Thursday, 2 August 2018

The origins of flammable vegetation

by Alastair Potts

Examples of plant communities that require fire for sustained existence are found around the world. In South Africa, both fynbos and savanna ecosystems need fire at some point for their component species to either complete their life history cycle or reduce competition with other plants. Thus, some vegetation types have  traits that make them more flammable than others (think of fynbos [e.g. small leaves = "fine" fuel] versus forest [e.g. large leaves = "coarse" fuel]).

However, the evolution of these flammable traits at a community level is an evolutionary conundrum. In a seminal paper published in 1970, Robbert Mutch from the U.S. Department of Agriculture (Forest Service), proposed that...
"Fire-dependent plant communities burn more readily than non-fire-dependent communities because natural selection has favoured the development of characteristics that make them more flammable."
 A straightforward argument on the surface. But as dip a bit deeper into this idea, problems arise. This has primarily to do with what is the biological unit that is being selected?

Mutch opens his paper with the following bold proposition...

"If species have developed reproductive mechanisms (underground rhizomes, root sprouting, serotinous cones) and anatomical mechanisms (thick bark, epicormic sprouting) to survive periodic fires, then fire-dependant plants might also possess characteristics obtained through natural selection that actually enhance the flammability of these communities." [emphasis added]
Thus,  he suggests in a vague way that plant communities, and not species, are the units under selection. This type of argument is known as "group selection"; this type of selection has experienced extreme criticism in the literature, as altruistic behaviour where, in its extreme form, an individual sacrifices itself "for the good of the species" does not make evolutionary sense: if any individuals evolve that do not behave altruistically, then they will have a higher likelihood of passing on their genes in a population of altruistic kamikazis — and thereby such defectors will, over generations, come to dominate the population.

What is additionally interesting is that group selection usually applied to a group within the same species. However, almost all flammable vegetation types are comprise a highly diverse suite of species which have flammable traits. So even arguments of "kin selection" (i.e. sacrificing individuals do pass on their gene via relatives who survive), which is another angle of group selection, still fail to explain such behaviour.

Enter the "kill-thy-neighbour" hypothesis proposed by Bond and Midgley in 1995. This hypothesis that
"...flammability may enhance inclusive fitness if the resulting fires kill neighbouring less flammable individuals and also open up recruitment possibilities".
They also state that
"Alteration of the fire regime through the evolution of flammability, even in a single species contributing heavily to fuel loads,would result in the selective exclusion or admission of other species to an ecosystem depending on the compatibility of their pre-existing traits with fire."
Yet how does this hypothesis apply within a flammable fire-driven community? [Note: flammability and fire-surviving traits need to be considered separately]. If flammable traits are costly to maintain (e.g. dead branch retention), then defectors (in this case, species with low flammability but fire-surviving traits [either of the current individual or of seeds]) should come to dominate the community and thereby decrease its overall flammability. This would allow the invasion of other species that were previously excluded by fire; for example, remove fire from fynbos vegetation and forest species invade and ultimately transform the vegetation into a forest community.  Thus, flammable species need to remain dominant components of the community, and non-flammable defectors a minority component. Thus, from the Bond and Midgley model, we can predict that there should be a combination of flammable and non-flammable fire-adapted (or pre-existing fire survival traits) species.

But this is where the "kill-thy-neighbour" model ends. It still does not adequately explain how flammability and the necessary post-fire seedling advantage could co-evolve. This is where I think there is a geological explanation for the origin of flammable vegetation with fire-adapted traits, but that is a blog for another day...


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