Risk sensitive reproductive strategies : the effect of environmental unpredictability

Abstract

A crucial task in ecology is to quantify trade offs between competing demographic processes for experienced by individuals that inhabit unpredictable environments. Perhaps the most widely studied trade off is that between current reproduction and future survival (‘the cost of reproduction’). While experimental studies have been widely used to quantify life history strategies in birds, virtually no experimental studies have been carried out on large and free ranging mammals. This thesis quantifies how female reindeer Rangifer tarandus subject to variability in food availability, trade their resources between reproduction and body mass to ensure own survival. By combining two experiments, one observational study and one theoretical model, this thesis show that: (1) Individuals subject to reduced food availability in one winter feeding promptly reduced their reproductive allocation the following summer to increased their autumn body mass. On the other hand, short-term improved conditions did not result in increased reproductive allocation. (2) Long-term improved winter feeding conditions did, however, result in increased reproductive allocation. (3) Reproduction was costly, especially for smaller females, as occasional harsh winters and high population density resulted in reduced reproduction and lowered female body mass. Moreover, a successfully reproducing female produced a smaller offspring in the coming year relative to a barren one. Reindeer also differ in their intrinsic quality as successfully reproducing females’ showed an increased probability of reproducing also in the following year. (4) In harsh and unpredictable winter conditions, the optimal reproductive strategy involved a low reproductive allocation per unit female spring body mass. Under such conditions females increased their autumn body mass to enhance their own survival. Conversely, the optimal reproductive strategy in benign and predictable conditions involved a higher reproductive allocation. (5) Reproductive strategies and environmental conditions had significant effects on population dynamics. Female reindeer do not to jeopardize their own survival and adjust their reproductive allocation in order to buffer periods of low food availability in a risk sensitive manner.

Klima og klimaendringer påvirker reproduksjon og overlevelse hos alle levende organismer. Denne avhandlingen inkluderer flere eksperimenter der vinterbetingelsene ble forbedret eller forverret ved å manipulere fôrtilgangen til reinsimler. En reduksjon i vinterbetingelsene førte til at simlene neste sommer reduserte investeringen i kalv, og isteden satset på oppbygging av kroppsreserver. En tilsvarende forbedring av vinterbetingelsene måtte imidlertid foregå over flere år før simlene var villige til å øke sin reproduktive investering. Et observasjonsstudie viste at vinterforholdene ikke bare påvirker investeringen i kalv, men også kostnadene knyttet til dette. De asymmetriske konsekvensene av forbedring og forverring koblet til et uforutsigbart vintermiljø viser at reinsdyr har utviklet en risikosensitiv investeringsstrategi der simlene avpasser bygging av fettreserver basert på hva slags vinter de forventer skal komme. I et modellarbeide blir det vist at denne typen strategi vil være optimal, og videre hvordan slike strategier vil påvirke bestandene under ulike klimascenerier. Økt nedbør og mildere vintre i Finnmark vil i tråd med disse studiene på sikt gi betydelig lavere reintall og større dyr med lav villighet til å investere i kalveproduksjon