A ‘Darwinian Demon’ is a hypothetical, ideal organism having optimal adaptations to all relevant environmental factors at the same time, implying unlimited access to all kind of resources and the absence of biological constraints. You might have wished to become such a ‘Demon’ when facing multiple deadlines at work. However, if ’Darwinian Demons’ would really exist in nature, it would be a drab and monotonous world, as these perfect all-rounders would drive all other species towards extinction. Hence, the observed high diversity in nature is an indirect evidence for the existence of biological constraints or trade-offs, which prevent organisms from simultaneously optimizing all their functional traits. However, there is surprisingly little direct empirical evidence for the existence of trade-offs between functional traits in natural systems, not to mention quantitative information on their shape.
During my PhD in the group of Ursula Gaedke at the University of Potsdam, I developed much theory on trade-off curves and showed that their exact shape (e.g. convex or concave) is crucial to species coexistence (published in a previous paper). At that time, I was not sure whether my theory will ever come to life and will be tested in the lab or even in the field, considering the difficulties of trade-off measurements. It turned out that I hadn’t to wait so long for this favourable occasion. Nadja Kath, a PhD in the same group, analysed the phytoplankton community of Lake Constance, a large lake in the middle of Europe, where Ursula Gaedke worked after her PhD during the nineties. She combined phytoplankton data from a sampling period of 21 years with a comprehensive literature collection of phytoplankton traits, thoroughly compiled by Jorn Bruggeman in 2011, and found an intriguing result: the phytoplankton species showed a distinct, concave trade-off between defence against predation and growth. This was the starting shot for a study bringing together theory, lab-derived trait data and field measurements of plankton biomasses and environmental factors.
In this paper, we show that, in line with theory for concave trade-offs, phytoplankton species with intermediate defence levels and relatively high maximum growth rates dominate in the community. Neither ‘Darwinian Demons’ with high values nor ‘losers’ with low values for both traits were found. The latter may be physiologically feasible, but these ‘losers’ have likely been outcompeted. Interestingly, we were also able to predict the alternation in the community composition among seasons by including information on the environmental drivers. As predicted, we could observe a continuous shift in the ‘winning’ phytoplankton trait combinations towards higher defence levels at the cost of lower growth rates from early spring to summer, when zooplankton grazing pressure increased. To complete the picture, we explained the observed maintenance of diversity apart from these ‘winning strategies’ by exploring further trait dimensions and the fitness landscape using a mathematical model.
In conclusion, our study adds an important piece in the puzzle of biodiversity research, by providing quantitative insights on a frequently postulated trade-off and revealing its importance for community dynamics.