Ecological and evolutionary responses of consumer-resource systems facing environmental disturbances

Consumer-resource interactions and trait variation are intimately linked. Numerous traits govern the quantity and the quality of the energy exchange between the consumer and the resource by defining the morphology, the physiology and the behavior of these organisms. By controlling the outcome of the trophic interaction, traits have a strong influence on the fitness of the consumer and the resource. Traits also shape the responses of consumer-resource systems to environmental disturbances by being major contributors of ecological, evolutionary and eco-evolutionary processes. Therefore, traits have repercussions on all levels of biological organization, from species to ecosystems. While extensive research has been conducted on the effects of single traits, less is known about how multiple traits of both consumer and resource interact to determine the responses of consumer-resource systems to environmental disturbances. In my thesis, I studied the effects of trait variation on the response of planktonic predator-prey and host-virus systems and the persistence of consumers facing environmental disturbances combining laboratory experiments and model simulations. I used a rotifer (Brachionus calyciflorus) as predator and either 6 strains of a green alga (Chlamydomonas reinhardtii) or 5 green alga species (Acutodesmus obliquus, Chlamydomonas reinhardtii, Chlorella vulgaris, Cryptomonas ovata and Monoraphidium minutum) as prey. I used a chlorovirus (Paramecium bursaria chlorella virus) as a virus and 1 strain of a green alga (Chlorella variabilis) as a prey. In Chapter 1, I showed the presence of an intraspecific defense-competitiveness trade-off relationship among prey strains using multiple predator and prey traits. Defense traits related to predator ingestion (i.e., attack rate and handling time) and competitive traits related to prey growth (i.e., maximum growth rate) were the most relevant traits to describe this relationship. I also showed the consequences of the defense-competitiveness trade-off on the fitness of the predator and the prey. The growth of the predator population was lower when feeding on defended (slow growing) prey strains and vice versa when feeding on undefended (fast growing) prey strains. These results help to comprehend the underlying mechanisms maintaining intraspecific trait variation in predator-prey systems. In Chapter 2, I showed that large trait variation in defense and competitive traits among prey strains allowed the persistence of the predator facing an environmental disturbance which reduces resource acquisition (i.e., microplastics). I demonstrated that 2 indirect evolutionary mechanisms favored the persistence of the predator population: indirect evolutionary rescue (IER) where the predator avoided extinction and indirect evolutionary facilitation (IEF) where the predator avoided density reduction. This occurred via the presence of large trait variation among prey strains due to the intraspecific defense-competitiveness trade-off described in Chapter 1, which led to a shift in frequency between defended and undefended prey strains favoring the comeback of the predator after the disturbance. These results contribute to the growing recognition of the eco-evolutionary processes shaping the response of predator-prey systems to environmental disturbances. In Chapter 3, I showed that the quantity and the quality of prey species drove the energetic balance of the predator facing an environmental disturbance which destabilizes energy acquisition (i.e., warming). The thermal responses of predator metabolism (i.e., energy losses) and predator ingestion (i.e., energy supplies) were uncoupled and differed among prey species, which revealed differences in the quantity of prey consumed by the predator. The thermal responses of prey stoichiometry (i.e., C:N and C:P ratios) were also uncoupled and differed among prey species, which revealed differences in the quality of prey consumed by the predator. I also showed the consequences of the predator’s energetic balance and prey stoichiometry on the fitness of the predator. The growth of the predator population was higher when feeding on high quantity and quality prey species. These results highlight the importance of considering the predator energetic balance for the response of predator-prey systems to environmental disturbances. In Chapter 4, we showed that virus lysis time depended on the initial density of viruses relative to the density of hosts (i.e., multiplicity of infection) and was not impacted by the presence of bacteria in the environment of the host. We also showed that the change in virus lysis time was not linked to the change in virus burst time. The virus lysis time was increasing but the virus burst time was changing inconsistently with increasing multiplicity of infection. My thesis demonstrates the role of intraspecific and interspecific trait variation for the maintenance of biodiversity and the ecological and evolutionary processes shaping the response of predator-prey systems to environmental disturbances. I highlight the need for measuring and linking multiple traits of both predator and prey or virus and host and connecting them to their respective fitness as well as combining theorical and empirical approaches to predict the response of populations and communities to changing environments.