ISSN:0012-9658    

DOI:10.2307/2265648

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

We examined the reproductive success and long—term population dynamics of Neotropical migrant birds in the fragmented landscapes of Illinois. Our primary objective was to assess whether annual variation in abundances and persistence of populations within woodlots was related to variation in reproductive success. Nesting success of migrants was so low in the woodlots where we sampled that it is unlikely that these populations are self—sustaining. Moreover, populations frequently disappear from and recolonize these woodlots. These data suggest that the source—sink metaphor for population dynamics within spatially structured populations is applicable to migratory birds in Illinois. The scale of this dynamic is unknown, but may be regional (i.e., midwestern U.S.). An important implication of this result is that census data must be interpreted carefully. For one woodlot, we detected no systematic trend in the overall abundances of migratory birds over much of the 20th century. Yet the viability of populations within that woodlot likely decreased dramatically over that time owing to increasing rates of nest predation and brood parasitism. Lack of data on dispersal is a major gap in understanding the population dynamics of Neotropical migrants and prescribing effective conservation measures.

ISSN:0012-9658    

DOI:10.2307/2265649

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Although there is strong evidence that a few Neotropical migrant land birds have severely declining populations, we know of no evidence of general overall declines. One widely cited analysis of data from the Breeding Bird Survey (BBS), the only continental—scale monitoring program for land birds in the breeding season, indicates declining populations in eastern forests in the 1980s, but other analyses conclude that most species have had stable or increasing populations in the last 25 yr. These conflicting results call for a new perspective. Here, we propose two approaches to the analysis of BBS data that emphasize the detection of temporal and geographic variation in population trends. The first approach uses nonlinear regression to detect temporal changes in population trends, and permits the construction of graphs that compare nonlinear trends among regions. The second approach provides statistical tests of multispecies patterns of geographic variation in population trends. We report analyses of BBS data for 1966 to 1992 for the 26 species of wood warblers (Parulinae) for which the most data are available and the chance of obtaining reliable estimates is best. Using pooled data for all 26 species for eastern and central North America, nonlinear regression indicated declining populations in the Appalachian Mountains and the Eastern Foothills in the 1970s, and stability elsewhere. As examples of the use of this first approach in single—species analyses, we give maps, graphs, and statistical tests to illustrate variation in population trends by physiographic strata in the Blue—winged Warbler (Vermivora pinus) and the Ovenbird (Seiurus aurocapillus). As an example of the second approach, we applied a probit—normal model to a matrix of increases and decreases in the 26 species in 37 geographic strata. The analysis identified specific highland areas as having unusually high proportions of declining populations. These results suggest that correlates of elevation should be considered as factors possibly regulating populations of warblers and other land birds. Our perspective is that focus on studies of temporal and geographic variation in population trends of all birds, including Neotropical migrants, could help conservation biologists identify where species and groups of species are in the most trouble.

ISSN:0012-9658    

DOI:10.2307/2265650

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The hypothesis that Neotropical migrant birds may be undergoing widespread declines due to land use activities on the breeding grounds has been examined primarily by synthesizing results from local studies. Growing concern for the cumulative influence of land use activities on ecological systems has heightened the need for large—scale studies to complement what has been observed at local scales. We investigated possible landscape effects on Neotropical migrant bird populations for the eastern United States by linking two large—scale inventories designed to monitor breeding—bird abundances and land use patterns. The null hypothesis of no relation between landscape structure and Neotropical migrant abundance was tested by correlating measures of landscape structure with bird abundance, while controlling for the geographic distance among samples. Neotropical migrants as a group were more sensitive to landscape structure than either temperate migrants or permanent residents. Neotropical migrants tended to be more abundant in landscapes with a greater proportion of forest and wetland habitats, fewer edge habitats, larger forest patches, and with forest habitats well dispersed throughout the scene. Permanent residents showed few correlations with landscape structure and temperate migrants were associated with habitat diversity and edge attributes rather than with the amount, size, and dispersion of forest habitats. The association between Neotropical migrant abundance and forest fragmentation differed among physiographic strata, suggesting that landscape context affects observed relations between bird abundance and landscape structure. Finally, associations between landscape structure and temporal trends in Neotropical migrant abundance were counter to those observed in space. Trends in Neotropical migrant abundance were negatively correlated with forest habitats. These results suggest that extrapolation of patterns observed in some landscapes is not likely to hold regionally, and that conservation policies must consider the variation in landscape structure associations observed among different types of bird species and in physiographic strata with varying land use histories.

ISSN:0012-9658    

DOI:10.2307/2265651

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Recent declines in Neotropical—Nearctic migrant songbird populations are often attributed to events during the nonbreeding season, such as tropical habitat conversion and drought. Support for this hypothesis in most species, however, is largely anecdotal or conjectural. There is a dearth of demographic information about migrants on their Neotropical winter grounds. Such data are needed to identify specific ecological factors influencing survival, dispersal, and, ultimately, population abundances aggregated over multiple habitats at regional spatial scales. In this paper, we review several lines of evidence, emphasizing results of our research on paruline warblers in Jamaica, which indicate that migrant passerines often compete intraspecifically in winter for preferred quality habitats and that their populations may be limited at least in part by ecological conditions in winter. The demographic and ecological evidence supporting this hypothesis for migrant passerines includes: (1) differing densities among habitats, suggesting variation in habitat suitability; (2) strong territoriality, site attachment, and site fidelity; (3) experimental demonstrations of habitat saturation; (4) nonrandom distributions of sex and age classes among habitats; (5) overwinter decline of body mass by individuals occupying the most drought—stressed habitats; and (6) different residence times among habitats, suggesting differences in survival or dispersal. We review ecological and behavioral explanations for these demographic patterns, and make conservation recommendations based on our understanding of how local demographic circumstances affect broader scale population processes.

ISSN:0012-9658    

DOI:10.2307/2265652

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Monitoring long—term population change is an integral part of effective conservation—oriented research and management, and is central to the current debate on the status of Neotropical migrant land birds. However, the analysis of count data such as the Breeding Bird Survey is complicated by the subjective nature of trend estimation, and by limitations inherent to extensive, volunteer—based surveys, such as measurement error and missing data. A number of analysis methods have been used that differ in their approach to dealing with these complications and produce different estimates of population change when applied to the same data. There is, however, no consensus as to which method is the most suitable. Many analytical issues remain unresolved, such as model of trend, observer effects, treatment of missing observations, distribution of counts, and data selection criteria. These issues make it difficult to evaluate the relative merits of the methods, although a number of new approaches (nonlinear regression, Poisson regression, estimating equations estimates) offer promising solutions to some problems. I suggest the use of Monte Carlo simulations to empirically test the performance of the methods under realistic, spatially explicit scenarios of population change, and provide an example of the approach.

ISSN:0012-9658    

DOI:10.2307/2265653

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Data from the North American Breeding Bird Survey (BBS) indicate significant declines in the populations of several species of songbirds, including several Neotropical migrants. These declines have been attributed to habitat destruction and fragmentation 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 reported in particular species at local and regional scales are undeniable, gaps in our knowledge still prevent us from incorporating these trends into a global model of the annual cycle of Neotropical migrants.

ISSN:0012-9658    

DOI:10.2307/2265654

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

We report on a 32—yr study of a population of Spanish Imperial Eagles, Aquila adalberti, which increased during the first 16 yr of study but remained stable during the last 16 yr. We analyzed changes in the mean and variance of fecundity in relation to population density to test predictions of two hypotheses of density—dependent fecundity. According to the interference" hypothesis, as density increases, frequency of agonistic encounters increases, resulting in a relatively uniform decrease in habitat quality. Consequently, mean fecundity decreases, and no relationship is expected between density and variance in fecundity. For the habitat heterogeneity hypothesis, however, as density increases, a greater proportion of individuals are forced to occupy lower quality habitats. Thus, mean fecundity decreases and fecundity variance must increase. Additionally, for this hypothesis, fecundities in good sites are expected to be equal in both low— and high—density situations. An inverse relationship between fecundity and population size was found in this eagle population. Annual variance in productivity showed significant increases over the study period. This trend was inversely related to mean productivity. Variance in productivity was related to the year of pair establishment, being higher in recently occupied territories. Mean and variance of the longer term territories remained constant during the study period. These results are in accordance with the habitat heterogeneity hypothesis.

ISSN:0012-9658    

DOI:10.2307/2265655

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

A statistical hypothesis test for a difference between the spatial distributions of two populations is presented. The test is based upon a generalization of the two—sample Cramer—von Mises test for a difference between two univariate probability distributions. It is designed to be sensitive to differences in the way the populations are distributed across the study area but insensitive to a difference in abundance between the two populations. The procedure is nonparametric and uses the methodology of a randomization test to determine the level of significance of the test statistic.

ISSN:0012-9658    

DOI:10.2307/2265656

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

I examine the suggestion of Holling (1992) that distributions of body sizes for animals show clumps corresponding to basic biotic and abiotic processes operating at different scales in time and space. This is done by comparing the properties of observed sample distributions with what is obtained from computer—generated samples taken from distributions with different numbers of modes. A combination of kernel density estimation and smoothed bootstrap resampling provides a test for whether a distribution with k + 1 modes fits significantly better than a distribution with k modes. The analysis of three data sets discussed by Holling shows sample size distributions that are consistent with the hypothesis that the underlying distributions are unimodal or bimodal. A fourth data set has a sample distribution that seems unusual even for observations from a distribution with eight modes.

ISSN:0012-9658    

DOI:10.2307/2265657

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY