Russell J. Schmitt

THE COMBINED EFFECTS OF PREDATION RISK AND FOOD REWARD ON PATCH SELECTION

Experiments were performed to examine the effects of predation risk on selection of food patches by a mobile forager, young black surfperch (Embiotocajacksoni). The goals were to investigate (1) how predation risk affected relative selectivity among patches that differed in density of food, and (2) whether the foragers response depended on how variations in food level and predation risk were encountered. In the experiments, density of food (crustaceans on algae) was manipulated among spatially uniform patches of a foliose alga. Safety of the forager was varied by manipulating the presence in patches of kelp bass (Paralabrax clathratus), a principal predator of young surfperch. Patch choice was determined for an array of situations where food and risk were varied separately and together in space and time. The response of foragers to variation in food level and risk depended upon the specific manner in which the factors were encountered. When patches with a high food level contained a predator, foragers shunned the risky food-rich areas for safe food-poor patches. When food was equal between safe and risky patches, the degree to which patches with a predator were avoided remained constant over a range of absolute food densities. By contrast, selectivity between patches of different food densities was dampened when all available patches contained a predator. Finally, predators enhanced relative selectivity for food-rich areas when foragers had simultaneous access to the complete combination of variation in risk and food level. Heightened selectivity in the presence of predators resulted from two hierarchical responses: foragers always took less risk to feed in the food-poor patch, and, when the difference in comparable value of available food patches was suffi- ciently great, they took more risk to eat where the density of food was higher. Overall, these results indicate that juvenile black surfperch possess an array of responses to food and predators, which together may act to minimize the sublethal effect of predators on feeding at a minimal increase in probability of death. Such plasticity may be a general characteristic of foragers that experience variation in food and risk at several spatial scales and temporal combinations.

INDIRECT INTERACTIONS BETWEEN PREY: APPARENT COMPETITION, PREDATOR AGGREGATION, AND HABITAT SEGREGATION'

Field experiments were performed to explore the nature of indirect interactions between two groups of ecologically distinct prey that occur on subtidal rocky reefs at Santa Catalina Island, California. Mobile gastropods (Tegula aureotincta, Tegula eiseni, and Astraea undosa) and sessile bivalves (mostly Chama arcana) share a common set of invertebrate predators (lobster Panulirus interrupts, cephalopod Octopus bimaculatus, and whelk Kelletia kelletii). The gastropods, which are secondarily preferred prey, principally occur on cobble reefs and less commonly on high-relief boulder reefs. Sessile bivalves and other species of favored prey are common on high-relief reefs and are rare or absent in cobble areas. The density of each predator species is greatest in high-relief areas containing abundant favored prey. Addition of bivalves to replicate cobble plots resulted in marked increases in predator density, relative to controls, because predators aggregated to areas containing favored prey. Greater densities of predators in the presence of experimentally added bivalves resulted in greater mortality and lower population densities of gastropods relative to controls. Conversely, mortality of Chama was higher where gastropods were common compared with cobble areas containing fewer gastropods. This occurred because the density of predators within a cobble reef, although low overall, was positively correlated with density of gastropods. Thus each group of prey was negatively affected by the presence of the other because each alternative prey increased the local density of predators. Such a doubly negative indirect interaction between prey, mediated by a shared predator, is known as a pparent competition; this is the first experimental demonstration of its existence. Because gastropods and bivalves are superior at withstanding predation in different habitats, shared predation and apparent competition may be sufficient mechanisms to maintain the pattern of habitat segregation displayed by these prey groups.

Analysis of abrupt transitions in ecological systems

The occurrence and causes of abrupt transitions, thresholds, or regime shifts between ecosystem states are of great concern and the likelihood of such transitions is increasing for many ecological systems. General understanding of abrupt transitions has been advanced by theory, but hindered by the lack of a common, accessible, and data-driven approach to characterizing them. We apply such an approach to 30–60 years of data on environmental drivers, biological responses, and associated evidence from pelagic ocean, coastal benthic, polar marine, and semi-arid grassland ecosystems. Our analyses revealed one case in which the response (krill abundance) linearly tracked abrupt changes in the driver (Pacific Decadal Oscillation), but abrupt transitions detected in the three other cases (sea cucumber abundance, penguin abundance, and black grama grass production) exhibited hysteretic relationships with drivers (wave intensity, sea-ice duration, and amounts of monsoonal rainfall, respectively) through a variety of response mechanisms. The use of a common approach across these case studies illustrates that: the utility of leading indicators is often limited and can depend on the abruptness of a transition relative to the lifespan of responsive organisms and observation intervals; information on spatiotemporal context is useful for comparing transitions; and ancillary information from associated experiments and observations aids interpretation of response-driver relationships. The understanding of abrupt transitions offered by this approach provides information that can be used to manage state changes and underscores the utility of long-term observations in multiple sentinel sites across a variety of ecosystems.

Herbivory, Connectivity, and Ecosystem Resilience: Response of a Coral Reef to a Large-Scale Perturbation

Coral reefs world-wide are threatened by escalating local and global impacts, and some impacted reefs have shifted from coral dominance to a state dominated by macroalgae. Therefore, there is a growing need to understand the processes that affect the capacity of these ecosystems to return to coral dominance following disturbances, including those that prevent the establishment of persistent stands of macroalgae. Unlike many reefs in the Caribbean, over the last several decades, reefs around the Indo-Pacific island of Moorea, French Polynesia have consistently returned to coral dominance following major perturbations without shifting to a macroalgae-dominated state. Here, we present evidence of a rapid increase in populations of herbivorous fishes following the most recent perturbation, and show that grazing by these herbivores has prevented the establishment of macroalgae following near complete loss of coral on offshore reefs. Importantly, we found the positive response of herbivorous fishes to increased benthic primary productivity associated with coral loss was driven largely by parrotfishes that initially recruit to stable nursery habitat within the lagoons before moving to offshore reefs later in life. These results underscore the importance of connectivity between the lagoon and offshore reefs for preventing the establishment of macroalgae following disturbances, and indicate that protecting nearshore nursery habitat of herbivorous fishes is critical for maintaining reef resilience.

Patch selection by juvenile black surfperch (Embiotocidae) under variable risk: Interactive influence of food quality and structural complexity

Abstract The influence of risk on the selection of foraging patches by young-of-year black surfperch, Embiotoca jacksoni Agassiz, was investigated by laboratory and field experiments. These foragers harvest crustacean prey from a variety of benthic algal substrata. In field environments, patch types vary in two ways. First, substrata differ in structural complexity and probably afford different degrees of protection from predators. Second, substratum types vary in prey richness. There was no correlation between structural complexity and prey richness, and either or both factors could be a component of foraging patch value. Each patch is small and individual foragers are simultaneously confronted with arrays of patches encompassing the full range of variation in structure and prey richness. Furthermore, a major predator of young-of-year black surfperch, the kelp bass, Paralabrax clathratus (Girard), is patchily distributed in space and time. Thus similar arrays of patch types can be characterized by different levels of overall risk. Risk to foragers is dependent on light level as well as the presence and density of predators. The interplay between food quality and shelter potential in influencing patch choice was examined under different regimes of risk. Both laboratory and field experiments indicated patch preference was based primarily on food quality. However, the physical structure of a patch did become a component of patch choice as risk increased. The relative value of physical structure under high risk was dependent on the prey richness of a patch; food-poor substrata with high shelter potential remained unfavored even in situations of high risk.

Gape-limitation, foraging tactics and prey size selectivity of two microcarnivorous species of fish

SummaryPatterns of prey size selectivity were quantified in the field for two species of marine microcarnivorous fish, Embiotoca jacksoni and Embiotoca lateralis (Embiotocidae) to test Scott and Murdochs (1983) size spectrum hypothesis. Two mechanisms accounted for observed selectivity: the relative size of a fish in relation to its prey, and the type of foraging behavior used. Juvenile E. jacksoni were gape limited and newborn individuals achieved highest selectivity for the smallest prey size by using a visual picking foraging strategy. As young E. jacksoni grew, highest preference shifted to the next larger prey sizes. When E. jacksoni reached adulthood, the principal mode of foraging changed from visual picking to relatively indiscriminant winnowing behavior. The shift in foraging behavior by adults was accompanied by a decline in overall preference for prey size; sizes were taken nearly in proportion to their relative abundance. Adult E. lateralis retained a visual picking strategy and achieved highest selectivity for the largest class of prey. These differences in selectivity patterns by adult fish were not explained by gape-limination since adults of both species could ingest the largest prey items available to them. These results support Scott and Murdochs (1983) hypothesis that the qualitative pattern of size selectivity depends largely on the range of available prey sizes relative to that a predator can effectively harvest.

Habitat biodiversity as a determinant of fish community structure on coral reefs

Increased habitat diversity is often predicted to promote the diversity of animal communities because a greater variety of habitats increases the opportunities for species to specialize on different resources and coexist. Although positive correlations between the diversities of habitat and associated animals are often observed, the underlying mechanisms are only now starting to emerge, and none have been tested specifically in the marine environment. Scleractinian corals constitute the primary habitat-forming organisms on coral reefs and, as such, play an important role in structuring associated reef fish communities. Using the same field experimental design in two geographic localities differing in regional fish species composition, we tested the effects of coral species richness and composition on the diversity, abundance, and structure of the local fish community. Richness of coral species overall had a positive effect on fish species richness but had no effect on total fish abundance or evenness. At both localities, certain individual coral species supported similar levels of fish diversity and abundance as the high coral richness treatments, suggesting that particular coral species are disproportionately important in promoting high local fish diversity. Furthermore, in both localities, different microhabitats (coral species) supported very different fish communities, indicating that most reef fish species distinguish habitat at the level of coral species. Fish communities colonizing treatments of higher coral species richness represented a combination of those inhabiting the constituent coral species. These findings suggest that mechanisms underlying habitat-animal interaction in the terrestrial environment also apply to marine systems and highlight the importance of coral diversity to local fish diversity. The loss of particular key coral species is likely to have a disproportionate impact on the biodiversity of associated fish communities.

Crucial knowledge gaps in current understanding of climate change impacts on coral reef fishes

SUMMARY Expert opinion was canvassed to identify crucial knowledge gaps in current understanding of climate change impacts on coral reef fishes. Scientists that had published three or more papers on the effects of climate and environmental factors on reef fishes were invited to submit five questions that, if addressed, would improve our understanding of climate change effects on coral reef fishes. Thirty-three scientists provided 155 questions, and 32 scientists scored these questions in terms of: (i) identifying a knowledge gap, (ii) achievability, (iii) applicability to a broad spectrum of species and reef habitats, and (iv) priority. Forty-two per cent of the questions related to habitat associations and community dynamics of fish, reflecting the established effects and immediate concern relating to climate-induced coral loss and habitat degradation. However, there were also questions on fish demographics, physiology, behaviour and management, all of which could be potentially affected by climate change. Irrespective of their individual expertise and background, scientists scored questions from different topics similarly, suggesting limited bias and recognition of a need for greater interdisciplinary and collaborative research. Presented here are the 53 highest-scoring unique questions. These questions should act as a guide for future research, providing a basis for better assessment and management of climate change impacts on coral reefs and associated fish communities.

Symbiotic crabs maintain coral health by clearing sediments

Stony corals are the foundation of coral reef ecosystems and form associations with other reef species. Many of these associations may be ecologically important and play a role in maintaining the health and diversity of reef systems, rendering it critical to understand the influence of symbiotic organisms in mediating responses to perturbation. This study demonstrates the importance of an association with trapeziid crabs in reducing adverse effects of sediments deposited on corals. In a field experiment, mortality rates of two species of branching corals were significantly lowered by the presence of crabs. All outplanted corals with crabs survived whereas 45–80% of corals without crabs died within a month. For surviving corals that lacked crabs, growth was slower and tissue bleaching and sediment load were higher. Laboratory experiments revealed that corals with crabs shed substantially more of the sediments deposited on coral surfaces, but also that crabs were most effective at removing grain sizes that were most damaging to coral tissues. The mechanism underlying this symbiotic relationship has not been recognized previously, and its role in maintaining coral health is likely to become even more critical as reefs worldwide experience increasing sedimentation.

Resource overlap, prey dynamics, and the strength of competition

Patterns of resource use and the strength of interspecific competition were explored for two temperate marine reef fishes, black surfperch (Embiotoca jacksoni) and striped surfperch (E. lateralis). These species occupy a depth—related gradient in abundance of food (crustaceans) and preferred foraging microhabitats (species of foliose algae that contain food items). Field experiments revealed that the intensity of competition varied greatly along the resource gradient. At the shallow end of the gradient, which contains seasonally high food levels and large amounts of the most preferred foraging microhabitat (the red alga Gelidium), interspecific competition was intense during seasons when food was scarce. This was accompanied by low overlap in use of feeding microhabitats. We detected no competition in deep areas that lacked Gelidium. In this portion of the resource gradient, low overlap occurred because there was no preferred microhabitat that both fishes could share. Low overlap in the deep habitat was not a consequence of competition but rather of the slightly different foraging adaptations of each species. In the surfperch system, static, among—habitat patterns of resource overlap provided little insight into the existence or strength of competition. High overlap occurred when competition was dampened by abundant food (in the shallow zone during food—rich seasons). Low overlap was either associated with strong competition (during periods of relative scarcity of food in the shallow zone) or its absence (deep areas which lacked the shared, preferred microhabitat). Spatial and temporal variation in the strength of interspecific competition was better predicted by dynamic (seasonal) patterns of microhabitat overlap, but only when coupled with knowledge of food dynamics, foraging adaptations of the competitors, and availability of preferred shared microhabitat.