Ecological and evolutionary causes and consequences of intra-population variability in foraging niche: predator and prey in a marine archipelago
Date
2008-05-21T20:34:02Z
Authors
Darimont, Christopher Tod
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Abstract
The niche concept, which provides a tractable measure of the environment encountered by organisms, figures prominently in ecological and evolutionary theory. Although neglected, valuable information may be gained by examining niche variation at hierarchies nested within its historical roots at the species level. Herein I examine intrapopulation variation in foraging niche - at the sub-population, social group, and individual levels - and investigate its ecological causes and evolutionary consequences in a predator-prey system within a marine archipelago.
I used two analytical techniques. My primary tool, stable isotope analysis of δ13C and δ15N, coalesces the multiple dimensions of the foraging niche into two dimensions. Variation in prey remains identified in faeces served as an independent data set.
I examined how different levels of resource availability over space and time as well as competition could account for observed intrapopulation niche variation in a terrestrial carnivore (wolf, Canis lupus). Major shifts in resource use were associated with biogeographic region, from dominance of in situ terrestrial resources (deer, Odocoileus hemionus) on the mainland to prevalence of allochthonous (marine) resources
islands. The probability of deer remains in wolf faeces declines as a function of island isolation. Correspondingly, marine enrichment in isotopic signatures of wolves increases with isolation. Estimates of marine biomass assimilation vary from roughly 25 to 50 to 75% among mainland, inner, and outer island sub-populations respectively
How animals select resources also can contribute to intrapopulation variability in niche. A `resource selection' approach indicated that, when available, wolves select spawning salmon (Oncorhynchus spp.) over deer. Although only available during autumn, salmon contribute up to 25% of biomass assimilation in wolves over the 6 month period for which it was estimated. Moreover, previously undocumented salmon hunting by wolves I report identifies them as among a few primary biological vectors that transfer salmon from marine into adjacent terrestrial ecosystems.
I also ask whether intrapopulation variation exists at finer scales, and specifically at the individual level. Approximately 40% of total variation in `isotopic niche' exists within social groups, likely a function of strong intraspecific (i.e. intra-pack) competition in this social carnivore. Moreover, the magnitude of inter-individual variation was significantly higher on inner islands compared with the mainland and outer islands, likely because of differences in prey species richness and interspecific competition, which likewise vary among biogeographic regions.
On a smaller spatial scale, 1 examined the niches of deer to ask whether microspatial variation in vegetation patterns also create conditions suitable for inter-individual niche divergence. Such variation presents a potentially central object on which natural selection can act. Using intra-generational comparisons of deer that had either survived or perished from wolf predation, I detected resource-specific fitness. Individuals with isotopic signatures that suggested they foraged in nutritionally-rich forest stands were more likely to be killed by wolves, suggesting a trade-off between predation risk and nutritional benefit. Moreover, non-survivors diverged more than survivors from median isotopic niches, suggesting selection against foraging specialization. Both selection against specialization and proposed trade-offs would maintain observed variation in niche, and possibly also traits associated with the niche. Stable isotope analysis in an evolutionary framework provides novel opportunity to integrate the ecological and selective landscapes to identify underlying ecological mechanisms of selection and provide insight into the maintenance of variability.
Variability in intrapopulation niche can be predicted by ecological and evolutionary conditions and processes in the environment. Specifically, niche variability is largely a Function of habitat variability, which large and varied protected areas would best provide. Moreover, management that aims to safeguard an 'average niche' for a population may not adequately protect diverse populations and the processes underlying such readily observed variability.
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Keywords
niche, ecology, wolves, salmon, marine