Rethinking invasive species

This commentary posted to Mongabay questions whether eradicating invasive species is a good idea. Is invasive species management doing more harm than good?

I have to say that I have asked similar questions. This morning I was out walking the local bike trail. As in most of the Bay Area lowlands, especially populated zones, there is almost no native herbaceous vegetation left; yet the mostly-invasive herbaceous flora supports native fauna. Today I saw a small (presumably native) bee foraging on bristly oxtongue – one of the coarsest and most ubiquitous weeds. On numerous previous occasions, I have seen Lesser Goldfinches eating the seeds of dandelion, chicory, and sow thistle – also among our most ubiquitous weeds.

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I’m skeptical. Insect-plant partnerships are much more fine-tuned than those with vertebrates, which tend to be generalists. And insects appear to have taken a large and little-remarked hit to their welfare, shaking the foundations of food webs globally.

That said, I do think the concept must continue to evolve. Climate is changing and ‘native’ ecosystems will have to deal with the disruption somehow. We’re gonna have to get smarter about this stuff fast.

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Native animals that visit non-native plants are almost definitionally generalists, which also means that they will be least affected by the removal of invasives.

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I think this question, broadly speaking, is an interesting one and my intuition is that there is a lot more room for subtlety than some of the public discussions would have us believe. Along these lines, in the interest of fleshing out my thinking, there are two books I would like to read when I finally find the time. These might be interesting to you too:

  • Where Do Camels Belong?: Why Invasive Species Aren’t All Bad by Ken Thompson
  • The New Wild: Why Invasive Species Will Be Nature’s Salvation by Fred Pearce

(No financial or other relationship with the authors to disclose)

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A lot of invasive species are definitely here to stay, especially around human disturbance and habitation, but the species which always feel the brunt of habitat destruction/disturbance are always the more specialized ones. Native species that are more generalist in terms of diet and habitat are generally way more tolerant of disturbed habitats full of invasive plants, and will live around human habitation just as well as they will more pristine habitat

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I view removing introduced plants as the flip side of promoting native plants. Plants in general are in competition for space, light, water, fertilizer. Every spot occupied by an introduced species can’t be occupied by its native competitors. The introduced species also excludes any insects that may be specialized to use the native plant. So I believe our basic mind-set has to be “remove the invaders!”

That said, there can be complications. Will the invader be replaced by different invaders? Then maybe removing it isn’t worthwhile. Does the invader provide some important ecosystem service that can’t be provided by the natives in the currently seriously disturbed ecosystem? Is the process of removing the invader more disturbing than the invader itself, and if so, is that worthwhile? Can we / will we keep the invader out if we remove it?

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Books have been written on this topic. Indeed, the presence of invasive species brings complexity to conservation and the politics thereof.

So much effort and time have been dedicated to alien species, both (often first) in propagating them, and (later) trying to eradicate them.

The majority of alien species never take off and fail to establish when introduced, and in many cases are maintained only by human attention and care (hybrid rose cultivars come to mind).

Others spread beyond human control, due to suitable environmental conditions (climate, soil, etc). In many cases however, they are simply responding to the changes humankind has already made to the landscape. Many native species will never grow in hard, compacted soil, but the alien dandelion will. Many native species will also not germinate in areas that might otherwise by suitable, because mankind has changed the chemistry of the soil (due to agricultural activities and/or pollution). Water hyacinth loves waterways that are high in nutrients, so it will spread in lakes and rivers with high agricultural run-off and sewage. In other words, they are simply filling in the empty niches available. In such cases, is the alien “invasive” species really taking over from the native vegetation, when mankind has already made conditions unsuitable for the latter?

Some species are invasive because they proliferate without some form of control, whether that is a pathogen or a herbivore. An example is the invasive species Mimosa pigra from the Neotropics, which had invaded Gorongosa National Park in Mozambique. Researchers found that this species decreased dramatically after the recovery of the park’s large herbivores.

Other invasive species have been brought in by mankind and are useful economically, and if removed will impact the livelihoods of people who may depend on it. These species include prickly pear, eucalyptus, etc.

Ironically, this means that some species get labelled as invasive when they actually would otherwise mitigate the ecological impact of the economically useful alien species. Here I am thinking of various insects that attack economically useful but ecologically harmful species including pines and eucalyptus, such as the Sirex woodwasp and eucalyptus longhorn beetle.

And as noted, rare and endangered native species may come to depend on alien species for survival. An example is the willow flycatcher with tamarisk, and various cases where the alien species created conditions suitable for native species to live and thrive in.

The presence of invasive species often tells more about people’s mindsets, attitudes, and actions, than it does about the actual impact of the species in question.

One thing is for sure: many alien species are here to stay and there are few places in the world where not one alien species has established due to human presence. There is no turning back the clock. Some alien species are minor, while others that are brought under some form of biocontrol can naturalise into the local community. Others may be problematic for years or decades to come as long as conditions favourable to them remain.

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I’m generally in favor of slowing the spread of invasives to allow natives to adapt, but eradicating them is a fool’s errand.

Random disclaimer: I’m not one of those people who doesn’t want to kill anything.

I spent several years conducting research in Lake Erie coastal wetlands dominated by invasive Phragmites australis and Typha angustifolia, equipped with your standard meltdown components like Eurasian watermilfoil, common carp, round goby, zebra mussel, invasive honeysuckle-dominated understory in adjacent forest, etc. The reactionist/government solution to this was to bomb everything with herbicide, wetlands included, every 3 years, with a helicopter and a specialized boat. (During nesting season. Good job, Ohio.)

The local songbirds and herps of conservation concern didn’t care about the invasives. Some, like the marsh wren, even preferred them for nesting, or, like the lake erie watersnake, preferred to prey on invasives. The macroinvertebrates didn’t seem to be affected by them (or the herbicides, for that matter). Fish diversity didn’t decline significantly beyond a handful of species.

The whole thing was a waste of money and an unnecessary release of toxic chemicals into the environment. The most damaging thing in that particular ecosystem outside of the herbicide, imo, is actually the population of subsidized mesopredators, raccoons specifically.

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This pretty much aligns with my views. Eradicating invasives is generally a good goal, but there are several huge caveats.

  1. All time and money spent on removing invasives is time/money that could also be spent doing something else with a positive impact. The real question isn’t whether removing invasives is good or not - it’s whether it’s the best use of resources. In my opinion, it often isn’t (though prevention is).

  2. Not all invasives are equal - while some have huge impacts, most have minimal impacts. Trying to remove those with lesser imapcts isn’t worth it.

  3. A lot of invasive removal won’t have serious benefits in the long run. Unless there’s a plan to continue suppression/manage an environment, it’s generally not worth it.

In fact, I think the situations in which invasive removal is worth it are likely rare. The conditions under which invasive removal is worth considering (that immediately come to mind) are:

  • An invasive species is in the early stages of establishment, and it’s actually possible to completely eradicate it and prevent future spread.

  • The invader has a devastating impact, and even partially reducing that is worthwhile.

  • There’s a good chance that certain native species that might otherwise be extirpated/go extinct will adapt if they are given enough time by managing invasive populations.

A good example of what I consider to be pointless invasive removal is Burmese python hunts in the Everglades. These pythons do have a huge impact, but there’s no way to effectively control their populations in the inaccessible habitat of the Everglades. It’s good PR perhaps, and may help raise general awareness of problems with invasive species, but the pythons and their impacts are here to stay. I would guess this isn’t a good use of resources.

An example of worthwhile invasive removal is eradication of giant African land snails in Florida. These snails have huge negative impacts for plants and humans. They have been successfully eradicated before, and the investment of time/money in eradicating them a third time (hopefully) is likely a good won, weighed against the potential costs of their spread in the US.

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Look at fields of 3+ m high https://www.inaturalist.org/taxa/499936 (actually hybrids of different Caucasian species) that kill off everything else, or different Asian Impatiens or American Solidago species and all questions end, invasives destroy native habitats and all trophic links with them.

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The worst invasive species is Homo sapiens, and somehow no-one ever wants to eliminate that one!

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as far as I know the pythons in the everglades are a bit overblown as well? They are there, but, from what i’ve heard being a python in south florida sucks, temperatures still get too cold in the winter for a large tropical snake to really thrive, high mortality for young snakes, adult snakes never get as big on average as they do in their native range. Feral cats and hogs are likely still bigger issues but yknow, “invasive hogs ruin everything” doesn’t sitck in people’s minds like “giant invasive maneater snake that will KILL you in the everglades” does (albeit this is all things i’ve heard over the years, haven’t been down to south florida myself ever)

I think one of the things to remember is that we (ie humans) have altered all the environments that we somewhat commonly see, at least to a certain level. Even preserved areas in the US are directly impacted by decisions that humans have made since the start of the industrial revolution. One thing that’s quite common is that non-native species flourish in more altered (not necessarily urban, just altered) environments, as opposed to restored habitats. That means that we must understand species that become, not just introduced, but invasive, with the context of our own behavior. And that demands that we ask ourselves what our relationship is to the natural world.

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Our current understanding locally (Minnesota) is that “invasive” means “spreads aggressively.” Native species can be invasive. Where we live, certain Cedars and Sumac are native and invasive. Non-natives can also be invasive or not. Common Buckthorn is not native here and is invasive, crowds out native and other non-native species. Does anyone else use these definitions?

In scientific circles, usually “invasive” = non-native + aggressive, and “aggressive” = native + aggressive.

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Humans have been living in concert with the ecosystems on this planet since the dawn of history. Humans as a species aren’t the problem. You may in fact be thinking of capitalism or colonization.

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I would not say that they are overblown, though I’m not sure exactly what you’re referring to. They can have clutches >70, the temps really don’t get too low for them, and they get plenty large (18 ft is the record for FL). Hogs are definitely an issue, but I don’t think feral cats are breeding throughout the Everglades.

If someone is claiming that pythons are a serious threat to humans, that’s pretty much bunk. Burmese pythons attacking humans in S Fl, while theoretically possible for a small child or something, is not really happening (I haven’t heard of any serious incidents).

On the other hand, pythons are having a pretty massive effect at the ecosystem level. They have caused huge reductions in mammal populations in areas where they have been established. This has cascading impacts on other organisms - it can even be beneficial for some that have reduced nest predation, for instance. But by and large, they are likely disrupting ecosystem functioning though we don’t have enough evidence yet to really understand/quantify this.

For an overview, this peer-reviewed article and pop sci coverage are a good place to start.

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Can’t argue against something (the weedy invasive) being better than nothing. Anecdotally though, it seems like natives support more biodiversity.

I might see some other lifeform maybe 1 in 5 times, usually a Western Honeybee (Apis mellifera), but in this patch of Narrowleaf Milkweed (Asclepias fascicularis), within less than 5 minutes, I saw:

May go back again because it was so easy to observe all of those, almost like topic: Pollinators at the garden centre: Like stealing candy from baby fish in a barrel.

So I think, maybe it’s not so much a question of whether invasives are useful, but whether, given a piece of land, whether it’s better to have invasives or natives grow on it.

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Definitely agree with the land use conservation decision aspect. There’s just some patches of land that are too degraded and will remain too degraded for many native plant species. So, at least having the introduced ones that native generalists can feed/live on is worthwhile to maintain some semblance of ecological integrity. I keep thinking of highway-adjacent “meadows” dominated by introduced and invasive plants where the soil salt content and particulate matter sort of excludes anything else from growing there.

Of course, this would open up the new aspect of “which introduced plants would we rather have in those degraded areas”.

Caveat - if a severely degraded area was formerly a key ecological area, how many resources would we be willing to put into restoring the area given the probable need for long-term sustained resource investment

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I admit to similar feelings. Whenever I see a bee or butterfly alight ontop of an invasive thistle, or a goldfinch eating common mullein seeds I think they aren’t so bad. But then I see a recovering natural area (from agriculture or other human uses) invaded with Canadian thistle or swathes of mullein and curly dock and think, no they are that bad. Here in Colorado we have many native thistles that grow at the same elevations as these invasive ones. The natives do not form large, monoculture colonies the way the invasives do.

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