Dispersal dynamics
Despite recent progress, physical and biological processes that regulate organismal dispersal are still poorly understood (particularly in marine systems). Yet dispersal is vital to determining the extent of demographic and genetic interactions among life history stages, individuals, and populations. My research has focused on the role of the production and supply of planktonic propagules (zoospores) in limiting kelp recruitment. After developing a spectrophotometric technique for identifying kelp zoospores from field plankton samples, I studied spatio-temporal variability in zoospore abundance, the relative importance of local vs. remote reproduction to zoospore supply, and the utility of treating kelp population dynamics as either open or closed. I found that the role of physical transport and reproduction in regulating zoospore abundance was scale-dependent, as the drag of adult plants altered flow characteristics of the water column, and ultimately the distance zoospores dispersed. Currents within large giant kelp forests (Macrocystis pyrifera) had low net displacement whereas currents outside of forests had high net displacement. As a result, zoospores were retained near their parents in forest centers (coupling zoospore supply to local reproduction), but transported away from their parents along forest edges (de-coupling zoospore supply from local reproduction). Small kelp forests have less of an effect on currents, and thus dispersal. I am currently building particle-dispersal models of the effect of spatio-temporal variability in forest size and location on kelp dispersal.

Emergent properties of organismal aggregations
My finding that coupling between the production and supply of zoospores was related to kelp forest size demonstrated that scaling ecological processes from individuals to aggregations can yield unpredictable patterns and identify novel population dynamics. For example, as a newly colonized kelp population develops from a few founding individuals into a large forest, enhanced flow-dampening likely shifts the population from being a zoospore exporter to a zoospore retainer. Such dispersal modification may facilitate self-recruitment, hasten the expansion of individual kelp forests, regulate the demographic and genetic linkages among kelp forests, and alternate individual forests between serving as propagule sources or sinks. I am addressing the following questions through a series of modeling and field studies: How large must kelp forests be before flow is modified enough to affect coupling between the production and supply of zoospores? Can the relationship between reproductive coupling and forest size be quantified? If so, is there a minimum size above which kelp forests shift from zoospore exporters to zoospore retainers? I am also investigating whether the patterns observed for giant kelp are characteristic of marine habitat-forming species in general (i.e. other kelps and seaweeds), with specific applications to the design of Marine Protected Areas and the construction of Artificial Reef Habitats.

Historical vs. contemporary processes
Given an understanding of the effect of population size on dispersal, much of population and genetic structure may be understood from present-day patterns of distribution and abundance. In natural systems, however, distribution and abundance are not static as variability in climactic (e.g. temperature, solar insolation) and geological (e.g. sea-level, tectonics) processes are important constraints on organismal physiology and survival, and vary greatly over both ecological and evolutionary time scales. In various collaborations, I have been investigating the role of historical fluctuations in population size and isolation on kelp population and genetic structure. We have used GIS and remote sensing of giant kelp surface canopies to study the relationship between decadal-scale variability in kelp forest size and abundance, and to reconstruct late-Quaternary fluctuations (millennial-scale) in giant kelp distribution from coastal bathymetries, sea-level fluctuations, and paleo-temperatures. These historical distribution data were then used to develop specific hypotheses concerning the demographic and structure of present-day giant kelp forest organisms, and even humans, to be tested with various proxy data (stable isotopes, molecular markers, etc.). We are also using these data to compare insular species-area relationships among kelp forest, rocky intertidal, and terrestrial communities for the California Channel Islands, with the benefit of predicting from the GIS studies how island area and isolation have changed over the last 20,000 yrs. These techniques are equally applicable to island systems of volcanic origin, and in Summer 2006 I will be leading a National Geographic exploration to the Galapagos Islands to study similar processes in tropical seaweed systems.

Comparative life histories
Despite over 50 years of intensive field and laboratory research, much remains to be understood about the processes that determine the dynamics of seaweed populations, and the subsequent consequences to the diversity and productivity of their associated communities. This limitation is due largely to the fact that processes regulating seaweed system productivity, dynamics and diversity are determined by how different species interact with environmental variability over ecological and evolutionary timescales, yet present-day paradigms in macroalgal ecology have been motivated by studies focusing on particular life history stages of conspicuous taxa. In the absence of broad comparative life history studies, it is impossible to determine how different life history stages work together to drive seaweed population dynamics, and the extent to which among-taxa variability in “key” life history traits constrains the diversity and productivity of global ecosystems. Through a series of laboratory and field experiments I am testing the null hypothesis that kelp life history evolution is independent of kelp recruitment variability, a rejection of which would indicate that all kelp forests are not “created equal”, and establish the utility of kelp biogeography and evolutionary history as predictors of the productivity and diversity potential of global kelp ecosystems. I am using field experiments to study the physiological tradeoff between kelp sporophyte growth and reproduction, and flexibility in the timing of zoospore release; laboratory culture experiments to determine whether kelp gametogenesis, fertilization, and growth of kelp microscopic stages are driven by exogeneous versus endogenous cues; and field experiments to test the relative contribution of pre- versus post-settlement processes on kelp recruitment. This study includes 19 species of intertidal and subtidal kelp from California and Chile, spanning all 3 extant kelp families, and represents ~20% of all known kelp taxa. Each of these species represents a different combination of the various life history traits considered important to the regulation of kelp recruitment, allowing for a unique opportunity to study the interaction between life history variability and population persistence. In addition to testing the generality of experimental results across many taxa, the broad taxonomic sampling allows for the assessment of the extent to which kelp life history evolution is phylogenetically constrained and convergent evolution of particular traits has occurred.

Ecological consequences of variable seaweed diversity
Understanding variability in population dynamics of habitat-forming species is vital to studies of the functioning of their associated communities, as processes regulating habitat structure and energy flow are no longer mysterious. In many systems, multiple habitat-forming species may be present and it has been hypothesized that such increases in producer diversity may benefit the functioning of their associated systems. Although I have worked extensively on population and community processes in kelp systems, the sheer magnitude of species richness in these systems precludes the experimental investigation of kelp diversity effects on ecosystem function. As such, I have begun to explore this question in a more simple seaweed-based system, the mid-intertidal of the central California coast. In collaboration with Dr. Jay Stachowicz (UC Davis), I have been conducting a series of field and laboratory experiments to examine the effects of intertidal macroalgal (seaweed) diversity on primary production and the diversity, abundance, and reproduction of higher trophic levels (consumer species). Specifically, we are: examining the effects of macroalgal diversity manipulation on plot-wide primary production and associated variables in the field; using laboratory experiments with assembled communities to assess the effect of diversity on community-wide seaweed photosynthetic rate in air vs. water and nutrient utilization; and using field experiments to examine the effects of algal diversity on the diversity, growth, recruitment and abundance of mobile consumer species. This combined and integrated approach is allowing us to assess, for this community, the role that producer diversity plays in determining a suite of critical ecosystem functions. Our experiments specifically target key forage species and are providing the first test of the effects of primary producer species diversity on marine ecosystem function. We view this work as a critical first step toward assessing the importance of accounting for basal species diversity in the design and implementation of conservation and management efforts such as MPAs.