Parasites are ecological dark matter – and they need protecting

The two species of African rhinoceros are conservation icons. Large, charismatic, and vitally important for their ecosystems, to lose either species would be a devastating blow.

But what of their parasites? If the black and white rhinoceros went extinct, so too would four species of host-specific tick that call the rhinos their home – should we not also be worried about their extinction?

This provocative question was first raised over 30 years ago and presents a significant challenge to the common view of parasites as a threat to conservation. If the goal is to preserve biodiversity, early supporters argued, then this should include all species and not just the most charismatic ones.

Over the past three decades, a range of ecological evidence suggests that parasites are indeed worthy of our attention. In simple numerical terms, parasites make up a significant proportion of animal diversity (estimates range from a third to over half of all species) and are likely to make up most species extinctions. More importantly, parasites play vital roles in evolutionary and ecological processes.

Parasites have been likened to “ecological dark matter” – an unseen force vital for sustaining life on earth as we know it, supporting ecosystems that are more diverse, more resilient, and more productive.

Take the flatworm parasite Euhaplorchis californiensis, for example. When it infects the small, colourful California killifish, the fish swims in a jerky, conspicuous way close to the water surface. This makes it more susceptible to being eaten by birds such as herons, which are the flatworm’s ultimate hosts.

Its eggs are then shed in bird faeces, with further stages cycling through snails and back to fish. This helps transfer nutrients through the ecosystem.

Parasites can even make a whole ecosystem more resilient to droughts. By infecting herbivorous Littorina snails, parasites make the snails less likely to eat marsh vegetation. Fewer parasites mean the snails will do more grazing, exacerbating vegetation loss during droughts and potentially making their ecosystem less stable.

When a species dies, so do its parasites

Parasites are vulnerable to conventional threats such as climate change and habitat loss, but also to declining populations of their host species. Many recent extinctions are likely to have been caused by the extinction of a species the parasite depended upon.

Conservation actions may be as simple as stopping the unnecessary use of anti-parasitic chemicals or as complex as breeding “insurance populations” of parasites in captivity. Although there is increasing expert support, there is still a distinct lack of practical work.

The handful of positive examples include a captive breeding programme for the Ryuku rabbit tick, which lives on a rabbit found only on two small islands hundreds of kilometres south of the Japanese mainland. Or there’s the pygmy hog-sucking louse which has been officially classified as “critically endangered”. Its host, a small pig from the Himalayan foothills, is fast losing its habitat.

It is still far more common for threatened parasites to persist through chance rather than through deliberate conservation action.

The neglected social side of parasite conservation

In our latest research we suggest that an understudied aspect of parasite conservation is the attitudes of people towards parasites and their conservation. Little is known about the level of support among professional conservationists or the wider public. Nor do we know what factors might be driving support or opposition.

There are many social and cultural reasons that people may be reluctant to conserve parasites. People often have strong feelings of disgust towards parasites and the signs of their presence (such as sick, dying, or dead animals). These feelings are part of a set of innate responses that help us avoid infection risks.

Others have suggested that parasites might be too ugly or alien to us to gain popular conservation support. Consider a real-world host-parasite relationship such as the roundworm Baylisascaris schroederi which lives in the bowels of its host the giant panda. We can anticipate why some people might be drawn to the charismatic host, but apathetic if not outright hostile to the idea of conserving its unique parasite.

Beyond human perceptions of parasites, there are a range of social and economic factors that might create barriers. Most people don’t know how important parasites can be, for instance, and technical resources and guidelines on how to actually conserve parasites can be limited. Sometimes its simply because there is a lack of available money and effort at a time when so much in the natural world is at risk.

Ethical opposition

People may also hold an ethical or philosophical opposition. They may fail to see the value of wildlife parasites, or think this value is outweighed by the parasites harming their hosts or the risk of diseases spilling over into humans and livestock. Although this could be due to incomplete awareness, we argue that these concerns also need to be taken seriously if we are to realise the best outcomes for parasites, hosts and people.

Parasites present an interesting conundrum for environmental philosophers. Conservationists often talk about the value of a species or biodiversity in general, but should this extend to parasite diversity? If the eradication of smallpox was a good thing, why should the extinction of a wildlife parasite such as the panda roundworm be any different in terms of good and bad?

For parasite conservation to be successful we need to unpack the values and harms associated with individual parasites so that we can make more informed judgements about how they should be treated. This will ensure that human biases against parasites don’t lead to the widespread removal or neglect of a group of species that many would argue have just as much right to be here as any others.

Take the tuatara tick as an example. Evolutionary biologists believe the lizard-like tuatara and its unique tick have been co-evolving together for the past 80 million years, ever since the New Zealand landmass split from the supercontinent Gondwana.

Ticks have many potential uses in the production of medicines due to anti-inflammatory and anaesthetic chemicals they produce for feeding. The tuatara tick also appears to cause relatively little stress to its host. Its extinction, in large part due to the human-driven decline of its host, would represent the loss of a unique part of the natural world with little benefit for its host or humans. It is hard to imagine how such a loss does not make the world a poorer place.

Don’t have time to read about climate change as much as you’d like?

Get a weekly roundup in your inbox instead. Every Wednesday, The Conversation’s environment editor writes Imagine, a short email that goes a little deeper into just one climate issue. Join the 40,000+ readers who’ve subscribed so far.