Effectiveness of resilience assessment tools in different case studies

Tese de doutoramento, Biologia (Biologia Marinha e Aquacultura), Universidade de Lisboa, Faculdade de Ciências, 2015

Systems at times undergo abrupt, often undesirable changes termed regime shifts. Anticipating critical transitions requires the understanding of ecosystem resilience; however establishing operational resilience assessment tools remains a major hurdle assessed by the current study using different case studies. The first analysis, aiming at analysing the recovery time of macrobenthos in a coastal lagoon, evidenced the system previously underwent a regime shift. Only one taxon showed a significant recovery in response to the management action with the objective of improving the status of the lagoon. A lag in response time, hysteresis, constituted a major challenge for sound restoration management. The following studies assessed recently proposed statistical signals capable of anticipating shifts, early warning signals (EWS) as resilience assessment tools. First EWS were calculated on oyster landings with an oscillating trend and also catch rates characterized by an irreversible decline. In both cases the tools accurately reflected the stability of the respective trends of the case studies. Three bottlenecks related to the use of EWS were identified: the quality of the time series, EWS behaviour in cyclic dynamics and the influence of the turnover rate of the measures upon which EWS are calculated. Next EWS were applied to phytoplankton data of a regime shift in the North Sea, as an example of a biotic signal with fast turnover rate. In one case, the resilience loss was detected 12 years prior to the shift. The high-resolution data enabled advanced methodological testing confirming the robustness of the indicators and their trends. Finally a model was used to assess whether EWS might indicate a loss of cyclostationarity. It simulated three species in a competitive cycle: the first scenario oscillated stably while the second scenario was programmed with a gradually decaying resilience. Indeed EWS can be used to detect instabilities in cyclic phenomena, however the choice of the indicator remains essential. While hurdles do exist, we conclude that overall EWS are promising resilience assessment tools, and particularly recommendable considering the ease of their calculation.

Fundação para a Ciência e a Tecnologia (FCT), SFRH/BD/48402/2008; MARE (Centro de Ciências do Mar e do Ambiente)