There seems to be a pervasive pattern in the way naturalists and the media talk about invasives species. Invasives are qualified and their merits as a species evaluated by the impact they have ecosystems. They universally receive negative marks by those who grade them, getting dinged because their fruits are less nutritious than native species (honeysuckle), they turn over nutrients too quickly and remove the insulative layer of leaves (earthworms), they evict native species from their nests (European starlings). Somehow native species get a pass for demonstrating the same characteristics, and our native flora and fauna (I’m looking at you nectarless Hepatica – source – and parasitic cowbirds) are praised for just being clever species living well in place. As I’ve said before, invasive plants are native somewhere and the same adaptations or behaviors that we locals may find so abhorrent, in another land we might admire for the inventive way that species has maximized its competitive edge.
The Opossum-Tick Myth
The same thing happens in reverse with our ugly native fauna. There seems to be a need somehow to justify why something so ugly is actually “good” for the environment. Surely all things must serve a purpose? Nowhere is this more true than with the opossum. And nowhere has this rankled me more than when I first heard that opossums, which are objectively ugly, awkward, and more than a little viscerally repugnant (though adored by me, your humble grumbler), should be loved because they eat ticks and will save us all from Lyme disease. Don’t get me wrong, they should absolutely be loved (I for one, adore opossums) Well, turns out it was all one big April Fools.
If we’re to believe the agitprop of the National Wildlife Federation, “opossums act like little vacuum cleaners when it comes to ticks—with a single opossum hoovering up and killing an estimated 5,000 ticks in a season.” Pretty impressive stat block on this little marsupial. This nonsense can be traced back to a 2009 study (Hosts as ecological traps for the vector of Lyme disease) where possums and squirrels were inoculated with 100 ticks and then observed over the next 4 days (roughly the time it takes for a tick to feed on its host and drop off). The authors summarized: “we found that some host species (e.g. opossums, squirrels) that are abundantly parasitized in nature kill 83–96% of the ticks that attempt to attach and feed, while other species are more permissive of tick feeding.”
One big problem is that the authors extrapolated from their poor hosts held in captivity to behavior of opossums in the wild. This is similar to the bunk that buckthorn is a laxative for birds. This myth is based on a study where robins and catbirds were forced to eat fruits with artificially high levels of emodin (the equivalent of eating 50 unripe fruits), they developed diarrhea, even though other studies have concluded that emodin poses no real threat to birds (link). I’d develop diarrhea too if I were force fed an overabundance of unripe fruit.
The other major problem is that the authors in the 2009 study did not check for ticks when they released the opossums after 4 days?! So it’s quite possible that 4 days wasn’t long enough for the ticks to feed and then drop from the opossums rather than the opossums ingested the 90% of the ticks as they groomed them over (source). In a follow up study in 2021, researchers dissected and examined the stomach contents of wild caught opossums and found 0, that’s right 0 evidence of tick remains (source). So not quite the tick-vacuums we’d pinned all our Lyme-fighting hopes on.
We don’t need to exaggerate ecosystem services to fabricate a fascination with our wild neighbors. They are endlessly fascinating and equally valuable whether or not they have a functional value. Be critical and be curious, but don’t be a fool!
