Population cycles and outbreaks of small rodents: ten essential questions we still need to solve

Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

Indirect effects of prey fluctuation on survival of juvenile arctic fox (Vulpes lagopus): a matter of maternal experience and litter attendance

Reproductive experience affects juvenile survival in a wide range of species with possible links to differences in foraging capacity and predation. Using supplementary feeding, we aimed to limit direct effect of prey abundance to investigate indirect effects of small-rodent availability and maternal experience on juvenile summer survival rates in an endangered population of arctic fox (Vulpes lagopus (L., 1758)). We used data spanning 7 years, included a complete small-rodent cycle, comprising 49 litters and 394 cubs. The effect of small-rodent abundance on juvenile survival depended on maternal breeding experience. Cubs born by first-time-breeding females had lower survival rate when small-rodent abundance was low compared with juveniles born to experienced mothers who remained unaffected. It was unlikely due to starvation, as physical condition was unrelated to survival. Instead, we favour the explanation that intraguild predation was an important cause of mortality. There was a negative relationship between survival and amount of time cubs were left unattended, suggesting that parental behaviour affected predation. We propose that a prey switch related to small-rodent abundance caused fluctuations in intraguild predation pressure and that inexperienced females were less able to cope with predation when small rodents were scarce.

Predator swamping and supplementary feeding do not improve reintroduction success for a threatened Australian mammal, Bettongia lesueur

Broad-scale Australian mammal declines following European settlement have resulted in many species becoming regionally or globally extinct. Attempts to reintroduce native mammals are often unsuccessful due to a suboptimal number of founders being used, high rates of predation and a lack of knowledge of the reintroduction biology for the species concerned. We trialled predator swamping and supplementary feeding in an attempt to offset predation and improve reintroduction success for the burrowing bettong (Bettongia lesueur) in arid South Australia. We compared population longevity of a large release group (1266 animals) with five releases of smaller groups (~50 animals at each). We compared release sites with (n = 5) and without (n = 1) supplementary food to determine whether site fidelity, body condition and reproduction were affected, and whether these traits aided population establishment. Predator swamping did not facilitate reintroduction success, with no bettongs detected more than 122 days after release. While supplementary food increased site fidelity and persistence at release sites, bettongs failed to establish successfully at any site. Neither predator swamping nor supplementary feeding enhanced reintroduction success at our sites but results suggested that supplementary feeding should be explored as an aid to reintroduction success for Australian mammals.

Avian predators as a biological control system of common vole (Microtus arvalis) populations in north-western Spain: experimental set-up and preliminary results

BACKGROUND

Ecologically based rodent pest management using biological control has never been evaluated for vole plagues in Europe, although it has been successfully tested in other systems. The authors report on the first large-scale replicated experiment to study the usefulness of nest-box installation for increasing the breeding density of common kestrels (Falco tinnunculus) and barn owls (Tyto alba) as a potential biological control of common vole (Microtus arvalis) abundance in agricultural habitats in north-western Spain.

RESULTS

The results show that: (1) population density of both predator species increased in response to both nest-site availability and vole density; (2) voles are a major prey for the common kestrels during the breeding period; (3) vole density during the increase phase of a population cycle may be reduced in crop fields near nest boxes.

CONCLUSION

The installation of nest boxes provides nesting sites for barn owls and kestrels. Kestrel populations increased faster than in areas without artificial nests, and the common vole was one of their main prey during the breeding season. The results suggest that local (field) effects could be found in terms of reduced vole density. If so, this could be an environmentally friendly and cheap vole control technique to be considered on a larger scale.

Cyclic voles and shrews and non-cyclic mice in a marginal grassland within European temperate forest

Cyclic population dynamics of small mammals are not restricted to the boreal and arctic zones of Eurasia and North America, but long-term data series from lower latitudes are still less common. We demonstrated here the presence of periodic oscillations in small mammal populations in eastern Poland using 22-year (1986–2007) trapping data from marginal meadow and river valley grasslands located in the extensive temperate woodland of Białowieża Primeval Forest. The two most common species inhabiting meadows and river valleys, root vole Microtus oeconomus and common shrew Sorex araneus, exhibited synchronous periodic changes, characterised by a 3-year time lag as indicated by an autocorrelation function. Moreover, the cycles of these two species were synchronous within both habitats. Population dynamics of the striped field mouse Apodemus agrarius was not cyclic. However, this species regularly reached maximum density 1 year before the synchronized peak of root voles and common shrews, which may suggest the existence of interspecific competition. Dynamics of all three species was dominated by direct density-dependent process, whereas delayed density dependent feedback was significant only in the root vole and common shrew. Climatic factors acting in winter and spring (affecting mainly survival and initial reproduction rates) were more important than those acting in summer and autumn and affected significantly only the common shrew. High temperatures in winter and spring had positive effects on autumn-to-autumn changes in abundance of this species, whereas deep snow in combination with high rainfall in spring negatively affected population increase rates in common shrew.

Impacts of rodent outbreaks on food security in Asia

Since 2007, a spate of rodent outbreaks has led to severe food shortages in Asia, affecting highly vulnerable and food-insecure families. Little has been documented about wildlife-management issues associated with these outbreaks. The aims of the present study were to synthesise what we know about rodent outbreaks in Asia, and identify important gaps in our knowledge. We compiled information from agencies of the United Nations, non-government organisations and the authors. The authors conducted site visits to areas affected by outbreaks of rodent populations, and convened an international conference in October 2009 to share knowledge. Bamboo masting is clearly implicated as the primary cause of the rodent-population outbreaks that led to severe food shortages in Mizoram (India), Chin State (Myanmar), Chittagong Hill Tracts (Bangladesh) and upland provinces of Lao PDR. In Laos, emergency food assistance was required for 85 000–145 000 people. In 2009, high rodent losses occurred also in lowland irrigated rice-based systems in the Philippines, Myanmar and Indonesia, not related to bamboo masting. Asynchronous or aseasonal growing of rice crops was a common element in these outbreaks. In the Ayeyarwaddy delta, Myanmar, 2.6 million rats were collected in 3 months through community activities; this outbreak appeared to be related to an extreme climatic event, Cyclone Nargis. There are two key features of rodent outbreaks that make the future uncertain. First, climate change and extreme climatic events will increase impacts of rodents on agricultural production. Second, there is food-security pressure in some countries to grow three crops per year. Increased cropping intensity will reduce fallow periods and create ideal conditions for rodents to breed nearly continuously. Implications of the research are as follows: (i) rodent outbreaks are a consequence of enhanced reproduction and natural mortality is of minor importance, particularly with rapidly increasing populations; therefore, we need to focus more on methods for disrupting reproduction; (ii) a stronger understanding of the ecology of pest species and community dynamics will enable ecologically sustainable management; (iii) we need landscape approaches that focus on crop synchrony, and timely coordinated community action to manage pest species and conserve desirable species; and (iv) a simple monitoring system can help implement ecologically based rodent management.

Ungulate densities and biomass in the tropical dry deciduous forests of Gir, Gujarat, India

Data on densities, biomass and ecological factors governing the distribution of various wild ungulate species in the difFerent management units of Gir forest from 1987 to 1989 were collected. Density of ungulates ranged from 50.8 km−2 to 0.42 km−2, the highest for chital (Axis axis), followed by chinkara (Gazella gazella), sambar (Cervus unicolor), nilgai (Boselaphus tragocamelus) and chowsingha (Tetracerus quadricornis). The density of chital did not vary significantly between different censuses and management units. The wild ungulate biomass ranged from 3290 kg km−2 in the National Park to 1900 kg km−2 in the Sanctuary East. Following the partial removal of people and livestock in the mid-1970s, there was an increase in the population of all wild ungulates except nilgai and wild pig (Sus scrofa). Concurrently, there was an increase in the proportion of wild ungulate prey in the lion's diet. Chital density has shown a 1320% increase. An increase in suitable habitats and a decrease in direct competition with livestock are the most likely factors to have triggered the eruption in chital population. These density estimates are discussed in relation to the prevailing ecological conditions in different management units of Gir.

Advances in wildlife ecology and the influence of Graeme Caughley

Graeme Caughley produced substantial advances in our understanding of interactions between large mammalian herbivores and the environments they occupy. The strength of his work lay in the logical approach to answering fundamental questions. While his life work contributed to our understanding of animal population dynamics, it is in the application of his research and ideas that we have greatly advanced the science of conservation biology. Two central legacies of Caughley’s lifelong work are that an understanding of basic science leads to more appropriate management, and that underlying assumptions must be explicitly stated and tested. By arguing that efficient management of ecosystems requires an understanding of the underlying mechanisms, he moved forward the application of basic research to management. Future advances in wildlife conservation must focus on three aspects: (1) the rules for stability in ecosystems, and how humans cause instability; (2) the decline in native habitats, mostly from agriculture, and how to renew and reconstruct them while expanding threatened populations; and (3) how to breed species in captivity, and then reintroduce them as a last line of defence.

Does the house mouse self-regulate its density in maturing sorghum and wheat crops?

1. One of the central questions in population ecology and management is: what regulates population growth? House mouse Mus domesticus L. populations erupt occasionally in grain-growing regions in Australia. This study aimed to determine whether mouse populations are self-regulated in maturing sorghum and wheat crops. This was assessed by examining food supply to mice (i.e. yield) and the relationship between initial mouse density (D(I)) and density at harvest (D(H)). Eight levels of D(I) ranging from 89 to 5555 mice ha(-1) were introduced to sorghum at the hard dough stage and to wheat crops at the milky stage in mouse-proofed pens. D(H) was measured by trapping out mice 49 days after the introduction. 2. There were at least 3.11 tonnes ha(-1) of wheat and 1.85 tonnes ha(-1) of sorghum grain available for mice at harvest. The estimated relationship between D(I) and D(H) was asymptotic exponential, with D(H) initially increasing almost linearly with D(I). When D(I) was above c. 500 mice ha(-1), D(H) increased asymptotically with D(I) and then saturated at c. 3100 mice ha(-1). The asymptotic increases in and saturation of D(H) was due partly to more young mice being born and recruited in pens treated with lower levels of D(I). 3. Our findings indicated that mouse densities in maturing cereal crops were driven by a numerical response of mice to the abundant supply of grain, modified by some unknown self-regulation mechanism that reduced this numerical response of mice at higher mouse densities. The mechanism was possibly spacing behaviours. Although the nature of this self-regulation mechanism is not known our model is, nevertheless, useful for predicting increases and eruptions in mouse population density in sorghum and wheat crops. Understanding the nature of this mechanism may provide insights into population processes that can be exploited in controlling mice in cereal crops.

Short- and long-term demographic changes in house mouse populations after control in dryland farming systems in Australia

In Australia, outbreaks of house mice (Mus domesticus) cause significant damage to agricultural crops. Rodenticides are used to reduce damage to crops, but the demographic consequences of applying rodenticides are poorly understood. Furthermore, it is not known whether the reduction induced by rodenticides would be similar to that of a natural crash in abundance at the end of mouse outbreaks. I compared the demographic responses of populations of mice to broad-scale field application of fast-acting, acute rodenticides (strychnine and zinc phosphide) in three grain-growing regions of Australia on baited and unbaited sites through live-trapping of mouse populations before baiting and up to four months after baiting. The reductions in population density in each region immediately after baiting were <40%, 92% and 98%. There were few consistent changes in demographic responses across the three regions for bodyweight (no change, increased or decreased), proportion of juveniles (increased or decreased), sex ratio (no change or bias towards females), survival (no change or decreased) and relative body condition (no change or increased). The differences in demographic responses appeared to be related to differences in the efficacy of the rodenticide. A natural crash in densities occurred over a 2–4-week period after baiting and induced a >85% decline in population densities across all regions on baited and unbaited sites. The natural crash caused increases and decreases in bodyweights, a reduction in the proportion of juveniles, male bias, poor survival and poor relative body condition. Poor survival was the only demographic parameter that was consistent for baiting and the natural crash. Five of seven demographic responses for mice during the natural crash were similar to those found in the literature for the decline phase of cyclic vole and lemming populations in the Northern Hemisphere. These results raise the question of whether mouse populations should be baited if a natural crash would occur anyway, but the timing of the natural crash is always uncertain and rodenticides are inexpensive.

Kin interactions and changing social structure during a population outbreak of feral house mice

Populations of feral house mice (Mus domesticus L.) in Australia undergo multiannual fluctuations in density, and these outbreaks may be partly driven by some change in behavioural self-regulation. In other vertebrate populations with multiannual fluctuations, changes in kin structure have been proposed as a causal mechanism for changes in spacing behaviour, which consequently result in density fluctuations. We tested the predictions of two alternative conceptual models based on kin selection in a population of house mice during such an outbreak. Both published models (Charnov & Finerty 1980; Lambin & Krebs 1991) propose that the level of relatedness between interacting individuals affects their behavioural response and that this changes with population density, though the nature of this relationship differs between the two models. Neither of the models was consistent with all observed changes in relatedness between interacting female mice; however, our results suggested that changes in kin structure still have potential for explaining why mouse outbreaks begin. Therefore, we have developed a variant of one of these conceptual models suggesting that the maintenance of female kin groups through the preceding winter significantly improves recruitment during the subsequent breeding season, and is therefore necessary for mouse outbreaks. We provide six testable predictions to falsify this hypothesis.

Mammal population regulation, keystone processes and ecosystem dynamics

The theory of regulation in animal populations is fundamental to understanding the dynamics of populations, the causes of mortality and how natural selection shapes the life history of species. In mammals, the great range in body size allows us to see how allometric relationships affect the mode of regulation. Resource limitation is the fundamental cause of regulation. Top-down limitation through predators is determined by four factors: (i). body size; (ii). the diversity of predators and prey in the system; (iii). whether prey are resident or migratory; and (iv). the presence of alternative prey for predators. Body size in mammals has two important consequences. First, mammals, particularly large species, can act as keystones that determine the diversity of an ecosystem. I show how keystone processes can, in principle, be measured using the example of the wildebeest in the Serengeti ecosystem. Second, mammals act as ecological landscapers by altering vegetation succession. Mammals alter physical structure, ecological function and species diversity in most terrestrial biomes. In general, there is a close interaction between allometry, population regulation, life history and ecosystem dynamics. These relationships are relevant to applied aspects of conservation and pest management.

The numerical response: rate of increase and food limitation in herbivores and predators

Two types of numerical response function have evolved since Solomon first introduced the term to generalize features of Lotka-Volterra predator-prey models: (i) the demographic numerical response, which links change in consumer demographic rates to food availability; and (ii) the isocline numerical response, which links consumer abundance per se to food availability. These numerical responses are interchangeable because both recognize negative feedback loops between consumer and food abundance resulting in population regulation. We review how demographic and isocline numerical responses have been used to enhance our understanding of population regulation of kangaroos and possums, and argue that their utility may be increased by explicitly accounting for non-equilibrium dynamics (due to environmental variability and/or biological interactions) and the existence of multiple limiting factors. Interferential numerical response functions may help bridge three major historical dichotomies in population ecology (equilibrium versus non-equilibrium dynamics, extrinsic versus intrinsic regulation and demographic versus isocline numerical responses).

Complex numerical responses to top-down and bottom-up processes in vertebrate populations

Population growth rate is determined in all vertebrate populations by food supplies, and we postulate bottom-up control as the universal primary standard. But this primary control system can be overridden by three secondary controls: top-down processes from predators, social interactions within the species and disturbances. Different combinations of these processes affect population growth rates in different ways. Thus, some relationships between growth rate and density can be hyperbolic or even have multiple nodes. We illustrate some of these in marsupial, ungulate and rabbit populations. Complex interactions between food, predators, environmental disturbance and social behaviour produce the myriad observations of population growth in nature, and we need to develop generalizations to classify populations. Different animal groups differ in the combination of these four processes that affect them, in their growth rates and in their vulnerability to extinction. Because conservation and management of populations depend critically on what factors drive population growth, we need to develop universal generalizations that will relieve us from the need to study every single population before we can make recommendations for management.

Response of American kestrels and gray-tailed voles to vegetation height and supplemental perches

We tested the behavioral and demographic responses of American kestrels, Falco sparverius, and gray-tailed voles, Microtus canicaudus, to vegetation height and addition of perches. We conducted our experiment in sixteen 0.2-ha rodent enclosures with four replicates assigned to each of the following treatments: tall vegetation without perches, tall vegetation with perches, short vegetation without perches, and short vegetation with perches. The enclosures were stocked with 20 gray-tailed voles in early November 1998. Before perches were erected during the 12th week of the experiment, kestrels showed a preference for short-vegetation enclosures (P < 0.05). After perches were erected, kestrels used enclosures with perches, showing the greatest preference for short-vegetation enclosures with a perch. Vole populations and recruitment rates were higher in tall-vegetation enclosures than in short-vegetation enclosures, but supplemental perches did not affect vole populations or recruitment. In many agricultural areas where perches are not available, providing supplemental perches may increase accessibility to prey species that cause crop damage. Facilitating predation by raptors may reduce vole populations and reduce the need to use potentially harmful chemicals in pest population management.

The effects of supplemental perch sites on avian predation and demography of vole populations

We erected supplemental perches to evaluate their effectiveness in attracting perching raptors and to determine if an increase in raptor visitation could affect vole demography. Our model experimental system consisted of six 0.2-ha enclosures containing gray-tailed vole, Microtus canicaudus. The availability of supplemental perch sites had no effect on northern harriers, Circus cyaneus, but resulted in an 11-fold increase in visitation by American kestrels, Falco sparverius. This increased predation pressure did not affect vole population size, growth rate, or adult survival. However, juvenile recruitment and the proportion of reproductive females were lower in treatment sites than in control sites. Male voles had smaller home ranges and were less active than expected, which may have been an avoidance response to predation risk. High densities of voles, adequate cover in treatment sites, and good foraging areas with less cover in surrounding habitat may have negated the effects of the availability of supplemental perch sites. We conclude that adding perches can increase raptor visitation substantially but may have little effect on high-density vole populations.

Factors regulating kit fox populations

Recent declines in several populations of kit foxes (Vulpes macrotis) have been attributed to decreased prey abundance and increased coyote-related mortalities. However, it is not known if short-term fluctuations in prey abundance or interference competition by coyotes can regulate the population dynamics of kit foxes. We coalesced data from all pertinent studies of kit and swift foxes and examined their numerical responses to changes in prey abundance. We also explored the influence of coyote-related mortalities on fox population dynamics. Fox density was positively correlated with leporid abundance, and fox reproduction rates were density-dependent at low prey densities. The proportion of the fox population killed by coyotes increased linearly over low to moderate fox densities, and survival rates of adult foxes decreased as the proportion of mortalities caused by coyotes increased. Hence, prey abundance and interference competition by coyotes may regulate fox populations. We suspect that prey abundance, and behavioral spacing mechanisms, are the major factors regulating fox densities. Coyote-related mortalities may be a less predominant factor, but could still act in concert with prey abundance to reduce the amplitude of fox population dynamics and keep foxes at lower densities than they might otherwise attain.

Do olfactory cues mediate interactions between rodents on northern shortgrass prairie?

We counted captures of free-ranging deer mice (Peromyscus maniculatus) in odor-baited traps to determine whether deer mice use olfactory cues to detect and evade grasshopper mice (Onychomys leucogaster), a predator and possible competitor on northern shortgrass prairie. Avoidance was measured using the frequency of captures in traps containing grasshopper mouse odors compared with that in traps containing odors of an innocuous rodent (Reithrodontomys megalotis) and in clean traps. We predicted that deer mice would be most deterred by odors in areas where grasshopper mice were common, and during winter, when alternative prey for grasshopper mice are less abundant. We also expected reproductive females to show greater avoidance because of the vulnerability of litters in burrows. Surprisingly, deer mice showed no evidence of avoidance in any experiments. These results are consistent with trapping records from capture–recapture studies on our site. We assert that the presence of grasshopper mice may affect the surface activity of deer mice, but that deer mice apparently do not use olfactory cues to avoid grasshopper mice. Taken with other studies noting the lack of avoidance of predator odors by deer mice and similar species, our results suggest that the response of rodents to predator odors is more variable than was previously appreciated.

Mortality factors in a cyclic vole population

The causes of cyclic fluctuations in microtine rodent populations are still a bone of contention. In particular, the actual causes of mortality in the different phases of the 3-4-year vole are an enigma. We present results from studies of radio-collared voles (Microtus agrestis, M. rossiaemeridionalis and Clethrionomys glareolus), which show that small mustelid predation was the major mortality factor of voles in the decline phase, but had less importance in the increase phase of the 3-year population cycle. After the initial decline in the non-breeding season (winter), vole-kill rate from predators increased to a point where mortality substantially exceeded the reproductive capacity of microtine prey. Our results suggest that predators may alone cause a decline in the density of these vertebrate-prey populations.

Snake odor alters behavior, but not pain sensitivity in mice

It is known that predator cues (both mammalian odor or avian vocalization) elicit marked analgesia in rodents. The present experiment used olfactory cues produced by an opportunistic rodent predator snake species (100 cc of sawdust scented by Elaphe quatuorlineata). Upon exposure to snake odor (for 30 s, 20 min, or 40 min), adult mice of both the CD-1 and DBA2 strains were assessed for tail-flick or hot-plate analgesia at different times after exposure (from 0 to 40 or 45 min, respectively). In both strains, snake odor exposure induced significant alteration in the frequencies of sniffing, self-grooming, and digging, while it inhibited habituation of locomotor activity in DBA/2 mice. No analgesia emerged with both tests as a consequence of exposure to snake odor. Results suggest that although endogenous analgesia has been demonstrated by other authors to be elicited in response to cues emanating from common and widely distributed mouse predators (such as carnivores or owls), predators such as reptiles, which under natural conditions exert a limited predatory pressure on the house mouse gene pool, may only induce fear-associated behavioral responses but cannot provide ethologically relevant stimuli triggering mouse analgesia.

Prevalence of viral antibodies and helminths in field populations of house mice (Mus domesticus) in southeastern Australia

A 13-month study of wild mice (Mus domesticus) in wheatlands in southeastern Australia contrasted changes in the seroprevalence of antibody to 13 viruses and the occurrence of helminths with changes in their population dynamics. Mice were seropositive for mouse hepatitis virus (MHV), rotavirus, minute virus of mice (MVM), mouse adenovirus (MAdV), reovirus (reo 3), and murine cytomegalovirus (MCMV). The seroprevalences of all but rotavirus varied significantly with time and increased with host density. Near the end of the study, host density declined rapidly and the seroprevalence of MVM and reo 3 increased significantly. These two viruses had low seroprevalence when host survival was high and high seroprevalence when host survival was low, indicating they may play a role in regulating mouse populations. In the case of MVM, there was evidence of a viral epizootic during the decline in mouse abundance. The prevalence of four helminths (Taenia taeniaeformis, Syphacia obvelata, and Vampirolepis spp.) differed significantly with time but showed no apparent association with host density. These findings highlight the need for further study on the effect of viruses on the population dynamics of mice.