John E. Gross

Chronic wasting disease in mule deer: disease dynamics and control.

We developed a mechanistic model to simulate dynamics of chronic wasting disease (CWD) in mule deer (Odocoileus hemionus) populations. The model projected age-specific disease dynamics, changes in population size. and effects of control strategies. We estimated parameters from observations of infected and uninfected mule deer in Colorado. We used Monte Carlo techniques to evaluate likely responses. Simulations of CWD epidemics were highly unstable. Disease was not sustained in projected populations when transmission rates were low, but CWD eliminated populations when more realistic transmission rates were used. We failed to produce stable coexistence of CWD in simulated mule deer populations. Even low CWD prevalence reduced potential harvest via combined effects of diminished per-capita production and decreased population density. Changes in CWD prevalence with-in populations were highly sensitive to transmission rate, and small decreases resulted in noticeable damping of prevalence increases. Simulated selective culling programs revealed the importance of initiating control while CWD prevalence was low (<0.01). Low selective culling rates (<20% of infected populations) effectively eliminated CWD if initiated when prevalence was low, but the likelihood of control diminished rapidly as prevalence increased. Management programs will likely require an effort sustained over many decades if eliminating CWD is the desired goal.

Spatial and temporal variability modify density dependence in populations of large herbivores

A central challenge in ecology is to understand the interplay of internal and external controls on the growth of populations. We examined the effects of temporal variation in weather and spatial variation in vegetation on the strength of density dependence in populations of large herbivores. We fit three subsets of the model ln(Nt) = a + (1 + b) x ln(N(t-1)) + c x ln(N(t-2)) to five time series of estimates (Nt) of abundance of ungulates in the Rocky Mountains, USA. The strength of density dependence was estimated by the magnitude of the coefficient b. We regressed the estimates of b on indices of temporal heterogeneity in weather and spatial heterogeneity in resources. The 95% posterior intervals of the slopes of these regressions showed that temporal heterogeneity strengthened density-dependent feedbacks to population growth, whereas spatial heterogeneity weakened them. This finding offers the first empirical evidence that density dependence responds in different ways to spatial heterogeneity and temporal heterogeneity.

Applying Patch Use to Assess Aspects of Foraging Behavior in Nubian Ibex

Consideration of optimal patch use by animals that may face danger from predators and that allocate time among several activities is an extension of the marginal value theorem (Brown 1988). According to the theorem, an animal should leave a food patch when its harvest rate of food equals the sum of its energetic and predation costs arising from foraging and its missed opportunity costs arising from foregoing other activities. A technique derived from this theory involves presenting foragers with depletable food patches and measuring food density remaining after foraging. The food density remaining is a measurement of an animals foraging efficiency and can be used to measure various foraging inputs

The dynamics and scaling of foraging velocity and encounter rate in mammalian herbivores

1. We developed a simple model of foraging locomotion for mammalian herbivores that contains two parameters, maximum foraging velocity (V max , m s -1 ) and acceleration (a o , m s -2 ) and a single dependent variable, the distance between plants (d, m). 2. We examined predictions of our model by varying the distance between plants offered to nine mammalian herbivores ranging in mass from 0.05 to 533 kg. 3. Application of simple laws of motion were adequate to explain the dynamics of foraging locomotion. As the animals travelled between food items, they accelerated to a maximum foraging velocity, which they maintained until they approached the next food item. Acceleration was achieved by increasing both the length and the frequency of strides. Mean foraging velocity increased up to 10-fold as plant spacing increased, but as plant spacing increased, mean velocity was asymptotic with distance between plants. 4. V max scaled in proportion to M M0.04, and a o scaled as M -0.17 . 5. Our results suggest that small herbivores achieve similar maximum foraging velocities as do large herbivores, but can accelerate faster and therefore encounter plants more rapidly when plants are spaced closely together (<2 m apart). We conclude that foraging models that do not include the effects of acceleration will overestimate encounter rate and affect the model solution.

Exposure of U.S. National Parks to land use and climate change 1900–2100

Many protected areas may not be adequately safeguarding biodiversity from human activities on surrounding lands and global change. The magnitude of such change agents and the sensitivity of ecosystems to these agents vary among protected areas. Thus, there is a need to assess vulnerability across networks of protected areas to determine those most at risk and to lay the basis for developing effective adaptation strategies. We conducted an assessment of exposure of U.S. National Parks to climate and land use change and consequences for vegetation communities. We first defined park protected-area centered ecosystems (PACEs) based on ecological principles. We then drew on existing land use, invasive species, climate, and biome data sets and models to quantify exposure of PACEs from 1900 through 2100. Most PACEs experienced substantial change over the 20th century (> 740% average increase in housing density since 1940, 13% of vascular plants are presently nonnative, temperature increase of 1 degree C/100 yr since 1895 in 80% of PACEs), and projections suggest that many of these trends will continue at similar or increasingly greater rates (255% increase in housing density by 2100, temperature increase of 2.5 degrees-4.5 degrees C/100 yr, 30% of PACE areas may lose their current biomes by 2030). In the coming century, housing densities are projected to increase in PACEs at about 82% of the rate of since 1940. The rate of climate warming in the coming century is projected to be 2.5-5.8 times higher than that measured in the past century. Underlying these averages, exposure of individual park PACEs to change agents differ in important ways. For example, parks such as Great Smoky Mountains exhibit high land use and low climate exposure, others such as Great Sand Dunes exhibit low land use and high climate exposure, and a few such as Point Reyes exhibit high exposure on both axes. The cumulative and synergistic effects of such changes in land use, invasives, and climate are expected to dramatically impact ecosystem function and biodiversity in national parks. These results are foundational to developing effective adaptation strategies and suggest policies to better safeguard parks under broad-scale environmental change.

Delineating the Ecosystems Containing Protected Areas for Monitoring and Management

Park managers realized more than 130 years ago that protected areas are often subsets of larger ecosystems and are vulnerable to change in the unprotected portions of the ecosystem. We illustrate the need to delineate protected area—centered ecosystems (PACEs) by using comprehensive scientific methods to map and analyze land-use change within PACEs around 13 US national park units. The resulting PACEs were on average 6.7 times larger than the parks in upper watersheds and 44.6 times larger than those in middle watersheds. The sizes of these PACEs clearly emphasized the long-term reliance of park biodiversity on surrounding landscapes. PACEs in the eastern United States were dominated by private lands with high rates of land development, suggesting that they offer the greatest challenge for management. Delineating PACEs more broadly will facilitate monitoring, condition assessment, and conservation of the large number of protected areas worldwide that are being degraded by human activities in the areas that surround them.

Density dependence in northern ungulates: interactions with predation and resources

Variation in the abundance of animals has traditionally been explained as the outcome of endogenous forcing from density dependence and exogenous forcing arising from variation in weather and predation. Emerging evidence suggests that the effects of density dependence interact with external influences on population dynamics. In particular, spatial heterogeneity in resources and the presence of capable predators may weaken feedbacks from density dependence to growth of populations. We used the Kalman filter to analyze 23 time series of estimates of abundance of northern ungulate populations arrayed along a latitudinal gradient (latitude range of 40°–70°N) to evaluate the influence of spatial heterogeneity in resources and predation on density dependence. We also used contingency tables to test whether density dependence was independent of the presence of carnivores (our estimate of predation) and multiple regressions to determine the effects of spatial heterogeneity in resources, predation, and latitude on the strength of density dependence. Our results showed that the strength of density dependence of ungulate populations was low in the presence of large carnivores, particularly at northern latitudes with low primary productivity. We found that heterogeneity in elevation, which we assume acted as a surrogate for spatial heterogeneity in plant phenology, also reduced effects of density dependence. Thus, we show that external forces created by heterogeneity in resources and predation interact with internal feedbacks from population density to shape dynamics of populations of northern ungulates.

Evolving plans for the USA National Phenology Network

Phenology is the study of periodic plant and animal life cycle events, how these are influenced by seasonal and interannual variations in climate, and how they modulate the abundance, diversity, and interactions of organisms. The USA National Phenology Network (USA-NPN) is currently being organized to engage federal agencies, environmental networks and field stations, educational institutions, and citizen scientists. The first USA-NPN planning workshop was held August 2005, in Tucson, Ariz. (Betancourt et al. [2005]; http://www.uwm.edu/Dept/Geography/npn/; by 1 June 2007, also see http://www.usanpn.org). With sponsorship from the U.S. National Science Foundation, the U.S. Geological Survey (USGS), the U.S. Fish and Wildlife Service, and NASA, the second USA-NPN planning workshop was held at the University of Wisconsin-Milwaukee on 10–12 October 2006 to (1) develop lists of target species and observation protocols; (2) identify existing networks that could comprise the backbone of nationwide observations by 2008; (3) develop opportunities for education, citizen science, and outreach beginning in spring 2007; (4) design strategies for implementing the remote sensing component of USA-NPN; and (5) draft a data management and cyberinfrastructure plan.

A new framework for selecting environmental surrogates

Surrogate concepts are used in all sub-disciplines of environmental science. However, controversy remains regarding the extent to which surrogates are useful for resolving environmental problems. Here, we argue that conflicts about the utility of surrogates (and the related concepts of indicators and proxies) often reflect context-specific differences in trade-offs between measurement accuracy and practical constraints. By examining different approaches for selecting and applying surrogates, we identify five trade-offs that correspond to key points of contention in the application of surrogates. We then present an 8-step Adaptive Surrogacy Framework that incorporates cross-disciplinary perspectives from a wide spectrum of the environmental sciences, aiming to unify surrogate concepts across disciplines and applications. Our synthesis of the science of surrogates is intended as a first step towards fully leveraging knowledge accumulated across disciplines, thus consolidating lessons learned so that they may be accessible to all those operating in different fields, yet facing similar hurdles.

Changing Patterns of Land Use and Tenure in the Dalrymple Shire, Australia

Australia is the world’s flattest continent, a testament to its ancient, wellweathered geological landforms, and consequently the associated soils are generally of low fertility (Flannery 1994). It is also the world’s driest inhabited continent, a situation that is exacerbated by erratic rainfall and high evaporation. In comparison with other countries, agriculture in Australia is characterized by: dependence on low productivity environments that are prone to drought and degradation; the large scale of agricultural activities; concentration on a limited range of products; heavy dependence on overseas markets; and a relatively high standard of living in the agricultural community (Laut 1988). Almost three-quarters of the country is classed as rangelands, mainly arid and semi-arid lands that are not suitable for intensive agriculture. European settlement and agricultural development of the continent, and rangelands in particular, has been marked by bitter experiences of coming to terms with the climatic and edaphic constraints of this environment. Fragmentation of Australian rangelands has been a relatively recent phenomenon. Although Aboriginal land-use practices shaped Australian