Ontogenetic niche shifts as a driver of seasonal migration

Abstract

Ontogenetic niche shifts have helped to understand population dynamics. Here we show that ontogenetic niche shifts also offer an explanation, complementary to traditional concepts, as to why certain species show seasonal migration. We describe how demographic processes (survival, reproduction and migration) and associated ecological requirements of species may change with ontogenetic stage (juvenile, adult) and across the migratory range (breeding, non-breeding). We apply this concept to widely different species (dark-bellied brent geese (Branta b. bernicla), humpback whales (Megaptera novaeangliae) and migratory Pacific salmon (Oncorhynchus gorbuscha) to check the generality of this hypothesis. Consistent with the idea that ontogenetic niche shifts are an important driver of seasonal migration, we find that growth and survival of juvenile life stages profit most from ecological conditions that are specific to breeding areas. We suggest that matrix population modelling techniques are promising to detect the importance of the ontogenetic niche shifts in maintaining migratory strategies. As a proof of concept, we applied a first analysis to resident, partial migratory and fully migratory populations of barnacle geese (Branta leucopsis). We argue that recognition of the costs and benefits of migration, and how these vary with life stages, is important to understand and conserve migration under global environmental change.

Keywords: Barnacle goose; Dark-bellied brent goose; Humpback whale; Matrix population modelling; Ontogeny; Pacific salmon; Reproduction; Seasonal migration.

Fig. 1

Scheme representing the life cycle of a migratory species. The inner circle shows the life stages (in black), the main life history processes (in color), in both the breeding and non-breeding range and the transition between the different life stages (colored arrows). The boxes represent the ecosystem contexts of the different life stages. These ecosystem contexts influences the life history processes through factors (connected to the life history stages with black arrows). The ecosystem context, and the factors within those, will differ for the different life stages (for instance, juveniles in the breeding range will perceive a different ecosystem context than the adults). The colour of the arrows and of the life history processes next to those indicates which ecosystem context is relevant for that life history process

Fig. 2

Outline of how our conceptual framework can be captured in a matrix population model. The model has five time steps to describe a whole migratory life cycle. The second column shows in color the part of the life cycle which is associated with the time step indicated in the first column. The vital rates are noted like a_yx, which indicated the rate with which individuals transition from stage x to stage y, or in case of reproduction, the contribution of stage x to stage y. The “non-active” parts of the life cycle are presented in grey. The third column shows the associated matrix formulation to calculate numbers of juveniles (J), subadults (S), reproductive adults (R) and non-reproductive adults (N) at any time step for a migratory population

Fig. 3

Example life cycle of a marine mammal, the humpback whale. Humpback whales are a clear example of a species in which migration to the equatorial breeding range is mainly beneficial for the juveniles, which cannot cope with the cold conditions of the arctic feeding range. Adults do not profit from migration in terms of resources, since those are largely lacking in the warmer equatorial waters

Fig. 4

Matrices for three different migration strategies in the barnacle goose. Data is obtained from literature (see supplementary methods and results). Lambda, the population growth rate, of the resident population is 1.139, that of the short distance migrating population is 1.157 and the Lambda of the long-distance migrating population is 1.034. These differences are caused by the differences in partial vital rates as can be seen from the values in the matrices