10 Oct 2012
State dependent life-history strategies: a long-term study on Oystercatchers

BRANTA — Martijn van de Pol

State dependent life-history strategies: a long-term study on Oystercatchers

Institution: University of Groningen, the Netherlands
Supervisors: RH Drent, JM Tinbergen, FJ Weissing, S Verhulst
Details: PhD 2006 (Completed)

Address: University of Groningen, Kerklaan 30, 9750AA, Haren , The Netherlands (Sep 2006) Email

Subject Keywords: Life-history evolution, population ecology, aging, cooperation
Species Keywords: Eurasian Oystercatcher Haematopus ostralegus
Thesis Online here



The research described in this thesis is part of a long-term field study on free-living Oystercatchers (Haematopus ostralegus), on the Dutch Wadden Sea island of Schiermonnikoog. The study was started in 1983 by Jan Hulscher, and since then a lineage of PhD-students has investigated life-history decisions in this long-lived bird species. Most of their work has focused on understanding individual variation in settlement decisions, partner choice and reproductive decisions. Their strategy to investigate these life-history decisions has been to combine detailed behavioral observation with a long-term dataset collected over many years, and to conduct critical experiments whenever possible. The research conducted in this thesis is aimed to be a continuation, and it investigates some of the questions which are the direct result of previous work. Furthermore, we will integrate the results from many of these studies to better understand why this breeding population is declining so rapidly in recent years.
This thesis is partitioned into four main parts: (i) a general introductory part, (ii) a section on age-dependent life-history decisions, (iii) a section on state-dependent life-history strategies, and (iv) a concluding part. Each part consists of several chapters, with each chapter discussing one specific research topic. Some of the chapters are followed by a box, which discusses a topic related to the chapter, but in less detail. We end with a general discussion in the final chapter, in which some of the results from individual chapters are brought together. In chapter 2, we continue our General introduction with an introduction to the study system by describing how this population has changed over the last twenty-one years. Breeding numbers of Oystercatchers have declined strongly in recent years on Schiermonnikoog, as well as in most other breeding areas in the Netherlands. We investigate in detail which demographic parameters have changed on Schiermonnikoog and how this relates to changes in food availability and climate change. Subsequently, we discuss what might have caused Oystercatcher breeding numbers to decline rapidly all over the Dutch Wadden Sea.
The second section on age-dependent life-history decision we discuss how life-history traits change during a lifetime. In chapter 3, we first develop a new statistical model to separate within and between-individual effects. Our model can be useful in many biological situations, but we will illustrate our approach with two examples on age-dependent reproduction in Oystercatchers. We show that old Oystercatchers have a higher reproductive performance than young Oystercatchers for two different reasons. Firstly, individuals perform better as they age (within-individual improvement), and secondly Oystercatchers that start breeding late in life perform better than individuals that start breeding early in life (selective appearance, between-individual change). Furthermore, our method can give much more insight in underlying mechanisms of age-dependent changes than alternative methods. In box A, we give an additional example in which the statistical model developed in chapter 3 can be used. We show that patterns of extra-pair copulation behavior not only change during the duration of a pair bond, but that pairs with high levels of extra-pair copulation behavior are also more likely to split up the next year. Following up on the role of the pair bond we show in chapter 4 that variation in reproductive performance during a lifetime is more likely to depend on how long a pair had been together, than on the age or breeding experience of individual pair members. Using observational as well as experimental data, we suggest mate-familiarity is one of the main determinants of reproductive success in Oystercatchers.  
In box B, we integrate patterns of age-dependent probabilities of breeding, reproduction and survival into an age-structured population model. From this model we calculate age-specific reproductive values and sensitivities which we use to discuss how the selection for earlier or delayed maturity depends on the population growth rate in Oystercatchers.
In the third section on state-dependent life-history strategies we investigate how life-history strategies depend on the conditions, mainly in relation to the habitat quality individuals breed in. In chapter 5, we first give a general description of what are the main causes of phenotypic variation in life-history traits of Oystercatchers, and what causes life-history traits to co-vary. We show that both reproductive output and survival not only depend on individual quality, but also on the quality of the partner, suggesting that life-history traits in Oystercatchers are not simple sex-linked traits. Furthermore, we discuss how our analyses can be used as a powerful ally to life-history experiments. In chapter 6, we focus on one specific life-history trait, egg size. In Oystercatchers there is substantial variation in egg size, and parents that lay larger eggs produce more offspring. However, it is unclear whether a large egg really constitute an advantage to the chick or that chicks from larger eggs do better because they are also born in a better rearing environment. We performed a cross-foster experiment to disentangle the effect of egg quality and the quality of the rearing conditions on the survival of chicks hatched from small and large eggs. In chapter 7, we investigate the long-term consequences of being reared under favorable conditions. We show that rearing under high quality conditions not only affects life-history traits early in life, but also settlement decisions later in life. Even future offspring benefit. We discuss whether early conditions are likely to have long-term effects in many other species too, and discuss the problems associated with using short-term fitness measures. In box C, we follow up on chapter 7 by discussing why offspring reared under favorable conditions do much better, and what determines offspring quality. In chapter 8, we discuss the relationship between the life-history decisions where and when to reproduce. Oystercatchers strongly vary in the quality of their breeding habitat, as well as in their age at first reproduction. Following up on a previous study by Bruno Ens and colleagues, we further develop the idea that variation in both these traits might be the result of individuals queuing for high quality breeding territories. We improve their original model; incorporate asymmetries in settlement behavior (as suggested by chapter 7), and compare model predictions with observed patterns in the field. Finally, we discuss how queuing behavior might help us understand patterns of population decline in this species.
In the General discussion section, chapter 9 concludes with integrating some of the conclusions from separate chapters into a more general framework. We will specifically return to what we have learned about what are the most important causes of variation in life-history traits. Furthermore, we will discuss how life-history decisions are affected by the population composition, and how in turn life-history decisions might affect population dynamics. Finally, we will discuss what might be some of the future challenges in the study of life-history evolution, and for the Oystercatcher study population on Schiermonnikoog.


Published Papers

van de Pol, M., Bruinzeel, L.W., Heg, D., Kuijper, A.L.W. & Verhulst, S. 2006. Experimental and observational evidence that reproductive performance improves with pair bond duration in oystercatchers (Haematopus ostralegus). Behavioral Ecology, in press.
van de Pol, M. & Verhulst, S. 2006. Age-Dependent Traits: a new statistical model to separate within- and between- individual effects. The American Naturalist 167: 764-771.
Van de Pol, M., Bruinzeel, L.W., Heg, D., Van der Jeugd, H.P. & Verhulst, S. 2006. A silver spoon for a golden future: long-term effects of natal origin on fitness prospects of Oystercatchers (Haematopus ostralegus). Journal of Animal Ecology 75: 616-626.
Van de Pol, M., Bakker, T, Saaltink, D.J. & Verhulst, S. 2006. Rearing conditions determine offspring survival independent of egg quality: a cross-foster experiment with Oystercatchers. Ibis 148: 203-210.
Bruinzeel, L.W., Van de Pol, M. & Trierweiler, C. 2006. Competitive abilities of Oystercatchers (Haematopus ostralegus) occupying territories of different quality. Journal of Ornithology 147: 457-463.
Bruinzeel, L.W. & van de Pol, M. 2004. Site attachment of floaters predicts success in territory acquisition.Behavioral Ecology 15: 290-296.

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