This work started in the scope of the“Thresholds of Environmental Sustainability” project (http://www.thresholds-eu.org) that was an integrated project in which the study of ecological discontinuities and thresholds effects as the consequence of human perturbations on ecological systems were the main related aspects. There I had the opportunity to work with two different streams, Stream 2: Thresholds and points of no-return: thresholds definition, theory and identification and Stream 4: Thresholds and drivers of contaminants. Researchers in the Stream 4 developed several kinds of experiments and collected data from them. Specifically they performed a mesocosm experiment which had the main purpose to study effects of pyrene as a contaminant on a pelagic system including three trophic levels (algae, bacteria, zooplankton), under two nutrient conditions. Such a experiment was used to investigate direct and indirect effects of the contaminant on the studied species. Firstly my collaborators and I tried to propose models that could validate these experimental data, however our studies drove into a different path with a more general point of view, others in the group kept working in the original direction. Hence I could contribute with two different works studying regime shifts of marine ecosystems, not just considering the pyrenne case. One related to marine trophic chains affected by contaminants, changing differently the mortalities in each trophic level. And another work related to effects of eutrophication process in shallow marine ecosystems and competition between floating and submerged algae.

Then at the same perspective of the Threshold project and global change studies I drove my attention deeper inside to ecosystems that present complex behavior, discontinuities and non-linearities, i.e., systems that act through a number of motors, including climate change, desertification, land erosion, water shortage, eutrophication, pollution, biodiversity loss, succession, adaptation, natural disturbances, etc. Then I could develop different ecological models and show the versatility of a new kind of professional might be to carry out interdisciplinary projects. A professional who not only might dominate mathematical tools and bioxivlogical knowledge but might have the insight to solve, interpret and make prediction to problems in nature. These are the qualities required in a professional that work in joint programs of different areas of knowledge and could be the major challenge scientists shall face in the 21st Century. This thesis is divided in two parts. The first part is related to mathematical tools and modeling ideas used throughout the second part where the original contributions are presented. The main purpose of the first part is to introduce mathematical concepts with some illustrative examples and give the reader as many references as possible. This is maybe useful in order to help further students in preliminary studies and guide them in searching articles and books driving them deeper in the cited topics

I acknowledge support from the European Commission through the integrated project THRESHOLDS (contract 003933) and from MEC (Spain) and FEDER, through project FISICOS (FIS2007-60327) and from Govern de les Illes Balears.

Tesis leída en l'Universitat de les Illes Balears el 2 de noviembre de 2010

Peer reviewed