Size, growth, and density data for shallow-water sea urchins from Mexico to the Aleutian Islands, Alaska, 1956-2016.

Size, growth, and density have been studied for North American Pacific coast sea urchins Strongylocentrotus purpuratus, S. droebachiensis, S. polyacanthus, Mesocentrotus (Strongylocentrotus) franciscanus, Lytechinus pictus, Centrostephanus coronatus, and Arbacia stellata by various workers at diverse sites and for varying lengths of time from 1956 to present. Numerous peer-reviewed publications have used some of these data but some data have appeared only in graduate theses or the gray literature. There also are data that have never appeared outside original data sheets. Motivation for studies has included fisheries management and environmental monitoring of sewer and power plant outfalls as well as changes associated with disease epidemics. Studies also have focused on kelp restoration, community effects of sea otters, basic sea urchin biology, and monitoring. The data sets presented here are a historical record of size, density, and growth for a common group of marine invertebrates in intertidal and nearshore environments that can be used to test hypotheses concerning future changes associated with fisheries practices, shifts of predator distributions, climate and ecosystem changes, and ocean acidification along the Pacific Coast of North America and islands of the north Pacific. No copyright restrictions apply. Please credit this paper when using the data.

Direct and indirect effects of giant kelp determine benthic community structure and dynamics.

Indirect facilitation can occur when a species positively affects another via the suppression of a shared competitor. In giant kelp forests, shade from the canopy of the giant kelp, Macrocystis pyrifera, negatively affects understory algae, which compete with sessile invertebrates for space. This raises the possibility that giant kelp indirectly facilitates sessile invertebrates, via suppression of understory algae. We evaluated the effect of giant kelp on the relative abundance of algae and invertebrates by experimentally manipulating kelp abundance on large artificial reefs located off San Clemente, California, USA. The experiments revealed a negative effect of giant kelp on both light availability and understory algal abundance and a positive effect on the abundance of sessile invertebrates, which was consistent with an indirect effect mediated by shade from the kelp canopy. The importance of these processes to temporal variability in benthic community structure was evaluated at 16 locations on natural reefs off Santa Barbara, California, over an eight-year period. Interannual variability in the abundance of understory algae and in the abundance of sessile invertebrates was significantly and positively related to interannual variability in the abundance of giant kelp. Analysis of these observational data using Structural Equation Modeling (SEM) indicated that the magnitude of the indirect effect of giant kelp on invertebrates was six times larger than the direct effect on invertebrates. Results suggest that the dynamics of this system are driven by variability in the abundance of a single structure-forming species that has indirect positive, as well as direct negative, effects on associated species.

The recruitment sweepstakes has many winners: genetic evidence from the sea urchin Strongylocentrotus purpuratus.

As a consequence of free spawning in the unpredictable nearshore environment, marine species with large fecundities and high pre-reproductive mortality may be subject to extreme variance in reproductive success. If the unpredictability of the ocean results in only a small subset of the adult population contributing to each larval cohort, then reproduction may be viewed as a sweepstakes, with chance events determining which adults are successful each spawning season. Such a reproductive sweepstakes scenario may partially account for large reductions in effective population sizes relative to census population sizes in marine species. We evaluated two predictions of the sweepstakes reproductive success hypothesis by testing: (1) whether sea urchin recruits contain reduced genetic variation relative to the adult population; and (2) whether cohorts of sea urchin recruits are genetically differentiated. Mitochondrial DNA sequences were collected from 283 recently settled Strongylocentrotus purpuratus recruits from four annual cohorts spanning seven years in locations throughout California. Observed haplotype numbers and haplotype diversities showed little evidence of reduced genetic variation in the recruits relative to the diversity estimated from a previously reported sample of 145 S. purpuratus adults. Different cohorts of recruits were in some cases mildly differentiated from each other. A computer simulation of sweepstakes recruitment indicates that our sampling strategy had sufficient statistical power to detect large variances in reproductive success.

Detecting the Ecological Effects of Environmental Impacts: A Case Study of Kelp Forest Invertebrates.

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.

Recolonization of Deep-Water Hard-Substrate Communities: Potential Impacts From Oil and Gas Development.

Many regions of the California, USA, outer continental shelf are subject to ongoing and planned oil and gas development, but there is very limited information on the recolonization and recovery of deep-water (e.g., 60-300 m) hard-substrate communities that may be affected by these activities. The purpose of this study was to summarize existing information on these communities, thereby aiding regulatory agencies and the scientific community in assessing potential impacts, and to determine whether additional studies may be necessary. Impacts from anchoring and increased sedimentation (e.g., from discharges of drilling muds and cuttings) are particularly likely. Sedimentation effects can be natural of human-induced, but habitat disruption from anchoring has few natural parallels. Disturbances from anchoring and sedimentation generally produce Type 1 and Type 2 patches (sensu Sousa 1985, Connell and Keough 1985), respectively. Recolonization and recovery of these patches in deep-water environments are expected to vary in accordance with current models of succession (i.e., inhibition, facilitation, and tolerance). However, inhibition responses likely will predominate in Type 1 patches, particularly high-relief (e.g., > 1 m) areas, where regrowth from the margins often is possible from sheet-like or mound-like forms such as those represented by many sponge taxa. In contrast, recovery of Type 2 patches, particularly low-relief (e.g., <1 m) areas, also may be influenced primarily by inhibition responses, but the relative lack of nearby colonizers suggests greater numbers of species interactions in accordance with facilitation and tolerance models. This is due to the greater stochastic component associated with recolonization by long-range larval dispersers. Recovery is expected to require a few to several years to accomplish for these deep-water hard-substrate communities. This is based on estimated times for recovery of slow-growing, generally long-lived taxa, such as some vase sponges and anemones characteristic of many high-relief areas, and the uncertainties of long-range recruitment coupled with variable sediment movement in many low-relief areas.

Effects of the starfish Patiria miniata on the distribution of the sea urchin Lytechinus anamesus in a southern Californian kelp forest.

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.

Latitudinal variation in intertidal algal community structure: the influence of grazing and vegetative propagation.

The hypothesis that sea urchin grazing and interactions with turf-forming red algae prevent large brown algae from forming an extensive canopy in the low intertidal zone of southern California was tested with field experiments at two study sites. Experimental removal of sea urchins resulted in rapid algal recruitment. Crustose coralline algae which typically dominate the substratum in areas with dense urchin populations were quickly overgrown by several species of short-lived green, brown and red algae. The removal of urchins also significantly increased the recruitment of two long-lived species of large brown algae (Egregia laevigata and Cystoseira osmundacea at one study site and E. laevigata and Halidrys dioica at the other). The experimental plots at both sites were eventually dominated by perennial red algae.A two-factorial experiment demonstrated that sea urchin grazing and preemption of space by red algae in areas where urchins are less abundant are responsible for the rarity of large brown algae in the low intertidal of southern California. The three dominant perennial red algae, Gigartina canaliculata, Laurencia pacifica and Gastroclonium coulteri, recruit seasonally from settled spores but can rapidly fill open space with vigorous vegetative growth throughout the year. These species encroach laterally into space created by the deaths of large brown algae or by other disturbances. Once extensive turfs of these red algae are established further invasion is inhibited. This interaction of algae which proliferate vegetatively with algae which recruit only from settled spores is analogous to those which occur between solitary and colonial marine invertebrates and between solitary and cloning terrestrial plants.It is suggested that a north-south gradient in the abundance of vegetatively propagating species, in grazing intensity and in the frequency of space-clearing disturbances, may account for latitudinal variation in intertidal algal community structure along the Pacific coast of North America.