Overview

Research in my lab is focused on understanding the processes that determine the spatial and temporal distribution of biological diversity in the sea, emphasizing not just the traditional counts of species, but also functional groups and morphological diversity. In particular we are using marine mollusks as a focal group to (i) test hypotheses about the origin and maintenance of the spatial patterns of species diversity in the ocean; (ii) better understand the effects of climatic and environmental change on shallow marine species and communities; and (iii) quantify spatial patterns of morphological diversity in marine invertebrates and explore the ecological and evolutionary basis of these patterns. In addition, we have just initiated a large research project quatifying the effects of human impacts on the rocky intertidal biota of southern California. Research in my lab is interdisciplinary in nature and combines ecological and biogeographic data from living populations with the deep time perspective afforded by the rich fossil record of mollusks, as well as collaborative work on molecular phylogeny and population structures of selected clades.

Latitudinal diversity gradients

Latitudinal diversity gradients (high species richness in the tropics and a declining trend towards high latitudes) are one of the most fundamental biodiversity patterns shared by both marine and terrestrial ecosystems but the factors that control such gradients remain poorly understood. Over two dozen hypotheses have been proposed to explain this pattern, but none has gained general acceptance. In addition, most of the existing hypotheses have not been tested for marine organisms due to the lack of relevant ecological and biogeographic data. Over the last five years Kaustuv Roy, David Jablonski and James W. Valentine, have compiled a database of over 3000 species of eastern Pacific marine mollusks in order to test hypotheses about the marine latitudinal gradient (Roy et al. 1994, 1998, 2000). This is one of the largest existing databases of its kind for marine invertebrates and serves as a platform for asking a wide array of evolutionary and ecological questions. It is an ongoing project that requires regular updates of the database as new information becomes available, and analyses using these data have already produced novel results. In a 1998 paper in PNAS that received wide attention (including a feature article in New Scientist), we demonstrated the importance of energy related parameters (species-energy hypothesis) in determining latitudinal diversity trends in marine gastropods. In subsequent publications (Roy et al. 2000), using data for marine bivalves, we also found strong support for species-energy hypothesis, thereby establishing the generality of this process among marine invertebrates. In addition, we tested a longstanding and influential hypothesis in marine ecology that strong latitudinal gradients in diversity should occur in marine epifauna (taxa living on the surface of the substratum) but not in infauna (burrowing or boring into the substratum); a contrast that was attributed to the greater spatial and temporal environmental homogeneity of infaunal habitats. This hypothesis has been the subject of considerable debate but our analyses involving over 900 species of bivalves conclusively rejects it for one of the most important groups of benthic marine invertebrates. While both infauna and epifauna show positive latitudinal diversity gradients, these analyses also revealed interesting differences in trends between trophic categories and developmental modes. Current and planned work on these differences should provide a better understanding of the processes underlying latitudinal patterns of biodiversity.

Biotic effects of climate change

Climate is one of the most important determinants species range limits and understanding how species and ecosystems respond to climate change is a major challenge for biology. The problem is important in terms of theoretical issues of species distributions and community assembly as well as for predicting the impact of global change on natural and managed systems (e.g. agriculture, aquaculture). Our work on the biotic effects of climate change focus on both ecological (e.g. species range shifts and changes in community compositions, see Roy et al 1995, 1996, 2001) and evolutionary effects (Hellberg, Balch & Roy 2001). Insights about the evolutionary consequences of climate change have come largely from molecular phylogeographic data and little is known about phenotypic evolution in such systems. Also very few studies have tested the inferences from the phylogeographic studies against actual historical data such as those from the Pleistocene fossil record. We have recently completed one of the first studies that combines mitochondrial DNA sequences, phenotypic measurements and paleontological data to investigate how populations of a marine gastropod (Acanthinucella spirata) living along the California coast responded to Pleistocene climatic changes. This species ranges from Baja to Tomales Bay and mitochondrial sequences show a large genetic break at San Pedro with very little variation to the north. In contrast, southern populations show much lower morphological variance compared to the northern ones. Finally, the morphological variation in individuals from the Pleistocene terraces along the coast is similar to the southern populations and some northern morphs are not found in the fossil record. These results are consistent with the hypothesis that glacial-interglacial cycles led to the extinction of northern populations followed by later recolonization from the south. While phylogeographic studies involving other species also point in this direction, ours is among the few convincing demonstrations that such population bottlenecks in response to Pleistocene climatic shifts can also lead to a greater phenotypic variance in northern populations, a pattern rarely demonstrated in natural system (Hellberg, Balch and Roy 2001).

Morphological diversity

Spatial and temporal patterns of biological diversity are commonly quantified using species richness estimates. While useful, this metric captures only one aspect of biological diversity, and increasing attention is being focused on other biodiversity metrics, including morphological and ecological diversity (Roy & Foote, 1997; Roy et al. in press) Our work on spatial patterns of morphological diversity focuses on two related questions: (i) how does morphological diversity of marine invertebrate assemblages relate to the species richness; and (ii) what are the trends in morphological diversity along major environmental gradients such as those associated with latitude, and how do these patterns evolve? Morphological diversity can be measured using a number of different approaches (Roy & Foote, 1997), and our work on mollusks use two different measures, (i) body size and (ii) multivariate morphometric descriptions of shell shape. We have quantified changes in these parameters at the level of whole assemblages from the equator to the pole and have initiated more detailed studies at the level of individual clades. Again our work is interdisciplinary and involves integration of molecular phylogenetic, macroecological, biogeographic and paleontological data.

(i) Body size: Body size influences almost every aspect of the biology of a species and is considered to be one of the single most important attributes of organisms. However, most of the recent work on ecology and evolution of body size has focused on terrestrial animals, and the lack of empirical data for marine invertebrates is widely acknowledged. To investigate the latitudinal patterns of body size in northeastern Pacific marine bivalves we collected and analyzed data for 915 species of bivalves living on the continental shelf from the equator to the Arctic Ocean (Roy, Jablonski & Martien 2000; Roy & Martien, 2001). This study represents one of the most extensive analysis of latitudinal distribution of body size ever undertaken for a group of invertebrates, and the first of its kind for marine invertebrates. Contrary to expectations, the size-frequency distributions (SFDs) of northeastern Pacific bivalves at the provincial level are statistically indistinguishable from the tropics to the Arctic despite a four-fold change in species richness, and changes in a wide array of variables held to influence body size, such as temperature, seasonality, and productivity. Thus latitudinal patterns of species richness are decoupled from patterns of body size, a fundamental aspect of species ecology and life history. This pattern has been previously suspected but never quantified over spatial scales comparable to the one we used. In terms of the underlying processes, the modal sizes and shapes of these SFDs are consistent with the predictions of an influential but highly controversial energetic model previously applied only to terrestrial mammals and birds (Roy, Jablosnki & Martien 2001). However, this and other optimization models fail in their predictions of the evolutionary dynamics of lineages; analyses of the Miocene-Recent history of body sizes within 82 molluscan genera show little support for the expectation that the modal size is an evolutionary attractor over geological time. Thus the density of occupation of a size range, or any other part of the morphospace, need not reflect a long-term evolutionary optimum, and our results suggest that the SFDs are maintained by origination and extinction dynamics within and among size classes rather than a strong directional evolution towards the mode, as previously suggested.

Organisms are arrayed along environmental gradients besides latitude and for marine orgnanisms the most important is depth. I have quantified changes in body size distributions of northeast Pacific marine gastropods along bathymetry (Roy in press). The results highlight the importance of clade-specific adaptations in determining bathymetric size trends and contradicts predictions of existing models that highlight environmental influences. This is the first analyses of its kind ever undertaken for shallow marine mollusks and predictions from this study should provide a framework for future work on understanding changes in benthic community structure as well as evolutionary dynamics along depth.

(ii) Multivariate morphometrics: Our clade level work focuses on the family Strombidae. This group exhibits a wide spectrum of morphologies, including some of the most extreme forms seen in marine gastropods. Thus, this large group of tropical marine gastropods provides an ideal system for understanding the evolution of morphological novelties in marine invertebrates. We have recently finished quantifying the spatial patterns of morphological diversity in this group throughout the tropical Pacific (Roy, Balch & Hellberg in press). Spatial patterns of morphological diversity show remarkably poor correspondence to patterns of species richness. Parts of the Pacific that may not be as rich in species diversity, nonetheless harbor an impressive variety of morphologies that sometimes even exceed the more speciose regions in this respect. This result not only challenges the traditional view of biological diversity across the Pacific but also has important conservation implications. Certain regions of the Pacific are among the parts of the ocean most threatened by human activities and previous studies have highlighted the need for conserving regions with high species richness. But our results suggest that increased attention also needs to be focused on these areas of the Pacific that may have low species richness but anomalously high morphological diversity (Roy, Balch & Hellberg in press). The loss of even a few species in these regions can lead to a disproportionately high loss of morphological variety and potential regional extinction of distantly related evolutionary lineages. Now that the spatial trends have been quantified, we are going to add the temporal dimension by generating a molecular phylogeny of these gastropods (in collaboration with Dr. M. Hellberg at Louisiana State University) and using the fossil record. These data will be used to understand the evolution of the documented spatial trends in morphology and will focus on questions such as whether the most divergent morphologies are the most ancient, or whether they evolved convergently in different regions at different times.

Conservation of coastal ecosystems

We are currently working on quantifying the impacts of anthropogenic activities on the rocky intertidal biota of southern California. See the CBRISC site for details of this work.