What limits the geographic distribution of organisms? -- Populations cannot increase without limit -- Favorable and unfavorable habitats exist for every species -- Overexploited populations will collapse -- Plant and animal communities can recover from disturbances -- Communities can exist in several configurations -- Keystone species may be essential to the functioning of biological communities -- Natural systems are products of evolution -- Natural systems recycle essential materials -- Solar energy powers natural ecosystems -- Climates change, communities and ecosystems change -- Extinction is forever and species losses caused by humans are avoidable
How did rodent outbreaks in Germany help to end World War I? What caused the destructive outbreak of rodents in Oregon and California in the late 1950s, the large population outbreak of lemmings in Scandinavia in 2010, and the great abundance of field mice in Scotland in the spring of 2011? Population fluctuations, or outbreaks, of rodents constitute one of the classic problems of animal ecology, and in Population Fluctuations in Rodents, Charles J. Krebs sifts through the last eighty years of research to draw out exactly what we know about rodent outbreaks and what should be the a.
Ecologists that study the population dynamics of large and small herbivorous mammals operate in two worlds that overlap only partly, and in this paper I address whether the conjecture that these worlds represent two distinct and valid paradigms is currently justified. I argue that large mammals fall into three groups depending on whether they have effective predators or not, and whether they are harvested by humans. Because of human persecution of large predators, more and more large herbivorous mammals are effectively predator-free and are controlled bottom-up by food. But in less disturbed systems, large herbivorous mammals should be controlled top-down by effective predators, and this can lead to a trophic cascade. Small herbivorous mammals have been suggested to be controlled top-down by predators but some experimental evidence has challenged this idea and replaced it with the notion that predation is one of several factors that may affect rates of population increase. Intrinsic control (territoriality, infanticide, social inhibition of breeding) appears to be common in small herbivorous mammals with altricial young but is absent in species with precocial young, in ecosystems with strong stochastic weather variation (deserts) and in areas of human-induced habitat fragmentation in agricultural monocultures. The extrinsic control of large herbivores with precocial young validates part of Graeme Caughley's Grand Vision, but the relative role of intrinsic and extrinsic mechanisms for small herbivores with altricial young is still controversial. An improved knowledge of population control mechanisms for large and small herbivores is essential for natural resource management.
The study of population dynamics addresses three questions that are not always separated in discussions with empirical data. Two questions address population regulation. What stabilises population density is the first question, and, in spite of much theory, little progress has been made in answering this question empirically. The assumption of an equilibrium density is impossible to test and direct experimental tests to answer this question are rare. What prevents population growth is a second question, and is the classic question of population regulation. To answer this question requires an increasing population, and, with adequate experimental manipulations, the density dependent factors preventing increase can be identified. Surprisingly, answering this question has provided little assistance in solving practical problems in population dynamics, possibly because most populations are rarely in the state of growth and show a limited range of densities. What limits population density in good and poor habitats is a third question, which addresses population limitation rather than regulation, and has been the most useful question for empirical ecologists to ask. Population limitation admits of little theory and no elegant models, and highlights the gap between theory and practice in much of ecology. Defining the question clearly and adopting an experimental approach with clear alternative hypotheses will be essential to avoiding the controversies of the past while building useful generalisations for the practical problems of population management.
Analyzes fluctuations in the size of both government- and Eskimo-owned herds of Rangifer tarandus in the Reindeer Grazing Preserve of northern Mackenzie District. Each of the six Eskimo-owned herds increased in numbers for a few years, then declined and ultimately, except one, was returned to the nucleus, government herd. Annual variation in herd size is examined in relation to the birth, death, and dispersal rates. The last which represents losses by straying, approx. 17,000 since 1938, usually yearlings, is the most important, birth rate the least. Effects of herd size and composition (tabulated by age and sex) on the birth and dispersal rates are deemed negligible. Dispersal is apparently caused by factors extrinsic to the herd itself, e.g. insects, weather, man, and their interactions.
Context Variable demographic rates can manifest themselves between habitat types in the form of source–sink dynamics where populations in sink habitats would not exist without the addition of migrants from source habitats.
Aims Arctic ground squirrels (Urocitellus parryii pleisus (Osgood, 1900)) occupy a large geographic area in northern Canada and live in a variety of habitat types, including boreal forest, low-elevation meadows and alpine meadows, providing an opportunity to investigate the possible existence of source–sink dynamics.
Methods We hypothesised that arctic ground squirrels in the south-western Yukon exhibit demographic characteristics indicative of source–sink dynamics. Boreal forest habitat could be a sink in spite of previous high squirrel densities, whereas meadows could be a source. We investigated this by mark–recapture live-trapping and radio-telemetry.
Key Results In the boreal forest in the Kluane region, we found reduced recruitment, reduced population growth rates (λ), and reduced survivorship for radio-collared individuals that moved from low-elevation meadows into the boreal forest. There was no evidence from radio-collared juveniles of dispersal from high-density ground squirrel populations in alpine meadows down into boreal forest.
Conclusions Boreal forest is a sink habitat for arctic ground squirrels. Source–sink dynamics observed between low-elevation meadow and boreal forest habitats appear to result from increased predation pressure in the boreal forest. The result has been a near extirpation of boreal forest arctic ground squirrels in the Kluane region since 1998.
Implications Because the source areas of low-elevation meadows occupy only 7–9% of the lowland habitat, recolonisation of boreal forest sites has been very slow. Whereas alpine populations remain high in 2011, boreal forest populations remain near zero. Alpine populations do not appear to be a source for the boreal forest.
Context Predator dynamics may be related to prey abundance and influenced by environmental effects, such as climate. Predator–prey interactions may be represented by mechanistic models that comprise a deterministic skeleton with stochastic climatic forcing.
Aims The aim of this study was to evaluate the effects of climate on predator–prey dynamics. The lynx and snowshoe hare predator–prey system in the Kluane region of the Yukon, Canada, is used as a case study. The specific hypothesis is that climate influences the relationship between lynx and hare abundance.
Methods We evaluate 10 linear relationships between predator and prey abundance and effects of climate. We use data on lynx and snowshoe hare abundance over 21 years in the Yukon as the predator–prey system, and three alternative broad-scale climate indices: the winter North Atlantic Oscillation (NAO), the Pacific North American (PNA) index and the North Pacific index (NPI).
Key results There was more support, as assessed by Akaike weights (ωi = 0.600), evidence ratio (=4.73) and R2 (=0.77) for a model of predator (lynx) and prior prey (hare) abundance with an effect of prior climate (winter NAO) when combined in a multiplicative, rather than in an additive, manner. The results infer that climate changes the amplitude of the lynx cycle with lower predator (lynx) abundance with positive values of winter NAO for a given hare density.
Conclusions The study provides evidence that predator–prey dynamics are related to climate in an interactive manner. The ecological mechanism for the interactive effect is not clear, and alternative hypotheses are proposed for future evaluation.
Implications The study implies that changes in climate may alter predator–prey relationships.
Context The broad concepts and generalisations that guide conduct of applied ecology, including wildlife management, have been reviewed and synthesised recently into 22 prescriptive and three empirical principles. Aims The aim of this study was to use these principles to evaluate three on-ground wildlife management programs and assess the utility of the principles themselves. Key results Case studies of long-term management of national park biodiversity impacted by feral pigs (Sus scrofa), and of conservation and harvest of red kangaroos (Macropus rufus) and mallards (Anas platyrhnchos), were selected to provide a representative range of management objectives, spatial scales and land tenures, and to include both native and introduced species. Management documents and a considerable scientific literature were available for all three programs. The results highlight similarities and differences among management activities and demonstrate the 25 principles to differing degrees. Most of the prescriptive principles were demonstrated in both the management and the scientific literature in all three programs, but almost no use was made of the three empirical principles. We propose that use of the prescriptive principles constitutes evidence that these programs meet both societal and scientific expectations. However, the limited use of the empirical principles shows gaps in the three programs. Conclusions The results suggest that evaluating other wildlife management programs against the principles of applied ecology is worthwhile and could highlight aspects of those programs that might otherwise be overlooked. Little use was made of the empirical principles, but the the Effort–outcomes principle in particular provides a framework for evaluating management programs. Implications The effort–outcomes relationship should be a focus of future applied research, and both prescriptive and empirical principles should be integrated into wildlife management programs.
Context The arctic ground squirrel (Urocitellus parryii) comprised 17% of the biomass of herbivores in the Yukon boreal forest during the summer months from 1987 to 1996 and was responsible for 23% of the energy flow at the herbivore level. By 2000, ground squirrel populations in this region collapsed to nearly zero and have remained there.
Aims We summarise the population monitoring (since 1975) and recent experimental work that has been done on this key herbivore in the Kluane area of the southern Yukon to test one mechanistic hypothesis as the possible explanation for this population collapse and subsequent lack of recovery: predation.
Methods Ground squirrels are the preferred summer prey of bird and mammal predators when snowshoe hare (Lepus americanus) populations are declining. We used translocations into formerly occupied habitat and radiotelemetry to determine movements and causes of death from 2009 to 2014. We surveyed 158 sites between 2008 and 2013 to measure the disappearance of colonies in alpine and forest habitats over 25 000 km2.
Key results Ground squirrels from 2000 to 2013 comprised a small fraction of the herbivore biomass in the boreal forest zone, down from 17% earlier. Most forest populations (~95%) are currently extinct, whereas just over half (65%) of low-elevation meadow populations are locally extinct. One hypothesis is that ground squirrels in the forest have been driven into a predator pit from which they cannot recover. They remain abundant in alpine tundra (93% occupancy rate) and around airport runways and human habitations (97% occupancy), but there is no apparent dispersal from alpine areas down into the boreal forest.
Conclusion The predator pit hypothesis is a likely explanation for the initial collapse and sustained decline in population size from 2000 to 2013. Recent attenuation of the hare cycle and milder winter climate have allowed shrubs to expand throughout the forest, thereby reducing visibility and increasing predation risk. This conclusion will be tested in further research using reintroductions to formerly occupied sites.
Implication If the loss of this herbivore from the boreal forest is not reversed, predator pressure on the other major herbivores of the montane forest zone is likely to change significantly.
Wildlife populations on both land and in the sea are under increasing pressure from both direct and indirect anthropogenic impacts. Threats to wildlife, such as over-exploitation, habitat loss, invasive species, infectious disease and climate change persist; yet in some situations, wildlife populations are the subject of management to reduce their unwanted impacts on economic, agricultural and conservation interests. The contrasting requirements of declining and overabundant populations, sometimes existing as part of the same ecosystem, mean that wildlife management needs to be viewed increasingly as part of an integrated social–ecological system. Moreover, solutions to wildlife-management problems may require new combinations of ecological knowledge, technological innovation and an understanding of the social and economic factors involved. The disciplinary scope of Wildlife Research will be broadening to reflect these changes. Wildlife Research has tended to focus on terrestrial systems in the past, but research approaches and findings are increasingly relevant across different environments, and consequently, we will also be expanding our coverage of freshwater and marine systems. We are planning to publish at least one Special Issue each year, to highlight new areas of research and the increasing internationalisation of wildlife management, and we welcome suggestions for these. We are also pleased to be able to offer fast-track publication of papers that contain results of high topical significance or policy relevance. We hope that the increased topical, disciplinary and geographical coverage of Wildlife Research will enhance its position as one of the leading international journals in applied ecology and environmental management.
Abstract ContextCapture–recapture sampling is one of the most commonly used methods for monitoring population demographics and is needed in a wide variety of studies where repeat sampling of individuals is desired. Although studies employing capture–recapture methods often assume unbiased sampling, it is well established that inherent capture biases can occur with these methods, including those related to baits. Reducing sources of sampling bias and augmenting recapture reliability is necessary for capture-dependent studies. However, few studies have examined the efficacy of baits on individuals with variable capture experience. AimsTo investigate the use of an attractant-augmented bait in enhancing capture–recapture probabilities for snowshoe hares (Lepus americanus). MethodsTo examine the efficacy of different attractant-augmented bait types, a variety of baits were created, with bait preference tested on a captive hare. Because a strawberry jam-based bait was preferentially consumed (in comparison with other tested baits), the effectiveness of this attractant in enhancing capture–recapture rates was subsequently examined in wild hares, using paired live-trapping field trials (n=6 trials). ResultsLive-trapping trials showed that although overall hare capture rates were not affected by the use of a jam-based bait, recaptures were 33.1% higher in capture-naïve individuals exposed to our attractant. This was not the case for hares with prior capture experience; such hares had an equal likelihood of being recaptured regardless of the bait type used. ConclusionsThe tested attractant improved recapture rates of capture-naïve hares. ImplicationsStudies relying on high recapture rates should use methods that maximise recapture rates wherever possible, including the use of baits that may augment recaptures in capture-naïve animals.
