possums, predators, and biocontrol

Featured image from https://www.environment.nsw.gov.au/topics/animals-and-plants/native-animals/native-animal-facts/brush-tailed-possum

Recently I shared this Spinoff article about extinction on Twitter, & tagged the Science Learning Hub as the NZ focus makes the article a good fit with their mahi supporting student learning. But I was somewhat surprised to have someone else pop up saying that they wouldn’t read it because, well – here, read the comment in its full glory.

“Pure Democracy” is guily of making a sweeping assumption here, because the article isn’t about pest control (by poison or any other means). But he also made a specific, testable claim. So, I asked for supporting evidence:

“The scientifically supported solution to rodents and possums is to bring in predators.”

Citation please. Because NZ tried that when rabbits first got out of control, bringing in mustelids. And look what a disaster that was for our native species.

— alison campbell (@AcampbelTeacher) April 11, 2023

As you’ll see if you go to the thread, PD went on to spam it with a lot of links, none of which supported their claims about possum control. (The great majority were about control of insect pests, which is something that I should probably write about as well.) But let’s focus on the possums. It turns out that in Australia, the main predators of brushtail possums (Trichosurus vulpecula) are dingoes, foxes, cats, & pythons, though apparently the endangered Powerful Owl (Ninox strenua) has recently made something of a comeback and is known to take possums in urban areas.

Well, apart from cats we don’t have any of those predators here in Aotearoa, and I doubt that their importation is the “scientifically supported solution” to the presence of possums.  One of the key questions that is asked in looking for suitable biocontrol agents for pest control is this: how likely is it that the proposed control agent will stick with the target species and not move on to taking others? So, how do the natural predators of Australian possums measure up?

While it’s true that N.strenua’s main prey in Australia are possums and greater gliders, there’s an extensive list of other species on its menu. It is clearly a generalist & would pose a threat to our native birds (& potentially, free-range domestic poultry). We already know that cats prey on a range of species that includes rodents, birds, & reptiles – and that they have not to date made any obvious inroads into the possum population. (It would be a big cat that would take on an adult possum.) Dingoes, too, are opportunistic generalist predators and don’t rely solely on possums. The same is true for foxes, which were introduced to Australia in the mid-1880s: they have a very wide dietary range and are considered a threat to more than 70 native Australian species. And pythons are also generalist feeders.

In other words, PD’s claim fails on that key question; it’s not a scientifically-supported fix for the possum problem¹. Bringing in these generalist predators, with their wide dietary tastes, would be a move pretty much guaranteed to put our native animals at even more risk of being added to the “endangered” list – or worse, the “extinct” one. And as the Spinoff article I shared says, we have reason to care about this:

humans are part of ecosystems, not separate from them. The food we eat is grown in soil enriched by other species, and it’s pollinated by them too. The carbon we produce is absorbed by trees and the ocean. The waste we throw away is decomposed by microbes and invertebrates, ready to begin the cycle all over again.

Beyond the services that ecosystems offer humans – a transactional way to see the species we depend on – the risk of extinction is worrying if you think that other living beings have an inherent right to exist, Lee says. “We know what the problems are, and we should do our bit to fix them. If we let species decline, the world will be a less vibrant and rich place.

¹ The idea wasn’t even touched on in this 2003 paper by Parkes & Murphy, which specifically discusses control of mammalian pests in Aotearoa New Zealand. However, biocontrol methods that interfere with fertility have definitely been investigated, discussed eg here, here, & here, for example, along with investigations of the potential for genome-based control methods.