Applications of Evolution 1: The Erythrina Gall Wasp

This is a repost: Unlike some of the folks here, there really aren't that many of my articles over at the old blog that I thought were worth bringing over here. This is one of the exceptions. It's the first post in a series about the effects of a new invasive insect species on an endemic tree found in Hawaii. I'll be bringing the remaining posts in the series over here over the course of the next week or so. Once I've moved them all over here, I'll post an update on the entire situation.

This article was originally posted on the old blog on 16 August, 2005. I have not updated this post in any way, and I did not confirm that the links are still valid.

Invasive species are nothing new to the Islands of Hawai'i. The first invasive species arrived with, and included, the first Polynesian settlers. Although there does appear to be some evidence that they may have caused the extinction of a few endemic species, the effects of these invasions were most likely relatively minor. Since the first western contact with the islands, the number of invasive species present has skyrocketed, causing a massive ecological disaster. If you want proof of the severity, you need not look any farther than the fact that Hawaii contains well less than 1% of the total land area of the US, but has over a third of the listed endangered species in the US.

At the moment, there is a new invasive species that is making the news here in Hawai'i: a species of "gall wasp" that has been wrecking havoc on trees of the genus Erythrina in Singapore, Taiwan, and a number of other places was found in a valley on Oahu in April. Since then, it has been found in a large number of other places on Oahu, and has started to turn up on other islands, including Maui, and a number of scientists believe that it poses a serious threat to a culturally-significant endemic plant - the Wiliwili (Erythrina sandwichensis). The threat is being taken so seriously that scientists have reportedly begun to bank Wiliwlil seeds as a precaution in case the extant population is completely lost.

So what does this have to do with evolution?

Individual species do not evolve in a vacuum. They evolve in an environment, and natural selection favors the preservation of variations that increase the chances for an organism to survive within that environment. This is common knowedge. What people sometimes forget, however, is that when we discuss the "environment" that an organism evolves in, we are talking about much, much more than just the climate. The evolutionary environment also includes every other species of organism that has any sort of effect on it. Species evolve within the ecosystem or ecosystems that they inhabit, and they evolve as a part of those ecosystems.

So what happens when people - either intentionally or inadvertantly - introduce a species to a new habitat? That's a broad question, and one where the answer is obviously going to depend on a lot of things - not least, what the new species is, and where it is being introduced. (Biology can be really annoying that way, with the answers to so many questions depending on specific circumstances.)

Since the broad question isn't really answerable, let's narrow it in a way that is tailored to these specific circumstances: what happens if you introduce a pest species (either parasite or predator) into a new, hospitable environment that contains a species that is closely related to the pest's usual target? That is a very complexly worded question, but it has a simple answer. Nothing good is going to happen.

In the case of the Erythrina gall wasp, this is exactly what happened. The gall wasp is not native to Hawai'i, and it did not evolve here. This means that up until now, it has not been a part of the evolutionary environment for any of the native species. As a result, it should come as no surprise that the native species has no natural defences against the gall wasp. At the same time, the gall wasp has now found itself in an environment where it has lots of access to a number of species that it can use, and which completely lacks any of it's normal predators. In short, it's gall wasp heaven out here - at least until the Erythrina are all gone.

In fact, the wild success of the gall wasp is one of the reasons that we can tell that it is an invasive, rather than a previously undiscovered native. Its mode of reproduction damages the trees that it uses, if too many wasps use the same tree. Right now, that is exactly what is happening in Hawaii, and it is happening to such an extent as to kill the trees. If it continues to happen at current rates, it is quite possible that the entire genus Erythrina will be wiped out across the Hawaiian Islands. The wasps are loving life right now, but they are actually not in a situation that bodes well for their long-term success in Hawaii - if the current trend continues, they will fall victim to their own success. This indicates that it is not likely that this species evolved here. If it had, one would expect that it would have a lifestyle that would not be so massively counterproductive in the long term. That is not, I should add, because evolution looks forward to see what will work in the long term, but because an organism that has evolved here has already been here for the "long term".

Evolution explains why invasive species can be such a major problem. And evolution, combined with a bit of thought, also can provide us with some possible solutions. With an invasion like this, where the insects involved are tiny and the number of hosts enormous, chemical methods of insect control are normally ineffective. The only real hope for permanently controlling the problem is to find some sort of biological control method. This is normally a natural parasite or predator of the pest. Of course, this can be risky, since the biological control agent is itself an invasive species. If you are not careful, you can easily wind up with a situation that is similar to the children's song about the old lady who swallowed the fly:

She swallowed the goat to catch the dog ...

She swallowed the dog to catch the cat.

She swallowed the cat to catch the bird ...

She swallowed the bird to catch the spider

That wriggled and jiggled and wiggled inside her.

She swallowed the spider to catch the fly.

But I dunno why she swallowed that fly

Ultimately, however, it may be impossible to save the Wiliwili without some sort of biological control. So let's assume that a decision is made to look for one. Normally, we would look to the species' native habitat to find a parasite or predator. Unfortunately, we don't actually know what the native range of this species actually is. It was only described for the first time last year, and it seems to be an invasive in all of the places that it has been found. The world is a really big place, and we have a limited amount of time until the Wiliwili follows countless other Hawaiian species into extinction. So where do we start to look?

Africa. Since this species hasn't been described from there at all yet, what makes us think that we should look there?

Evolution.

There are a number of related species of gall wasp in South Africa, including some that utilize Erythrina trees. There are also Erythrina trees that are native to this area. As I understand it from conversations with other grad students here, these trees do not appear to be experiencing the same sort of massive infestation that is being seen here in Hawaii. This indicates that one of two things is happening there. Either the South African Erythrina have an innate defence mechanism protecting them from the gall wasps (this is unlikely, as the gall wasps can and do breed using these trees) or there is something else, most likely a predator or parasite, keeping their population in check. This is the type of more stable ecological relationship that we expect to see in areas where the species evolved.

I don't know whether or not sufficient funding will be found to do this, but I predict that a thorough study of the gall wasps in South Africa will turn up a predator and/or parasite capable of controlling the gall wasp population. (Or that the Erythrina trees have a defence mechanism protecting them from gall wasps.) I also predict that, again pending sufficient funding, that molecular studies will show that the wasps invading Hawaii are relatively closely related to species from South Africa.

Looking back over this long post, I see that evolution is actually extremely informative in this case - both in understanding what is happening, and as a tool to help develop responses to the situation. To summarize, this is what evolution allows us to do in this situation:

1) Identify the gall wasp as being more likely to be invasive than endemic

2) Understand why this invasive has such a bad effect on the Wiliwili.

3) Identify biological control as a possible method of containment.

4) Narrow the search for the home range of the organism.

5) Predict future findings.

This is all just something to keep in mind the next time that a creationist tries to convince you that evolution is just airy speculation, with no real practical use. That might be what creationists wish were the case, but the reality is actually much different. Evolution is used as a basic tool in the biological sciences, and it often has real-world, practical implications. This particular example comes from the field of invasion biology, but other examples can be found in all other areas of the biological sciences.