Beatrice Van Horne

Ecological mechanisms and landscape ecology

Landscape ecology deals with the effects of the spatial configuration of mosaics on a wide variety of ecological phenomena. Because problems in many areas of conservation biology and resource management are related to landscape use, development of a rigorous theoretical and empirical foundation for landscape ecology is essential. We present an approach to research that focuses on how individual-level mechanisms operating in a heterogeneous mosaic produce ecological patterns that are spatially dependent. The theoretical framework that we develop considers the density and distribution of a population among patches as a function of (a) within-patch movement patterns of individuals; (b) emigration from patches as a function of population density, patch configuration, patch context, and within-patch movement; and (c) loss of individuals as they disperse through landscape elements

Habitat occupancy patterns of North American shrubsteppe birds: the effects of spatial scale

Understanding the processes that underlie how birds select and use habitats depends on an accurate representation of the patterns of habitat occupancy. These patterns are sensitive to the spatial scale on which they are viewed. Densities of several shrubsteppe bird species in North America varied in relation to features of habitat structure at a biogeographic scale, but these associations disappeared at a regional scale within the shrubsteppe. In another regional comparison involving a different array of shrubsteppe plots and sites, densities of both sage trashers (Oreoscoptes montanus) and sage sparrows (Amphispiza belli) varied with habitat features in quite different ways than in the other regional analysis. A consideration of the pattens of distribution of the bird species in a multivariate habitat space created by Principal Components Analysis of the regional habitat data revealed several clear patterns, but these relationships generally failed to hold when the spatial scale was further reduced, to consider differences between plots at the same location. At this scale other bird-habitat relationships were apparent, but these patterns differed for populations of the same species at different sites. Consideration of habitat differences between areas within occupied territories versus unoccupied areas within plots revealed still other patterns of habitat occupancy. Some of these were consistent with those obtained in analyses at other scales, but many were not. How one characterizes the habitat occupancy of a species is thus dependent on the spatial scale used. We explore several factors that complicate bird-habitat analyses at different spatial scales, and conclude that these problems of scale are most likely to be circumvented by conducting studies at several hierarchically nested scales arranged on clearly defined environmental gradients over a reasonably long period of time.

A LESSON IN THE LIMITATIONS OF FIELD EXPERIMENTS: SHRUBSTEPPE BIRDS AND HABITAT ALTERATION'

We manipulated the habitat composition and structure within territories of birds breed- ing in the Artemisia-dominated shrubsteppe of central Oregon in order to assess how closely individuals track habitat in features such as territory placement or size or behavioral budgeting and space use. We removed 75, 50, 25, and 0% of the shrub individuals from 625-M2 blocks in a checkerboard design. Over the following 7 yr we monitored territorial locations and sizes and breeding densities of Homed Larks (Eremophila alpestris), Sage Sparrows (Amphispiza belli), and Brewers Sparrows (Spi- zella brewer) on the manipulated area and an adjacent unaltered control area. Activity and substrate- use patterns and intensity of use of the different treatment blocks within the manipulated area were also determined for the two sparrow species. Sage Sparrows, which from our previous studies appear to be more closely linked to shrub (sagebrush) coverage than the other species, did not respond to the manipulation at a local population level. The manipulation apparently did affect territory placement and densities of Brewers Sparrows and Homed Larks in 1980-1982, but these trends were not consistent over the entire postmanipulation period. Densities of all species varied among years. For Brewers Sparrows and Homed Larks these variations did not parallel those in nearby census locations, but Sage Sparrow densities varied in the same ways over all the locations. These variations appeared to parallel variations in bioyear (October-April) precipitation, with a 1-yr time lag. Individuals of both sparrow species altered the details of their activity budgeting, but not their substrate-use patterns, in response to the manipulation. Both species clearly expressed an affinity for unmanipulated blocks within the manipulation area. This experimental manipulation is complicated by influences of time lags on individual and pop- ulation responses that may stem in part from site tenacity by breeding adults, leading them to return to previous breeding locations in years following the manipulation despite the habitat changes. Further, the spatial scale on which the manipulation was conducted may have been inappropriate to gauge responses at the population level. We suggest that these complications may plague many field exper- iments in ecology.

Niches of Adult and Juvenile Deer Mice (Peromyscus Maniculatus) In Seral Stages of Coniferous Forest

I analyzed food habits and habitat utilization patterns of deer mice (Peromyscus maniculatus) in seral stages of coastal coniferous forest in southeast Alaska. Adult deer mice consumed a higher proportion of hard—bodied arthropods than did juveniles. Juveniles consumed a lower proportion of soft—bodied arthropods in high—density populations than in low—density populations. High—and low—density habitats were described for adults and juveniles using discriminant function analysis. Adult high—density habitat had more cover than juvenile high—density habitat and was more favorable, as evidenced by a greater overwinter survival of both juveniles and adults. Intraspecific competition appeared to responsible for niche displacement of juvenile deer mice along food and habitat dimensions. See full-text article at JSTOR

GENETIC STRUCTURE OF A METAPOPULATION OF BLACK-TAILED PRAIRIE DOGS

Abstract Habitat alteration, agricultural control, recreational shooting, and most recently, sylvatic plague (caused by Yersinia pestis) contributed to local extinctions and a steady decline of black-tailed prairie dog (Cynomys ludovicianus) throughout its range. As a consequence, prairie dogs currently live in metapopulations, where their overall persistence will depend on a balance between extinction of colonies and recolonization from extant colonies. Patterns of genetic similarity among colonies, as measured by neutral molecular markers, provide an estimate of the dispersal and gene flow among colonies within prairie dog metapopulations. We sampled 13 colonies of black-tailed prairie dogs in short-grass prairie of northern Colorado, 100-km east of Fort Collins, Colorado. We used historical records and genetic analysis to show that colonies undergo regular extinctions, which subsequently are recolonized by individuals from multiple source colonies. We examined 155 individuals for variation at 7 microsatellite loci and found moderate levels of genetic differentiation among colonies (Θ [=FST] = 0.118). We also used assignment and exclusion tests based on multilocus genotypes of individuals to determine the probability that individuals originated from the same colony in which they were captured. About 39% of individuals could not be assigned to colonies where they were captured, indicating they were either immigrants (adults) or the offspring of immigrants (adults and juveniles). We tested for genetic isolation by distance among colonies by comparing genetic distances to geographic distances between colonies. Akaikes Information Criterion for model selection revealed that dispersal most likely occurred along low-lying dry creek drainages connecting isolated colonies. Genetic distances between colonies were also related to ages of colonies; older colonies were more similar genetically than younger colonies. This underscores the importance of dispersal among prairie dog colonies and has important implications for persistence of prairie dog metapopulations, in which all colonies, regardless of size, are vulnerable to extinction from plague.

Influence of habitat on behavior of townsend's ground squirrels (Spermophilus townsendii)

Trade-offs between foraging and predator avoidance may affect an animals survival and reproduction. These trade-offs may be influenced by differences in vegetative cover, especially if foraging profitability and predation risk differ among habitats. We examined above-ground activity of Townsends ground squirrels ( Spermophilus townsendii ) in four habitats in the Snake River Birds of Prey National Conservation Area in southwestern Idaho to determine if behavior of ground squirrels varied among habitats, and we assessed factors that might affect perceived predation risk (i.e. predator detectability, predation pressure, population density). The proportion of time spent in vigilance by ground squirrels in winterfat ( Krascheninnikovia lanata ) and mosaic habitats of winterfat-sagebrush ( Artemisia tridentata ) was more than twice that of ground squirrels in burned and unburned sagebrush habitats. We found no evidence for the “many-eyes” hypothesis as an explanation for differences in vigilance among habitats. Instead, environmental heterogeneity, especially vegetation structure, likely influenced activity budgets of ground squirrels. Differences in vigilance may have been caused by differences in predator detectability and refuge availability, because ground squirrels in the winterfat and mosaic habitats also spent more time in upright vigilant postures than ground squirrels in burned-sagebrush or sagebrush habitats. Such postures may enhance predator detection in low-growing winterfat.

Arthropod dynamics on sagebrush (Artemisia tridentata): effects of plant chemistry and avian predation

We conducted a field experiment to assess interrelationships between leaf- tissue secondary chemistry, avian predation, and the abundance and diversity of arthropods occurring on sagebrush (Artemisia tridentata) in central Oregon. Arthropods were removed from individual shrubs, some of which were then caged to exclude birds. Secondary chem- istry and arthropods were sampled at intervals up to 56 wk following the defaunation/ caging treatment. Recovery rates differed among arthropod taxa and functional groups. Several sap- sucking homopterans and hemipterans reached control levels within 2-4 wk of the treat- ment, whereas abundances ofparasitoids and predators recovered to match control numbers only 6 wk after defaunation. Abundances of several herbivorous leaf-chewing taxa (pri- marily lepidopteran larvae) remained significantly depressed even 56 wk after the treatment. Fungivores (oribatid mites) reached greater abundances on defaunated than on control shrubs by the end of the experiment. There were also significant changes in the concen- trations or frequency of occurrence of several chemical compounds following the defauna- tion treatment. Several hydrocarbons, sesquiterpenes, and monoterpenes that were present in most shrubs exhibited sharply reduced concentrations in leaf tissues within 4 wk of the treatment, whereas some alcohols and ketones (linalool, borneol, thujone), which occurred at relatively low frequencies among control plants, increased dramatically in their fre- quencies of occurrence following arthropod removal. Both changes persisted for the du- ration of the experiment. We found several significant associations between the abundance or occurrence of arthropod taxa or groups and concentrations or frequencies of secondary compounds, but these were most prevalent among the leaf-chewing lepidopterans. We suggest that the shrubs responded to the removal of herbivorous, leaf-chewing arthropods by altering chemical allocation patterns; the changes in chemistry persisted for over a year because recolonization of the defaunated plants by these herbivores was slow. Effects of the caging treatment were much less obvious. The recovery of the diversity of arthropods known to constitute prey for birds in this system was slower on shrubs

Effects of drought and prolonged winter on Townsend's ground squirrel demography in shrubsteppe habitats

During a mark–recapture study of Townsend’s ground squirrels (Spermophilus townsendii) on 20 sites in the Snake River Birds of Prey National Conservation Area, Idaho, in 1991 through 1994, 4407 animals were marked in 17639 capture events. This study of differences in population dynamics of Townsend’s ground squirrels among habitats spanned a drought near the extreme of the 130-yr record, followed by prolonged winter conditions. Townsend’s ground squirrels have a short active season (≈4 mo) in which to reproduce and store fat for overwintering. Their food consists largely of succulent grasses and forbs in this dry shrubsteppe and grassland habitat. The drought in the latter half of the 1992 active season produced early drying of Sandberg’s bluegrass (Poa secunda) and was associated with low adult and juvenile body masses prior to immergence into estivation/hibernation. The following prolonged winter was associated with late emergence of females in 1993. Early-season body masses of adults were low in 1993 r...

EFFECTS OF FOOD SUPPLEMENTATION ON THE TIMING OF NEST INITIATION IN BELTED KINGFISHERS

Numerous food supplementation experiments have tested the hypothesis that timing of reproduction in birds is limited by energy constraints. These studies have generally concluded that food availability is the primary proximate factor affecting the timing of nesting in birds. None of these studies, however, has examined the relationship between food availability and the timing of nesting in piscivorous birds. To determine whether food was the primary proximate factor affecting timing of nest initiation in the piscivorous Belted Kingfisher (Ceryle alcyon), we conducted a food supplementation experiment from 1993 through 1995; additional descriptive data were collected in 1992. Belted Kingfishers that nested early had heavier nestlings and were more likely to renest, in the event of nest failure, than were late-nesting birds. This indicates a reproductive advantage in early nesting. In 1994 and 1995, when we began the feeding treatment early in the prebreeding season (8 March) and provided enough fish to meet the estimated energy requirements of a pair of kingfishers (90 g/d), nests of supplemented birds were initiated earlier than those of unsupplemented birds as predicted by the energy constraint model. In 1993, when we began the feeding treatment later (20 April) and provided less fish (20-30 g/d), there was no difference in the initiation dates of supplemented and unsupplemented nests. Although these data support the primary prediction of the energy constraint model, two patterns in our data were not explained by this model. First, few unsupplemented nests were initiated early in the breeding season relative to an unmanipulated reference population. Second, no sup- plemented nests were initiated outside the range of nesting dates recorded in our unma- nipulated reference population. Based on these patterns, we argue that variation in arrival time of females on the breeding grounds, coupled with their selection of territories based on food availability, offers a feasible and testable alternative to the energy constraint model. This habitat selection model predicts that food availability influences the location of re- production of individuals, but does not alter the timing of their reproduction. A literature review revealed few data that could distinguish the energy constraint and habitat selection models. We think that it is important to consider the habitat selection alternative before concluding that the results of food supplementation studies support the energy constraint model. We suggest that future studies use spatial and temporal controls to partition variation in nest initiation dates into multiple causes (e.g., energy constraints, habitat selection).

Relationships between the thermal environment and activity of Piute ground squirrels (Spermophilus mollis)

(1) We examined relationships between ground squirrel activity and operative temperature (Te) in two habitats in southwestern Idaho, USA to determine how activity patterns are tied to environmental conditions and how habitat can mediate the thermal environment. (2) Midday environmental conditions were too hot throughout much of the active season for Piute ground squirrels to remain active in a grassland habitat. (3) Ground squirrels were most active in grassland at a Te of approximately 25°C and showed a strong bimodal activity pattern during the latter half of their active season (after 15 April). (4) Suitable aboveground microclimates existed throughout most of the season in sagebrush (Artemisia tridentata) habitats and ground squirrels remained active throughout the day. (5) The availability of cooler microhabitats may make sagebrush a better habitat than grassland for Piute ground squirrels, especially during the latter portion of the active season and in hotter than normal years.