Since 1977 we have been conducting experiments in which we add supplemental seeds or remove certain combinations of species of seed—eating rodents and ants from 0.25—ha plots in the Chihuahuan Desert of southeastern Arizona. These experiments evaluate the extent to which food availability and interspecific competition influence rodent populations. Monitoring with live traps revealed that: (1) the addition of seed at the rate of 96 kg°plot—1°yr—1 resulted in an increased density of the largest granivorous rodent species (Dipodomys spectabilis), decreases in the densities of the two next—to—largest species (D. merriami and D. ordii), and no detectable changes in the densities of other rodents; (2) the removal of D. spectabilis, as well as other experimentally induced changes in the abundance of this species, resulted in reciprocal shifts in the densities of the two congeneric species, D. merriami and D. ordii, and no significant changes in densities of other rodents; and (3) the removal of all three Dipodomys species resulted in large increases in density of four of the five species of smaller seed—eating rodents, but had no effect on two species of insectivorous rodents. Taken together, these results indicate that limited food resources and interspecific competition pay major roles in regulating the density of rodent populations and determining the organization of desert rodent communities. However, the responses of the rodent populations to our manipulations were unexpectedly complex; they included long time lags, asymmetrical interactions, and little compensation in energy consumption. This indicates how much remains to be learned about the processes that determine the structure and function of even this relatively simple and well—studied community.
We outline a computer model of heat and mass transfer through flesh, fat and porous fur for endotherms of any dimensions. We then validate it with a series of laboratory studies. Finally, we explore applications of the model to Bergmann's rule, predicting the mouse-to-elephant curve, climatedisease-toxicant interactions, animal 'design' via genetic engineering and energetic constraints on community structure.As a first test of the model we present calculations and metabolic chamber measurements for mammals ranging in size from mice to Holstein calves. We then compare simultaneous measurements on deer mice, Peromyscus maniculatus, of oxygen consumption, doubly labelled water turnover and food consumption with calculations of metabolic rate using body temperature radio-telemetry as input to the endotherm model. The endotherm model derived in the Appendix requires data on allometry (body dimensions, surface area), fur properties, core temperature, air and radiant temperatures and wind speed. The model is useful for calculating energetic expenditure in different microclimates without the need for extensive physiological measurements in the laboratory. Model predictions of metabolic rate at 12°C and at 22°C were well correlated with each of the three empirical estimates. The model shows that the posture an animal assumes can influence measurements of metabolic rate. Model calculations of metabolic rate using postures ranging from a curled-up ball-like geometry to a sprawled-out, cylinder or ellipsoid geometry bracket all three sets of simultaneous empirical data taken on the same animals.Applications of the model show that it can be applied in a wide variety of circumstances to gain insight into physiological and ecological problems.
Larger species of seed-eating desert rodents were excluded from experimental plots while smaller, potentially competing species were allowed to enter. Density of small granivores on these plots increased to nearly 3.5 times that on control plots but only after 8 months. These results indicate that interspecific competition affects the abundance of desert rodents; they also support indirect evidence that competition for seeds influences the organization of desert rodent communities.
One way to study a parasite's effect on the individual and population ecology of its host is to examine effects on the host's energy budget. A relatively innocuous effect of a gut parasite, such as decreased digestive efficiency, can potentially translate into an effect costly to host fitness, such as decreased reproduction, if other compensations (such as increased rate of food intake) do not occur. We found that infection by the tapeworm Hymenolepis citelli caused a 2% drop in dry—matter digestibility in host white—footed mice (Peromyscus leucopus). A crowding experiment indicated that this result should be applicable to a wide range of intensities of infection. However, we detected no compensation for this decreased digestive efficiency either in amount of food consumed or in mass change (which would indicate use of fat stores or changes in growth). In field experiments we used doubly labeled water to measure effects of tapeworms on field metabolic rate and water influx (potentially a measure of food intake rate), and temperature—sensitive transmitters to measure body temperature. We detected no compensations via these routes either. Our failure to detect compensations indicates that in the nonreproductive mice studied the decrease in digestive efficiency is not of sufficient importance to engender substantial compensations and is therefore unlikely to lead to fitness—altering effects. It is in reproductive animals or in animals subjected to food shortage that such effects would be expected.
Aim The introduction of non‐native species into aquatic environments has been linked with local extinctions and altered distributions of native species. We investigated the effect of non‐native salmonids on the occupancy of two native amphibians, the long‐toed salamander (Ambystoma macrodactylum) and Columbia spotted frog (Rana luteiventris), across three spatial scales: water bodies, small catchments and large catchments.Location Mountain lakes at ≥ 1500 m elevation were surveyed across the northern Rocky Mountains, USA.Methods We surveyed 2267 water bodies for amphibian occupancy (based on evidence of reproduction) and fish presence between 1986 and 2002 and modelled the probability of amphibian occupancy at each spatial scale in relation to habitat availability and quality and fish presence.Results After accounting for habitat features, we estimated that A. macrodactylum was 2.3 times more likely to breed in fishless water bodies than in water bodies with fish. Ambystoma macrodactylum also was more likely to occupy small catchments where none of the water bodies contained fish than in catchments where at least one water body contained fish. However, the probability of salamander occupancy in small catchments was also influenced by habitat availability (i.e. the number of water bodies within a catchment) and suitability of remaining fishless water bodies. We found no relationship between fish presence and salamander occupancy at the large‐catchment scale, probably because of increased habitat availability. In contrast to A. macrodactylum, we found no relationship between fish presence and R. luteiventris occupancy at any scale.Main conclusions Our results suggest that the negative effects of non‐native salmonids can extend beyond the boundaries of individual water bodies and increase A. macrodactylum extinction risk at landscape scales. We suspect that niche overlap between non‐native fish and A. macrodactylum at higher elevations in the northern Rocky Mountains may lead to extinction in catchments with limited suitable habitat.
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