W. Daniel Edge

RIPARIAN BIRD COMMUNITY STRUCTURE IN PORTLAND, OREGON: HABITAT, URBANIZATION, AND SPATIAL SCALE PATTERNS

Abstract In 1999, we surveyed breeding bird and plant communities along 54 streams in the Portland, Oregon, metropolitan region to link bird community metrics with structural and spatial characteristics of urban riparian areas. Canonical correspondence analysis produced two explanatory axes relating to vegetation and road density. Total and non-native bird abundance was higher in narrow forests. Native bird abundance was greater in narrow forests surrounded by undeveloped lands; native species richness and diversity were greater in less-developed areas. Native resident and short-distance-migrant abundance was higher in narrow forests, and diversity was positively associated with developed lands. Neotropical migrant abundance, richness, and diversity were greater in open-canopied areas with fewer roads. We examined spatial relationships by regressing bird variables on satellite-derived forest canopy cover, area of undeveloped lands, and street density in a series of 50-m buffers within a 500-m radius around study sites. Non-native bird abundance decreased with increasing canopy cover within 450 m, but most other relationships were strongest at smaller scales (50–100 m). Our results suggest that increasing urban canopy cover is the most valuable land management action for conserving native breeding birds. A hierarchical scheme for Neotropical migrant conservation might include increasing forest canopy within 450 m of streams to control non-native species and cowbirds; reducing street density within a 100-m radius of streams; and conserving or planting onsite native trees and shrubs. Estructura de Comunidades Riparias de Aves en Portland, Oregon: Hábitat, Urbanización y Patrones de Escala Espacial Resumen. Censamos las comunidades de aves reproductivas y plantas a lo largo de 54 arroyos en el área metropolitana de Portland, Oregon en 1999 para conectar medidas de comunidades de aves con características estructurales y espaciales de zonas riparias urbanas. Análisis de correspondencia canónica produjeron dos ejes explicativos relacionados con la vegetación y la densidad de carreteras. La abundancia total de aves y la de aves no nativas fueron mayores en bosques estrechos. La abundancia de aves nativas fue mayor en bosques estrechos rodeados por terrenos rurales y la riqueza y diversidad de especies fueron mayores en áreas menos desarrolladas. La abundancia de residentes nativas y migratorias de corta distancia fue mayor en bosques estrechos y su diversidad estuvo asociada positivamente con terrenos desarrollados. La abundancia, riqueza y diversidad de las migratorias neotropicales fueron mayores en áreas de dosel abierto y con pocas carreteras. Examinamos las relaciones espaciales mediante regresiones entre variables de aves y la cobertura del dosel derivada de imágenes satelitales, el área de terrenos sin desarrollar y la densidad de calles en una serie de áreas de 50 m de ancho en un radio de 500 m alrededor de los sitios de estudio. La abundancia de aves no nativas disminuyó con aumentos en la cobertura del dosel hasta 450 m, pero la mayoría de las demás relaciones fueron más fuertes a escalas menores (50–100 m). Nuestros resultados sugieren que el incremento de la cobertura del dosel en áreas urbanas es la estrategia de manejo más valiosa para conservar las aves nativas que se reproducen en el área. Un esquema jerárquico para la conservación de las migratorias neotropicales podría incluir aumentar la cobertura de bosque a menos de 450 m de los arroyos para controlar a las especies no nativas y a los Molothrus, reducir la densidad de calles dentro de un radio de 100 m alrededor de los arroyos y conservar o plantar árboles y arbustos nativos.

Evaluating Population-Size Estimators: An Empirical Approach

We used nine closed populations of gray-tailed voles ( Microtus canicaudus ) in 0.2-ha enclosures to empirically select the best fit among 11 probabilistic estimators of population size. We also examined the influence of population size and number of trap occasions on performance of estimators. Population size was known in all instances, providing a basis for comparison of performance of estimators. Three replicates of three population sizes (30, 60, and 90 voles/enclosure) were used in this experiment. The most accurate and precise estimators, selected on the basis of four consecutive trapping occasions, were the Pollock and Ottos Mbh, Chaos Mh, and jackknife estimators. Examination of the hypothesis tests included in the Model Selection Procedure of the program CAPTURE identified individual heterogeneity as the prevailing source of variation in capture probabilities and suggested that the appropriate estimator would be the jackknife. Reliability of the heterogeneity estimators (the jackknife and Chaos estimators for Mh and Mth) was positively related to population size, whereas reliability of almost all of the other estimators varied inversely with population size. The jackknife estimator was unique in the stability and quality of its performance in the first few trap occasions. Using the jackknife estimator and three trap occasions offered the best tradeoff between reliability and trapping effort.

Density-dependent responses of gray-tailed voles to mowing

Voles (Microtus spp.) commonly inhabit forage crops and may cause excessive damage to these crops. However, cover removal by mowing or haying may cause vole populations to decline. To determine if gray-tailed voles (M. canicaudus) responded to mowing of alfalfa in a density-dependent manner, we livetrapped 4 populations in each of low- ( 90) 0.2-ha enclosures before and after mowing in 1992. Survival rates of both sexes, population size and growth rates, and proportion of total captures that were recruits declined (P 0.22). Greater than 50% of dispersing adult females were pregnant, and we suggest that mowing disrupted social organization of voles causing some animals to abandon home ranges or territories. Although mowing reduces populations of gray-tailed voles and disrupts social organization, the high reproductive potential of the species enables populations to recover rapidly to premowing levels

Small Mammal Survival and Downed Wood at Multiple Scales in Managed Forests

Abstract Conservation of small mammals requires knowledge of ecologically meaningful spatial scales (e.g., individuals or populations) at which species respond to habitat heterogeneity. Between July and October of 1998, we sampled small mammals, understory vegetation, and downed wood (DW) at multiple scales (trap sites, 1-hectare forest patches, and stands) in 2 Douglas-fir (Pseudotsuga menziesii) forests in western Oregon. Our objectives were to determine if DW or understory vegetation varied among or within forest patches or among forest stands and whether variation in survival of small mammals coincided with the scale in which these varied. Understory vegetation explained most of the variation within patches, but did not vary among patches or stands. Survival of the 2 most abundant species, the deer mouse (Peromyscus maniculatus) and creeping vole (Microtus oregoni), also varied within patches by differing among individual home ranges, and was most related to DW volume (cubic meters per 0.01 hectare) and herb and gass cover (%). Survival of deer mice was explained by a 2nd-degree polynomial function of DW within individual home ranges, and peaked at 2.0 m3 per 0.01 ha. Survival of creeping voles was dependent on a negative loge function of DW within home ranges, and was highest in home ranges lacking DW. Our results demonstrate that these species may not be generalists, as previously suspected, but rather specialists tied to specific amounts of particular habitat components within home ranges. We recommend that future studies of relationships between small mammals and habitats consider multiple spatial scales that are ecologically meaningful for the species of interest (e.g., home ranges), and examine demographic parameters including survival.

Reproductive and Behavioral Biology of the Gray-Tailed Vole

We describe the reproductive biology of the gray-tailed vole, Microtus canicaudus , and relate behavioral and morphological features to the mating system. The breeding season extended from March through December, mean litter size was 4.4 young, and hip (scent) glands were prominent in adult males and females throughout the breeding season. Juvenile dispersal was male-biased, and home ranges of opposite-sex relatives did not overlap when juveniles became reproductively active. Gray-tailed voles were sexually dimorphic, and males had larger home ranges than did females, indicative of a polygynous or promiscuous mating system. However, males had relatively small testes, which is characteristic of monogamy or single-male harems. We conclude that the mating system of gray-tailed voles cannot be inferred from behavioral and morphological correlates, as has been attempted for other species of Microtus .

INSECTICIDE EFFECTS ON SMALL MAMMALS: INFLUENCE OF VEGETATION STRUCTURE AND DIET

We tested whether differences in the diets of nontarget organisms or in vegetation structure could result in adverse ecological effects not predicted by the Quotient Method, a laboratory-based risk assessment methodology used by the Environmental Pro- tection Agency to evaluate pesticides for registration and use. We established populations of herbivorous gray-tailed voles (Microtus canicaudus) and omnivorous deer mice (Pero- myscus maniculatus) in 24 0.2-ha enclosures planted with alfalfa (Medicago sativa) and monitored the populations by live trapping. Alfalfa in twelve enclosures was mowed on 22 June to reduce vegetation height, and 3 wk later, we applied azinphos-methyl at 0, 0.88, and 3.61 kg/ha. We compared predictions of risk with observed effects on the small mam- mals. Treatment with azinphos-methyl at 3.61 kg/ha reduced population density and growth, survival, recruitment, and body growth of voles in both mowed and unmowed enclosures. Survival of female voles exposed to 3.61 kg/ha was lower in mowed than in unmowed enclosures, and the 0.88 kg/ha treatment affected body growth of male voles only in mowed enclosures. Vole densities in 3.61 kg/ha enclosures remained depressed for ?6 wk after spraying. Azinphos-methyl did not detectably affect reproductive activity of female voles. Precipitation after spraying may have increased exposure of the mammals to azinphos- methyl, resulting in greater effects on voles than reported in a previous, similar experiment. Deer mouse densities in mowed enclosures receiving 3.61 kg/ha decreased 47% within 5 d after spraying. No other adverse insecticide effects on deer mice were significant, but confidence intervals for such effects were wide. Analysis of deer mouse feces indicated that consumption of arthropods just after spraying was greater in insecticide-treated en- closures than in controls. In general, the Quotient Method adequately predicted effects on the small mammals, but its performance may be affected by vegetation structure and pre- cipitation.

Small Mammal Responses to Fine Woody Debris and Forest Fuel Reduction in Southwest Oregon

Abstract Despite its importance for wildlife, most forests in the Pacific Northwest contain low volumes of large downed wood compared to fine woody debris (FWD). We used a replicated experiment to compare short-term responses of deer mice (Peromyscus maniculatus) and western red-backed voles (Clethrionomys californicus) among 3 arrangements of FWD: piled, lopped and scattered, and pile burning, a commonly used method of fuel reduction in commercial Douglas fir (Pseudotsuga menziesii) forests in southwest Oregon, USA. We assessed habitat use, density, and survival of mice and voles during 2 consecutive summers (Jun–Aug 1999 and 2000). Both mice and voles used FWD cover disproportionately from its availability, and they differed in their responses to specific FWD arrangements. Mice used piled FWD (proportional use = 37.0%, 90% CI = 33.0–44.0) 43% more than expected (26.0). Number of mice captured (𝑥̄ = 1.9 mice, 90% CI = 1.5–2.5) and index of home range size (𝑥̄ = 4.8 m, 90% CI = 0.7–8.9) at individual FWD piles decreased up to 16% and increased up to 50%, respectively, for each 1-m increase in distance from piles. Voles used all FWD cover classes in proportion to availability, but number of voles captured increased slightly (𝑥̄ = 0.016 voles/m, 90% CI = 0.001–0.031) for each 1-m increase in distance from piles. Piled FWD had no discernable effect on population density and apparent survival of mice, but analyses had low power (0.25, 0.67). Our results suggest that piling FWD would benefit deer mice, whereas lopped and scattered FWD might benefit voles. Thus, a combination of methods to reduce fire risk should be considered to accommodate multiple small mammal species.

EFFECTS OF ADULT SEX RATIOS ON RECRUITMENT OF JUVENILE GRAY-TAILED VOLES, MICROTUS CANICAUDUS

Abstract Juvenile recruitment, growth, survival, and time to sexual maturation of rodents are all negatively affected by high densities of adults. But whether density of males, of females, or total density of adults has the greatest effect on juveniles is not known. We maintained populations of gray-tailed voles, Microtus canicaudus, in 0.2-ha enclosures with densities of 30 males, 10 females (male-biased); 20 males, 20 females (control); and 10 males, 30 females (female-biased) breeding adults. With juvenile recruitment, population densities increased to 180 voles/enclosure (700 voles/ha) in male-biased enclosures and 200 voles/enclosure (1,000 voles/ha) in control and female-biased enclosures. Recruitment, growth, and survival of either sex juvenile were not affected negatively by adult males but recruitment and time to sexual maturation of young females were negatively affected by high densities of females. Reduced juvenile recruitment was likely due to neonatal death before emergence from nest. We conclude that females have a greater negative effect on juvenile recruitment and reproductive development than do males. This difference was slight, however, and under normal densities of wild populations probably has minimal impact on vole demography.

Demographic uncertainty in ecological risk assessments

Abstract We built a Rickers model incorporating demographic stochasticity to simulate the effects of demographic uncertainty on responses of gray-tailed vole ( Microtus canicaudus ) populations to pesticide applications. We constructed models with mark-recapture data collected from populations in 1998 and 1999. We ran 30 simulations of a single pesticide application for small (∼30 voles), medium (∼50 voles), and large (∼100 voles) population sizes for 1998 data. Significantly less uncertainty in detecting pesticide effects was exhibited at large population sizes. Fifty percent of the simulations for small or medium population sizes suggested no differences between control and treatments. Due to population fluctuations resulting from demographic stochasticity and small population sizes, we detected no significant differences in the simulations using 1999 data. Population sizes may affect the recovery ability of vole populations following pesticide-induced mortality. Vole population-size declines were significant for pesticide applications at large population sizes, but greater uncertainty existed in the simulations of low and medium population sizes. Our results suggested that the Quotient Method (QM), an ecological risk assessment model, should differentiate the short-term risk of a chemical to small and large populations. The QM uses the ratio of the expected environmental concentration to the median lethal dose or concentration of chemicals for a species to assess the risk of chemicals to nontarget wildlife. Our results also suggested that the QM could not predict or may underestimate the long-term extinction risk of rare or endangered wildlife species from contamination by pesticides.

Population Characteristics and Group Composition of Capra aegagrus in Kirthar National Park, Pakistan

Population characteristics and group composition of wild goats ( Capra aegagrus ) were studied in Kirthar National Park, Pakistan, January–October 1986. Estimated density was 11.8–16.3/km2. Based upon 5,351 observations, the male: female ratio for adults (0.56:1) and yearlings (0.59:1) differed from parity. Age structure of the population was distributed evenly except for 5- and 6-year-old females. Number of young/100 adult females declined from 46.5 in February to 26.9 during September-October. Size of 283 groups ranged from two to 131, and both size and composition varied seasonally depending on the type of group. Population and group characteristics appear to depend strongly on annual and seasonal levels of precipitation.