William Z. Lidicker

Ecological Observations on a Feral House Mouse Population Declining to Extinction

It is clearly an unusual opportunity for an ecologist to be able to observe in detail the extinction of a species over a large area. Quite understandably, ecologists tend to study common species which are not likely to disappear during the time of study. This report documents the events occurring in a population of house mice (Mus musculus), living on 55 acre Brooks Island in San Francisco Bay, as it declined to extinction in a period of little over one year. The study was not initiated with the expectation that the population would become extinct, but rather because the mice occurred on the island in very high densities, and it was hoped that something could be learned about density regulation in this species under these congested, yet completely feral, conditions. Almost exactly coincident with the beginning of my studies on Brooks Island house mice in September of 1958, a few individuals of Microtus californicus were accidentally introduced on to the island from a neighboring islet, and rapidly colonized the entire island. The history of the colonization and exploitation of the island by Microtus is documented in detail in an earlier paper (Lidicker and Anderson 1962). Refer also to that report for a more complete description of the island and its recent history. For our purposes here, it is perhaps sufficient to

Solving the Enigma of Microtine “Cycles”

A frequently expressed opinion is that despite massive efforts the multi-annual cycles in density exhibited by many microtine rodents remain mysterious; moreover, understanding awaits completion of one or a few clever experiments or elegant mathematical models that will discredit all but the one true hypothesis. I contend in this brief, personal essay that, on the contrary, we are actually quite close to an adequate understanding of microtine cycles. This optimistic perspective requires that we adopt a multi-factorial model of vole demography, a position that allows us to comprehend how many extrinsic and intrinsic factors act synergistically and sequentially to produce the density changes we observe. I argue that this approach is not only supported by a modern systems view of reality, but is consistent with the extensive data base accumulated over a 60 year period. A multi-factorial perspective is illustrated by data for Microtus californicus , a well-studied species that shows considerable geographic and temporal variation in demographic pattern. At least eight key factors (four extrinsic and four intrinsic) are required to explain the multi-annual cycles in this species. The resulting model is complex, but not chaotic or non-testable.

The relationship between habitat heterogeneity, space use, and demography in a population of California voles.

The relationships between habitat heterogeneity, space use, and demography were analyzed in a population of California voles (Microtus californicus). Live-trapping was conducted on a monthly basis from 1977 to 1981 on four grids, each sampling a different microhabitat type within a meadow. High quality habitat patches (having high percent cover of the perennial grass Elymus triticoides) were characterized by higher peak densities, more strongly female-biased sex ratios, longer average persistence (= survivorship), and higher rates of juvenile recruitment than were lower quality patches (Conium maculatum and annual grasses). For females, probability of persistence varied markedly in response to seasonal (wet vs. dry) variation in resource quality, while for males, probability of persistence was lower and constant across all seasons. The study included a period of low density (1977-1979) followed by an irruption (spring 1980) and subsequent crash in vole numbers. Persistence of females did not vary with population density, but persistence of males was inversely related to density. Females tended to aggregate in high quality areas but the distribution of males was more uniform. Females co-occurred at individual trap sites within 2-3 d trapping periods significantly more frequently than did males. In addition, the frequency of co-occurrence of females was higher in better quality habitat patches than in poorer microhabitat, whereas the frequency of co-occurrence of males was spatially unvarying. Both sexes showed an increased frequency of co-occurrence with increased density; however, co-occurrence of females was often high even at low densities, while co-occurrence of males was virtually non-existent at low densities. We believe that the responsiveness of females to both spatial and temporal variation in resource quality reflects a direct reliance on resource acquisition for reproductive success, and that their tendency to overlap intrasexually indicates a lack of strong intrasexual interference. In contrast, the lack of responsiveness of males to resource variation may reflect a relative independence from resource acquisition in seeking reproductive success, while their low co-occurrence and greater sensitivity to density variation indicates the presence of strong intrasexual interference.

To disperse or not to disperse: who does it and why?

Dispersal is an integrative and synthetic phenomenon bridging the disciplines of ecology, genetics, behaviour, and evolution. It is therefore a key element in the understanding of a great many population processes (Chapter 1). Moreover, it becomes critical to our understanding of these processes to know which individuals disperse and what factors cause them to initiate this behaviour.

The study of dispersal: a conceptual guide

Probably, the best known example of dispersal is the Biblical story of the Israelites traversing the desert in search of a new land. Since then many examples have been recorded in the literature. Today we know dispersal — or movement from one home site to another — as a phenomenon of potentially great importance to the demographic and evolutionary dynamics of populations. The most obvious effect of dispersal is that it changes the spatial distribution of individuals, i.e. dispersal can lead to changes in dispersion (sensu Odum, 1953; Southwood, 1978). These changes may cause increased clumping, greater randomness, or more even spacing. Unfortunately, considerable confusion persists in the literature over the terms ‘dispersal’ and ‘dispersion’ (in fact in some languages, e.g. Spanish and Norwegian, only a single word is available for both concepts). It is therefore necessary to be alert to this problem in reading the literature.

A Phylogeny of New Guinea Rodent Genera Based on Phallic Morphology

The morphology of the male phallus is described for 19 of the 23 genera of New Guinea rodents; 28 species are included among the 72 penises examined. A total of 66 characters was studied on each penis and the resulting data were carefully analysed for phylogenetic evidence. An interpretation of the relationships among the taxa examined, based solely on the phallic data, is presented in a phylogenetic tree. Comparisons are then made with the earlier phylogenetic hypotheses, which were based on dental and external body features only. Both approaches indicate four major subdivisions in the endemic fauna. An hypothesis is proposed that accounts for the evolution of all endemic forms from only two invading stocks, one in the Pleistocene giving rise to endemic species of Rattus, and one in the Miocene leading to all the other native rodents. In addition, four widespread modern species have reached New Guinea with the help of man. The extant genus most like the hypothetical original murid immigrant is Anisomys. Evidence is presented indicating that Hyomys should be placed in the XJromys group rather than with Pogonomys and Mallomys. Finally, Pogonomelomys is shown to be almost certainly polyphyletic.

Communal winter nesting and food sharing in Taiga Voles

Summary1.In central Alaska, Taiga Voles live in communal groups of five to ten individuals (mean = 7.1) for eight months of the year. During this winter period, they share a common stored food cache.2.Evidence from both field monitoring of nest temperatures and laboratory studies indicates that thermoregulatory advantages accrue from communal nesting. Cooperative defense against predators and food thieves probably also occurs.3.Group members are generally not from the same immediate family, although occasionally female litter mates join the same group. Sex and age composition appear to be the result of random sampling of the population at large in late August.4.We speculate that the non-relatedness of midden groups can be explained partly by a spreading of the risk against predation and/or inbreeding avoidance. However, this does not satisfactorily explain several aspects of this behavior.5.The midden groups provide circumstances favorable to the operation of group selection.

Extra-large body size in California voles : causes and fitness consequences

Extra-large body size in microtine rodents is a ubiquitous feature of peak population densities, and it has been hypothesized that these giant individuals represent a genetically based morphotype that has high fitness under increasing and high densities, and may represent a key element of a genetic polymorphism driving multi-annual cycles (Chitty/Krebs model). We examine this large-size phenomenon (Chitty Effect) in the California vole utilizing three approaches : analyzing the weight distribution in a non-cyclic population over a 13 yr period with comparisons to cyclic populations, analyzing body composition, especially fat content, as a function of body size, and observing the microhabitat distribution of extra-large males in two populations, one of which cycles

The Allee Effect: Its History and Future Importance~!2010-05-06~!2010-06-09~!2010-09-03~!

The role of mutually beneficial interactions (++, cooperation) is a rapidly growing research field in population dynamics, microevolution, and conservation biology. Such positive influences cause destabilizing pressures in population dynamics (anti-regulating factors), and can generate Allee effects. Not only can large demes benefit from such cooperation, but the loss of cooperation in small demes can produce a minimum threshold density. Interest in these phenomena grew rapidly to the middle of the 20th century, followed by about four decades in which interest waned. In the last 20 years attention to Allee effects has burgeoned once again. This renewal has produced new perspectives, including a more realistic framework for the way populations and communities are organized. A core concept for Allee effects emerges from the historical record and current views on population dynamics: Allee effects are demographic consequences of the collective actions of anti-regulating influences. Recent developments, including proposals for much new terminology, are reviewed and found to be helpful in building mechanistic understanding of the core concept. Support for the growing relevance of Allee effects to conservation biology as well as population and community dynamics is emphasized. Some new avenues for future research directions include improving our abilities to predict life history and environmental features that favor strong anti-regulation and hence Allee effects, the role of mutually positive interspecific relations in community function, and possible role of anti-regulation in restoration ecology.

Responses of Small Mammals to Habitat Edges

Ecologists understand the world as a seamless fabric in space and time and are fond of saying that everything is connected to everything else. They also know, however, that said fabric is not homogeneous nor necessarily changing only gradually. Discontinuities are rampant, and so we have “things” defined by those discontinuities or boundaries. Habitat edges are one such type of discontinuity. These edges are critical features in the life of organisms and are essential components to our understanding of how they respond to habitat heterogeneity (Lidicker and Koenig 1996). Although edges have always been important in the functioning of the biosphere, their prevalence is accelerating in a world increasingly dominated by anthropogenic influences.