S. K. Morgan Ernest

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.

LIFE HISTORY CHARACTERISTICS OF PLACENTAL NONVOLANT MAMMALS

The purpose of this data set was to compile general life history characteristics for a variety of mammalian species in order to perform comparative life history analyses among different taxa and different body size groups. Data were collected from the literature, and data sources are documented for each species within the data file. Since life history characteristics will show minor variation with environmental conditions (resource availability, climate, competitive environment, and predation pressure) a general life history for each species was sought to average over minor differences in local populations. To create a general life history for each species, life history values are often an average from several literature sources. Life history variables included in the data set are: maximum lifespan (months), age of first reproduction (months), gestation time (months), weaning age (months), weaning mass (grams), litter size, litters per year, newborn mass (grams), and adult body mass (grams). Since these d...

Rain and rodents: complex dynamics of desert consumers

W is the lifeblood of the desert. It comes in rains that are typically scant and sporadic, but can be so intense as to cause flooding. Because water is the resource in shortest supply, the amount and timing of precipitation directly limits plant growth and primary production. Seasons of exceptionally heavy and frequent rains produce the spectacular desert blooms shown in nature films and magazines. Seasons of exceptionally high rainfall are also thought to cause increases in rodent populations and outbreaks of rodentborne diseases such as hantavirus and plague. El Niño is supposed to cause exceptionally heavy winter rainfall in the deserts of southwestern North America, leading in turn to plant growth, abundant seeds and insects, high populations of small mammals, density-dependent increases in parasites and diseases, and increased contact between rodents, their pathogens, and humans, resulting in disease epidemics. Thus, the outbreak of the Sin Nombre strain of hantavirus that killed 27 people in the Four Corners region of the southwestern United States in the summer of 1993 was attributed to the rains, plant production, and rodent increases triggered by the El Niño events of 1991–1992 and 1992–1993 (Harper and Meyer 1999). Ecologists have long been interested in these kinds of complicated pathways of interactions and particularly in how relationships between resources and consumers affect the structure and dynamics of ecosystems. The “bottom-up” pattern of regulation described above occurs when pulsatile resource inputs are transmitted up food chains, causing increases first in plants and then in successively higher trophic levels. This contrasts with “top-down” regulation, in which the feeding activities of top carnivores cascade down food chains to affect successively lower trophic levels (e.g., Hairston et al. 1960, Oksanen et al. 1981, Carpenter and Kitchell 1988). But ecological systems are complex, and there is reason to believe that resource–consumer relationships can exhibit chaotic or other forms of complicated nonlinear dynamics (e.g., Schaffer and Kot 1985, Hanski et al. 1993, Hastings et al. 1993, Lima et al. 1999). Long-term ecological studies provide unique opportunities to study resource–consumer relationships in realistically complex natural settings. Since 1977 we have been monitoring the weather, plants, and rodents in the Chihuahuan Desert near Portal, Arizona (figure 1; Brown 1998, Ernest et al. 2000). The resulting data allow us to evaluate the relationship between El Niño events and rainfall, the dependence of plants on precipitation, and the ways in which episodic rains affect desert rodent populations. After 23 years of study, we are far from understanding the dynamics of this ecosystem. One thing that is clear, however, is that simple bottom-up regulation does not occur. The responses of desert consumers to precipitation are complex and nonlinear.

Homeostasis and compensation: the role of species and resources in ecosystem stability

A synthesis of community and ecosystem ecology should yield insights into the role of species in ecosystem function. Concepts from these subdisciplines of ecology, specifically species compensation and ecosystem homeostasis, can be linked by analyzing the effect of changes in the abundance of species on ecosystem processes. Compensatory changes in species populations in response to environmental fluctuations can maintain an approximate steady state between rates of resource supply and resource consumption. We predict that ecosystem-level properties, such as species richness, total population, biomass, and energy use, will exhibit less variability in response to environmental change than will species composition. We tested this prediction using long-term data of a desert rodent community responding to natural environmental fluctuations and of a plant community responding to experimental manipulations. For the rodents, species composition was twice as variable as the ecosystem properties. This result was the same for both the analysis of variability around the 22-yr average and the analysis of variability from one time period to the next. For the plant communities, species composition was more variable among treatments in most years than stem count or species richness. Using the variance ratio proposed by J. L. Klug et al. we detected negative covariances in the rodent community, confirming the presence of compensatory dynamics.

Zero-sum, the niche,and metacommunities: long-term dynamics of community assembly

Recent models of community assembly, structure, and dynamics have incorporated, to varying degrees, three mechanistic processes: resource limitation and interspecific competition, niche requirements of species, and exchanges between a local community and a regional species pool. Synthesizing 30 years of data from an intensively studied desert rodent community, we show that all of these processes, separately and in combination, have influenced the structural organization of this community and affected its dynamical response to both natural environmental changes and experimental perturbations. In addition, our analyses suggest that zero‐sum constraints, niche differences, and metacommunity processes are inextricably linked in the ways that they affect the structure and dynamics of this system. Explicit consideration of the interaction of these processes should yield a deeper understanding of the assembly and dynamics of other ecological communities. This synthesis highlights the role that long‐term data, especially when coupled with experimental manipulations, can play in assessing the fundamental processes that govern the structure and function of ecological communities.

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.

Species‐level and community‐level responses to disturbance: a cross‐community analysis

Communities are comprised of individual species that respond to changes in their environment depending in part on their niche requirements. These species comprise the biodiversity of any given community. Common biodiversity metrics such as richness, evenness, and the species abundance distribution are frequently used to describe biodiversity across ecosystems and taxonomic groups. While it is increasingly clear that researchers will need to forecast changes in biodiversity, ecology currently lacks a framework for understanding the natural background variability in biodiversity or how biodiversity patterns will respond to environmental change. We predict that while species populations depend on local ecological mechanisms (e.g., niche processes) and should respond strongly to disturbance, community-level properties that emerge from these species should generally be less sensitive to disturbance because they depend on regional mechanisms (e.g., compensatory dynamics). Using published data from terrestrial animal communities, we show that community-level properties were generally resilient under a suite of artificial and natural manipulations. In contrast, species responded readily to manipulation. Our results suggest that community-level measures are poor indicators of change, perhaps because many systems display strong compensatory dynamics maintaining community-level properties. We suggest that ecologists consider using multiple metrics that measure composition and structure in biodiversity response studies.

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.