The populations of two coexisting species of Dipodomys (Heteromyidae, Rodentia) were manipulated on 10, large, unenclosed, trapping grids. These manipulations revealed that, although many kangaroo rats are established residents in an area, a large number are transient individuals who quickly occupy vacated habitats. On plots from which residents had been removed, transients settled at rates of up to 5% of carrying capacity per day. These immigrants were invariably of the same species that was removed, indicating a strong element of intraspecific competition with little or no evidence of competition between the species.Trapping records suggest that these species avoid competition through habitat selection. Dipodomys ordii prefer a grassier habitat, and D. merriami a habitat dominated by creosote bush. Apparent overlap in their utilization of habitats, based on sites of capture, predicts competition coefficients to be higher than those permitted by the theory of limiting similarity and much higher than those actually shown by the perturbation experiments.This study demonstrates the dangers of estimating alpha without experimentation. This is especially true in cases where habitat selection may be important, since organisms may travel in habitats without collecting resources therein. Our results are discussed in light of a theory which examines the optimal (rather than tolerable) amount of overlap in habitat utilization between two potential competitors in a mixed habitat. This theory predicts that the pressure of natural selection should eliminate the interspecific competition entirely.However, the conclusion that the interspecific competitive alpha is zero does not lead to the conclusion that interspecific competition is unimportant in the system. Instead, if our interpretation is correct, such competition has molded the system, and were there not a continual threat of interspecific competition, the habitat specializations would soon disappear.

Radiotracking and live—trapping techniques were used to determine the movement patterns of Dipodomys spectabilis in central New Mexico, USA. In contrast with the live—trapping results which suggest extensive home range overlap, 133 h of radiotracking over a 2—yr period revealed that most individuals exclude other bannertails from a circular home range of approximately 0.05 ha with a centrally located burrow. Home range size remained relatively constant during the warmer months. The extent and intensity of activity varied greatly among rats, and although the contents of food caches changed qualitatively during the year, no seasonal foraging pattern was evident. The average bannertail spent 58% of the night hours inside or on the mound and less than 22% of the time more than 6 m from the mound. There was no evidence that moonlight affected activity. Foraging is an efficient process, perhaps accomplished concurrently with other activities. The tendency of this species to store large quantities of seed heads collected at an optimal stage of development from a feeding territory suggests that the bannertial may manage its food resources to an extent seen in few other rodent species.

High-resolution radio-tracking was used to study the habitat affinities of Dipodomys merriami, D. ordii, and D. spectabilis in an area of mixed shrubs and grasses in central New Mexico. Detailed habitat maps were examined to place the locations of stops in one of seven habitat classes, and preferences were identified by comparing the number and duration of stops with that expected if habitat use were proportional to habitat abundance. Neither Dipodomys merriami nor Dipodomys ordii used habitats in the available proportions. Both stopped more frequently at burrow locations beneath Larrea, the dominant shrub, and also used sand dunes more than expected. Dipodomys merriami avoided grassy areas entirely. These two species also used significantly different mixes of habitats. Analyses limited to stops either in the open or beneath shrubs showed that D. ordii preferred the shrubs and that D. merriami had no relative preference for either habitat. Limited observations of D. spectabilis showed that they avoided shrubby habitats.These results support the hypothesis that habitat selection facilitates coexistence between Ord's and Merriam's kangaroo rats. However, the adequacy of this mechanism to account fully for the coexistence of these species is questioned. An alternative, nonequilibrium hypothesis is proposed, which is based on the assumptions that neither species has a clear competitive advantage in the mixture of habitats where they coexist, and that the course of the interaction is frequently disturbed by migration from nearby, allopatric populations.

Lead and cadmium concentrations were measured in eggs and tissues of downy young, prefledgling, and adult Royal and Sandwich Terns collected from Galveston Bay, Texas. These species do not differ significantly in the amount of heavy metals they accumulate; however, they differed from other species of the same trophic level from Galveston Bay in degree and pattern of temporal accumulations of these metals. Lead levels were lower in terns, but cadmium levels in adult terns were higher than those of other species. The prefledglings of other species at this trophic level have lead and cadmium concentrations which are similar to those found in adults. In contrast, the concentrations of the metals in adult terns are markedly higher than those found in prefledgling terns. This suggests that accumulation of metals continues as the birds age.

The concentrations of lead, cadmium, and manganese in the tissues of cattle egrets (Bubulcus ibis) and laughing gulls (Larus atricilla) gathered from the Galveston Bay region of Texas were compared, to determine if different patterns of accumulation exist. Their levels in these species were within the range reported for other bird species. Lead levels in bone were comparable, but gulls had more lead in brain, kidney and liver tissues than the egrets, which suggested a higher rate of accumulation or exposure. Due to their high abundance and comparable positions in the estaurine and terrestrial food webs, it is suggested that Bubulcus ibis and Larus atricilla may serve as convenient biological indicators to monitor potentially toxic substances in these ecosystems.