Clash of the Titans

Posted by Graeme Lyons , Tuesday, 7 August 2018 07:56

Yesterday I completed the 5th (out of 6) survey visits planned at Iping & Stedham Commons this year to monitor invertebrates. The highlights was stumbling upon a Hornet Robber-fly carrying a huge black mass. It struggled to fly more than a few metres at a time due to the weight of its food but eventually I got a look at the prey item. It was none other than Tachina grossa, our largest tachinid! This alone is a huge fly so this really was quite the sight! Go large or go home!

In the black corner, we have Tachina grossa. They spent their informative years living inside the body of an unsuspecting Fox Moth larvae devouring it from within (the most likely host at Iping). In the yellow corner, Hornet Robber-fly, who as a youth lived under a cow pat and devoured Minotaur Beetle larvae in their subterranean burrows (the most likely host at Iping). They had never met until today but now one has become the other's lunch. It's a fly eat fly world out there!

I had a couple of lifers, both were also new to the SWT reserve network. These were the ladybird Scymnus suturalis beaten off pine and the mirid Trigonotylus caelestialum. Neither particularly rare and I was surprised to see the latter was a first for West Sussex.

Just to clarify what I was talking about in my last post, "what exactly IS a heathland invertebrate?". The point of this post was to show my course attendees just how few species are often strictly tied to a single habitat. And that this description itself is therefore inherently subjective depending on how we define this. For example, you could say they are all heathland invertebrates as we found them all on a heathland. Not very helpful.

You could take the extreme approach and describe only those that are say Calluna/Erica obligates as being heathland specialists, also not very helpful as it restricts you to an extremely small number of species. Such as this Phytocorois insignis which feeds on heathers and is now the most westerly record in Sussex (we recorded it at Stedham last year).

But that doesn't take into account (for example) the species that are using structural types, or a particular colour of flower that is provided in that environment. Such as these two. Now they're not heather obligates. But they do have a strong tie to heathlands. Thomisus onustus occurs mainly in heather flowers waiting to ambush its prey but I have found it on Dodder and Common Cotton-sedge over the years where it can clearly survive without the heather. Is that enough reason to not consider it a heathland specialist? I don't think so. Likewise, Evarcha arcuata is common on heathlands in Sussex in both heathers and Molinia (which I see as just another component of heathland). In Sussex I know of only one record away from heathland of this species and that's from Butcherlands. So is that enough to not consider it a heathland specialist? I would also say no. It clearly has a strong association with dwarf ericaceous shrubs in a structural way but to ignore this just because very occasionally it doesn't is wrong in my opinion. 

Equally I would also consider many of the species that occur on bare sandy ground to be heathland invertebrates. They often outnumber the heather obligates hugely and are a vitally important part of the heathland. So my whole point of asking "what exactly IS a heathland invertebrate?" was to show how few heather obligates there were but to also show how this is a subjective question in the first place! What exactly is a heathland? Is it just the heather  or is it all the components together functioning as a whole? And also to give people an idea on how resource analysis works with invertebrates but when you only have space for ONE factor (you'd normally use several), you're bound to have disagreements.

Now that's cleared up I can go back to my specimens but not until I show you this weird larva which I believe is a Scalloped Hook-tip.

Oh and one last thing, if you want to see Thomisus onustus, it's having a really good year at Iping and sweeping Bell Heather would yield one within a few sweeps this year where usually you could work all day and see only one, IF you were lucky.

2 Response to "Clash of the Titans"

John Says:

Graeme, surely whilst "heathland" is defined as a habitat that supports "heathers" it supports other species, both animal and plant that require similar conditions. So a heathland species is defined as a species that only occurs on heathland, that species does not have to be dependent on "heathers" per se. Does that make sense??:-)

Graeme Lyons Says:

My point exactly John!

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