- Email: [email protected]
Are some plants born to burn?
NEWS first documented role for this type of predation by lizards in a wet tropical forest eco system; other lizard-removal experiments have been in more xeric systems. While in no way disagreeing with these conclusions, I was mildly surprised by the fact that they make no attempt to place these fascinating results (gathered with great skill, effort and not a little courage) into a wider theoretical context. There are, after all, millions of species in thousands of habitats, and to conclude that species of type X have such-and-such an impact in habitats of type Y may need cataloguing, but it is not a very gripping conclusion. Their results are important in at least three wider contexts. First, there has been considerable interest recently in the evolutionary and ecological processes that mould the distribution of body sizes in fauna1 assemblagesis, including tropical forest arthropodsi6. Ecological explanations for these patterns have tended to focus on habitat structure as a key parameter, and have ignored predation. Such a position is no longer tenable. Second, ornithologists interested in the role of interspecific competition for food as a force structuring
guilds of canopy insectivores need to make lizards honorary birds. Correlational evidence already exists for significant competition between birds and lizards in sclero phyllous scrub on West Indian islandsa; Dial and Roughgarden’s study suggests that similar effects must also be operating in tropical forest canopies. Third, a growing body of theory addresses the related problems of food-web omnivory4 and intraguild predation5; it is a pity that these new data touch on, but do not critically address, some of these issues. Hopefully, all three (and other broader questions) will be examined by Dial and Roughgarden in later publications. Such hard-won data need to be exploited to the full. Oh yes, I nearly forgot. Who is Tarzan? It is the lizard that ‘was observed to jump from 1 m above a spider, grabbing the 8 mm Casteracantha.. .in passing, then landing on leaves below’. Who (or what) else did you think it might be? John H. Lawton NERC Centre for Population Biology Imperial College, Silwood Park, Ascot, UK SLS 7PY
Are some plants born to burn? hy should a plant want to burn? To W city-dwelling humans, wildfire seems a bad thing, and a superficial analysis might suggest that the same view should be held by plants. Surely it would make sense for them to evolve toward being fire proof? It is, therefore, a bit surprising, even to pyrophilic ecologists, that almost the reverse is true. Where climatic conditions favor fire, the plants seem to possess traits that encourage fire spread and increase fire intensity. This pattern is most striking in fire-prone shrublands such as the North American chaparral, Australian mallee and South African fynbos. Plant attributes in these ecosystems include fire survival features, such as serotinous fruiting bodies and lignotubers, but there are also fireenhancing traits: fine-branching, dense canopies with a high proportion of dead wood, high volatile oil content and loose flaky bark (Fig. 1). The effectiveness of these traits in propagating fire cannot be doubted. In chaparral, hillsides exposed to the intense heat from a fire on an opposing slope will seem to explode as the gases distilled from the fine branches and small leaves ignite over large areas. Noting these points, R. Mutchi proposed that the extreme flammability of fireTREE
vol.
10, no.
10 October
1995
prone vegetation was not accidental. He hypothesized that ‘natural selection has favored development of characteristics that make [plants of fire-prone regions] more flammable’. The ‘Mutch hypothesis’ struck a chord with fire ecologists, but there have always been doubters. To many, it seemed that Mutch’s arguments lacked the rigor of the kind advocated by Williams in his influential book2 on adaptation, which appeared a few years before Mutch’s paper. Mutch did not explain how individual selection could favor flammability, or acknowledge the problems with his implicit assumption of group selection3. It was difficult to see how choosing to burn could significantly increase the fitness of the self-immolator4. Most of the flammability traits had other possible explanations, and right-thinking eco-evolutionists tended to dismiss the hypothesis. In a new paper, Bond and Midgleys have reinvigorated the Mutch hypothesis by supplying a plausible explanation of how individual selection might operate. The nub of their argument is that individual fitness of a self-immolator could be significantly increased if one of the consequences of its burning were the death of neighboring
&
COMMENT
References 1 Hunter, M.D. and Price, P.W. (1992) Eco/osy 73, 724-732 2 Wiens, J.A.er al. (1991) Ecol. Monogr. 61, 299321 3 Wright, S.J.(1981)Am. Nat. 117, 181-192 4 Pimm, S.L. and Lawton, J.H. (1978) Nature 275, 542-544 5 Polis, G.A. and Holt, R.D. (1992) Trends Ecot. Evot. 7, 151-154 6 Dial, R. and Roughgarden, J. (1995) Ecology 76, 1821-1834 7 Dial, R. and Tobin, S.C.(1994) SeIbyana 15, 24-37 8 Pacala, SW. and Roughgarden, J. (1984) Oecologia 64,160-162 9 Schoener, T.W. and Spiller, D.A. (1987) Science 236,949-952 10 Spiller, D.A. and Schoener, T.W. (1994) Ecology 75,182-196 11 Schoener, T.W. and Spiller, D.A. (1995) Science 267, 1811-1813 12 Spiller, D.A. and Schoener, T.W. (1990) Natvre 347,469-472 13 Oksanen, L. et al. (1981) Am. Nat. 118,240-262 14 Holt, R.D. and Lawton, J.H. (1994)Annu. Rev. Ecol. Syst. 25,495-520 15 Blackburn, T.M. and Gaston, K.J. (1994) Trends Ecol. Evol. 9,471-474 16 Stork, N.E.and Blackburn, T.M. (1993) Oikos 67,483-489
plants. To demonstrate this they used a cellular automaton spatial model in which the contest for occupancy of grid cells is between two fixed types - ‘torches’ and ‘damps’. Each run of the model is a fire that burns all torches, and each torch burns a variable number of damp neighbors. Death of an individual releases the cell for reoccupancy by recruitment from the ‘soil seedbank’. The outcome of competition be tween damp and torch is random, with the probabilities based on relative seed numbers weighted by fitness. Stochastic simulations were started with a single torch, and continued until one or the other plant type went extinct in their fixed and finite population of 400 plants. The sensitivity of the variable ‘probability of the triumph of torch’ was evaluated by varying several parameters: life history type (sprouter and non-sprouter); fitness in the seedling stage (always assumed equal or to favor torch); fecundity; and, for sprouters only, the relative survival of burned individuals (always assumed to be equal or greater for the torch). The model results support the idea that individual selection based on killing neighbors would favor a flammable life history. The probability of torch spreading in the population was indeed very sensitive to the torch’s effect on the survival of its neighbors. As the radius of neighbors affected increased, the probability of torch winning increased.
0 1995, Elsevier
Science
Ltd
393
NEWS
&
COMMENT
Fig. 1. The function and mode of origin of ‘fire adaptations’ have always been matters of debate. (a) Lignotubers, such as the one on this manzanita (Xy/ococcus bicolor), clearly enhance post-fire resprouting, but some have said that resprouting is the general condition of woody plants, needing no special explanation. Barks show intriguing variation. (b) Many trees, like this Quercus agrifolia (coast live oak), have bark that is very thick, to protect the cambium, and smooth, perhaps to minimize the probability of fire spreading up the trunk. In contrast, a similar size stem of Q. dumosa (scrub oak), a shrubby oak that co-occurs with Q. agrifolia, has much thinner, more flammable bark. A reasonable explanation of the difference is that investment in fire-resistant aboveground stems is pointless for a small plant that will inevitably suffer severe crown fire. (c) Bark flakiness is carried to an extreme in Adenostoma sparsifolium (ribbon bush), and it is tempting to think that the function of the ribbons is to carry fire into the canopy of this relatively large shrub. Some Australian Eucalyptus have similar accumulations of bark. All species illustrated are from southern California, USA.
But in clearing out competitors, the torch also clears out itself. Thus, the death of the parent must be offset by the greater success of the offspring. If fecundity and seedling performance are comparable between torch and damp, the sacrifice of the parent would not yield any clear advantage, and this is what Bond and Midgley’s model shows. The slight advantage to the torch, resulting from the seed shadow decreasing exponentially away from each individual and the circle of destruction being centered about the torch, was not sufficient. For the seeder life history, in which both damp and torch individuals die if burned, increase of the torch was likely only when it had higher fecundity or superior seedling performance. For the ‘sprouter’ case, in which a proportion of burned individuals survive, the fire-caused mortality of the damps had to be substantially greater than the torches for the torch to win. Thus, a major conclusion of Bond and Midgley is that for mutations that increase flammability to spread, they must confer other benefits. For example, a mutation that raised the volatile oil content might spread if it also decreased seedling herbivory6. The kill-thy-neighbor model is intriguing, but neo-Mutchism still faces difficulties. There is the obvious criticism that the Bond-Midgley model does not consider population genetics. From the historical perspective, as Bond and Midgley acknowledge, there is still the challenge to show that fire selection is a necessary element in the explanation of specific traits. Because of its requirement that other benefits must accompany the increased flammability, their model does not encourage us to expect clean answers. Fine branches, for 394
example, make bushes burn better, but they can also be explained as the only efficient canopy structure for a small-leaved plant in a drought-prone climate. It seems unlikely that if fires were suspended a fine-branched shrub would evolve a completely different canopy type. The question then is probably one of degree: how much finer is the branching because of fire selection? Similar arguments can be raised about volatile oils. Assuming their only other function is herbivore deterrence, would other means of reducing herbivory be selected if fire were not a factor? Bond and Midgley’s suggestion for demonstrating the potency of flammability selection is to study features that would seem least likely to be advantageous, except in promoting fire. Their most-intriguing example is the patterns of branch shedding in presumed fire-selected and non-fireselected species. Except perhaps for their shade function in shrubs in light-saturated deserts, dead twigs are a liability for a plant that does not burn. They are a source of disease and infestation and decrease light penetration into the canopy. But dead wood, because of its low moisture content, increases flammability. Bond and Midgley therefore predict that torch species should tend to retain dead twigs, whereas damp species should shed them as soon as they become non-functional. This prediction should be easily testable by suitable pairing of taxa in more and less fire-prone regions. But is it necessarily the case that being flammable puts a plant at a disadvantage relative to less-flammable competitors? Probably, if flammable plants spontaneously cornbusted in a setting in which
fire was otherwise rare, but possibly not in a more historically reasonable scenario. Fire is an inevitable consequence of bio mass accumulation, and not-burning is often not a choice. When a species is exposed to frequent fire, a degree of fire related selection seems certain. One response, available mostly to trees, is to develop means of survival and ways of protecting the investment in aboveground biomass, such as thick non-flammable bark (Fig. 1). But where vegetation structure and the laws of physics dictate that crown fires are inevitable, preserving the canopy is not possible. The choices for species in these situations are to suffer massive population crashes from time to time (not recommended) or to improve survival (e.g. sprouting) and the exploitation of post-fire conditions (e.g. seeding, rapid regrowth). Assuming trade-offs, it would be inevitable that a post-fire exploiter would to some degree become fire-dependent. A shrub, for example, may be able to thrive only in the open, or develop special fire-related germination cuesr. Once this shift to fire dependence occurs, failure to burn can become a liability. For the fire-dependent species the risks of incomplete burning or failing to burn (partial kill, a poorly prepared seedbed, heat kill without the canopy being removed, death from old age) may be greater than the cost of death and the risk of failing to occupy gaps in the post-fire lottery. For such individuals, fire is an opportunity more than a danger. Burning well creates a favorable post-fire patch in which the probability of recapturing the space is very high. Killing neighbors, especially neighbors less able to exploit the TREE
uol.
IO,
no.
10 October
1995
NEWS post-burn conditions, could, however, provide a powerful additional benefit. The debate will continue, but Bond and Midgley have added an interesting new dimension to it.
3
Biology Dept, SanDiego State University, San Diegol CAg2182-005~ usA
79-85
6 Rundel, P.W. (1981) in Fire Regimes and Ecosystem Properties, pp. 183-207, US Forest Service General Technical Report WO-26 7 Keeley, J. (1991) Bat. Rev. 57,81-116
References 1 Mutch, R.W. (1970) Ecology51, 1046-1051 2 Williams, C.C. (1966) Adaptation and Natural Setection, Princeton University Press
S. Bliss, K. Cummins and S. Shapiro commented on the manuscript.
Unwelcome immigrants?
E
ndowed with the power of flight, birds are able to reach remote islands and to travel between continents with relative ease. Even so, their powers of dispersal are insufficient to allow each species to colonize every part of the world in which it could survive. We know this because many of the birds that have been moved about the globe by man have established permanent populations in their new homes. Often they have become accepted and familiar members of the local avifauna. Sometimes they have become pests. Such translocations were the subject of a recent conference organized by the British Ornithologists’ Union (BOU) and the Joint Nature Conservation Committee (JNCC)in Peterborough, UK. David Stroud (JNCC, Peterborough, UK) proposed some useful definitions that were approved by the conference: ‘introduction’ refers to non-native species; ‘re-establishment’ refers to native species that have gone extinct in the area that they are being moved to; l ‘restocking’ refers to the boosting of wild populations by the release of individuals that are taken from other wild populations, or that have been bred in captivity - only populations derived from domestic stock should, the conference agreed, be described as ‘feral’. l l
Christopher Lever (Winkfield, UK) laid the groundwork for the conference by reviewing not only these various types of translocation but also their manifold results and, particularly, the reasons why translocations have been made. Some are accidental: parrots (popular cagebirds), waterfowl (often kept full-winged on unenclosed waters), and raptors (flown free for falconry) are particularly prone to escape into the wild. The provision of food or sport hunting are among the deliberate reasons for introductions and re stocks: game birds and wildfowl are common subjects. Commonly, birds have also been introduced for aesthetic reasons. In addition, there is a long and continuing TREE
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COMMENT
Agee, J.K. (1993) Fire Ecology of the Pacific Northwest, Island Press 4 Snyder, J.R. (1984) Oikos 43,404-405 5 Bond, W.J. and Midgley, J.J. (1995) Oikos 73,
Paul H. Zedler
Acknowledgements
&
history of introducing birds as potential agents of biological control: for example, house (English) sparrows (Passer domesticus) were introduced to North America to combat insect pests. Yet another reason for translocating birds is illustrated by the movement of kakapo (Striops habrophilus) and kiwi (Apteryx australis) from mainland New Zealand to Resolution Island, to save them from the (introduced) stoat, (Mustelu ermineu). This was done 100 years ago, establishing a tradition of conservation translocations that has been of great value in New Zealand, and which was reviewed by Brian Bell (Wildlife Management International, Wellington, New Zealand). Not all the work has involved finding refuge from predators. Thus, North Island saddle backs (Philestumus curunculutus rufuster) had their world range increased from one small (though suitable) island to two, simply to improve their chances of surviving accidents, while takahes (Notomis muntelli) benefited from being moved into the lowlands from the suboptimal highland areas to which they had become confined. Bell’s conclusions were that prior study of habitat requirements was an important determinant of success, and that monitoring of the translocations was essential if lessons were to be learned from either success or failure. Much of the conference was concerned with British examples. John Marchant [British Trust for Ornithology (BTO), Thetford, UK] pointed out that nine of over 200 bird species breeding regularly in Britain and h-eland were introductions, including such common and familiar birds as the pheasant (Phusiunus cofchicus). Several other introduced species are breeding (or have bred) in the wild in Britain without it yet being clear that they will be permanent additions to the avifauna. Yet others, such as the Canada goose (Bruntu cunudensis), have established themselves as breeders only because of introductions, even though occasional birds make an unassisted passage to Britain. As in New
Zealand, the conservation value of translocations in Britain has mainly been for native species. For example, the red kite (Miluus miluus), though formerly wide spread in Britain, has been confined to midWales for many years. Despite protection, the remnant population has increased slowly, perhaps because the Welsh habitat is not good for it or perhaps because it is inbred. Through careful planning of re leases of Swedish and Spanish birds, the species is now re-established in Scotland and England, with a total of 28 breeding pairs. Ian Evans (JNCC), a key worker on the red kite programme, reviewed British experience of such re-establishment and restocking work. Like Bell, he stressed the value of prior evaluation, suggesting that the reason why there-establishment of the great bustard (Otis turdu) failed was that suitable habitat was no longer present in Britain. He also considered cases, such as the barn owl (Tyto u&z) and grey partridge (Perdixperdk), in which large-scale restocking masks underlying problems, by artificially and continuously boosting the apparent size of the populations. Successful re-establishment does not always depend on prior evaluation and careful methods. The goshawk (Accipter gentifis), as Steve Petty (The Forestry Authority, Dunoon, UK) showed, has come back to Britain through haphazard escapes and releases by individual enthusiasts. Nor are benefits of introduction always foreseen, as the case of the mandarin duck (Aix gulericulutu) shows: the small introduced British population may now comprise as much as 30-50% of the total world population and so is of not inconsiderable conservation value. The golden pheasant (Chrysolophus pictus) is another example of a species that is scarce in its native range but for which Britain provides a second home. In this case, its previously unknown ecology and behaviour in the wild have been uncovered by Mark Rehfisch, Dawn Balmer and Stephen Browne (BTO) through work in Britain-logistically much easier than research in the forested hills of China. Golden pheasants and mandarins have not become common in Britain and they appear harmless, so they are generally welcome. The goshawk, however, expanding
0 1995, Elsevier
Science
Ltd
395
guilds of canopy insectivores need to make lizards honorary birds. Correlational evidence already exists for significant competition between birds and lizards in sclero phyllous scrub on West Indian islandsa; Dial and Roughgarden’s study suggests that similar effects must also be operating in tropical forest canopies. Third, a growing body of theory addresses the related problems of food-web omnivory4 and intraguild predation5; it is a pity that these new data touch on, but do not critically address, some of these issues. Hopefully, all three (and other broader questions) will be examined by Dial and Roughgarden in later publications. Such hard-won data need to be exploited to the full. Oh yes, I nearly forgot. Who is Tarzan? It is the lizard that ‘was observed to jump from 1 m above a spider, grabbing the 8 mm Casteracantha.. .in passing, then landing on leaves below’. Who (or what) else did you think it might be? John H. Lawton NERC Centre for Population Biology Imperial College, Silwood Park, Ascot, UK SLS 7PY
Are some plants born to burn? hy should a plant want to burn? To W city-dwelling humans, wildfire seems a bad thing, and a superficial analysis might suggest that the same view should be held by plants. Surely it would make sense for them to evolve toward being fire proof? It is, therefore, a bit surprising, even to pyrophilic ecologists, that almost the reverse is true. Where climatic conditions favor fire, the plants seem to possess traits that encourage fire spread and increase fire intensity. This pattern is most striking in fire-prone shrublands such as the North American chaparral, Australian mallee and South African fynbos. Plant attributes in these ecosystems include fire survival features, such as serotinous fruiting bodies and lignotubers, but there are also fireenhancing traits: fine-branching, dense canopies with a high proportion of dead wood, high volatile oil content and loose flaky bark (Fig. 1). The effectiveness of these traits in propagating fire cannot be doubted. In chaparral, hillsides exposed to the intense heat from a fire on an opposing slope will seem to explode as the gases distilled from the fine branches and small leaves ignite over large areas. Noting these points, R. Mutchi proposed that the extreme flammability of fireTREE
vol.
10, no.
10 October
1995
prone vegetation was not accidental. He hypothesized that ‘natural selection has favored development of characteristics that make [plants of fire-prone regions] more flammable’. The ‘Mutch hypothesis’ struck a chord with fire ecologists, but there have always been doubters. To many, it seemed that Mutch’s arguments lacked the rigor of the kind advocated by Williams in his influential book2 on adaptation, which appeared a few years before Mutch’s paper. Mutch did not explain how individual selection could favor flammability, or acknowledge the problems with his implicit assumption of group selection3. It was difficult to see how choosing to burn could significantly increase the fitness of the self-immolator4. Most of the flammability traits had other possible explanations, and right-thinking eco-evolutionists tended to dismiss the hypothesis. In a new paper, Bond and Midgleys have reinvigorated the Mutch hypothesis by supplying a plausible explanation of how individual selection might operate. The nub of their argument is that individual fitness of a self-immolator could be significantly increased if one of the consequences of its burning were the death of neighboring
&
COMMENT
References 1 Hunter, M.D. and Price, P.W. (1992) Eco/osy 73, 724-732 2 Wiens, J.A.er al. (1991) Ecol. Monogr. 61, 299321 3 Wright, S.J.(1981)Am. Nat. 117, 181-192 4 Pimm, S.L. and Lawton, J.H. (1978) Nature 275, 542-544 5 Polis, G.A. and Holt, R.D. (1992) Trends Ecot. Evot. 7, 151-154 6 Dial, R. and Roughgarden, J. (1995) Ecology 76, 1821-1834 7 Dial, R. and Tobin, S.C.(1994) SeIbyana 15, 24-37 8 Pacala, SW. and Roughgarden, J. (1984) Oecologia 64,160-162 9 Schoener, T.W. and Spiller, D.A. (1987) Science 236,949-952 10 Spiller, D.A. and Schoener, T.W. (1994) Ecology 75,182-196 11 Schoener, T.W. and Spiller, D.A. (1995) Science 267, 1811-1813 12 Spiller, D.A. and Schoener, T.W. (1990) Natvre 347,469-472 13 Oksanen, L. et al. (1981) Am. Nat. 118,240-262 14 Holt, R.D. and Lawton, J.H. (1994)Annu. Rev. Ecol. Syst. 25,495-520 15 Blackburn, T.M. and Gaston, K.J. (1994) Trends Ecol. Evol. 9,471-474 16 Stork, N.E.and Blackburn, T.M. (1993) Oikos 67,483-489
plants. To demonstrate this they used a cellular automaton spatial model in which the contest for occupancy of grid cells is between two fixed types - ‘torches’ and ‘damps’. Each run of the model is a fire that burns all torches, and each torch burns a variable number of damp neighbors. Death of an individual releases the cell for reoccupancy by recruitment from the ‘soil seedbank’. The outcome of competition be tween damp and torch is random, with the probabilities based on relative seed numbers weighted by fitness. Stochastic simulations were started with a single torch, and continued until one or the other plant type went extinct in their fixed and finite population of 400 plants. The sensitivity of the variable ‘probability of the triumph of torch’ was evaluated by varying several parameters: life history type (sprouter and non-sprouter); fitness in the seedling stage (always assumed equal or to favor torch); fecundity; and, for sprouters only, the relative survival of burned individuals (always assumed to be equal or greater for the torch). The model results support the idea that individual selection based on killing neighbors would favor a flammable life history. The probability of torch spreading in the population was indeed very sensitive to the torch’s effect on the survival of its neighbors. As the radius of neighbors affected increased, the probability of torch winning increased.
0 1995, Elsevier
Science
Ltd
393
NEWS
&
COMMENT
Fig. 1. The function and mode of origin of ‘fire adaptations’ have always been matters of debate. (a) Lignotubers, such as the one on this manzanita (Xy/ococcus bicolor), clearly enhance post-fire resprouting, but some have said that resprouting is the general condition of woody plants, needing no special explanation. Barks show intriguing variation. (b) Many trees, like this Quercus agrifolia (coast live oak), have bark that is very thick, to protect the cambium, and smooth, perhaps to minimize the probability of fire spreading up the trunk. In contrast, a similar size stem of Q. dumosa (scrub oak), a shrubby oak that co-occurs with Q. agrifolia, has much thinner, more flammable bark. A reasonable explanation of the difference is that investment in fire-resistant aboveground stems is pointless for a small plant that will inevitably suffer severe crown fire. (c) Bark flakiness is carried to an extreme in Adenostoma sparsifolium (ribbon bush), and it is tempting to think that the function of the ribbons is to carry fire into the canopy of this relatively large shrub. Some Australian Eucalyptus have similar accumulations of bark. All species illustrated are from southern California, USA.
But in clearing out competitors, the torch also clears out itself. Thus, the death of the parent must be offset by the greater success of the offspring. If fecundity and seedling performance are comparable between torch and damp, the sacrifice of the parent would not yield any clear advantage, and this is what Bond and Midgley’s model shows. The slight advantage to the torch, resulting from the seed shadow decreasing exponentially away from each individual and the circle of destruction being centered about the torch, was not sufficient. For the seeder life history, in which both damp and torch individuals die if burned, increase of the torch was likely only when it had higher fecundity or superior seedling performance. For the ‘sprouter’ case, in which a proportion of burned individuals survive, the fire-caused mortality of the damps had to be substantially greater than the torches for the torch to win. Thus, a major conclusion of Bond and Midgley is that for mutations that increase flammability to spread, they must confer other benefits. For example, a mutation that raised the volatile oil content might spread if it also decreased seedling herbivory6. The kill-thy-neighbor model is intriguing, but neo-Mutchism still faces difficulties. There is the obvious criticism that the Bond-Midgley model does not consider population genetics. From the historical perspective, as Bond and Midgley acknowledge, there is still the challenge to show that fire selection is a necessary element in the explanation of specific traits. Because of its requirement that other benefits must accompany the increased flammability, their model does not encourage us to expect clean answers. Fine branches, for 394
example, make bushes burn better, but they can also be explained as the only efficient canopy structure for a small-leaved plant in a drought-prone climate. It seems unlikely that if fires were suspended a fine-branched shrub would evolve a completely different canopy type. The question then is probably one of degree: how much finer is the branching because of fire selection? Similar arguments can be raised about volatile oils. Assuming their only other function is herbivore deterrence, would other means of reducing herbivory be selected if fire were not a factor? Bond and Midgley’s suggestion for demonstrating the potency of flammability selection is to study features that would seem least likely to be advantageous, except in promoting fire. Their most-intriguing example is the patterns of branch shedding in presumed fire-selected and non-fireselected species. Except perhaps for their shade function in shrubs in light-saturated deserts, dead twigs are a liability for a plant that does not burn. They are a source of disease and infestation and decrease light penetration into the canopy. But dead wood, because of its low moisture content, increases flammability. Bond and Midgley therefore predict that torch species should tend to retain dead twigs, whereas damp species should shed them as soon as they become non-functional. This prediction should be easily testable by suitable pairing of taxa in more and less fire-prone regions. But is it necessarily the case that being flammable puts a plant at a disadvantage relative to less-flammable competitors? Probably, if flammable plants spontaneously cornbusted in a setting in which
fire was otherwise rare, but possibly not in a more historically reasonable scenario. Fire is an inevitable consequence of bio mass accumulation, and not-burning is often not a choice. When a species is exposed to frequent fire, a degree of fire related selection seems certain. One response, available mostly to trees, is to develop means of survival and ways of protecting the investment in aboveground biomass, such as thick non-flammable bark (Fig. 1). But where vegetation structure and the laws of physics dictate that crown fires are inevitable, preserving the canopy is not possible. The choices for species in these situations are to suffer massive population crashes from time to time (not recommended) or to improve survival (e.g. sprouting) and the exploitation of post-fire conditions (e.g. seeding, rapid regrowth). Assuming trade-offs, it would be inevitable that a post-fire exploiter would to some degree become fire-dependent. A shrub, for example, may be able to thrive only in the open, or develop special fire-related germination cuesr. Once this shift to fire dependence occurs, failure to burn can become a liability. For the fire-dependent species the risks of incomplete burning or failing to burn (partial kill, a poorly prepared seedbed, heat kill without the canopy being removed, death from old age) may be greater than the cost of death and the risk of failing to occupy gaps in the post-fire lottery. For such individuals, fire is an opportunity more than a danger. Burning well creates a favorable post-fire patch in which the probability of recapturing the space is very high. Killing neighbors, especially neighbors less able to exploit the TREE
uol.
IO,
no.
10 October
1995
NEWS post-burn conditions, could, however, provide a powerful additional benefit. The debate will continue, but Bond and Midgley have added an interesting new dimension to it.
3
Biology Dept, SanDiego State University, San Diegol CAg2182-005~ usA
79-85
6 Rundel, P.W. (1981) in Fire Regimes and Ecosystem Properties, pp. 183-207, US Forest Service General Technical Report WO-26 7 Keeley, J. (1991) Bat. Rev. 57,81-116
References 1 Mutch, R.W. (1970) Ecology51, 1046-1051 2 Williams, C.C. (1966) Adaptation and Natural Setection, Princeton University Press
S. Bliss, K. Cummins and S. Shapiro commented on the manuscript.
Unwelcome immigrants?
E
ndowed with the power of flight, birds are able to reach remote islands and to travel between continents with relative ease. Even so, their powers of dispersal are insufficient to allow each species to colonize every part of the world in which it could survive. We know this because many of the birds that have been moved about the globe by man have established permanent populations in their new homes. Often they have become accepted and familiar members of the local avifauna. Sometimes they have become pests. Such translocations were the subject of a recent conference organized by the British Ornithologists’ Union (BOU) and the Joint Nature Conservation Committee (JNCC)in Peterborough, UK. David Stroud (JNCC, Peterborough, UK) proposed some useful definitions that were approved by the conference: ‘introduction’ refers to non-native species; ‘re-establishment’ refers to native species that have gone extinct in the area that they are being moved to; l ‘restocking’ refers to the boosting of wild populations by the release of individuals that are taken from other wild populations, or that have been bred in captivity - only populations derived from domestic stock should, the conference agreed, be described as ‘feral’. l l
Christopher Lever (Winkfield, UK) laid the groundwork for the conference by reviewing not only these various types of translocation but also their manifold results and, particularly, the reasons why translocations have been made. Some are accidental: parrots (popular cagebirds), waterfowl (often kept full-winged on unenclosed waters), and raptors (flown free for falconry) are particularly prone to escape into the wild. The provision of food or sport hunting are among the deliberate reasons for introductions and re stocks: game birds and wildfowl are common subjects. Commonly, birds have also been introduced for aesthetic reasons. In addition, there is a long and continuing TREE
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Agee, J.K. (1993) Fire Ecology of the Pacific Northwest, Island Press 4 Snyder, J.R. (1984) Oikos 43,404-405 5 Bond, W.J. and Midgley, J.J. (1995) Oikos 73,
Paul H. Zedler
Acknowledgements
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history of introducing birds as potential agents of biological control: for example, house (English) sparrows (Passer domesticus) were introduced to North America to combat insect pests. Yet another reason for translocating birds is illustrated by the movement of kakapo (Striops habrophilus) and kiwi (Apteryx australis) from mainland New Zealand to Resolution Island, to save them from the (introduced) stoat, (Mustelu ermineu). This was done 100 years ago, establishing a tradition of conservation translocations that has been of great value in New Zealand, and which was reviewed by Brian Bell (Wildlife Management International, Wellington, New Zealand). Not all the work has involved finding refuge from predators. Thus, North Island saddle backs (Philestumus curunculutus rufuster) had their world range increased from one small (though suitable) island to two, simply to improve their chances of surviving accidents, while takahes (Notomis muntelli) benefited from being moved into the lowlands from the suboptimal highland areas to which they had become confined. Bell’s conclusions were that prior study of habitat requirements was an important determinant of success, and that monitoring of the translocations was essential if lessons were to be learned from either success or failure. Much of the conference was concerned with British examples. John Marchant [British Trust for Ornithology (BTO), Thetford, UK] pointed out that nine of over 200 bird species breeding regularly in Britain and h-eland were introductions, including such common and familiar birds as the pheasant (Phusiunus cofchicus). Several other introduced species are breeding (or have bred) in the wild in Britain without it yet being clear that they will be permanent additions to the avifauna. Yet others, such as the Canada goose (Bruntu cunudensis), have established themselves as breeders only because of introductions, even though occasional birds make an unassisted passage to Britain. As in New
Zealand, the conservation value of translocations in Britain has mainly been for native species. For example, the red kite (Miluus miluus), though formerly wide spread in Britain, has been confined to midWales for many years. Despite protection, the remnant population has increased slowly, perhaps because the Welsh habitat is not good for it or perhaps because it is inbred. Through careful planning of re leases of Swedish and Spanish birds, the species is now re-established in Scotland and England, with a total of 28 breeding pairs. Ian Evans (JNCC), a key worker on the red kite programme, reviewed British experience of such re-establishment and restocking work. Like Bell, he stressed the value of prior evaluation, suggesting that the reason why there-establishment of the great bustard (Otis turdu) failed was that suitable habitat was no longer present in Britain. He also considered cases, such as the barn owl (Tyto u&z) and grey partridge (Perdixperdk), in which large-scale restocking masks underlying problems, by artificially and continuously boosting the apparent size of the populations. Successful re-establishment does not always depend on prior evaluation and careful methods. The goshawk (Accipter gentifis), as Steve Petty (The Forestry Authority, Dunoon, UK) showed, has come back to Britain through haphazard escapes and releases by individual enthusiasts. Nor are benefits of introduction always foreseen, as the case of the mandarin duck (Aix gulericulutu) shows: the small introduced British population may now comprise as much as 30-50% of the total world population and so is of not inconsiderable conservation value. The golden pheasant (Chrysolophus pictus) is another example of a species that is scarce in its native range but for which Britain provides a second home. In this case, its previously unknown ecology and behaviour in the wild have been uncovered by Mark Rehfisch, Dawn Balmer and Stephen Browne (BTO) through work in Britain-logistically much easier than research in the forested hills of China. Golden pheasants and mandarins have not become common in Britain and they appear harmless, so they are generally welcome. The goshawk, however, expanding
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