The response of model food webs to species loss : secondary extinctions and relaxation times

• Current destruction and fragmentation of natural habitats is predicted to cause great future losses in biodiversity (Pimm et al. 1995; Hughes et al. 1997; Sala et al. 2000). Due to complex interactions between species in food webs, loss of a species can set up a cascade of secondary extinctions (Paine 1966; Estes & Palmisano 1974; Pace et al. 1999).In this thesis we have studied the effects of species deletion in model food webs with different degrees of structural redundancy, measured as number of species per functional group. We found that the risk of secondary species extinction, as well as the fraction of secondary extinctions, decreases with increasing number of species per functional group. Hence, species-rich communities seem to be more stable than species-poor ones as they experience proportionally less secondary extinctions. Further, it is shown that more redundant food webs often have longer relaxation times (time elapsed from initial species loss until the new community composition is reached) compared with less redundant webs. This indicates that species-rich communities are more stable because in those communities there is more time available for dispersal between local communities and hence time for recolonizations to take place. Thus, redundancy seems to enhance food web stability. This lends additional support to the hypothesis (Walker 1992; Naeem 1998) and empirical result (McGrady-Steed et al. 1997) that high biodiversity can be seen as an insurance against radical ecosystem changes even though species may have largely redundant functional roles.The degree of stability of decaying food webs was further found to depend on the type of species being lost from the community. Some studies indicate that extinctions following habitat fragmentation are disproportionately experienced by species at high trophic levels (discussed by Holt et al. 1999). Although species at higher trophic levels might be more prone to extinction than species at lower trophic levels, this study indicates that the loss of species from lower trophic levels causes greater risks of losing additional species.Thus, when it comes to conservational efforts our results show that it might not be enough only to care about those species tending to go extinct in the first place but also take into account what happens as a consequence of the loss of other less extinction-prone species.

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