S. Kathleen Lyons

BODY MASS OF LATE QUATERNARY MAMMALS

The purpose of this data set was to compile body mass information for all mammals on Earth so that we could investigate the patterns of body mass seen across geographic and taxonomic space and evolutionary time. We were interested in the heritability of body size across taxonomic groups (How conserved is body mass within a genus, family, and order?), in the overall pattern of body mass across continents (Do the moments and other descriptive statistics remain the same across geographic space?), and over evolutionary time (How quickly did body mass patterns iterate on the patterns seen today? Were the Pleistocene extinctions size specific on each continent, and did these events coincide with the arrival of man?). These data are also part of a larger project that seeks to integrate body mass patterns across very diverse taxa (NCEAS Working Group on Body Size in Ecology and Paleoecology: linking pattern and process across space, time, and taxonomic scales). We began with the updated version of D. E. Wilson an...

Similarity of Mammalian Body Size across the Taxonomic Hierarchy and across Space and Time

Although it is commonly assumed that closely related animals are similar in body size, the degree of similarity has not been examined across the taxonomic hierarchy. Moreover, little is known about the variation or consistency of body size patterns across geographic space or evolutionary time. Here, we draw from a data set of terrestrial, nonvolant mammals to quantify and compare patterns across the body size spectrum, the taxonomic hierarchy, continental space, and evolutionary time. We employ a variety of statistical techniques including “sib‐sib” regression, phylogenetic autocorrelation, and nested ANOVA. We find an extremely high resemblance (heritability) of size among congeneric species for mammals over ∼18 g; the result is consistent across the size spectrum. However, there is no significant relationship among the body sizes of congeneric species for mammals under ∼18 g. We suspect that life‐history and ecological parameters are so tightly constrained by allometry at diminutive size that animals can only adapt to novel ecological conditions by modifying body size. The overall distributions of size for each continental fauna and for the most diverse orders are quantitatively similar for North America, South America, and Africa, despite virtually no overlap in species composition. Differences in ordinal composition appear to account for quantitative differences between continents. For most mammalian orders, body size is highly conserved, although there is extensive overlap at all levels of the taxonomic hierarchy. The body size distribution for terrestrial mammals apparently was established early in the Tertiary, and it has remained remarkably constant over the past 50 Ma and across the major continents. Lineages have diversified in size to exploit environmental opportunities but only within limits set by allometric, ecological, and evolutionary constraints.

The Evolution of Maximum Body Size of Terrestrial Mammals

How Mammals Grew in Size Mammals diversified greatly after the end-Cretaceous extinction, which eliminated the dominant land animals (dinosaurs). Smith et al. (p. 1216) examined how the maximum size of mammals increased during their radiation in each continent. Overall, mammal size increased rapidly, then leveled off after about 25 million years. This pattern holds true on most of the continents—even though data are sparse for South America—and implies that mammals grew to fill available niches before other environmental and biological limits took hold. Maximum mammal size increased at the beginning of the Cenozoic, then leveled off after about 25 million years. The extinction of dinosaurs at the Cretaceous/Paleogene (K/Pg) boundary was the seminal event that opened the door for the subsequent diversification of terrestrial mammals. Our compilation of maximum body size at the ordinal level by sub-epoch shows a near-exponential increase after the K/Pg. On each continent, the maximum size of mammals leveled off after 40 million years ago and thereafter remained approximately constant. There was remarkable congruence in the rate, trajectory, and upper limit across continents, orders, and trophic guilds, despite differences in geological and climatic history, turnover of lineages, and ecological variation. Our analysis suggests that although the primary driver for the evolution of giant mammals was diversification to fill ecological niches, environmental temperature and land area may have ultimately constrained the maximum size achieved.

An analytical model of latitudinal gradients of species richness with an empirical test for marsupials and bats in the New World

Although latitudinal gradients in species richness are well-documented for a plethora of taxa in terrestrial, freshwater, and marine environments, little consensus exists concerning the predominant biological factor that is responsible for the pattern. We produced an analytical null model to assess the degree to which gradients in species richness could be a consequence of the random determination of the limits of species ranges. The model predicts a parabolic increase in species richness toward the middle of a latitudinal domain in the absence of underlying environmental gradients. Our stochastic model accounted for a significant portion of variation in marsupial and bat species richness for each of three different latitudinal domains in the New World: continental limits, the latitudinal extent of each higher taxon, and the smallest latitudinal extent which comprises 95% of the species in the higher taxon. A unique prediction of the stochastic model, which distinguishes it from all other hypotheses, is that parabolic latitudinal gradients in richness should exist for species wholly contained within random latitudinal subsets. Observed gradients for New World marsupials and bats document that this is true. Regardless of taxon or domain, differences between observed and expected species richness (residuals) were not related appreciably to latitudinal band area (r2 i 0.15). The ubiquity and similarity of latitudinal gradients in species richness for different taxa could be a consequence of pervasive stochastic mechanisms rather than a product of a dominant underlying environmental gradient to which all species respond. Application of our null model to other gradients (e.g., depth, productivity, disturbance) may provide insight into mechanisms affecting patterns of species richness in other ecological or biogeographic settings.

A QUANTITATIVE ASSESSMENT OF THE RANGE SHIFTS OF PLEISTOCENE MAMMALS

Abstract Much attention has been focused on the response of species to the climate change associated with the last deglaciation during the Pleistocene. Generally, species respond in an individualistic manner to climate change, and expand and contract their ranges independently; consequently, community composition is extremely variable over time. Although data are available to examine range shifts of species by mapping species ranges over time, most investigations to date have been qualitative. I quantitatively assessed changes in distributions of species to determine the degree to which species shifted their ranges independently over broad time scales. Data on Pleistocene mammal assemblages from the FAUNMAP database were divided into 4 time periods (Pre-Glacial, full Glacial, Post-Glacial, and Modern). Range shifts were characterized by change in the median position of the range from 1 time period to another, change in range size, and direction of the shift. The degree to which species were shifting their ranges independently of one another was evaluated by examining frequency distributions of each range shift parameter and comparing directions wherein species were shifting their range centroids. Many species are shifting their ranges in similar ways. Many species in each time transition change their range size very little. Species shift their range centroids, on average, between 1,200 and 1,400 km. Moreover, if the United States is divided into quadrats and the direction in which species with those quadrats shift their ranges is examined, it becomes clear that in each quadrat there are some directions that are favored over others. The majority of these distributions in the 2 oldest transitions differ from a uniform distribution. Therefore, the prediction that the individualistic responses of species to climate change should result in nonanalog communities likely is oversimplified.

A HEMISPHERIC ASSESSMENT OF SCALE DEPENDENCE IN LATITUDINAL GRADIENTS OF SPECIES RICHNESS

Considerable controversy surrounds the importance of historical, evolution- ary, and ecological factors affecting continental patterns in species richness. Although the importance of area and latitude are both well documented, few attempts have been made to integrate their effects in a single model. Most studies have been conducted by super- imposing grids on equal-area projection maps and counting the number of species occurring within grid cells (i.e., quadrats). Unfortunately, different grid-based studies use different quadrat sizes, making comparisons tenuous. We developed a hierarchical model to evaluate the degree to which area (based on a nested series of quadrats of five sizes, 1000-25 000 km 2 ) affects the latitudinal gradient in species richness. The model allows the relationship between latitude and area to be nonlinear and, in its simple form, evaluates how well species richness can be predicted by the additive influences of latitude and area. The complex model evaluates whether an area 3 latitude interaction accounts for significant additional variation in species richness above that in the simple model (i.e., assesses the scale dependence of the latitudinal gradient). For bats and marsupials, the simple model included only latitudinal effects and accounted for over half of the variation in species richness of each of the two taxa. The interactive effect was nonsignificant for each taxon, accounting for ,0.1% of additional variation in species richness in each case. If other taxa or land masses produce similar relationships, then the form of the latitudinal gradient is relatively invariant with respect to area at 1000-25 000 km 2 scales, and comparisons among studies at this spatial

SPECIES RICHNESS, LATITUDE, AND SCALE-SENSITIVITY

The latitudinal gradient of species richness is well documented for a variety of taxa in both terrestrial and aquatic environs. Moreover, a number of recent attempts to assess the effects of scale on the relationship have concluded that the latitudinal pattern is scale-invariant. Nonetheless, the power of those approaches is predicated on precise knowledge of the forms of the latitudinal gradient, the area relationship, and their interaction. We used a model developed by J. Pastor, A. Downing, and H. E. Erickson for assessing the effects of scale on the productivity–diversity gradient to avoid such complications. More specifically, for 253 sets of nested quadrats (1000–25 000 km2) located throughout the New World, we parameterized the power function and determined whether those parameters varied in a systematic fashion with latitude. Significant latitude-induced monotonic variation in the rate of species accumulation with area (z parameter) documented scale-sensitivity for both bats and marsupials, with z decreasing toward tropical latitudes. Variation in the intercept parameter (C) reflected the latitudinal gradient in richness after adjustment for the latitude-specific effects of area. Both bats and marsupials exhibited strong gradients of richness, with modal values in the tropics. Mechanisms affecting species richness, or the size of ranges and the juxtapositioning of their boundaries, may initiate scale-sensitivity in many systems. A number of mechanistic or phenomenological models (environmental, geometric constraint, and Rapoport-rescue hypotheses) thought to produce the latitudinal gradient also enhance the likelihood of scale-sensitivity. Consequently, investigations of other macroecological gradients of richness (e.g., elevation, depth, and productivity) that should be affected by such factors are probably also scale-sensitive.

Latitudinal patterns of range size : methodological concerns and empirical evaluations for New World bats and marsupials

Controversy surrounds the existence and causes of latitudinal gradients in range size, as well as the methodologies for detecting them. We show that results based on traditional methods used to evaluate Rapoports Rule (i.e., a positive correlation between range size and latitude) for New World bats and marsupials are conflicting and subject to problems associated with statistical independence and mathematical bias. To avoid these shortcomings, we used simulation models to assess the degree to which latitudinal patterns are a product of stochastic or deterministic processes. Two different kinds of simulations were used to generate range sizes. The simulations differed in the kinds of spatial constraints that were incorporated into random algorithms. The first model randomly produced upper and lower latitudinal limits, without any spatial constraint except that species ranges were entirely within the continental New World. To reflect aspects of empirical latitudinal gradients of diversity, the second model incorporated the constraint that the set of randomly generated ranges had a distribution of mid-latitudes or most-distal points that corresponded exactly to the distribution of mid-latitudes or most-distal points in each taxon. The correlation between latitudinal range size and latitude was calculated separately for each taxon. Random distributions of correlation coefficients were generated from 1000 simulations for each taxon. When mid-latitude was used as a descriptor, New World bats and marsupials had ranges that are smaller in the tropics and larger in the temperate zone than would be expected by chance alone. In contrast, when most-distal point was used as a descriptor, relationships were consistently indistinguishable from those produced by stochastic processes.

Integrating spatial and temporal approaches to understanding species richness

Understanding species richness patterns represents one of the most fundamental problems in ecology. Most research in this area has focused on spatial gradients of species richness, with a smaller area of emphasis dedicated to understanding the temporal dynamics of richness. However, few attempts have been made to understand the linkages between the spatial and temporal patterns related to richness. Here, we argue that spatial and temporal richness patterns and the processes that drive them are inherently linked, and that our understanding of richness will be substantially improved by considering them simultaneously. The species–time–area relationship provides a case in point: successful description of the empirical spatio-temporal pattern led to a rapid development and testing of new theories. Other areas of research on species richness could also benefit from an explicitly spatio-temporal approach, and we suggest future directions for understanding the processes common to these two traditionally isolated fields of research.

The maximum rate of mammal evolution

How fast can a mammal evolve from the size of a mouse to the size of an elephant? Achieving such a large transformation calls for major biological reorganization. Thus, the speed at which this occurs has important implications for extensive faunal changes, including adaptive radiations and recovery from mass extinctions. To quantify the pace of large-scale evolution we developed a metric, clade maximum rate, which represents the maximum evolutionary rate of a trait within a clade. We applied this metric to body mass evolution in mammals over the last 70 million years, during which multiple large evolutionary transitions occurred in oceans and on continents and islands. Our computations suggest that it took a minimum of 1.6, 5.1, and 10 million generations for terrestrial mammal mass to increase 100-, and 1,000-, and 5,000-fold, respectively. Values for whales were down to half the length (i.e., 1.1, 3, and 5 million generations), perhaps due to the reduced mechanical constraints of living in an aquatic environment. When differences in generation time are considered, we find an exponential increase in maximum mammal body mass during the 35 million years following the Cretaceous–Paleogene (K–Pg) extinction event. Our results also indicate a basic asymmetry in macroevolution: very large decreases (such as extreme insular dwarfism) can happen at more than 10 times the rate of increases. Our findings allow more rigorous comparisons of microevolutionary and macroevolutionary patterns and processes.