- Email: [email protected]
Fragments past, present and future
NEWS & COMMENT
Fragments past, present and future
A
t the New York World’s Fair of 1964 I saw the Juggernaut. It was a deforestation machine, the centerpiece of a futurist diorama in an auto company pavilion. One side of the diorama was a dark Amazon forest, the other a sunny land of corn and cows. At the center, giant Juggernaut pushed forward on huge wheels, its long arms and lobster claws ripping trees from the earth, triumphantly turning forest to farm. Thirty-four years later the deforestation juggernaut is unrelenting, and conservationists are concerned with the deteriorating biodiversity in the forest fragments left behind. Fragments lose species because of fire and wind on their edges, lack of mutualists, displacement of resident by weedy species, and the vulnerability of small populations to chance extinction. However, recent work, showing what can happen in fragments over the long term, suggests that we might be able to salvage diversity in these fragments. Martin Kellman and his co-workers study forest fragments that were created thousands of years in the past and are rich in tree species at present1. By studying such fragments, we can learn lessons for conserving diversity in the future1. Kellman studies gallery forests in savannas of Belize and Venezuela2,3. These are fragments of once-continuous forest that during Pleistocene drought retreated to narrow, discontinuous strips of forest along streams, where nutrient and water supplies were greater and fires fewer than in the surrounding savanna. In the Belize study area, these fragments range from 0.5 to 160 ha; fragments are larger in Venezuela. In contrast to current theory, there are as many tree species in 1 ha of these old fragments as in 1 ha of continuous forest in the region, and richness accumulates from hectare-to-hectare within and among fragments. In addition, richness is not depressed along gallery forest peninsulas4, and most of the tree species in these fragments are also found in continuous forest. Whatever processes have conserved species in these fragments might also have conserved them in fragments in similar topographic settings during Pleistocene drought. This would explain the rapid reappearance of species-rich, continuous forest in some areas after rainfall increased and where no large Pleistocene refugia are postulated5. Tree diversity in these gallery forest fragments (studied mainly in Belize) is maintained by several mechanisms. First, the trees are small, permitting a high
382
density of stems and, therefore, species3,6. Small trees could also permit relatively large populations per species in a given area, consequently reducing the extinction rate. Second, bird-dispersed species dominate the tree flora, and their vagility appears to enrich individual fragments by exchange among fragments3. Third, species respond differentially to environmental and disturbance gradients from edge to core of the forest1,7. Concentrated at the edge is a group of species that tolerates fire incursions from the savanna and thrives in high light. When fire enters the forest it does not usually kill canopy trees but burns the ground layer and creates a seedbed suitable for these edge species. They form a thick wall of foliage that limits penetration by high light to 7–12 m into the forest8. Further into the forest, treefalls are small and occur mainly on upper valley slopes, promoting regeneration of a few species requiring gaps for establishment. Both fire and treefalls support specialists but are restricted enough to prevent disturbance of much of the forest core, which is suitable for a diversity of shadetolerant, fire-sensitive species that also occur in continuous forest. Soil variability is minor in these forests but accounts for some additional specialization. There are also generalist tree species occurring all along the edge-to-core gradient. These three mechanisms of species enrichment in natural fragments (high stem density, high immigration and edge–core characteristics) do not generally operate in tropical forest fragments recently created by humans and predicted to lose tree diversity. As remnants of previously continuous forests, stem densities tend to be lower in anthropogenic fragments, and there are more species requiring dispersal by less mobile mammals and large birds, whereas the newly created agricultural matrix these dispersers must cross is less hospitable than the savanna matrix surrounding gallery forests. Most obvious is the contrast between the deleterious edge effects in anthropogenic fragments and the species-enriching edge effects in natural fragments. Fire and high light penetrate further and treefall rates are higher on edges of recent anthropogenic fragments. This promotes invasion by weedy pioneer trees and vines that disrupt regeneration of diverse shade-tolerant species9. By contrast, gallery forest edges are sealed off by a fringe of edge specialists that protect interior microclimates, while fire maintains this guild of edge trees.
Speed of creation is the reason for these contrasts between natural and anthropogenic fragments. The natural fragments were formed over a long period, slowly adjusting to slowly changing conditions, with ample time for a protective edge to develop and for species adapted to fragment conditions to replace less fit species. By contrast, anthropogenic fragments form abruptly and are dominated by short-term processes of adjustment to abruptly changed conditions, leading to a rapid collapse in community organization and richness (especially evident for animals10). Studies of recently created fragments describe these short-term processes and subsequent deleterious conditions and results, but ancient, natural fragments might better indicate the long-term, steadystate that could be conserved in new fragments. Thus, to conserve tree diversity in forest fragments, the management goal is to bring modern fragments into a condition resembling ancient fragments as quickly as possible. To achieve this, at least three actions seem necessary1,2. The first is to create a stable, protective community of edge species around new fragments to protect the diverse core species from altered microclimates and disturbance regimes. This requires helping likely members of the regional flora to establish on edges. The second is to control fire. New edges are especially vulnerable to fire, and the frequent fires characteristic of tropical agriculture might overwhelm even the edge specialists. The third action might be to assist in tree migration between fragments to balance extinctions within fragments. Migration might be promoted by maintaining even the slenderest connections for bird dispersers, such as hedgerows or isolated pasture trees11. Kellman emphasizes that managed forest fragments cannot be microcosms of continuous forest12, an essential point to understand if we hope to conserve species in fragments and over a landscape. Compared with the biota in continuous forest, fragment biota are organized by different conditions and processes, involving small canopy trees, small treefall gaps, edge fires, a guild of edge trees, edge-to-core gradients and broadly dispersed plant species. Fragments become new kinds of communities and, to conserve species in them, managers must work with these new conditions and processes and not try to maintain fragments as small pieces of the old, continuous forest for which appropriate conditions and processes no longer exist. Over a landscape, a system of managed fragments, however species-rich, will not substitute for conserving large blocs of continuous forest. The natural fragments in Belize, although rich in tree species, lack
Copyright © 1998, Elsevier Science Ltd. All rights reserved. 0169-5347/98/$19.00 PII: S0169-5347(98)01429-3
TREE vol. 13, no. 10 October 1998
NEWS & COMMENT many mammal-dispersed and dioecious tree species found in continuous forest in the region3, probably because these fragments lack the diversity of animals that disperse seed and pollinate plants in continuous forest. This work in gallery forests shows for the first time that fragments can conserve tree diversity over millennia, but it also shows for the first time that tree species dependent on large mutualist animals are eventually lost. Although a natural savanna-forest mosaic can support diverse animal species13, their value as mutualists might differ from that of animals in continuous forest and, in any case, in modern fragments the agricultural matrix, hunting, and within-fragment habitat changes would reduce many animal populations. Thus, to maintain the diversity of all tropical plants and animals, conservation of large blocs of continuous forest is still the highest priority. In much of the tropics, however, the juggernaut of deforestation has left no large blocs to conserve, although recently created fragments still rich in plant species might remain. The present state of natural fragments formed in the distant past suggests that loss of a rich tree flora from these recent fragments is not inevitable if they are managed well. Just as Pleistocene fragments apparently coalesced to form rich Holocene forests, so these modern fragments could be the nuclei of expanding forests in the future14, as is happening now in Puerto Rico where much farming has been abandoned15. Meanwhile, for
tree species in managed fragments of the future, this study of ancient fragments inspires ‘cautious optimism’12. Nicholas Brokaw Manomet Center for Conservation Sciences, PO Box 1770, Manomet, MA 02345, USA ([email protected])
References 1 Kellman, M., Tackaberry, R. and Rigg, L. (1998) Structure and function in two tropical gallery forest communities: implications for forest conservation in fragmented systems, J. Appl. Ecol. 35, 195–206 2 Kellman, M., Tackaberry, R. and Meave, J. (1996) The consequences of prolonged fragmentation: lessons from tropical gallery forests, in Forest Patches in Tropical Landscapes (Schelhas, J. and Greenberg, R., eds), pp. 37–58, Island Press 3 Meave, J. and Kellman, M. (1994) Maintenance of rain forest diversity in riparian forests of tropical savannas: implications for species conservation during Pleistocene drought, J. Biogeogr. 21, 121–135 4 Tackaberry, R. and Kellman, M. (1996) Patterns of tree species richness along peninsular extensions of tropical forests, Glob. Ecol. Biogeogr. Lett. 5, 85–90 5 Leyden, B.W. (1984) Guatemalan forest synthesis after Pleistocene aridity, Proc. Natl. Acad. Sci. U. S. A. 81, 4856–4859 6 Denslow, J.S. (1995) Disturbance and diversity in tropical rain forests: the density effect, Ecol. Appl. 5, 962–968
7 Kellman, M. and Tackaberry, R. (1993) Disturbance and tree species coexistence in tropical riparian forest fragments, Glob. Ecol. Biogeogr. Lett. 3, 1–9 8 MacDougall, A. and Kellman, M. (1992) The understory light regime and patterns of tree seedlings in tropical riparian forest patches, J. Biogeogr. 19, 667–675 9 Laurance, W.F. and Bierregaard, R.O., Jr, eds (1997) Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities, The University of Chicago Press 10 Terborgh, J. et al. (1997) Transitory states in relaxing ecosystems of land bridge islands, in Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities (Laurance, W.F. and Bierregaard, R.O., Jr, eds), pp. 256–274, The University of Chicago Press 11 Greenberg, R. (1996) Managed forest patches and the diversity of birds in southern Mexico, in Forest Patches in Tropical Landscapes (Schelhas, J. and Greenberg, R., eds), pp. 59–90, Island Press 12 Kellman, M. (1996) Redefining roles: plant community reorganization and species preservation in fragmented systems, Glob. Ecol. Biogeogr. Lett. 5, 111–116 13 Tutin, C.E.G., White, L.J.T. and Mackanga-Missandzou, A. (1997) The use by rain forest mammals of natural forest fragments in an equatorial African savanna, Conserv. Biol. 11, 1190–1203 14 Turner, I.M. and Corlett, R.T. (1996) The conservation value of small, isolated fragments of lowland tropical rain forest, Trends Ecol. Evol. 11, 330–333 15 Thomlinson, J.R. et al. (1996) Land-use dynamics in a post-agricultural Puerto Rican landscape (1936–1988), Biotropica 28, 525–536
The stoop of large falcons
L
arge falcons hunt by diving from great altitude – ‘stooping’ – on their quarry. In these spectacular aerobatic displays, the prey is sometimes seized and even retrieved in the air1 (Fig. 1). The legendary stoop of the peregrine Falco peregrinus has stimulated exaggerated estimates of the top speeds achieved. Some early observers reported2 fantastic speeds of up to nearly 150 m s−1 (576 km h−1). The highest figure is based on an observation of a falcon overtaking an aircraft at an estimated speed twice that of the aeroplane. Other methods based on timed observations of falcons diving a known distance yielded speeds of nearly 100 m s−1 (Ref. 3) but, as Derek Ratcliffe4 wrote in 1980, ‘the speed achieved by a stooping peregrine is a favourite topic, but one still not yet satisfactorily resolved by the marvels of modern electronic gadgetry’. TREE vol. 13, no. 10 October 1998
In 1987 Thomas Alerstam reported the first speeds of a stooping peregrine (and of a stooping goshawk, Accipiter gentilis) obtained by a tracking radar5. This falcon was tracked during three consecutive stoops, interspersed with soaring in preparation for the next stoop, and it reached a maximum speed of 39 m s−1 during ten seconds – well below the previously reported speeds. The radar measurements were compared with calculations of theoretical diving speeds, and Alerstam concluded that the falcon controlled its stooping speed well below the theoretical maximum, presumably because high-speed manoeuvring towards a prey is facilitated by a moderate diving speed5. Therefore, the question remained whether falcons ever achieve top speeds of up to 100 m s−1 or if the much lower speeds recorded by highly accurate radar5 are typical.
Fig. 1. The typical shape of a stooping peregrine with the wings held slightly cupped against the body and tail.
Theory versus reality In two new papers, Tucker and colleagues6,7 shed new light on the question of top speed in stooping falcons. The first
Copyright © 1998, Elsevier Science Ltd. All rights reserved. 0169-5347/98/$19.00 PII: S0169-5347(98)01435-9
383
Fragments past, present and future
A
t the New York World’s Fair of 1964 I saw the Juggernaut. It was a deforestation machine, the centerpiece of a futurist diorama in an auto company pavilion. One side of the diorama was a dark Amazon forest, the other a sunny land of corn and cows. At the center, giant Juggernaut pushed forward on huge wheels, its long arms and lobster claws ripping trees from the earth, triumphantly turning forest to farm. Thirty-four years later the deforestation juggernaut is unrelenting, and conservationists are concerned with the deteriorating biodiversity in the forest fragments left behind. Fragments lose species because of fire and wind on their edges, lack of mutualists, displacement of resident by weedy species, and the vulnerability of small populations to chance extinction. However, recent work, showing what can happen in fragments over the long term, suggests that we might be able to salvage diversity in these fragments. Martin Kellman and his co-workers study forest fragments that were created thousands of years in the past and are rich in tree species at present1. By studying such fragments, we can learn lessons for conserving diversity in the future1. Kellman studies gallery forests in savannas of Belize and Venezuela2,3. These are fragments of once-continuous forest that during Pleistocene drought retreated to narrow, discontinuous strips of forest along streams, where nutrient and water supplies were greater and fires fewer than in the surrounding savanna. In the Belize study area, these fragments range from 0.5 to 160 ha; fragments are larger in Venezuela. In contrast to current theory, there are as many tree species in 1 ha of these old fragments as in 1 ha of continuous forest in the region, and richness accumulates from hectare-to-hectare within and among fragments. In addition, richness is not depressed along gallery forest peninsulas4, and most of the tree species in these fragments are also found in continuous forest. Whatever processes have conserved species in these fragments might also have conserved them in fragments in similar topographic settings during Pleistocene drought. This would explain the rapid reappearance of species-rich, continuous forest in some areas after rainfall increased and where no large Pleistocene refugia are postulated5. Tree diversity in these gallery forest fragments (studied mainly in Belize) is maintained by several mechanisms. First, the trees are small, permitting a high
382
density of stems and, therefore, species3,6. Small trees could also permit relatively large populations per species in a given area, consequently reducing the extinction rate. Second, bird-dispersed species dominate the tree flora, and their vagility appears to enrich individual fragments by exchange among fragments3. Third, species respond differentially to environmental and disturbance gradients from edge to core of the forest1,7. Concentrated at the edge is a group of species that tolerates fire incursions from the savanna and thrives in high light. When fire enters the forest it does not usually kill canopy trees but burns the ground layer and creates a seedbed suitable for these edge species. They form a thick wall of foliage that limits penetration by high light to 7–12 m into the forest8. Further into the forest, treefalls are small and occur mainly on upper valley slopes, promoting regeneration of a few species requiring gaps for establishment. Both fire and treefalls support specialists but are restricted enough to prevent disturbance of much of the forest core, which is suitable for a diversity of shadetolerant, fire-sensitive species that also occur in continuous forest. Soil variability is minor in these forests but accounts for some additional specialization. There are also generalist tree species occurring all along the edge-to-core gradient. These three mechanisms of species enrichment in natural fragments (high stem density, high immigration and edge–core characteristics) do not generally operate in tropical forest fragments recently created by humans and predicted to lose tree diversity. As remnants of previously continuous forests, stem densities tend to be lower in anthropogenic fragments, and there are more species requiring dispersal by less mobile mammals and large birds, whereas the newly created agricultural matrix these dispersers must cross is less hospitable than the savanna matrix surrounding gallery forests. Most obvious is the contrast between the deleterious edge effects in anthropogenic fragments and the species-enriching edge effects in natural fragments. Fire and high light penetrate further and treefall rates are higher on edges of recent anthropogenic fragments. This promotes invasion by weedy pioneer trees and vines that disrupt regeneration of diverse shade-tolerant species9. By contrast, gallery forest edges are sealed off by a fringe of edge specialists that protect interior microclimates, while fire maintains this guild of edge trees.
Speed of creation is the reason for these contrasts between natural and anthropogenic fragments. The natural fragments were formed over a long period, slowly adjusting to slowly changing conditions, with ample time for a protective edge to develop and for species adapted to fragment conditions to replace less fit species. By contrast, anthropogenic fragments form abruptly and are dominated by short-term processes of adjustment to abruptly changed conditions, leading to a rapid collapse in community organization and richness (especially evident for animals10). Studies of recently created fragments describe these short-term processes and subsequent deleterious conditions and results, but ancient, natural fragments might better indicate the long-term, steadystate that could be conserved in new fragments. Thus, to conserve tree diversity in forest fragments, the management goal is to bring modern fragments into a condition resembling ancient fragments as quickly as possible. To achieve this, at least three actions seem necessary1,2. The first is to create a stable, protective community of edge species around new fragments to protect the diverse core species from altered microclimates and disturbance regimes. This requires helping likely members of the regional flora to establish on edges. The second is to control fire. New edges are especially vulnerable to fire, and the frequent fires characteristic of tropical agriculture might overwhelm even the edge specialists. The third action might be to assist in tree migration between fragments to balance extinctions within fragments. Migration might be promoted by maintaining even the slenderest connections for bird dispersers, such as hedgerows or isolated pasture trees11. Kellman emphasizes that managed forest fragments cannot be microcosms of continuous forest12, an essential point to understand if we hope to conserve species in fragments and over a landscape. Compared with the biota in continuous forest, fragment biota are organized by different conditions and processes, involving small canopy trees, small treefall gaps, edge fires, a guild of edge trees, edge-to-core gradients and broadly dispersed plant species. Fragments become new kinds of communities and, to conserve species in them, managers must work with these new conditions and processes and not try to maintain fragments as small pieces of the old, continuous forest for which appropriate conditions and processes no longer exist. Over a landscape, a system of managed fragments, however species-rich, will not substitute for conserving large blocs of continuous forest. The natural fragments in Belize, although rich in tree species, lack
Copyright © 1998, Elsevier Science Ltd. All rights reserved. 0169-5347/98/$19.00 PII: S0169-5347(98)01429-3
TREE vol. 13, no. 10 October 1998
NEWS & COMMENT many mammal-dispersed and dioecious tree species found in continuous forest in the region3, probably because these fragments lack the diversity of animals that disperse seed and pollinate plants in continuous forest. This work in gallery forests shows for the first time that fragments can conserve tree diversity over millennia, but it also shows for the first time that tree species dependent on large mutualist animals are eventually lost. Although a natural savanna-forest mosaic can support diverse animal species13, their value as mutualists might differ from that of animals in continuous forest and, in any case, in modern fragments the agricultural matrix, hunting, and within-fragment habitat changes would reduce many animal populations. Thus, to maintain the diversity of all tropical plants and animals, conservation of large blocs of continuous forest is still the highest priority. In much of the tropics, however, the juggernaut of deforestation has left no large blocs to conserve, although recently created fragments still rich in plant species might remain. The present state of natural fragments formed in the distant past suggests that loss of a rich tree flora from these recent fragments is not inevitable if they are managed well. Just as Pleistocene fragments apparently coalesced to form rich Holocene forests, so these modern fragments could be the nuclei of expanding forests in the future14, as is happening now in Puerto Rico where much farming has been abandoned15. Meanwhile, for
tree species in managed fragments of the future, this study of ancient fragments inspires ‘cautious optimism’12. Nicholas Brokaw Manomet Center for Conservation Sciences, PO Box 1770, Manomet, MA 02345, USA ([email protected])
References 1 Kellman, M., Tackaberry, R. and Rigg, L. (1998) Structure and function in two tropical gallery forest communities: implications for forest conservation in fragmented systems, J. Appl. Ecol. 35, 195–206 2 Kellman, M., Tackaberry, R. and Meave, J. (1996) The consequences of prolonged fragmentation: lessons from tropical gallery forests, in Forest Patches in Tropical Landscapes (Schelhas, J. and Greenberg, R., eds), pp. 37–58, Island Press 3 Meave, J. and Kellman, M. (1994) Maintenance of rain forest diversity in riparian forests of tropical savannas: implications for species conservation during Pleistocene drought, J. Biogeogr. 21, 121–135 4 Tackaberry, R. and Kellman, M. (1996) Patterns of tree species richness along peninsular extensions of tropical forests, Glob. Ecol. Biogeogr. Lett. 5, 85–90 5 Leyden, B.W. (1984) Guatemalan forest synthesis after Pleistocene aridity, Proc. Natl. Acad. Sci. U. S. A. 81, 4856–4859 6 Denslow, J.S. (1995) Disturbance and diversity in tropical rain forests: the density effect, Ecol. Appl. 5, 962–968
7 Kellman, M. and Tackaberry, R. (1993) Disturbance and tree species coexistence in tropical riparian forest fragments, Glob. Ecol. Biogeogr. Lett. 3, 1–9 8 MacDougall, A. and Kellman, M. (1992) The understory light regime and patterns of tree seedlings in tropical riparian forest patches, J. Biogeogr. 19, 667–675 9 Laurance, W.F. and Bierregaard, R.O., Jr, eds (1997) Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities, The University of Chicago Press 10 Terborgh, J. et al. (1997) Transitory states in relaxing ecosystems of land bridge islands, in Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities (Laurance, W.F. and Bierregaard, R.O., Jr, eds), pp. 256–274, The University of Chicago Press 11 Greenberg, R. (1996) Managed forest patches and the diversity of birds in southern Mexico, in Forest Patches in Tropical Landscapes (Schelhas, J. and Greenberg, R., eds), pp. 59–90, Island Press 12 Kellman, M. (1996) Redefining roles: plant community reorganization and species preservation in fragmented systems, Glob. Ecol. Biogeogr. Lett. 5, 111–116 13 Tutin, C.E.G., White, L.J.T. and Mackanga-Missandzou, A. (1997) The use by rain forest mammals of natural forest fragments in an equatorial African savanna, Conserv. Biol. 11, 1190–1203 14 Turner, I.M. and Corlett, R.T. (1996) The conservation value of small, isolated fragments of lowland tropical rain forest, Trends Ecol. Evol. 11, 330–333 15 Thomlinson, J.R. et al. (1996) Land-use dynamics in a post-agricultural Puerto Rican landscape (1936–1988), Biotropica 28, 525–536
The stoop of large falcons
L
arge falcons hunt by diving from great altitude – ‘stooping’ – on their quarry. In these spectacular aerobatic displays, the prey is sometimes seized and even retrieved in the air1 (Fig. 1). The legendary stoop of the peregrine Falco peregrinus has stimulated exaggerated estimates of the top speeds achieved. Some early observers reported2 fantastic speeds of up to nearly 150 m s−1 (576 km h−1). The highest figure is based on an observation of a falcon overtaking an aircraft at an estimated speed twice that of the aeroplane. Other methods based on timed observations of falcons diving a known distance yielded speeds of nearly 100 m s−1 (Ref. 3) but, as Derek Ratcliffe4 wrote in 1980, ‘the speed achieved by a stooping peregrine is a favourite topic, but one still not yet satisfactorily resolved by the marvels of modern electronic gadgetry’. TREE vol. 13, no. 10 October 1998
In 1987 Thomas Alerstam reported the first speeds of a stooping peregrine (and of a stooping goshawk, Accipiter gentilis) obtained by a tracking radar5. This falcon was tracked during three consecutive stoops, interspersed with soaring in preparation for the next stoop, and it reached a maximum speed of 39 m s−1 during ten seconds – well below the previously reported speeds. The radar measurements were compared with calculations of theoretical diving speeds, and Alerstam concluded that the falcon controlled its stooping speed well below the theoretical maximum, presumably because high-speed manoeuvring towards a prey is facilitated by a moderate diving speed5. Therefore, the question remained whether falcons ever achieve top speeds of up to 100 m s−1 or if the much lower speeds recorded by highly accurate radar5 are typical.
Fig. 1. The typical shape of a stooping peregrine with the wings held slightly cupped against the body and tail.
Theory versus reality In two new papers, Tucker and colleagues6,7 shed new light on the question of top speed in stooping falcons. The first
Copyright © 1998, Elsevier Science Ltd. All rights reserved. 0169-5347/98/$19.00 PII: S0169-5347(98)01435-9
383