.     

Maternal effects, paternal effects and sexual selection

Maternal and paternal effects can lead to complicated evolutionary dynamics, including evolution in the opposite direction to selection. Recent studies demonstrate that parental effects on sexually selected traits, as well as preferences for those traits, might be large. Although these findings are likely to have consequences for both the evolutionary dynamics and equilibria of sexual selection, theory is lacking. Because parents are expected to maximize their own fitness, rather than that of a specific offspring, the magnitude (and even direction) of parental effects are context dependent. By extension, this dynamic nature of parental effects might help to explain the maintenance of variation in many sexually selected traits.     

Agent-based models to investigate sound impact on marine animals: bridging the gap between effects on individual behaviour and population level consequences

Marine ambient sound levels have risen due to noisy human activities, such as shipping, fishing, seismic surveys and piling for windfarms. Marine mammals and fishes are two prominent taxonomic groups that are exposed to this noise pollution, which may experience detrimental effects at the population level. Acoustic effects on individual behaviour such as deterrence, disturbance, distraction and masking of biologically relevant sounds, can be translated energetically to changes in vital rates (growth, maturation, reproduction and survival) in a population consequences of acoustic disturbance (PCAD) approach. However, we typically neglect spatial variation in species distributions and noise pollution, while abiotic factors like temperature, bathymetry and currents, as well as habitat quality in terms of feeding or hiding opportunities, will also have a geographically variable impact on potential consequences. We here address the conceptual integration of agent based models (ABM) into the PCAD framework, as a suitable theoretical tool with high potential for the exploration of these spatial factors and their modifying role in noise impact assessment studies. We review five ABM case studies, including investigations into: 1) effects of movement strategy on the impact of explosions in harbour porpoise; 2) effects of disturbance sensitivity on pile driving impact on migrating cod; 3) impact of seismic survey sounds on Atlantic mackerel distribution and movement; 4) population-level impact of mitigation of harbour porpoise bycatch with pingers; and 5) population effects of alternative windfarm construction scenarios in harbour porpoise. We discuss similarities and differences among these studies in sound and species mapping approaches and we evaluate model realism and pattern validation. We believe that ABMs are a valuable tool for integrating spatial information into ecological impact studies that investigate acoustic disturbance, for any type of sound source, and for both marine mammals and fish.