Fitting species into the complexity-stability debate

Ecological communities – groups of interacting species – are subject to a variety of disturbances. Understanding responses to these disturbances is a primary goal of community ecology. The structural complexity of the community and the traits of the community’s constituent species are both known to have a significant impact on a community’s response to a disturbance. In this thesis, we investigated how these two scales – the community level and the species level – interactively affected community responses to both short and long term disturbances. Our first hypothesis was that interaction strength would be weaker in species with many interactions when compared to species with fewer interactions. To test this hypothesis, we used simulated food webs and found that, in locally stable food webs, species with many interactions tended predominantly to have interactions with predators or with prey. While these many predator or prey interactions were weak, they tended to be balanced by a few interactions of the opposite type (with prey or predators) which were stronger than average. The structure of the network, where species had predominantly one type of interaction, was essential for this relationship between the number and strength of interactions to arise. Our second study investigated how food webs of varying size and connectance respond to press and pulse disturbances. Many studies of food web stability only focus on the response to short term or “pulse” disturbances, however, as anthropogenic impacts on food webs increase, it is important to increase our understanding of food web responses to long term or “press” disturbances and determine whether they follow the same pattern as pulse disturbances. We found that more species rich and connected food webs were less stable to both types of disturbance and the more stable a food web was to a pulse disturbance, the more stable it was to a press disturbance as well. We also found that the traits – trophic level and number of interactions – of the disturbed species impacted a food web’s resistance to a press disturbance. Food webs were less resistant to the disturbance of species with many interactions or low trophic level than species with few interactions or high trophic level. The strength of species’ effects on stability was also moderated by the structural complexity of the food web. Together the work that makes up this thesis suggests that, to understand the stability of food webs to any kind of disturbance, we should consider both the structure of the network and the traits of the species embedded within it. While we found that networks were more vulnerable to disturbance of certain species than others, this observation also depended on the structure and complexity of the community they existed in. This has important implications for communities subject to disturbances, especially those disturbances which alter the way in which communities are structured and species interact