Todd W. Anderson

PREDATOR RESPONSES, PREY REFUGES, AND DENSITY-DEPENDENT MORTALITY OF A MARINE FISH

Detection of density dependence in animal populations is a primary goal of population ecology, and the processes causing density dependence play a major role in population regulation. Predation can strongly regulate populations by populational and behavioral responses of predators to their prey. Here I evaluate the existence and strength of density-dependent mortality in local populations of a reef fish, the kelp perch (Bra- chyistius frenatus), caused by its predator, the kelp bass (Paralabrax clathratus). Specifi- cally, I examine both the functional response and a potential aggregative response by kelp bass as mechanisms underlying predator-induced density-dependent mortality. I calculated the per capita mortality of kelp perch as a result of the functional response of its predator by subjecting a range of densities of kelp perch to a low, constant number of kelp bass and different amounts of giant kelp (Macrocystis pyrifera) as habitat structure and a potential prey refuge. The potential for an aggregative response by kelp bass was determined by exposing densities of kelp perch to natural levels of kelp bass in the field. Per capita mortality of kelp perch in the laboratory was inversely density dependent to density independent with increasing habitat structure. By contrast, per capita mortality in the field was strongly density dependent, with evidence for an aggregative response by kelp bass. Furthermore, emigration and other nonpredatory losses of kelp perch from field plots were negligible, indicating that the product of the functional and aggregative responses by kelp bass induced density-dependent mortality in kelp perch. My results indicated that increasing habitat structural complexity at low densities of kelp perch, coupled with a strong aggregative response by kelp bass, was responsible for the observed pattern of density-dependent mortality. In addition, the distribution and relative abundances of kelp perch and kelp bass at larger spatial scales (entire reefs) were consistent with predation as an important process structuring local populations of kelp perch. The contrasting patterns of short-term per capita mortality found here underscore the need to evaluate both the functional and aggregative responses of predators to their prey. Ultimately, the pattern of mortality expressed will be determined by the relative strengths of these behavioral re- sponses and the degree to which habitat structure provides an effective prey refuge.

Biodiversity, population regulation, and the stability of coral-reef fish communities

Unprecedented population declines and extinctions because of human activities, combined with a growing recognition that such losses affect the stability of ecosystems, underscore the need to better understand how populations persist naturally. We provide field experimental evidence that high biodiversity—in particular, the combined effects of predators and competitors—acts in a way that regulates the size of local fish populations within their coral-reef community. These results indicate that complex interactions among multiple species are necessary for the stability of a highly diverse community, and so forewarn that overexploiting such species may have cascading negative consequences for the entire system.

Influence of an artificial reef on the surrounding infaunal community

Artificial reefs have been constructed throughout the world, but their effects on adjacent soft-bottom communities are largely unknown. In December 1986, we investigated the influence of Pendleton Artificial Reef (PAR) in Southern California on the abundance of infauna in the surrounding sand bottom. PAR was constructed in 1980 of quarry rock placed in eight piles, or modules. The artificial reef altered the grain-size distribution of sediments around the reef; sediments close to the modules were coarser than those 10 or 20 m away from the modules. Densities of one of the two most common species, the polychaetePrionospio pygmaeus, were lower near the reef, perhaps due to foraging by reef-associated predators or because the habitat near the reef was less suitable. We found no evidence that foraging by reef-associated fishes caused a widespread reduction in infaunal densities near the reef, and in fact the other most common taxon,Spiophanes spp., had higher densities near the reef. The most conspicuous effect of the artificial reef concerned the tube-dwelling wormDiopatra ornata, which only occurred in close association with the modules. In addition, total infaunal density and the densities of decapods, echinoderms and sipunculids were higher withinD. ornata beds than outside the beds. These results indicate that the densities of some species were enhanced, and others depressed, around the reef, but that the overall effect of the artificial reef on the surrounding infauna was limited to a small area near the modules.

THE ROLE OF REPRODUCTIVE SYNCHRONY IN THE COLONIZATION POTENTIAL OF KELP

Dispersal is a key element in the recovery of populations that have been locally destroyed by disturbance. Surprisingly, many sessile species that seemingly have limited potential to disperse often rapidly colonize areas that have been recently disturbed. The synchronous release of propagules during periods that promote advection may extend the dispersal of such species and promote their rapid colonization. Alternatively, rapid recovery may result from colonization by dormant stages that survive disturbance. Here we test for reproductive synchrony as a way of extending the dispersal potential of two common seaweeds, the kelps Macrocystis pyrifera and Pterygophora californica. Synchrony in spore release in their case is likely to be particularly useful in extending colonization distance because fertilization in these species occurs after spore dispersal; synchrony in- creases the chance of fertilization by increasing the concentration of spores. We also evaluate experimentally the relative importance of dormant stages vs. recently settled spores in accounting for the rapid recovery of local kelp populations following severe disturbances. Reproductive synchrony was evaluated by following weekly changes in the reproductive condition of adult kelp. The degree of reproductive synchrony in both Macrocystis and Pterygophora was significantly greater than -that expected under conditions of asynchronous reproduction. In Macrocystis, periods of synchronous spore production and release occurred sporadically over a 2-yr period. At least 75% of the sampled population exhibited the same directional change in reproductive condition in 38 of 82 sample periods. Episodes of sig- nificant spore release varied in duration from as little as 1 wk to as much as 2 mo. The sharpest decline in reproductive condition occurred during a severe storm. In contrast to Macrocystis, relatively well defined cycles of spore production, maturation, and release were observed in Pterygophora. More than one cycle was observed within a single repro- ductive season, and each cycle lasted -3-4 wk. The different patterns and degrees of synchrony observed between the two species likely reflect the degree to which their pro- duction of spores is influenced by environmental conditions; spore production in Macro- cystis is greatly influenced by fluctuations in seawater temperature and nutrients while spore production in Pterygophora is not. Results from field experiments comparing the recruitment of small plants among rocks placed in the kelp bed for varying lengths of time indicated that microscopic life stages of Macrocystis and Pterygophora have little capacity for dormancy, and that the vast majority of recruitment resulted from recently settled spores. These results contrasted with those observed for the annual brown alga Desmarestia ligulata which showed a dormancy period of several months. Our findings suggest that mechanisms such as reproductive synchrony that extend the distances over which kelp spores can effectively colonize are likely to play a critical role in the dynamics of kelp populations, which often fluctuate greatly in time due to disturbance.

Density dependence and population regulation in marine fish: a large-scale, long-term field manipulation

Do small-scale experiments showing spatial density dependence in marine fishes scale-up to temporal density dependence and regulation of relatively large local populations? If so, what are the causative mechanisms and their implications? We conducted an eight-year multigeneration study of population dynamics of bicolor damselfish (Stegastes partitus) inhabiting four large coral reefs in the Bahamas. After a four-year baseline period, it was clear that two populations naturally received very few settlement-stage larvae, so recruitment of recently settled fish was artificially enhanced at one low-settlement reef and reduced at one high-settlement reef to ensure a broad range of population sizes over which to test for regulation. Over all eight years, populations on the two naturally high-settlement reefs experienced temporal density dependence in multiple per capita demographic rates: mortality, survival to adulthood, and fecundity. These local populations also displayed components of regulation: persistenc...

POSITIVE INDIRECT EFFECTS OF REEF FISHES ON KELP PERFORMANCE: THE IMPORTANCE OF MESOGRAZERS

It has been suggested that microcarnivorous reef fishes may play an important role in giant kelp forest communities by preventing infestations of mesograzers that could severely impact or potentially destroy recovering kelp forests after extreme disturbance events. However, these trophic linkages, specifically the direct and indirect effects of fishes on the biomass of mesograzers, grazing intensity, and the performance of giant kelp, have not been sufficiently quantified and evaluated as to their importance and in the absence of such disturbance events. We examined experimentally the effects of mesograzers on the growth and performance of giant kelp in the presence and absence of their fish predators near Santa Catalina Island, California (U.S.A.). Mesograzer biomass and grazing intensity were significantly higher when fishes were excluded from giant kelp, which in turn, lowered kelp performance. This pattern was consistent both on experimental plots of kelp as habitat isolates, and on a continuous reef. Moreover, the abundance of mesograzers was inversely related to the abundance of kelp perch among several kelp-forested reefs, suggesting that these effects can occur at larger spatial scales. Because of differences in the diet and behavior of two microcarnivorous fishes, the kelp perch and señorita, we conducted an experiment manipulating each species and its density independently to determine their separate effects on mesograzers and kelp performance. Concurrently we examined the growth and mortality of juvenile kelp. Grazing intensity decreased, estimates of kelp performance increased, and the growth of juvenile kelp increased with increasing densities of fish but with no detectable effects between fishes. Our results demonstrate that these microcarnivorous fishes have positive indirect effects on kelp performance by reducing mesograzer biomass and grazing intensity, and the early life stages of other fishes also may be important. More specifically, these fishes have a positive effect on the density of fronds of giant kelp that can result in greater recruitment success and the abundance of kelp-associated invertebrates and fishes. Indeed, this study suggests that mesograzers have the potential to be one of the most important herbivores in kelp forest ecosystems.

Top‐down control of epifauna by fishes enhances seagrass production

Predators can influence the structure and function of ecosystems by altering the composition or behavior of herbivore communities. Overexploitation of predators, therefore, may lead to habitat loss by altering important top-down interactions that facilitate habitat-forming species. In seagrass beds, top-down control of algal growth by mesograzers appears to facilitate seagrass production. The indirect consequences of higher-order trophic interactions, however, remain unclear. Although predators may limit the beneficial effects of algal mesograzers, it is also possible that they limit the abundance of invertebrates that consume and foul seagrasses. We used experimental enclosure and exclosure cages to explore the direct and indirect effects of microcarnivorous fishes on epifaunal invertebrates, epiphytic loads, and seagrass growth in a natural eelgrass (Zostera marina) bed in San Diego Bay, California, USA. Contrary to expectations, when fishes were excluded, invertebrate abundance increased by 300-1000%, fouling on eelgrass leaves increased by 600%, and eelgrass production declined by 50%. Despite high densities of predators in enclosures, subsequent effects did not differ from ambient conditions. When predators were excluded, however, abundances of epifauna (including tube-building crustaceans and an eelgrass-grazing limpet) increased dramatically, resulting in reduced seagrass production. Our results are supported by several studies of eelgrass communities in the northeastern Pacific, characterized by coastal upwelling, inverse estuaries, and a voracious seagrass-consuming limpet. These strong, positive, indirect effects of microcarnivores on seagrass production contrast with the beneficial mesograzer paradigm, highlighting the need for hypotheses to be tested across a variety of ecosystems with varying biophysical characteristics.

Bincke: a highly efficient net for collecting reef fishes

The efficient collection of fishes from structurally complex environments (e.g., coral reefs, kelp forests) is difficult because conventional collecting methods generally cannot be used and many of the fishes are mobile and active. We describe the design, operation, and application of a diver-propelled net for efficiently collecting many species of benthic fish that reside on coral reefs and on kelp-forested rocky reefs. The overall size of the net and mesh size of the netting can be adjusted according to the size and behavior of targeted species to minimize drag and damage to specimens. Altering these dimensions combined with proper use of the net can result in a high rate of capture success.

Correspondence between food availability and growth of a planktivorous temperate reef fish

Although food availability generally has not been considered to be an important influence on the demography of reef fishes, a few studies have indicated that it can affect individual growth and the local abundance of some species. We examined the relationship between food availability and growth of the kelp perch, Brachyistius frenatus Gill, a planktivorous temperate reef fish. Among sites, there was a high degree of concordance in the rank abundance of planktonic taxa, even though prey biomass differed significantly. The growth of juvenile kelp perch among sites was positively and linearly related to prey biomass, suggesting that food can be a limiting resource for this fish over a wide range of prey abundance. Young kelp perch are closely associated with the giant kelp, Macrocystis pyrifera (L.) C.Ag. and appear to be restricted in their movements to reefs in which they were born. Therefore, there may be important consequences of differential food availability in that differences in the rate of delivery of zooplankton to these reefs, further affected by the physical structure of Macrocystis, may substantially influence juvenile growth rates and other demographic characteristics of kelp perch populations.

Intermittent Noise Induces Physiological Stress in a Coastal Marine Fish.

Anthropogenic noise in the ocean has increased substantially in recent decades, and motorized vessels produce what is likely the most common form of underwater noise pollution. Noise has the potential to induce physiological stress in marine fishes, which may have negative ecological consequences. In this study, physiological effects of increased noise (playback of boat noise recorded in the field) on a coastal marine fish (the giant kelpfish, Heterostichus rostratus) were investigated by measuring the stress responses (cortisol concentration) of fish to increased noise of various temporal dynamics and noise levels. Giant kelpfish exhibited acute stress responses when exposed to intermittent noise, but not to continuous noise or control conditions (playback of recorded natural ambient sound). These results suggest that variability in the acoustic environment may be more important than the period of noise exposure for inducing stress in a marine fish, and provide information regarding noise levels at which physiological responses occur.