Influence of warming on microbial ecosystems

Climate change is progressing fast causing losses in biodiversity to the extent that scientists believe the world to be on the brink of the sixth wave of mass extinction. While some global change drivers like pollution, nutrient enrichment or extended land-use on the expense of natural habitats may pose more obvious threats to ecosystems, even seemingly small changes in temperature as they are predicted for this century can have detrimental effects on populations and entire ecosystems. Temperature influences ecological processes through their underlying biological rates like metabolism, growth rate, carrying capacity and various feeding parameters, determining population stability and species interactions. However, the influence of temperature is not uniform, and unsynchronised changes in these rates can alter the strength of species interactions which are an important indicator of population stability. While previous studies suggested an increase in predator-prey oscillations, this does not conform with experimental data, and the mechanistic understanding is still lacking. Therefore, I conducted time series experiments in a microbial predator-prey system with the ciliated predator Tetrahymena pyriformis preying on the bacteria Pseudomonas fluorescens along a temperature gradient combined with theoretical simulations based on a new global database for the temperature dependency of carrying capacity, half-saturation density, maximum feeding rate and metabolism (Chapter 2). Increasing predator-prey oscillations with warming as predicted by previous studies were only reported in 8.9 % of one million simulations and caused by a faster increase in maximum resource density with temperature than in foraging efficiency. In 91.1 % of simulations, predator-prey oscillations were stabilised with warming based on a faster increase in foraging efficiency with warming than in maximum resource density. Despite stabilising dynamics, in 73.6 % of simulations, predators went into extinction due to a faster increase in metabolism with temperature than in maximum feeding rate. This mismatch leads to predator starvation even under high prey abundances and stabile population dynamics, posing a challenge for conservation programs trying to preserve biodiversity in a changing world. However, species have shown to adapt to changing environmental conditions on ecological time scales raising the question whether temperature adaptation of predators could provide a feasible way out of the extinction scenario. While only very few studies focus on predator adaptation, temperature adaptation of predator interference has not been documented to date although it is a common pattern in ecological systems. Therefore, I conducted functional response experiments along an experimental temperature gradient within the microbial framework with predators adapted to a range of adaptation different temperatures to test for temperature adaptation of feeding parameters (Chapter 3) and predator interference (Chapter 4). My results show that the ciliated predator Tetrahymena pyriformis is able to increase activation energies for maximum feeding rate after an adaptation period of approximately 20 generations to higher temperatures compared to predators adapted to colder temperatures. Further, predators adapted to higher temperatures developed smaller body sizes, reducing their energetic demands potentially counteracting a mismatch between energy gain and energetic demand with rising temperatures. Predator interference increased with warming with the highest rates and shallowest increase with experimental temperature in warm-adapted predators, corroborating the assumptions of an improved energy budget in predators adapted to warmer temperatures. Due to higher levels of predator interference, maximum feeding rates were lowered in warm adapted predators, especially at low experimental temperatures which could have stabilising effects on predator prey oscillations. Further, as a result of a stronger increase in attack rates and simultaneously a shallower decrease in handling times with increasing experimental temperature, half-saturation densities decrease for predators adapted to colder temperatures. The opposite is the case for warm-adapted predators. Increasing half-saturation densities with experimental temperature in warm-adapted predators potentially have an additional stabilising effect on predator-prey dynamics by controlling the energy flux together with carrying capacity. My results suggest that the stabilising effect of temperature on predator-prey dynamics might be increased by temperature adaptation of predators. Further, a potential adaptation of feeding rates as well as metabolism, either through body size or potentially through physiological adaptation, might increase a predator’s energy budget and prevent a mismatch between energy gain and energetic demand to prevent extinction. However, smaller body sizes can increase the vulnerability of a predator towards further, more sudden increases in temperature. In my thesis, I provide the mechanistic understanding for a reported trend towards food webs with smaller organisms and fewer links as well as findings that taxa adapted to tropical environments might be more vulnerable to short-term changes in temperature.