When resources are spatially and temporally variable, consumers can increase their foraging success by moving to track ephemeral feeding opportunities as these shift across the landscape; the best examples derive from herbivoreplant systems, where grazers migrate to capitalize on the seasonal waves of vegetation growth. We evaluated whether analogous processes occur in watersheds supporting spawning sockeye salmon (Oncorhynchus nerka), asking whether seasonal activities of predators and scavengers shift spatial distributions to capitalize on asynchronous spawning among populations of salmon. Both glaucous-winged gulls and coastal brown bears showed distinct shifts in their spatial distributions over the course of the summer, reflecting the shifting distribution of spawning sockeye salmon, which was associated with variation in water temperature among spawning sites. By tracking the spatial and temporal variation in the phenology of their principal prey, consumers substantially extended their foraging opportunity on a superabundant, yet locally ephemeral, resource. Ecosystem-based fishery management efforts that seek to balance trade-offs between fisheries and ecosystem processes supported by salmon should, therefore, assess the importance of life-history variation, particularly in phenological traits, for maintaining important ecosystem functions, such as providing marine-derived resources for terrestrial predators and scavengers.
Adjacent communities and ecosystems often differ in underlying productivity but are connected by flows of nutrients, energy, and matter. Pacific salmon (Oncorhynchus spp.) transport substantial quantities of nutrients from marine ecosystems to coastal freshwater habitats when they return to spawn and die. Nutrients from their carcasses are initially concentrated in spawning streams and lakes, but are subsequently dispersed by abiotic (floods, hyporheic flow) and biotic processes (predators and scavengers). In southwest Alaska, mobile avian scavengers (gulls; Larus spp.) breed on small islands within salmon nursery lakes and consume large quantities of spawning salmon during the chick‐rearing period. However the role of birds as vectors of salmon‐derived nutrients remains unknown. We examined how gulls – by transporting salmon tissues to their chicks – create hotspots of biological productivity in the aquatic habitats surrounding their nesting colonies. We found that algal production was ∼10× higher at islands with high gull densities compared to islands without nesting gulls, but was concentrated within 40 m of island shorelines. Carbon stable isotopes (δ13C) confirmed that gulls enhance primary production in local benthic communities and demonstrated that this production was transferred up the food web to grazers (snails) and carnivores (blackfish). Nitrogen stable isotopes (δ15N) confirmed that salmon dominated the diet of gulls and that nutrients from gull guano were incorporated into algae and passed up the food web. By relocating and concentrating salmon‐derived nutrients into new and distant locations, gulls alter and magnify production in local aquatic communities. We offer the first evidence that the avian community can move salmon‐derived nutrients great distances, enriching otherwise isolated habitats.
Quantifying the spatial distributions of organisms in simple and meaningful ways is important for understanding the ecology and habitat needs of species subject to anthropogenic disturbances. Most multi‐species conservation and management programs do not yet account for the variation of space‐use patterns exhibited by species preferring the same habitat type. To measure species' space‐use patterns as a step toward determining habitat needs, we suggest using evenness indices. Although commonly used in characterizing communities (i.e., as a measure of species diversity), these indices are suitable for characterizing many other ecological patterns, including patterns of site use by individuals. We investigate the statistical properties of five indices (Camargo's index of evenness, E′; Simpson's index of evenness, E1/D^; Lloyd's index of mean crowding, J; Smith‐Wilson index Evar; and Dispersion index, DL, a variant of the Shannon diversity index) to evaluate their utility for quantifying broad‐scale spatial patterns of migratory shorebirds. We use a Monte Carlo simulation approach to compare these indices for their (1) ability to characterize a wide range of spatial patterns (from even to patchy); (2) ability to discriminate among distributions; and (3) robustness to incomplete sampling. In addition, we compare the ability of these indices to characterize spatial dispersion for four species of shorebirds migrating through the United States: Killdeer (Charadrius vociferus), Semipalmated Plover (C. semipalmatus), Sanderling (Calidris alba), and Red Knot (C. canutus). Overall, we recommend Camargo, Simpson, and Lloyd indices for quantifying spatial dispersion. All three indices gave precise and unbiased estimates once 35 or more sites (3.5% of 1000 simulated sites) were sampled randomly, regardless of the degree of patchiness. Nonrandom sampling resulted in biased estimates until a much greater proportion of sites (at least 30–50%) were sampled, highlighting the importance of site selection in sampling programs. Our analysis of shorebird spatial patterns revealed that evenness indices discriminate well among the species we considered, ranking Killdeer as most dispersed and Red Knot as most aggregated. Our results agree with earlier (independent) assessments of these species' migration strategies and illustrate how simple univariate metrics may be useful tools for characterizing complex spatial patterns.
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