David J. Currie

Energy, water, and broad-scale geographic patterns of species richness

It is often claimed that we do not understand the forces driving the global diversity gradient. However, an extensive literature suggests that contemporary climate constrains terrestrial taxonomic richness over broad geographic extents. Here, we review the empirical literature to examine the nature and form of the relationship between climate and richness. Our goals were to document the support for the climatically based energy hypothesis, and within the constraints imposed by correlative analyses, to evaluate two versions of the hypothesis: the productivity and ambient energy hypotheses. Focusing on studies extending over 800 km, we found that measures of energy, water, or water-energy balance explain spatial variation in richness better than other climatic and non-climatic variables in 82 of 85 cases. Even when considered individually and in isolation, water/ energy variables explain on average over 60% of the variation in the richness of a wide range of plant and animal groups. Further, water variables usually represent the strongest predictors in the tropics, subtropics, and warm temperate zones, whereas energy variables (for animals) or water-energy variables (for plants) dominate in high latitudes. We conclude that the interaction between water and energy, either directly or indirectly (via plant productivity), provides a strong explanation for globally extensive plant and animal diversity gradients, but for animals there also is a latitudinal shift in the relative importance of ambient energy vs. water moving from the poles to the equator. Although contemporary climate is not the only factor influencing species richness and may not explain the diversity pattern for all taxonomic groups, it is clear that understanding water-energy dynamics is critical to future biodiversity research. Analyses that do not include water-energy variables are missing a key component for explaining broad-scale patterns of diversity.

ENERGY AND LARGE-SCALE PATTERNS OF ANIMAL- AND PLANT- SPECIES RICHNESS

Many hypotheses have been proposed to explain the great variation among regions in species richness. These were tested by first examining patterns of species richness of birds, mammals, amphibians, and reptiles in 336 quadrats covering North America. These patterns were then compared with the regional variation of 21 descriptors of the environment suggested by the hypotheses. I found that, in the four vertebrate classes studied, 80%-93% of the variability in species richness could be statistically explained by a monotonically increasing function of a single variable: annual potential evapotranspiration (PET). In contrast, tree richness is more closely related to actual evapotranspiration (AET). Both AET and PET appear to be measures of available environmental energy. The relationships between tree and vertebrate richness are strikingly poor. Species richness in particular orders and families of the Vertebrata is also closely related to PET, but not always monotonically, often resembling a replacement series along an environmental gradient. The present results are consistent with the hypothesis that environmentally available energy limits regional species richness. However, my observations are not completely consistent with earlier species-energy theory. The energy-richness relationship appears to depend on scale, and it is affected differently by variations in area and in areal energy flux.

THE DIVERSITY-DISTURBANCE RELATIONSHIP: IS IT GENERALLY STRONG AND PEAKED?

The contemporary literature accepts that disturbance strongly influences pat- terns of species diversity, and that the relationship is peaked, with a maximum at inter- mediate levels of disturbance. We tested this hypothesis using a compilation of published species diversity-disturbance relationships that were gleaned from a literature search of papers published from 1985 through 1996 and from references therein. We identified 116 species richness-, 53 diversity-, and 28 evenness-disturbance relationships in the literature, which we grouped according to shape of relationship (nonsignificant, peaked, negative monotonic, positive monotonic, or U-shaped). We tested the relationships between the strength and shapes of these relationships and attributes of the community, disturbance, and sampling and study design. Nonsignificant relationships were the most common, com- prising 35% of richness, 28% of diversity, and 50% of evenness studies. Peaked responses were reported in only 16% of richness, 19% of diversity, and 11% of evenness cases. Explained variation in the three measures of diversity was variable among studies but averaged -50%. It was higher when few samples and few disturbance levels were examined and when organisms within the samples were not exhaustively censused, suggesting that procedural artifact contributes to these relationships. Explained variation was also higher in studies in which disturbance was measured as a gradient of time passed since the last disturbance (mean r2 = 61%), vs. studies of spatial variation in richness (mean r2 = 42%). Peaked richness relationships had the greatest odds of being observed when sampled area and actual evapotranspiration were small, when disturbances were natural rather than an- thropogenic in origin, and when few disturbance levels were examined. Thus, on average, diversity-disturbance relationships do not have consistently high r2 and are not as consis- tently peaked as the contemporary consensus would suggest.

Global Change in Forests: Responses of Species, Communities, and Biomes

G change is often perceived as human-induced modifications in climate. Indeed, human activities have undeniably altered the atmosphere, and probably the climate as well (Watson et al. 1998). At the same time, most of the world’s forests have also been extensively modified by human use of the land (Houghton 1994). Thus, climate and land use are two prongs of human-induced global change. The effect of these forces on forests is mediated by the organisms within forests. Consideration of climate, land use, and biological diversity is key to understanding forest response to global change. Biological diversity refers to the variety of life at organizational levels from genotypes through biomes (Franklin 1993). The responses of ecological systems to global change reflect the organisms that are within them. While ecologists have sometimes not seen the forest for the trees, so to speak, it is also true that forests cannot be understood without knowledge of the trees and other component species. It is the responses of individual organisms that begin the cascade of ecological processes that are manifest as changes in system properties, some of which feed back to influence climate and land use (Figure 1). Beyond its role in ecosystems, biodiversity is invaluable to humans for foods, medicines, genetic information, recreation, and spiritual renewal (Pimentel et al. 1997). Thus, global changes that affect the distribution and abundance of organisms will affect future human well-being and land use, as well as, possibly, the climate. This article serves as a primer on forest biodiversity as a key component of global change. We first synthesize current knowledge of interactions among climate, land use, and biodiversity. We then summarize the results of new analyses on the potential effects of human-induced climate change on forest biodiversity. Our models project how possible future climates may modify the distributions of environments required by various species, communities, and biomes. Current knowledge, models, and funding did not allow these analyses to examine the population processes (e.g., dispersal, regeneration) that would mediate the responses of organisms to environmental change. It was also not possible to model the important effects of land use, natural disturbance, and other factors on the response of biodiversity to climate change. Despite these limitations, the analyses discussed herein are among the most comprehensive projections of climate change effects on forest biodiversity yet conducted. We conclude with discussions of limitations, research needs, and strategies for coping with potential future global change.

A Globally Consistent Richness‐Climate Relationship for Angiosperms

Species richness, the simplest index of biodiversity, varies greatly over broad spatial scales. Richness‐climate relationships often account for >80% of the spatial variance in richness. However, it has been suggested that richness‐climate relationships differ significantly among geographic regions and that there is no globally consistent relationship. This study investigated the global patterns of species and family richness of angiosperms in relation to climate. We found that models relating angiosperm richness to mean annual temperature, annual water deficit, and their interaction or models relating richness to annual potential evapotranspiration and water deficit are both globally consistent and very strong and are independent of the diverse evolutionary histories and functional assemblages of plants in different parts of the world. Thus, effects of other factors such as evolutionary history, postglacial dispersal, soil nutrients, topography, or other climatic variables either must be quite minor over broad scales (because there is little residual variation left to explain) or they must be strongly collinear with global patterns of climate. The correlations shown here must be predicted by any successful hypothesis of mechanisms controlling richness patterns.

Compensatory dynamics are rare in natural ecological communities

In population ecology, there has been a fundamental controversy about the relative importance of competition-driven (density-dependent) population regulation vs. abiotic influences such as temperature and precipitation. The same issue arises at the community level; are population sizes driven primarily by changes in the abundances of cooccurring competitors (i.e., compensatory dynamics), or do most species have a common response to environmental factors? Competitive interactions have had a central place in ecological theory, dating back to Gleason, Volterra, Hutchison and MacArthur, and, more recently, Hubbells influential unified neutral theory of biodiversity and biogeography. If competitive interactions are important in driving year-to-year fluctuations in abundance, then changes in the abundance of one species should generally be accompanied by compensatory changes in the abundances of others. Thus, one necessary consequence of strong compensatory forces is that, on average, species within communities will covary negatively. Here we use measures of community covariance to assess the prevalence of negative covariance in 41 natural communities comprising different taxa at a range of spatial scales. We found that species in natural communities tended to covary positively rather than negatively, the opposite of what would be expected if compensatory dynamics were important. These findings suggest that abiotic factors such as temperature and precipitation are more important than competitive interactions in driving year-to-year fluctuations in species abundance within communities.

THE SPECIES RICHNESS-ENERGY HYPOTHESIS IN A SYSTEM WHERE HISTORICAL FACTORS ARE THOUGHT TO PREVAIL: CORAL REEFS

Much of the variance in species richness of terrestrial organisms has been related to levels of available energy (the species richness-energy hypothesis). In contrast, the global patterns of coral diversity have been hypothesized to depend mainly on disturbance and historical factors. In this study, we test several general diversity hypotheses as they relate to hermatypic corals by examining the relationships between coral generic richness at 130 sites worldwide and descriptors of the environment that would be suggested by the hypotheses. The best environmental predictors of diversity are mean annual ocean temperature and an estimate of regional coral biomass, which suggests that available energy limits regional generic richness. In contrast, we found little evidence supporting other ecological hypotheses, including the hypotheses that disturbance or environmental stability is an important control of diversity. We also investigated historical hypotheses proposed to explain coral distributions. We found a relationship between coral richness and up-current island density that is consistent with vicariance models of speciation and theories of coral dispersal. Using multiple regression, 71% of the variation in coral generic richness could be statistically explained using a combination of variables representing both ecological and historical factors. Similar patterns exist for both coral species and reef fishes.

Global patterns of animal abundance and species energy use

There is enormous variation among animal species in population density and population energy use. Density (D) is known to vary strongly with body weight (W), while allometric scaling of population energy use is disputed. The present study examines this variability in reported species densities and energy consumption to test the hypothesis that the patterns are related to environmental energy levels and to the efficiency of energy utilization. We found that the intercepts, but not slopes, of density-body size relationships of the form log D = a + b log W differ significantly among broad groups: invertebrates, vertebrate ectotherms, mammals and birds

Global patterns of tree species richness in moist forests : another look

After examining some perceived inconsistencies in the relationship between species richness and energy, Latham and Ricklefs rejected the Energy Diversity Theory in favour of historical explanations of the geographical patterns of tree species richness. We have re-examined Latham and Ricklefss data, both by itself and pooled with Currie and Paquins data. We find that Latham and Ricklefss data are, in fact, consistent with the richness-energy hypothesis. Because of strong collinearity between annual evapotranspiration and region in their particular data sets, the data cannot be used to distinguish between the hypotheses that contemporary climate, versus some other interregional difference, is responsible for observed species richness patterns. Finally, there is no particular reason to attribute differences among regions to historical factors rather than to contemporary ones.

Projected effects of climate change on patterns of vertebrate and tree species richness in the conterminous United states

General circulation models (GCM) predict that increasing levels of atmospheric carbon dioxide (CO2) and other greenhouse gases will lead to dramatic changes in climate. It is known that the spatial variability of species richness over continental spatial scales is strongly correlated with contemporary climate. Assuming that this relationship between species richness and climate persists under conditions of increased CO2, what changes could we expect to occur in terms of species richness? To address this question, I used observed relationships between contemporary richness and climate, coupled with climate projections from five GCM, to project these future changes. These models predict that the richness of vertebrate ectotherms will increase over most of the conterminous United States. Mammal and bird richness are predicted to decrease in much of the southern US and to increase in cool, mountainous areas. Woody plant richness is likely to increase throughout the North and West and to decrease in the southwestern deserts. These projections represent changes that are likely to occur over long time scales (millennia); short-term changes are expected to be mainly negative.