The interaction between the spatial distribution of resource patches and population density: consequences for intraspecific growth and morphology

Abstract

Summary1. How individuals within a population distribute themselves across resource patches of varying quality has been an important focus of ecological theory. The ideal free distribution predicts equal fitness amongst individuals in a 1 : 1 ratio with resources, whereas resource defence theory predicts different degrees of monopolization (fitness variance) as a function of temporal and spatial resource clumping and population density. 2. One overlooked landscape characteristic is the spatial distribution of resource patches, altering the equitability of resource accessibility and thereby the effective number of competitors. While much work has investigated the influence of morphology on competitive ability for different resource types, less is known regarding the phenotypic characteristics conferring relative ability for a single resource type, particularly when exploitative competition predominates. 3. Here we used young-of-the-year rainbow trout (Oncorhynchus mykiss) to test whether and how the spatial distribution of resource patches and population density interact to influence the level and variance of individual growth, as well as if functional morphology relates to competitive ability. Feeding trials were conducted within stream channels under three spatial distributions of nine resource patches (distributed, semi-clumped and clumped) at two density levels (9 and 27 individuals). 4. Average trial growth was greater in high-density treatments with no effect of resource distribution. Within-trial growth variance had opposite patterns across resource distributions. Here, variance decreased at low-population, but increased at high-population densities as patches became increasingly clumped as the result of changes in the levels of interference vs. exploitative competition. Within-trial growth was related to both pre-and post-trial morphology where competitive individuals were those with traits associated with swimming capacity and efficiency: larger heads/bodies/caudal fins and less angled pectoral fins. 5. The different degrees of within-population growth variance at the same density level found here, as a function of spatial resource distribution, provide an explanation for the inconsistencies in within-site growth variance and population regulation often noted with regard to density dependence in natural landscapes.

Partial migration alters population ecology and food chain length: evidence from a salmonid fish

“…In our system, we found that high density of young fish was associated with migratory fish, a pattern that has been detected in other partially migratory populations of O. mykiss (McMillan et al 2015) and salmonid fishes (e.g., Salmo trutta, Bohlin et al 2001). Many aspects of salmonid population ecology are density-dependent; higher juvenile density reduces growth rates (Jenkins et al 1999) but increases body size variation (Jacobson et al 2015). In other species, seasonal differences in density associated with migratory individuals (pulsed, seasonal increase in densities) vs. resident individuals (year-round presence) can influence ecological dynamics.…”

Phenotype‐dependent selection underlies patterns of sorting across habitats: the case of stream‐fishes

“…low velocity areas within riffles) may have occurred across all density levels. At low densities, individuals have been found to compete via antagonistic interactions (Brown et al , Tregenza et al , Jacobson et al ), individuals excluded to the use of alternative habitats (akin to dispersers in the resident versus disperser dichotomy in the non‐random movement/dispersal literature; Clobert et al , Edelaar and Bolnick , Bolnick and Otto ) those less specialized (i.e. adapted) and thereby less competitive (Jacob et al ).…”

“…Since, however, a greater number of individuals were able to utilize preferred habitat at higher densities than the number found within such sections at low density, we consider habitat choice in our low density trials to be independent of competitive effects and this is the more unlikely explanation for the habitat use in these trials. In systems with high densities, individuals have been found to increasingly compete via scramble interactions as their ability to defend optimal locations decreases (Tregenza et al , Jacobson et al ). Phenotypic‐sorting thereby decreases as a function of density under this process as all adapted relatively competitive individuals are able to remain within preferred (i.e.…”

Diet and body shape among populations of Bryconamericus iheringii (Otophysi: Characidae) across the Campos Sulinos ecosystem

“…This feeding flexibility is an adaptive response (Brandão-Gonçalves et al, 2009) to fluctuations in the availability of food resources in the environment (Abelha et al, 2001). Such fluctuations may be caused by several factors such as physicochemical, spatial and temporal components (Lawlor, 1980;Costa et al, 2013;Jacobson et al, 2015). For example, the presence of riparian vegetation is an important source of food subsidies for fishes (Esteves, Aranha, 1999;Burnett, Reeves, 2006) as well as for shading the water (Pusey, Arthington, 2003).…”