Delayed density-dependent onset of spring reproduction in a fluctuating population of field voles

High housing density increases stress hormone- or disease-associated fecal microbiota in male Brandt's voles (Lasiopodomys brandtii)

Density-dependence is an important mechanism in the population regulation of small mammals. Stressors induced by high-density (e.g., crowding and aggression) can cause physiological and neurological disorders, and are hypothesized to be associated with alterations in gut microbiota, which may in turn reduce the fitness of animals by increasing stress- or disease-associated microbes. In this study, we examined the effects of housing density on the hormone levels, immunity, and composition of gut microbiota in male Brandt's voles (Lasiopodomys brandtii) by conducting two specific housing density experiments with or without physical contact between voles. Voles in high density groups exhibited higher serum corticosterone (CORT), serotonin (5-HT), and immunoglobulin G (IgG) levels, as well as higher testosterone (T) levels only in the experiment with physical contact. Meanwhile, high-density treatments induced significant changes in the composition of gut microbiota by increasing disease-associated microbes. The levels of hormones and immunity (i.e., CORT, 5-HT, and IgG) elevated by the high density treatment were significantly correlated with some specific microbes. These results imply that high-density-induced stress may shape the fitness of animals under natural conditions by altering their gut microbiota. Our study provides novel insights into the potential roles of gut microbiota in the density-dependent population regulation of small rodents as well as the potential mechanisms underlying psychological disorders in humans and animals under crowded conditions.

Population-level manipulations of field vole densities induce subsequent changes in plant quality but no impacts on vole demography

Grazing-induced changes in plant quality have been suggested to drive the negative delayed density dependence exhibited by many herbivore species, but little field evidence exists to support this hypothesis. We tested a key premise of the hypothesis that reciprocal feedback between vole grazing pressure and the induction of anti-herbivore silicon defenses in grasses drives observed population cycles in a large-scale field experiment in northern England. We repeatedly reduced population densities of field voles (Microtus agrestis) on replicated 1-ha grassland plots at Kielder Forest, northern England, over a period of 1 year. Subsequently, we tested for the impact of past density on vole life history traits in spring, and whether these effects were driven by induced silicon defenses in the voles' major over-winter food, the grass Deschampsia caespitosa. After several months of density manipulation, leaf silicon concentrations diverged and averaged 22% lower on sites where vole density had been reduced, but this difference did not persist beyond the period of the density manipulations. There were no significant effects of our density manipulations on vole body mass, spring population growth rate, or mean date for the onset of spring reproduction the following year. These findings show that grazing by field voles does induce increased silicon defenses in grasses at a landscape scale. However, at the vole densities encountered, levels of plant damage appear to be below those needed to induce changes in silicon levels large and persistent enough to affect vole performance, confirming the threshold effects we have previously observed in laboratory-based studies. Our findings do not support the plant quality hypothesis for observed vole population cycles in northern England, at least over the range of vole densities that now prevail here.

Effects of density on sex organ development and female sexual maturity in laboratory-bred Microtus fortis

Density dependence plays a key role in determining the population sizes of rodents. To explore density-dependent effects on sexual development, we documented and analyzed the development of the sex organs and hormone concentrations in both sexes, and the time to maturity in females of the reed vole in response to different population densities under laboratory conditions. Weaned voles were put into either same-sex or mixed-sex groups. Upon maturity, organ coefficients were calculated for sex organs as the length or weight of the sex organ divided by the length or weight of the body, respectively. The results demonstrate that, for individuals in same-sex groups, the coefficients for uterine length and short diameter of the testis decreased as population density increased. Population density had a highly significant effect on hormone concentrations as well as time to maturity in females. Population density in mixed-sex groups affects hormone concentrations, and increases the organ coefficients for ovarian weight, uterine weight, and uterine length; however, population density had no significant effect on the time to maturity of female voles in mixed-sex groups. These experiments showed that the effect of density dependence on the development of the vole differed between same-sex and mixed-sex conditions, the effects of increased density being greater in same-sex groups. We conclude that the effect of promoting sexual development between individuals might be greater than the effect of inhibition between individuals in mixed-sex groups.

Seasonal demography of a cyclic lemming population in the Canadian Arctic

1. The causes of cyclical fluctuations in animal populations remain a controversial topic in ecology. Food limitation and predation are two leading hypotheses to explain small mammal population dynamics in northern environments. We documented the seasonal timing of the decline phases and demographic parameters (survival and reproduction) associated with population changes in lemmings, allowing us to evaluate some predictions from these two hypotheses. 2. We studied the demography of brown lemmings (Lemmus trimucronatus), a species showing 3- to 4-year population cycles in the Canadian Arctic, by combining capture-mark-recapture analysis of summer live-trapping with monitoring of winter nests over a 10-year period. We also examined the effects of some weather variables on survival. 3. We found that population declines after a peak occurred between the summer and winter period and not during the winter. During the summer, population growth was driven by change in survival, but not in fecundity or proportion of juveniles, whereas in winter population growth was driven by changes in late summer and winter reproduction. 4. We did not find evidence for direct density dependence on summer demographic parameters, though our analysis was constrained by the paucity of data during the low phase. Body mass, however, was highest in peak years. 5. Weather effects were detected only in early summer when lemming survival was positively related to snow depth at the onset of melt but negatively related to rainfall. 6. Our results show that high mortality causes population declines of lemmings during summer and fall, which suggests that predation is sufficient to cause population crashes, whereas high winter fecundity is the primary factor leading to population irruptions. The positive association between snow depth and early summer survival may be due to the protective cover offered by snow against predators. It is still unclear why reproduction remains low during the low phase.

Overcompensation and phase effects in a cyclic common vole population: between first and second-order cycles

Population cycles in voles are often thought to be generated by one-year delayed density dependence on the annual population growth rate. In common voles, however, it has been suggested by Turchin (2003) that some populations exhibit first-order cycles, resulting from strong overcompensation (i.e. carrying capacity overshoots in peak years, with only an effect of the current year abundance on annual growth rates). We focus on a common vole (Microtus arvalis) population from western France that exhibits 3-year cycles. Several overcompensating nonlinear models for populations dynamics are fitted to the data, notably those of Hassell, and Maynard-Smith and Slatkin. Overcompensating direct density dependence (DD) provides a satisfactory description of winter crashes, and one-year delayed density dependence is not responsible for the crashes, thus these are not classical second-order cycles. A phase-driven modulation of direct density dependence maintains a low-phase, explaining why the cycles last three years instead of two. Our analyses suggest that some of this phase dependence can be expressed as one-year delayed DD, but phase dependence provides a better description. Hence, modelling suggests that cycles in this population are first-order cycles with a low phase after peaks, rather than fully second-order cycles. However, based on the popular log-linear second-order autoregressive model, we would conclude only that negative delayed density dependence exists. The additive structure of this model cannot show when delayed DD occurs (here, during lows rather than peaks). Our analyses thus call into question the automated use of second-order log-linear models, and suggests that more attention should be given to non-(log)linear models when studying cyclic populations. From a biological viewpoint, the fast crashes through overcompensation that we found suggest they might be caused by parasites or food rather than predators, though predators might have a role in maintaining the low phase and spatial synchrony.

Host-parasite biology in the real world: the field voles of Kielder

Research on the interactions between the field voles (Microtus agrestis) of Kielder Forest and their natural parasites dates back to the 1930s. These early studies were primarily concerned with understanding how parasites shape the characteristic cyclic population dynamics of their hosts. However, since the early 2000s, research on the Kielder field voles has expanded considerably and the system has now been utilized for the study of host-parasite biology across many levels, including genetics, evolutionary ecology, immunology and epidemiology. The Kielder field voles therefore represent one of the most intensely and broadly studied natural host-parasite systems, bridging theoretical and empirical approaches to better understand the biology of infectious disease in the real world. This article synthesizes the body of work published on this system and summarizes some important insights and general messages provided by the integrated and multidisciplinary study of host-parasite interactions in the natural environment.

Is quantity or quality of food influencing the reproduction of rice-field rats in the Philippines?

Context Asynchronous or aseasonal planting of rice crops can extend the period when high-quality food is available to rodents. Consequently, rodents may extend their breeding season, increasing population densities. An improved understanding of the effects of food availability and quality on rodent reproduction may enable better forecasts of high rodent population densities in response to asynchronous or aseasonal planting of crops. Aim The present study examined the association between the quality and quantity of food and the reproductive success of female rice-field rats, Rattus tanezumi and Rattus argentiventer, in a lowland rice landscape in the Philippines. Methods We evaluated the main dietary components of female rats on two different islands through a cropping season during the 2010 wet season. The breeding performance of 60 female R. tanezumi and 60 R. argentiventer individuals was measured. Key results Our findings indicated the following: (1) the main dietary items for females of both rodent species during the main breeding season (the booting stage to harvest) were rice panicles and rice seeds; (2) the high protein content of the rice crop at the tillering stage triggered the onset of the main breeding season, leading to the highest rates of conception during the booting and ripening stages; (3) the quantity of food available at the stubble stage provided sufficient nutrient to maintain pregnancy and lactation by females; and (4) asynchronous planting and poor harvest technology could extend the breeding season of rice-field rats. Conclusions We contend that the extension of the growing season by 3–4 weeks provides high-quality food for rodents, which in turn provides sufficient conditions for higher population densities. The availability of spilled rice grain at the stubble stage is a source of good-quality food for pregnant and lactating females, allowing extension of the breeding season. Implications Synchronous planting (within 2 weeks) with good post-harvest management of rice stubble are important to prevent high population densities of rice-field rats in lowland rice landscapes in the Philippines.

Delayed induced silica defences in grasses and their potential for destabilising herbivore population dynamics

Some grass species mount a defensive response to grazing by increasing their rate of uptake of silica from the soil and depositing it as abrasive granules in their leaves. Increased plant silica levels reduce food quality for herbivores that feed on these grasses. Here we provide empirical evidence that a principal food species of an herbivorous rodent exhibits a delayed defensive response to grazing by increasing silica concentrations, and present theoretical modelling that predicts that such a response alone could lead to the population cycles observed in some herbivore populations. Experiments performed under greenhouse conditions revealed that the rate of deposition of silica defences in the grass Deschampsia caespitosa is a time-lagged, nonlinear function of grazing intensity and that, upon cessation of grazing, these defences take around one year to decay to within 5 % of control levels. Simple coupled grass-herbivore population models incorporating this functional response, and parameterised with empirical data, consistently predict population cycles for a wide range of realistic parameter values for a (Microtus) vole-grass system. Our results support the hypothesis that induced silica defences have the potential to strongly affect the population dynamics of their herbivores. Specifically, the feedback response we observed could be a driving mechanism behind the observed population cycles in graminivorous herbivores in cases where grazing levels in the field become sufficiently large and sustained to trigger an induced silica defence response.