Exploring the influence of density-dependence and weather on the spatial and temporal variation in common vole (Microtus arvalis) abundance in Castilla y León, NW Spain

BACKGROUND

The common vole has invaded the agroecosystems of northwestern Spain, where outbreaks cause important crop damage and management costs. Little is yet known about the factors causing or modulating vole fluctuations. Here, we used 11 years of vole abundance monitoring data in 40 sites to study density-dependence and weather influence on vole dynamics. Our objective was to identify the population dynamics structure and determine whether there is direct or delayed density-dependence. An evaluation of climatic variables followed, to determine whether they influenced vole population peaks.

RESULTS

First- and second-order outbreak dynamics were detected at 7 and 33 study sites, respectively, together with second-order variability in periodicity (2-3 to 4-5-year cycles). Vole population growth was explained by previous year abundance (mainly numbers in summer and spring) at 21 of the sites (52.5%), by weather variables at 11 sites (27.5%; precipitation or temperature in six and five sites, respectively), and by a combination of previous abundance and weather variables in eight sites (20%).

CONCLUSIONS

We detected variability in vole spatiotemporal abundance dynamics, which differs in cyclicity and period. We also found regional variation in the relative importance of previous abundances and weather as factors modulating vole fluctuations. Most vole populations were cyclical, with variable periodicity across the region. Our study is a first step towards the development of predictive modeling, by disclosing relevant factors that might trigger vole outbreaks. It improves decision-making processes within integrated management dealing with mitigation of the agricultural impacts caused by voles. © 2023 Society of Chemical Industry.

Effects of harvesting and stubble management on abundance of pest rodents (Mus musculus) in a conservation agriculture system

BACKGROUND

The shift to more environmentally sensitive agricultural practices over the last several decades has changed farmland landscapes worldwide. Changes including no-till and retaining high biomass mulch has been coincident with an increase in rodent pests in South Africa, India, South America and Europe, indicating a possible conflict between conservation agriculture (CA) and rodent pest management. Research on effects of various crop management practices associated with CA on pest rodent population dynamics is needed to anticipate and develop CA-relevant management strategies.

RESULTS

During the Australian 2020-2021 mouse plague, farmers used postharvest stubble management practices, including flattening and/or cutting, to reduce stubble cover in paddocks to lessen habitat suitability for pest house mice. We used this opportunity to assess the effects of both harvest and stubble management on the movement and abundance of mice in paddocks using mouse trapping and radio tracking. We found that most tracked mice remained resident in paddocks throughout harvest, and that mouse population abundance was generally unaffected by stubble management.

CONCLUSION

Recent conversions to CA practices have changed how pest house mice use cropped land. Management practices that reduce postharvest habitat complexity do not appear to reduce the attractiveness of paddocks to mice, and further research into new management strategies in addition to toxic bait use is required as part of an integrated pest management approach. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Timing outweighs magnitude of rainfall in shaping population dynamics of a small mammal species in steppe grassland

Disentangling the effects of rainfall timing and magnitude on animal and plant populations is essential to reveal the biological consequence of diverse climate change scenarios around the world. We conducted a 10-y, large-scale, manipulative experiment to examine the bottom-up effects of changes in rainfall regime on the population dynamics of Brandt’s voles in the steppe grassland of Inner Mongolia, China. We found that a moderate rainfall increase during the early growing season could produce marked increases in vole population size by increasing the biomass of preferred plant species, whereas large increases in rainfall produced no additional increase in vole population growth. Our study highlights the importance of rainfall magnitude and timing on the nonlinear population dynamics of herbivores.

Climate change–induced shifts in species phenology differ widely across trophic levels, which may lead to consumer–resource mismatches with cascading population and ecosystem consequences. Here, we examined the effects of different rainfall patterns (i.e., timing and amount) on the phenological asynchrony of population of a generalist herbivore and their food sources in semiarid steppe grassland in Inner Mongolia. We conducted a 10-y (2010 to 2019) rainfall manipulation experiment in 12 0.48-ha field enclosures and found that moderate rainfall increases during the early rather than late growing season advanced the timing of peak reproduction and drove marked increases in population size through increasing the biomass of preferred plant species. By contrast, greatly increased rainfall produced no further increases in vole population growth due to the potential negative effect of the flooding of burrows. The increases in vole population size were more coupled with increased reproduction of overwintered voles and increased body mass of young-of-year than with better survival. Our results provide experimental evidence for the fitness consequences of phenological mismatches at the population level and highlight the importance of rainfall timing on the population dynamics of small herbivores in the steppe grassland environment.

Multiple ecological processes underpin the eruptive dynamics of small mammals: House mice in a semi-arid agricultural environment

Mouse plagues are a regular feature of grain-growing regions, particularly in southern and eastern Australia, yet it is not clear what role various ecological processes play in the eruptive dynamics generating these outbreaks.This research was designed to assess the impact of adding food, water, and cover in all combinations on breeding performance, abundance, and survival of mouse populations on a typical cereal growing farm in northwestern Victoria.Supplementary food, water, and cover were applied in a 2 × 2 × 2 factorial design to 240 m sections of internal fence lines between wheat or barley crops and stubble/pasture fields over an 11-month period to assess the impact on mouse populations.We confirmed that mice were eating the additional food and were accessing the water provided. We did not generate an outbreak of mice, but there were some significant effects from the experimental treatments. Additional food increased population size twofold and improved apparent survival. Both water and cover improved breeding performance. Food and cover increased apparent survival.Our findings confirm that access to food, water, and cover are necessary for outbreaks, but are not sufficient. There remain additional factors that are important in generating mouse plagues, particularly in a climatically variable agricultural environment.

Modeling ecological traps for the control of feral pigs

Ecological traps are habitat sinks that are preferred by dispersing animals but have higher mortality or reduced fecundity compared to source habitats. Theory suggests that if mortality rates are sufficiently high, then ecological traps can result in extinction. An ecological trap may be created when pest animals are controlled in one area, but not in another area of equal habitat quality, and when there is density-dependent immigration from the high-density uncontrolled area to the low-density controlled area. We used a logistic population model to explore how varying the proportion of habitat controlled, control mortality rate, and strength of density-dependent immigration for feral pigs could affect the long-term population abundance and time to extinction. Increasing control mortality, the proportion of habitat controlled and the strength of density-dependent immigration decreased abundance both within and outside the area controlled. At higher levels of these parameters, extinction was achieved for feral pigs. We extended the analysis with a more complex stochastic, interactive model of feral pig dynamics in the Australian rangelands to examine how the same variables as the logistic model affected long-term abundance in the controlled and uncontrolled area and time to extinction. Compared to the logistic model of feral pig dynamics, the stochastic interactive model predicted lower abundances and extinction at lower control mortalities and proportions of habitat controlled. To improve the realism of the stochastic interactive model, we substituted fixed mortality rates with a density-dependent control mortality function, empirically derived from helicopter shooting exercises in Australia. Compared to the stochastic interactive model with fixed mortality rates, the model with the density-dependent control mortality function did not predict as substantial decline in abundance in controlled or uncontrolled areas or extinction for any combination of variables. These models demonstrate that pest eradication is theoretically possible without the pest being controlled throughout its range because of density-dependent immigration into the area controlled. The stronger the density-dependent immigration, the better the overall control in controlled and uncontrolled habitat combined. However, the stronger the density-dependent immigration, the poorer the control in the area controlled. For feral pigs, incorporating environmental stochasticity improves the prospects for eradication, but adding a realistic density-dependent control function eliminates these prospects.

Reproduction and survival of rodents in crop fields: the effects of rainfall, crop stage and stone-bund density

Context Reproduction and survival are two of the most important demographic factors that play a major role in changing population abundances of pest species over time and space, solid understanding of which is a useful input to forecast future population changes for proactive management. Aims We investigated the effects of rainfall, crop-development stage and density of stone bunds on reproductive patterns, and the effects of stone-bund density and sex on survival probabilities of two widespread rodent species (Mastomys awashensis and Arvicanthis dembeensis) in Ethiopian highlands. Methods Rodent population dynamics were monitored from April 2007 to February 2011, using capture–mark–recapture (CMR) technique in four 60 × 60 m permanent square grids for four consecutive cropping seasons. Two of the grids represented fields with low stone-bund density (LSBD, ~15 m apart) and the other two represented fields with high stone-bund density (HSBD, ~10 m apart). Key results Reproduction was seasonal, commencing during the wet season following the rain and continuing through the early dry season. We found an increase in the abundance of reproductively active female individuals of both species towards the milky and fruiting crop stages and around harvest period. We found no strong difference in survival probability between the two rodent species with variation in stone-bund density and sex. Conclusion Stone bunds play a minor role in the reproduction and survival of the rodent species at the observed abundances. Implications In terms of pest management, the high local survival rates estimated for both rodent species matter more than survival differences owing to variations in stone-bund density and sex.

Population ecology of the Asian house rat (Rattus tanezumi) in complex lowland agroecosystems in the Philippines

Context Rattus tanezumi (the Asian house rat) is the principal rodent pest of rice and coconut crops in the Philippines. Little is known about the population and breeding ecology of R. tanezumi in complex agroecosystems; thus, current methods of rodent control may be inappropriate or poorly implemented. Aims To investigate the habitat use, population dynamics and breeding biology of R. tanezumi in complex lowland agroecosystems of the Sierra Madre Biodiversity Corridor, Luzon, and to develop ecologically based rodent management (EBRM) strategies that will target specific habitats at specific times to improve cost-efficiency and minimise non-target risks. Methods An 18-month trapping study was conducted in rice monoculture, rice adjacent to coconut, coconut groves, coconut-based agroforest and forest habitats. Trapped animals were measured, marked and assessed for breeding condition. Key results Five species of rodent were captured across all habitats with R. tanezumi the major pest species in both the rice and coconut crops. The stage of the rice crop was a major factor influencing the habitat use and breeding biology of R. tanezumi. In rice fields, R. tanezumi abundance was highest during the tillering to ripening stages of the rice crop and lowest during the seedling stage, whereas in coconut groves abundance was highest from the seedling to tillering stage of nearby rice crops. Peaks in breeding activity occurred from the booting stage of the rice crop until just after harvest, but >10% of females were in breeding condition at each month of the year. Conclusions In contrast with the practices applied by rice farmers in the study region, the most effective time for lethal management based on the breeding ecology of R. tanezumi is likely to be during the early stages of the rice crop, before the booting stage. Farmers generally apply control actions as individuals. We recommend coordinated community action. Continuous breeding throughout the year may necessitate two community campaigns per rice cropping season. To limit population growth, the most effective time to reduce nesting habitat is from the booting stage until harvest. Implications By adopting EBRM strategies, we expect a reduction in costs associated with rodent control, as well as improved yield and reduced risk to non-target species.

Developing, testing and application of rodent population dynamics and capture models based on an adjusted Leslie matrix-based population approach

Small rodents in general and the multimammate rat Apodemus agrarius in particular, damage crops and cause major economic losses in China. Therefore, accurate predictions of the population size of A. agrarius and an efficient control strategy are urgently needed. We developed a population dynamics model by applying a Leslie matrix method, and a capture model based on optimal harvesting theory for A. agrarius. Our models were parametrized using demographic estimates from a capture–mark–recapture (CMR) study conducted on the Qinshui Forest Farm in Northwestern China. The population dynamics model incorporated 12 equally balanced age groups and included immigration and emigration parameters. The model was evaluated by assessing the predictions for four years based on the known starting population in 2004 from the 2004–2007 CMR data. The capture model incorporated two functional age categories (juvenile and adult) and used density-dependent and density-independent factors. The models were used to assess the effect of rodent control measures between 2004 and 2023 on population dynamics and the resulting numbers of rats. Three control measures affecting survival rates were considered. We found that the predicted population dynamics of A. agrarius between 2004 and 2007 compared favorably with the observed population dynamics. The models predicted that the population sizes of A. agrarius in the period between 2004 and 2023 under the control measure applied in August 2004 were very similar to the optimal population sizes, and no significant difference was found between the two population sizes. We recommend using the population dynamics and capture models based on CMR-estimated demographic schedules for rodent, provided these data are available. The models that we have developed have the potential to play an important role in predicting the effects of rodent management and in evaluating different control strategies.

Potential changes in disease patterns and pharmaceutical use in response to climate change

As climate change alters environmental conditions, the incidence and global patterns of human diseases are changing. These modifications to disease profiles and the effects upon human pharmaceutical usage are discussed. Climate-related environmental changes are associated with a rise in the incidence of chronic diseases already prevalent in the Northern Hemisphere, for example, cardiovascular disease and mental illness, leading to greater use of associated heavily used Western medications. Sufferers of respiratory diseases may exhibit exacerbated symptoms due to altered environmental conditions (e.g., pollen). Respiratory, water-borne, and food-borne toxicants and infections, including those that are vector borne, may become more common in Western countries, central and eastern Asia, and across North America. As new disease threats emerge, substantially higher pharmaceutical use appears inevitable, especially of pharmaceuticals not commonly employed at present (e.g., antiprotozoals). The use of medications for the treatment of general symptoms (e.g., analgesics) will also rise. These developments need to be viewed in the context of other major environmental changes (e.g., industrial chemical pollution, biodiversity loss, reduced water and food security) as well as marked shifts in human demographics, including aging of the population. To identify, prevent, mitigate, and adapt to potential threats, one needs to be aware of the major factors underlying changes in the use of pharmaceuticals and their subsequent release, deliberately or unintentionally, into the environment. This review explores the likely consequences of climate change upon the use of medical pharmaceuticals in the Northern Hemisphere.

Population dynamics of house mice in Queensland grain-growing areas

Context Irregular plagues of house mice cause high production losses in grain crops in Australia. If plagues can be forecast through broad-scale monitoring or model-based prediction, then mice can be proactively controlled by poison baiting. Aims To predict mouse plagues in grain crops in Queensland and assess the value of broad-scale monitoring. Methods Regular trapping of mice at the same sites on the Darling Downs in southern Queensland has been undertaken since 1974. This provides an index of abundance over time that can be related to rainfall, crop yield, winter temperature and past mouse abundance. Other sites have been trapped over a shorter time period elsewhere on the Darling Downs and in central Queensland, allowing a comparison of mouse population dynamics and cross-validation of models predicting mouse abundance. Key results On the regularly trapped 32-km transect on the Darling Downs, damaging mouse densities occur in 50% of years and a plague in 25% of years, with no detectable increase in mean monthly mouse abundance over the past 35 years. High mouse abundance on this transect is not consistently matched by high abundance in the broader area. Annual maximum mouse abundance in autumn–winter can be predicted (R2 = 57%) from spring mouse abundance and autumn–winter rainfall in the previous year. In central Queensland, mouse dynamics contrast with those on the Darling Downs and lack the distinct annual cycle, with peak abundance occurring in any month outside early spring. On average, damaging mouse densities occur in 1 in 3 years and a plague occurs in 1 in 7 years. The dynamics of mouse populations on two transects ~70 km apart were rarely synchronous. Autumn–winter rainfall can indicate mouse abundance in some seasons (R2 = ~52%). Conclusion Early warning of mouse plague formation in Queensland grain crops from regional models should trigger farm-based monitoring. This can be incorporated with rainfall into a simple model predicting future abundance that will determine any need for mouse control. Implications A model-based warning of a possible mouse plague can highlight the need for local monitoring of mouse activity, which in turn could trigger poison baiting to prevent further mouse build-up.

Population characteristics of house mice (Mus musculus) on southern Yorke Peninsula, South Australia

Seasonal population characteristics of house mice (Mus musculus), including the effect of season on body mass, were studied at Innes National Park, southern Yorke Peninsula. Mice were caught with Elliott traps, ear-notched, and released. Over 1550 trap-nights (January to December 2006, excluding May), 202 mice were caught. The overall capture success rate was 13.03 mice per 100 trap-nights. The recapture rate was 42.57%. Body mass of adult house mice varied significantly among seasons (P = 0.009). In particular, mouse body mass varied between autumn and winter (P = 0.018), and spring and winter (P = 0.023). The body mass of mice captured in autumn and then recaptured in winter was also significantly different (P = 0.006). This study is the first published for M. musculus population characteristics on Yorke Peninsula and adds to the relatively limited information available on house mouse populations in non-agricultural habitats.

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.

Annual flooding, survival and recruitment in a rodent population from the Niger River plain in Mali

Multimammate rats of the genus Mastomys are among the most widespread pest species in Africa. Previous studies of Mastomys population dynamics have generally reported variation in abundance but few have investigated the demographic parameters underlying this variation, and in particular recruitment. Capture-mark-recapture data were collected for Mastomys erythroleucus several times a year from 2000 to 2004 at a site annually flooded by the Niger River in Mali. Closed-population models were used to estimate population abundance. Both seniority (a parameter inversely linked to recruitment) and survival probabilities were estimated by capture-mark-recapture models. The impacts of water level, population abundance and cumulative rainfall were assessed for each demographic parameter. Survival probabilities (local survival) were negatively correlated with water level, suggesting that rodents emigrated out of the study zone during flooding. As for seniority probabilities, 86% of temporal variation was explained by a model with season, abundance, water level and the interaction between abundance and water level. This suggests that density-dependence in recruitment was mediated by intraspecific competition for food or refuge from floodwaters, or by predation. The flood of the Niger River greatly impacts Mastomys erythroleucus population dynamics, affecting both survival and seniority probabilities.

Ecology of the rare but irruptive Pilliga mouse (Pseudomys pilligaensis). I. Population fluctuation and breeding season

During a 4-year study in the Pilliga Scrub, trappable densities of the Pilliga mouse, Pseudomys pilligaensis (Muridae), were low before a wildfire in November 1997, higher in late 1999 and February 2000 (5–30 mice ha−1) and very high (up to 83 mice ha−1) in April 2000; however, the densities fell sharply by July 2000, remaining low (0–5 mice ha−1) until trapping ended in October 2001. Site-specific densities and their fluctuations differed among the four trapping sites, although fluctuations were broadly synchronised by the irruption peak. Within-site distribution changed as density fluctuated, from sparse to almost ubiquitous and back to sparse, and within-grid pre-irruption distributions did not predict those after the irruption. After the population decline, mice virtually disappeared from three of the four sites. The species’ breeding season spanned at least October–April; some females bred repeatedly within a season. Prolonged good rains soon after the wildfire may have facilitated the irruption. The study suggested that P. pilligaensis is distributed in disjunct patches of (refuge) habitat within its range except when environmental conditions are favourable, and that it is able to irrupt and become briefly ubiquitous before suddenly declining to a low density and sparse distribution. We suggest approaches for monitoring of this rare species.

Self-regulation within outbreak populations of feral house mice: a test of alternative models

1. Outbreaks of feral house mice, Mus domesticus, in Australia represent a fundamental failure of the behavioural control mechanisms of population density, as proposed in the hypothesis of self-regulation. 2. Mice have the potential to keep numbers in check via a suite of spacing behaviours; however, the self-regulation hypothesis implies that some social change occurs that permits the population to erupt. It also suggests that at different phases of an outbreak, distinct patterns of social activity are evident. 3. We compare predictions from two models encapsulating the self-regulation hypothesis as applied to feral house mice in south-eastern Australia. Each model may be distinguished by the timing of aggressiveness between mice that leads to a closed social system. We compare individual turnover, residency and territoriality in each sex and age cohort during the increase, peak and low phases of a population outbreak that peaked in 2001. 4. The activity of 438 mice was monitored via intensive mark-recapture trapping and an automated event recording system that detected the activity of 300 marked individuals at burrow entrances. 5. Our findings support the second model, which suggests that mice switch from an almost asocial structure at low densities to a territorial system as abundance increases. Adult females appear more likely than males or juveniles to make the significant social shift. The trigger for this change remains unclear and several alternative mechanisms are proposed.

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.

Shifting age structure of house mice during a population outbreak

A technique to age wild house mice, Mus domesticus, in Australia using the dry weight of the eye lens based on known-age mice from semi-natural enclosures is described and presented for 3–32-week-old mice. At four sampling periods from November 2000 to September 2001, the age frequency distributions of free-living house mice were determined using this relationship. The distributions of ages shifted between seasons from relatively young animals at the beginning of the breeding season (November 2001), coinciding with low mouse abundance, to progressively older distributions in each sample as breeding continued, ending with the cessation of breeding and a population crash before the last sample. No significant difference was detected between the sexes at any of the four periods. These results are consistent with the suggestion that the formation of mouse outbreaks requires a shift in age structure towards younger mice.

Can outbreaks of house mice in south-eastern Australia be predicted by weather models?

Outbreaks of house mice (Mus domesticus) occur irregularly in the wheat-growing areas of south-eastern Australia, and are thought to be driven by weather variability, particularly rainfall. If rainfall drives grass and seed production, and vegetation production drives mouse dynamics, we should achieve better predictability of mouse outbreaks by the use of plant-production data. On a broader scale, if climatic variability is affected by El Niño–Southern Oscillation (ENSO) events, large-scale weather variables might be associated with mouse outbreaks. We could not find any association of mouse outbreaks over the last century with any ENSO measurements or other large-scale weather variables, indicating that the causal change linking mouse numbers with weather variation is more complex than is commonly assumed. For the 1960–2002 period we were only partly successful in using variation in cereal production to predict outbreaks of mice in nine areas of Victoria and South Australia, and we got better predictability of outbreaks from rainfall data alone. We achieved 70% correct predictions for a qualitative model using rainfall and 58% for a quantitative model using rainfall and spring mouse numbers. Without the detailed specific mechanisms underlying mouse population dynamics, we may not be able to improve on these simple models that link rainfall to mouse outbreaks.

Is reproduction of the Australian house mouse (Mus domesticus) constrained by food? A large-scale field experiment

Food quantity and especially food quality are thought to be key factors driving reproductive changes in the house mouse, Mus domesticus, leading to outbreaks of house mouse populations in the Australian grain-growing region. Characteristic changes during an incipient mouse plague are an early start of breeding, a high proportion of females breeding at a young age and a prolonged breeding season. We conducted a large-scale food manipulation during an incipient mouse plague, which started with early breeding and relatively high spring numbers of mice. We measured background food availability in four farms throughout the study and conducted a food manipulation experiment from November to March in two of them. After harvest in December 100-200 kg/ha spilled grain remained in the stubble. This was depleted by March. In two treatment farms we added high-protein food pellets on a weekly basis between November and March and two farms served as controls. We measured changes in mouse numbers by capture-mark-recapture trappings and changes in reproduction by scoring embryos and recent placental scars at necropsy. Mouse numbers did not differ between treatments and controls. There were no differences in the litter size or the proportion of females breeding between treatments and controls. We observed the normal pattern of high litter size in spring and decreasing litter size towards the end of summer in treatments and controls. In all farms reproduction stopped in March. Mouse numbers were high but not at plague densities. Contrary to our prediction we did not observe food constraint affecting the reproduction of female mice. Our field experiment seems to rule out food quality as the driving factor for improved reproduction and formation of an outbreak of mice. We suggest that physiological mechanisms in mice might not enable them to take advantage of food with a high protein content in arid summers in southeastern Australian grain fields because of the lack of free-standing water.

Populations in variable environments: the effect of variability in a species' primary resource

Mechanistic models for herbivore populations responding to rainfall-driven pasture are used to explore the effect of temporal variability in a primary resource on the abundance and distribution of a species. If the numerical response of the herbivore to pasture is a convex function, then gains made over time intervals with above average rainfall do not compensate for losses incurred when rainfall is below average. Populations therefore fare worse when rainfall is variable compared with when rainfall is reliable. It is demonstrated that this result is independent of the distribution of rainfall. Sensitivity of a species to variability, and hence the limit to its distribution in variable environments, is directly proportional to the difference between population growth rate under ideal conditions and the estimated rate of decline as the species' resource tends to zero. When density dependence is included in the numerical response, the average abundance of a species declines with increasing variability in its primary resource. However, a model for the dynamics of pasture and rabbits (Oryctolagus cuniculus) and red foxes (Vulpes vulpes) in southern Australia, is used to illustrate that trophic interactions can reverse the effect of variability: in the absence of foxes, the mean abundance of rabbits declines with variability as expected, but in the full model the mean abundance of rabbits increases.

Population growth rate and its determinants: an overview

We argue that population growth rate is the key unifying variable linking the various facets of population ecology. The importance of population growth rate lies partly in its central role in forecasting future population trends; indeed if the form of density dependence were constant and known, then the future population dynamics could to some degree be predicted. We argue that population growth rate is also central to our understanding of environmental stress: environmental stressors should be defined as factors which when first applied to a population reduce population growth rate. The joint action of such stressors determines an organism's ecological niche, which should be defined as the set of environmental conditions where population growth rate is greater than zero (where population growth rate = r = log(e)(N(t+1)/N(t))). While environmental stressors have negative effects on population growth rate, the same is true of population density, the case of negative linear effects corresponding to the well-known logistic equation. Following Sinclair, we recognize population regulation as occurring when population growth rate is negatively density dependent. Surprisingly, given its fundamental importance in population ecology, only 25 studies were discovered in the literature in which population growth rate has been plotted against population density. In 12 of these the effects of density were linear; in all but two of the remainder the relationship was concave viewed from above. Alternative approaches to establishing the determinants of population growth rate are reviewed, paying special attention to the demographic and mechanistic approaches. The effects of population density on population growth rate may act through their effects on food availability and associated effects on somatic growth, fecundity and survival, according to a 'numerical response', the evidence for which is briefly reviewed. Alternatively, there may be effects on population growth rate of population density in addition to those that arise through the partitioning of food between competitors; this is 'interference competition'. The distinction is illustrated using a replicated laboratory experiment on a marine copepod, Tisbe battagliae. Application of these approaches in conservation biology, ecotoxicology and human demography is briefly considered. We conclude that population regulation, density dependence, resource and interference competition, the effects of environmental stress and the form of the ecological niche, are all best defined and analysed in terms of population growth rate.