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3.3.2.      Tipping point or gradual change?

Ecosystems can change gradually over time or may have a tipping point that can be triggered by an extreme event or meeting a certain threshold value for an important system component. The external drivers discussed in section 3.3.1 can change gradually over time (e.g. habitat fragmentation, overharvesting) or represent a catastrophic change to the system (e.g. dam installation, significant pollution event). Natural perturbations’ such as floods and droughts can cause a significant shift in the timing, quantity and quality of flows in river systems. However change is caused or manifests it will to differing degrees upset the functioning state of the ecosystem in some way and cause a reduction in resilience. Ecological research has shown that with reduced resilience from human alteration of the freshwater system a sudden event may trigger a critical threshold to be reached from which the system will move into a less desirable state with reduced ecosystem service provision (Scheffer et al. 2001). Much of this can be explained by considering the pathways in which the system is able to return to a previous state and how particular species can re-colonize. Any disturbance in a natural system will act like a selective force, moulding traits so that species can persist. This can be expressed in traits such as resistance to high flow and capacity to recover following a flood, or resistance to high temperatures and low oxygen during droughts (Lake 2007). Also important are the availability of refugia that bolster resilience after disturbance by providing sources for decolonization after the disturbance. Any reduction in the natural flow regime will thus render a less adaptive set of species to flood events or low flow conditions. Also by un-coupling the river form the floodplain in order to provide flood defence structures there is a reduction in the availability of refugia and re-colonization pathways for biota following either a gradual change or extreme event.

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  • aurelie.dubois@developpement-durable.gouv.fr (invited by Wouter Vanneuville) 24 Aug 2012 09:17:20

    I would like to transmit the comments of a colleague from French Office of Water:

    I think the text could make a more explicit distinction between two different modalities of selection. There is on the one hand natural selection as an evolutionary force that drives genetic adaptation, selecting those individuals which are best adapted to given environmental conditions, and on the other hand the processes of extinction/recolonization of local populations from biological refuges. They act on different time scales and their importance also depends on the life cycle length of organisms: taxa with short life cycles (eg diatoms, invertebrates) will be more prone to the former whereas vertebrates will be more susceptible to the latter (though of course both mechanisms apply to all taxa).

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