For most people diseases and parasites are only seen as something to be eradicated because of their negative impacts on the health and well being of humans, or agricultural and wild species
Collecting Galapagos giant tortoise poo to hunt for intestinal nematode eggs
However, pathogens and parasites actually comprise much of world’s biodiversity since every species, be they of interest to people, or barely noticed, hosts its own unique co-evolved community of disease causing organisms and other parasites. This unregarded realm of biodiversity is fundamentally important to maintaining ecosystem diversity and function through regulatory effects on hosts, and by driving the evolution of host species. In the current biodiversity crisis, the extinction of a host species is likely to also cause the extinction of its associated parasites, magnifying the biodiversity loss.
A dilemma for conservation scientists and managers therefore is what interventions should be made for wildlife disease in threatened species. On the one hand some parasites may put species at risk, but pathogens are also important biodiversity worthy of conservation in their own right. In a recent paper our team from the Royal Veterinary College; School of Biology, University of Leeds; Zoological Society of London; University of Guayaquil; and Galapagos National Park Service shed some light on this issue. As part of a programme assessing the health of endangered endemic Galapagos giant tortoises, we surveyed the different giant tortoise species scattered across the Galapagos Islands for eggs from parasitic worms which live in tortoise intestines, examining hundreds of tortoise poo samples from wild individuals, and tortoises which are part of a captive breeding programme run by the Galapagos National Park. Different worm species produce eggs of different shapes and sizes, so cataloguing the numbers of different egg types seen in different individuals and tortoise species gives a measure of worm parasite diversity.
We found that each tortoise species has a unique complement of worm parasites, with the presence and relative abundance of worm species varying between tortoise species and islands. This is puzzling since all the tortoise species share a common ancestor, and so the parasites which must have arrived with the first tortoise colonists from the mainland, should be similar across the archipelago. This suggests the tortoises and parasites must have co-evolved with each other during the subsequent colonisation of the archipelago by the tortoises. We are currently working to understand the mechanisms underlying these differences.
The observations in the wild tortoise raise some important points for the management of the tortoise captive breeding programme and for conservation programmes globally. In Galapagos, tortoises are bred or raised in captivity for release back into the wild to help restore populations that were devastated by the combined effects of human hunting, habitat loss and invasive species. Since the wild tortoises have unique coevolved parasite mixtures, biosecurity measures are needed to prevent possible cross-contamination of parasites between tortoise species within the captive breeding programme. The maintenance of parasite diversity is seldom considered in conservation programmes, but possible short-term impacts on animal health and long-term impacts on species evolution through disruption of co-evolved parasites communities should be considered when planning conservation interventions.
Currently it is not completely clear if the worms are detrimental to tortoises, but it appears that most of the time the tortoises tolerate the worms, and they may only affect health if the tortoises suffer some other kind of disease or stress.
Guillaume Fournié, Simon J Goodman, Marilyn Cruz, Virna Cedeño, Alberto Vélez, Leandro Patiño, Caroline Millins, Lynda M Gibbons, Mark T Fox, Andrew A Cunningham (2015) Biogeography of parasitic nematode communities in the Galápagos giant tortoise: implications for conservation management, PLOS ONE 10(9): e0135684. doi:10.1371/journal.pone.0135684