The spatial scale of competition from recruits on an older cohort in Atlantic salmon

Competitive effects of younger cohorts on older ones are frequently assumed to be negligible in species where older, larger individuals dominate in pairwise behavioural interactions. Here, we provide field estimates of such competition by recruits on an older age class in Atlantic salmon (Salmo salar), a species where observational studies have documented strong body size advantages which should favour older individuals in direct interactions. By creating realistic levels of spatial variation in the density of underyearling (YOY) recruits over a 1-km stretch of a stream, and obtaining accurate measurements of individual growth rates of overyearlings (parr) from capture–mark–recapture data on a fine spatial scale, we demonstrate that high YOY density can substantially decrease parr growth. Models integrating multiple spatial scales indicated that parr were influenced by YOY density within 16 m. The preferred model suggested parr daily mass increase to be reduced by 39% when increasing YOY density from 0.0 to 1.0 m−2, which is well within the range of naturally occurring densities. Reduced juvenile growth rates will in general be expected to reduce juvenile survival (via increased length of exposure to freshwater mortality) and increase generation times (via increased age at seaward migrations). Thus, increased recruitment can significantly affect the performance of older cohorts, with important implications for population dynamics. Our results highlight that, even for the wide range of organisms that rely on defendable resources, the direction of competition among age classes cannot be assumed a priori or be inferred from behavioural observations alone

The spatial scale of density-dependent growth and implications for dispersal from nests in juvenile Atlantic salmon

By dispersing from localized aggregations of recruits, individuals may obtain energetic benefits due to reduced experienced density. However, this will depend on the spatial scale over which individuals compete. Here, we quantify this scale for juvenile Atlantic salmon (Salmo salar) following emergence and dispersal from nests. A single nest was placed in each of ten replicate streams during winter, and information on the individual positions (±1 m) and the body sizes of the resulting young-of-the-year (YOY) juveniles was obtained by sampling during the summer. In six of the ten streams, model comparisons suggested that individual body size was most closely related to the density within a mean distance of 11 m (range 2–26 m). A link between body size and density on such a restricted spatial scale suggests that dispersal from nests confers energetic benefits that can counterbalance any survival costs. For the four remaining streams, which had a high abundance of trout and older salmon cohorts, no single spatial scale could best describe the relation between YOY density and body size. Energetic benefits of dispersal associated with reduced local density therefore appear to depend on the abundance of competing cohorts or species, which have spatial distributions that are less predictable in terms of distance from nests. Thus, given a trade-off between costs and benefits associated with dispersal, and variation in benefits among environments, we predict an evolving and/or phenotypically plastic growth rate threshold which determines when an individual decides to disperse from areas of high local density

Variation in carbon isotope discrimination in relation to plant performance in a natural population of Cryptantha flava. Oecologia 145:541–548

Abstract 1 Few studies of phenotypic selection have focused on physiological traits, especially in 2 natural populations. The adaptive significance of plant water-use efficiency, the ratio of 3 photosynthesis to water loss through transpiration, has rarely been examined. In this study, 4 carbon isotopic discrimination, , an integrated measure of water-use efficiency, was repeatedly 5 measured in juveniles and adults in a natural population of the herbaceous desert perennial 6 Cryptantha flava over a four-year period and examined for plasticity in , consistency between 7 years in values of , and evidence for selection on phenotypes. There was significan

Shading by shrubs in a desert system reduces the physiological and demographic performance of an associated herbaceous perennial

A 2‐year field study examined the demographic consequences of association with shrubs in an herbaceous perennial, Cryptantha flava. Physiological data were collected to evaluate whether shrub effects were mediated primarily through water, nutrient or light availability. Microclimatic conditions under the north side of shrubs differed from open microhabitats, primarily in light availability. Due to little photosynthetic acclimation to light, daily photosynthesis for plants under shrubs was reduced proportionally to the light regime. Shading did not reduce stomatal conductance proportionally to photosynthesis, which led to decreased water use efficiency for plants under shrubs. Few differences were found in leaf water potential between microhabitats, indicating that little competition for water was occurring. There was little evidence for shrub‐induced nutrient island effects. Soil nitrogen, phosphorus and organic content did not differ between open and shrub microhabitats. Leaf nitrogen content also differed little between plants in the two microhabitats. Growth and flowering responses of individuals under shrubs were reduced relative to those in the open, even for plants located on the south side of shrubs. Over this 2‐year period of average to above‐average rainfall, association of C. flava with shrubs was dominated by competition for light, rather than for water or nutrients. Future investigations will address whether this asymmetric competitive interaction changes during years with below‐average rainfall to a facilitative interaction, or one of increased competition for water