The objective of my research is to understand general processes and mechanisms that influence (1) abundance and dynamics of populations and (2) species composition and diversity of communities. I make extensive use of field experiments and observations to explore such fundamental issues as consumer - resource interactions (e.g., predator-prey, exploitation competition, apparent competition), population dynamics and regulation, and bio-diversity and coexistence of competitors. All of my work has been conducted in subtidal reef environments of temperate and tropical marine ecosystems using both benthic invertebrates and reef fishes as model systems. Because many marine organisms have dispersing life stages (and therefore can have local populations that are demographically open), reef systems provide the opportunity to explore issues related to scale dependency and to the causes and consequences of variation in the contributions of different processes to abundance, dynamics and regulation. In addition to my fundamental research, I am interested in the application of ecological principles to the resolution of coastal marine environmental problems. This perspective includes the development and application of techniques to estimate the effect size of ecological impacts and to ameliorate those impacts through scientifically rigorous restoration and conservation approaches.
Much of my work is done in collaboration with other field ecologists and theoreticians here at UC Santa Barbara and elsewhere. My current research is funded primarily by the National Science Foundation, the W.M. Keck Foundation and the Gordon and Betty Moore Foundation. The field locations of my current projects include the Santa Barbara Channel (particularly Santa Cruz Island) off southern California and the South Pacific island of Moorea in the Society Islands of French Polynesia. Below are brief descriptions of some of my ongoing projects and findings.
Population Dynamics and Regulation: Reef Fish as Model Systems
This work, all done in collaboration with Dr. Sally J. Holbrook, addresses several related issues: causes of fluctuations in local abundances of reef fish populations; identifying the vital rates and life stages where density-dependence (hence regulation) occurs; estimating the relative contributions of an external supply of colonists, and subsequent density-independent and density-dependent mortality on setting patterns of local abundance; understanding the mechanisms producing density-dependence; and exploring biological and physical features that influence replenishment (recruitment) of local populations.
In our coral reef project involving damselfishes with dispersing larvae, we found that mortality of young damselfishes on coral reefs was strongly density-dependent, but only for the first few days after settlement. Notably, we found that the per capita probability of death and the strength of density-dependence were inversely related to body size of an individual, which explained different patterns of mortality among young of different age classes (because of fast body growth) and species (because of variation in size at settlement). Fish We also found that spatial patterns of variation in settlement within and between closely related species determines the strength of intra- and inter-specific density effects on per capita survival rates, and we identified the resource in short supply. Our work confirms that predators are the agent of density-dependent death of these tropical reef species, and that counter to current ideas for these systems, most successful predation events occurred during the night not during daylight hours. The latter was confirmed by direct observation using a novel infrared video imaging system that we developed. We found that spatial variation in the amount of appropriate shelter space used by juveniles and adults (which often differs) was an excellent predictor of spatial variation in abundance of several coral reef fishes. We subsequently tested experimentally the extent to which spatial variation in adult abundance of several reef fishes was related to availability of suitable shelter habitat and differences in settlement levels. These long-term (4 and 6 years) experiments revealed that for the locations, species and spatial scales examined, spatial variation in adult abundance was due entirely to availability of shelter space. One of the species examined was completely 'recruitment limited' (i.e., no density-dependence) whereas we found shelter space became limited for two other species. The limitation arose from the effects of residents reducing subsequent settlement. Fish These findings highlight the need to consider explicitly patterns of co-variation in larval supply and availability of suitable resources and to examine density effects on local input as well as loss rates. They also reinforce the growing appreciation by workers in these systems that local abundance typically is determined by a number of simultaneously acting processes. To advance our understanding of this issue further, I collaborated with Drs. Sally Holbrook and Craig Osenberg to develop one of the first operational frameworks to quantify the influence of an external supply of colonists and subsequent density-independent and density-dependent mortality. We collected the experimental and observational data necessary to use the approach, which illustrated the nature of interactions and nonlinearities among the processes. This approach provides a technique to compare across systems and thus enhance our ability to draw general conclusions regarding the processes that shape local abundance of species with demographically open populations. A meta-analysis of published experiments framed two major unsolved issues in this regard: general features that cause differences in ambient population densities among systems, and factors that create heterogeneity in the per capita strength of density-dependence. Our work on the latter issue revealed that fine-scale (~ 1 km) spatial heterogeneity in the strength of temporal density dependence in mortality of juvenile damselfish was well predicted by spatial variation in damselfish predators, which in turn was associated with structural complexity of the reef environment.
Our research involving surfperches on temperate reefs illustrates how fluctuations in local resources obscure straightforward 'stock-recruitment' relationships because per capita production of young can vary with food supply (or other critical resource). Food supply on a reef was an excellent predictor of the number of surfperches born as this is the commodity that adults transform into young. Local dynamics were well predicted by annual fluctuations in the food supply across reefs, revealing the underlying 'production-recruitment' relationship. We also have estimated the demographic and population consequences of interspecific competition for food between surfperches, as well as explored the nature of the interaction.
Consumer - Resource Interactions: Invertebrate Grazers as a Model System
My work in this area has examined both predator-prey interactions (particularly predator-mediated apparent competition) as well as exploitation competition. In addition to damselfishes and surfperches, a model system that I have focused on in this context consists of a pair of grazers (marine gastropods in the genus Tegula) that share the same predators and a common food resource (microalgae). My past work revealed that differences in foraging behaviors between the consumers result in predictable patterns of density-dependent effects on growth within and between the gastropods. The complementary foraging behaviors exhibited by the Tegula appear commonly in pairs of marine microherbivores that compete for food but also coexist. This motivated collaboration with Drs. Will Wilson, Roger Nisbet and Craig Osenberg that resulted in a model, parameterized by empirical data I collected, that showed such complementary foraging behaviors can be a sufficient mechanism of coexistence for species that compete exploitatively. The model subsequently has been generalized by others and the result was found to be robust.
Population and Community Responses to Environmental Forcing
I have been collaborating with Drs. Sally Holbrook and Andrew Brooks to determine how populations of reef organisms have responded to changes in potentially important environmental conditions in Southern California. During the past 3 decades, ocean surface temperatures have warmed substantially in the Southern California Bight, and we have found changes in the composition of reef fish communities that are consistent with expectations of such warming. In particular, the relative abundances of cold-water affinity species have declined and the representation of warm-affinity species has increased. However, abundances of > 90% of all species of reef fishes have declined an average of about 70% during the past decade. These changes mirror those observed in macrozooplankton in the California Current, and suggest there has been a widespread decline in environmental productivity that has affected both nearshore pelagic and benthic food webs. Our long-term data on abundances of organisms on 3 (linked) trophic levels of a benthic food web indicate that all trophic levels have declined concurrently by about the same magnitude. This finding is not consistent with current models of control and structure of benthic marine communities, and we are now addressing this issue. We are conducting 'meta-analyses' based on a number of long-term data sets. Our preliminary findings suggest that species on different trophic levels in different nearshore systems have all responded similarly to recent environmental changes.
UCSB's Marine Science Institute is home to the NSF-funded Santa Barbara Coastal Long Term Ecological Research (SBC LTER) program (http://sbc.lternet.edu/), which involves an interdisciplinary group of about 25 scientists led by Dr. Daniel Reed. The aim of the SBC LTER is to investigate the relative importance of land and ocean processes in structuring giant kelp forest ecosystems. I am associated with the 'Marine Reef' group of this integrated watershed - nearshore marine program and will be involved in estimating the relative importance of terrestrial and marine influences on population dynamics and community structure.
I also serve as Lead Investigator of the NSF-funded Moorea Coral Reef Long Term Ecological Research (MCR LTER) program (http://mcr.lternet.edu/. The MCR LTER site is the complex of coral reefs that surround the 60 km perimeter of Moorea in the Society Islands of French Polynesia in the South Pacific (4400 km south of Honolulu). The program is a partnership between UCSB and the California State University Northridge that includes researchers and students from 3 additional UC campuses (Davis, Santa Cruz, San Diego) and the University of Hawaii. Field operations are conducted from the UCB Gump Research Station on Moorea (http://moorea.berkeley.edu). The research program focuses on understanding the long-term consequences of disturbance and climate regimes in coral reef ecosystems, which requires knowledge of the major controls over reef dynamics.
Detection & Amelioration of Ecological Impacts
Detection of Ecological Impacts from Human Activities
I am interested in the application of ecological principles to environmental issues. My interest in estimating effect sizes from point source disturbances is well represented in a book, published by Academic Press, that I edited with Dr. Craig Osenberg, entitled Detecting Ecological Impacts: Concepts and Application in Coastal Environments (ISBN: 0126272557). In addition, I am interested in the effects of stressors on population dynamics, particularly involving species with demographically open populations. Ameliorating Ecological Impacts - Ecotechnology Approaches to Restoration
With my colleagues in biology and ecology, I recently have become involved in research to surmount a widely recognized obstacle to effective conservation: the scarcity of proven conservation and management tools. Current conservation efforts emphasize preservation and protection. In general, more - and more effective - techniques are needed to slow the rate at which ecosystems are being degraded and, in particular, to speed the rate at which they recover from human and natural disturbances. Research Dive The latter issue largely has been ignored despite evidence that degraded ecosystems frequently take an extraordinarily long time to recover. Better approaches are needed to resolve such urgent problems as the maintenance of biodiversity, the control of exotic species, the enhancement of depleted populations and the restoration of degraded habitats. Our broad objective is to develop and test a completely new genre of modern conservation tools that will be effective in resolving a range of problems.
To develop modern tools, we are pioneering a new, interdisciplinary approach that we termed Ecotechnology. The approach is to alter the behavior of individuals within ecosystems to obtain a desired ecological outcome (e.g., enhanced abundance of depleted species, restored diversity, control of invasive species, speedy recovery of damaged habitats). Our primary tactic will be to alter behaviors by harnessing key environmental signals that individuals use naturally and which govern the rate limiting process. Biologists identify ecological bottlenecks and signals that could be harnessed to rebuild populations, and test the effectiveness of developed tools in the field. Engineers develop and employ new technologies to package and deliver the signals to the targeted species, and to monitor their response in the field. Development of each new tool will arise only through focused integration of the work of biologists and engineers. We have developed and tested a practical tool to enhance the abundance and diversity of reef fishes by harnessing settlement cues of larval fishes.
- Schmitt, R.J. and S.J. Holbrook. 2007. The scale and cause of spatial heterogeneity in strength of temporal density dependence. Ecology in press.
- Thompson, A.R., R.M. Nisbet and R.J. Schmitt. 2006. Dynamics of mutualist populations that are demographically open. Journal of Animal Ecology 75: 1239-1251.
- Stewart, H.L., S.J. Holbrook, R.J. Schmitt and A.J. Brooks. 2006. Symbiotic crabs maintain coral health by clearing sediments. Coral Reefs 25: 609-615.
- Holbrook, S.J. and R.J. Schmitt. 2005. Growth, reproduction and survival of a tropical sea anemone (Actiniaria): benefits of hosting anemonefish. Coral Reefs 24: 67-73.
- Holbrook, S.J. and R.J. Schmitt. 2004. Population dynamics of a damselfish: effects of a competitor that also is an indirect mutualist. Ecology 85: 979-985.
- Schmitt, R.J. and S.J. Holbrook. 2003. Mutualism can mediate competition and promote coexistence. Ecology Letters 6: 898-902.
- Bernardi, G., S.J. Holbrook, R.J. Schmitt and N.L. Crane. 2003. Long-distance dispersal in an edge population of the coral reef three-spot damselfish Dascyllus trimaculatus. Marine Biology 143: 485-490.
- Holbrook, S.J. and R.J. Schmitt. 2003. Spatial and temporal variation in mortality of newly settled damselfishes: patterns, causes and co-variation with settlement. Oecologia 135:532-541.
- Holbrook, S.J. and R.J. Schmitt. 2002. Competition for shelter space causes density-dependent predation mortality in damselfishes. Ecology 83: 2855-2868.
- Osenberg, C.W., C.M. St. Mary, R.J. Schmitt, S.J. Holbrook, P. Chesson and B. Byrne. 2002. Rethinking ecological inference: density-dependence in reef fishes. Ecology Letters 5: 715-721.