Roland R. Roth

Spatial Heterogeneity and Bird Species Diversity

A heterogeneity index, D, derived from the point—quarter technique was significantly correlated with bird species diversity (BSD) for several shrub and forest areas. It predicted BSD for a series of similar brushlands where other indices had failed. Species richness increased faster than species overlap in a series of increasing complex habitats up to the forest stage. Species overlap was negatively correlated with patchiness. Additional species may be accommodated in preforest habitats primarily by horizontal spatial segregation facilitate by the presence of additional patches. New patches result from the addition of layers of vegetation. In late shrub or forest stages other kinds of segregation such as vertical segregation become important to species packing. Patchiness, as measured here, has a proximate effect on avian diversity. The extent or existence of latitudinal gradients in habitat patchiness and the effect on bird species diversity is unknown. The need remains for a universal simple, yet meaningful, heterogeneity index which incorporates both horizontal and vertical variability of vegetation.

TEMPORAL PATTERNS OF FITNESS AND SURVIVAL IN THE WOOD THRUSH

Data from a 22-yr study of individually marked Wood Thrushes (Hylocichla mustelina) in a Delaware forest fragment were analyzed to determine temporal and demographic patterns of nesting success, recruitment of local young into the population (fitness), and adult return rates (survival). Clutch size, number of eggs incubated, and number of eggs hatched were greater in early nests than in late nests. Although no difference existed in the average number of fledglings produced from early and late nests, fledglings from early nests were more likely to be recruited as breeders in the local population. Pairs of yearling (SY) birds produced equal numbers of recruits per nest from early and late nests, whereas pairs of older (≥2-yr-old, ASY) birds produced more recruits from early nests. The probability of fledging at least one young tended to be greater for late nests, and the percentage of eggs that eventually fledged was also greater for late nests. All nests were most likely to fail in the laying stage than du...

Consequences of a Temporally and Spatially Variable Food Supply for an Unexploited Gray Squirrel (Sciurus carolinensis) Population

-The demography and spatial distribution of a gray squirrel (Sciurus carolinensis) population was studied with respect to differences in habitat quality and food production in a heterogeneous Delaware woodlot from 1972-1973. In 1971 a large mast crop was produced, followed by a poor 1972 crop and a partial recovery in 1973. The squirrel population declined from 116 individuals in summer 1972 to 82 in fall 1973. Major components of the decline were losses of 1972 juveniles and 2-5-year-old females, and curtailed breeding in 1973. Squirrels were segregated among five study plots that represented areas of differing habitat quality and food production within the woodlot. The distribution of the sexes among the plots was significantly heterogeneous in summer and fall 1972 and summer 1973 (P < 0.10, G-test); plots with poorer mast production had a preponderance of females (R percent of individuals = 62%) while males predominated in plots with the best mast resources (R = 64%). In particular, adult males and yearlings (both sexes) were skewed towards high-mast plots while adult females were more abundant in low-mast plots. Juveniles showed no discernable pattern of distribution among plots. Losses were greater than expected from low-mast plots and less than expected from high-mast plots, especially for juveniles, yearlings, and females. The pattern of spatial variation of food abundance, sex (and possibly age) ratios and disappearance rates suggests a model of population regulation in which subordinates are relegated to suboptimal sites and bear major stress and losses when population density is high and food resources dwindle.

Population Ecology of the Eastern Box Turtle in a Fragmented Landscape

Abstract In the mid-Atlantic region, urban sprawl and development have resulted in habitat alterations and fragmentation; however, the effects on eastern box turtle (Terrapene carolina carolina) populations are unclear. To investigate the status of eastern box turtle populations in a fragmented landscape, we used mark–recapture and radiotelemetry to estimate population density, sex ratio, age structure, and survival on 4 study areas with differing degrees of isolation and human disturbance in northern New Castle County, Delaware, USA. We estimated adult population densities ranging from 0.81 turtles/ha to 3.62 turtles/ha among our 4 study areas. Sex ratios were male-biased at 2 study areas and balanced at 2 study areas. Proportion of juveniles ranged from 0% to 31%. Estimated annual survival rate ranged from 0.813 to 0.977. Mortality of radiotagged and marked turtles was primarily due to natural causes, but mowing was the primary cause of human-induced mortality. We found evidence of population decline at one study area due to low survival and recruitment. Human disturbances, isolation, and habitat composition appear to have the greatest influence on the box turtle populations we studied. To minimize mortality from human disturbance, we suggest planting crops adjacent to forest habitat that require no mowing or mowing at a height ≥15 cm.

Predicting Metapopulation Responses to Conservation in Human-Dominated Landscapes

Loss of habitat to urbanization is a primary cause of population declines as human-dominated landscapes expand at increasing rates. Understanding how the relative effects of different conservation strategies is important to slow population declines for species in urban landscapes. We studied the wood thrush Hylocichla mustelina, a declining forest-breeding Neotropical migratory species, and umbrella species for forest-breeding songbirds, within the urbanized mid-Atlantic United States. We integrated 40 years of demographic data with contemporary metapopulation model simulations of breeding wood thrushes to predict population responses to differing conservation scenarios. We compared four conservation scenarios over a 30-year time period (2014–2044) representing A) current observed state (Null), B) replacing impervious surface with forest (Reforest), C) reducing brown-headed cowbird Molothrus ater parasitism pressure (Cowbird removal), and D) simultaneous reforesting and cowbird removal. Compared to the Null scenario, the Reforest scenario increased mean annual population trends by 54 % , the Remove cowbirds scenario increased mean annual population trends by 38 %, and the scenario combining reforestation and cowbird removal increased mean annual population trends by 98 %. Mean annual growth rates (λ) per site were greater in the Reforest (λ = 0.94) and Remove cowbirds (λ = 0.92) compared to the Null (λ = 0.88) model scenarios. However, only by combining the positive effects of reforestation and cowbird removal did wood thrush populations stop declining (λ = 1.00). Our results suggest that independently replacing impervious surface with forest habitat around forest patches and removing cowbirds may slow current negative population trends. Furthermore, conservation efforts that combine reforestation and cowbird removal may potentially benefit populations of wood thrushes and other similarly forest-breeding songbird species within urbanized fragmented landscapes that typify the mid-Atlantic United States.