Carol A. Blanchette

The Keystone Species Concept: Variation in Interaction Strength in a Rocky Intertidal Habitat

The usefulness and generality of the keystone species concept has recently been questioned. We investigated variation in interaction strength between the original keystone predator, the seastar Pisaster ochraceus, and its primary prey, mussels (Mytilus californianus and M. trossulus). The study was prompted by differences in community structure at two low zone sites along the central Oregon coast, Boiler Bay (BB) and Strawberry Hill (SH). Predators, especially seastars, were larger and more abundant at SH than at BB. Further, sessile animals were more abundant and macrophytes were less abundant at SH. Predators were more abundant at wave—exposed sites at both sites, and at SH, sessile invertebrates were more abundant at the wave—exposed location and sand cover was high at the wave—protected location. To test the hypothesis that variation in predation strength explained some of these differences, we examined the seastar—mussel interaction at locations with high and low wave exposure at both sites. Predation intensity was quantified by determining the survival of mussels in clumps (50 mussels per clump, shell length 4—7 cm) transplanted to large plots (18—163 m2) with or without seastars in the low intertidal zone. Predation effects were quantified by determining prey recolonization rates in marked quadrats in the same large plots. Spatial variation in interaction strength was quantified by examining predation at scales of metres (among transplants within plots), 10s of metres (between replicate plots within each exposure at each site), 100s of metres (between wave exposures within locations), and 10 000s of metres (between sites). Temporal variation was evaluated by performing the experiments in 1990 and 1991. The relation between prey (mussel) recruitment and growth to differences in community structure was evaluated by quantifying recruitment density in plastic mesh balls (collectors) and growth of individually marked transplanted mussels, respectively, at each site ° exposure ° tide level combination each month for 4 yr. Predation intensity varied greatly at all spatial scales. At the two largest spatial scales (10s of kilometres, 100s of metres), differences in both survival of transplanted mussels and prey recolonization depended on variation in seastar abundance with site, wave exposure, prey recruitment and growth, and at SH protected, the extent of sand burial. Variation at the two smallest scales (metres, 10s of metres) was high when seastars were scarce and low when seastars were abundant. Transplanted mussels suffered 100% mortality in 2 wk at wave—exposed SH, but took >52 wk at wave—protected BB. Seastar effects on prey recolonization were detected only at the SH wave—exposed site. Here, where prey recruitment and growth were unusually high, the mussel M. trossulus invaded and dominated space within 9 mo. After 14 mo, whelks, which increased in both size and abundance in the absence of Pisaster, arrested this increase in mussel abundance. Similar changes did not occur at other site ° exposure combinations, evidently because prey recruitment was low and possibly also due to whelk predation on juveniles. Longer term results indicate that, as in Washington state, seastars prevent large adult M. californianus from invading lower intertidal regions, but only at wave—exposed, not wave—protected sites. Thus, three distinct predation regimes were observed: (1) strong keystone predation by seastars at wave—exposed headlands; (2) less—strong diffuse predation by seastars, whelks, and possibly other predators at a wave—protected cove, and (3) weak predation at a wave—protected site buried regularly by sand. Comparable experimental results at four wave—exposed headlands (our two in Oregon and two others in Washington), and similarities between these and communities on other West Coast headlands suggest keystone predation occurs broadly in this system. Results in wave—protected habitats, however, suggest it is not universal. In Oregon, keystone predation was evidently contingent on conditions of high prey production (i.e., recruitment and growth), while diffuse predation occurred when prey production was low, and weak predation occurred when environmental stress was high. Combining our results with examples from other marine and non—marine habitats suggests a need to consider a broader range of models than just keystone predation. The predictive and explanatory value of an expanded set of models depends on identifying factors distinguishing them. Although evidence is limited, a survey of 17 examples suggests (1) keystone predation is evidently not distinguished from diffuse predation by any of 11 previously proposed differences, but (2) may be distinguished by rates of prey production. Further, (3) differential predation on competitively dominant prey does not distinguish keystone from nonkeystone systems, since this interaction occurs in both types of community. Instead, differential predation on dominant prey evidently distinguishes strong—from weak—predation communities. While the keystone predation concept has been and will continue to be useful, a broadened focus on testing and developing more general models of community regulation is needed.

WHAT DETERMINES THE STRENGTH OF A TROPHIC CASCADE

Trophic cascades have been documented in a diversity of ecological systems and can be important in determining biomass distribution within a community. To date, the literature on trophic cascades has focused on whether and in which systems cascades occur. Many biological (e.g., productivity : biomass ratios) and methodological (e.g., experiment size or duration) factors vary with the ecosystem in which data were collected, but ecosystem type, per se, does not provide mechanistic insights into factors controlling cascade strength. Here, we tested various hypotheses about why trophic cascades occur and what determines their magnitude using data from 114 studies that measured the indirect trophic effects of predators on plant community biomass in seven aquatic and terrestrial ecosystems. Using meta-analysis, we examined the relationship between the indirect effect of predator ma- nipulation on plants and 18 biological and methodological factors quantified from these studies. We found, in contrast to predictions, that high system productivity and low species diversity do not consistently generate larger trophic cascades. A combination of herbivore and predator metabolic factors and predator taxonomy (vertebrate vs. invertebrate) explained 31% of the variation in cascade strength among all 114 studies. Within systems, 18% of the variation in cascade strength was explained with similar predator and herbivore char- acteristics. Within and across all systems, the strongest cascades occurred in association with invertebrate herbivores and endothermic vertebrate predators. These associations may result from a combination of true biological differences among species with different phys- iological requirements and bias among organisms studied in different systems. Thus, al- though cascade strength can be described by biological characteristics of predators and herbivores, future research on indirect trophic effects must further examine biological and methodological differences among studies and systems.

More than a meal… integrating non‐feeding interactions into food webs

Organisms eating each other are only one of many types of well documented and important interactions among species. Other such types include habitat modification, predator interference and facilitation. However, ecological network research has been typically limited to either pure food webs or to networks of only a few (<3) interaction types. The great diversity of non-trophic interactions observed in nature has been poorly addressed by ecologists and largely excluded from network theory. Herein, we propose a conceptual framework that organises this diversity into three main functional classes defined by how they modify specific parameters in a dynamic food web model. This approach provides a path forward for incorporating non-trophic interactions in traditional food web models and offers a new perspective on tackling ecological complexity that should stimulate both theoretical and empirical approaches to understanding the patterns and dynamics of diverse species interactions in nature.

SPECIES INTERACTION STRENGTH: TESTING MODEL PREDICTIONS ALONG AN UPWELLING GRADIENT

A recent model predicts that species interactions in benthic marine communities vary predictably with upwelling regimes. To test this model, we studied the Pisaster–Mytilus interaction at 14 rocky intertidal sites distributed among three oceanographic regions along a 1300-km stretch of the U.S. West Coast. Regions included an intermittent-upwelling region (northern), a persistent-upwelling region (central), and a region of weak and infrequent upwelling (southern). We quantified predation rates by the sea star Pisaster ochraceus on its main prey Mytilus californianus by transplanting mussels into the sea stars low-zone foraging range and comparing the rate of mussel loss in +Pisaster plots to those in −Pisaster plots. To evaluate the relation between predation rates and key ecological processes and conditions, we quantified phytoplankton concentration and rates of mussel recruitment, mussel growth, mussel abundance, and sea star abundance. Predictions of the model are expressed as responses of predator an...

SPATIAL AND TEMPORAL PATTERNS OF INVERTEBRATE RECRUITMENT ALONG THE WEST COAST OF THE UNITED STATES

Patterns of recruitment in marine ecosystems can reflect the distribution of adults, dispersal by ocean currents, or patterns of mortality after settlement. In turn, patterns of recruitment can play an important role in determining patterns of adult abundance and community dynamics. Here we examine the biogeographic structure of recruitment variability along the U.S. West Coast and examine its association with temperature variability. From 1997 to 2004 we monitored monthly recruitment rates of dominant intertidal invertebrates, mussels and barnacles, at 26 rocky shore sites on the West Coast of the United States, from northern Oregon to southern California, a span of 1750 km of coastline. We examined spatial variation in the dynamics of recruitment rates and their relationship to coastal oceanography using satellite-derived time series of monthly sea surface temperature (SST). Recruitment rates showed a biogeographic structure with large regions under similar dynamics delimited by abrupt transitions. The seasonal peak in recruitment rates for both mussels and barnacles changed from a late summer-early fall peak in Oregon to winter or early spring in northern California, and then back toward summer in southern California. Recruitment rates varied greatly in magnitude across the latitudinal range. The barnacle Balanus glandula and mussels (Mytilus spp.) showed a decline of two orders of magnitude south of Oregon. In contrast, recruitment rates of barnacles of the genus Chthamalus showed a variable pattern across the region examined. The spatial distribution of associations between raw SST and recruitment rates for all species showed positive associations, indicating recruitment during warm months, for all species in Oregon, northern California, and several sites in south-central California. By considerably extending the spatial and temporal scales beyond that of previous studies on larval recruitment rates in this system, our study has identified major biogeographic breaks around Cape Blanco and Point Conception despite considerable spatial and temporal variation within each region and among species. These large differences in recruitment rates across biogeographic scales highlight the need for a better understanding of larval responses to ocean circulation patterns in the conservation and management of coastal ecosystems.

Scales of Dispersal and the Biogeography of Marine Predator‐Prey Interactions

Striking differences in the dispersal of coexisting species have fascinated marine ecologists for decades. Despite widespread attention to the impact of dispersal on individual species dynamics, its role in species interactions has received comparatively little attention. Here, we approach the issue by combining analyses of simple heuristic predator‐prey models with different dispersal patterns and data from several predator‐prey systems from the Pacific coasts of North and South America. In agreement with model predictions, differences in predator dispersal generated characteristic biogeographic patterns. Predators lacking pelagic larvae tracked geographic variation in prey recruitment but not prey abundance. Prey recruitment rate alone explained more than 80% of the biogeographic variation in predator abundance. In contrast, predators with broadcasting larvae were uncorrelated with prey recruitment or adult prey abundance. Our findings reconcile perplexing results from previous studies and suggest that simple models can capture some of the complexity of life‐history diversity in marine communities.

Detecting the Unexpected: A Research Framework for Ocean Acidification

The threat that ocean acidification (OA) poses to marine ecosystems is now recognized and U.S. funding agencies have designated specific funding for the study of OA. We present a research framework for studying OA that describes it as a biogeochemical event that impacts individual species and ecosystems in potentially unexpected ways. We draw upon specific lessons learned about ecosystem responses from research on acid rain, carbon dioxide enrichment in terrestrial plant communities, and nitrogen deposition. We further characterize the links between carbon chemistry changes and effects on individuals and ecosystems, and enumerate key hypotheses for testing. Finally, we quantify how U.S. research funding has been distributed among these linkages, concluding that there is an urgent need for research programs designed to anticipate how the effects of OA will reverberate throughout assemblages of species.

CLIMATE AND RECRUITMENT OF ROCKY SHORE INTERTIDAL INVERTEBRATES IN THE EASTERN NORTH ATLANTIC

Studies of the impacts of climate and climate change on biological systems often attempt to correlate ecological responses with basin-scale indices such as the North Atlantic Oscillation (NAO). However, such correlations, while useful for detecting long-term trends, are unable to provide a mechanism linking the physical environment and ecological processes. Here we evaluate the effects of the NAO on recruitment variability of rocky intertidal invertebrates in the North Atlantic examining two possible climate-related pathways. Using a highly conservative test we interpret associations with the NAO integrated over a season (three months) as an indicator of atmospheric effects on newly settled recruits (NAO3), and the effects of the NAO integrated over six months (NAO6) as an indicator of changes in ocean circulation affecting patterns of larval transport. Through an extensive literature survey we found 13 time series, restricted to southwest Ireland and Britain and comprising five species, that could be used for statistical analysis. Significant correlations with NAO3, our proxy for atmospheric effects, were observed in the south-central domain of our study region (southwest Ireland and south England). Significant correlations with NAO6, the proxy for ocean circulation effects, were detected on southwest Ireland. The associations were detected for three (two barnacles and a topshell) at two sites. These results suggest that the NAO can have effects on the recruitment of intertidal invertebrates through different pathways linked to climate and be distributed heterogeneously in space. Based on previous evidence and the sign and geographic location of significant correlations, we suggest that winter NAO effects are likely to occur as a result of effects on the survival of early life stages settling during spring or through changes in phenology. Our results argue that a combination of modeling and synthesis can be used to generate hypotheses regarding the effects of climate on recruitment and aid in the design of field-based tests of explicit ecological mechanisms.

Persistent spatial structuring of coastal ocean acidification in the California Current System

The near-term progression of ocean acidification (OA) is projected to bring about sharp changes in the chemistry of coastal upwelling ecosystems. The distribution of OA exposure across these early-impact systems, however, is highly uncertain and limits our understanding of whether and how spatial management actions can be deployed to ameliorate future impacts. Through a novel coastal OA observing network, we have uncovered a remarkably persistent spatial mosaic in the penetration of acidified waters into ecologically-important nearshore habitats across 1,000 km of the California Current Large Marine Ecosystem. In the most severe exposure hotspots, suboptimal conditions for calcifying organisms encompassed up to 56% of the summer season, and were accompanied by some of the lowest and most variable pH environments known for the surface ocean. Persistent refuge areas were also found, highlighting new opportunities for local adaptation to address the global challenge of OA in productive coastal systems.

A metadata-driven framework for generating field data entry interfaces in ecology

Abstract With the advent of affordable yet powerful handheld computers, many ecologists now capture data electronically in the field, while avoiding transcription errors found in more traditional field notebook or slate-based approaches of recording data. Ecologists either use simple forms from generic handheld software that doesn’t allow for needed optimizations, or else they must spend substantial effort in the design and optimization of handheld software user interfaces. These efforts often produce highly customized software that is not easily repurposed, and that produce non-standard, undocumented data formats. Our research is focused on reducing this effort by providing a flexible framework that uses highly structured metadata to drive the generation of customized data entry interfaces. Our approach relies on formalized metadata encoded in either XML Schema or XML instance documents, which facilitate the creation of graphical interfaces from arbitrary data schemas. Specifically, we leverage the rich metadata descriptions and structures found in the Ecological Metadata Language to create data entry forms, and output data sets that adhere to this specification. The interfaces are refined by using data-typing and domain information found in the structured metadata to generate validation routines that assist in maintaining data integrity during the entry process. The framework utilizes a rule-based engine to promote both reuse and extensibility of the modules that are created for any given data entry effort. Interfaces are rendered using open, Internet-based standards to maintain portability and to advocate the sharing of software components. The framework represents an initial implementation of a customizable user interface generator, which should have broad applicability to researchers in need of rapidly creating field and laboratory-based forms for varied collection efforts.