Search

It has been long established that the richness of vascular plant species and many animal taxa decreases with increasing latitude, a pattern that very generally follows declines in actual and potential evapotranspiration, solar radiation, temperature, and thus, total productivity. Using county-level data on vascular plants from the United States (3000 counties in the conterminous 48 states), we used the Akaike Information Criterion (AIC) to evaluate competing models predicting native and normative plant species density (number of species per square kilometer in a county) from various combinations of biotic variables (e.g., native bird species density, vegetation carbon, normalized difference vegetation index), environmental/topographic variables (elevation, variation in elevation, the number of land cover classes in the county, radiation, mean precipitation, actual evapotranspiration, and potential evapotranspiration), and human variables (human population density, cropland, and percentage of disturbed lands in a county). We found no evidence of a latitudinal gradient for the density of native plant species and a significant, slightly positive latitudinal gradient for the density of normative plant species. We found stronger evidence of a significant, positive productivity gradient (vegetation carbon) for the density of native plant species and nonnative plant species. We found much stronger significant relationships when biotic, environmental/topographic, and human variables were used to predict native plant species density and nonnative plant species density. Biotic variables generally had far greater influence in multivariate models than human or environmental/topographic variables. Later, we found that the best, single, positive predictor of the density of nonnative plant species in a county was the density of native plant species in a county. While further study is needed, it may be that, while humans facilitate the initial establishment invasions of nonnative plant species, the spread and subsequent distributions of normative species are controlled largely by biotic and environmental factors. Key words: native plant species; non-indigenous plant species; patterns of invasion; species densities.

Changes in the forest habitats in the US may cause an error in the statistics acquired by the The Breeding Bird Surveys conducted by the National Biological Service. The surveys typically estimate bird populations from observations of populations near roads. However, a study of changes in forest cover in Ohio found that the habitat has changed significantly for data to be unreliable if based only on observations made from the roads.

Population trend data from the North American Breeding Bird Survey (BBS) showed that during 1966-1991, increasing species of woodland birds exceeded decreasing species except in central North America. Neotropical migrants had four increasing and sixteen decreasing species during 1980-1991. Residents and short distance migrants had the same number of increasing and decreasing species.

Because count data collected in many bird surveys are only an index to population size, factors that can influence the counts must be identified and incorporated into analyses. Observer quality is often ignored in analyses of population changes from survey data, but observers differ in methods and capabilities and, hence, tend to count different numbers of birds. We assess the consequences of between-observer differences in counts for estimation of population trends in the North American Breeding Bird Survey. Observer differences in numbers of birds counted were found in 50% of the 369 species we examined. For many species, observers in later years tended to count more birds than observers in earlier years, suggesting an increase in observer quality over time. Analysis of population trends from 1966 through 1991 indicates that failure to include observers as covariables in the analysis results in an overly optimistic view of population trends.

Peterson (2001) used Genetic Algorithm for Rule-set Prediction (GARP) models to predict distribution patterns from Breeding Bird Survey (BBS) data. Evaluations of these models should consider inherent limitations of BBS data: (1) BBS methods may not sample species and habitats equally; (2) using BBS data for both model development and testing may overlook poor fit of some models; and (3) BBS data may not provide the desired spatial resolution or capture temporal changes in species distributions. The predictive value of GARP models requires additional study, especially comparisons with distribution patterns from independent data sets. When employed at appropriate temporal and geographic scales, GARP models show considerable promise for conservation biology applications but provide limited inferences concerning processes responsible for the observed patterns. Key words: avian distribution patterns, BBS, GARP models, North American Breeding Bird Survey, predictive distribution models, roadside surveys.

Concerns over the use of the index methods to estimate trends in population size are discussed. It is suggested that index methods are often a cost effective component of valid wildlife monitoring, but that double-sampling or another procedure that corrects bias or establishes bounds on bias is essential.

SHOREBIRDS OF NORTH AMERICA: THE PHOTOGRAPHIC GUIDE. By Dennis R. Paulson. Princeton University Press, Princeton, New Jersey. 2005:384 pp., 534 color photographs. ISBN: 0691102740, $65 (cloth). ISBN: 0691121079, $29.95 (paper).-As [...]

OKLAHOMA BREEDING BIRD ATLAS. Edited by Dan L. Reinking. University of Oklahoma Press, Norman, Oklahoma. 2004: 528 pp., 222 color illustrations, 230 color maps. ISBN: 0806136146. $34.95 (paper).--The first North [...]