Gregory D. Hayward

Minimum viable populations: is there a 'magic number' for conservation practitioners?

Establishing species conservation priorities and recovery goals is often enhanced by extinction risk estimates. The need to set goals, even in data-deficient situations, has prompted researchers to ask whether general guidelines could replace individual estimates of extinction risk. To inform conservation policy, recent studies have revived the concept of the minimum viable population (MVP), the population size required to provide some specified probability of persistence for a given period of time. These studies conclude that long-term persistence requires ≥5000 adult individuals, an MVP threshold that is unaffected by taxonomy, life history or environmental conditions. Here, we re-evaluate this suggestion. We find that neither data nor theory supports its general applicability, raising questions about the utility of MVPs for conservation planning.

Using Surrogate Species and Groups for Conservation Planning and Management

ABSTRACT In species management and conservation, surrogate species or groups of species can be used as proxies for broader sets of species when the number of species of concern is too great to allow each to be considered individually. However, these surrogate approaches are not applicable to all situations. In this article we discuss how the nature of the ecological system, the objectives and scale of management, and the level of available knowledge influence the decision about using a surrogate approach. We use species-area relations to define a “surrogate zone” in which the approach may be most effective. Using the Interior Columbia Basin of the northwestern United States as an example, we outline 10 steps that may enhance the effectiveness of surrogate approaches. Using a surrogate approach necessarily entails a trade-off between management tailored to individual species and less precise practices that may apply to a broader array of species. Ultimately, the use of a surrogate approach depends on the level of uncertainty that is acceptable in conducting management or conservation activities—in other words, “How good is good enough?”

Effects of the Exxon Valdez Oil Spill on Marine Bird Communities in Prince William Sound, Alaska

The supertanker Exxon Valdez ran aground on 24 March 1989, spilling 41 × 106 L of oil into Prince William Sound, Alaska. To examine effects of this oil spill on the marine bird community, we analyzed data from 11 survey cruises between June 1989 and August 1991. Cruises were conducted in 10 study bays differing in the magnitude of initial oiling. We gauged bird responses to the spill in terms of habitat use, measured by frequency of bay occupancy and species abundances as functions of initial bay oiling. We focused on community-level measures to obtain a broader perspective than can be obtained from studies directed toward individual species of concern. Effects of the oil spill on community measures were most apparent shortly after the spill but diminished rapidly. Species richness was significantly lower in 1989 than at the same season 1-2 yr later, especially in heavily oiled bays. Species diversity (log-series α) was also significantly reduced in more heavily oiled bays in early summer 1989 and 1990, but impacts evident in midsummer and fall 1989 were absent 1 yr later, and there were no significant relationships between diversity and bay oiling after midsummer 1990. Species occurrence in bays was more restricted immediately following the spill than 1-2 yr later, and widespread species were less abundant in early summer and fall 1989 than at the same seasons 1 yr later. This latter pattern was reversed in the midsummer surveys, perhaps because spill clean-up activities attracted large numbers of nonbreeding gulls. We used cluster analysis to define six avian guilds based on ecological characteristics of the species. Species richness of several guilds of birds feeding on or close to the shoreline was negatively related to initial oiling level until early or midsummer 1990, but not thereafter. Of these guilds, the richness of a guild of winter visitant and resident species showed the greatest negative association with initial oiling. However, the richness of guilds of solitary or colonial species that dive and/or feed on fish showed no significant relationships with oiling at any time. Correspondence analysis based on bird community composition indicated clear differences between heavily oiled and unoiled bays in 1989, but overall community composition converged between these sets of bays in subsequent years. Our analyses indicated that the Exxon Valdez oil spill had significant initial impacts on marine bird community structure, although they were not evenly distributed among ecological guilds. Even during the first survey, many species were present in the most heavily oiled bays. Although a few species continued to show spill impacts in late 1991, none of the community measures indicated continuing negative oiling effects. This suggests that, at the community level, recovery was well underway, consistent with observations that seabird habitat had apparently returned to normal in all but a few localized areas by mid-1991. Seabird communities appear to have considerable resiliency to such severe but relatively short-term perturbations, possibly because birds move over a regional scale. It may, therefore, be important to consider regional processes in evaluating recovery following environmental accidents.

Lichens as Nesting Material for Northern Flying Squirrels in the Northern Rocky Mountains

We examined the composition of nest material used by northern flying squirrels ( Glaucomys sabrinus ) occupying artificial nest boxes in central Idaho and western Montana. Nests were constructed almost entirely of arboreal lichens (96% lichen by volume). In both regions, three species of Bryoria dominated nesting material although a total of 15 species of lichens were identified in 159 nest samples from central Idaho. Species of lichens used for nests differed across four types of forest vegetation. Nests in stands of lodgepole pine ( Pinus contortd ) had more Bryoria fremontii and less B. pseudofuscescens than nests in other forest types. The rank-order abundance of lichens used in nests was related to the rank-order abundance of lichens collected from forests at nest sites, although the relationship was not strong. We suggest that lichen nests may function to reduce thermal energy expenditures during winter or act as food caches. The dominant lichen used in nests lacked acids and other secondary compounds found in other arboreal lichens and, thus, may be more palatable to squirrels.

Effects of an Introduced Piscivore on Native Trout: Insights from a Demographic Model

Introductions of exotic species pose a significant threat to the persistence of many native populations, including many inland fishes. In 1994, piscivorous lake trout (Salvelinus namaycush) were discovered in Yellowstone Lake, Yellowstone National Park, Wyoming, USA, one of the last strongholds of the native Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri). Predation by lake trout is expected to lead to a substantial decline in the native cutthroat trout population, which may have significant negative consequences for terrestrial predators that depend on cutthroat trout for prey and for the recreational fishery of the Park. We developed a matrix demographic model for the cutthroat trout population in Yellowstone Lake to identify the life stages that are most critical for understanding population dynamics. Parameter estimates (vital rates) were manipulated to explore the possible consequences of lake trout invasion. Comparisons of our results with current estimates of population trend and age structure suggested that our model reflected current conditions of the system. Elasticity analysis of the model revealed that population growth was most sensitive to annual survival of young trout, the group that is expected to be most vulnerable to lake trout predation. Projection of our deterministic model suggested that, in addition to a decline in abundance of cutthroat trout, the effects of lake trout may be manifest as changes in age and breeding structure of the population. Simulations of a stochastic version of the model indicated that a 60% or greater decline in the cutthroat trout population could be expected within 100 years if the lake trout population were permitted to grow uncontrolled. However, an effective control strategy that prevented the establishment of a large population of lake trout substantially reduced population decline, although the reduction in the availability of adult trout to terrestrial predators and anglers may be still be substantial (20–40%). In addition to current control activities in place in the Park, we recommend a renewed emphasis on understanding and monitoring juvenile life stages of cutthroat trout. Our results demonstrate the value of existing data sets for developing models to estimate the potential impact of biological invasions on the management and conservation of native populations, especially when opportunities and resources for additional empirical studies are limited.

HIGH CONNECTIVITY AND MINIMAL GENETIC STRUCTURE AMONG NORTH AMERICAN BOREAL OWL (AEGOLIUS FUNEREUS) POPULATIONS, REGARDLESS OF HABITAT MATRIX

Abstract Habitat connectivity and corridors are often assumed to be critical for the persistence of patchily distributed populations, but empirical evidence for this assumption is scarce. We assessed the importance of connectivity among habitat patches for dispersal by a mature-forest obligate, the Boreal Owl (Aegolius funereus). Boreal Owls demonstrated a lack of genetic structure (θ = 0.004 ± 0.002 [SE]) among subpopulations, regardless of matrix type and extent, which indicates that unforested matrix does not act as a barrier to dispersal for this vagile species. We found only slightly higher genetic distances (Cavalli-Sforza chord distances ranged from 0.015 to 0.025) among patchily distributed Rocky Mountain subpopulations as compared with largely contiguous boreal-forest subpopulations (0.013 to 0.019) and no evidence of a genetic split across the expansive high plains of Wyoming. Even the most isolated subalpine patches are connected via gene flow. As northern boreal forests continue to experience intensive harvest of mature stands, geographic dispersion of Boreal Owl habitat may begin to more closely resemble that found in the Rocky Mountains. We suggest that decreased connectivity poses much less of a threat to continued abundance of this mature-forest obligate than overall loss of nesting and foraging habitat. Assessment of the importance of corridors and connectivity should be conducted on a species-by-species basis, given the variation in response of species to discontinuity of habitat, even among closely related taxa or guilds. Alta Conectividad y Estructura Genética Mínima entre Poblaciones Norteamericanas de Aegolius funereus, Independientemente de la Matriz del Hábitat

Estimates of Predator Consumption of Yellowstone Cutthroat Trout (Oncorhynchus clarki bouvieri) in Yellowstone Lake

ABSTRACT We estimated consumption of Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri) by resident terrestrial predators and human anglers to place into context the potential predatory impact of exotic lake trout (Salvelinus namaycush) in Yellowstone Lake, Yellowstone National Park, Wyoming, USA. Estimates of consumption rates of piscivorous birds and mammals from the literature were combined with recent estimates of abundance of these predators in the park. Angling mortality was estimated by creel surveys from park management records. Results from a demographic model were used to estimate population size and age distribution of the cutthroat trout population. We estimated that terrestrial predators consumed approximately 7% of the trout population in the lake; angler mortality represented only 1–4% of the adult population. Piscivorous birds accounted for most (99%) of predator mortality, which was focused largely on juvenile trout. American white pelicans (Pelecanus erythrorhynchos) and grizzly bears (Ursus arctos) ate an estimated 5% of the adult trout spawning in streams. Depending on the ultimate size of the lake trout population, predation by introduced lake trout is expected to increase juvenile mortality by 13–67%. Consumption of young adult cutthroat trout by a large population of lake trout would effectively increase angling mortality by 50%. The resulting decline in the native cutthroat trout population is projected to have a significant effect on piscivorous birds, mammalian carnivores, and anglers in Yellowstone National Park.

Historical Environmental Variation in Conservation and Natural Resource Management: Wiens/Historical Environmental Variation in Conservation and Natural Resource Management

In North America, concepts of Historical Range of Variability are being employed in land-management planning for properties of private organizations and multiple government agencies. The National Park Service, U.S. Fish & Wildlife Service, Bureau of Land Management, U.S. Forest Service, and The Nature Conservancy all include elements of historical ecology in their planning processes. Similar approaches are part of land management and conservation in Europe and Australia. Each of these user groups must struggle with the added complication of rapid climate change, rapid land-use change, and technical issues in order to employ historical ecology effectively.

RED-BACKED VOLE (CLETHRIONOMYS GAPPERI) RESPONSE TO DISTURBANCE IN SUBALPINE FORESTS: USE OF REGENERATING PATCHES

Abstract We investigated scale-dependent habitat selection by the southern red-backed vole (Clethrionomys gapperi) in subalpine forests and 20- to 30-year-old regenerating timber harvests of the central Rocky Mountains. At the macrohabitat level, C. gapperi preferred forest stands, which had more overstory canopy cover, more uniformly distributed coarse woody debris (CWD), and more dwarf huckleberry (Vaccinium scoparium) than regenerating harvested stands. Further, C. gapperi was captured in forest stands up to harvest boundaries but crossed them significantly less than expected by chance. Analysis of microhabitat selection indicated that, in forest stands, C. gapperi showed selection at a fine spatial scale, strongly favoring CWD microhabitats and avoiding those dominated by V. scoparium. Our results suggest that older, regenerating clear-cuts in the Rocky Mountains can have clearly defined impacts on C. gapperi similar to those immediately after timber harvests, but that regenerating clear-cuts do support some use by C. gapperi. The results support the association of C. gapperi with CWD and forest overstory in western montane habitats and indicate a noticeable response to forest boundaries but no direct edge effect.

Monitoring boreal owl populations with nest boxes : sample size and cost

Evaluating the economic and sampling efficiency of potential monitoring programs is a first step in validation. Thus, we established a system of nest boxes in the Payette National Forest to evaluate the feasibility of using a system of nest boxes to monitor response of boreal owl (Aegolius funereus) populations to habitat change. We recorded nest site occupancy and productivity as measures of foraging habitat trend. Using monitoring results from 3 years, we evaluated alternative survey sampling techniques for occupancy and determined sample sizes necessary to estimate occupancy and clutch size within specified relative bounds. We also examined the cost of establishing a nest box system and monitoring nest box use