Insect diversity: when humans move in

When humans move into an environment, we inevitably change it: trees are felled, brush is cleared, crops are planted and non-native animals are reared. The soil underfoot is of course still there, but what are the consequences of these land-use changes for the insects and other invertebrates that live underfoot? The answer is that their diversity shrinks too. Chris Smith spoke to Auckland-based ecologist Andrew Dopheide…

Andrew - We used a technique called DNA meta-barcoding. We use DNA sequencing to identify lots of different insects and other small animals in a very efficient and large scale way. The team of researchers visited 75 sites throughout New Zealand, each of which was in a different land use type such as agriculture or horticulture or forest and collected a number of soil cores. The invertebrates got separated out and then were taken into the lab and analysed.

Chris - Presumably you grind this lot up and then just say, right, what DNA sequences are in here and then compare it to a database.

Andrew - Ah yes that's right.

Chris - To what extent is this quantitative? Cause I can see how that would enable you to tick things off as you discover them: I've got the genetic sequence of this, I've got the genetic sequence of that, but how does that relate to numbers? Can you tell roughly how many of any given thing is there?

Andrew - Not exactly. We use a technique called PCR to copy or amplify the bits of DNA that we're interested in. The amount of DNA we get from different organisms is quite variable and therefore doesn't accurately reflect its abundance.

Chris - So it's reasonably good for working out the broad spectrum of what is there, but it's not going to tell you very much about how much of any particular thing is there, that would take a different kind of study to elicit that information.

Andrew - Yes, that's right. However, if you have preexisting information about how a particular organism, how its abundance works, you can actually do that.

Chris - And what were you comparing then? Were you just saying, well I'm going to compare the forest to this area, which has had animals on it. And what are you actually then comparing those things to, what's the gold standard?

Andrew - We compared five different land use types. So at the one extreme we've got native forest, forest that's always been here, relatively undegraded and unimpacted. Then we've got pine forest, which has been planted for forestry purposes. Two different levels of agriculture, so one of them is quite low intensity, just tussock grass, few sheep, that sort of thing. Another is more high intensity agriculture, so just a monoculture of grass and lots of fertiliser inputs. And then at the other end we've got perennial crop land, which is basically horticulture, fruit and nut orchards primarily. We figured that the native forest sites represented a relatively natural state for New Zealand. So by comparing all the other land uses to the natural forest, that gives us a measure of how things have changed as a result of agriculture.

Chris - And how have they changed?

Andrew - We observed a general decline in the number of species as you go from natural forest to grassland to cropland. Each of our native forest sites had quite a unique set of species that weren't found anywhere else. But as we looked at grassland sites and horticulture sites, we found that the communities of animals became more and more similar everywhere. So it goes from native forest having very distinct and unique sets of organisms or animals to agriculture, just having the same things everywhere.

Chris - And do you know why that might be happening? What do you think the mechanism is?

Andrew - It's probably due to factors such as removal of trees and replacement with grass. Really just simplification of the habitat. So there's fewer places for different animals to live, fewer food sources. And also you've got these chemicals which are designed to suppress these same animals as well, all in order to make a more friendly environment for things such as cows and sheep. But at the same time you're removing the habitat for all the smaller invertebrates.

Chris - It sounds pretty worrying, doesn't it? Is this a permanent thing? If you were to revert the environment back to what it would have been originally, do you get your species back?

Andrew - That's something we do not know, but I think it would be worth trying.

Chris - And what might be the consequences of not having these species there if you lose these animals? Obviously a loss is a loss and that's sad on so many levels, but what are the more direct consequences of not having some of these species there?

Andrew - All these species of animals have a role in their ecosystem. All together, they're contributing to the fertility and the stability and the health of the soil basically. So if you remove these invertebrates, you're left with a soil community which may be unstable and less resilient to environmental stress, such as drought or climate change.