12 October 2009

More on invasive species

I've been reading about invasive plants. In my post about purple loosestrife, I promised more about invasive plants which actually have been shown to have significant negative effects on native species.

Easier said than done.

I though I had hit what I was looking for when I found a paper titled Ecology and ecosystem impacts of common buckthorn (Rhamnus cathartica): a review The paper starts elucidating why it is so successful: Shade tolerance, rapid growth, high photosynthetic rates, etc etc. Sounds great. But when it gets to showing how it hurts native plants, it all sort of falls apart – even though the authors clearly WANT common buckthorn to be a bad guy. Allow me to quote:

“Although the surveys and removal experiements do not definitively implicate R. catharica in the decline of native species, and the controlled experiments under individual R. cathartica threes did not show detrimental effects on understory plants, it is quite likely that R. cathartica thickets have negative effects on native species in North America.”

Which I translate as:
“All the evidence shows it really isn't a problem, but we bet it is bad anyhow. Just because.”

I could keep plowing through other species, but I decided to get a bird's eye view of the issues of non-native species, so I read the book Invasion Biology by Mark A Davis (Not to be confused with another -- much less reputable -- book by the same title by David Theodoropoulos). It's aimed at a professional academic audience (professors, grad students) but is very readable, and pretty interesting.
Basically, they say that there is good evidence of serious damage caused to native species by non-native predators and diseases – eg: emerald ash borer, dutch elm disease – especially on islands and small lakes, which ecosystems tend to be more fragile. But most non-native plant species are off the hook (which makes the practice of introducing new non-native predator insects to control non-native plants even more suspicious... let's import something very likely to be a problem to control something which most likely isn't a problem!)
There are plant species which cause significant damage --  not by out-competing native species, but by changing the environment. The best example (actually the only example I've been able to track down so far) are the European grasses in the genus Bromus which changing parts of the Western US. They are much more flammable than native species so areas they have invaded burn more frequently, causing enormous impacts on the entire ecosystem. So some non-native species are bad. Some non-native plants are bad. But my impression from all the reading I've been doing is that in most cases, people have simply seen a non-native species outside of cultivation and ASSUMED it must be doing damage even though indepth studies rarely support that assumption.
So... as a gardener and someone who cares about the environment, what do I do with this? First, I'm certainly not going to support organizations that spend large amounts of time and money trying to eradicate invasive species unless they have very good evidence that it is actually worth it. Pollution, urban sprawl, and global warming are issues I'd much rather spend time and money fighting against. And in my garden? I'll still avoid planting species with the potential to invade natural areas – no one is arguing that invasive species are a GOOD thing – but honestly (and I know this is going to make people mad...), I'm not going to make it the priority I used to. It looks like the carbon dioxide my car emits driving to the nursery to buy the Iris pseudacorus is way more damaging than the presence of that non-native species in my garden.


Wm Jas Tychonievich said...

While the contrarian in me enjoys seeing a fashionable cause deflated, he also enjoys -- well, being a contrarian. So what do you think of this line of reasoning:

1. The vast majority of invasive plant species seem to be ecologically harmless.

2. But some (e.g. Bromus) are clearly harmful.

3. It's hard to predict, prior to introduction, whether or not a given species will turn out to be one of the harmful ones.

4. Once an invasive species has taken root, it's virtually impossible to get rid of it -- and efforts to do so will cost a lot of money and perhaps (as in your beetle example) do more harm than good.

5. Therefore, prudence dictates that we not import non-native plants unless we're very, very sure we know what we're doing.

For a fun, politically radioactive exercise, you can apply the same reasoning to human populations and conclude from such cautionary tales as North America and Taiwan (whose native ethnic groups were decimated by invasive races, European and Chinese, respectively) that it would be prudent to oppose pretty much all immigration. This is called, appropriately enough, nativism and, unlike its horticultural counterpart, is a Very Bad Thing.

Joseph said...

Your argument makes a great deal of sense in the case of the introduction of an entirely new species, never before been in a given region. Then it would be well to be quite cautious and do preliminary testing of potential to invade.

But the strange thing about the anti-invasive plant community is that all the focus is on stuff that is already here -- once a species is seen to move out of cultivation, everyone is encouraged to stop growing it. Well, at that point I think the cat is out of the bag a little. If the species is Bromus like and you decide it is worth exterminating, then forbidding people to grow it seems a reasonable part of an extermination program. But unless you are going to set out to eliminate a species entirely, (which will always involve a LOT of other disruption of native ecology) preventing people from growing a plant seems like fairly pointless.