Resilience of aquatic metacommunities. Implications for disturbance recovery

Resumo

Disturbances are main determinants of community assembly and have an effect throughout all system levels. Their role in shaping community outcome is key to better understand mechanisms and dynamics affecting species presence. Nevertheless, not only specific disturbance effects play a role in determining the post-disturbance community outcome. Landscape structure and the regional context of each local community also modulates its resilience. These two layers —disturbance effects and landscape structure— acts at both local and regional scales. The interaction among several delimited local communities through regional dispersal of individuals constitutes what is called a metacommunity. Thus, a metacommunity perspective is as a consequence, essential to disentangle how disturbances that have a regional affectation such as wildfires, can determine species assembly processes. Joining metacommunity theory within disturbance theory is a current need in the sense of advance toward a theory of metacommunity disturbance. However, how disturbances interact with metacommunities is still unclear, blurring the creation of a general framework that would help to better comprehend and consequently, cope, with future disturbance scenarios (e.g., wildfire increase, habitat fragmentation, species loss). Therefore, we currently are in the need of more integrative studies (e.g., including simulation models, experiments and observational studies) that will account with disturbances (e.g., wildfires) from a metacommunity perspective (e.g., network of temporary ponds) in order to better disentangle and comprehend the full picture of disturbances interaction with metacommunities. Wildfires are among the most worldwide distributed disturbances and are considered main natural community shaping agents. However, wildfire impacts on aquatic systems have been historically poorly addressed in comparison to their impacts on terrestrial systems. Furthermore, within aquatic systems, ponds, including temporary ones, have received still less attention regarding wildfire impacts. The future scenario pictured regarding wildfires in dry regions such as the Mediterranean, where temporary ponds are abundantly found, draws an increase in wildfire intensity and recurrence. Consequently, there is an urgent need in comprehending wildfire impacts if some considerations must be taken to maintain and conserve these endangered systems that hold unique communities. The main objective of the current thesis was to study wildfire impacts on Mediterranean temporary ponds faunal communities. In this thesis we analysed the implications of such disturbance and pond network relevance through three different approaches: observational field study through the analysis of samples from a natural wildfire event, which partially affected a temporary pond network (Chapter I & Chapter II), an experimental field study, using mesocosms around temporary ponds at different locations within the network (Chapter III) and a simulation model, which accounted with landscape structure and post-wildfire recovery patterns (Chapter IV). To carry all these analyses the Albera temporary ponds (lowland Mediterranean temporary ponds) and Guils de Cerdanya temporary ponds (high altitude Pyrenean temporary ponds), located in north-eastern Iberian Peninsula, were sampled. Their macrofaunal community, including macroinvertebrates and amphibians, was identified to the maximum taxonomic possible level in order to develop all these analyses. Our results indicated that in the observational study, temporary ponds were affected both directly and indirectly by the wildfire, providing a new perspective on wildfire impacts on aquatic systems (i.e., direct burning of aquatic organisms aestivating in the pond sediment) and highlighting species dispersal ability relevance in post-disturbance recovery (Chapter I). These factors affected pond metacommunity assembly mechanisms, which fluctuated along the neutral-niche gradient, but were mainly influenced by hydroperiod successional changes which still determined successional changes overcoming wildfire consequences (Chapter II). Moreover, the results also pointed out the different behaviour according to the dispersal ability of the organisms and highlighted the relevance of location within the pond network. Although distance and direction from the source also modulated dispersal dynamics along colonization trends (Chapter III). Thus, to consider several landscape perspectives (i.e., dispersal abilities) and the network structure appeared key in order to better approach metacommunity dynamics. Finally, the simulation models showed how dispersal ability —species landscape perception— was a key resilience and recovery driver determining post-disturbance dynamics with a strong link with pond network structure (Chapter IV). The studied wildfire did not compromise community recovery at a landscape scale, but an increase on this disturbance intensity might represent a rapid and pronounced decline in ecosystem resilience at the regional level (i.e., marked non-linear fall of community resilience at determined wildfire intensities). In general, a high resilience to wildfire was observed in the studied metacommunity, mainly due to their intrinsic drought-adapted community and the dense network structure that fostered recolonization of affected ponds and that was helped by the important role of dispersal in these systems (i.e., metacommunity rescue effect). Nevertheless, future scenarios, which encompass an increase in wildfire regimes and greater habitat fragmentation, do not ensure that such intrinsic resilience of temporary ponds will be maintained. Consequently, future wildfires may really compromise Mediterranean temporary ponds faunal communities and the current metacommunity rescue effect. More studies aimed to disentangle disturbance effects on metacommunities are needed and they must open the current metacommunity perspective and not focus on which mechanisms is being affected or fostered. New integrative approaches and the current perspective of several driving forces acting together along a changing gradient may help to improve the picture of a metacommunity disturbance theory, which is necessary to face the present incendiary future