"Some things still need to be said about the world". Pliny the Elder, Naturalis Historia
November 2016: “What checks the natural tendency of each species to increase in numbers is most obscure”. Such wrote Charles Darwin one and a half century ago in “Origin of Species”. The statement is still true: empirical knowledge about density dependent regulation of animal populations is exceedingly difficult to obtain and interpret. In the new paper “When in life does density dependence occur in fish populations?” published in Fish and Fisheries, we develop the missing theoretic understanding about density dependent regulation in fish populations. Why care? We show that a correct understanding of density dependence is crucial for a central instrument in Maximum Sustainable Yield fisheries management – the minimum landing size regulations – that are used to avoid landing young fish by fisheries. See the paper in Fish and Fisheries here or download a preprint.
Fisheries yield vs. minimum landing size limit for four types of density-dependent regulation (thick lines) compared to the classic stock-recruitment theory that are used today (thin black dashed line). Fisheries yield is maximized with a size limit around size at maturation (vertical dotted line) only when density-dependent regulation occurs early.
July 2016: Sharks vs telests: How does the life-history strategy differ between elasmobranchs (sharks, rays etc) and teleosts (bony fish)? With respect to the size of their offspring, one difference is striking: elasmobranchs make offspring that is roughly a factor 100 (in weight) smaller than the adults, while most teleosts make offspring that is around 1 mg, independent of the adult size (see figure). We show that the difference in the offspring size strategy is unrelated to whether offspring is live or eggs, and there are no significant differences between growth, mortality etc. of elasmobranchs and teleosts. How, then, can evolution maintain these two very different offspring size strategies among very similar groups of organisms? In a recent paper in Journal of Theoretical Biology we develop an evolutionary model to show how the two offspring size strategies can co-exists evolutionary — in technical terms, we can say that there exists two local evolutionary stable states. The difference between the two strategies boils down to a difference in density-dependent control in the populations: if newly hatched offspring compete with one another, the evolutionary stable strategy is one where offspring size proportional to the adult size (the elasmobranch strategy), while if early life is devoid of density-dependent competition, the strategy with small offspring independent of adult size emerges.
July 2015: Review: Characteristic Sizes of Life in the Oceans, from Bacteria to Whales (download here). Imagine picking a random organism out of the ocean. Which one trait characterizes the largest part of this organism’s physiology, encounter rate with prey (and predators), abundance of conspecifics, reproductive rate etc.: it’s body size. Body size also determines a large part of the organism’s fundamental design and strategy towards life: If it is small (unicellular) it is likely to photosyntheize, while very small bacteria do not photosynthesize. If the organism is large (larger than about a cm) it is likely to be slender and streamlined, while smaller organisms come in odd shapes with appendages necessary for sensing movements of the water to compensate for lack of vision. Larger organisms, such as fish, however, are largely visual predators. The largest organisms in the ocean are warm blooded and breathe air, while all organisms less than about 10 kg are cold blooded and take oxygen from the water.
The five aspects of pelagic marine life: body temperature, resource encounter strategy, mobility regime, sensing mode, and life history strategy. The art at the top represents the seven realms of life defined by body size. Artwork by Jan Heuschele.
We show how these and other differences in design and strategies of life in the ocean are due to physical and ecological processes dependent on body size, and we develop simple expressions to describe the characteristic body sizes where there is a transition from one type of life to another, i.e., between bacteria, phytoplankton, mixotrophs, unicellular and multicellular zooplankton, fish, and marine mammals. In this way we stengthen the fundamental theoretical framework about body size, initiated by some of the founding fathers of evolutionary and ecological thinking, Haldane and Elton.
See archive for older news.
Students and post docs:
National Institute of Aquatic Resources, AQUA
Technical University of Denmark
Jægersborg Allé 1