Small Mam Research Articles

Predicting the direction and magnitude of small mammal disturbance effects on plant diversity across scales

Despite years of research on small mammal disturbance effects on plant diversity, predicting the direction and magnitude of these effects remains elusive. Models such as the intermediate distur- bance hypothesis, the perturbation hypothesis, or the ecosystem engineering hypothesis of small mam- mal disturbance, show considerable overlap but fail to account for key variable interactions and thus provide mainly post hoc explanations. Recent re views have emphasized the importance of small mam- mals to basic and applied ecology. Re-examining the mechanisms underlying their disturbance effects is thus timely. Here I present the Slope-Hump Model, which integrates previous models and insights from the literature, and which is capable of predicting the direction and relative magnitude of disturbance effects on plant diversity. These predictions qualitatively match the results of recent meta-analyses. The model also suggests new patterns and predictions that can stimulate both pure and applied ecology re- search.

Breeding System and Interaccessional Hybridization ofPurshia tridentataPlants Grown in a Common Garden

Purshia spp. (Rosaceae) comprise a widespread western North American species complex that is impor- tant as landscape dominants, wildlife habitat, browse for wild and domestic ungulates, and seed reserves for small mam- mals. This study examined aspects of the phenology, compatibility, pollination biology, and progeny fruit characteristics of multiple accessions of bitterbrush (Purshia tridentata), as well as a putative hybrid between bitterbrush and Purshia stansburiana. Except for open- and wind-pollination treatments, mass pollination was accomplished at anthesis within white paper sacks, which were used to isolate treatment branches. Bitterbrush appears to be largely self-incompatible and requires some agent, either wind or animal, to effect pollination. While the majority of pollination of bitterbrush flowers is undoubtedly accomplished by insects, some pollination by wind is possible (ambophily) when shrubs are closely spaced, as flowering is fairly synchronous. Flower duration varied among shrubs, but generally lasted from 12 to 24 hours, depending on weather conditions. Flower development stage had little effect on fruit set, indicating prolonged stigma receptivity. Populations of bitterbrush exhibit considerable interfertility, as well as substantial hybridization with congeners. Intrapopulation crosses were only slightly more successful than interpopulation crosses. One of the popula- tions analyzed exhibits characteristics consistent with its putative hybrid nature, having more than one pistil per recep- tacle, smaller but longer fruits, and a greater reliance on insects for pollination. Both maternal and paternal effects are important in fruit development. Taxonomic relationships within the complex are discussed.

Comparing three Live trapping methods for small mammal sampling in cultivated areas of NE Spain

We aimed to investigate trap efficiency and specificity of three widely used live trapping methods (Sherman, mesh, and pitfall traps) in an agricultural landscape of NE Spain. We trapped 243 small mammals of 8 different species. Sherman traps yielded more species (6) than mesh (5) and pitfall (3) traps. Log-linear analysis for contingency tables showed that the three trapping methods used were species-specific and the analysis of the standardized residuals pointed out that Apodemus sylvaticus and Mus spretus were under- sampled by pitfall traps, whereas Suncus etruscus and Microtus duodecimcostatus were significantly over-sampled by pitfall traps. Finally, Suncus etruscus was significantly un- der-sampled by Sherman and mesh traps. The composition of the small mammal commu- nity studied was rather similar when using Sherman and mesh traps, but differed strongly from the community sampled by pitfall traps. As previously pointed out by many authors, a combination of trapping techniques is necessary to assess the composition of small mam- mal communities.

CORRIDOR USE BY DIVERSE TAXA

One of the most popular approaches for maintaining populations and con- serving biodiversity in fragmented landscapes is to retain or create corridors that connect otherwise isolated habitat patches. Working in large-scale, experimental landscapes in which open-habitat patches and corridors were created by harvesting pine forest, we showed that corridors direct movements of different types of species, including butterflies, small mam- mals, and bird-dispersed plants, causing higher movement between connected than between unconnected patches. Corridors directed the movement of all 10 species studied, with all corridor effect sizes .68%. However, this corridor effect was significant for five species, not significant for one species, and inconclusive for four species because of small sample sizes. Although we found no evidence that corridors increase emigration from a patch, our results show that movements of disparate taxa with broadly different life histories and functional roles are directed by corridors.

Experimental Analysis of the Role of Emigration in Population Regulation of Desert Pupfish

Pupfishes (genus Cyprinodon) have existed for long periods in small desert springs and streams that often lack competitors, major predators, and large environmental changes. How their populations are regulated in these systems is poorly known. We experimentally examined the role of emigration in regulating populations of pupfish by (1) comparing the dynamics of populations held in four pools open to emigration with those of populations held in four pools closed to emigration over a 22—mo period and (2) comparing the rates of emigration from open pools having different densities of pupfish in relation to resources. When pupfish populations were prevented from emigrating, they exhibited symptoms of overpopulation similar to those well documented for fenced populations of small mammals. Their numbers increased more than populations in open pools, mortality was high, and body condition and recruitment were low. Again, as in small—mammal populations, these responses were absent in pupfish populations in open pools, where 84% of the total numbers of fish produced emigrated. The timing and magnitude of pupfish emigration was similar in all four open pools and was related to seasonal temperature changes. More males emigrated than females, and emigrants had significantly lower condition factors than residents. When densities of pupfish in relation to resources were doubled in two open pools, the percentages of pupfish that emigrated (41.8 and 42.2%) were almost equal to the 50% drop in resources. In contrast, only 15.5 and 16.0% emigrated from two control pools, where pupfish densities in relation to resources were left unchanged. Our results, demonstrating that emigration is a potentially significant mechanism for regulating populations of desert pupfish, are consistent with data from a variety of other animals (small mammals, hydra, roe deer), suggesting that population regulation via a behavioral spacing mechanism may be common among mobile animals.

The Role of Reproduction and Life Histories in Models of Small Rodent Population Dynamics

Mammalian reproduction consists of internal fertilization, a period of intrauterine development, parturition, a period of postnatal dependence (lactation), and weaning. Quantitative aspects of this process include the time for development (both intrauterine and postnatal), the number of offspring raised, the age at maturation and the frequency of breeding. These characteristics are governed, within the constraints of morphology, physiology and genetics, by natural selection. Life historians have attempted to understand how organisms are adapted to their environments by considering age specific breeding patterns and the probability of reproductive success in relation to the survival costs of reproduction. Their studies have emphasized the constraints on reproductive patterns, the coevolved nature of reproductive characteristics, and the relationships between these patterns and nutritive resources or social systems (see Clutton-Brock and Harvey 1979, Zeveloff and Boyce 1980, Stearns 1983, Gittleman 1986, Harvey 1986 for examples). Population processes represent the cumulative effects of life history patterns. Many of the consequences of life histories (e.g. population growth) are too far removed from the immediate environment of organisms to be of great interest to life historians. Similarly, some aspects of life histories (e.g. neonatal weights and development) have no direct effects on population growth or density and are of little interest to population ecologists. Nevertheless, there are areas of mutual interest where improved communication may be beneficial. These comments will be restricted to life history principles that may be useful to population ecologists, particularly those who incorporate individual responses into their hypotheses. Reproductive processes are fueled by energy, with specific requirements dictated by the rate of metabolism and number of offspring raised. Among small mammals, reproduction is energetically expensive because basal metabolic rates are generally high (McNab 1986) and several young are produced per litter. Small mam-

Home Range Shape and Activity in Peromyscus leucopus

Shape of the home range and the distribution of activity within the home range were analyzed for 15 female and seven male Peromyscus leucopus. Ten of the 22 home ranges were circular. In all of the home ranges, the frequency with which animals were recorded in any unit of area in the home range decreased normally away from the center of activity. During the last 20 years, investigators of home range behavior in small mam- mals have been increasingly concerned with the way activity is distributed within home ranges. The statistical analysis of home range activity, an ap-