Predicting Physical Disturbance in a Changing Environment: The Effect of Spatial and Temporal Scale

Stanford University, Stanford CA

Investigators

Abstract

Current ideas regarding the roles of competition, predation, recruitment, succession, and disturbance in community ecology are based in part on experiments in the intertidal zone of wave-swept rocky shores, and these studies serve as benchmarks for the ability of ecological theory to make predictions about the real world. Much of the experimental utility of the shore is due to the rapid turnover of individuals in the community. On exposed shores where turnover is due primarily to environmental effects, the ultimate ability to predict the distribution and abundance of organisms rests on our proximal ability to predict the physical environment at all relevant spatial and temporal scales, and on our ability to account for the biological consequences of environmental stress. Field experiments have shown that it is feasible to predict from offshore wave height the maximum wave forces imposed at a given location on the shore. However, these forces (an important environmental stress) vary substantially through both space and time in a pattern known as 1/f-noise: the larger the spatial or temporal scale at which the shore is examined, the larger the variation measured. As a result, it is difficult to specify unambiguously the wave exposure of a site. Because many aspects of intertidal community dynamics are closely tied to wave exposure, the 1/f-noise characteristic of the shore becomes potentially problematic when an attempt is made to generalize ("scale up") the results of small-scale experiments. Preliminary measurements of species diversity suggest, however, that it is possible for plants and animals to interact with the physical environment in a fashion that produces well-defined spatial structure even in the presence of physical 1/f-noise. This project builds on these results by examining how several basic ecological processes (e.g., recruitment, growth, and predation intensity) vary across spatial and temporal scales, and how these scales contribute to the overall pattern in community dynamics. Field experiments will be conducted along a horizontal transect in the intertidal zone for which previous work has characterized the spatial variation in maximum wave-induced force and maximum temperature. Simultaneous measurements will be made at 2-m intervals along a 200-m transect of: the rate of recruitment of mussels and barnacles, the rate of growth of mussels and barnacles, the intensity of gastropod predation on barnacles, disturbance in the mussel bed, and the course of succession on previously unoccupied substratum. Quarterly measurements will be made of species abundances at each location, and from these species diversity is calculated at each of the 100 points on the transect. Because these measurements are made at equally spaced points, it is possible to use spectral analysis to examine the scale-specific cross-correlation between the variation in each ecological process or biological attribute and the co-occurring variation of the physical environment (wave force and temperature). In conjunction with the experiment on predation intensity, measurements will be made of the adhesive tenacity and foraging speed of the primary gastropod predators of barnacles (Nucella and Acanthina). These field experiments will provide a direct measure of whether there are defined scales for ecological processes, and (if so) how biology interacts with environmental 1/f-noise to yield the relevant scale(s) of community dynamics. In addition to exploring the spatial and temporal variation in wave forces, previous work has shown that maximum wave-induced forces on the shore correspond to water velocities that are approximately twice those predicted by standard theories of wave breaking. The cause of these extreme velocities (up to 25 m/s) is thought to be tied to the interaction of breaking waves with the complex topography of the rocky shore. For example, when breaking waves are refracted so that two wave fronts collide, a jet of water is produced the velocity of which is substantially greater than that of either wave. Experiments will be conducted in a laboratory wave tank to characterize the flows resulting when waves break on a variety of model shores, thereby to delineate the circumstances under which enhanced velocities can be expected. In conjunction with these experiments measurements will be made of the force exerted on objects held fixed to the substratum. These measurements (in addition to similar measurements conducted in the field) will be used to test the possibility that the apparently extreme water velocities calculated from force measurements are in fact an artifact of the manner in which the leading edge of a wave impacts plants and animals on the shore. This work will allow one to quantify for the first time the inherent spatial and temporal scales of processes that govern community dynamics on wave-swept shores, and thereby to forge mechanistic links among the wave "weather," the shoreline topography, and the processes of recruitment, growth, predation, and succession. The elucidation of these mechanisms represents an important first step toward an ability to make large-scale mechanistic predictions from small-scale interactions, predictions that may be important inputs into the appropriate design of coastal marine protected areas.