NEW DATA: Ecological consequences of a single introduced species: Part 1

This is going to be divided into two parts as the data comes in and I present it at conferences: First up is the impact that the invasive midge Eretmoptera murphyi is having on the Antarctic island of Signy, with a focus on nitrogen levels – the first of the big results to come in. I’m currently collecting more data and will hopefully be in a position to present the whole picture in December at the British Ecological Society Annual meeting in Birmingham, UK.

I was very thankful to be given an opportunity to present this early data at the Entomological Society of America’s joint annual meeting with their Canadian and British Columbia counterparts, this last week in Vancouver. I won the ESA – Physiology, Biochemistry and Toxicology travel prize, and without this I wouldn’t of been able to afford the flights. So thank you, the academy! I was in an odd session with evolutionary biologists and behaviourists, so made a fuss on twitter to try and let people know where I was. It worked, and as it edged closer to my time slot the room filled up at an entirely coincidental rate with the butterflies in my belly! Thankfully the sub-lethal dose of caffeine I had had that morning prior gave me an unnecessary amount of nervous energy, so I quite literally leapt into my presentation, manic smiles and all.

...densities of up to 150,000 larvae per m2...

The talk described the distribution survey of E. murphyi that I conducted in 2016/17 on Signy Island, where I extended a previous survey by Hughes et al 2010 that found densities of up to 150,000 larvae per m2 covering an area of 35’000 m2 . Their findings found an association with footpaths, leading to the conclusion that we are moving the midge larvae around on the treads of our boots. I repeated this work, extending the area and finding that the larvae can still be found in equivalent densities but much farther out, now to an area closer to 85,000m2. This distribution data will be published in full soon, but for now you can find it in poster form here.

Behold, The Trophic Grid!

In order to establish how this spread may be affecting the ecosystem I designed what I call my ‘trophic grid’. A grid of 4 x 4 points 10m apart covering a moss bank with known varied distribution of E. murphyi. From these grid points I took substrate cores, to analyse the following: E. murphyi abundance; collembola and mite abundance (the only other bugs on the island – neither of which is an insect by the way!); microbiology, that is looking at the load of fungi and bacteria; soil organic carbon; and other abiotic factors such as water content and pH. I also noted what percentage of the core was peat or vegetation and what percentage was soil. Additional to this I measured vegetation cover and diversity using a m2 quadrat; and in a first for Antarctica (we think!) used in-situ ion-exchange membranes to assess soil biogeochemistry. To get an idea of how these result fit into the overall patterns across the island, I also sampled random sites around the area local to the new distribution of the midge, and surveyed wildlife colonies and random non-midge colonised moss banks around the island. Then, and now actually as I’ve not finished, I ran some correlations and built maps of relative abundances of each input to get an initial idea of E. murphyi’s influence on the terrestrial ecosystem.

...they increase levels by 3-5 times as much...
Nitrogen levels in the area surround distribution (shaded). Shows total nitrogen, and then the ammonia and nitrite/nitrate constituents.

And the verdict? Well, I can say in a cautious but excited way, that so far, E. murphyi is having a positive effect on invertebrate abundance. This is possibly because of the following result: A strong positive relationship with total nitrogen, of which most of the impact is on nitrate/nitrite levels over ammonias. In 2013 Stef Bokhurst found that the mite and collombola populations on Signy tended to aggregate and/or proliferate in areas with higher nitrogen levels, so this increase in other invertebrate communities was not a huge surprise. What was a surprise was the level on nitrogen that the midge is inputting. When compared with colonies around the island, we find that they increase levels by 3-5 times as much as background levels in similar habitats without E. murphyi present. This brings the lowly moss bank up to the level of seal colonies! This is a small bug, not even a centimeter long. And there you have it ladies and gents, size (at least in ecological terms) does not matter! What is interesting is that the signal from the midge is mostly nitrites/nitrates, and not ammonia. This tells me that the levels we are seeing are not likely to be as a result of sea bird guano confounding the results. As you can see below, sea birds have much higher ammonia signals.

Nitrogen levels around the island, focussing on control habitats near and away from wildlife colonies, and then measurements from within seal, penguin, giant petrel and midge colonies.

We have also find a relationship with substrate depth and the percentage of soil that occurs with E. murphyi. As their numbers go up, so does soil. But the depth of the moss bank goes down. What I think is happening here is that this midge, who is a detritovore feeding on the peat and dead organic material in the moss banks, is degrading the litter faster, and turning it into denser soil. Gone is the springy mats of moss and peat, hello dense fertilised soil. This is what I suspected would be happening, but to see it in the data is really cool. This impacts study has been a pet project of mine, so to be seeing some early traces of a good story is really cool!

Keep an eye on this blog if this interests you as I will be getting the data from the microbiology next week. The full results will be ready in time for the British Ecological Society meeting in December, so I will give an update then. In the mean time remember the significance of the small stuff, and I say that with no personal bias being short myself!

If you are interested in seeing the whole talk, you can find the presentation as a powerpoint file, here.

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© 2018 by Jesamine Bartlett