Steven R. Beissinger

Effects of Urbanization on Avian Community Organization

-The avian community of a mature residential area was studied and compared with an undisturbed climax beech-maple forest. Urbanization was presumed to be responsible for decreasing species richness and diversity, increasing biomass and density, and favoring dominance by a few species. Foraging guilds shifted from forest insectivores that were canopy foliage gleaners or bark drillers to urban ground gleaners. Analyses of habitat structure showed that although urban foliage height diversity was like that of the forest, the urban area contained only one-third of the total percent vegetative cover. As compared to the forest, urban vegetative cover was: (1) significantly less in all but the middle layer; (2) replaced by man-made structures, ground cover and ornamental vegetation in the low and middle layers but dominated the high layer; and (3) highly discontinuous, existing as isolated strata. Differences in avian community organization between the forest and urban area are discussed in relation to urban habitat manipulation and population-suppressing factors. Bird communities of residential and urban areas contain higher bird densities than outlying natural areas (Graber and Graber 1963, Emlen 1974), with only forest edge communities supporting greater densities in temperate zones. In addition to the factors controlling natural communities (Lancaster and Rees 1979) the diversity of birds in urban areas is affected by the age of the neighborhood (Lucid 1974) type of housing (Geis 1974), and degree of urbanization (Batten 1972). Few studies have compared the avifauna of cities with that of outlying natural areas and have measured habitat structure in both communities. The difficulties arise in selecting comparable study areas and quantifying the synthetic urban habitat in relation to natural parameters. In this study, we determined how urbanization affected avian community organization by comparing the ecological characteristics of the birds of a mature residential area with those of the regional vegetative climax, an outlying forest.

Population Viability Analysis

As human populations and the resources required to support societies continue to grow, an increasing number of plant and animal species around the world are facing extinction. Given limited time, space, and money, how do we decide which management actions will be most effective to avert extinctions? In this book, many of the worlds leading conservation and population biologists evaluate what has become a key tool in estimating extinction risk and evaluating potential recovery strategies - population viability analysis, or PVA. PVA integrates data on the life history, demography, and genetics of a species with information on environmental variability, using computer models ranging from simple measures of population growth rate to complex spatial simulations, to predict whether a given population will remain viable (i.e., not go extinct) under various management options. A synthetic and objective overview of the latest theoretical and methodological advances, Population Viability Analysis will be crucial reading for conservationists, land managers, and policy makers.

Identification of 100 fundamental ecological questions

Summary 1. Fundamental ecological research is both intrinsically interesting and provides the basic knowledge required to answer applied questions of importance to the management of the natural world. The 100th anniversary of the British Ecological Society in 2013 is an opportune moment to reflect on the current status of ecology as a science and look forward to high-light priorities for future work.

Detecting range shifts from historical species occurrences: new perspectives on old data

The difficulty of making valid comparisons between historical and contemporary data is an obstacle to documenting range change in relation to environmental modifications. Recent statistical advances use occupancy modeling to estimate simultaneously the probability of detection and the probability of occupancy, and enable unbiased comparisons between historical and modern data; however, they require repeated surveys at the same locations within a time period. We present two models for explicitly comparing occupancy between historical and modern eras, and discuss methods to measure range change. We suggest that keepers of historical data have crucial roles in curating and aiding accessibility to data, and we recommend that collectors of contemporary specimen data organize their sampling efforts to include repeated surveys to estimate detection probabilities.

Hatching Asynchrony and the Onset of Incubation in Birds, Revisited

In most animals, offspring from a reproductive bout usually hatch, emerge, or are born within a relatively short time of each other compared to the time required for their development. Thus, hatching or birthing in most animals is synchronous. This is especially likely to be true for animals with internal fertilization and development, where the birth of alll offspring occurs simultaneously (e.g., some fihes, snakes, and most mammals). Synchronous reproduction also occurs in animals with external fertilization or development when all zygotes are subject to the same environmental conditions (e.g., many insects, anurans, and fishes). Thus, in most the behavior of parents has little effect on the time between the emergence of their first and last young.

Trans–shell infection by pathogenic micro–organisms reduces the shelf life of non–incubated bird's eggs: a constraint on the onset of incubation?

Many birds initiate incubation before clutch completion, which results in asynchronous hatching. The ensuing within–brood size disparity often places later–hatched nestlings at a developmental disadvantage, but the functional significance of the timing of the onset of incubation is poorly understood. Early incubation may serve to maintain the viability of early–laid eggs, which declines over time owing to the putative effects of ambient temperature. An unexplored risk to egg viability is trans–shell infection by micro–organisms. We experimentally investigated the rate and magnitude of microbial trans–shell infection of the egg, and the relative effects of ambient temperature and micro–organisms on hatching success. We show that infection of egg contents is prevalent and occurs within the time required to lay a clutch. The probability of infection depends on the climatic conditions, the exposure period and the phylogenetic composition of the eggshell microbiota. We also demonstrate that microbial infection and ambient temperature act independently to reduce egg viability considerably. Our results suggest that these two factors could affect the onset of avian incubation in a wide range of environments.

Modeling Extinction in Periodic Environments: Everglades Water Levels and Snail Kite Population Viability

The effects of periodic environmental fluctuations on population viability are examined based on the use of environmental states. The approach is applied to the Florida population of the Snail Kite, an endangered wetland hawk that feeds almost solely on one species of snail. A preliminary assessment based on stochastic population fluctuations indicated that populations became viable when initial size surpassed 300 individuals. However, changes in population size between consecutive years, nesting success, and the length of the breeding season were all highly and positively related to water level and rainfall characteristics, which are highly periodic. Low water conditions cause Snail Kites to disperse and result in low recruitment, increased adult mortality, and population declines. The effects of cyclic drought were explored using stage-based life tables for three different water conditions or environmental states (drought, lag years following drought, and high years). Population sizes predicted by the model were closely associated with actual kite population counts. Deterministic projections indicated that kite populations would increase when intervals between droughts exceeded 3.3 yr, but stochastic simulations found that populations did not become viable unless intervals exceeded 4.3 yr. The model was sensitive to estimates of survivorship. The use of the environmental state approach is compared to standard techniques for population viability analyses (PVA), and the implications of the model for Everglades water management are discussed.

Detecting diversity: emerging methods to estimate species diversity

Estimates of species richness and diversity are central to community and macroecology and are frequently used in conservation planning. Commonly used diversity metrics account for undetected species primarily by controlling for sampling effort. Yet the probability of detecting an individual can vary among species, observers, survey methods, and sites. We review emerging methods to estimate alpha, beta, gamma, and metacommunity diversity through hierarchical multispecies occupancy models (MSOMs) and multispecies abundance models (MSAMs) that explicitly incorporate observation error in the detection process for species or individuals. We examine advantages, limitations, and assumptions of these detection-based hierarchical models for estimating species diversity. Accounting for imperfect detection using these approaches has influenced conclusions of comparative community studies and creates new opportunities for testing theory.

Minimum viable populations: is there a 'magic number' for conservation practitioners?

Establishing species conservation priorities and recovery goals is often enhanced by extinction risk estimates. The need to set goals, even in data-deficient situations, has prompted researchers to ask whether general guidelines could replace individual estimates of extinction risk. To inform conservation policy, recent studies have revived the concept of the minimum viable population (MVP), the population size required to provide some specified probability of persistence for a given period of time. These studies conclude that long-term persistence requires ≥5000 adult individuals, an MVP threshold that is unaffected by taxonomy, life history or environmental conditions. Here, we re-evaluate this suggestion. We find that neither data nor theory supports its general applicability, raising questions about the utility of MVPs for conservation planning.

SURVIVAL RATES OF A NEOTROPICAL PARROT: IMPLICATIONS FOR LATITUDINAL COMPARISONS OF AVIAN DEMOGRAPHY

Latitudinal variation in avian demography played an important early role in the development of life history theory, especially in the idea of a cost of reproduction. Recent attempts to determine the survivorship of tropical birds with mark-recapture sta- tistics have proved controversial. Here, we use a small neotropical bird, the Green-rumped Parrotlet (Forpus passerinus), as a model system for investigating sources of heterogeneity that might bias interspecific comparisons. Mark-resighting data were collected on 1334 adult parrotlets over a decade. We expected adult survival to be low because this parrot lays a large clutch (mean 5 7 eggs), is a cavity nester, and breeds in a highly seasonal environment. A two-age-class term in local survival was nonsignificant, indicating that an age or transience effect was unimportant. Local survival of males did not vary annually, but 19.3% of the yearly variation in female survival was explained by rates of nest loss during stages when females were incubating or brooding young. The overall local survival rate of parrotlets ( f5 0.565) was identical to temperate hole-nesting species of the same body size but was lower than that of tropical birds that lay smaller clutches. However, we also detected considerable heterogeneity in parrotlet survival. Females and males that were sighted but did not breed comprised a mean 23.5% and 52.9% of our population, respec- tively. Using multistate models, we found that breeders had significantly higher probabilities of local survival ( f5 0.678 vs. 0.486), of retaining their status as breeders ( c5 0.719 vs. 0.279), and of detection ( p 5 0.997 vs. 0.375) than did nonbreeders. Overall, males and females had comparable local survival rates (breeders f5 0.698 vs. 0.658, nonbreeders f5 0.536 vs. 0.436). Our estimates of local survival could be affected by breeding dispersal, but site fidelity of parrotlets was strong: 95% of adults moved ,500 m in consecutive years. A literature review for tropical birds showed that mark-resighting studies usually report return rates based on resightings of breeding or territorial adults, whereas mist net studies rely on recaptures and pool birds of different age and social status in their calcu- lations of local survival. Future studies should attempt to compare subsets of avian pop- ulations that are similar in demography. Because rates of site fidelity and social system may differ among species, these factors must also be considered in interspecific comparisons of avian life histories.