Energy flow through small mammal populations

In order to compile an annual or seasonal energy budget for a small mammal population, it is necessary to derive estimates of standing crop, consumption, egestion, digestion, excretion, assimilation, respiration, growth and reproduction. This review gives an account of the methods available for compiling such budgets, and uses results obtained from a study of Microtus agrestis in south west Britain as a worked example. This population was live–trapped on twenty occasions during the course of 2 years. Total energy flow was 309 kJ/m² in the first year and 173 kJ/m² in the second. The average coefficient of digestibility of the natural diet was 54–3%, the average net production efficiency was 1 –0% and the average gross ecological efficiency was 1 –2%. Peak energy flow occurred in spring and late summer each year, and coincided with peaks in the abundance and growth rate of the grasses which formed the main source of food. Previously published estimates show that the annual flow of energy is higher amongst small mammal populations inhabiting open habitats, such as grasslands, than it is in successionally more mature habitats, such as woodlands.     

Production efficiency in small mammal populations

Summary The data used to analyse the relationship between production (P) and respiration (R) in small mammals (Grodziski and French 1983) are reanalysed and different conclusions are drawn. Differences in production efficiency cannot be attributed to any particular trophic classe, but in the higher taxonomic categories the murids have higher production effeciency than the rest. However examination of numerous populations of the same species shows that production efficiencies of congeneric species sometimes differ significantly and show differences which exceed those between higher taxa. The same applies to trophic classes. The low slope described for the regression of P on R for Insectivora is due to an aberrant set of data for Sorex araneus. Production efficiency is directly related to litter size in rodents and inversely to survival of adults. This result conflicts with much earlier work and is discussed at length.     

A mathematical analysis of small mammal populations

Populations of Microtus montanus, the montane vole, have been extensively studied. It is known that their reproductive activity is closely linked to the availability of the chemicals in growing plants. We use a mathematical model here to study how the length of the vegetative season and the natural reproduction rhythm of voles are involved in the long term dynamics of the population numbers. In particular, we use data obtained from Timpie Springs, Utah, and from Jackson Hole, Wyoming, to formulate a model. The novelty of this model is its use of littering curves that highlight the temporally discrete nature of vole reproduction. The model shows how the timing of the vegetative season can influence vole population sizes.This work was supported in part by NSF Grant No. MCS-82-08986 and DMS85-14000 (FCH) and by the University of Utah where Dr. Murphy was a visitor     

Environmental heterogeneity and population density affect the functional diversity of personality traits in small mammal populations

Understanding factors affecting the functional diversity of ecological communities is an important goal for ecologists and conservationists. Previous work has largely been conducted at the community level; however, recent studies have highlighted the critical importance of considering intraspecific functional diversity (i.e. the functional diversity of phenotypic traits among conspecifics). Further, a major limitation of existing literature on this topic is the lack of empirical studies examining functional diversity of behavioural phenotypes—including animal personalities. This is a major shortcoming because personality traits can affect the fitness of individuals, and the composition of personalities in a population can have important ecological consequences. Our study aims to contribute to filling this knowledge gap by investigating factors affecting the functional diversity of personality traits in wild animal populations. Specifically, we predicted that the richness, divergence and evenness associated with personality traits would be impacted by key components of forest structure and would vary between contrasting forest types. To achieve our objective we conducted a fully replicated large-scale field experiment over a 4 year period using small mammal populations as a model system. We found that greater heterogeneity in the cover of shrubs, coarse woody debris and canopy cover was associated with a greater richness, lower divergence and lower evenness in personality traits. Greater population density was associated with greater functional richness and lower functional divergence and evenness of personality traits. To maintain a behaviourally diverse population and its associated functions, managers may promote heterogeneity in vegetation and increased population density, which we found to be the most important determinants driving functional diversity of personality traits.     

Survival of small populations under demographic stochasticity.

We estimate the mean time to extinction of small populations in an environment with constant carrying capacity but under stochastic demography. In particular, we investigate the interaction of stochastic variation in fecundity and sex ratio under several different schemes of density dependent population growth regimes. The methods used include Markov chain theory, Monte Carlo simulations, and numerical simulations based on Markov chain theory. We find a strongly enhanced extinction risk if stochasticity in sex ratio and fluctuating population size act simultaneously as compared to the case where each mechanism acts alone. The distribution of extinction times deviates slightly from a geometric one, in particular for short extinction times. We also find that whether maximization of intrinsic growth rate decreases the risk of extinction or not depends strongly on the population regulation mechanism. If the population growth regime reduces populations above the carrying capacity to a size below the carrying capacity for large r (overshooting) then the extinction risk increases if the growth rate deviates from an optimal r-value.     

On the problem of trap-response types of small mammal populations

1. So far as the population data thus far amassed go, commensal rats in residential habitats are of type II; house mice may be of type I or III in fields but might show type II in residences, and fieral species or subspecies may range from type I to III. Altogether alloresponsive populations seem to be much commoner than the isoresponsive.
2. It is as yet little disclosed to what degree the differentiation of the type is correlated with speciation or subspeciation, even if only a proof that a subspecific population would hold type III as its fixed feature is given. The response types are supposed to be associated with the levels of population shyness. Hence some characters reflected in those may possibly be connected with sexual isolation provoking speciation.
3. Some awkward respects due to the heterogeneous trappability, depending upon the trap-response type, among a mixed population of marked and unmarked animals have been confirmed regarding the methodology for estimating parameters.

     

The ecological connections of dispersal in small mammal populations]

The dispersal of 27 Holarctic species of rodents was quantitatively described on the base of long-term catch method. The relative proportion of non-territorial individuals is an important parameter that reflects the intensity of dispersal. This parameter is statistically related to population (abundance and population type) and species characters (the size of species range and the level of intraspecific differentiation). The contribution of species characters to the variance of migrant portion is lower than contribution of population ones. Isolation and population specialisation are accompanied by the decrease in the portion of disperser, while in pessimal environment the increase in population fluidity is observed. Intensity of dispersal is positively correlated with the size of species range and negatively--with the level of intraspecific differentiation. Thus the dispersal in populations of small mammals serves for colonisation and reparation, providing the stable existence of given group in variable environment.     

Genetic and demographic dynamics of small populations of Silene latifolia

Small local populations of Silene alba, a short-lived herbaceous plant, were sampled in 1994 and again in 1999. Sampling included estimates of population size and genetic diversity, as measured at six polymorphic allozyme loci. When averaged across populations, there was very little change between samples (about three generations) in population size, measures of within-population genetic diversity such as number of alleles or expected heterozygosity, or in the apportionment of genetic diversity within and among populations as measured by F(st). However, individual populations changed considerably, both in terms of numbers of individuals and genetic composition. Some populations doubled in size between samples, while others shrank by more than 75%. Similarly, expected heterozygosity and allele number increased by more than two-fold in individual populations and decreased by more than three-fold in others. When population-specific change in number and change in measures of genetic diversity were considered together, significant positive correlations were found between the demographic and genetic variables. It is speculated that some populations were released from the demographic consequences of inbreeding depression by gene flow.     

Habitat structure and disturbance affect small mammal populations in Mediterranean forests

Habitat structure, the physical arrangement of objects in space, affects animal populations, ecological interactions, species diversity, and ecosystem functioning. Conventional forest management practices that reduce habitat structural complexity are thus under reappraisal, and there are increasing efforts to understand how habitat structure affects functionally important species. We investigate here the effect of habitat structures on small mammal dens and their interactions with widespread antagonistic species (wild boars). Among habitat structures we considered tree dimension (diameter), shrub cover, rock cover, dead wood volume, and stump area. Ground cover with rooting signs measured wild boar disturbance. The number of small mammal dens was related positively to rock cover and tree diameter, and negatively to wild boar disturbance. Additionally, a positive interaction between wild boar disturbance and tree diameter emerged, showing that the effect of big trees on small mammal nesting was more evident with higher levels of disturbance. This study suggests that habitat structures that can be affected by forest management provide effective refuges for functionally important species. The effect of habitat structure on species survival and interactions thus needs to be addressed to understand ecosystem management and functioning.     

Synchrony in small mammal populations of montane forest patches in southern India

1. Small mammal populations were studied in montane evergreen forests in the Nilgiris, southern India, using live-trapping from January 1994 to September 1996. Two sites were selected, one with a single large forest patch and adjoining plantations, and the other with several small forest patches separated by grassland.
2. Nine species were recorded, of which eight were trapped in the forest patches, two in the grasslands and five in plantations. Rattus rattus was the most abundant species (2–36 individuals ha⁻¹) followed by Suncus montanus (0–11 individuals ha⁻¹). Densities of seven other species recorded were very low.
3. Synchrony in various population variables – density, biomass, mean weight, density of adults and adult females, and proportion of adults, adult females and sub-adults – was examined for Rattus rattus populations in the forest patches and plantations. Density and biomass were studied in seven other species recorded at these sites. Spearman's rank order correlation was used as a measure of synchrony between the population variables.
4. Within-site synchrony was higher than between-site synchrony in population characters. Synchrony was also higher between plots within the unfragmented site than they were between plots in the fragmented site. Relatively high synchrony in proportions of adults, adult females and juveniles in the forest patches implied that breeding is probably influenced by climate and food availability, which are seasonal in this habitat.
5. Given the small patch sizes (≈1–10 ha) and low population sizes, asynchrony is likely to be an outcome of demographic and environmental stochasticity, and low dispersal rates may impede establishment of synchrony.     

The effects of fire and grazing pressure on vegetation cover and small mammal populations in the Maasai Mara National Reserve

An extensive study of vegetation changes as a consequence of fire and grazing pressure and their effect on small mammal populations inside the Maasai Mara National Reserve, Kenya, was carried out during May–June 1997. Comparison of vegetation maps from 1979 and 1998 suggested that vegetation in 46% of the Reserve area converted from shrubland to grassland, possibly as a result of fire and grazing pressure. We tested the hypothesis that in areas with high fire and grazing impact the population of small mammals was negatively affected. A low density of rodents was recorded in all habitats except in areas of human activity, where artificial resources are constantly present. Capture efforts were unsuccessful in grasslands. Our results confirm those of Norton‐Griffiths (1979) and Dublin (1995) , i.e. that fire and grazing pressure impact the vegetation of the Serengeti–Mara ecosystem and limit the natural regeneration of woodlands. This indirectly affects the small mammal community, which is limited in its long‐term establishment.     

Marine mammal culling programs: review of effects on predator and prey populations

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Seasonal regulation in fluctuating small mammal populations: feedback structure and climate

We studied fluctuating populations of six small mammal species in the Appalachian Plateau of Pennsylvania, USA for 20 yr. We analyzed the feedback structure of these species using statistical time series models for spring and autumn abundances. All species showed a seasonal density-dependent structure, and in five of them first-order feedbacks were dominant in winter and summer. Instead, southern red-backed voles ( Clethrionomys gapperi ) showed a different feedback structure during winter and summer. In three species ( C. gapperi , Napaeozapus insignis and Peromyscus maniculatus ), environmental factors were more important during summer, while the opposite pattern was found in Blarina brevicauda and Tamias striatus . Snowfall influenced positively the winter population growth rates of southern red-backed voles, white-footed mice, woodland jumping mice and eastern chipmunks. We found seasonal differences in the effects of the small mammals assemblage on population growth rates of the two Peromyscus species. The common feedback structure between seasons observed in most of the species, particularly among voles and mice, points to a different feedback structure from northern cyclic small mammals. We conclude that a seasonal feedback structure dominated by intra- and inter-specific competitive interactions may be at the basis of the population dynamics of these species.     

Inbreeding depression and drift load in small populations at demographic disequilibrium

Inbreeding depression is a major driver of mating system evolution and has critical implications for population viability. Theoretical and empirical attention has been paid to predicting how inbreeding depression varies with population size. Lower inbreeding depression is predicted in small populations at equilibrium, primarily due to higher inbreeding rates facilitating purging and/or fixation of deleterious alleles (drift load), but predictions at demographic and genetic disequilibrium are less clear. In this study, we experimentally evaluate how lifetime inbreeding depression and drift load, estimated by heterosis, vary with census (N_c) and effective (estimated as genetic diversity, H_e) population size across six populations of the biennial Sabatia angularis as well as present novel models of inbreeding depression and heterosis under varying demographic scenarios at disequilibrium (fragmentation, bottlenecks, disturbances). Our experimental study reveals high average inbreeding depression and heterosis across populations. Across our small sample, heterosis declined with H_e, as predicted, whereas inbreeding depression did not vary with H_e and actually decreased with N_c. Our theoretical results demonstrate that inbreeding depression and heterosis levels can vary widely across populations at disequilibrium despite similar H_e and highlight that joint demographic and genetic dynamics are key to predicting patterns of genetic load in nonequilibrium systems.     

A long term study of small mammal populations in a Brazilian agricultural landscape

Long-term monitoring of small mammal populations is very important to understand the variations in temporal abundance on a large time scale, which are related to ecological, economic and epidemiological phenomena. The aim of this study is to monitor the populations of the marsupials Didelphis aurita and Philander frenatus and the rodents Nectomys squamipes, Akodon cursor and Oligorysomys nigripes in a locality of typical Brazilian rural landscape, Sumidouro Municipality, Rio de Janeiro State, Brazil. A mark-recapture study was conducted during five years. We analyzed the population dynamics, the reproduction and age structure of these species. Both marsupials presented higher population sizes in the end of wet period and beginning of the dry period, which can be explained by the seasonal reproduction which begins in the middle of the dry period and ends in the last months of the wet period. N. squamipes reproduced throughout the year but mostly during rainy periods, due to the close association of this rodent to resources found in the water. Higher survivorship and recruitment rates were in the end of the wet season. The rodent A. cursor had an opportunist reproduction, resulting in high turnover rates. Survivorship increased with the effects of the dry periods. O. nigripes showed a clear annual pattern of population cycle with peaks during the dry season. The rodents did not show potential to present outbreaks and become agricultural pests. The annual population cycles of O. nigripes and the unique peak of A. cursor population during five years highlight attention to their importance as wild reservoirs of the hantavirus disease. Their ecological characteristics associated to their opportunistic behavior make these species prone to be good reservoirs of zoonoses.     

Genetic analysis reveals demographic fragmentation of grizzly bears yielding vulnerably small populations

Ecosystem conservation requires the presence of native carnivores, yet in North America, the distributions of many larger carnivores have contracted. Large carnivores live at low densities and require large areas to thrive at the population level. Therefore, if human-dominated landscapes fragment remaining carnivore populations, small and demographically vulnerable populations may result. Grizzly bear range contraction in the conterminous USA has left four fragmented populations, three of which remain along the Canada-USA border. A tenet of grizzly bear conservation is that the viability of these populations requires demographic linkage (i.e. inter-population movement of both sexes) to Canadian bears. Using individual-based genetic analysis, our results suggest this demographic connection has been severed across their entire range in southern Canada by a highway and associated settlements, limiting female and reducing male movement. Two resulting populations are vulnerably small (< or =100 animals) and one of these is completely isolated. Our results suggest that these trans-border bear populations may be more threatened than previously thought and that conservation efforts must expand to include international connectivity management. They also demonstrate the ability of genetic analysis to detect gender-specific demographic population fragmentation in recently disturbed systems, a traditionally intractable yet increasingly important ecological measurement worldwide.