Limits to Ecological Resilience
In many regions of Europe, the transition from abandoned to semi-natural land takes less than 15 years, followed by another 15–30 years before reforestation (Cramer et al. 2008; Verburg and Overmars 2009). Passive regeneration can therefore be a slow process, particularly in a dry environment such as the Mediterranean (Rey Benayas et al. 2008), or when the soils have been modified by past agriculture, that is, the "cultivation legacy" (Cramer et al. 2008), or the "grazing history" (Chauchard et al. 2007). The revegetation also depends on the availability and quality of the native seed bank (Rey Benayas et al. 2008).
If the abandoned land is too degraded assisted regeneration may be needed (Cramer et al. 2008). Active restoration would involve large-scale native trees plantation and tree growth management (Rey Benayas et al. 2008). An intermediate level of intervention involves the creation and management of forest regeneration sources or “woodland islets” (Rey Benayas and Bullock 2012; see Chap. 7). Another problem often requiring intervention is the vulnerability of intermediate stages of natural succession to natural perturbations, such as invasive species (Kull et al. 2004; Stoate et al. 2009) and fire (Pausas et al. 2008). Fire is a particularly acute problem as it has impacts not only on biodiversity but also on human health (Proença and Pereira 2010b). If fire regime is not appropriately managed, frequent fires will favor fire-prone scrubland and halt succession towards forest, in a selfreinforcing feedback loop (Proença and Pereira 2010a).
One of the strategies to manage fire regimes is to maintain open spaces in the landscape (see Chap. 8), minimizing also the impacts of revegetation on species that prefer open areas (Fig. 1.2). This strategy can be implemented by increasing the populations of large herbivores (Hodder and Bullock 2009; Sutherland 2002), including reintroduction of extinct species (Svenning 2002). In the case of species regionally extinct, it is possible to use individuals from other populations. For instance, seven European bison were recently reintroduced in northern Spain, 1,000 years after their extinction (Burton 2011). A more complex situation occurs with species that are globally extinct, such as wild relatives of some domesticated species. A possible solution is to release into the wild individuals of breeds that are most likely to be successful in replacing the ecological role of their wild ancestors. For instance, Iceland ponies have been released in the former arable fields of the Dutch-Belgian border (Kuiters and Slim 2003): their grazing favored a dense grass sward and after 27 years open grassland still represented 98 % of the area.
Natural colonization of abandoned land by carnivores can also be limited by the availability of prey, as is the case for the Iberian lynx ( Lynx pardinus) currently negatively affected by the scarcity of rabbits, decimated by diseases (DelibesMateos et al. 2008), or as can be expected for some populations of wolves and bears currently preying on livestock (Russo 2006).
Rewilding may be a future option in areas that are undergoing agricultural development or intensification today. There is currently a debate between land sharing and land sparing approaches to reconcile food production with biodiversity (Phalan et al. 2011). In land sharing, biodiversity conservation and food production goals are met on the same land, with biodiversity friendly agricultural practices and extensive agriculture, whereas in land sparing, land is divided between areas of intensification and of exclusion of agriculture. In practice, it is difficult to determine which is the best option because species respond differently to the alteration of their habitat (Phalan et al. 2011). To maintain future options for rewilding, both land sparing and land sharing are needed. On the one hand, land sharing is essential to limit land degradation and to maintain the appropriate seed bank for future passive revegetation (see Chap. 7). On the other hand, land sparing would allow for the conservation of populations of species that are currently in conflict with human activities, making “cohabitation” very difficult.