Invading introduced species in insular heterogeneous environments

We are concerned with the development and analysis of a predator–prey system designed for heterogeneous insular environments; populations are native preys exposed to introduced and invading predators and competitor preys. We first look at the unstructured model; this yields a singular system of ordinary differential equations having interesting dynamical features, such as finite time extinction or persistence of populations. Next we build a spatially heterogeneous structured model upon developing a reaction–diffusion system; then, using numerical experiments we analyze some typical effects of spatial heterogeneities on the persistence or extinction of native or introduced and invading species. The case of Kerguelen sub-Antartic heterogeneous islands where both domestic cats and alien preys have been introduced is taken as an example.     

Predator hunting mode and habitat domain alter nonconsumptive effects in predator-prey interactions

Predators can affect prey populations through changes in traits that reduce predation risk. These trait changes (nonconsumptive effects, NCEs) can be energetically costly and cause reduced prey activity, growth, fecundity, and survival. The strength of nonconsumptive effects may vary with two functional characteristics of predators: hunting mode (actively hunting, sit-and-pursue, sit-and-wait) and habitat domain (the ability to pursue prey via relocation in space; can be narrow or broad). Specifically, cues from fairly stationary sit-and-wait and sit-and-pursue predators should be more indicative of imminent predation risk, and thereby evoke stronger NCEs, compared to cues from widely ranging actively hunting predators. Using a meta-analysis of 193 published papers, we found that cues from sit-and-pursue predators evoked stronger NCEs than cues from actively hunting predators. Predator habitat domain was less indicative of NCE strength, perhaps because habitat domain provides less reliable information regarding imminent risk to prey than does predator hunting mode. Given the importance of NCEs in determining the dynamics of prey communities, our findings suggest that predator characteristics may be used to predict how changing predator communities translate into changes in prey. Such knowledge may prove particularly useful given rates of local predator change due to habitat fragmentation and the introduction of novel predators.     

Consumptive and nonconsumptive effects of predators on metacommunities of competing prey

Although predators affect prey both via consumption and by changing prey migration behavior, the interplay between these two effects is rarely incorporated into spatial models of predator-prey dynamics and competition among prey. We develop a model where generalist predators have consumptive effects (i.e., altering the likelihood of local prey extinction) as well as nonconsumptive effects (altering the likelihood of colonization) on spatially separated prey populations (metapopulations). We then extend this model to explore the effects of predators on competition among prey. We find that generalist predators can promote persistence of prey metapopulations by promoting prey colonization, but predators can also hasten system-wide extinction by either increasing local extinction or reducing prey migration. By altering rates of prey migration, predators in one location can exert remote control over prey dynamics in another location via predator-mediated changes in prey flux. Thus, the effect of predators may extend well beyond the proportion of patches they visit. In the context of prey metacommunities, predator-mediated shifts in prey migration and mortality can shift the competition-colonization trade-off among competing prey, leading to changes in the prey community as well as changes in the susceptibility of prey species to habitat loss. Consequently, native prey communities may be susceptible to invasion not only by exotic prey species that experience reduced amounts of mortality from resident predators, but also by exotic prey species that exhibit strong dispersal in response to generalist native predators. Ultimately, our work suggests that the consumptive and nonconsumptive effects of generalist predators may have strong, yet potentially cryptic, effects on competing prey capable of mediating coexistence, fostering invasion, and interacting with anthropogenic habitat alteration.     

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.     

The growth-predation risk trade-off under a growing gape-limited predation threat

Growth is a critical ecological trait because it can determine population demography, evolution, and community interactions. Predation risk frequently induces decreased foraging and slow growth in prey. However, such strategies may not always be favored when prey can outgrow a predator's hunting ability. At the same time, a growing gape-limited predator broadens its hunting ability through time by expanding its gape and thereby creates a moving size refuge for susceptible prey. Here, I explore the ramifications of growing gape-limited predators for adaptive prey growth. A discrete demographic model for optimal foraging/growth strategies was derived under the realistic scenario of gape-limited and gape-unconstrained predation threats. Analytic and numerical results demonstrate a novel fitness minimum just above the growth rate of the gape-limited predator. This local fitness minimum separates a slow growth strategy that forages infrequently and accumulates low but constant predation risk from a fast growth strategy that forages frequently and experiences a high early predation risk in return for lower future predation risk and enhanced fecundity. Slow strategies generally were advantageous in communities dominated by gape-unconstrained predators whereas fast strategies were advantageous in gape-limited predator communities. Results were sensitive to the assumed relationships between prey size and fecundity and between prey growth and predation risk. Predator growth increased the parameter space favoring fast prey strategies. The model makes the testable predictions that prey should not grow at the same rate as their gape-limited predator and generally should grow faster than the fastest growing gape-limited predator. By focusing on predator constraints on prey capture, these results integrate the ecological and evolutionary implications of prey growth in diverse predator communities and offer an explanation for empirical growth patterns previously viewed to be anomalies.     

Indirect effects of prey swamping: differential seed predation during a bamboo masting event

Resource pulses often involve extraordinary increases in prey availability that "swamp" consumers and reverberate through indirect interactions affecting other community members. We developed a model that predicts predator-mediated indirect effects induced by an epidemic prey on co-occurring prey types differing in relative profitability/preference and validated our model by examining current-season and delayed effects of a bamboo mass seeding event on seed survival of canopy tree species in mixed Patagonian forests. The model shows that predator foraging behavior, prey profitability, and the scale of prey swamping influence the character and strength of short-term indirect effects on various alternative prey. When in large prey-swamped patches, nonselective predators decrease predation on all prey types. Selective predators, instead, only benefit prey of similar quality to the swamping species, while very low or high preference prey remain unaffected. Negative indirect effects (apparent competition) may override such positive effects (apparent mutualism), especially for highly preferred prey, when prey-swamped patches are small enough to allow predator aggregation and/or predators show a reproductive numerical response to elevated food supply. Seed predation patterns during bamboo (Chusquea culeou) masting were consistent with predicted short-term indirect effects mediated by a selective predator foraging in large prey-swamped patches. Bamboo seeds and similarly-sized Austrocedrus chilensis (ciprés) and Nothofagus obliqua (roble) seeds suffered lower predation in bamboo flowered than nonflowered patches. Predation rates on the small-seeded Nothofagus dombeyi (coihue) and the large-seeded Nothofagus alpina (rauli) were independent of bamboo flowering. Indirect positive effects were transient; three months after bamboo seeding, granivores preyed heavily upon all seed types, irrespective of patch flowering condition. Moreover, one year after bamboo seeding, predation rates on the most preferred seed (rauli) was higher in flowered than in nonflowered patches. Despite rapid predator numerical responses, short-term positive effects can still influence community recruitment dynamics because surviving seeds may find refuge beneath the litter produced by bamboo dieback. Together, our theoretical analysis and experiments indicate that indirect effects experienced by alternative prey during and after prey-swamping episodes need not be universal but can change across a prey quality spectrum, and they critically depend on predator-foraging rules and the spatial scale of swamping.     

Adaptive prey behavior and the dynamics of intraguild predation systems

Intraguild predation constitutes a widespread interaction occurring across different taxa, trophic positions and ecosystems, and its endogenous dynamical properties have been shown to affect the abundance and persistence of the involved populations as well as those connected with them within food webs. Although optimal foraging decisions displayed by predators are known to exert a stabilizing influence on the dynamics of intraguild predation systems, few is known about the corresponding influence of adaptive prey decisions in spite of its commonness in nature. In this study, we analyze the effect that adaptive antipredator behavior exerts on the stability and persistence of the populations involved in intraguild predation systems. Our results indicate that adaptive prey behavior in the form of inducible defenses act as a stabilizing mechanism and show that, in the same direction that adaptive foraging, enhances the parameter space in which species can coexist through promoting persistence of the IG-prey. At high levels of enrichment, the intraguild predation system exhibits unstable dynamics and zones of multiples attractors. In addition, we show that the equilibrium density of the IG-predator could be increased at intermediate values of defense effectiveness. Finally we conclude that adaptive prey behavior is an important mechanism leading to species coexistence in intraguild predation systems and consequently enhancing stability of food webs.     

Effects of density-dependent dispersal behaviours on the speed and spatial patterns of range expansion in predator-prey metapopulations

Dispersal can strongly affect the spatiotemporal dynamics of a species (its spread, spatial distribution and persistence). We investigated how two dispersal behaviours, namely prey evasion (PE) and predator pursuit (PP), affect the dynamics of a predator-prey system. PE portrays the tendency of prey avoiding predators by dispersing into adjacent patches with fewer predators, while PP describes the tendency of predators to pursue the prey by moving into patches with more prey. Based on the Beddington predation model, a spatially explicit metapopulation model was built to incorporate PE and PP. Numerical simulations were run to investigate the effects of PE and PP on the rate of spread, spatial synchrony and the persistence of populations. Results show that both PE and PP can alter spatial synchrony although PP has a weaker desynchronising effect than PE. The predator-prey system without PE and PP expanded in circular waves. The effect of PE can push the prey to distribute in a circular ring front, whereas the effect of PP can change the circular waves to anisotropic expansion. Furthermore, weak PE and PP can accelerate the spread of prey while strong and disproportionate intensities slow down the range expansion. The effects of PE and PP further enhance the population size, break down the spatial synchrony and promote the persistence of populations.     

Outbreak suppression by predators depends on spatial distribution of prey

The capacity of a predator population to suppress a prey population that varies in abundance and spatial distribution is explored in a lattice simulation model. The model is based on empirically derived parameters for particular species. Within season predation by Pterostichus cupreus (Coleoptera: Carabidae) of varying densities and distributions of the prey Rhopalosiphum padi (Homoptera: Aphididae) in spring cereals was simulated. From these spatially explicit simulations prey population suppression was found to be largely dependent on the spatial distribution of the prey. A possible mechanism was that high degrees of prey aggregation provided refuge for the prey that, when aggregated, escaped detection by P. cupreus. In contrast, P. cupreus was found to efficiently suppress incipient outbreaks for evenly distributed prey populations, even at high prey densities. A higher predator density compensated for the lowered control ability of the predators for highly aggregated prey populations and hastened the decline of the prey population.     

Predator and prey space use: dragonflies and tadpoles in an interactive game

Predator and prey spatial distributions have important population and community level consequences. However, little is known either theoretically or empirically about behavioral mechanisms that underlie the spatial patterns that emerge when predators and prey freely interact. We examined the joint space use and behavioral rules governing movement of freely interacting groups of odonate (dragonfly) predators and two size classes of anuran (tadpole) prey in arenas containing two patches with different levels of the prey's resource. Predator and prey movement and space use was quantified both when they were apart and together. When apart from predators, large tadpoles strongly preferred the high resource patch. When apart from prey, dragonflies weakly preferred the high resource patch. When together, large prey shifted to a uniform distribution, while predators strongly preferred the high resource patch. These patterns qualitatively fit the predictions of several three trophic level, ideal free distribution models. In contrast, the space use of small prey and predators did not deviate from uniform. Three measures of joint space use (spatial correlations, overlap, and co-occurrence) concurred in suggesting that prey avoidance of predators was more important than predator attraction to prey in determining overall spatial patterns. To gain additional insight into behavioral mechanisms, we used a model selection approach to identify behavioral movement rules that can potentially explain the observed, emergent patterns of space use. Prey were more likely to leave patches with more predators and more conspecific competitors; resources had relatively weak effects on prey movements. In contrast, predators were more likely to leave patches with low resources (that they do not consume) and more competing predators; prey had relatively little effect on predator movements. These results highlight the importance of investigating freely interacting predators and prey, the potential for simple game theory models to predict joint spatial distributions, and the utility of using model choice methods to identify potential key factors that govern movement.     

Predation affects the susceptibility of hard clam Meretrix lusoria to Hg-stressed birnavirus

Predator–prey interaction in aquatic ecosystem is one of the simplest drivers affecting the species population dynamics. Predation controls are recognized as important aspects of ecosystem husbandry and management. In this paper we investigated how predation control cause an increase in host growth in the abundance of hard clam (Meretrix lusoria) populations subject to mercury (Hg)-stressed birnavirus. Here we linked predator–prey relationships with a bioenergetic matrix population model (MPM) associated with a susceptible–infectious–mortality (SIM) model based on a host–pathogen–predator framework to quantify the predator effects on population dynamics of disease in hard clam populations. Our results indicated that relative high predation rates could promote the hard clam abundances in relation to predators that selectively captured the infected hard clam, by which the disease transmission was suppressed. The results also demonstrated that predator-induced modifications in host behavior could have potential negative or positive effects on host growth depending on relative species density and resource dynamics. The most immediate implication of this study for the management of aquatic ecosystem is that, beyond the potential for causing a growth in abundance, predation might provoke greater predictability in aquatic ecosystem species populations and thereby increase the safety of ecosystem production from stochastic environmental events.     

The influence of intraguild predation on prey suppression and prey release: a meta-analysis

Intraguild predation (IGP) occurs when one predator species consumes another predator species with whom it also competes for shared prey. One question of interest to ecologists is whether multiple predator species suppress prey populations more than a single predator species, and whether this result varies with the presence of IGP. We conducted a meta-analysis to examine this question, and others, regarding the effects of IGP on prey suppression. When predators can potentially consume one another (mutual IGP), prey suppression is greater in the presence of one predator species than in the presence of multiple predator species; however, this result was not found for assemblages with unidirectional or no IGP. With unidirectional IGP, intermediate predators were generally more effective than the top predator at suppressing the shared prey, in agreement with IGP theory. Adding a top predator to an assemblage generally caused prey to be released from predation, while adding an intermediate predator caused prey populations to be suppressed. However, the effects of adding a top or intermediate predator depended on the effectiveness of these predators when they were alone. Effects of IGP varied across different ecosystems (e.g., lentic, lotic, marine, terrestrial invertebrate, and terrestrial vertebrate), with the strongest patterns being driven by terrestrial invertebrates. Finally, although IGP theory is based on equilibrium conditions, data from short-term experiments can inform us about systems that are dominated by transient dynamics. Moreover, short-term experiments may be connected in some way to equilibrium models if the predator and prey densities used in experiments approximate the equilibrium densities in nature.     

Habitat structure affects intraguild predation

Intraguild predation is thought to be ubiquitous in natural food webs. Yet, theory on intraguild predation predicts the intraguild prey to persist only under limited conditions. This gap between theory and empirical observations needs scrutiny. One reason might be that theory has focused on equilibrium dynamics and a limited set of species (usually three) that interact in well-mixed populations in unstructured habitats, and these assumptions will often not hold in natural systems. In this review, we focus on the effects of habitat structure on intraguild predation. Habitat structure could reduce encounter rates between predators and prey and could create refuges for prey. In both cases, habitat structure could reduce the strength of intraguild interactions, thereby facilitating species coexistence. A meta-analysis of studies on manipulation of habitat structure shows that intraguild prey indeed suffer less from intraguild predation in structured habitats. This was further confirmed by a meta-analysis in which studies on intraguild predation were classified according to habitat structure. Intraguild predation reduced densities of the intraguild prey significantly more in habitats with little structure than in habitats rich in structure. The effect of intraguild predation on the shared prey was negative, and not significantly affected by habitat structure. We conclude that habitat structure may increase persistence of the intraguild prey by decreasing the strength of the interaction between intraguild predator and intraguild prey.     

Assessing effects of non-native crayfish on mosquito survival

Introductions of non-native predators often reduce biodiversity and affect natural predator–prey relationships and may increase the abundance of potential disease vectors (e.g., mosquitoes) indirectly through competition or predation cascades. The Santa Monica Mountains (California, U.S.A.), situated in a global biodiversity hotspot, is an area of conservation concern due to climate change, urbanization, and the introduction of non-native species. We examined the effect of non-native crayfish (Procambarus clarkii) on an existing native predator, dragonfly nymphs (Aeshna sp.), and their mosquito larvae (Anopheles sp.) prey. We used laboratory experiments to compare the predation efficiency of both predators, separately and together, and field data on counts of dragonfly nymphs and mosquito larvae sampled from 13 local streams. We predicted a lower predation efficiency of crayfish compared with native dragonfly nymphs and a reduced predation efficiency of dragonfly nymphs in the presence of crayfish. Dragonfly nymphs were an order of magnitude more efficient predators than crayfish, and dragonfly nymph predation efficiency was reduced in the presence of crayfish. Field count data showed that populations of dragonfly nymphs and mosquito larvae were strongly correlated with crayfish presence in streams, such that sites with crayfish tended to have fewer dragonfly nymphs and more mosquito larvae. Under natural conditions, it is likely that crayfish reduce the abundance of dragonfly nymphs and their predation efficiency and thereby, directly and indirectly, lead to higher mosquito populations and a loss of ecosystem services related to disease vector control.     

Predators usurp prey defenses? Toxicokinetics of tetrodotoxin in common garter snakes after consumption of rough-skinned newts

Snakes are common predators of organisms, such as amphibians, with toxic defenses that can be lethal to other predators. Because snakes do not have the option of dissecting prey into edible versus inedible components, they face a full dose of any chemical defenses encountered during attempted predation. This limitation has likely resulted in intense selection favoring the evolution of alternative mechanisms for dealing with prey toxins. These mechanisms can be physiological (e.g., resistance to prey toxins) or behavioral (e.g., toxin sampling and rejection). When physiological resistance arises, the possibility of bioaccumulation of a toxin results. We examined the coevolutionary interaction between the common garter snake (Thamnophis sirtalis) and the rough-skinned newt (Taricha granulosa), which contains a powerful neurotoxin called tetrodotoxin (TTX). In some populations syntopic with newts, individuals of T. sirtalis have evolved resistance to TTX. We examined the persistence of TTX in T. sirtalis after administration of an oral dose of TTX to investigate the possibility that snakes are sequestering TTX. The half-life of TTX in snake liver was estimated at 8.1?days. Accordingly, clearance of 99% of a single dose of TTX averages 61?days. Negative fitness consequences of intoxication during and after newt consumption may be balanced by co-opting the newts?? chemical defense for protection from the snakes?? own predators. Accounting of the coevolutionary dynamic between snakes and newts must incorporate post-consumption affects of lingering TTX.     

Local interactions between predators and prey call into question commonly used functional responses

Commonly used functional response models (Holling’s type I and type II models) assume that the encounter rate of a predator increases linearly with prey density, provided that the predator is searching for prey. In other other words, aN (a is the baseline encounter rate and N is prey density) describes the encounter rate. This study examined whether the models are adequate when predators and prey interact locally by using a spatially explicit individual based model because local interactions affect the spatial distribution of predators and prey, which also affects the encounter rate. Predators were assumed to possess a spatial perception range that influenced their foraging behavior (e.g., if a prey is in the perception range, the predator moves towards the prey). The effect of antipredator behavior by prey was also examined. The results suggest that prey and predator densities as well as handling time affect the baseline rate (i.e., parameter a) as opposed to the common assumption that the parameter is constant. The nature of model deviations depended on both the antipredator behavior and the predators’ perception range. Understanding these deviations is important as they qualitatively affect community dynamics.     

Pre-encounter versus post-encounter inducible defenses in predator–prey model systems

It has been reported that, in order to reduce mortality, prey are able to change their phenotype in response to cues released from predators. These short-time responses constitute effective antipredator strategies in variable environments, and involve changes in morphology, behavior, physiology or life-history traits of prey individuals belonging to a wide spectrum of taxa. Defenses can be classified into pre-encounter and post-encounter, depending on the phase of the predation process in which they take place. Also, inducible defenses should be costly.Despite the current knowledge of inducible defenses at the individual level, our understanding of their dynamic consequences at the population and community level is limited. In this work we construct and analyze numerically a predator–prey system, parameterized from published experimental data, in which prey exhibit inducible defenses of the type pre-encounter (affecting attack rate) or post-encounter (affecting handling time) and entailing either metabolic or feeding costs. The above assumptions were analyzed over a gradient of resource availability.Our results indicated that both types of cost have a similar effect on the dynamics of the model system, but we expect that different costs will produce different outcomes in a more complex model community. Conversely, pre-encounter and post-encounter IDs define domains of attraction with different size and shape within the studied sections of the multidimensional parameter space. Roughly speaking, post-encounter IDs determine a more rich dynamics when plausible parameter values are chosen, and the effect of resource density is different if the ID is handling-time based or attack-rate based. In agreement with previous works, our analyses indicate that IDs can damp population oscillations and prevent the paradox of enrichment.     

Effect of Small‐Scale Heterogeneity of Prey and Hunter Distributions on the Sustainability of Bushmeat Hunting

Abstract: Bushmeat is the main source of protein and the most important source of income for rural people in the Congo Basin, but intensive hunting of bushmeat species is also a major concern for conservationists. Although spatial heterogeneity in hunting effort and in prey populations at the landscape level plays a key role in the sustainability of hunted populations, the role of small‐scale heterogeneity within a village hunting territory in the sustainability of hunting has remained understudied. We built a spatially explicit multiagent model to capture the dynamics of a system in which hunters and preys interact within a village hunting territory. We examined the case of hunting of bay duikers (Cephalophus dorsalis) in the village of Ntsiété, northeastern Gabon. The impact of hunting on prey populations depended on the spatial heterogeneity of hunting and prey distribution at small scales within a hunting area. Within a village territory, the existence of areas hunted throughout the year, areas hunted only during certain seasons, and unhunted areas contributed to the sustainability of the system. Prey abundance and offtake per hunter were particularly sensitive to the frequency and length of hunting sessions and to the number of hunters sharing an area. Some biological parameters of the prey species, such as dispersal rate and territory size, determined their spatial distribution in a hunting area, which in turn influenced the sustainability of hunting. Detailed knowledge of species ecology and behavior, and of hunting practices are crucial to understanding the distribution of potential sinks and sources in space and time. Given the recognized failure of simple biological models to assess maximum sustainable yields, multiagent models provide an innovative path toward new approaches for the assessment of hunting sustainability, provided further research is conducted to increase knowledge of prey species’ and hunter behavior.     

The effect of predation on artificial reef juvenile demersal fish species

There is a concern that artificial reefs (AR) may act purely as fishing aggregation devices. Predators attracted to ARs can influence the distribution and abundance of prey fish species. Determining the role of predators in AR is important in advancing the understanding of community interactions. This paper documents the effects of predation on fish assemblages of AR located near a coastal lagoon fish nursery. The Dicentrarchus labrax is a very opportunistic species preying on juveniles (0⁺ and 1⁺ age classes) of several demersal fish species on the ARs. Reef prey and sea bass abundance were negatively correlated. The mean numbers of prey per sea bass stomach increased with the increase of reef fish prey abundance, suggesting that predation has a significant influence, resulting in a decrease in prey abundance. Prey mortality (4–48%) of demersal reef fish associated species depends on bass density. Prey selection was related both with prey abundance and vulnerability. Results showed that D. labrax predation on AR-fish associated species can increase prey natural mortality. However, the role of bass predation on the ecological functioning of exploited ARs is not clear. There may be increases in local fishing yields due either to an increase in predator biomass through aggregation of sea bass attracted to ARs or to greater production. In contrast, predation on juveniles of economically important reef fish preys, especially the most frequent and abundant (Boops boops), can contribute to a decrease in recruitment to the fishery. Our results indicate that inter-specific interactions (predator–prey) are important in terms of conservation and management, as well as for the evaluation of the long-term effects of reef deployment. Thus, it is necessary to consider ecological interactions, such as predation, prior to the development and deployment of artificial habitats as a tool for rehabilitation.     

Consequences of patch reef spacing for density-dependent mortality of coral-reef fishes

The spatial configuration of habitat patches can profoundly affect a number of ecological interactions, including those between predators and prey. I examined the effects of reef spacing on predator-prey interactions within coral-reef fish assemblages in the Bahamas. Using manipulative field experiments, I determined that reef spacing influences whether and how density-dependent predation occurs. Mortality rates of juveniles of two ecologically dissimilar species (beaugregory damselfish and yellowhead wrasse) were similarly affected by reef spacing; for both species, mortality was density dependent on reef patches that were spatially isolated (separated by 50 m), and density independent on reef patches that were aggregated (separated by 5 m). A subsequent experiment with the damselfish demonstrated that a common resident predator (coney) caused a substantial proportion of the observed mortality, independent of reef spacing. Compared to isolated reefs, aggregated reefs were much more likely to be visited by transient predators (mostly yellowtail snappers), regardless of prey density, and on these reefs, mortality rates approached 100% for both prey species. Transient predators exhibited neither an aggregative response nor a type 3 functional response, and consequently were not the source of density dependence observed on the isolated reefs. These patterns suggest that resident predators caused density-dependent mortality in their prey through type 3 functional responses on all reefs, but on aggregated reefs, this density dependence was overwhelmed by high, density-independent mortality caused by transient predators. Thus, the spatial configuration of reef habitat affected both the magnitude of total predation and the existence of density-dependent mortality. The combined effects of the increasing fragmentation of coral reef habitats at numerous scales and global declines in predatory fish may have important consequences for the regulation of resident fish populations.