Abundance, Dynamics and Coexistence of Interacting Damselfishes: Quantifying the Contributions of Causal Processes Across Spatial Scales

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

The aim of this study is to understand the causes of variation in abundance of reef fishes whose local populations are linked via exchange of reproductive output. Research over the past two decades has revealed that the size of a local population of these organisms typically is determined by several processes that operate simultaneously, and that the key to understanding the relative importance of these processes is the interplay between larval supply and density -dependent interactions. There also has been growing appreciation that a local population cannot be considered in isolation of other sub-populations or species, and that multiple spatial scales need to be examined to fully understand the dynamics of the system. Finally, several frameworks now exist that can allow the contributions of various processes in setting local abundance to be quantified and the consequences of patterns of covariation in environmental features (e.g., larval supply of potential competitors, habitat availability, food supply, predation pressure) to be evaluated. These quantitative frameworks need to be applied to real systems to resolve several fundamental - and largely unanswered - questions for reef fishes. These include the relative importance of the multiple processes that affect local abundance and dynamics, the degree of variability in their importance, the determinants of relative abundance and coexistence of competitors, where and how population regulation occurs, and whether the dynamical behavior of the system as a whole differs from that predicted from dynamics at smaller spatial scales. Quantifying the effects of various processes for multiple species will enable ecologists to ask how the intensity of these processes varies as a function of general characteristics of species (e.g., life history traits, trophic status, types of environments). Examining covariation between environmental features and density dependence will deepen our insight into the structure, regulation and behavior of such systems. This study will focus on two pairs of co-occurring species of damselfishes that inhabit lagoons of French Polynesia. These planktivores compete intra- -and inter-specifically for shelter space from predators. Using short- and long-term experiments and observations, the variety of data needed to apply two different quantitative approaches to understanding dynamics of stage--structured, demographically open populations will be obtained. In particular, spatial variation in output (time-averaged adult density) will be estimated as a function of input (larval supply per unit suitable habitat) and relevant parameters that determine local abundance (density-independent survivorship; adult saturation or peak density; density dependence in settlement from conspecifics and heterospecifics; density dependence in survivorship from interactions with conspecifics and heterospecifics). In addition to larval supply, variation in other environmental features that will be estimated include microhabitat availability and predation pressure. These variables will be measured across several spatial scales (from -5 in to 200 km). Data from empirical work will be used in the quantitative frameworks to compare the relative importance of various processes among species and spatial scales, to address issues related to coexistence of competing species with open populations, and to examine whether dynamics at larger scales differ from that predicted from local dynamics.