Department of Biosciences, University of Helsinki, 00100 Helsinki, FINLAND.
Organisms interact continuously with their environment. They change the quality of the environment by consuming the resources or otherwise modifying the conditions. Changes in environmental conditions in turn affect fundamentally the dynamics of populations and communities. This two-directional interaction between organisms and environment can also be an important evolutionary force.
In my thesis I have studied how changes in environmental conditions affect organisms on different levels of biological hierarchy ranging from individuals to communities. I have focused on the ecological and evolutionary effects of temporal changes in resource availability. Intermediate resource fluctuations are predicted to promote community diversity. Furthermore, rare fluctuations in resource availability together with inter- or intraspecific competition can shape the evolution of life-history traits. The questions of my thesis were addressed using experimental evolution techniques and heterotrophic bacteria.
The results of my thesis exemplify a system where the interaction between organisms and their environment is dynamic, changing, and reciprocal. I found support for the view that temporal environmental variation promotes diversity within communities. However, also other factors contributed to the species coexistence such as spatial heterogeneity due to bacterial growth as biofilms, and evolutionary changes in life-history traits. I monitored the evolution of fitness related traits (mortality, growth rate, biomass and biofilm production) over hundreds of bacterial generations. When bacteria experienced rare, seasonal changes in their resource availability, the evolutionary response was decreased mortality during resource scarcity. The growth rate increased over evolutionary time in one of my study species, when it experienced interspecific competition and resource fluctuations. In addition, my results show that bacteria modify their habitat, and these changes alter the between species interaction from antagonistic to positive. The temporal changes in species interaction type can partly explain the long-term coexistence of the study species.
Altogether, the results of my thesis show that biological interactions can be complex even in a relatively simple system. Changes in environmental conditions affect population dynamics and coexistence of species, and in turn, the activity of organisms changes the environmental conditions - and evolution adds complexity in these dynamics.