Brian A. Maurer

Macroecology: the division of food and space among species on continents

Analyses of statistical distributions of body mass, population density, and size and shape of geographic range offer insights into the empirical patterns and causal mechanisms that characterize the allocation of food and space among the diverse species in continental biotas. These analyses also provide evidence of the processes that couple ecological phenomena that occur on disparate spatial and temporal scales—from the activities of individual organisms within local populations to the dynamics of continent-wide speciation, colonization, and extinction events.

Evolution of Species Assemblages: Effects of Energetic Constraints and Species Dynamics on the Diversification of the North American Avifauna

Characteristics of the terrestrial avifauna of North America can be viewed as adaptations by a taxonomically, geographically, and ecologically defined assemblage of many species to the constraints imposed by its own biology and by the environment. We have identified distinctive patterns in the variation among species in population density, body size, area of geographical range, and trophic status. The patterns observed in bivariate plots of log-transformed variables can be characterized provisionally in terms of polygons that enclose combinations of the variables exhibited by species. The sides of these polygons may be either abrupt or indistinct. We suggest that sharp, clear-cut boundaries separating combinations of characteristics that species possess from those combinations that are not observed in any species are the result of absolute constraints. As a trivial example, the maximum size of the geographical range is determined by the size of the continent. A more interesting example of an apparently absolute constraint is an energetic trade-off between maximum population density and body size. Boundaries separating combinations of characteristics that species possess from those not possessed can also be diffuse and relatively poorly defined. We suggest that such boundaries result from the probabilistic processes of origination and extinction, such that the number of species declines gradually across the boundary. An example is the increase in minimum area of geographical range with increasing body size, which is hypothesized to reflect the probability of extinction. We summarize our hypotheses to account for the observed patterns in a model for the adaptive evolution of the North American terrestrial avifauna. With appropriate modifications, a similar model could be developed for any biota. Our analyses provide neontological evidence for the kinds of patterns observed in the fossil record and used by paleontologists to argue that a process analogous to natural selection at the level of individuals within populations operates at the level of species within biotas. It is useful to view certain attributes of contemporary species as the result of some combination of absolute constraints and the dynamics of speciation, colonization, and extinction processes.

Empirical evaluation of neutral theory.

We describe a general framework for testing neutral theory. We summarize similarities and differences between ten different versions of neutral theory. Two central predictions of neutral theory are that species abundance distributions will follow a zero-sum multinomial distribution and that community composition will change over space due to dispersal limitation. We review all published empirical tests of neutral theory. With the exception of one type of test, all tests fail to support neutral theory. We identify and perform several new tests. Specifically, we develop a set of best practices for testing the fit of the zero-sum multinomial (ZSM) vs. a lognormal null hypothesis and apply this to a data set, concluding that the lognormal outperforms neutral theory on robust tests. We explore whether a priori parameterization of neutral theory is possible, and we conclude that it is not. We show that non-curve-fitting predictions readily derived from neutral theory are easily falsifiable. In toto, there is a current overwhelming weight of evidence against neutral theory. We suggest some next steps for neutral theory.

Relationship between cystic fibrosis respiratory tract bacterial communities and age, genotype, antibiotics and Pseudomonas aeruginosa

Polymicrobial bronchopulmonary infections in cystic fibrosis (CF) cause progressive lung damage and death. Although the arrival of Pseudomonas aeruginosa often heralds a more rapid rate of pulmonary decline, there is significant inter-individual variation in the rate of decline, the causes of which remain poorly understood. By coupling culture-independent methods with ecological analyses, we discovered correlations between bacterial community profiles and clinical disease markers in respiratory tracts of 45 children with CF. Bacterial community complexity was inversely correlated with patient age, presence of P. aeruginosa and antibiotic exposure, and was related to CF genotype. Strikingly, bacterial communities lacking P. aeruginosa were much more similar to each other than were those containing P. aeruginosa, regardless of antibiotic exposure. This suggests that community composition might be a better predictor of disease progression than the presence of P. aeruginosa alone and deserves further study.

The micro and macro in body size evolution

The diversity of body sizes of organisms has traditionally been explained in terms of microevolutionary processes: natural selection owing to differential fitness of individual organisms, or to macroevolutionary processes: species selection owing to the differential proliferation of phylogenetic lineages. Data for terrestrial mammals and birds indicate that even on a logarithmic scale frequency distributions of body mass among species are significantly skewed towards larger sizes. We used simulation models to evaluate the extent to which macro‐ and microevolutionary processes are sufficient to explain these distributions. Simulations of a purely cladogenetic process with no bias in extinction or speciation rates for different body sizes did not produce skewed log body mass distributions. Simulations that included size‐biased extinction rates, especially those that incorporated anagenetic size change within species between speciation and extinction events, regularly produced skewed distributions. We conclude that although cladogenetic processes probably play a significant role in body size evolution, there must also be a significant anagenetic component. The regular variation in the form of mammalian body size distributions among different‐sized islands and continents suggests that environmental conditions, operating through both macro‐ and microevolutionary processes, determine to a large extent the diversification of body sizes within faunas. Macroevolution is not decoupled from microevolution.

DISTRIBUTION OF ENERGY USE AND BIOMASS AMONG SPECIES OF NORTH AMERICAN TERRESTRIAL BIRDS

The distribution of biomass and energy use among species with different body sizes provides an empirical basis for studying ecological processes that determine species diversity. Biomass and energy use distributions were determined for North Amer- ican terrestrial birds from data on population density and body mass of 380 species and data on energy use obtained from the literature. Using these data, several hypotheses regarding the specific form of biomass (summed for all species in a body size category) and energy use distributions were evaluated. Biomass continued to increase in successive log body mass intervals, but this was not due simply to increasing species numbers. Energy use initially increased in these same intervals but leveled off above a body mass of 80 g. Energy used by average populations of individual species was uniformly distributed between the lower and upper bounds of each log body mass interval. In addition, the upper boundaries on biomass and energy use for individual species paralleled closely the biomass and energy use distributions. Quali- tatively similar patterns were obtained for plant- and animal-eating birds considered sep- arately, and for birds in 14 arbitrarily defined subregions of the North American continent. There were important quantitative differences among energy use distributions for the 14 subregions. Subregions at lower latitudes had energy use distributions that were nearly an order of magnitude higher than those of regions at higher latitudes. These results imply that previous hypotheses to explain biomass and energy use distributions were not of sufficient generality to account for both similarities among distributions of very different systems (e.g., birds and aquatic plankton) and spatial variation among systems composed of similar species. A more general hypothesis should consider the importance of inherent physiological constraints on energy use and environmental limitations on energy avail- ability. The processes that influence resource allocation in a large assemblage of many species may result in statistical patterns of energy use and biomass that tend to maximize ecological quantities analogous to entropy in statistical physical systems.

GEOSTATISTICS AS A TOOL FOR EXAMINING HYPOTHESIZED DECLINES IN MIGRATORY SONGBIRDS

Data from the North American Breeding Bird Survey (BBS) indicate sig- nificant declines in the populations of several species of songbirds, including several Neo- tropical migrants. These declines have been attributed to habitat destruction and fragmen- tation on the breeding grounds, in strategic migratory stopover sites, and on the wintering grounds. Using BBS data from the 1967-1989 period and universal kriging, we produced maps of abundance change for two declining species of wood warblers to test hypothetical spatial scenarios of decline over entire breeding ranges. These species were the Cerulean Warbler (Dendroica cerulea) and the Prairie Warbler (D. discolor). We found considerable variability in the location of areas of decline when comparing successive 5-yr periods. In some comparisons, areas of decline were concentrated in the centers of abundance of these species, and in others, they were scattered throughout their range. We also found that the direction and intensity of population trends was quite sensitive to the methods used for calculating abundance. Our results indicate that, even for species where significant long- term declines have been reported, considerable variation exists in the direction of abundance change, both geographically and temporally. Although most of the long-term declines re- ported in particular species at local and regional scales are undeniable, gaps in our knowl- edge still prevent us from incorporating these trends into a global model of the annual cycle of Neotropical migrants.

The relationship between distribution and abundance in a patchy environment

A number of researchers have documented that the average density per patch occupied increases with number of patches occupied.Explanations for this pattern include the idea that extinction probability varies among species as a function of the number of sites they occupy and the hypothesis that widely distributed species can use more kinds of resources than rare species.These ideas can be compared by developing a statistical model of the consequences of patch selection by individual organisms.The model predicts a nonlinear increase in the average number of individuals per patch as the proportion of patches occupied increases

Entropy and information in evolving biological systems

Integrating concepts of maintenance and of origins is essential to explaining biological diversity. The unified theory of evolution attempts to find a common theme linking production rules inherent in biological systems, explaining the origin of biological order as a manifestation of the flow of energy and the flow of information on various spatial and temporal scales, with the recognition that natural selection is an evolutionarily relevant process. Biological systems persist in space and time by transfor ming energy from one state to another in a manner that generates structures which allows the system to continue to persist. Two classes of energetic transformations allow this; heat-generating transformations, resulting in a net loss of energy from the system, and conservative transformations, changing unusable energy into states that can be stored and used subsequently. All conservative transformations in biological systems are coupled with heat-generating transformations; hence, inherent biological production, or genealogical proesses, is positively entropic. There is a self-organizing phenomenology common to genealogical phenomena, which imparts an arrow of time to biological systems. Natural selection, which by itself is time-reversible, contributes to the organization of the self-organized genealogical trajectories. The interplay of genealogical (diversity-promoting) and selective (diversity-limiting) processes produces biological order to which the primary contribution is genealogical history. Dynamic changes occuring on times scales shorter than speciation rates are microevolutionary; those occuring on time scales longer than speciation rates are macroevolutionary. Macroevolutionary processes are neither redicible to, nor autonomous from, microevolutionary processes.

Taking species abundance distributions beyond individuals

The species abundance distribution (SAD) is one of the few universal patterns in ecology. Research on this fundamental distribution has primarily focused on the study of numerical counts, irrespective of the traits of individuals. Here we show that considering a set of Generalized Species Abundance Distributions (GSADs) encompassing several abundance measures, such as numerical abundance, biomass and resource use, can provide novel insights into the structure of ecological communities and the forces that organize them. We use a taxonomically diverse combination of macroecological data sets to investigate the similarities and differences between GSADs. We then use probability theory to explore, under parsimonious assumptions, theoretical linkages among them. Our study suggests that examining different GSADs simultaneously in natural systems may help with assessing determinants of community structure. Broadening SADs to encompass multiple abundance measures opens novel perspectives in biodiversity research and warrants future empirical and theoretical developments.