A couple of weeks ago, something he wrote really caught my attention:
In the aftermath of any prescribed fire, there are winners and losers. Fire rapidly and dramatically alters habitat and growing conditions in ways that favor some plant and animal species and put others at a disadvantage. Fires also kill some insects and other animals outright. For example, dormant season (late fall through early spring) fires burn up a lot of invertebrates that overwinter in prairie thatch. Growing season fires, of course, can kill numerous small animals – especially slow-moving non-flying ones. We usually don’t see the evidence of those impacts, but when we do, it’s no fun. Over the years, I’ve seen way too many fried snakes and scorched nests, in addition to animals who suffered injuries from our fires. It can be tough to deal with the knowledge that I made the decision to light the fires that killed or maimed those animals.
I was interested in this insight because I'm sure it's also very relevant to the fires we light for management purposes, particularly in native grasslands and grassy woodlands in southern Australia. Some of you may grapple with exactly the same issues.
|Burnt grassland near Horsham|
I've always accepted that some animals will die in fires, unpalatable as this may be, and that for the long-term function of the system, such losses are both inevitable and acceptable. After-all, we know that fires kill plants and their propagules, but this event also creates the necessary conditions for seedling regeneration and species turnover. Indeed, I know this better than most. My PhD was looking at exactly the issue of recruitment by native forbs in grasslands under different fire regimes.
If timed correctly, fire need not cause excessive animal mortality, knowing full well that no mortality is highly unlikely. There is very little data, however, that I know of that quantifies when the 'best' time to burn is to reduce animal deaths.
Intuitively, it is likely that summer burns will have the least effect on animals because they occur when there are cracks in the soil where reptiles, frogs and ground dwelling invertebrates can escape, and bird and invertebrate breeding is largely over. Summer burns, however, are very difficult (if not impossible) to implement so an alternative time to burn needs to be found.
Spring burning, I suspect, likely impacts on breeding in many species as it coincides with the most productive phase of temperate grasslands. Very late autumn burning (even stretching into winter) often occur after cracks have disappeared as soils re-wet with the autumn rains; it's also a time of emergence of many invertebrates after the summer drought and hence, likely to be sub-optimal if the aim was to reduce animal mortality.
|Early dry season burning in Darwin. Note low flame height.|
There is one place I had never though of as a refuge from fire that I recently observed while burning savannah in the Top End. Last week, as part of the CSIRO Burning for Biodiversity project that I help out on (with Dick Williams and friends), we observed something very curious. Something I'd never considered before, but that might be very relevant to fires in grassy woodlands in southern Australia.
After fire ignition, but well before the fire front approached, frogs, geckoes, lizards and invertebrates started moving up tree stems into the canopy. I presume that many of these were in the grassy sward initially. Jumping up the trunk must be hard work for frogs, in particular, (and dangerous, because it probably exposes them to predators), but in early season savannah fires, this is probably an excellent refuge. Flame heights are rarely more than 2-3 m at this time of year (and tree canopies are 8-12 m high, well away from the flames). Residence time of the fire at any point is mostly only 30-90 seconds as the fire passes, and smoke clears quickly once the fire front has moved on. Hence, moving up to escape fire seems to me a very logical thing to do for an animal to survive the fire event. As a refuge from fire, it shouldn't be under-estimated given the probability of canopy fire is exceedingly low.
|Early season savanna fire in Darwin.|
Flames rarely reach the lower tree canopy.
I, nor my invertebrate ecologist friend Michael Nash who observed this phenomenon with me, have little idea what the cues are for this behaviour. Apparently there is some research that says frogs respond to the sound of fire (and they make their way into the ground). I'd also think that smoke (or some chemical cue in smoke) likely triggers this response. It's like a siren has gone off, and the critters are well-drilled to respond. One thing I think we could rule out is that these critters were responding to heat generated by the fire itself. They began moving well before any fire was near, and besides, waiting for it to get hot before responding is probably a strategy fraught with too much danger to be successful in the long-term.
So, if you manage a system with trees and grasses in it, think about the role that individual trees might play on creating refuges for animals from the fire event itself. And document this if it does occur. There is so much still to be learnt about fire response by animals yet simple observations, like the one I have described above, could be crucial for understanding how plants and animals coexist with fire.