ABSTRACT
Detecting the environmental impacts of human activities on natural communities is a central problem in applied ecology. It is a difficult problem because one must separate human perturbations from the considerable natural temporal variability displayed by most populations. In addition, most human perturbations are generally unique and thus unreplicated. This raises the problem of deciding whether observed local effects are due to human intervention or to the natural differences in temporal patterns that often occur among different sites. These problems can be successfully addressed with the Before-After/Control-Impact (BACI) sampling design, in which Impact and Control sites are sampled contemporaneously and repeatedly in periods Before and After the human perturbation of interest. In the present case, we use this design to examine the ecological effects of the cooling water discharge from a coastal nuclear power plant in southern California. The results suggest some general lessons about the process of impact assessments that are applicable in many ecological contexts. In systems where plants and animals are long-lived and recruit sporadically, the rates of change in density are often so low that sampling more than a few times per year will introduce serial correlations in the data. As a result, for studies of few years duration, few samples will be taken. A small sample size means that the tests of the assumptions underlying the statistical analyses, e.g., independence and additivity, will have low power. This injects uncertainty into the conclusions. Small sample size also means that the power to detect any but very large effects will be low. In our study, sampling periods of 2- yr both Before and After the impact were not long enough to detect a halving or doubling of populations at the impact site. We concluded that there were significant environmental impacts because: (1) the effect size was generally very large (°-75%); (2) there was a consistent pattern among species; (3) there were two Impact sites, and effects were larger at the site nearest the discharge; (4) the observed effects accorded with physical changes that could be linked with the source of impact; and (5) a number of alternative mechanisms, unrelated to the source of impact, were examined and rejected. Relative to control populations, there were statistically significant reductions in density of snails, sea urchins, and sea stars, all of which occurred primarily on rocky substrates. All of the reductions were larger at the Impact station about 0.4 km from the discharge than at a second Impact station 1.4 km away. The most plausible mechanisms for the declines seem to be linked to the turbidity plume created by the power plant and the resultant increase in suspended inorganic and organic materials (+46% at the Impact site nearest the discharge). Any associated flux of fine particles on rocks would have deleterious effects on many of the hard benthos. Populations of two filter-feeding species, a gorgonian coral and a sponge, showed relative increases in density. Although the increase in populations of filter feeders could be related to the ingestion, killing, and discharge of tons of plankton by the cooling system, an alternative natural mechanism was also considered reasonable. Monitoring studies or relatively long-lived organisms will often have low power to detect ecologically significant changes in density. The present study of kelpforest organisms extended over nearly 6 yr, yet the resulting statistical tests generally had power of <30% to detect a doubling or halving in density at a significance level of .05. In such a community it would be a mistake to conclude that there were no significant ecological effects based on conventional hypothesis tests. Unless there is a willingness to accept the fact that changes in natural populations on the order of 50% will often go undetected, the standards and types of evidence used to demonstrate environmental impacts must be changed.
ABSTRACT
At two stations in the San Onofre kelp bed near San Clemente, California the abundance of sea urchins, Lytechinus anamesus, and starfish, Patiria miniata, were negatively correlated. At one station, urchins were more abundant outside compared to inside the kelp bed, a pattern which generally occurs in southern California. In contrast, at the other station urchins were more abundant inside the bed. This exception to the general rule indicates that the distributions were not simply determined by something associated with the presence of kelp. Samples of substrates, sea urchins and sea stars suggested that the distribution of Patiria but not Lytechinus was controlled by the availability of suitable substrates. Further experiments showed that: 1) Patiria elicit an escape response from Lytechinus, the strength of which is positively related to the size of the urchins, 2) small urchins, which react less strongly to Patiria, are preyed on more heavily than are larger ones, and 3) escape responses elicited by Patiria cause local reductions of Lytechinus in the field. We conclude from these results that predation by Patiria controls the distribution of Lytechinus at our study sites, and may account for this species distribution in kelp forests throughout southern California. The possible effects of this predator-prey interaction on the structure of kelp forest communities are discussed.
ABSTRACT
The biological and physical factors that affect the distribution of the subtidal, epibenthic, sand-dwelling cnidarian Renilla kollikeri along a depth gradient from 2 to 13 m were determined by manipulative field and laboratory experiments. The sea pansy's nearshore distributional limit (2.5 m in depth) was set by the animal's inability to remain anchored on the bottom in the face of increasing wave surge associated with shallow water. Renilla was outcompeted for space by the sand dollar Dendraster excentricus in an area between 6 and 9 m in depth and was largely excluded from the substrate when sand dollar densities exceeded 350/m2. Predator-prey interactions also influence the Renilla distribution. The asteroid Astropecten brasiliensis feeds on and removes Renilla 40 mm and less in diameter from areas seaward of the sand dollar bed (9 m in depth). Astropecten did not forage in the shallow area where Renilla was most abundant because of its intolerance to wave surge and because of its difficulty in crossing the sand dollar bed. The nudibranch Armina californica fed on Renilla throughout the depth gradient but did not remove colonies greater than 20 mm in diameter because of the Armina- specific escape behavior of Renilla. The foraging behavior of Armina was also curtailed by strong surge conditions. The nearshore area (3-6 m in depth) of high Renilla density was a refuge in which animals that had settled from the plankton could escape predation. As the animals grow they lose their ability to anchor in the substrate in shallow water and thus move into deeper water. The results of multiway interactions among sets of three species, particularly the beneficial results of the competitive interaction with Dendraster in light of the activities of the predators are discussed with reference to the abundance and distribution of Renilla.
