Parasites are gross. Some of them are downright awful. Fleas are nasty. Plasmodium, the genus of protozoa that causes malaria, deserves our scorn.
But that doesn't mean we shouldn't protect them.
Ecologists Andrés Gómez and Elizabeth Nichols surveyed 77 different conservation biology textbooks published over 39 years, and found that almost three quarters of the books "either portray parasites uniquely as threats to conservation goals established for free-living species or do not mention parasites at all." Only one parasite, the pygmy hog-sucking louse, is listed as endangered on the IUCN Red List.
Parasites don't have a whole lot of friends or admirers because they can harm and kill wildlife, livestock, pets, and humans. Even for people who want to study or save them, parasites' small size, often-complex life cycles, and tendency to live inside other animals makes them hard to keep tabs on. This doesn't mean we should exclude them when we talk about conservation, though, say Gómez and Nichols. Rather, we need to include them, because parasitism is perhaps the most popular lifestyle on Earth. Of all the animal phyla (the taxonomic rank right below kingdom), nine are entirely parasitic, 22 are predominantly parasitic, and the rest have parasitic groups scattered throughout them. There are an estimated 75,000 to 300,000 species that parasitize vertebrates, never mind the ones that choose spineless hosts. Overall, Gómez noted in 2011, parasites may "outnumber free-living biodiversity by as much as 50 percent." If we forget about them or actively ignore them, we're taking an awful lot of organisms off the table.
Parasites are major ecological and evolutionary movers and shakers. Through the disease and death they bring, parasites can affect species distribution and density. By leeching resources from their hosts, they alter animals' energy needs — which affects growth, reproduction, competition, and survival. Some parasites even directly alter their hosts' behavior and force them to do their bidding. Parasites also drive adaptation and evolution as they and their hosts engage in "evolutionary arms races," with each trying to gain the upper hand in either infecting or resisting the other.
Any of these effects can be good or bad. Sometimes they're bad for some individuals or populations of plants and animals, and good for others. Either way, Gómez and Nichols say, parasites are "critical components of both the patterns and the processes that form natural ecosystems." They are gears in a complex moving machine, and losing them has repercussions that we might not yet fully understand or predict.
For all of that, Gómez and Nichols say, parasites should be considered "meaningful conservation targets no less relevant than their hosts."
That's no small order, but these ecologists have some ideas on how to get there.
First, as they said in 2011, conservation textbooks need to start making some important points about parasites: (1) that they're diverse and represent the majority of life on the planet, (2) that they're crucial to ecological and evolutionary processes, (3) that they do present challenges to conservation because of their role in population declines but (4) also deserve the same consideration to "intrinsic, aesthetic, and utilitarian value" as their hosts, and (5) that they're rarely considered conservation targets.
From there, the scientists' new work argues that working parasites into conservation efforts requires improving our knowledge of their role in ecosystems, creating conservation strategies for specific parasite species (e.g., maintaining parasites with endangered hosts in alternative species in captivity), continued focus on ecosystem-level conservation that indirectly benefits parasites and improving their public perception.
It's probably a bridge too far to suggest that we all cozy up to parasites — but we should recognize that wiping them out isn't good either.