Effects Of Biotic Interactions Research Articles

Microgeographical variation in brown trout reproductive traits: possible effects of biotic interactions

This study documents substantial variation in reproductive traits among populations of stream‐dwelling brown trout (Salmo truttaL.) at a very small geographic scale. Within two streams, we found a parallel pattern of variation, where females living above major waterfalls produced fewer and larger eggs than conspecifics from below the waterfalls. Four additional streams were represented with either a below‐waterfall site (n=2) or an above‐waterfall site (n=2). When these streams were included in the analyses, there was no consistent difference in reproductive traits between females from above‐ and below‐waterfall sites. There was no significant difference in total reproductive investment among sites within streams, but considerable variation among streams. Female first‐year growth rates was estimated from scales, and differed significantly among populations. Within streams, females from below waterfalls experienced higher first‐year growth rates as compared to females from above the waterfalls. Within seven out of eight populations, egg size increased significantly with increasing female body length. Within three populations, we found evidence for a trade‐off between offspring size and offspring number, as a negative association between fecundity and egg size independently of adult body size. Within three populations egg size decreased significantly with increasing maternal first‐year growth, independently of adult body size. We suggest that the within‐stream differences in offspring size/number strategies are influenced by population density and growth effects. Earlier, we have shown that population densities are consistently lower below the waterfalls in these streams. The Alpine bullhead (Cottus poecilopus) is found only below the waterfalls and could influence brown trout demography.

Caddisfly life histories along permanence gradients in high‐altitude wetlands in Colorado (U.S.A.)

SUMMARY1. Larvae of cased caddisflies (Limnephilidae and Phryganeidae) are among the most abundant and conspicuous invertebrates in northern wetlands. Although species replacements are often observed along permanence gradients, the underlying causal mechanisms are poorly understood. In this paper, we report on the distributional patterns of caddisflies in permanent and temporary high‐altitude ponds, and how those patterns reflect differences in life history characteristics that affect desiccation tolerance (fundamental niches) versus constraints related to biotic interactions (realised niches).2. Species (Hesperophylax occidentalisandAgrypnia deflata) that were encountered only in permanent ponds are restricted in distribution by life history (no ovarian diapause, aquatic oviposition, and/or inability to tolerate desiccation). Although the egg masses ofH. occidentalistolerate desiccation, the larvae leave the protective gelatinous matrix of the egg mass because adults oviposit in water.3. Three species (Asynarchus nigriculus, Limnephilus externusandL. picturatus) have life history characteristics (rapid larval growth, ovarian diapause and terrestrial oviposition of desiccation‐tolerant eggs) that should facilitate the use of both permanent and temporary habitats. However,A. nigriculusis rare or absent in most permanent ponds, andL. externusandL.picturatusare rare or absent in most temporary ponds. Experimental data from a previous study on the combined effects of salamander predation and interspecific interactions among caddisflies (e.g. intraguild predation) suggest that biotic interactions limit each species to a subset of potentially exploitable habitats.4. Many wetland invertebrates exhibit species replacements along permanence gradients, but few studies have separated the relative importance of the effects of dryingper sefrom the effects of biotic interactions. Our results emphasise the complementary roles of comparative data on life histories and experimental data on competition and predation for understanding invertebrate distributions along permanence gradients.

Tree species diversity in temperate and tropical forest gaps: The role of lottery recruitment

How canopy gaps promote tree species coexistence in temperate and tropical forests is reviewed. Given that evidence for traditional resource-based niche partitioning of canopy gaps is weak, lottery recruitment, whereby colonization of vacant living space is random with respect to species identity, may be key to maintaining diversity. Gap formation events are not highly predictable and species’ propagules tend to be patchily distributed. For these reasons the predictability of gap-phase recruitment is low and lottery principles apply. Recruitment limitation from discontinuities in species’ propagule supplies in space and time may permit lottery recruitment in temperate and tropical forests. However, the relative importance of recruitment limitation in species-rich versus species-poor communities is unclear. Although lottery models with purely random recruitment in vacant sites can be applied, relatively complex models appear to better capture the essential features of forest community dynamics. For example, models with recruitment probabilities weighted by species abundance can produce the non-random trajectories often observed in communities. Models with local competitive displacement by individuals of dominant species can also produce non-random trajectories. Models with spatial structure, localized competition, and dispersal limitations may provide further insight into the effects of biotic interactions and recruitment limitation on forest dynamics.

Indirect Effects in Experimental Studies of Marine Soft-Sediment Communities

Most experimental studies that deal with the effects of biotic interactions in marine soft sediments are designed to examine the direct effects of one species (or group of species) on some subset of the community. However, species within a community influence one another through a complex web comprising both direct and indirect effects. Ignoring this complexity in the development of hypotheses and experimental designs will likely lead to unexpected or conflicting findings. Few studies of marine soft-sediment communities have emphasized indirect effects and even fewer have included experimental designs capable of testing for them. To illustrate the potential importance of indirect effects in marine soft sediments, I provide several examples from the literature and describe the type of experimental design required to demonstrate a simple indirect effect involving three species. Perspective on indirect effects in a particular community changes as more information is collected about individual species and their effects. The influence of one species or guild may be confounded by that of another and even effects expected to be strong may appear weak or nonexistent as a result of unidentified indirect effects. Recognizing indirect effects in soft-sediment communities may help to explain apparent paradoxical results in past experiments and provide an improved template for the design of future experiments.

A study of population changes, production and survivorship of river fish: results from a long-term student practical exercise

The paper is presented in the form of a ‘case study’ of the changes in a fish population monitored over a period of several years. Fish from a 100 m section of a drainage canal in Suffolk, England were netted annually by undergraduate students as part of a practical exercise on fish population ecology. Data collected from 1981 to 1987 showed changes in the fish community both in terms of overall abundance (numbers and biomassj and in the relative abundance of different species. A variety of factors including human exploitation of the fish stock, the effect of biotic interactions and climatic conditions are considered as possible influences on fish recruitment and mortality. These possible explanations are examined against the background of other studies which have considered the causes of change in populations of freshwater fish.