generation of biodiversity
Created | Updated Jan 28, 2002
Controls on the generation of biological diversity in tropical forests: a state of knowledge review
Abstract
Aim: a review of recent research into the generation of biological diversity, with particular reference to tropical forests.
Location: the tropical regions of the world, approximately 20° north and south of the Equator. In addition, some attention is paid to laboratory experiments.
Methods: research through relevant academic journals and on the internet.
Results: new techniques of investigation into palaeo-environments have allowed researchers to question long-established hypotheses of rainforest refugia and speciation; the species-area relationship has been questioned and condemned as simplistic and non-biological; the relationship between species richness and productivity has proven difficult to empirically substantiate. Research seems to be focused on site-specific and local-scale environments, with a general consensus that more than one mechanism exists for the generation of biodiversity.
Main conclusions: contemporary environmental heterogeneity precludes the empirical substantiation of a single mechanism for the generation of biodiversity. The link between local and regional biodiversity remains elusive, but perhaps offers invaluable clues to explaining generation. Global and continental-scale biodiversity may be generally explained by productivity, area, disturbance, or stability but at regional and local scales these relationships collapse.
Keywords: biodiversity, palaeo-environments, productivity, species-area relationship, refugia, stability.
Introduction
The biological diversity within tropical forests is extraordinary. Fossil data shows that tropical forests have existed along a broad equatorial swathe since at least the Miocene (Hooghiemstra and van der Hammen 1998), expanding and contracting in response to climatic fluctuations in temperature and precipitation during the late Pleistocene (Aide and Rivera 1998, Schneider and Moritz 1999). The Amazon rainforests (where the highest biodiversity exists) were established by the mid-Miocene, with many modern genera present by then (Colinvaux and de Oliveira 2001).
Such longevity forms the basis of so-called museum models, which explain the generation of biodiversity in terms of prolonged existence and minimal disturbance: in other words, time and stability generate biodiversity. These models assume tropical species to be ancient relicts of a past climate: Hooghiemstra and van der Hammen (1998) suggest plant diversity reached its peak in the Miocene or Pliocene, and today's diversity is thus a legacy of the Tertiary rather than an evolutionary product of the Quaternary. Chown and Gaston (2000) discuss the idea of the tropics as museums in terms of the latitudinal gradient in diversity.
In contrast, other models suggest the tropics are the cradle of diversity, due to high origination rates caused by favourable growing conditions, and the evolution of specialist species to fill niches and microhabitats.
The generation of biodiversity is likely to have been driven by different factors than those which maintain biodiversity. Givnish (1999) proposes three hypotheses for generation:
Climate stability causes low rates of extinction
Climate stability causes high rates of speciation via the evolution of specialists
Large tropical area causes more speciation (due to greater heterogeneity of habitats), less extinction (because larger populations can be supported) and tighter species packing (causing resource partitioning between competitors)
Biodiversity is scale-dependent, and it is likely that more than one mechanism generates diversity. The link between local and regional biodiversity remains a focus for future research, because mechanisms change with scale (Gaston 2000). Within tropical regions, levels of biodiversity are heterogeneous: South America has far greater species richness than the African forests. Torti et al (2001) examine mono-dominance by Gilbertiodendron dewevrei within the Ituri forest of the Congo, and suggest that conventional models of tropical diversity either ignore mono-dominance or pretend it does not exist. In fact, large expanses of tropical forest exist with a single late-succession tree species comprising more than 60% of the canopy (Torti et al 2001).
Weiher (1999) suggests scale and productivity are the two most important influences on species richness, and Lomolino (2000a) uses scale to criticize the species area relationship, stating the relationship is sigmoidal because species richness is independent of area at small extents (the small-island effect). Productivity and area are two mechanisms held to explain the latitudinal gradient in biodiversity (Gaston 2000), but only at large extents of scale. At finer scales, other mechanisms influence community structure and composition, and thus control the species pool from which community diversity can develop.
Scale, then, is fundamental to the generation of biodiversity. The link between local and regional diversity is crucial…but not known fully. The difficulty of observing natural ecosystems may make laboratory experiments preferable (Morin 2000, Kassen et al 2000), but then we are left with the necessity of inferring lab results upon complex real-life systems. The enormity of tropical diversity precludes our ability to observe and measure it, so inferential hypotheses may be the best we have. As Tilman (1999) suggests, there is a diversity of explanations for diversity.
Hypothesis: area generates diversity
The species-area relationship (SAR) is one of ecology’s few laws, and has pre-occupied researchers for decades. The theory is that tropical areas north and south of the Equator abut, thus creating a larger climatically similar total area than any other ecoclimatic zone on Earth. It has long been noticed that, at large scales, more species exist toward the Equator than toward the Poles. Yet this gradient in biodiversity is probably generated not by latitude or area per se, but rather by a number of other mechanisms. For example, tropical environmental conditions (such as high productivity, or stability) may provide the mechanisms for the latitudinal gradient.
Gaston (2000) discusses the competing theories, and concludes that no single mechanism adequately explains all examples, and that observed patterns of biodiversity vary with spatial scale. If latitude controls biodiversity, we would expect the areas of highest biodiversity to lie on the Equator: in fact, hotspots of the highest biodiversity exist some distance from the Equator. Gaston (2000) suggests that mechanisms for the generation of biodiversity change with scale.
Chown and Gaston (2000) examine the theory's assumptions concerning strong relationships between range size and area, speciation and extinction; they conclude the area hypothesis is logical but lacks empirical evidence to support it. The mechanisms of the SAR are not mutually exclusive (eg: habitat heterogeneity, available energy, larger species range, etc), and fragmented areas may have fewer species due to the degree of their isolation as well as the reduction in area:an important consideration for the refugia hypothesis.
Williamson et al (2001) state the SAR is scale-dependent, and that distance, habitat, history, and climate all affect species composition of a community before area has an effect. Lomolino (2001a) suggests that understanding the effect of area on species richness depends most on comparing the thresholds at which each scale-dependent process has a dominant influence on community structure (eg: disturbance regimes, speciation, migration).
Larger areas may generate higher diversity simply because they include greater heterogeneity of habitat. Webb et al (1999) found diversity varied with topography because of gradients in hydrology, soil structure, and exposure. Lomolino (2001b) states that diversity decreases with elevation, but is unsure whether this is due to area (mountains have smaller surface area), climate gradients, or isolation from other environments. Svenning (2001) suggests environmental heterogeneity generates diversity at regional scales, but recruitment limitation dominates locally.
Plotkin et al (2000) use the SAR to determine species richness in tropical forests, and Vallan (2000) uses the SAR to identify the impact of fragmentation upon populations of amphibians in the Madagascar rainforest, broadly confirming the relationship. Area per se is unlikely to generate diversity, however, and the SAR may be a statistical relationship rather than a biological one.
Hypothesis: energy generates diversity
The energy hypothesis incorporates a number of other theories, each of which suggest the tropics experience favourable environmental conditions for the production of biomass, thus creating resources and niches for a plethora of species (ie: the theory is based on climatic conditions). The time hypothesis suggests these conditions have persisted for a long enough period for high diversity to evolve (ie: evolutionary time); the stability hypothesis suggests the typically aseasonal conditions of the tropics affords a suitable environment for diversity to flourish (ie: ecological time).
The theory holds that tropical forests experience enough warmth and water all year for optimum plant growth to continue uninterrupted. Givnish (1999) found species richness to increase with precipitation and soil fertility, and decreasing seasonality. However, research shows that the highest levels of diversity are not commensurate with the highest levels of productivity. Kassen et al (2000) found a unimodal relationship in heterogeneous environments between productivity and diversity, with diversity peaking at intermediate productivity levels. At continental scales, diversity does increase with productivity, but productivity could simply be correlated with other factors that actually generate biodiversity (Morin 2000).
Dynesius and Jansson (2000) suggest global patterns of diversity are controlled by Milankovitch climate oscillations, which operate on a scale of hundreds of thousands of years. Waide et al (1999) suggest that no universal patterns exist in the species-energy relationship, citing the fact that patterns appear to change with scale. At local scales, no clear relationships exist between species and energy, but at continental scales a positive relationship predominates. Great variability in environmental conditions within tropical forests hampers the detection of energy patterns. Furthermore, Ungerer et al (1999) found that species do not only react to changes in available energy, but also to other environmental factors.
Hypothesis: disturbance generates diversity
Disturbance is held to create the conditions necessary for communities to develop along successional lines. Since tropical forests experience frequent local disturbances, such as treefall, biodiversity is generated by a dynamic patchwork of gaps in which pioneer species colonise the disturbed area, followed by second-level succession species, and so on. This theory assumes a positive relationship exists between disturbance regimes and diversity; as tropical forests are species-rich, they are assumed to experience more disturbance than other ecosystems.
Orgeas and Andersen (2001) found that fire regimes in tropical savanna did not affect plant richness or composition, but superior ecological conditions after the fire event caused a long-term increase in the abundance and richness of beetle species (despite short-term population crashes due to mortality in fire). Basset et al (2001) found insect species richness and abundance increased in areas of open tropical canopy, yet evenness decreased as the increase in abundance occurred almost exclusively in species with wide ecological tolerances and large geographical ranges.
Fukanii (2001) used laboratory experiments to show that the sequence in which disturbances occur may be more important than their relative intensity or frequency. Arriaga (2000) found that type and cause of tree mortality in tropical montane forest affects the sequential replacement of plant species. Buckling et al (2000) found a relationship exists between disturbance and diversity only in heterogeneous environments, and is caused by competition between niche specialists.
Disturbance is likely to be locally important, but there is no evidence to suggest tropical regions experience more disturbance than any other ecosystems.
Hypothesis: refugia generate diversity
Did fragmentation of the rainforests during periods of glaciation create refugia of species rich forest, which underwent subsequent speciation and thus increased the pool of available species for colonisation when glaciation ended? Such theory requires proof that glacial aridity reduced and fragmented the forest, that speciation occurred in remnant populations, and that surviving species were able to disperse from refugia as conditions improved.
Colinvaux and De Oliveira (2001) cite evidence that suggests neither glacial aridity nor forest fragmentation occurred in the Amazon basin. The authors suggest the main response to climate change was a change in the relative abundance of species, but mostly the cyclical nature of climate change was within species tolerance. Linder (2001) found no link between climate during the last glacial maximum and centres of endemism in sub-Saharan Africa; these centres (or proposed refugia) may actually have lost their flora during the glacial maximum.
Genetic evidence cited by Aide and Rivera (1998) suggests that expanding populations did not emerge from single refugia as climate warmed. They found no relationship between proposed sites of refugia and genetic diversity, and state that if the rainforest was restricted by glacial aridity, it is impossible for it to have expanded to its current range in only 12000 years. They suggest an alternative hypothesis: rainforest species existed outside the forest during the Pleistocene, possibly along coasts, and then migrated to the forest as it expanded.
Schneider and Moritz (1999) found DNA evidence in three endemic species of Australia's wet tropics to suggest clear population-level response to forest contraction during Pleistocene glaciation, and subsequent expansion. Vicariant events provided the opportunity for phenotypic divergence via drift and founder effects, but the evidence shows that this did not occur: even in the most extreme case of isolation, phenotypic divergence is not in evidence. The authors suggest that the selective regimes of persistent species in forest fragments were strong enough to resist divergence. They conclude that long-term isolation is unlikely to be the primary process of divergence and speciation.
Schlaepfer and Gavin (2001) found inconclusive evidence of a detrimental effect on lizard and frog populations by forest fragmentation. Each species is unique, thus so too is the set of conditions that determine its response to environmental change. Turner et al (2001) found local processes, like dispersal and sympatric speciation, disrupt large-scale patterns of biogeography and prevent confident prediction of species sources in south-east Asia. Gascon et al (2000) found that geographical boundaries do not explain species distribution within the Amazon. Thus, fragmentation is held to impact less on species richness than the refugia hypothesis would suggest.
The refugia hypothesis fundamentally rests on assumptions about the dispersal abilities of species, claiming tropical forest species were able to disperse from forest refugia as the climate became warmer and thus expand their ranges. Grehan (2001) suggests there is no relationship between individual means of dispersal of organisms and geographical patterns of dispersal. That is, taxa with efficient means of dispersal appear to be no more widely dispersed than those taxa with poor means of dispersal. Migration is thus not a predictable mechanism for biogeographical patterns (Grehan 2001).
Hooghiemstra and van der Hammen (1998) suggest that refugia did exist, but the bulk of the forest remained untouched as the impacts of climate change were not uniform. Diversity was thus generated by both stability in the unaffected forest, and by dynamism within the refugia. This appears to be something of a Goldilocks hypothesis (ie: not too much fragmentation, but not too little) and could be criticised as tautological.
Conclusions
Hill and Hill (2001) discuss the various theories for diversity generation, and suggest a combination of large scale processes (climate change, productivity) and smaller scale factors (disturbance, gap dynamics, habitat heterogeneity) combine to generate diversity. Tilman (1999) states that large scale gradients (climate, resource availability, productivity) combine with inter-specific trade-offs. Thus, recent research focuses less on the search for a single mechanism of diversity generation, and more on the local interactions that produce the great local diversity within tropical forests.
Since diversity is unevenly distributed across the tropics, and in some places diversity is quite low (Torti et al 2001), it is appropriate that research should embrace this heterogeneity and focus on the generation of local diversity. The link between local and regional diversity requires further research. The search for a single all-embracing mechanism of diversity generation seems to have been abandoned in favour of researching site-specific ecological processes.
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References
Aide T.M. and Rivera E. (1998) ‘Geographic patterns of genetic diversity in Poulsenia armata (Moraceae): implications for the theory of Pleistocene refugia and the importance of riparian forest’ Journal of Biogeography 25; 695-705
Arriaga L. (2000) ‘Types and causes of tree mortality in a tropical montane cloud forest of Tamaulipas, Mexico’ Journal of Tropical Ecology 16; 623-636
Basset Y., Charles E., Hammond D.S., and Brown V.K. (2001) ‘Short-term effects of canopy openness on insect herbivores in a rain forest in Guyana’ Journal of Applied Ecology 38; 1045-1058
Buckling A., Kassen R., Bell G., and Rainey P.B. (2000) ‘Disturbance and diversity in experimental microcosms’ Nature 408; 961-964
Chown S.L. and Gaston K.J. (2000) ‘Areas, cradles, and museums: the latitudinal gradient in species richness’ Trends in Ecology and Evolution 15 (8); 311-315
Colinvaux P.A. and de Oliveira P.E. (2001) ‘Amazon plant diversity and climate through the Cenozoic’ Palaeogeography, Palaeoclimatology, Palaeoecology 166; 51-63
Dynesius M. and Jansson R. (2000) ‘Evolutionary consequences of changes in species’ geographical distributions driven by Milankovitch climate oscillations’ Proceedings of the National Academy of Sciences 97 (16); 9115-9120
Fukanii T. (2001) ‘Sequence effects of disturbance on community structure’ Oikos 92; 215-224
Gascon C., Malcolm J.R., Patton J.L., da Silva M.N.F., Bogart J.P., Lougheed S.C., Peres C.A., Neckel S., and Boag P.T. (2000) ‘Riverine barriers and the geographic distribution of Amazonian species’ Proceedings of the National Academy of Sciences 97 (25); 13672-13677
Gaston K.J. (2000) ‘Global patterns in biodiversity’ Nature 405; 220-227
Givnish T.J. (1999) ‘On the causes of gradients in tropical tree diversity’ Journal of Ecology 87; 193-210
Grehan J.R. (2001) ‘Panbiogeography from tracks to ocean basins: evolving perspectives’ Journal of Biogeography 28; 413-429
Hill J.L. and Hill R.A. (2001) ‘Why are tropical rain forests so species rich? Classifying, reviewing and evaluating theories’ Progress in Physical Geography 25 (3); 326-354
Hooghiemstra H. and van der Hammen T. (1998) ‘Neogene and Quaternary development of the neotropical rain forest: the forest refugia hypothesis, and a literature overview’ Earth Science Reviews 44; 147-183
Kassen R., Buckling A., Bell G., and Rainey P.B. (2000) ‘Diversity peaks at intermediate productivity in a laboratory microcosm’ Nature 406; 508-512
Linder H.P. (2001) ‘Plant diversity and endemism in sub-Saharan tropical Africa’ Journal of Biogeography 28; 169-182
Lomolino M. (2001a) ‘The species-area relationship: new challenges for an old pattern’ Progress in Physical Geography 25 (1); 1-21
Lomolino M. (2001b) ‘Elevation gradients of species-density: historical and prospective views’ Global Ecology and Biogeography 10; 3-13
Morin P.J. (2000) ‘Biodiversity’s ups and downs’ Nature 406; 463-464
Nuismer S.L. and Thompson J.N. (2001) ‘Plant polyploidy and non-uniform effects on insect herbivores’ Proceedings of the Royal Society of London B 268; 1937-1940
Orgeas J. and Andersen A.N. (2001) ‘Fire and biodiversity: responses of grass-layer beetles to experimental fire regimes in an Australian tropical savanna’ Journal of Applied Ecology 38; 49-62
Plotkin J.B., Potts M.D., Yu D.W., Bunyavejchewin S., Condit R., Foster R., Hubbell S., LaFrankie J., Manokaran N., Seng L.H., Sukumar R., Nowak M.A., and Ashton P.S. (2000) ‘Predicting species diversity in tropical forests’ Proceedings of the National Academy of Sciences 97 (20); 10850-10854
Schlaepfer M.A. and Gavin T.A. (2001) ‘Edge effects on lizards and frogs in tropical forest fragments’ Conservation Biology 15 (4); 1079-1090
Schneider C. and Moritz C. (1999) ‘Rainforest refugia and evolution in Australia’s Wet Tropics’ Proceedings of the Royal Society of London B 261; 191-196
Svenning J-C. (2001) ‘Environmental heterogeneity, recruitment limitation and the mesoscale distribution of palms in a tropical montane forest (Maquipucuna, Ecuador)’ Journal of Tropical Ecology 17; 97-113
Tilman D. (1999) ‘Diversity by default’ Science 283; 495-496
Torti S.D., Coley P.D., and Kursar T.A. (2001) ‘Causes and consequences of monodominance in tropical lowland forests’ American Naturalist 157 (2); 141-153
Turner H., Hovenkamp P. and van Welzen P.C. (2001) ‘Biogeography of Southeast Asia and the West Pacific’ Journal of biogeography 28; 217-230
Ungerer M., Ayres M., and Lombardero M. (1999) ‘Climate and the northern distribution limits of Dendroctonus frontalis Zimmerman (Coleoptera: Scolytidae)’ Journal of Biogeography 26 (6); 1133-1145
Vallan D. (2000) ‘Influence of forest fragmentation on amphibian diversity in the nature reserve of Ambohitantely, highland Madagascar’ Biological Conservation 96; 31-43
Waide R.B., Willig M.R., Steiner C.F., Mittelbach G., Gough L., Dodson S.I., Juday G.P., and Parmenter R. (1999) ‘The relationship between productivity and species richness’ Annual Review of Ecology and Systematics 30; 257-300
Webb E.L., Stanfield B.J., and Jensen M.L. (1999) ‘Effects of topography on rainforest tree community structure and diversity in American Samoa, and implications for frugivore and nectarivore populations’ Journal of Biogeography 26; 887-897
Weiher E. (1999) ‘The combined effects of scale and productivity on species richness’ Journal of Ecology 87; 1005-1011
Williamson M., Gaston K.J., and Lonsdale W.M. (2001) ‘The species-area relationship does not have an asymptote!’ Journal of Biogeography 28; 827-830
Abstract
Aim: a review of recent research into the generation of biological diversity, with particular reference to tropical forests.
Location: the tropical regions of the world, approximately 20° north and south of the Equator. In addition, some attention is paid to laboratory experiments.
Methods: research through relevant academic journals and on the internet.
Results: new techniques of investigation into palaeo-environments have allowed researchers to question long-established hypotheses of rainforest refugia and speciation; the species-area relationship has been questioned and condemned as simplistic and non-biological; the relationship between species richness and productivity has proven difficult to empirically substantiate. Research seems to be focused on site-specific and local-scale environments, with a general consensus that more than one mechanism exists for the generation of biodiversity.
Main conclusions: contemporary environmental heterogeneity precludes the empirical substantiation of a single mechanism for the generation of biodiversity. The link between local and regional biodiversity remains elusive, but perhaps offers invaluable clues to explaining generation. Global and continental-scale biodiversity may be generally explained by productivity, area, disturbance, or stability but at regional and local scales these relationships collapse.
Keywords: biodiversity, palaeo-environments, productivity, species-area relationship, refugia, stability.
Introduction
The biological diversity within tropical forests is extraordinary. Fossil data shows that tropical forests have existed along a broad equatorial swathe since at least the Miocene (Hooghiemstra and van der Hammen 1998), expanding and contracting in response to climatic fluctuations in temperature and precipitation during the late Pleistocene (Aide and Rivera 1998, Schneider and Moritz 1999). The Amazon rainforests (where the highest biodiversity exists) were established by the mid-Miocene, with many modern genera present by then (Colinvaux and de Oliveira 2001).
Such longevity forms the basis of so-called museum models, which explain the generation of biodiversity in terms of prolonged existence and minimal disturbance: in other words, time and stability generate biodiversity. These models assume tropical species to be ancient relicts of a past climate: Hooghiemstra and van der Hammen (1998) suggest plant diversity reached its peak in the Miocene or Pliocene, and today's diversity is thus a legacy of the Tertiary rather than an evolutionary product of the Quaternary. Chown and Gaston (2000) discuss the idea of the tropics as museums in terms of the latitudinal gradient in diversity.
In contrast, other models suggest the tropics are the cradle of diversity, due to high origination rates caused by favourable growing conditions, and the evolution of specialist species to fill niches and microhabitats.
The generation of biodiversity is likely to have been driven by different factors than those which maintain biodiversity. Givnish (1999) proposes three hypotheses for generation:
Climate stability causes low rates of extinction
Climate stability causes high rates of speciation via the evolution of specialists
Large tropical area causes more speciation (due to greater heterogeneity of habitats), less extinction (because larger populations can be supported) and tighter species packing (causing resource partitioning between competitors)
Biodiversity is scale-dependent, and it is likely that more than one mechanism generates diversity. The link between local and regional biodiversity remains a focus for future research, because mechanisms change with scale (Gaston 2000). Within tropical regions, levels of biodiversity are heterogeneous: South America has far greater species richness than the African forests. Torti et al (2001) examine mono-dominance by Gilbertiodendron dewevrei within the Ituri forest of the Congo, and suggest that conventional models of tropical diversity either ignore mono-dominance or pretend it does not exist. In fact, large expanses of tropical forest exist with a single late-succession tree species comprising more than 60% of the canopy (Torti et al 2001).
Weiher (1999) suggests scale and productivity are the two most important influences on species richness, and Lomolino (2000a) uses scale to criticize the species area relationship, stating the relationship is sigmoidal because species richness is independent of area at small extents (the small-island effect). Productivity and area are two mechanisms held to explain the latitudinal gradient in biodiversity (Gaston 2000), but only at large extents of scale. At finer scales, other mechanisms influence community structure and composition, and thus control the species pool from which community diversity can develop.
Scale, then, is fundamental to the generation of biodiversity. The link between local and regional diversity is crucial…but not known fully. The difficulty of observing natural ecosystems may make laboratory experiments preferable (Morin 2000, Kassen et al 2000), but then we are left with the necessity of inferring lab results upon complex real-life systems. The enormity of tropical diversity precludes our ability to observe and measure it, so inferential hypotheses may be the best we have. As Tilman (1999) suggests, there is a diversity of explanations for diversity.
Hypothesis: area generates diversity
The species-area relationship (SAR) is one of ecology’s few laws, and has pre-occupied researchers for decades. The theory is that tropical areas north and south of the Equator abut, thus creating a larger climatically similar total area than any other ecoclimatic zone on Earth. It has long been noticed that, at large scales, more species exist toward the Equator than toward the Poles. Yet this gradient in biodiversity is probably generated not by latitude or area per se, but rather by a number of other mechanisms. For example, tropical environmental conditions (such as high productivity, or stability) may provide the mechanisms for the latitudinal gradient.
Gaston (2000) discusses the competing theories, and concludes that no single mechanism adequately explains all examples, and that observed patterns of biodiversity vary with spatial scale. If latitude controls biodiversity, we would expect the areas of highest biodiversity to lie on the Equator: in fact, hotspots of the highest biodiversity exist some distance from the Equator. Gaston (2000) suggests that mechanisms for the generation of biodiversity change with scale.
Chown and Gaston (2000) examine the theory's assumptions concerning strong relationships between range size and area, speciation and extinction; they conclude the area hypothesis is logical but lacks empirical evidence to support it. The mechanisms of the SAR are not mutually exclusive (eg: habitat heterogeneity, available energy, larger species range, etc), and fragmented areas may have fewer species due to the degree of their isolation as well as the reduction in area:an important consideration for the refugia hypothesis.
Williamson et al (2001) state the SAR is scale-dependent, and that distance, habitat, history, and climate all affect species composition of a community before area has an effect. Lomolino (2001a) suggests that understanding the effect of area on species richness depends most on comparing the thresholds at which each scale-dependent process has a dominant influence on community structure (eg: disturbance regimes, speciation, migration).
Larger areas may generate higher diversity simply because they include greater heterogeneity of habitat. Webb et al (1999) found diversity varied with topography because of gradients in hydrology, soil structure, and exposure. Lomolino (2001b) states that diversity decreases with elevation, but is unsure whether this is due to area (mountains have smaller surface area), climate gradients, or isolation from other environments. Svenning (2001) suggests environmental heterogeneity generates diversity at regional scales, but recruitment limitation dominates locally.
Plotkin et al (2000) use the SAR to determine species richness in tropical forests, and Vallan (2000) uses the SAR to identify the impact of fragmentation upon populations of amphibians in the Madagascar rainforest, broadly confirming the relationship. Area per se is unlikely to generate diversity, however, and the SAR may be a statistical relationship rather than a biological one.
Hypothesis: energy generates diversity
The energy hypothesis incorporates a number of other theories, each of which suggest the tropics experience favourable environmental conditions for the production of biomass, thus creating resources and niches for a plethora of species (ie: the theory is based on climatic conditions). The time hypothesis suggests these conditions have persisted for a long enough period for high diversity to evolve (ie: evolutionary time); the stability hypothesis suggests the typically aseasonal conditions of the tropics affords a suitable environment for diversity to flourish (ie: ecological time).
The theory holds that tropical forests experience enough warmth and water all year for optimum plant growth to continue uninterrupted. Givnish (1999) found species richness to increase with precipitation and soil fertility, and decreasing seasonality. However, research shows that the highest levels of diversity are not commensurate with the highest levels of productivity. Kassen et al (2000) found a unimodal relationship in heterogeneous environments between productivity and diversity, with diversity peaking at intermediate productivity levels. At continental scales, diversity does increase with productivity, but productivity could simply be correlated with other factors that actually generate biodiversity (Morin 2000).
Dynesius and Jansson (2000) suggest global patterns of diversity are controlled by Milankovitch climate oscillations, which operate on a scale of hundreds of thousands of years. Waide et al (1999) suggest that no universal patterns exist in the species-energy relationship, citing the fact that patterns appear to change with scale. At local scales, no clear relationships exist between species and energy, but at continental scales a positive relationship predominates. Great variability in environmental conditions within tropical forests hampers the detection of energy patterns. Furthermore, Ungerer et al (1999) found that species do not only react to changes in available energy, but also to other environmental factors.
Hypothesis: disturbance generates diversity
Disturbance is held to create the conditions necessary for communities to develop along successional lines. Since tropical forests experience frequent local disturbances, such as treefall, biodiversity is generated by a dynamic patchwork of gaps in which pioneer species colonise the disturbed area, followed by second-level succession species, and so on. This theory assumes a positive relationship exists between disturbance regimes and diversity; as tropical forests are species-rich, they are assumed to experience more disturbance than other ecosystems.
Orgeas and Andersen (2001) found that fire regimes in tropical savanna did not affect plant richness or composition, but superior ecological conditions after the fire event caused a long-term increase in the abundance and richness of beetle species (despite short-term population crashes due to mortality in fire). Basset et al (2001) found insect species richness and abundance increased in areas of open tropical canopy, yet evenness decreased as the increase in abundance occurred almost exclusively in species with wide ecological tolerances and large geographical ranges.
Fukanii (2001) used laboratory experiments to show that the sequence in which disturbances occur may be more important than their relative intensity or frequency. Arriaga (2000) found that type and cause of tree mortality in tropical montane forest affects the sequential replacement of plant species. Buckling et al (2000) found a relationship exists between disturbance and diversity only in heterogeneous environments, and is caused by competition between niche specialists.
Disturbance is likely to be locally important, but there is no evidence to suggest tropical regions experience more disturbance than any other ecosystems.
Hypothesis: refugia generate diversity
Did fragmentation of the rainforests during periods of glaciation create refugia of species rich forest, which underwent subsequent speciation and thus increased the pool of available species for colonisation when glaciation ended? Such theory requires proof that glacial aridity reduced and fragmented the forest, that speciation occurred in remnant populations, and that surviving species were able to disperse from refugia as conditions improved.
Colinvaux and De Oliveira (2001) cite evidence that suggests neither glacial aridity nor forest fragmentation occurred in the Amazon basin. The authors suggest the main response to climate change was a change in the relative abundance of species, but mostly the cyclical nature of climate change was within species tolerance. Linder (2001) found no link between climate during the last glacial maximum and centres of endemism in sub-Saharan Africa; these centres (or proposed refugia) may actually have lost their flora during the glacial maximum.
Genetic evidence cited by Aide and Rivera (1998) suggests that expanding populations did not emerge from single refugia as climate warmed. They found no relationship between proposed sites of refugia and genetic diversity, and state that if the rainforest was restricted by glacial aridity, it is impossible for it to have expanded to its current range in only 12000 years. They suggest an alternative hypothesis: rainforest species existed outside the forest during the Pleistocene, possibly along coasts, and then migrated to the forest as it expanded.
Schneider and Moritz (1999) found DNA evidence in three endemic species of Australia's wet tropics to suggest clear population-level response to forest contraction during Pleistocene glaciation, and subsequent expansion. Vicariant events provided the opportunity for phenotypic divergence via drift and founder effects, but the evidence shows that this did not occur: even in the most extreme case of isolation, phenotypic divergence is not in evidence. The authors suggest that the selective regimes of persistent species in forest fragments were strong enough to resist divergence. They conclude that long-term isolation is unlikely to be the primary process of divergence and speciation.
Schlaepfer and Gavin (2001) found inconclusive evidence of a detrimental effect on lizard and frog populations by forest fragmentation. Each species is unique, thus so too is the set of conditions that determine its response to environmental change. Turner et al (2001) found local processes, like dispersal and sympatric speciation, disrupt large-scale patterns of biogeography and prevent confident prediction of species sources in south-east Asia. Gascon et al (2000) found that geographical boundaries do not explain species distribution within the Amazon. Thus, fragmentation is held to impact less on species richness than the refugia hypothesis would suggest.
The refugia hypothesis fundamentally rests on assumptions about the dispersal abilities of species, claiming tropical forest species were able to disperse from forest refugia as the climate became warmer and thus expand their ranges. Grehan (2001) suggests there is no relationship between individual means of dispersal of organisms and geographical patterns of dispersal. That is, taxa with efficient means of dispersal appear to be no more widely dispersed than those taxa with poor means of dispersal. Migration is thus not a predictable mechanism for biogeographical patterns (Grehan 2001).
Hooghiemstra and van der Hammen (1998) suggest that refugia did exist, but the bulk of the forest remained untouched as the impacts of climate change were not uniform. Diversity was thus generated by both stability in the unaffected forest, and by dynamism within the refugia. This appears to be something of a Goldilocks hypothesis (ie: not too much fragmentation, but not too little) and could be criticised as tautological.
Conclusions
Hill and Hill (2001) discuss the various theories for diversity generation, and suggest a combination of large scale processes (climate change, productivity) and smaller scale factors (disturbance, gap dynamics, habitat heterogeneity) combine to generate diversity. Tilman (1999) states that large scale gradients (climate, resource availability, productivity) combine with inter-specific trade-offs. Thus, recent research focuses less on the search for a single mechanism of diversity generation, and more on the local interactions that produce the great local diversity within tropical forests.
Since diversity is unevenly distributed across the tropics, and in some places diversity is quite low (Torti et al 2001), it is appropriate that research should embrace this heterogeneity and focus on the generation of local diversity. The link between local and regional diversity requires further research. The search for a single all-embracing mechanism of diversity generation seems to have been abandoned in favour of researching site-specific ecological processes.
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