The sixth mass coextinction: are most endangered species parasites and mutualists?

Abstract

The effects of species declines and extinction on biotic interactions remain poorly understood. The loss of a species is expected to result in the loss of other species that depend on it (coextinction), leading to cascading effects across trophic levels. Such effects are likely to be most severe in mutualistic and parasitic interactions. Indeed, models suggest that coextinction may be the most common form of biodiversity loss. Paradoxically, few historical or contemporary coextinction events have actually been recorded. We review the current knowledge of coextinction by: (i) considering plausible explanations for the discrepancy between predicted and observed coextinction rates; (ii) exploring the potential consequences of coextinctions; (iii) discussing the interactions and synergies between coextinction and other drivers of species loss, particularly climate change; and (iv) suggesting the way forward for understanding the phenomenon of coextinction, which may well be the most insidious threat to global biodiversity.

Figure 1.

Relationship between mean host specificity (number of host species associated with a given affiliate species) and the proportion of affiliate species estimated to go extinct given that 10 per cent of host species go extinct. Open circles represent affiliate extinction rates derived from simulated random extinction of host species based on empirical affiliate–host matrices; the solid line represents predicted affiliate extinction rates based on a nomographic model (Koh et al. 2004).

Figure 2.

The number of parasite and host extinctions and the proportion of parasites extinct as a function of the proportion of hosts extinguished. (a) The verticle line indicates the current proportion of carnivore hosts listed as threatened (IUCN 2008), filled circles the parasite extinctions (max.=518) and open circles the host extinctions (max.=29). (b) Each line represents a parasite taxon (triangles, viruses; squares and black solid line, trematodes; black solid line, ticks; grey solid line, namatodes; crosses and solid line, lice; dashed line, bacteria). Lines show the number (a) or proportion (b) of parasites extinct after random extinctions of different numbers of hosts. In (b), select parasite taxa are shown, since most taxa (including the result when all taxa are considered together) fall on a line coincident with the line shown for nematodes. Data in both panels are for all North American carnivores.