Dr Mark Harrison, OuTrop’s Director of Biodiversity Research, shares interesting findings from our recent butterfly surveys, which have us asking ‘What’s going on with our butterflies?’
As anyone who has read this blog over the last six months will know, Borneo’s forests and peatlands experienced severe forest fires, which reached their peak in September and October 2015. We’ve documented the devastating impacts of these fires in previous blogs, which include causing health problems for people in affected regions, huge carbon emissions and, of course, habitat loss for the island’s huge number of forest-dependent species.
|Photo by Anna Triggol/OuTrop|
Many animals that didn’t lose their forest homes during the fires will still have been negatively impacted. This may be through ill health, in the same way that humans suffer from toxic haze inhalation, behaviour impacts, such as reduced territorial singing in gibbons, influx of animals into unburned areas from nearby burned areas, and/or sight-dependent animals being less able to find food during thick smoke periods.
Against this background, we were therefore surprised when we completed our normal monthly fruit-feeding butterfly surveys in January 2016 and captured many times more butterflies than normal: a total 285 individuals! This high capture rate was then exceeded in February, when 309 individuals were caught.
Although captures in December 2015 were low (22 individuals), as might be expected, the relatively normal capture rates immediately following the fires in November (41) and during the earlier part of the fire season in July (43) and August (51) have also been surprising. Can butterflies really be bucking the trend, and benefiting from the fires and smoke?
|Photo by Connie Miller/OuTrop|
Well, we don’t yet know the answer to that question, but it’s interesting to speculate! In line with our results, studies of the large 1982-83 El Niño fire season also indicated explosions in butterfly populations in Kalimantan following fire, which was hypothesised to be a result of the abundance of nutrient-rich ash.
Although our butterfly survey transects were untouched by fire, forest only a few hundred metres away did burn, so it is possible that butterflies could have flown from these burned areas to our traps, leading to increased captures. If this is the case, then we might expect captures to decrease over the coming months, as more ash washes away.
Of course, any increase in captures along our transects is likely offset by much larger numbers of butterflies dying due to lost forest habitat. Indeed, this could even be part of the explanation: maybe butterflies have been displaced from the burned area to our unburned transects. Or, pupating caterpillars may have delayed their emergence during the smoky period, only to emerge on mass once the fires were over.
|Photo by Erik Frank/OuTrop|
We know that the stress experienced by forest trees during fire periods leads to widespread leaf shedding among forest trees, which may lead to increased new leaf production when rainy conditions return following fires. An alternative explanation is that this may have provided more food for caterpillars in November and December, which then emerged as adults and entered our traps in January and February.
|Photo by Bernat Ripoll/OuTrop|
Finally (or not…!*), the fire period stress may have resulted in forest trees producing less fruit, which may in turn make the rotting fruit bait in our traps more attractive to the butterflies. This could theoretically lead to increased captures in our traps without any actual increase in butterfly populations.
Any one – or more than one – of these explanations may be behind this observation. It will be fascinating to continue our surveys over the coming months to further explore the reasons behind it and see if this elevated butterfly capture rate persists, returns to normal or is a temporary blip followed by a longer-term decline.
* Do you have any other suggestions on why this increase may have occurred? If so, please email us at [email protected].