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.

Numerous studies have been made of rodent population and community dynamics, especially in arid ecosystems. Most have centered on understanding how t

En este estudio se determinó y comparó la dieta del coyote en sitios dentro y fuera de la Reserva de laBiosfera Mapimí. Se colectó un total de 201 excrementos de coyote, 62 en la temporada seca y 139 en lluvia, de marzode 2000 a marzo de 2001. Se identificaron 50 y 45 componentes alimenticios en cada temporada, respectivamente. Lasmuestras se analizaron de manera temporal, para toda el área y por sitios. De los 299 componentes-presas identificadosen la temporada seca y 894 en la de lluvia, la mayor frecuencia de consumo se presentó con los artrópodos,principalmente Acrididae, Cicadellidae, Coleoptera y Lepidoptera; mamíferos, sobresaliendo Lepus californicus ySylvilagus audubonii. El material vegetal, los reptiles y las aves se presentaron con una frecuencia menor. El grupo delos mamíferos aportó la mayor biomasa, con 86.47% en la temporada seca y 72.52% en lluvia. Se observó un mayorconsumo de materia vegetal en los sitios con menor grado de modificación. Estos resultados confirman que el coyotees un depredador generalista, omnívoro y oportunista; con una fuerte tendencia a consumir artrópodos durante ambastemporadas, aunque la mayor aportación de biomasa la obtenga de los mamíferos.

Familia monofilética constituida por tres géneros: Marsilea L., Pilularia L. y Regnedillium, y un estimado de 61 a 75 especies. En México se registran Pilularia con una especie y Marsilea con siete, este último es el que se registra en la región de estudio. Distribución subcosmopolita, principalmente en regiones templadas y subtropicales: Marsilea de amplia distribución, Pilularia distribuida en Norteamérica, Sudamérica, Europa y Australia y Regnedillium restringida a Brasil y Argentina.