Cohort variation in the life-history parameters of stoatsMustela erminea in relation to fluctuating food resources: a challenge to boreal ecologists

Distributions of LRS in varying environments

The lifetime reproductive success (LRS) of individuals is affected by random events such as death, realized growth or realized reproduction, and the outcomes of these events can differ even when individuals have identical probabilities. Another source of randomness arises when these probabilities also change over time in variable environments. For structured populations in stochastic environments, we extend our recent method to determine how birth environment and birth stage determine the random distribution of the LRS. Our results provide a null model that quantifies effects on LRS of just the birth size or stage. Using Roe deer Capreolus capreolus as a case study, we show that the effect of an individual's birth environment on LRS varies with the frequency of environments and their temporal autocorrelation, and that lifetime performance is affected by changes in the pattern of environmental states expected as a result of climate change.

Consecutive cohort effects driven by density-dependence and climate influence early-life survival in a long-lived bird

Conditions during early life, including maternal cohort effects, can influence the future fitness of individuals. This may be particularly true for long-distance migrating birds, because, apart from conditions experienced by cohorts during rearing, conditions during early life in regions far from breeding grounds may also influence their population dynamics. Very little is known about the fitness consequences of those conditions experienced by juveniles after independence, especially in wild populations and for long-lived birds. We used multi-event capture-recapture-recovery models and a unique 26-year dataset for the Audouin's gull (Larus audouinii) to assess for the first time whether survival was influenced by early conditions, both during the rearing period (i.e. a maternal cohort effect potentially affected by density dependence) and the first winter (i.e. a cohort effect driven by climate when birds disperse to wintering grounds). Our results show that juvenile survival was highly sensitive to early-life conditions and that survival decreased with stronger density dependence and harsh climate. The two consecutive cohort effects were of similar magnitude and they may represent a selection filter. Thus, early-life conditions had a strong impact on survival, and neglecting this complexity may hinder our understanding on how populations of long-lived animals fluctuate and respond to perturbations.

Measuring connectivity of invasive stoat populations to inform conservation management

Context Effective design of conservation management programs for long-term population control requires an accurate definition of the spatial extent of populations, along with a proper understanding of the ways that landscape patchiness influences demography and dispersal within these populations. Aims In the present study, genetic techniques are used to describe the population genetic structure and connectivity of invasive stoats (Mustela erminea) across the Auckland region, New Zealand, so as to assist planning for mainland stoat control, and define potential future eradication units. Methods A sample of stoats from across the region (n = 120), was genotyped at 17 microsatellite loci, and a combination of clustering, genetic population assignment and various migration estimation methods were applied to these data. Key results Moderate population structure was observed (FST = 0.03–0.21), with five geographic populations defined by genetic clustering. Almost all individuals were correctly assigned to the location of origin, and recent migration rates among forest patches were found to be low. Conclusions It is possible to define the origin of stoats at this regional scale using genetic measures. From this, we show that the stoat incursion on Rangitoto Island that occurred post-eradication in 2010 probably came from East Auckland (P < 0.0001), whereas the 2014 stoat incursion on Motutapu Island probably originated from a population linked to the Waitakeres. Also, the Waiheke Island stoat population has minimal connection to all other populations and it is therefore a potential eradication unit. Implications The low migration rates among forest patches indicated that if thorough control is imposed on a discrete forest patch, reinvasion from other forest patches will be relatively low. Importantly, for stoat control in the region, the isolation of the Waiheke Island stoat population means that eradication here is likely to be feasible with low reinvasion pressure.

Functional responses of an invasive top predator Mustela erminea to invasive meso-predators Rattus rattus and Mus musculus, in New Zealand forests

Context Management of suites of invasive mammal species can lead to perverse outcomes, such as meso-predator release, or can achieve desirable reductions in the abundance of top-order predators by controlling their prey. Predictive models for predator–prey systems require estimates of predator functional responses, i.e. predation rates as functions of prey density. Aims In New Zealand, estimates of the functional responses of stoats (Mustela erminea) to mice (Mus musculus) and ship (black) rats (Rattus rattus) are required to improve management models for these invasive species. Methods We derived fitted relationships between the presence or absence of mouse or ship-rat remains in stoat guts and corresponding indices of prey abundance in beech and podocarp forests, respectively. To convert field data on stoat-gut contents to minimum kill rates, we used data on feeding activity and estimates of gut-passage time, observed in captive stoats. Key results The most parsimonious fitted curves were Type II functional responses, with a steeper stoat–mouse curve for autumn–winter, indicating a more specialist feeding habit than that in spring–summer. Estimated kill rates of mice per stoat per day reached an asymptote of 1.13 during autumn–winter. Our maximum observed kill rate for spring–summer was 11% less than the extrapolated upper limit of 1.04 mice per stoat per day for New Zealand ecosystems. No asymptote was reached within the limits of the data for the stoat–rat relationship. Conclusions Recent models for trophic interactions between stoats and the primary rodent prey have overestimated kill rates by stoats in forested ecosystems, particularly at very low and very high densities of mice. We show how data on stoat-gut contents can be rescaled to estimate minimum kill rates of rodent prey. Implications The functional-response relationships we have derived can be used to improve modelled predictions of the effects of natural or management-driven perturbations of invasive stoats and their primary rodent-prey populations.

Primary and secondary resource pulses in an alpine ecosystem: snow tussock grass (Chionochloa spp.) flowering and house mouse (Mus musculus) populations in New Zealand

Context. Rodent populations in many parts of the world fluctuate in response to resource pulses generated by periodic high seed production (masting) by forest trees, with cascading effects on predation risk to other forest species. In New Zealand forests, populations of exotic house mice (Mus musculus) irrupt after periodic heavy beech (Nothofagus spp.) seedfall. However, in alpine grasslands, where snow tussock grasses (Chionochloa spp.) also flower and set seeds periodically, little is known about house mouse population dynamics. Aims. Our primary objective was to test for an increase in alpine mouse density following a summer when snow tussocks flowered profusely. We also estimated mouse density in adjacent montane forest over 2 years, and assessed mouse diet, to predict their potential impacts on native species. Methods. Flowering intensity of Chionochloa was assessed by counting flowering tillers on permanent transects (2003–06). Mouse density was estimated with capture–mark–recapture trapping in alpine (2003–07) and forest (2003–04) habitats. Mice were also collected and their stomach contents analysed. Flowering or fruiting of alpine shrubs and herbs, and beech seedfall at forest sites, were also measured. Key results. Chionochloa flowered profusely in austral summer 2005/06. Between autumn (May) and spring (November) 2006, mean alpine mouse density increased from 4 ha–1 to 39 ha–1, then declined to 8 ha–1 by autumn (May 2007). No mice were captured in 768 trap-nights during the following spring (November 2007). Prior to the mouse irruption, mouse density was consistently higher at alpine (0.4–4.0 mice ha–1) than at montane forest (0.02–1.8 mice ha–1) sites (in 2003–04). Alpine mouse diet was dominated by arthropods before mast flowering, and by seeds during it. Conclusions. The density and dynamics of alpine mice in relation to intensive snow-tussock flowering were similar to those in New Zealand beech forest in relation to beech masts. Implications. We predict the timing and duration of periods of heightened predation risk to native alpine fauna, as the result of pulses in mouse density and likely associated pulses in the density of stoats (Mustela erminea), a key exotic predator.

Estimating the potential for reinvasion by mammalian pests through pest-exclusion fencing

Pest mammals are completely excluded from Maungatautari Ecological Island, New Zealand, by a 47-km Xcluder pest-proof fence; however, they are commonly sighted directly outside, along the fenceline. Permanent pest exclusion relies on maintaining fence integrity, and enhancing knowledge of pest activity and behaviour at fenced reserves. We describe summer and winter periods of activity and behaviour of mammalian pests directly adjacent to the pest-proof fence. We (1) tested for the effects of adjacent habitat type, breach type and season on the rate of mammalian pest sightings directly at the fence, (2) determined how quickly pest mammals may locate a fence breach, and how likely they are to exploit it, and (3) developed a predictive model to help assess the probability of a pest gaining entry to the sanctuary if repair to a fence breach is delayed. Observations inside the rolled fence hood provided firm evidence that rats travel and forage extensively in this artificial although highly acceptable aboveground habitat, much more than on the ground. We confirm and emphasise that mammalian pests are constantly testing the pest-proof fence. Pests are very common directly outside the fence, and within 24 h there is a very high likelihood that a fence breach will be located and exploited. The greatest threat of reinvasion comes (1) nocturnally, (2) from rodents and (3) in the summer; however, these results also confirm that there is constant risk from multiple pest species, regardless of time of day or season.

Cohort effects in red squirrels: the influence of density, food abundance and temperature on future survival and reproductive success

1. Environmental conditions experienced early in life may have long-lasting effects on individual performance, thereby creating 'silver-spoon effects'. 2. We used 15 years of data from a North American red squirrel (Tamiasciurus hudsonicus Erxleben) population to investigate influences of food availability, density and spring temperature experienced early in life on reproduction and survival of female squirrels during adulthood. 3. We found that spring temperature and food availability did not affect female survival after 1 year of age, whereas higher squirrel densities led to lower survival, thereby affecting longevity and lifetime fitness. 4. In addition, both food availability experienced between birth and weaning, and spring temperature in the year of birth, had long-lasting positive effects on female reproductive success. These results emphasize the critical effect environmental conditions during the early life stages can have on the lifetime performance of small mammals. 5. These long-term effects of early food and temperature were apparent only once we controlled for conditions experienced during adulthood. This suggests that silver-spoon effects can be masked when conditions experienced early in life are correlated to some environmental conditions experienced later in life. 6. The general importance of silver-spoon effects for adult demographic performance might therefore be underestimated, and taking adult environment into account appears to be necessary when studying long-term cohort effects.

Understanding contributions of cohort effects to growth rates of fluctuating populations

1. Understanding contributions of cohort effects to variation in population growth of fluctuating populations is of great interest in evolutionary biology and may be critical in contributing towards wildlife and conservation management. Cohort-specific contributions to population growth can be evaluated using age-specific matrix models and associated elasticity analyses. 2. We developed age-specific matrix models for naturally fluctuating populations of stoats Mustela erminea in New Zealand beech forests. Dynamics and productivity of stoat populations in this environment are related to the 3-5 year masting cycle of beech trees and consequent effects on the abundance of rodents. 3. The finite rate of increase (lambda) of stoat populations in New Zealand beech forests varied substantially, from 1.98 during seedfall years to 0.58 during post-seedfall years. Predicted mean growth rates for stoat populations in continuous 3-, 4- or 5-year cycles are 0.85, 1.00 and 1.13. The variation in population growth was a consequence of high reproductive success of females during seedfall years combined with low survival and fertility of females of the post-seedfall cohort. 4. Variation in population growth was consistently more sensitive to changes in survival rates both when each matrix was evaluated in isolation and when matrices were linked into cycles. Relative contributions to variation in population growth from survival and fertility, especially in 0-1-year-old stoats, also depend on the year of the cycle and the number of transitional years before a new cycle is initiated. 5. Consequently, management strategies aimed at reducing stoat populations that may be best during one phase of the beech seedfall cycle may not be the most efficient during other phases of the cycle. We suggest that management strategies based on elasticities of vital rates need to consider how population growth rates vary so as to meet appropriate economic and conservation targets.

A silver spoon for a golden future: long-term effects of natal origin on fitness prospects of oystercatchers (Haematopus ostralegus)

1. Long-term effects of conditions during early development on fitness are important for life history evolution and population ecology. Using multistrata mark-recapture models on 20 years of data, we quantified the relation between rearing conditions and lifetime fitness in a long-lived shorebird, the oystercatcher (Haematopus ostralegus). We addressed specifically the relative contribution of short- and long-term effects of rearing conditions to overall fitness consequences. 2. Rearing conditions were defined by differences in natal habitat quality, in which there is a clear dichotomy in our study population. In the first year of life, fledglings from high-quality natal origin had a 1.3 times higher juvenile survival. Later in life (age 3-11), individuals of high-quality natal origin had a 1.6 times higher adult prebreeder survival. The most striking effect of natal habitat quality was that birds that were reared on high-quality territories had a higher probability of settling in high-quality habitat (44% vs. 6%). Lifetime reproductive success of individuals born in high-quality habitat was 2.2 times higher than that of individuals born in low-quality habitat. This difference increased further when fitness was calculated over several generations, due to a correlation between the quality of rearing conditions of parents and their offspring. 3. Long-term effects of early conditions contributed more to overall fitness differences as short-term consequences, contrary to common conceptions on this issue. 4. This study illustrates that investigating only short-term effects of early conditions can lead to the large underestimation of fitness consequences. We discuss how long-term consequences of early conditions may affect settlement decisions and source-sink population interactions.

Matching productivity to resource availability in a small predator, the stoat (Mustela erminea)

Stoats (Mustela erminea), introduced to New Zealand in the late nineteenth century, are common in New Zealand beech (Nothofagus sp.) forests, where populations of feral house mice (Mus musculus) fluctuate between years much as voles do in the northern hemisphere. We present new field evidence and two models demonstrating (i) a strong correlation between density indices for young stoats in summer and for mice in the previous spring, and (ii) a significant linear relationship between productivity per female and spring density of mice up to 25 mice captures per 100 trap-nights. These models confirm that short-lived small mustelid predators dependent on fluctuating populations of prey have evolved means of matching their productivity to the prospects of success across a wide range, from total failure in rodent crash years to >12 independent young per female in rodent peak years. We suggest that the enhanced reproductive success of female stoats when rodents are abundant is due to a combination of critical improvements in both the reproductive physiology and the foraging behaviour of female stoats in rodent peak years. Conversely, a drastic shortage of rodents increases the mortality of embryos and nestlings, while the adult females are able to survive, and even remain relatively fat, on other foods.