Tuesday, 8 December 2015

Inland Hunter inexpectata

Last Wednesday I took a lunchtime stroll along the northern edge of West Basin, a section of Lake Burley Griffin. It is just across Parkes Way from where I work and is in the heart of Canberra. I should confess that it wasn’t really just a stroll, so much as a survey of what odonates (dragonflies and damselflies) might have been present at the time. This is part of my fairly concerted effort this summer to get a feel for what odonate species occur in the ACT and when they are present through the season.

Apart from the expected damselflies - Blue Ringtails, Red & Blue Damsels and Common Bluetails (all of which have been present there for the past month or so) - I saw four or five individuals of a smallish black and yellow dragonfly. From the shape and body markings these were clearly a type of gomphid (Family Gomphidae) and at first I thought they might have been either Stout or Southern Vicetails (Hemigomphus heteroclytus and Hemigomphus gouldii respectively) which are not uncommon in some of the ACT’s rivers. But some decent views through the binoculars at close range of one individual made me think differently. It clearly had dorsal stripes down the abdominal segments and appeared to be some sort of ‘hunter’ rather than a ‘vicetail’, possibly a Unicorn Hunter Austrogomphus cornutus, but clearly not a Yellow-striped Hunter Austrogomphus guerini that I’m relatively familiar with. As I didn’t have my camera with me I took mental notes of what would hopefully be diagnostic ID features and jotted down a few comments in my little notebook.

Stout Vicetail Hemigomphus heteroclytus; Coppins Crossing, ACT; 5 Dec 2015.

Southern Vicetail Hemigomphus gouldii; Gudgenby River, ACT; 26 Dec 2014.

Unicorn Hunter Austrogomphus cornutus; Gudgenby River, ACT; 26 Dec 2014.

Yellow-striped Hunter Austrogomphus guerini; Gudgenby River, ACT; 26 Dec 2014.

When I got back to work I looked up the field guide (Theischinger & Hawking 2006) and the only thing that looked like what I had seen was Inland Hunter Austrogomphus australis. A species I'd never seen before.

The match was good for what I had seen, and nothing else really fitted. The problem was the field guide, and later the Atlas of Living Australia (ALA) when I looked it up, indicated that this species doesn’t occur in Canberra. The nearest records in ALA are from Wodonga (220 km to the south-west), Griffith (300 km to the west) and up near Tambar Springs (415 km to the north).

Distribution of Inland Hunter according to the Theischinger & Hawking field guide.

Distribution of records of Inland Hunter in the Atlas of Living Australia (ALA) at 4 Dec 2015.

It was kind of exciting but also frustrating that I had no visual record, and I determined to go back next day, with camera, and get some evidence/proof. So you can imagine my acute frustration when Thursday lunchtime revealed absolutely no sign of any of these dragonflies, despite walking back and forth and checking closely the locations where I had seen them the day before, under similar warm, bright sunny conditions.

What had been clear on the Wednesday was that these dragonflies were always associated with sandy/gravelly patches, be they gravelled pedestrian tracks along the edge of the lake or the tiny patch of gravelly sand that purports to be Acton Beach. I thought it unlikely that there would be several individuals at this location one day, and none the next, so I returned again on Friday lunchtime and this time was successful. I spent the better part of an hour photographing one of these dragonflies as it came and went, perching on the gravel and darting out over the water if I got too close or something else disturbed it. Just before heading back to work I saw a second individual and they had a brief skirmish the way dragonflies often do.

Inland Hunter Austrogomphus australis; Acton Beach, Canberra, ACT; 4 Dec 2015.

Inland Hunter Austrogomphus australis; Acton Beach, Canberra, ACT; 4 Dec 2015.

Inland Hunter Austrogomphus australis; Acton Beach, Canberra, ACT; 4 Dec 2015.

So I felt vindicated. Not only had I relocated at least two of these dragonflies, but I got quite a few photos, and most gratifyingly they clearly indicated that they were indeed Inland Hunters - and my mental note taking was validated.

Later that night as I went through the photos, culling the out-of-focus and leaving the small fraction that remained acceptable, it became clear, based on details of the colour development on the final four segments of the abdomen, and to a lesser extent the shape and completeness of the humeral stripe on the synthorax, that the photos represented at least six different individuals – all males. Only twice had the same individual returned for a second sitting. My initial assumption that the same individual had been coming and going was patently wrong.

But the question remains – what was this species doing in Canberra? The field guide indicates that the species “inhabits rivers and riverine pools, common and widespread in the inland river systems of eastern Australia.” Well that description is not at odds with the Canberra situation, but it doesn’t explain the near total lack of records of the species (at least in ALA) from the Murrumbidgee and Lachlan catchments.

So what was going on? Could it be that Canberra is within the species’ normal range but for whatever reason had not been reported from here before? Seems unlikely given that one of Australia’s foremost odonatologists, J.A.L. Watson had lived and worked in Canberra for many years. And why are there no records from along the lower courses of the ‘bidgee and Lachlan?

Or might it be that the species has expanded its range or increased its population size in this region? Could climate change be a factor? Or might the species be migratory and I had just happened across a cluster of migrants, or even wind-blown individuals from further west? Starting to scrape the barrel a bit here I think, particularly as, as far as I know, Gomphids tend to be rather territorial rather than migratory.

Anyway, I got to thinking if there were at least half a dozen of these things at one small beach right near the city, there must be others about. The closest place I could think of with similar habitat features was Grevillea Park at the eastern end of Lake Burley Griffin (LBG), about 3 km away. So early on Sunday afternoon I dropped by and scanned the ‘beach’ there and did indeed find and photograph another single male Inland Hunter.

Inland Hunter Austrogomphus australis; Grevillea Park, Canberra, ACT; 6 Dec 2015.

Inland Hunter Austrogomphus australis; Grevillea Park, Canberra, ACT; 6 Dec 2015.

But the more surprising discovery was of yet another individual at Coppins Crossing on my way home. Coppins Crossing is on the Molonglo River about 12 km downstream of the lake and presents quite a different habitat to the shores of LBG. The river there is fairly muddy (probably due to the massive multi-suburban development of ‘North Weston’ not far upstream) and not particularly fast-flowing except where it percolates through some rocky sections. The male I saw (which I had initially assumed was just another of quite a few Stout Vicetails present along that stretch of the river, and only discerned as an Inland Hunter later that night as I went through the photos) was perched on the top of a small rock in the middle of an expanded pondage where the water was murky and still.

Inland Hunter Austrogomphus australis; Coppins Crossing, ACT; 6 Dec 2015.

So it has been quite a special few days in which I’ve found a species that supposedly doesn’t occur in Canberra in three different Canberra locations. And I’m left wondering what to think about it and what to do about it. As a first step, I’ve written this blog post and will make the records available on ALA and Bowerbird…

And I'll keep hunting!