Rodent Removal Research Articles

Granivory and Competition as Determinants of Annual Plant Diversity in the Chihuahuan Desert

In the Chihuahuan Desert of southeastern Arizona, we monitored the responses of three seasonal classes of annual plants (winter, summer and biseasonal species) to experimental removal of seed-eating rodents and ants over a ten-year period (1978 through 1987). Rodent removal led to increased dominance of the winter annual flora by initially rare, but competitively superior, large-seeded annuals. The greater densities of these plants eventually suppressed populations of small-seeded annuals. In contrast, ant removal produced higher densities of small-seeded species, whose increases had no measurable effect on populations of large-seeded winter annuals. Competition between large-seeded and small-seeded winter annuals is highly asymmetric and consistently favors species with greater seed reserves. As selective predators of large-seeds, rodents behave as keystone consumers, differentially suppressing the dominant competitors and helping to maintain the diversity of winter annuals. The population dynamics of winter and summer annuals were linked through interactions with biseasonal annuals early in the study but not in later years when densities of the biseasonal annuals declined sharply in response to summer droughts. Nevertheless, the periodic abundance of biseasonal annuals (and some herbaceous perennials) during germination of summer annuals might help to explain the insignificance of large-seeded annuals in the summer flora. Large-seeded summer annuals were rare and showed little response to rodent removal, and both rodents and ants used the seeds of small-seeded summer plants. Although rodent predation on winter annuals may facilitate harvester ants indirectly by increasing the availability of small seeds, ants and rodents appear to compete exploitatively through populations of summer annuals. The conflicting influence of rodents on ant populations through the two seasonal classes of annuals may help to explain why ant populations have shown little response to rodent removal during the ten-year experiment. Results here contrast with earlier experiments in the Sonoran Desert, where ants and rodents interacted principally through populations of winter annuals, and increasing dominance of large-seeded annuals on rodent-removal plots eventually led ant populations to decline. However, the climatic and biotic differences between the Sonoran and Chihuahuan Deserts fail to mask the importance of rodents as keystone consumers of winter annuals. This strong indirect pathway is the most prominent feature of

Response of bank voles to a new source of food and its withdrawal

The response of bank voles Clethrionomys glareolus (Schreber, 1780) toward a new source of food and its later withdrawal was studied in a forest environment. Oat grain was used as a kind of food new to rodents in this natural habitat. The process of finding such new supply of food was studied in five groups of stations (23 in each) where oat grain was offered in 1 to 5-day periods. The response of voles toward withdrawal of the food source was studied in four groups of stations (again 23 in each) with variable time passing between food withdrawal and subsequent removal of rodents by snap traps. In the control group of 23 stations no oat grain was offered. The numbers of voles assessed by a regression line for subsequent days of trapping in stations without oat grain and in those where oat 'meals' were offered 1 to 4 days did not differ. It suggested rather rapid location of new food source. In 5th day there was an increase in numbers of voles coming to the sites with food offered. In the second part of experiment, i.e. this designed to assess the response of voles toward withdrawal of food source, the regression analysis revealed variable numbers of voles: the longer time passing from food withdrawal, the lower number of voles frequenting the site compared with the control sites, i.e. some voles avoided, for couple of days, the sites where new food had been withdrawn.

Factors Limiting Populations of Arid-Land Darkling Beetles (Coleoptera: Tenebrionidae): Predation by Rodents

The role of rodent predation as a limiting factor in the abundance of sixspecies of darkling beetles (Tenebrionidae) was examined in a sagebrush–steppe ecosystem (Kemmerer, Wyo.). Rodent predators were identified in feeding experiments using captive rodents and darkling beetles. Grasshopper mice ( Onychomys ) and deer mice ( Peromyscus ) successfully captured and consumed Eleodes extricatus Say. Ground squirrels ( Spermophilus ) and pocket mice ( Perognathus ) initially fed upon E. extricatus but developed an aversion to them after several exposures. Subsequent experiments demonstrated that the chemical aerosol produced by E. extricatus as an antipredator defense was effective in providing escape time during deer mouse attacks but not during grasshopper mouse attacks. A replicated field experiment, employing 10 rodent exclosures (15 by 15 m) and 10 control plots, showed that the removal of rodent predators resulted in population increases in five of the six species of darkling beetles ( Coniontis ovalis, E. extricatus, E. pimelioides, E. rileyi , and Helops difficilis ). The total population of beetles exhibited a 63% increase in abundance following rodent removal. Based on 1986 field density measurements of rodents (8/ha) and beetles (4,931/ha), an average daily consumption of fewer than two beetles per rodent was calculated to be sufficient to produce the observed differences in darkling beetle populations on the site. Therefore, it is concluded that rodent predation functioned as a significant limiting factor on darkling beetle populations in the sagebrush–steppe study area.

Granivory in the Chihuahuan Desert: Interactions within and between Trophic Levels

We investigated the effects of Chihuahuan Desert granivores on three seasonal classes of plant resource species, the effects of these resource classes on one another, and the way in which interactions through plant resources affect the abundances of seed consumers. At our study site, three seasonally distinct classes of annual plants produce the seeds used by ants and rodents, and two major taxa of resident granivores. Winter annuals and summer annuals have temporally nonoverlapping growth periods. However, one numerically prominent annual, Eriogonum abertianum, germinates with winter annuals, reproduces with summer annuals, and may link the population dynamics of plants in the two groups. Experiments showed that high densities of winter annuals inhibit populations of E. abertianum and that populations of this species can strongly suppress those of summer annuals. Densities of all three classes of annuals are regulated by the combined effects of competition and seed predation. In E. abertianum, interspecific and intraspecific competition appeared to predominate in alternate years and to produce 2—yr abundance cycles, accentuated in magnitude where ants, the major seed predators, had been removed. The period of these cycles appeared to lengthen where rodent removal intensified the effects of interspecific competition. Granivore removal and seed addition experiments showed that competition also helps to regulate some granivore populations. Although rodents reduced ant resources through their effects on seeds of both summer annuals and winter annuals, ants were competitors of rodents only for seeds produced during the weaker winter resource peak. Seeds of E. abertianum were used almost exclusively by ants. Rodents facilitated this plant species indirectly by reducing densities of other winter annuals. Ants and rodents were affected differently by one another's removal. After an initial time delay, workers of Pheidole xerophila increased in numbers and/or activity on rodent removal plots, but colony densities of a second ant species, Pogonomyrmex desertorum, simultaneously declined. Rodents did not compensate measurably in abundance, biomass, or reproductive activity where ants had been eliminated. The explanation for these responses is complex and includes such factors as: (1) seasonality in the production of seed resources and in their use by the two taxa, (2) specialization by ants and rodents on different density distributions of seeds; (3) diffuse compensation, or compensation spread over many species populations; and (4) indirect interaction pathways, mediated through competing resource classes. In general, our experiments show that, despite the climatic variability and unpredictability of desert environments, populations respond to the steady deterministic processes of competition and predation. Nevertheless, comparison of the results of similar experimental studies in the Sonoran and Chihuahuan Deserts demonstrates how climate and seasonality can alter the structure and intensity of interactions in ecosystems.

Granivory in a Desert Ecosystem: Experimental Evidence for Indirect Facilitation of Ants by Rodents

Two major groups of desert granivores, ants and rodents, coexist as permanent residents of local desert habitats in southwestern North America. At our Sonoran Desert Study site, both of the major taxa exhibited short—term increase in density when the other taxon was experimentally removed. Over the longer term, density compensation continued at a relatively constant level for rodents in the absence of ants. In contrast, beginning 82 yr after initiation of experiments, ant populations on rodent removal plots showed a gradual but significant decline relative to densities on control plots. Indirect interactions, mediated through ant and rodent resources, may account for these differences. Removal of harvester ants leads to higher annual plant densities only in small—seeded species. These plants are relatively poor competitors and do not displace the large—seeded annuals, on whose seeds rodents specialize. In contrast, rodent removal leads to a differential increase in large—seeded annuals, which competitively displace the small—seeded resource species of ants. The decline of ant populations on rodent removal plots preceded by several years the first detectable evidence for competitive suppression of small—seeded annuals. Because ants do not excavate buried seed, they probably experienced resource depression before buried seed reserves were exhausted through germination and subsequent competitive inhibition.

Interactions among unrelated species: Granivorous rodents, a parasitic fungus, and a shared prey species.

Granivorous rodents and a parasitic fungus in the Sonoran Desert utilize a common prey species, Erodium cicutarium, a desert annual plant. Experimental removal of rodents from field exclosures resulted in significantly higher densities of E. cicutarium. Fungal infection was significantly higher in the absence of rodents, suggesting that, while they do not interact directly, rodents and the fungus affect each other's densities by their use of a common prey species.

Особливості кримінально-правової регламентації призначення покарання за незакінчений злочин за законодавством зарубіжних країн

У статті аналізуються особливості техніко-юридичного оформлення інституту незакінченого злочину в кримінальному праві деяких зарубіжних країн, розглядаються конструкції приготування і замаху, їх правовий зміст, тлумачення і кваліфікаційне значення.