Robert B. Whitlatch

Linking climate change and biological invasions: Ocean warming facilitates nonindigenous species invasions.

The spread of exotic species and climate change are among the most serious global environmental threats. Each independently causes considerable ecological damage, yet few data are available to assess whether changing climate might facilitate invasions by favoring introduced over native species. Here, we compare our long-term record of weekly sessile marine invertebrate recruitment with interannual variation in water temperature to assess the likely effect of climate change on the success and spread of introduced species. For the three most abundant introduced species of ascidian (sea squirt), the timing of the initiation of recruitment was strongly negatively correlated with winter water temperature, indicating that invaders arrived earlier in the season in years with warmer winters. Total recruitment of introduced species during the following summer also was positively correlated with winter water temperature. In contrast, the magnitude of native ascidian recruitment was negatively correlated with winter temperature (more recruitment in colder years) and the timing of native recruitment was unaffected. In manipulative laboratory experiments, two introduced compound ascidians grew faster than a native species, but only at temperatures near the maximum observed in summer. These data suggest that the greatest effects of climate change on biotic communities may be due to changing maximum and minimum temperatures rather than annual means. By giving introduced species an earlier start, and increasing the magnitude of their growth and recruitment relative to natives, global warming may facilitate a shift to dominance by nonnative species, accelerating the homogenization of the global biota.

Scale‐Dependent Recolonization: The Role of Sediment Stability in a Dynamic Sandflat Habitat

An important ecological issue is developing an understanding of how patterns and processes vary with scale. We designed a field experiment to test how differences in the aerial extent of disturbance affected macrofaunal recolonization on a sandflat. Three different plot sizes (0.203 m 2 , 0.81 m 2 , and 3.24 m 2 ) were defaunated, and samples were collected to assess recovery over a 9-mo period. As the sandflat used for the experiment was prone to disturbance by wind-driven waves, we also measured changes in sediment bed height (an indicator of sediment stability) over the course of the experiment. Most common species revealed significant relationships between density and disturbance plot size. Scale-dependent recovery was also demonstrated by differences in species assemblage structure over the course of the experiment. Relative rates of colonization varied by 50% between large and small experimental plots. However, these differences were not related to specific species, particular functional groups, or potential modes of colonization. The results revealed an unusually slow rate of faunal recovery following defaunation. Both increasing numbers of colonists and density changes in ambient sediments made an important contribution to recovery. The relationship found between changes in sediment bed height and wind velocity indicated that wind-driven wave disturbance was an important factor influencing sediment instability. Sediment instability was higher in all experimental plots than in the ambient sediments, due to the initial removal of a dense spionid polychaete tube mat characteristically found at the study site. Sediment instability also increased with increasing plot size. Thus in this dynamic sandflat habitat, faunal emigration from recovering disturbed patches of sediment may significantly slow rates of recolonization. These results demonstrate that incorporating patch size, emigration, recovery time, and interactions between hydrodynamic conditions and habitat stability (particularly where colonists influence sediment stability) are crucial to generating a general understanding of recovery processes in soft-sediment habitats. While our results demonstrate the need for caution in scaling-up from small-scale studies, they do indicate that larger scale disturbances that destroy organisms with a role in maintaining habitat stability are likely to result in very slow recovery dynamics, particularly in wave-disturbed soft-sediment habitats.

INTERACTIONS AMONG ALIENS: APPARENT REPLACEMENT OF ONE EXOTIC SPECIES BY ANOTHER

Although many studies have documented the impact of invasive species on indigenous flora and fauna, few have rigorously examined interactions among invaders and the potential for one exotic species to replace another. European green crabs (Carcinus maenas), once common in rocky intertidal habitats of southern New England, have recently declined in abundance coincident with the invasion of the Asian shore crab (Hemigrapsus sanguineus). Over a four-year period in the late 1990s we documented a significant (40- 90%) decline in green crab abundance and a sharp (10-fold) increase in H. sanguineus at three sites in southern New England. Small, newly recruited green crabs had a significant risk of predation when paired with larger H. sanguineus in the laboratory, and recruitment of 0-yr C. maenas was reduced by H. sanguineus as well as by larger conspecifics in field- deployed cages (via predation and cannibalism, respectively). In contrast, recruitment of 0-yr H. sanguineus was not affected by larger individuals of either crab species during the same experiments. The differential susceptibility of C. maenas and H. sanguineus recruits to predation and cannibalism likely contributed to the observed decrease in C. maenas abundance and the almost exponential increase in H. sanguineus abundance during the period of study, While the Asian shore crab is primarily restricted to rocky intertidal habitats, C. maenas is found intertidally, subtidally, and in a range of substrate types in New England. Thus, the apparent replacement of C. maenas by H. sanguineus in rocky intertidal habitats of southern New England may not ameliorate the economic and ecological impacts attributed to green crab populations in other habitats of this region. For example, field experiments indicate that predation pressure on a native bivalve species (Mytilus edulis) has not nec- essarily decreased with the declines of C. maenas. While H. sanguineus has weaker per capita effects than C. maenas, its densities greatly exceed those of C. maenas at present and its population-level effects are likely comparable to the past effects of C. maenas. The Carcinus-Hemigrapsus interactions documented here are relevant in other parts of the world where green crabs and grapsid crabs interact, particularly on the west coast of North America where C. maenas has recently invaded and co-occurs with two native Hemigrapsus species.

Animal-sediment relationships in intertidal marine benthic habitats: Some determinants of deposit-feeding species diversity☆

Particulate sedimentary properties were examined in a variety of New England intertidal habitats. The sediments were composed of a complex array of both food and non-food particle types. Little evidence of temporal or vertical spatial variation was observed in the major particulate fractions. Organic-mineral aggregates (“detritus”), however, exhibited significant seasonal change in abundance at the sediment-water interface, possibly being related to annual productivity-decay cycles of the marsh grass Spartina alterniflora. Both particulate and bulk sedimentary characteristics were related to deposit-feeding species diversity. Species richness was correlated with the amount of surficial sedimentary organic carbon, and species diversity with total particulate and food particulate diversity. Food abundance and variety, therefore, may regulate the organization of deposit-feeding assemblages. Standard methods for describing sediments may largely be inadequate for understanding many aspects of organism-sediment relationships in marine benthic environments.

Spatial structure of bivalves in a sandflat:: Scale and generating processes

A survey was conducted during the summer of 1994 within a fairly homogeneous 12.5 ha area of sandflat off Wiroa Island, in Manukau Harbour, New Zealand, to identify factors controlling the spatial distributions of the two dominant bivalves, Macomona liliana Iredale and Austrovenus stutchburyi (Gray), and to look for evidence of adult–juvenile interactions within and between species. Most of the large–scale spatial structure detected in the bivalve count variables (two species, several size classes of each) was explained by the physical and biological variables. The results of principal component analysis and spatial regression modelling suggest that different factors are controlling the spatial distributions of adults and juveniles. Larger size classes of both species displayed significant spatial structure, with physical variables explaining some but not all of this variation. Smaller organisms were less strongly spatially structured, with virtually all of the structure explained by physical variables. The physical variables important in the regression models differed among size classes of a species and between species. Extreme size classes (largest and smallest) were best explained by the models; physical variables explained from 10% to about 70% of the variation across the study site. Significant residual spatial variability was detected in the larger bivalves at the scale of the study site. The unexplained variability (20 to 90%) found in the models is likely to correspond to phenomena operating at smaller scales. Finally, we found no support for adult–juvenile interactions at the scale of our study site, given our sampling scale, after controlling for the effects of the available physical variables. This is in contrast to significant adult–juvenile interactions found in smaller–scale surveys and in field experiments. Our perception of adult–juvenile interactions thus depends on the scale of study.

The Influence of Resident Adults on Recruitment: A Comparison to Settlement

For species recruiting into established sessile communities, the adult colonies and individuals already present form a significant part of the environment and have the potential to alter both larval settlement rates and post-settlement mortality. Settlement rates can be reduced by predation on larvae, by the removal or addition of substratum space, or by stimulation or prohibition of larvae from settling on adjacent substratum. Once attached, the recruiting individual can still be influenced by predation or overgrowth by residents, by the added physical structure for firmer attachment, or by being camouflaged from motile predators. To examine those processes by which residents affect recruitment we exposed experimental substrata with three densities of adults of a single species at a site in eastern Long Island Sound, USA for a 1-wk period. Seven different species of common invertebrates were used in nine separate experiments. The major effect of most resident species was the usurpation of space and the restricting of recruitment to adjacent unoccupied areas. This was particularly true for resident ascidians and bryozoans, but less so for barnacles and oysters. In fact several species recruited in higher densities on or next to oysters and barnacles. Comparison to 1-day settlement experiments indicated that the encrusting ascidian species Diplosoma and possibly Botryllus reduced recruitment relative to settlement, probably by overgrowing newly-settled individuals. However, in the presence of most resident species, recruitment patterns were not greatly different from settlement patterns, indicating that the effects of the attached community on recruitment may result from influences on settlement.

Recolonization and succession in soft-sediment infaunal communities: the spatial scale of controlling factors

Succession in marine soft-sediment habitats has been studied extensively and several general models of successional dynamics have been developed. However, few researchers have addressed how successional dynamics may change over different spatial scales. Here we extend a model that focuses on the factors that control recolonization and succession. These factors comprise three levels of a hierarchy which include environmental conditions, life history and population processes and biotic interactions. Using this hierarchical framework, we consider the spatial scales at which different factors operate, and argue that the relative mix and intensity of factors controlling succession change at different spatial scales. As a result, successional dynamics may vary considerably as the spatial scale of disturbance increases. At small scales, factors at each level of the hierarchy are important. The greater potential for biotic interactions at this scale may be particularly critical. At meso- to large scales, population processes and environmental conditions have the most influence on successional dynamics. Due to these differences, responses to small-scale (≳1 m2) as well as large-scale (≳1 hectare) disturbances may be quite variable. Within this range (≳1 m2 lsim;1 hectare), short- and long-term responses to disturbance may be relatively more predictable and conform to current models of succession in soft-sediment habitats.

SEASONAL CHANGES IN THE COMMUNITY STRUCTURE OF THE MACROBENTHOS INHABITING THE INTERTIDAL SAND AND MUD FLATS OF BARNSTABLE HARBOR, MASSACHUSETTS

1. A quantitative sampling survey of the benthic macrofauna inhabiting the intertidal sand and mud flats of Barnstable Harbor, Massachusetts, was conducted to describe general community structure and examine temporal changes in species composition.2. Classification analysis delimited coarse and fine sand, mud, muddy-sand, and gravel-mud benthic species associations. The 32 species used in the inverse classification analysis were partitioned into 10 species groups, reflecting spatial distributional patterns. Many of the species were both dominant and ubiquitous, masking discrete species groupings.3. The majority of macrobenthos at Barnstable Harbor were deposit-feeders which comprised more than 90% of all organisms sampled. The deposit-feeders normally dominate mud and muddy-sand sediments. Suspension-feeders were most abundant in fine sands. The relationship of sedimentary parameters affecting the distribution of both trophic groups proposed by Sanders is generally supported.4. While no significant change...

Scaling-up from experiments to complex ecological systems: Where to next?

a , b c d e * S.F. Thrush , D.C. Schneider , P. Legendre , R.B. Whitlatch , P.K. Dayton , a f a g h J.E. Hewitt , A.H. Hines , V.J. Cummings , S.M. Lawrie , J. Grant , a a i R.D. Pridmore , S.J. Turner , B.H. McArdle National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand Ocean Sciences Centre, Memorial University of Newfoundland, St. John’s, Canada ALC5S7 c ́ ́ ́ Departement de Sciences Biologiques, Universite de Montreal, C.P. 6128, succursale Centre-ville, ́ ́ Montreal, Quebec H3C 3J7, Canada Dept. Marine Sciences, University of Connecticut, Groton, CT 06340-6097, USA Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0201, USA Smithsonian Environmental Research Center, P.O. Box 28, Edgewater, MD 21037, USA Culterty Field Station, University of Aberdeen, Newburgh, AB40AA, Scotland Dept. of Oceanography, Dalhousie University, Halifax, Canada B3H 4JI Biostatistics Unit, School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand

Nonlinear foraging response of a large marine predator to benthic prey: eagle ray pits and bivalves in a New Zealand sandflat

The density-dependent foraging response of eagle rays (Myliobatis tenuicaudatus) to infaunal bivalves (Macomona lilliana) was measured in a New Zealand sandflat. Disturbance pits provided unequivocal indicators of ray feeding activity, and pits were counted on a plot (250 m×500 m) which had prey density mapped in a 200 cell (25 m×25 m) grid. Although foraging response increased significantly with prey density treated as a nominal (class, ANOVA-type) variable, treating bivalve density as a ratio scale (continuous, regression-type) variable provided more information about characteristics of the response. Eagle rays exhibited a nonlinear segmented response to prey density, in which ray foraging activity was low and independent of prey density at low Macomona densities, while foraging increased sharply above a threshold density of prey but did not reach satiation at the highest prey densities in our site. By counting ray pits repeatedly over a 31 day period, we showed that the levels and slope of the foraging response (no. of ray pits per 707 m2 per 4 days) varied temporally during the season, but the nonlinear characteristic and the threshold of prey density were consistent. Correlation analysis showed that the distribution of bivalve prey and ray foraging was spatially constant during the season. Comparison of 3 estimators of prey density showed that a fitted polynomial density was the best predictor of ray foraging, and indicated that rays were responding to prey patches on a scale of 75–100 m. The temporal features of the response to prey density were incorporated into a nonlinear segmented model and integrated with respect to time for each cell of the study grid. The impact of ray foraging estimated from the integral indicated that only about 1.6% of the Macomona population was consumed and 5.0% of the total plot was disturbed by rays during one month of study. However, the nonlinearity of response indicated that foraging impacts were concentrated disproportionately on high density patches of prey, which suffered up to 4% mortality and 13% disturbance. Macomona gained a refuge from predation and disturbance at low density, which would stabilize prey populations and even out prey distribution.