Life history adaptations to seasonality

Abstract

The thesis contains studies of animal life histories at high latitudes. Several topics are covered; from ecosystem consequences of life histories to tests of specific behavioural predictions put forward based on life history arguments. Work on complete life histories is also included in an attempt to understand the evolution of capital and income breeding in marine copepods. My main research questions, followed by main topics and findings, are as follows: • How are seasonal environments influencing life history traits and phenology? • What are the roles of storage as a reproductive adaptation? • How are individual states, particularly energy reserves, influencing optimal life histories and behaviour? Paper 1 deals with how growth of a high-latitude pelagic fish, the Norwegian spring-spawning herring, is scheduled during the annual cycle. Body mass data reveal a short period of rapid annual increase in body mass at the time when the copepod Calanus finmarchicus is the main prey. The food consumption (energy units) of the entire herring population is estimated, using a bioenergetics model, and compared with production estimates of C. finmarchicus. Large herring populations, particularly because of their selective predation on older stages, can influence population dynamics of C. finmarchicus, and may explain some of the copepod’s life history adaptations. Finally, estimates of spatial energy transport are presented; caused by herring migrating from oceanic feeding grounds to coastal overwintering and spawning sites. The large herring stock may be responsible for the world’s largest biomass transport caused by a migrating population. This flux of energy and nutrients is important for coastal species, and potentially for interactions not yet studied, such as between herring eggs and benthic invertebrates. Paper 2 is a short comment on recent studies of copepod life cycles that have suggested that the large lipid stores serve as a means of obtaining neutral buoyancy at a given depth - an ultimate explanation of energy stores. There is, however, a need to understand the use of energy reserves remaining after the winter, before we understand the ultimate explanation for storage. That is, we must study to what degree copepods are capital breeders (using energy reserves for egg production) as opposed to income breeders (relying on concurrent food intake for egg production). These ideas were pursued in papers 3 and 4 which are based on a life history model of the abundant Southern Ocean copepod Calanoides acutus. Life history tradeoffs arise in the model because bioenergetics, developmental constraints, and interactions with the environment (temperature, food availability and predation risk) are made explicit. Hence, trade offs need not be assumed. The model is statedependent and reproductive value of individuals with different developmental stage and condition is predicted by dynamic programming. The model predicts the optimal energy allocation and diapause strategies that maximise reproductive value at any time of the year, determining the entire life history strategy. A highly seasonal pattern in optimal egg-laying time is predicted. This optimality is, however, seen from an egg’s perspective. Actual egg production, as predicted from population simulations, do not match the seasonal peak in offspring fitness, which suggest that later life cycle stages are subject to trade-offs and constraints causing laying dates sub-optimal from the offspring’s perspective. Mechanism behind the mismatch are studied. Eggs from capital breeding have higher fitness than income bred eggs as capital breeding takes place early in the season, even before the phytoplankton bloom. The time an egg is produced influences its probability of: 1) reaching a stage capable of diapause; 2) developing large energy reserves, and; 3) developing to a stage capable of capital breeding next season. We conclude that seasonality in both growth potential and predation risk are key drivers of copepod life histories. Paper 5 is on the costs and benefits of capital breeding in aquatic environments, including ecosystem consequence that follow from capital breeding strategies such as the lipid transport of herring. Costs and benefits of carrying energy reserves are different for aquatic compared to terrestrial organisms. General analyses of the costs and benefits of capital breeding would improve by incorporating more of the findings from studies of marine organisms. Marine biologists, on the other hand, may better understand and appreciate the life history trade-offs of their study organism if theoretical concepts from the predominantly terrestrial literature were used more actively. Quantification of pre-breeding costs of a capital breeder strategy is one such issue. These costs may influence how we understand life cycle questions such as timing of diapause and other state-dependent decisions prior to breeding. Paper 6 is on a long lived seabird whose reproductive strategy includes sophisticated parental care. State-dependent behavioural responses are studied, and as in the papers above, an important state is body condition in terms of stored resources. The petrels are found to adjust chick feeding and guarding according to their own body condition, the body condition of their partner with whom they co-ordinate the chick-rearing period, and finally, adjustments in response to the chick’s needs for food and guarding. All these factors are found to play a role in determining individual behaviour. The results are obtained using a field experiment, and are discussed in relation to the life history strategies of seabirds.