Sunday, 5 February 2012

Notes on a butterfly eruption: a billion and counting!

 Just a brief post with the latest news, as it's coming in. The bulk of the movement seems to have passed Arusha Moshi now - there are still large numbers going through but (a) they've turned South West in most areas, and (b) they're coming through in groups now, not as a continuous stream. By contrast, I'm hearing of increasing numbers now in and around the Crater, with the first being reported (headed south and south west) from the broader Ndutu area. I've also confirmed that the movement hasn't been noted north of the Pare Mountains, suggesting the idea that they originated in the Maasai Steppe and headed north from there might well fit. The other very interesting observation comes from Tent with a View (I think that's their website!), who report 1000s appearing just recently headed south down the coast in Saadani NP. So, if you're in Dar, do look out over the next day or two. How this fits into the pattern, I'm not sure! But keep the records coming and we'll find out.

Meanwhile, I've been reading more papers on butterfly movements in Africa!

One thing that I have discovered is a fascinating account of a mass emergence in Sudan in 1928! What is most interesting about that account is what formed the original synchrony in the emergence (in this case it didn't result in a particular eruption): a few showers at the end of a drought resulted in a large growth of Maerua oblongifolia, all of which got laid on by a few butterflies. The next generation, as is to be expected, rapidly ate these plants and the adults emerged en masse, having eaten everything as caterpillars. So a couple of rain storms might be enough to synchronise massive numbers of butterflies, who will rapidly exhaust the available food and then they set off in search. Why they don't head in all directions from there isn't clear (maybe they do, and maybe that's why we're finally seeing animals on the coast now?), but the other observation in that paper that is very relevant to the current situation is the observation that many eruptive individuals are smaller and paler than the usual forms of these butterflies, a typical result of overcrowding and low food resources for the caterpillars. The individuals we've been seeing in this movement are also slightly smaller and paler than average, suggesting they may well have started off in very crowded conditions where competiton for food was high.

The next thing I liked in that paper was some estimates of how many individuals may be involved in some of these movements. Unfortunately, the calculations weren't given. But it's worth one of those back of an envelope calculations I'm so fond of, so let's give it a go!

During the peak movement here in Arusha last week, of a 20 front I was consistently recording over 100 butterflies, all on the move. Numbers elsewhere sound similar. Other days since then I've been getting around 50 per minute, dropping to an average of only 20 the last couple of days. So, from the map everyone's been contributing to, the movement front is at least 200km wide. If the butterflies are moving at this density (average of 50 per minute for 5 days now) across the whole front, then each minute about 500,000 butterflies move across the whole 200km front. They've been moving for about 7 hours each day, five days now. Which gives us 1,050,000,000 butterflies on the move! Yes, that's more than 1 billion individuals - and they're still moving! (To put that in context, reported a movement of 3 billion Painted Lady butterfly in 1949. But it's still impressive!) Out of interest, I've weight a few butterflies and they come to about 0.2g each, which gives us about 315 tons of butterfly on the move. Or put it another way, with a wingspan of 5cm, put them all together and you can easily go around the earth! Certainly numbers that might just impress enough to look away from a wildebeest for a while...

Anyway, please keep the records coming, especially as they move west and south. If you know anyone in these areas, get them on the case too, please! Keep checking the interactive map too, it's getting new points all the time. And with much thanks to MANY people who've been contributing records from across the country.

Main Reference:

ResearchBlogging.orgShields, O. (1974). Toward a theory of butterfly migration Journal of Research on the Lepidoptera, 13 (4), 238-238

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