So, first off, sorry that I did not post anything last month.
But let’s not dwell on the past. Instead, let’s focus on the future, and the role of current massive extinction rates on future biodiversity – and how parasites fit in to this picture.
Extinctions can occur through numerous mechanisms. Common ones include habitat loss, species invasion, overkill, climate change, and coextinction (Dunn et al. 2009). Regarding coextinction, it is easy to understand how the extinction of prey species could lead to the extinction of predators (for example, the extinction of krill could cause the extinction of baleen whale species). In a similar fashion, coextinction can apply to parasites. If a host species has species-specific parasites (and many hosts species do), then those parasites may go extinct along with the host species (Dunn et al. 2009). Furthermore, coextinction can include cascades of extinctions. If an insect species goes extinct, a bird species that uses that insect as prey may go extinct, and the parasites of that bird may then become extinct also. Whether such processes occur depends primarily on the host specificity of the parasite (Dunn et al. 2009). Host specificity, importantly, is very difficult to measure – for a variety of reasons, including that it is infeasible to expose all possible hosts to a particular parasite. In addition, because individuals within a parasite population likely have varying host specificities, the frequency of each host specificity level is probably more important than the average host specificity level (for example, knowing that 80% of the parasite population is highly general and 20% highly specific would be more important than knowing that the average parasite is moderately host specific) (Dunn et al. 2009). These considerations beg the question: If a host goes extinct, how likely is it that the parasite (or mutualist) will become extinct also or adapt to another available host? While studies often indicate that parasite are more general than previously assumed (Dunn et al. 2009), whether a parasite survives will depend highly on the specific parasite species in question; generalizations seem inadvisable.
But why should we care? Parasites are bad, right? Well, if we care about biodiversity for the sake of biodiversity, then we should protect parasites, which are often more diverse than their hosts (Dunn et al. 2009), implying that parasite species may outnumber host species. Furthermore, a common hypothesis for the evolution of diversity within sexual organisms relies upon parasites – to escape their parasites, hosts diversify. Without parasites, the force to diversify could thus decline, resulting in declines in host biodiversity (Dunn et al. 2009). Additionally, as stated above, host-specific parasites are more likely to become extinct than generalist parasites; generalist parasites are also often more pathogenic than host-specific parasites (Dunn et al. 2009). Zoonoses, parasites that infect humans but are maintained in the wild by non-human animal reservoirs, are by definition generalists. Therefore, as host-specific parasites become extinct, we should ask ourselves whether pathogenic zoonoses will become more common, filling the gaps left by the specific parasites, and whether such a result will negatively affect human health.
Dunn, R. R., N. C. Harris, R. K. Colwell, L. P. Koh, and N. S. Sodhi. 2009. The sixth mass coextinction: are most endangered species parasites and mutualists? Proceedings of the Royal Society B-Biological Sciences 276:3037-3045.