Defense mechanisms are very common in unicellular plankton. A wide variety of these mechanisms (toxins, shell etc.) is observed and it is rather difficult to quantify the cost of being defended against predators using laboratory experiments. We show that the implementation of the competition-defense tradeoff in a size based model of unicellular plankton affects the size distribution and trophic strategies (auto-, hetero- and mixotrophy) of the emerging community depending on environmental conditions.
Competition-defense tradeoff is a source of functional diversity in planktonic communities. Indeed, the most defended organisms are prevented to be the most dominant and outcompete their lower defended competitors by paying a cost relative to their defense mechanism. The cost of being defended materializes either as a reduction of resource affinity or as elevated energy expenditure. Moreover, the trade-off curve (cost vs. benefit) can be linear or non-linear with the cost being paid as diminishing returns as the investment in defense mechanism increases. In this paper, we examine the implications of the shape of the trade-off curve as well as whether the cost is paid as reduced resource (nutrient, food) affinity or as resource allocation to defense to the outcome of competition in a relatively complex size-based food-web model of unicellular plankton. The outputs of the simulations including the competition-defense tradeoff are compared to the simulations without defense implemented. The model is applied to both eutrophic and oligotrophic conditions at steady state and under seasonal forcing to assess the succession of plankton traits (size, trophic strategy and defense strategy) in a temperate, seasonally stratified environment.
Our model results show that the implementation of the competition-defense tradeoff promotes size coexistence and favors mixotrophic strategy in eutrophic conditions whereas in oligotrophic conditions, defense decreases size coexistence towards smaller phototrophic cells when defense are ‘cheap’ (diminishing returns) compared the scenario without defense. Further, when the competition-defense tradeoff curve is linear, it also matters how the cost of defense manifests. In the simulations with seasonal cycle, the implementation of defense leads to larger size coexistence and increased summer plankton biomass (when nutrients are depleted from the surface of the water column) with the promotion of highly defended mixotrophs in response to high predation pressure occurring in May-June.
By highlighting the importance of the details of the competition-defense trade-off on the composition of the microbial ecosystem, our work emphasizes the importance of a mechanistic understanding of the defense mechanisms and their associated cost through experimental measurements. This step appears as essential to allow further development of robust ecosystem models as, likewise trophic strategies or foraging behavior, defense mechanisms have a strong structuring effect of plankton communities as well as their biogeochemical implications on the whole ecosystem functioning (primary productions, vertical fluxes etc.).
The paper can be found here
Cadier M, Andersen KH, Visser AW & Kiørboe T (2019) Competition–defense tradeoff increases the diversity of microbial plankton communities and dampens trophic cascades. Oikos doi: 10.1111/oik.06101
Picture credits: 'Red algal bloom at Leigh, near Cape Rodney. Photo by Miriam Godfrey.'