Higher Order Interactions

Community ecology has long sought to understand structurally diverse ecosystems by decomposing communities into sets of pairwise interactions. However, summing the effects of these pairwise interactions to predict a whole community response has remained challenging. The leading hypothesis for the challenges associated with this approach is the existence of Higher-Order Interactions (HOI), whereby one species affects the strength of interactions between two other species. The goal of studying these interactions is to quantify the prevalence of HOIs, their underlying mechanisms, and to understand their consequences on community structure and dynamics. To date, nearly all studies describing the existence of HOIs have so identified them by isolating small groups of species from the greater community (e.g., two predators and one prey), and the use of modules has been key to gaining greater insight into trait- and density-mediated mechanisms driving HOIs. Modules are thought to be recurrent and empirically manageable structures within the tangled web of interactions found in diverse ecosystems; however, our understanding of whether or how the strength and direction of interactions measured at the module level operate similarly in diverse natural systems remains largely untested.

Our research in this area has been heavily focused on developing and testing null models for HOIs involving predation and mutualism. Our capacity to detect HOIs at either the module or whole community level is limited by the null models used to compare the expected effects of two guild members acting independently to the observed effects when HOIs, e.g., intraguild dynamics, are allowed to occur. Higher Order Interactions among predators are driven by direct predator-predator interactions (e.g. antagonism from interference or intraguild predation, or synergy from cooperative hunting), as well as indirectly through common or conflicting behavioral responses of prey to predators. While dozens of experiments have tested the prevalence of HOIs among predators, the classic null models used to evaluate these studies are restricted to a limited set of assumptions. During my PhD, I developed and tested a new null model to assess the strength of intraspecific predator-predator interactions and quantify how predator foraging behavior (i.e., attack and maximum-intake rates) changes with interactions between predators. We recently used simulations to demonstrate the bias in previous null models of HOIs between predator species and proposed an alternative that provides appropriate null models for these interactions. I have also developed null models for HOIs and tested them to show the benefits corals receive from a diverse community of crustacean mutualists that remove sediment and defend corals from predatory seastars.


McCoy M.W., Stier A.C., and C.W. Osenberg. 2012. Predicting emergent effects of multiple predators on prey survival: The importance of depletion and the functional response.  Ecology Letters. 15: 1449-1456. PDF

Stier A.C. and Leray M. Multiple predator effects drive community organization of reef invertebrates. Coral Reefs. 10.1007/s00338-013-1077-2. PDF

Stier A.C., M.A. Gil , C.S. McKeon , S. Lemer, M. Leray, S.C. Mills, and C.W. Osenberg. Housekeeping mutualisms: do more symbionts facilitate host performance. PLoS ONE. PDF in press

Stier A.C., McKeon C.S.G, Osenberg C.W. and J.S. Shima. 2010. Guard Crabs Alleviate Deleterious Effects of Vermetid Snails on a Branching Coral. Coral Reefs. 29 (4) 1019 – 1022. PDF

McKeon C.S.G, Stier A.C., and Boyer S.E. Multiple Defender Effects: Synergistic coral defense by exosymbiotic crustaceans. Oecologia. PDF 

Adrian Stier
How patchy landscapes drive spatial patterns

Habitat fragmentation is an increasing problem that threatens the integrity of a wide range of ecosystems. Once divided, networks of patchy habitat in different spatial configurations may vary in their capacity to attract and retain biological communities. In some cases, when habitat-forming organisms (such as corals or trees) benefit from a thriving community of residents, differences in colonization patterns or occupancy caused by variable patch arrangement could affect the entire system. One of the major mechanisms for this phenomenon, known as “propagule redirection”, occurs when nearby habitat dilutes the per-patch concentration of colonizing individuals by providing a refuge from density-dependent competition and mortality. In the paper “Landscape configuration drives persistent spatial patterns of occupant distributions”, recently published in Theoretical Ecology, we explored the circumstances under which the configuration of habitat patches would be expected to lead to differences in biological community structure. To do so, we created a model which was parameterized and applied to data collected from coral reef fishes. We found that persistent effects on community composition were more likely when settlement pulses were small and infrequent, and when density-dependent mortality was low. The influence of patch configuration was comparable to the effects predicted from variation in patch quality, a much more commonly described driver of colonization patterns in fragmented habitats. Our demonstration that different spatial configurations of habitat patches can affect biological communities is an important step toward understanding how ecosystems may respond to fragmentation, and the framework we have built may be useful to both scientists and managers studying the ecology of fractured landscapes.

Hamman E.A., Osenberg C.W., McKinley S.A., and A.C. Stier. Spatial patterns of symbionts arising from propagule redirection. Theoretical Ecology. PDF

post authored by J. Curtis.

Adrian Stier
Defining ecosystem thresholds for human activities and environmental pressures in the California Current

The oceans are changing more rapidly than ever before. Unprecedented climatic variability is interacting with unmistakable long-term trends, all against a backdrop of intensifying human activities. What remains unclear, however, is how to evaluate whether conditions have changed sufficiently to provoke major responses of species, habitats, and communities. We developed a framework based on multimodel inference to define ecosystem-based thresholds for human and environmental pressures in the California Current marine ecosystem. To demonstrate how to apply the framework, we explored two decades of data using gradient forest and generalized additive model analyses, screening for nonlinearities and potential threshold responses of ecosystem states (n = 9) across environmental (n = 6) and human (n = 10) pressures. These analyses identified the existence of threshold responses of five ecosystem states to four environmental and two human pressures. Both methods agreed on threshold relationships in two cases: (1) the winter copepod anomaly and habitat modification, and (2) sea lion pup production and the summer mode of the Pacific Decadal Oscillation (PDO). Considered collectively, however, these alternative analytical approaches imply that as many as five of the nine ecosystem states may exhibit threshold changes in response to negative PDO values in the summer (copepods, scavengers, groundfish, and marine mammals). This result is consistent with the idea that the influence of the PDO extends across multiple trophic levels, but extends current knowledge by defining the nonlinear nature of these responses. This research provides a new way to interpret changes in the intensities of human and environmental pressures as they relate to the ecological integrity of the California Current ecosystem. These insights can be used to make more informed assessments of when and under what conditions intervention, preparation, and mitigation may enhance progress toward ecosystem-based management goals.

This paper was authored by Jameal F. Samhouri, Kelly S. Andrews, Gavin Fay, Chris J. Harvey,
Elliott L. Hazen, Shannon M. Hennessey, Kirstin Holsman, Mary E. Hunsicker, Scott I. Large,
Kristin N. Marshall, Adrian C. Stier (me), Jamie C. Tam, Stephani G. Zador. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Adrian Stier
Rapid and direct recoveries of predators and prey through synchronized ecosystem management

One of the twenty-first century’s greatest environmental challenges is to recover and restore species, habitats and ecosystems. The decision about how to initiate restoration is best-informed by an understanding of the linkages between ecosystem components and, given these linkages, an appreciation of the consequences of choosing to recover one ecosystem component before another. However, it remains difficult to predict how the sequence of species’ recoveries within food webs influences the speed and trajectory of restoration, and what that means for human well-being. Here, we develop theory to consider the ecological and social implications of synchronous versus sequential (species-by-species) recovery in the context of exploited food webs. A dynamical systems model demonstrates that synchronous recovery of predators and prey is almost always more efficient than sequential recovery. Compared with sequential recovery, synchronous recovery can be twice as fast and produce transient fluctuations of much lower amplitude. A predator-first strategy is particularly slow because it counterproductively suppresses prey recovery. An analysis of real-world predator–prey recoveries shows that synchronous and sequential recoveries are similarly common, suggesting that current practices are not ideal. We highlight policy tools that can facilitate swift and steady recovery of ecosystem structure, function and associated services

This paper was authored by Jameal F. Samhouri, Adrian C. Stier (me), Shannon M. Hennessey, Mark Novak, Benjamin S. Halpern & Phillip S. Levin. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Costly stakeholder participation creates inertia in marine ecosystems

Ecosystems often shift abruptly and dramatically between different regimes in response to human or natural disturbances. When ecosystems tip from one regime to another, the suite of available ecosystem benefits changes, impacting the stakeholders who rely on these benefits. These changes often create some groups who stand to incur large losses if an ecosystem returns to a previous regime. When the participation cost in the decision-making process is extremely high, this can “lock in” ecosystem regimes, making it harder for policy and management to shift ecosystems out of what the majority of society views as the undesirable regime. Public stakeholder meetings often have high costs of participation, thus economic theory predicts they will be dominated by extreme views and often lead to decisions that do not represent the majority viewpoint. Such extreme viewpoints can create strong inertia even when there is broad consensus to manage an ecosystem towards a different regime. In the same manner that reinforcing ecological feedback loops make it harder to exit an ecosystem regime, there are decision-making feedback loops that contribute additional inertia.

This paper was authored by Lynham J, Halpern BS, Blenckner T, Essington T, Estes J, Hunsicker M, Kappel C, Salomon AK, Scarborough C, Selkoe KA, and Stier A (me). You can find a copy of the manuscript here, or contact me directly for a PDF. 

Biodiversity effects of the predation gauntlet

The ubiquity of trophic downgrading has led to interest in the consequences of mesopredator release on prey communities and ecosystems. This issue is of particular concern for reef-fish communities, where predation is a key process driving ecological and evolutionary dynamics. Here, we synthesize existing experiments that have isolated the effects of mesopredators to quantify the role of predation in driving changes in the abundance and biodiversity of recently settled reef fishes. On average, predators reduced prey abundance through generalist foraging behavior, which, through a statistical sampling artifact, caused a reduction in alpha diversity and an increase in beta diversity. Thus, the synthesized experiments provide evidence that predation reduces overall abundance within prey communities, but—after accounting for sampling effects—does not cause disproportionate effects on biodiversity.

This paper was authored by Adrian C. Stier (me), Christopher D. Stallings, Jameal F. Samhouri, Mark A. Albins, and Glenn R. Almany. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Using rarefaction to isolate the effects of patch size and sampling effort on beta diversity

Beta diversity describes how species composition varies across space and through time. Current estimators of beta diversity typically ignore the effects of within-patch sample size, determined jointly by local abundance and sampling effort. Many ecological processes such as immigration, predation, or nutrient limitation affect abundance and asymptotic beta diversity concurrently; thus, existing metrics may confound changes in asymptotic beta diversity with changes that result from differences in abundance or sampling. Results from a stochastic simulation model illustrate how decreasing within-patch sample size may either increase or decrease observed beta diversity, depending on the type of metric, sample size, and community properties; these changes are easy to understand, and predict, by considering the effects of sampling on variance. A modified, patch-level form of rarefaction controls for variation in within-patch sample size; two case studies illustrate the utility of this approach. Studies seeking a mechanistic link between ecological process and beta diversity will continue to benefit from explicit consideration of sampling effects.

This paper was authored by Adrian C. Stier (me), Benjamin M. Bolker, Craig W. Osenberg. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Adrian Stier
Intraspecific variation in body size does not alter the effects of mesopredators on prey

As humans continue to alter the species composition and size structure of marine food webs, it is critical to understand size-dependent effects of predators on prey. Yet, how shifts in predator body size mediate the effect of predators is understudied in tropical marine ecosystems, where anthropogenic harvest has indirectly increased the density and size of small-bodied predators. Here, we combine field surveys and a laboratory feeding experiment in coral reef fish communities to show that small and large predators of the same species can have similar effects. Specifically, surveys show that the presence of a small predator (Paracirrhites arcatus) was correlated with lower chances of prey fish presence, but these correlations were independent of predator size. Experimental trials corroborated the size-independent effect of the predator; attack rates were indistinguishable between small and large predators, suggesting relatively even effects of hawkfish in various size classes on the same type of prey. Our results indicate that the effects of small predators on coral reefs can be size-independent, suggesting that variation in predator size-structure alone may not always affect the functional role of these predators.

This paper was authored by Austin J. Gallagher, Simon J. Brandl, and Adrian C. Stier (me). A copy of this manuscript can be found here, or email me directly to request a PDF. 

Adrian Stier
The relative influence of abundance and priority effects on colonization success in a coral-reef fish

The sequence of species colonization is increasingly recognized as an important determinant of community structure, yet the significance of sequence of arrival relative to colonizer abundance is seldom assessed. We manipulated the magnitude and timing of coral-reef fish settlement to investigate whether the competitive dominance of early-arriving Ambon damselfish (i.e., a priority effect) decreased in strength with increasing abundance of late-arriving lemon damselfish. Sequence of arrival had a stronger effect on survival than the number of competing individuals. Relative to when both species arrived simultaneously, lemon damselfish were less aggressive, avoided competitive interactions more frequently and experienced depressed survival when they arrived later than Ambon damselfish, with these effects occurring independently of lemon damselfish abundance. These results suggest priority effects are more important than colonizer abundance and should motivate the integration of priority effects into future studies of density dependence to determine their relative importance.

This paper was authored by Shane W. Geange, Davina E. Poulos, Adrian C. Stier (me), and Mark I. McCormick. You can find a copy of the manuscript here, or contact me directly for a PDF

Adrian Stier
Ecosystem context and historical contingency in apex predator recoveries

Habitat loss, overexploitation, and numerous other stressors have caused global declines in apex predators. This “trophic downgrading” has generated widespread concern because of the fundamental role that apex predators can play in ecosystem functioning, disease regulation, and biodiversity maintenance. In attempts to combat declines, managers have conducted reintroductions, imposed stricter harvest regulations, and implemented protected areas. We suggest that full recovery of viable apex predator populations is currently the exception rather than the rule. We argue that, in addition to well-known considerations, such as continued exploitation and slow life histories, there are several underappreciated factors that complicate predator recoveries. These factors include three challenges. First, a priori identification of the suite of trophic interactions, such as resource limitation and competition that will influence recovery can be difficult. Second, defining and accomplishing predator recovery in the context of a dynamic ecosystem requires an appreciation of the timing of recovery, which can determine the relative density of apex predators and other predators and therefore affect competitive outcomes. Third, successful recovery programs require designing adaptive sequences of management strategies that embrace key environmental and species interactions as they emerge. Consideration of recent research on food web modules, alternative stable states, and community assembly offer important insights for predator recovery efforts and restoration ecology more generally. Foremost among these is the importance of a social-ecological perspective in facilitating a long-lasting predator restoration while avoiding unintended consequences.

This paper was authored by Adrian C. Stier (me), Jameal F. Samhouri, Mark Novak, Kristin N. Marshall, Eric J. Ward, Robert D. Holt, and Phillip S. Levin. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Variable perceptions of food webs have conservation consequences

One of the best parts about being a scientist is exploring the unknown and building a deeper understanding of how biological systems work. However, our uncertainty surrounding some of the seemingly basic questions about biological systems such as ‘what is a species?’, can present tremendous challenges when it comes to conservation and management. For example, how can we say whether a species is endangered if we can’t agree on what a ‘species’ even is? In this paper we explored how uncertainty in scientific expert’s perceptions of food web structure, can have tremendous consequences when it comes to conservation and management of Pacific Herring, a key forage fish. In particular, we quantified how scientific, local, and traditional knowledge experts vary in their perceptions of food webs centered on Pacific Herring—a valuable ecological, economic, and cultural resource in Haida Gwaii, BC, Canada. Expert perceptions of the herring food web varied markedly in structure, and a simulated herring recovery with each of these unique mental models demonstrated wide variability in the perceived importance of herring to the surrounding food web. Using this general approach to determine the logical consequences of expert perceptions of social-ecological system structure in the context of potential future management actions, decision-makers can work explicitly toward filling knowledge gaps while embracing a diversity of perspectives.

Stier A.C., Samhouri J.F., Gray S., Martone R., Mach M., Scarborough C., Kappel C., B. Halpern B., and  P.S. Levin. 2017. Integrating expert opinion into food web conservation and management. Conservation Letters. PDF 

Adrian Stier
Adaptive genetic variation mediates bottom-up and top-down control in an aquatic ecosystem

Research in eco-evolutionary dynamics and community genetics has demonstrated that variation within a species can have strong impacts on associated communities and ecosystem processes. Yet, these studies have centred around individual focal species and at single trophic levels, ignoring the role of phenotypic variation in multiple taxa within an ecosystem. Given the ubiquitous nature of local adaptation, and thus intraspecific variation, we sought to understand how combinations of intraspecific variation in multiple species within an ecosystem impacts its ecology. Using two species that co-occur and demonstrate adaptation to their natal environments, black cottonwood (Populus trichocarpa) and three-spined stickleback (Gasterosteus aculeatus), we investigated the effects of intraspecific phenotypic variation on both top-down and bottom-up forces using a large-scale aquatic mesocosm experiment. Black cottonwood genotypes exhibit genetic variation in their productivity and consequently their leaf litter subsidies to the aquatic system, which mediates the strength of top-down effects from stickleback on prey abundances. Abundances of four common invertebrate prey species and available phosphorous, the most critically limiting nutrient in freshwater systems, are dictated by the interaction between genetic variation in cottonwood productivity and stickleback morphology. These interactive effects fit with ecological theory on the relationship between productivity and top-down control and are comparable in strength to the effects of predator addition. Our results illustrate that intraspecific variation, which can evolve rapidly, is an under-appreciated driver of community structure and ecosystem function, demonstrating that a multi-trophic perspective is essential to understanding the role of evolution in structuring ecological patterns.

This paper was authored by Seth M. Rudman, Mariano A. Rodriguez-Cabal, Adrian Stier (me), Takuya Sato, Julian Heavyside, Rana W. El-Sabaawi, and Gregory M. Crutsinger. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Principles for managing marine ecosystems prone to tipping points

As climatic changes and human uses intensify, resource managers and other decision makers are taking actions to either avoid or respond to ecosystem tipping points, or dramatic shifts in structure and function that are often costly and hard to reverse. Evidence indicates that explicitly addressing tipping points leads to improved management outcomes. Drawing on theory and examples from marine systems, we distill a set of seven principles to guide effective management in ecosystems with tipping points, derived from the best available science. These principles are based on observations that tipping points (1) are possible everywhere, (2) are associated with intense and/or multifaceted human use, (3) may be preceded by changes in early-warning indicators, (4) may redistribute benefits among stakeholders, (5) affect the relative costs of action and inaction, (6) suggest biologically informed management targets, and (7) often require an adaptive response to monitoring. We suggest that early action to preserve system resilience is likely more practical, affordable, and effective than late action to halt or reverse a tipping point. We articulate a conceptual approach to management focused on linking management targets to thresholds, tracking early-warning signals of ecosystem instability, and stepping up investment in monitoring and mitigation as the likelihood of dramatic ecosystem change increases. This approach can simplify and economize management by allowing decision makers to capitalize on the increasing value of precise information about threshold relationships when a system is closer to tipping or by ensuring that restoration effort is sufficient to tip a system into the desired regime.

This paper was authored by Kimberly A. Selkoe, Thorsten Blenckner, Margaret R. Caldwell,
Larry B. Crowder, Ashley L. Erickson, Timothy E. Essington, James A. Estes, Rod M. Fujita,
Benjamin S. Halpern, Mary E. Hunsicker, Carrie V. Kappel, Ryan P. Kelly, John N. Kittinger,
Phillip S. Levin, John M. Lynham, Megan E. Mach, Rebecca G. Martone, Lindley A. Mease,
Anne K. Salomon, Jameal F. Samhouri, Courtney Scarborough, Adrian C. Stier (me),
Crow White, and Joy Zedler. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Adrian Stier
Conservation challenges of predator recovery

Predators are critical components of ecosystems. Globally, conservation efforts have targeted depleted populations of top predators for legal protection, and in many cases, this protection has helped their recoveries. Where the recovery of individual species is the goal, these efforts can be seen as largely successful. From an ecosystem perspective, however, predator recovery can introduce significant new conservation and legal challenges. We highlight three types of conflicts created by a single-species focus: (1) recovering predator populations that increase competition with humans for the same prey, (2) new tradeoffs that emerge when protected predators consume protected prey, and (3) multiple predator populations that compete for the same limited prey. We use two food webs with parallel conservation challenges, the Northeast Pacific Ocean and the Greater Yellowstone Ecosystem, to demonstrate legal/policy conflicts and the policy levers that exist to ameliorate conflicts. In some cases, scientific uncertainty about the ecological interaction hinders progress towards resolving conflicts. In others, available policy options are insufficient. In all cases, management decisions must be made in the face of an unknown future. We suggest a framework that incorporates multispecies science, policy tools, and tradeoff analyses into management.

This paper was authored by Kristin N. Marshall, Adrian C. Stier (me), Jameal F. Samhouri, Ryan P. Kelly, and Eric J. Ward. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Adrian Stier
Trophic Biogeography

Top predators are often absent from spatially isolated habitats leading to the pervasive view that fragmented ecological communities collapse from the top down. These species at the top of the food chain are a critical part of healthy ecosystems, often acting as keystones maintaining ecosystem diversity. If predators disperse widely, serial influx of colonists could rescue isolated predator populations from extinction. Our study reveals that marine predator species richness is less sensitive than prey richness to habitat isolation, suggesting that management strategies should be tailored to consider the role of biogeography in driving biodiversity within marine food webs. These results add to a growing body of evidence suggesting the careful consideration of organismal movement may help explain how ecological communities are structured across the globe. Given the suite of anthropogenic threats facing reef ecosystems and top predators more generally, a renewed focus on the mechanisms governing community structure is essential

The article is co-led by Andrew Hein and Adrian Stier (me) and was co-authored by Valeriano Parravicini and Michel Kulbicki.

The abstract and full text of this article are availabe here at Nature.com here: http://www.nature.com/ncomms/2014/141121/ncomms6575/full/ncomms6575.html

Adrian Stier
Assessing trade-offs to inform ecosystem-based fisheries management of forage fish

Twenty-first century conservation is centered on negotiating trade-offs between the diverse needs of people and the needs of the other species constituting coupled human-natural ecosystems. Marine forage fishes, such as sardines, anchovies, and herring, are a nexus for such trade-offs because they are both central nodes in marine food webs and targeted by fisheries. An important example is Pacific herring, Clupea pallisii in the Northeast Pacific. Herring populations are subject to two distinct fisheries: one that harvests adults and one that harvests spawned eggs. We develop stochastic, age-structured models to assess the interaction between fisheries, herring populations, and the persistence of predators reliant on herring populations. We show that egg- and adult-fishing have asymmetric effects on herring population dynamics - herring stocks can withstand higher levels of egg harvest before becoming depleted. Second, ecosystem thresholds proposed to ensure the persistence of herring predators do not necessarily pose more stringent constraints on fisheries than conventional, fishery driven harvest guidelines. Our approach provides a general template to evaluate ecosystem trade-offs between stage-specific harvest practices in relation to environmental variability, the risk of fishery closures, and the risk of exceeding ecosystem thresholds intended to ensure conservation goals are met.

This paper was authored by Andrew O. Shelton, Jameal F. Samhouri, Adrian C. Stier (me), and Philip S. Levin. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Predation in Patchy Habitat

Humans-altered ecosystems often experience concurrent declines in keystone predators and the loss and fragmentation of habitat. Whether and how habitat and predator modification interact to affect ecosystem structure and function remains poorly studied. New theory merges island biogeography and food web ecology theory to predict diversity patterns in in patchy landscapes. In this study we empirically test trophic island biogeography theory to quantify how the independent and interactive effects of habitat loss, fragmentation, and predator-prey interactions affect the assembly of coral reef fish communities. Our results demonstrate strong, but independent effects of predation and habitat characteristics in driving reef fish community assembly. Predators (Cephalopholis argus) exhibited an oddity effect – disproportionately consuming rare prey species and reducing fish abundance and biodiversity by ~50%. Additionally, fragmentation caused a 50% increase in reef fish biodiversity. Our findings suggest two independent pathways (i.e., habitat or predator shifts) by which natural and/or anthropogenic processes can drive variation in fish biodiversity and community assembly.

This paper is led by: Adrian Stier (me) (NOAA) and coauthored by Kate Hanson (University of Hawaii), and Russ Schmitt, Sally Holbrook, and Andrew Brooks (all UCSB). Download the paper here on the ESA website or e-mail me (adrian.stier@gmail.com) for a copy of the manuscript.

Adrian Stier
High mortality in a surgeonfish following an exceptional settlement event

Marine organisms occasionally settle at exceptional densities, whereby thousands of individuals arrive concurrently. High levels of mortality, which has historically been attributed to predation or competition, often follow this episodic settlement of reef fishes. Here, however, we observed large numbers of newly settled surgeonfish (Ctenochaetus striatus) with white lesions lying dead on the sand amongst patch reefs following separate episodic settlement events in 2006 and 2009 in Moorea, French Polynesia. Pathogens have been identified as an important driver of population dynamics in other marine organisms but less so for reef fishes. Our observations suggest that disease outbreaks may play an underappreciated role as a mechanism of mortality following episodic settlement events in reef fishes.

This paper was authored by Adrian C. Stier (me), Joshua A. Idjadi, Shane W. Geange, and Jada-Simone S. White. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Adrian Stier
Predator density and the functional responses of coral reef fish

Predation is a key process driving coral reef fish population dynamics, with higher per capita prey mortality rates on reefs with more predators. Reef predators often forage together, and at high densities, they may either cooperate or antagonize one another, thereby causing prey mortality rates to be substantially higher or lower than one would expect if predators did not interact. However, we have a limited mechanistic understanding of how prey mortality rates change with predator densities. We re-analyzed a previously published observational dataset to investigate how the foraging response of the coney grouper (Cephalopholis fulva) feeding on the bluehead wrasse (Thalassoma bifasciatum) changed with shifts in predator and prey densities. Using a model-selection approach, we found that per-predator feeding rates were most consistent with a functional response that declines as predator density increases, suggesting either antagonistic interactions among predators or a shared antipredator behavioral response by the prey. Our findings suggest that variation in predator density (natural or anthropogenic) may have substantial consequences for coral reef fish population dynamics.

This paper was authored by A.C. Stier (me) and J.W. White. You can find a copy of the manuscript here, or contact me directly for a PDF. 

Trophic Ecology of Cryptofauna

Studies of species interactions on coral reefs have primarily focused on fishes and corals that live conspicuously on the surface of the reef. However, the majority of the reef’s diversity comes from invertebrates and fishes that live embedded within the interstitial spaces of the reef, and we know little about the processes underlying the structure and stability of these more cryptic species. For example, a key group of fishes and decapods live embedded in the reef building coral Pocillopora. Many of these cryptic inhabitants are known coral mutualists that subsidize nutrients, clean the coral of sedimentation, and defend the coral from the predatory seastars. Our previous studies in this system suggest substantial context dependency to the degree of services provided by certain mutualists and that protective services vary with mutualist abundance. However, the processes that drive the structure and diversity of mutualist communities remain poorly understood. Here we experimentally test the hypothesis that predation promotes the diversity of mutualist reef cryptofauna. By quantifying the independent and combined effects of two predatory fishes on the abundance, diversity and composition of a diverse, but cryptic, coral reef fish and decapod community, we find strong effects of both predator density and identity on the abundance, diversity, and composition of resident organisms. Predators substantially altered the dynamics of coral-dwelling decapods (a poorly studied but extraordinarily beautiful and diverse guild). Our results suggest that predation on mutualist communities may have negative indirect effects on host coral’s ability to withstand increasing frequency and intensity of natural and anthropogenic stressors on coral reefs.

This manuscript is the newest in a series of studies that have examined the ecology of key mutualistic reef cryptofauna. Please check out our other papers that feature this same guild of symbionts.

Stier A.C., M.A. Gil, C.S. McKeon, S. Lemer, M. Leray, S.C. Mills, and C.W. Osenberg. 2012. Housekeeping mutualisms: do more symbionts facilitate host performance. PLoS ONE. 7(4): e32079PDF

McKeon C.S., Stier A.C., Boyer S.E., and Bolker BM. 2012.  Multiple Defender Effects: Synergistic coral defense by exosymbiotic crustaceans. Oecologia. 169:1095-1103. PDF

Stier A.C., McKeon C.S., Osenberg C.W. and J.S. Shima. 2010. Guard Crabs Alleviate Deleterious Effects of Vermetid Snails on a Branching Coral. Coral Reefs. 29 (4) 1019 – 1022. PDF

This paper is equally coauthored with Matthieu Leray (Smithsonian Institution). Download the paper here or e-mail me (adrian.stier@gmail.com) for a copy of the manuscript.

Adrian Stier