Identifying factors that may be responsible for regulating the size of animal populations is a cornerstone in understanding population ecology. The main factors that are thought to influence population size are either resources (bottom-up), or predation (top-down), or interspecific competition (parallel). However, there are highly variable and often contradictory results regarding their relative strengths and influence. These varied results are often interpreted as indicating "shifting control" among the three main factors, or a complex, nonlinear relationship among environmental variables, resource availability, predation, and competition. We argue here that there is a "missing link" in our understanding of predator-prey dynamics. We explore whether the landscape-of-fear model can help us clarify the inconsistencies and increase our understanding of the roles, extent, and possible interactions of top-down, bottom-up, and parallel factors on prey population abundance. We propose two main predictions derived from the landscape-of-fear model: (1) for a single species, we suggest that as the makeup of the landscape of fear changes from relatively safe to relatively risky, bottom-up impacts switch from strong to weak as top-down impacts go from weak to strong; (2) for two or more species, interspecific competitive interactions produce various combinations of bottom-up, top-down, and parallel impacts depending on the dominant competing species and whether the landscapes of fear are shared or distinctive among competing species. We contend that these predictions could successfully explain many of the complex and contradictory results of current research. We test some of these predictions based on long-term data for small mammals from the Chihuahuan Desert in the United States, and Mexico. We conclude that the landscape-of-fear model does provide reasonable explanations for many of the reported studies and should be tested further to better understand the effects of bottom-up, top-down, and parallel factors on population dynamics.

The diversity and organization of the mammalian community is related to the vegetation structure of the coffee plantations in the area of Barranca Grande, in the State of Veracruz, Mexico. Four transects (each 200 m in length) were used to study the vegetation structure within the coffee plantation, and 178 night/traps, tracks registration and information from local people, were used in order to become familiar with the mammalian community. The sample period was from October 1989 to February 1991. The 24 mammals of medium-size species present in the area were classified using two measures of ecological diversity, locomotion and foraging in order to understand the guild organization. If the complexity of the plantation is reduced, the number of guilds occupied could suffer a loss of 45% and 43% in ecological richness and diversity, respectively, and 24% in the equitability of the mammalian fauna. We recommend the maintenance of a high diversity in the tree stratum (shade species), in this case, species such as Inga jinicuil, banana (Musa sapientum), Citrus spp., coyo avocado (Persea schiedeana), mango (Mangifera indica); species which could provide food resources and protection for the mammals.Resumen. En el ~irea de Barranca Grande, Estado de Veracruz, M6xico, se estudi6 la relaci6n que tiene la estructura de la vegetaci6n de los cultivos de caf6 con la diversidad de la comunidad de mamiferos medianos. Para ello, se establecieron 4 transectos fijos de 200 m de largo cada uno, en los que se tomaron los datos de los diferentes estratos. Para conocer la fauna de mamiferos, se hizo un muestreo con trampas Tomahawk (178 noches/trampas), registro de rastros asi como encuestas a la gente que vive en la zona, entre Octubre de 1989 y Febrero de 1991. Se registraron 24 especies de mamiferos medianos que fueron clasificados utilizando dos medidas de diversidad ecol6gica, locomoci6n y forrajeo, para conocer la organizaci6n en gremios. Si se reduce la complejidad de los cafetales, puede existir una reducci6n hasta del 45% en el n~mero de gremios ocupados, 43% en la riqueza y en la diversidad ecol6gica, y 24% en la equitatividad. Se recomienda mantener una alta diversidad en el estrato arb6reo, en este caso especies como el jinicuil (lnga finicuil), plfitanos (Musa sapientum), citricos (Citrus spp.), pagua o aguacate (Persea schiedeana), mango (Mangifera indica), serian recomendables ya que proporcionan recursos alimenticios y protecci6n a la mayoria de los mamiferos.

Aim Coyote (Canis latrans) distribution in Mexico and Central America has expanded recently reaching the Yucatan peninsula, Belize and Panama, probably promoted by deforestation of tropical areas. Historically, the southern distribution of coyotes prior to European settlement in America was described as reaching only as far south as central Mexico and that introduction of livestock favoured migration of coyotes to southern Mexico and Central America. However, coyote fossil records in Central America and Yucatan, as well as observational records of travellers during the sixteenth century suggest that the coyote's arrival to the region was earlier. Because of the uncertainty of past coyote distribution and the possible economic and ecological impacts due to recent range expansion, the objectives of this study were to confirm if paleontological and historical evidence support the hypothesis that the southernmost limit of coyote distribution before the arrival of European settlers was the centre of Mexico, to discuss the possible factors that have influenced historical shifts in coyote distribution, and to model the present distribution of the coyote in Mexico and Central America, determining the areas where they could invade in the near future.

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The effect of an open landfill and the seasonality of a tropical deciduous forest in Jalisco, Mexico, was tested on the home range and group size of coyotes Canis latrans under the Resource Dispersion Hypothesis (RDH), which proposes that in social carnivores dispersion of patches of limited resources determines home-range size, whereas independently, abundance of resources affects group size. The predictions in this study were that coyotes using the landfill, where food is available all year due to the continuous arrival of food wastes that are concentrated in a single patch, would have smaller, seasonally constant home ranges than coyotes living outside, where food is distributed in several patches. In this area, coyotes would increase their home ranges during the dry season due to seasonal changes in resource availability. Also, a larger coyote group size should exist in the landfill, where food abundance is greater. Home-range size and group size of coyotes living in and outside the landfill were estimated by radio-tracking. Home ranges of coyotes in the landfill varied from 0.9 to 9.5 km 2 , whereas home-range sizes of coyotes outside the landfill varied from 10.9 to 43.7 km 2 . Seasonality had no effect on the home-range sizes. We identified a group of four adult coyotes in the landfill and no group formation in coyotes outside. These results support the predictions about home-range and group size of coyotes in relation to landfill presence, and indicate that under the circumstances of our study, coyotes follow the postulates proposed by the RDH.

The diet and pattern of food resource use by the pygmy skunk Spilogale pygmaea, an endemic and threatened species of western Mexico, was evaluated in the tropical dry forest of Chamela, Jalisco. Analysis of scats (n = 104) and stomach contents (n = 5) obtained during 1997, 1998 and 2000 showed that pygmy skunks fed mainly on invertebrates. Insect larvae constituted the most represented food item in samples. During 2000, food resource use was quantified by comparing occurrence of available prey with occurrence of items found in scats. Even though pygmy skunks seem to be opportunistic foragers, some invertebrates were positively selected, such as insect larvae and Myriapoda, thus suggesting that pygmy skunks may target prey of high nutritional value. The diet varied considerably between consecutive dry and rainy seasons of the studied years, with little overlap in food items. Pygmy skunks may adjust food resource use in response to temporal variations in prey abundance, by incorporating other food items in their diet during the dry season, when food is scarce. Dietary switching between seasons indicates that pygmy skunks may exhibit strategies similar to those of other tropical carnivores to deal with a highly seasonal environment.

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