Appendix S1
Density-dependent resource selection by a terrestrial herbivore in
response to sea-to-land nutrient transfer by seals
PHILIP D. MCLOUGHLIN1,*, KENTON LYSAK1, LUCIE DEBEFFE1, THOMAS PERRY1,
AND KEITH A. HOBSON2,3
1
Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon,
SK S7N 5E2, Canada
2
Environment Canada, 11 Innovation Boulevard, Saskatoon, SK S7N 3H5, Canada
3
Department of Biology, University of Western Ontario, London, ON N6A 5B7, Canada
2
APPENDIX S1. Spatial heterogeneity in sea-to-land nutrient subsidization leading to increased
persistence of consumer populations on islands via density-dependent habitat or resource
selection.
The process of adaptive resource or habitat selection is well described by concepts like the idealfree distribution (Fretwell and Lucas 1969) and related fitness-maximizing models of space use
(reviews in Rosenzweig 1981, 1991; Morris 2003). Optimizing behavior of individuals to acquire
resources may be especially important for terrestrial population dynamics and community
stability in the context of allochthonous nutrient transfers because: 1) inputs on islands are
expected to be spatially variable (concentrated around colonies of animal vectors or shorelines),
which may then promote fine-scale heterogeneity (patchiness) in local carrying capacities of
consumers and out-of-phase population dynamics (Coulson et al. 1997); and 2) density- and
habitat-dependent mixing of behavioral strategies by consumers to optimally select resources
should contribute to periodic movements and spatial genetic admixture (Fortin et al. 2008). For
example, we can imagine the role of density-dependent habitat selection in driving dynamics of a
‘patchy metapopulation’ (Harrison 1991) among regions of an island where seabird colonies
fertilize discrete ornithogenic pastures that drive local dynamics (Fig. S1). Presence of
enrichment will lead to direct or indirect increases in density of food or limiting nutrients for
terrestrial consumers at or near these patches. Population dynamics of island consumers
(secondary or tertiary) will thus tend towards that of a patchy metapopulation, whereby
subpopulations linked to patches are sufficiently close to function as a single population yet
differences in dynamics may exist due to stochasticity or, in this case, behavior of fitnessmaximizing individuals. For the latter, individuals will preferentially occupy different patches as
3
density increases locally, depending on quality of patches and dispersal costs following
expectations of density-dependent habitat selection (e.g., via ideal-free distribution when no
costs to movement are involved, or density-dependent habitat selection with costs to movement
[Rosenzweig 1981, 1991; Morris 1987, 2003; Fig. S1]). The overall effect of spatial
heterogeneity in sea-to-land nutrient transfer paired with behavioral strategies to select resources
in a fitness-maximizing manner is to increase persistence probability of a consumer which exists
as a patchy population (Harrison 1991), or by extension allow for species coexistence of
competitors and/or a predator and its prey, as expected from metapopulation theory (Hanski and
Gilpin 1991).
Figure S1. Patches (a, b, c) can be described by fitness isodars of associated consumer densities
(pairwise plots of densities, see Morris [1987, 2003]). Regions of the model island are linked by
4
patch dynamics through dispersal in response to competition (broad arrows). Density-dependent
costs of emigration from one area to another and perceived changes in quality amongst areas in
density-dependent fashion relates to slope of isodars (Morris 1987), with ideal-free distribution
(and no cost of dispersal) conveyed by slope 1.0 (dashed line) but non-zero intercept (as for
pairing a-c). For pairings (a-b) and (b-c) there is a density-dependent emigration cost, e.g.,
habitat (a) is perceived to decrease in quality relative to habitat (b) in a density-dependent
fashion (with expected net migration towards b, as indicated by broad arrows), fitness-density
curves are convergent and the isodar slope < 1.0 (vice-versa for pairing b-c). Background
including relevant equations in Morris (1987, 2003).
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