Oviposition Behavior Partitions Aquatic Landscapes Along Predation and Nutrient Gradients

That individuals attempt to minimize the ratio of mortality risk/growth rate (μ/g) when foraging within individual habitat patches is well established. Do species partition among spatially discrete communities embedded in complex landscapes in a similar manner? We investigated how 3 ovipositing species (2 Hyla treefrogs and a hydrophilid beetle, Tropisternus lateralis) responded to simultaneous gradients of nutrients and predation risk. Species partitioned our experimental metacommunity primarily by reducing oviposition with fish. Tropisternus positively responded to increased nutrients, but the effect decreased with increasing risk, as predicted by μ/g theory. Use of fish habitats by Tropisternus was unrelated to breeding intensity. In contrast, Hyla showed no nutrient response but oviposited with fish only on nights with high breeding activity. Behavioral responses to the spatial distribution of resources and risk among discrete patches generated substantial variation in habitat-specific colonization rates, which has been identified as a primary mechanism generating both community and metacommunity structure

Diffuse competition and continuous niche shifts in size-structured populations of predatory salamanders

Size-specific interactions between predators can affect both species population dynamics and the structure and biodiversity of communities they inhabit. Interactions between size-structured populations of predators, especially those with complex life-cycles, often change with resource-use changes driven by ontogenetic niche shifts. However, if resource use is determined largely by prey size, generalist predators may compete across a wider range of body sizes and life stages resulting in diffuse intra- and interspecific competition. We examined size- and stage-specific interactions between juvenile sirens (Siren intermedia) and adult newts (Notophthalmus viridescens dorsalis) in the context of previous experiments demonstrating competitive equality of larvae and strong effects of adult S. intermedia on adult N. viridescens. Competition between juvenile siren and adult newts was mutually negative and roughly symmetrical. Two S. intermedia reduced growth of three N. viridescens by 21%, while three N. viridescens reduced growth of two S. intermedia by 29%. Together with previous work, this implicates diffuse competition as a critical feature in the ecology of these species across the range of body sizes and suggests that intensity of competition varies more with size than species identity. Competition that varies incrementally with body size expands the realm of possibilities for continuous niche changes and diffuse competition across large size gradients. For generalist predators such as S. intermedia and N. viridescens, body size, except at the extremes, is not an adequate niche difference either intra- or interspecifically