Do parasites really have a charm deficiency?

While drafting yesterday’s post (which was my 150th for Increasing Disorder!), I included some discussion about parasites being intentionally lost as part of conservation efforts for their hosts. While it didn’t make the final cut, it is worth talking about as further evidence that parasites are not on a level playing field for conservation. Do parasites really have a charm deficiency? Or is there something else going on? There is a perceptible degree of arrogance with regard to conservation of species – that some species are more equal than others. It won’t matter if some little worms are lost, as long as the cute fluffies are saved, right? Wrong.

One way in which parasites can be lost is through translocation of their hosts. Translocation is a method used to counter loss of species. It is expected to be used more frequently in coming years as habitat loss, environmental perturbation and climate change affect populations of endangered species (Sainsbury & Vaughan-Higgins 2012). Translocation can be as simple as rounding up a number of individuals from one site and releasing them in another, but usually there are various holding periods where the individuals are kept in captivity to breed and increase the overall population size before being released. Translocated individuals could therefore potentially either lose parasites (or other pathogens, commensals or mutualists) or introduce parasites (or other pathogens, commensals or mutualists) into the new area. The former is of particular importance for plants, which often rely on certain invertebrates for pollination, or have invertebrates that rely on them for habitat (see Moir et al. 2012 for discussion on this).

A famous case of parasites being lost via captive breeding is that of the louse Colpocephalum californici. It was intentionally removed (as part of a comprehensive de-parasite treatment) from its host, the critically endangered Californian condor (Gymnogyps californianus), when the last 22 known condors from the wild were brought into captivity (Mihalca et al. 2011, Colwell et al. 2012). However, while many lice are highly species-specific, it appears that there has been little study on the released condors, or any co-occurring related birds to examine whether the condor’s ectoparasites have managed to survive on other hosts (Colwell et al. 2012). Although there is evidence that endoparasites (such as nematodes or cestodes) do stand a greater chance of survival in a translocation event than ectoparasites (Moir et al. 2012); if all hosts held in captivity are treated with anthelmintic drugs to remove parasites, then they will most likely be lost anyway. This is bad if there are either no other wild hosts (e.g., the Californian condors), or if the parasite is not distributed evenly across the whole geographic range of the host.

While the loss of parasites during captive breeding and translocation is bad for biodiversity, allowing parasites to travel with their hosts into the translocated area can cause problems too, by introducing new pathogens into wild populations. Parasites occurring in hosts being translocated may not occur in the population being supplemented. Or, translocated hosts may become a new host for an existing parasite/pathogen, increasing its presence in the environment. Such changes to host and parasite/pathogen balances can potentially cause outbreaks of disease, or could intensify the infection rate disease in the translocated naïve population (Sainsbury & Vaughan-Higgins 2012). It is critical that an adequate understanding of the parasites, pathogens, commensals and mutualists is required for any translocation or captive breeding to succeed. Interestingly, however, captive breeding programs are often subject to human intervention and interaction on a number of direct and indirect levels. In a recent study, it was found that captive breeding of brush-tailed rock wallabies (Petrogale penicillata) appeared to have conferred a degree of antibiotic resistance in their gut bacteria, via inclusion of mobile genetic elements (integrons) of bacteria (Power et al. 2013). Such integrons were absent in bacteria from wild populations, indicating that the gut flora of the wallabies acquired them during their time in captivity (Power et al. 2013).

There are obviously many elements to consider when making large steps towards conservation in form of species translocation. Parasites (and commensals and mutualists) are an essential – and immense – component of healthy ecosystems, and there is still much to learn about their role in regulating ecosystem function (Dobson et al. 2008). They should not be ignored when decisions on conservation are being made for their larger, more charismatic (and charming?) hosts.

placard

References

Colwell, R et al. (2012) Coextinction and persistence of dependent species in a changing world. Ann. Rev. Eco. Evol. Syst. 43, 183-203.

Dobson A et al. (2008) Homage to Linnaeus: How many parasites? How many hosts? PNAS 105, 11482-11489.

Mihalca, AD et al. (2011) Coendangered hard-ticks: threatened or threatening? Parasites & Vectors 4:71.

Moir, ML et al. (2012) Considering extinction of dependent species during translocation, ex situ conservation and assisted migration of threatened hosts. Cons. Biol. 26, 199-207.

Power, ML et al. (2013) Into the wild: dissemination of antibiotic resistance determinants via a species recovery program. Plos One 8, e63017.

Sainsbury, AW & Vaughan-Higgins, RJ (2012) Analyzing disease risks associated with translocations. Cons. Biol. 26, 442-452.

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