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Posts Tagged ‘Biogeography’

Introduction

Fire plays and has played a unique and varied role in shaping ecosystems and the geography of species.   Fire has been a factor for all of Earth’s 4 billion years of existence, and all life has evolved in its presence (Stott 1988).  Its effects on species can be varied.  At high intensities, it has the ability to completely destroy total assemblages of species, yet there are many species that are unable to reproduce without it.  At lower levels of heat and exposure, fire can hold back ecological succession and maintain biological diversity.  This extensive timeframe and wide range of potential impacts makes fire an important, and sometimes critical, ecosystem engineer.  Yet little work has been done to explore the large scale biogeographic effects of fire.

Fire and the History of Life

For most of Earth’s history fires were ignited by lightning strikes and burned until they ran out of fuel or were extinguished by rainfall.  These ignitions occurred with a certain stochastic regularity in frequency and intensity for a given place and during given timeframes.  Forest managers call the particular combination of frequency and intensity the fire regime of a region.  Ecosystems had to evolve with the fire regime of that particular region as one of the abiotic factors alongside amount and seasonality of precipitation, temperature ranges, and soil chemistry.  Species that were not able to tolerate a particular fire regime were excluded from regions which burned in that manner.  Others evolved ways of protecting themselves from relatively infrequent fires and so could withstand the regime of that area.  Yet others evolved in areas that burn so regularly that the organisms became dependent on fire during parts of their life cycle.  And all these processes continue today.

Throughout the history of earth fire has also effected the composition of climactic zones.  It was found on Mount Kenya that the fire regimes have changed cyclically during the Pleistocene (Rucina et al. 2009).   Zones of climatic conditions moved up and down the slope as glacial periods came and went; this would lead us to expect species assemblages to move altitudinally according to their climactic needs.  However, this is not the case.  While some general movements can be attributed to climate, there are others in each cycle that cannot.  With each glacial-interglacial cycle the individual species which makeup each assemblage changed.  They move beyond the climatic region they had occupied before in ways that would not have been predicted.  The factor that best explains these differences is the patterns of fire left by layers of charcoal in the soil.  With the different cycles of glaciation, the fire regime changed and so the different species were shuffled into different assemblages.  These unique combinations of species resulted in different pressures from predators, prey, and competitors.  Such biotic shuffling opened up many new niches for natural selection to fill.  This shuffling has been demonstrated in modern times.  Sara et al. (2006) found that after a fire burned through an area it greatly disrupted the co-occurrence of many vertebrates, and changed the assemblage of species in stochastic ways.

The different fire regimes coming and going during the Pleistocene could have acted as a “species pump” with relic areas of a particular fire regime being separated and left behind as refugia as the climate and large scale fire regime of the region changed.  Populations within these relic fire regimes could then have diverged and even speciated before the preexisting fire regime and related conditions returned, bringing the new and ancestral populations back into contact.

Local Fire Ecology

For many species fire can have important and beneficial effects.   In some areas regular relatively low intensity fires have been shown to encourage forest expansion by thinning juvenile trees and thereby reducing competition for resources (Grau and Veblen 2000).   Openings in forests created by small scale fires have been shown to make habitat that is necessary for species that require disturbed areas (Askins et al. 2007).  Fire can also maintain certain habitat types that are high in biodiversity (Cox and Jones 2009).  Fire also plays an important role in certain stages of the reproduction of numerous species.  Lodgepole Pine (Pinus contorta ) has cones that are sealed closed and will open and release seeds only after being burned.  The seeds of Point Reyes Ceanothus (Ceanothus gloriosus) will only germinate after being exposed to a heat pulse such as that from a low intensity fire.

Fires maintain heterogeneous landscapes with different stages of succession that offer different habitat types.  This fragmentation commonly results in archipelagos of one habitat type isolated from one another by a different habitat type (Trabaud and Prodon 1993).   This has been found to increase the biodiversity in some habitats such as Hemlock-Hardwood forests in the eastern United States by increasing the number of habitat types in some areas (Ziegler 2000).  However, it has also been found to decrease biodiversity in others due to local population extinctions because of the isolation of habitat patches just as in island biogeography (Sara et al. 2006).  In other areas fire has been shown to hold back forest expansion and thereby maintain grasslands and the species that rely on them (Silva et al. 2001).

The beneficial effects of a fire can be especially far reaching if the species that directly benefits is a keystone species such as the Saw Palmetto (Serenoa repens) which provides food or habitat for several hundred species from a wide array of taxa.  Fire has been experimentally demonstrated to increase the number of flowers and the amount of fruit produced by Saw Palmetto in pine flatwoods in Florida (Carrington and Mullahey 2006).  In this way, fire applied at the proper frequency during the appropriate season, can influence the geography of an ecosystem by influencing not only the distribution of certain species, but the trophic systems as well.  It can even determine an ecosystems existence.

However not all fire is beneficial fire.  Major stand replacing fires remove vegetative cover and organic detritus completely exposing the bare mineral soil to erosion by wind and rain and to the desiccation by the sun. This can retard many ecosystems from expanding.  In extreme cases such high intensity fires can even prevent recolonization (Lomolino et al. 2006).  Fire in French oak forests has been shown to kill oak seedlings and so reduce recruitment rates (Curt et al. 2009).  The Bush Karoo Rat (Otomys unisulcatus) of southern Africa builds large nests of sticks that are very vulnerable to burning therefore its range is limited to areas that have very low fire frequency (Kerley and Erasmus 1992).  Shrubsteppe habitat contains another assemblage of species that is harmed by fire because after a burn the shubsteppe tends to be replaced by grassland (Earnst et al. 2009).  The openings and forest fragmentation that results from fire leads to increased edge effects which frequently include higher rates of predation, invasion of new species, and altered microclimatic conditions all of which can reduce biodiversity and cause local population extinctions.

Large Scale Fire Effects

These effects on assemblages of species and distribution of habitat types, both positive and negative, can be extended to the global scale.  An abiotic force that has such extensive influence on individual species has equally extensive influence on the distribution of biomes as a whole.  Fire has been shaping the distribution of biomes for millions of years, and around the world, a large area burns regularly.  Fire dependant ecosystems have evolved in these areas, and now comprise a large part of it and contain a huge number of species.  Species that would be reduced in number or go extinct in the absence of fire.

Biodiversity is increased in regions with fire.  Beaty and Taylor (2001) and Ziegler (2000) showed that areas of forest that experienced frequent fires had higher diversity of tree species than areas in which fires had been excluded.  This is another aspect that has larger biogeographic effects when the whole community is considered.  By increasing the number of tree species, the diversity of seeds and cones available as food sources increases and so does the number of species that use them, and these effects continue throughout the trophic levels of the area.

Ecological secession is also held back at a huge scale when examined globally.  It is estimated that there are many grasslands that occupy climatic regions and have nutrient levels that would support forests, and that the primary force preventing forests from spreading into these grasslands is the occurrence of frequent fires that kill tree seedlings and allow the grasses to maintain their presence (Stott 1988).  Bond et al. (2005) predicted that if fire were completely suppressed the global area covered by forest would double and that occupied by grasslands would be halved.  Such strong effects have equally dramatic results.  The more contiguous a habitat is on a global scale, whether grassland or forest, the greater the amount of dispersal and mixing species will be within it.  Further, fire is frequently the primary mode of decomposition and nutrient cycling in these grassland ecosystems (DeBano et al. 1998).  Frequent fires, and the resulting high rate of nutrient turnover, are critical for these habitats to maintain their high levels of productivity.  Such extensive influence makes fire an abiotic force comparable to precipitation or temperature in the determination of community biogeography.

Fire can also influence evolution and extinction.  Theoretically, the trait of flammability could evolve if fire spread from a flammable individual to kill neighboring individuals and if the seeds of the more flammable plant could out compete the seeds of less the flammable plants (Bond and Midgley 2003).  There is strong phylogentic support for the association of pines that lack self pruning, the loss of dead lower branches, and cone serotiny (Schwilk and Ackerly 2001).  This association makes sense.  Serotinouse pines have a strong incentive to expose their cones to fire, so the individuals that retained fuel material on their trunks that would allow fire into the tree crown would produce more offspring than individuals which had to rely on high intensity crown fires to occur naturally.

Even within species, fire plays a role in natural selection.  In grasslands that burn frequently, saplings of the African tree Acacia karroo grow straight up which quickly takes the foliage above the level of the low surface fires which are common in that habitat.  However, in areas which burn only rarely A. karroo saplings form tangled cage-like structures.  This serves as an example of the selection pressures exerted by fire (Archibald and Bond 2003).

Fire and Native Peoples

Native peoples used landscape fire in a variety of ways as a tool to control habitat for their own gain.  Evidence for hominid use of fire dates back 1.4 million years (Gowlett et al. 1981).  Intermittent, low intensity fire was used by Aboriginal peoples in Australia to clear ground and replace nutrients in the soil for agriculture (Yibarbuk et al. 2001).  They also used fire around their dwelling places to clear away vegetation and allow for better visibility of their surroundings.  It is thought that this allowed them to see large movements of prey, and also to prevent enemies for approaching unnoticed.  Kershaw (1986) showed that dramatic increases in microscopic charcoal in core samples matched the receding of Araucaria forests and the replacement with Eucalyptus forests in north eastern Australia.  Differences in the transition between forest types and climatic conditions in the cores indicate that landscape fires set by colonizing Aboriginals are most likely source of these fires and the resulting habitat changes.  They also appear to be responsible for the extinction of at least one tree species, the rainforest conifer Dacrydium (Bowman 1998).  Aboriginal landscape fire is certainly responsible for the maintenance of a mosaic of fire intolerant rainforest and fire tolerant Eucalyptus forests during the Pleistocene (Clark 1983).

Native Americans used fire in several ways.  Native Californians set brush fires to drive game, such as rabbits, into waiting entrapments for easy capture, and seed meadows were routinely burned to increase yields in future years (Margolin 1978).  In so doing, native tribes affected the distribution of species.  They facilitated the presence and spread of fire tolerant species at the expense of species less fire tolerant.  They also altered habitat structure which resulted in more “park like” habitat.  This influenced which species persisted and which were excluded.

However, in more recent times fire has been viewed by humans as a force of destruction only and has been suppressed almost completely in many parts of the world.   This has lead to an increase in the amount of fuel material and a corresponding increase in the intensity of the fires when they do inevitably break out.  The suppression of fire has also lead ecosystems to change in ways that they had been otherwise prevented from, reversing the effects the native peoples and natural fire regimes had had on the landscape.  We are still not at all sure what the full consequences of these changes are.

Conclusions

Whether fire is used as a management tool, suppressed completely, or ignored all together there will be a corresponding effect on biogeography, and these biogeographic effects cannot be underestimated.  As forests expand or contract whole assemblages of species are shifted.  If the fire in a given location increases fragmentation, there will be a corresponding increase in edge effects and the disturbances that go along with them.  Such disturbances can ultimately lead certain species to local or total extinction.  However, other ecosystems have evolved to incorporate fire as an integral feature.  Whatever the effects are, their impact will be extensive and reach throughout the ecosystem.  All these effects compound when the global scale is considered.

Cited Literature

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