Earth Minus Invasives (No Comments)

ISSG Top100

The top 100 most destructive invasive species as determined by the IUCN-Invasive Species Specialist Group.

I think I deserve a pat on the back – using only the power of my mind, I have, in one fell swoop, done what every land manager has been trying to do for decades and deleted every non-native species from the planet – all without chemicals or having to do the dirty work of eradication.

Let’s put some bounds on this: does it mean those non-native species were wiped off the entire face of the earth? That doesn’t seem right. OK, the species wouldn’t be gone from the entire planet, but only from the spots where it would not currently be living if it weren’t for human intervention or activities. That is to say, from its non-native range, which, for the sake of argument, includes range expansion due to climate change as well.

Gone is the brown tree snake (Boiga irregularis) from Guam, the kudzu (Pueraria spp.) from the southeastern United States, the Nile perch (Lates niloticus) from Lake Victoria, and something like 90% of the biomass in San Francisco Bay. Incidentally, also gone is all of my research, which means I have to start my dissertation from scratch – so, that’s kind of a bummer.


The sigh of relief from native birds is audible as the brown tree snake disappears from Guam.

Oh, but wait, I think I also just deleted all of agricultural production in the world. Whoops, sorry about that. OK, hang on, I can fix this. Because this is happening only in my brain, I am perfectly happy setting more rules where only individuals of species that were moved by accident or that were moved intentionally, but are now known to have negative ecological or economic impacts, are erased. What about biocontrol for agricultural pests? What, like cane toads in Australia? Well, duh, definitely cane toads in Australia, are you kidding?! They never even really functioned as biocontrol.

Alright, I realize even with these rules, there are exceptions and many who will point at me with the “gotchya” finger and say, “But what about _____?!” And as a blanket response to all of you, “Sure, yes, there are exceptions, and discussing them is valuable, but the point of this thought experiment was to obliviate (turns out that’s only a word in Harry Potter but you get the point) those individual plants, animals, and pathogens that are considered invasive – inclusive of those that cause ecological if not economic harm.”

So, we wake up tomorrow and those plants, animals, pathogens, etc. are all kaput. What now? Would we see everything go back to a pre-global-homogenization utopia?

In the majority of cases where eradication has been achieved in the real world, removal of non-natives is good news for the native species. Many studies of invasive eradication come from islands, systems in which invasives have some of the most dramatic impacts on resident organisms, so it’s not entirely surprising to see evidence that natives bounce back. Some of the worst threats to islands are terrestrial vertebrates: Rats and cats, goats and stoats, pigs and … snakes – all have invaded islands and imperiled or extirpated native fauna and flora (e.g., Fritts and Rodda) and changed ecosystem processes (e.g. guano). For some birds in particular – I’m looking at you takahe – predator-free islands appear to be their only hope. A review of the effects of predator eradication on birds backs this up: combined monitoring results from 112 studies showed a global average of 25% increase in population growth rate following predator removal.

However, that’s not always the case, and eradication efforts have resulted in some ecological surprises. For instance on Macquerie Island, removal of invasive cats allowed for a temporary boom of invasive rabbits, which in turn mowed down a lot of native island flora . Naturally, in my [pipe]dream world, this wouldn’t have happened because both cats and rabbits would have gone “poof” from the island simultaneously. It’s also worth noting that there are a couple of well-publicized scenarios in which rare or endangered natives have become partially or entirely dependent on non-native species, such as the Island marble butterfly and the southwestern willow flycatcher. I would argue that these cases are pretty rare exceptions. Nevertheless, sudden disappearance of their favorite non-natives could lead to very bad days indeed for the natives in these scenarios.


The Island marble butterfly is only found on two of the San Juan Islands in Washington State, and now depends on non-native mustards to complete its lifecycle.

Then again, sometimes nothing happens! In another Biodiversity Challenge post, Fletcher Halliday suggested that removal of Chestnut blight wouldn’t necessarily result in recolonization by the American Chestnut. In many cases, the landscape faced by native species, even in the absence of non-natives, has dramatically and irreversibly changed. Sometimes, this is literal. For instance in areas previously dominated by ice plant and salt cedar, those plants have changed the soil characteristics, so even after the plants are gone, recolonization by native plants is not a foregone conclusion. More importantly, however, non-natives are not the only whole-scale ecological threat natives face; habitat change and degradation are larger and graver threats to the existence of most species. So unless, and perhaps even if, we also consider all humans outside of Africa to be invasive (and the IUCN-ISSG evidently does), and we remove them/us as well, many species would still need additional help to recover.

In short, my best guess is that the effects of wholesale deletion of invasive organisms from the planet would probably be as idiosyncratic as the wholesale global translocation of species has been to begin with. I think in the majority of cases, native species would indeed benefit, but we probably wouldn’t see a return to pre-Anthropocene community structure and function, and we would get some surprises along the way. This is not to say invasive species management is pointless – quite the contrary! Deletion of all invasive organisms would give us a major leg up in conservation of many natives, and is critical to the conservation of some systems. But it would also require that we follow through by changing our other destructive behaviors – and do a better job preventing future invasion.

Ecology and evolution have winners and losers contingent on history, context, and garden-variety luck (=stochasticity). Deletion of invasives is itself a kind of contingency, which would make it an exciting horse-race to watch, even if it’s hard to place winning bets.

June 6, 2016

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