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Plant facilitation is now recognized as an important process in severe environments. However, there is still no agreement on how facilitation changes as conditions become increasingly severe. The classic stress gradient hypothesis (SGH) predicts a monotonic increase in facilitation, which rises in frequency as conditions approach the extreme end of the environmental gradient. However, few studies have evaluated the validity of the SGH at the community level, the level at which it was formulated. Moreover, few studies have tested the SGH at either extreme of the gradient, and very few have excluded the effect of livestock on community response to stress. In line with the SGH, we hypothesized that several spatial pattern summary statistics would change monotonically from the least to the most arid sites, indicating increasingly aggregated patterns. In this study, we performed an evaluation of the SGH both within communities of shrub species and across a large portion of the Atacama Desert, and we isolated the abiotic component of the SGH. Our environmental gradient covered an extreme aridity gradient (

Understanding the nature of faunal assembly and community structure remains central to ecology. Research in North American deserts and some tropical forests provides evidence of energetic compensation and zero-sum dynamics, suggesting that species in some natural assemblages may be replaced with limited impact on ecosystem function. Experimental removal of a dominant small mammal (degu, Octodon degus) from replicate plots in semiarid coastal thorn-scrub habitat in north-central Chile revealed no evidence for energetic or functional compensation; energy consumption remained significantly lower on degu exclusions relative to control plots after 17 years of exclusion. This occurred in spite of the fact that the geographic species pools for South American sites generally are similar in size to those of most North American sites (mean and median number of species, 16.3 and 21.5 vs. 21.0 and 20, respectively). A macroecological assessment of energetically equivalent species at 394 arid sites in North America, t...

The abundance of exotic plants is thought to be limited by competition with resident species (including plants and generalist herbivores). In contrast, observations in semiarid Chile suggest that a native generalist rodent, the degu (Octodon degus), may be facilitating the expansion of exotic annual plants. We tested this hypothesis with a 20-year data set from a World Biosphere Reserve in mediterranean Chile. In this semiarid environment, rainfall varies annually and dramatically influences cover by both native and exotic annual plants; degu population density affects the composition and cover of exotic and native annual plants. In low-rainfall years, cover of both native and exotic herbs is extremely low. Higher levels of precipitation result in proportional increases in cover of all annual plants (exotic and native species), leading in turn to increases in degu population densities, at which point they impact native herbs in proportion to their greater cover, indirectly favoring the expansion of exotic plants. We propose that bottom-up control of consumers at our site results in top-down indirect facilitation of invasive annual herbs, and that this pattern may be general to other semiarid ecosystems.

We report on the foraging behavior of three small-mammal species over 2½ years using experimental foraging trays. Trays provisioned with oat seed were set out in the field at dawn and dusk, and the amount of seed consumed (with re-provisioning) was determined over three consecutive nights and days. Experiments simultaneously evaluated the role of lunar phase, season, and microhabitat (shrub vs. open); additionally, artificial exclosures (∼0.56 ha) were employed to evaluate the influence both of mammalian and avian predators and of a dominant competitor species, the large diurnal degu (Octodon degus). Other principal small mammals included Darwin's leaf-eared mouse (Phyllotis darwini) and the olive field mouse (Akodon olivaceus). Trays were enclosed by hardware-cloth cages with either small or large holes in the side to exclude avian consumers and provide selective access to all small mammals (large holes) or to exclude the largest species (small holes). Small-mammal population sizes varied greatly through...

We report on the spatial scaling of body size distributions in the mammalian fauna of South America, exclusive of fully aquatic or volant species. Although the frequency distribution of body sizes for 720 species of South American mammals was qualitatively similar to that observed for 464 species of North American mammals, different patterns emerged at smaller spatial scales in these two regions. At smaller scales in North America, body size frequency distributions became progressively more uniform on a log axis, whereas they remained modal with decreasing spatial scale in neotropical rain forest, and the mode shifted to larger body sizes. This unexpected scaling relationship appears to reflect the addition of a substantial third habitat dimension: the rain forest canopy. When rain forest mammals were separated by foraging stratum (terrestrial, scansorial, and arboreal), distributions of body sizes within strata exhibited scaling relationships similar to those observed in temperate areas of North and South America. These results are consistent with earlier interpretations of body size scaling across large geographic or taxonomic ranges, but they emphasize the pivotal role that habitat structure and heterogeneity can play in the ecological and evolutionary development of faunas. Additionally, these results underscore the role of habitat complexity as a factor contributing to the latitudinal gradient in species richness.

Recent theoretical developments to explain the unimodal and asymmetric distribution of body sizes among species in higher taxa have yielded predictions for related demographic and life history traits. In particular, it has been predicted that there is an energetically optimal body size (M*) for terrestrial mammals at ∼100 g, and that the relationships of many biological characteristics will change slope or even sign at this point. We reanalyze a well-known data set on the relationship between home range size and body size in mammals. If the distribution of home range sizes as a function of body size is energetically constrained, then it is reasonable to assume that the lower size limit of a home range for a given body mass will also be constrained. To evaluate this hypothesis, we predicted that the relationship between minimal home range size and body size is nonlinear, and that the smallest home ranges should correspond to species in the vicinity of M*. Our data tentatively support both hypotheses but constitute a clear call for more comprehensive analyses with larger data sets.

Presence/absence data for the small-mammal species at sites in seven deserts were analyzed for evidence of similarity in community structure. The deserts studied were located in North and South America (268 and 118 sites, respectively), Australia (245 sites), Israel (54 sites), and greater Eurasia (Thar, 15 sites; Turkestan, 36 sites; Gobi, 98 sites). Patterns observed in all deserts included: (1) low a diversity (2-4 species per site); (2) high P diversity (species turnover between sites); and (3) local coexistence of 20-30% of the species in the regional pool. Additionally, the number of species with which a species co-occurred increased with the number of sites at which that species occurred. Although these results suggested that some features of community structure were similar across deserts, other aspects, especially trophic structure, differed widely. Deserts in the northern hemisphere possessed more granivores, and the Turkestan Desert more folivores, than other deserts. Carnivorous small mammals were most strongly represented in Australia, and omnivores in South America, Australia, and the Thar. The structure of desert small-mammal communities is strongly influenced by historical factors; different taxonomic groups with distinctive trophic adaptations proliferate in different desert regions where they are subject to some common structuring processes of community assembly.

Competition has been shown to be an important factor in natural systems. Yet, its effect on community structure remains poorly understood and is often disputed. Null models have been employed by researchers in recent years, but these generally suffer from a lack of power estimation and unclear distinction between statistical significance and biological importance. Models that employ an appropriate alternative hypothesis allow for power estimates and for quantitative evaluation of the effects of competition. We have developed methodologies to (1) allow for testing the significance of competition in natural communities, (2) determine the power of these estimates, and (3) produce maximum likelihood estimates of interaction parameters for these communities. Using these methodologies we show that small mammal communities at the interface between Valdivian temperate rain forest and Patagonian steppe in southern South America appear significantly and importantly structured by competition, although we cannot distinguish between resource competition, interference competition, and apparent competition. Secondarily, we show that the geographic origin of the component species (Valdivian vs. Patagonian) is relatively more important than their habitat affinities in structuring these communities.

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The data provided here include home range and body sizes for 285 species of mammals in 177 genera. Data were extracted from 552 published sources, and have been used in multiple papers since we originally published results of analyses in 1999 and 2001. While this likely is the most comprehensive such data set available, it is arguably biased taxonomically and geographically. While some mammal orders are well represented taxonomically most are not. Hence, while all genera in Monotremata, Proboscidea, and Pholidota are represented, these comprise only 60, 66, and 20% of species, respectively; moreover, these are relatively minor radiations. In contrast, we have no species of Paucituberculata, Microbiotheria, Notoryctemorphia, Tubulidentata, Hyracoidea, Cingulata, Scandentia, Dermoptera, and Primates (the latter likely reflecting a bias in data acquisition). The modal proportion of genera and species across 27 terrestrial orders is zero; the median values are 10.8% (genera) and 3.7% (species), suggesting tha...

The data set covers a 16-year period (1989–2005) of monthly small-mammal surveys and associated habitat and environmental data. Small mammals are trapped every month on all experimental plots, and both ephemeral and perennial plant cover and soil seed bank composition are measured at regular intervals. Rainfall is recorded and compiled annually (through 1998) or monthly; information on captured small mammals and on all vegetative and rainfall data is contained in associated files. These data have been applied to diverse population- and community-level analyses and remain useful for investigations of population dynamics, species interactions, and the relative influence of biotic and abiotic factors. Sampling is ongoing, and data will be added over time.