Climate change is expected to cause shifts in the composition of tropical montane forests towards... more Climate change is expected to cause shifts in the composition of tropical montane forests towards increased relative abundances of species whose ranges were previously centered at lower, hotter elevations. To investigate this process of " thermophilization, " we analyzed patterns of compositional change over the last decade using recensus data from a network of 16 adult and juvenile tree plots in the tropical forests of northern Andes Mountains and adjacent low-lands in northwestern Colombia. Analyses show evidence that tree species composition is strongly linked to temperature and that composition is changing directionally through time, potentially in response to climate change and increasing temperatures. Mean rates of thermophilization [thermal migration rate (TMR), °C·y −1 ] across all censuses were 0.011 °C·y −1 (95% confidence interval = 0.002– 0.022 °C·y −1) for adult trees and 0.027 °C·y −1 (95% confidence interval = 0.009–0.050 °C·y −1) for juvenile trees. The fact that thermophi-lization is occurring in both the adult and juvenile trees and at rates consistent with concurrent warming supports the hypothesis that the observed compositional changes are part of a long-term process, such as global warming, and are not a response to any single episodic event. The observed changes in composition were driven primarily by patterns of tree mortality, indicating that the changes in composition are mostly via range retractions, rather than range shifts or expansions. These results all indicate that tropical forests are being strongly affected by climate change and suggest that many species will be at elevated risk for extinction as warming continues. climate change | conservation biogeography | dispersal modes | species migrations | thermal niches
3 Current affiliation: Center for Tropical Forest Science Harvard University–Arnold Arboretum Cam... more 3 Current affiliation: Center for Tropical Forest Science Harvard University–Arnold Arboretum Cambridge, Massachusetts 02138 USA Key words: forest fragmentation, forest dynamics, herbivory, Lago Guri, predation, predator removal, trophic cascade, vegetation dynamics, Venezuela
Aim To test relationships between the richness and composition of vascular plants and birds and a... more Aim To test relationships between the richness and composition of vascular plants and birds and attributes of habitat fragments using a model land-bridge island system, and to investigate whether the effects of fragmentation differ depending on species natural history traits.
Efforts to anticipate threats to biodiversity take the form of species richness predictions (SRPs... more Efforts to anticipate threats to biodiversity take the form of species richness predictions (SRPs) based on simple correlations with current climate and habitat area. We review the major approaches that have been used for SRP, species-area curves and climate envelopes, and suggest that alternative research efforts may provide more understanding and guidance for management. Extinction prediction suffers from a number of limitations related to data and the novelty of future environments.
Few studies have focused explicitly on the later stages of the fragmentation process, or “late-st... more Few studies have focused explicitly on the later stages of the fragmentation process, or “late-stage fragmentation”, during which habitat area and patch number decrease simultaneously. This lack of attention is despite the fact that many of the anthropogenically fragmented habitats around the world are, or soon will be, in late-stage fragmentation. Understanding the ecological processes and patterns that occur in late-stage fragmentation is critical to protect the species richness in these fragments.
Abstract: In the face of ongoing and future climate change, species must acclimate, adapt or shif... more Abstract: In the face of ongoing and future climate change, species must acclimate, adapt or shift their geographic distributions (ie," migrate") in order to avoid habitat loss and eventual extinction. Perhaps nowhere are the challenges posed by climate change more poignant and daunting than in tropical forests, which harbor the majority of Earth's species and are facing especially rapid rates of climate change relative to current spatial or temporal variability.
Pine wilt disease is a severe threat to the native pine forests in East Asia. Understanding the n... more Pine wilt disease is a severe threat to the native pine forests in East Asia. Understanding the natural regeneration of the forests disturbed by pine wilt disease is thus critical for the conservation of biodiversity in this realm. We studied the dynamics of composition and structure within different plant functional types (PFTs) in Masson pine forests affected by pine wilt disease (PWD).
Abstract The net primary productivity, carbon (C) stocks and turnover rates (ie C dynamics) of tr... more Abstract The net primary productivity, carbon (C) stocks and turnover rates (ie C dynamics) of tropical forests are an important aspect of the global C cycle. These variables have been investigated in lowland tropical forests, but they have rarely been studied in tropical montane forests (TMFs). This study examines spatial patterns of above-and belowground C dynamics along a transect ranging from lowland Amazonia to the high Andes in SE Peru.
Abstract Land-bridge islands formed by dam construction are considered to be “experimental” syste... more Abstract Land-bridge islands formed by dam construction are considered to be “experimental” systems for studying the effects of habitat loss and fragmentation, offering many distinct advantages over terrestrial fragments. The Thousand Island Lake in Southeast China is one such land-bridge system with more than 1000 islands.
Abstract Following habitat fragmentation, species are predicted to go locally extinct from remnan... more Abstract Following habitat fragmentation, species are predicted to go locally extinct from remnant patches in a predictable order due to differential extinction vulnerabilities. This selective species loss will result in nested distributions of species such that species found in depauperate patches will also tend to be found in larger, more speciose patches.
Abstract Tropical forests are threatened by many human disturbances-two of the most important of ... more Abstract Tropical forests are threatened by many human disturbances-two of the most important of which are deforestation and climate change. In order to mitigate the impacts of these disturbances, it is important to understand their potential effects on the distributions of species. In the tropics, such understanding has been hindered by poor knowledge of the current distributions and range limits of most species.
1 We tested the 'green world' hypothesis of Hairston, Smith and Slobodkin by monitoring vegetatio... more 1 We tested the 'green world' hypothesis of Hairston, Smith and Slobodkin by monitoring vegetation change on recently created predator-free land-bridge islands in a huge hydroelectric impoundment, Lago Guri, in the State of Bolivar, Venezuela. 2 Our results affirm the green world hypothesis and expose the operation of a strong top-down trophic cascade that negatively impacted nearly every plant species present, implying that community stability is maintained through the action of predators. 3 To test the hypothesis, we monitored vegetation on nine predator-free islands and compared demographic parameters to those observed at control sites supporting complete or nearly complete suites of predators. 4 Herbivore abundance was high on 'small' ( ≥ 0.5, < 2 ha) islands, moderate on 'medium' islands (> 3, < 15 ha) and low on the 'large' landmasses that served for reference. 5 Small sapling densities on small islands were only 37% of controls in 1997 (after 11 years of isolation), and when recensused in 2002, had fallen to 25% of controls. High mortality and, especially, low recruitment contributed to the decline in sapling cohorts. 6 Sapling decline occurred earlier on small islands, although recruitment failure had become equally pronounced on medium islands by the end of the monitoring period. 7 Several mechanisms could potentially account for suppressed sapling recruitment, but the weight of evidence points to herbivory on seedlings and small saplings by leaf-cutter ants ( Atta spp. and Acromyrmex sp.). Exposure to prevailing trade winds (windward vs. leeward slopes of islands) had no detectable effect on the density or diversity of seedlings or saplings.
The impacts of global change on tropical forests remain poorly understood. We examined changes in... more The impacts of global change on tropical forests remain poorly understood. We examined changes in tree growth rates over the past two decades for all species occurring in large (50-ha) forest dynamics plots in Panama and Malaysia. Stem growth rates declined significantly at both forests regardless of initial size or organizational level (species, community or stand). Decreasing growth rates were widespread, occurring in 24-71% of species at Barro Colorado Island, Panama (BCI) and in 58-95% of species at Pasoh, Malaysia (depending on the sizes of stems included). Changes in growth were not consistently associated with initial growth rate, adult stature, or wood density. Changes in growth were significantly associated with regional climate changes: at both sites growth was negatively correlated with annual mean daily minimum temperatures, and at BCI growth was positively correlated with annual precipitation and number of rainfree days (a measure of relative insolation). While the underlying cause(s) of decelerating growth is still unresolved, these patterns strongly contradict the hypothesized pan tropical increase in tree growth rates caused by carbon fertilization. Decelerating tree growth will have important economic and environmental implications.
This study analyzed the distribution of resident, forest-interior bird species nesting on islands... more This study analyzed the distribution of resident, forest-interior bird species nesting on islands in Lake Guri, Venezuela using several different community assembly rule models. The models that were tested included Diamond's Assembly Rules, Size Structure, Guild Proportionality, Favored States, and Nestedness. It was determined that the species composition of the study communities was only weakly influenced by competition, but that competition did appear to limit the size similarity which is permissible for co-occurring species. There was no tendency for the relative proportion of species within guilds (i.e. insectivore, omnivore, nectivore and frugivore) to remain stable among the islands. When only the insectivorous and omnivorous species were analyzed (using feeding strata as the functional groups) there was some support for the guild proportionality hypothesis. This study found no support for Fox's Favored State hypothesis, possibly due to the overrepresentation of insectivores and omnivores in the species pool. The island communities exhibit a highly nested structure. This high degree of nestedness supports the hypothesis that the assemblages are more strongly determined by differential extinction vulnerability and selective species loss than by interspecific or inter-guild competition. Understanding patterns of community assembly and their underlying forces has important implications for conservation ecology and reserve design.
In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and ... more In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO 2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16-52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (þ0.24 MgC ha À1 y À1 , 95% confidence intervals [0.07, 0.39] MgC ha À1 y À1 ), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (þ0.33 [0.09, 0.55] % y À1 ) compared with the tree community as a whole (þ0.15 % y À1 ); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (þ0.21 [0.02, 0.37] % y À1 ), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests.
The role of herbivores in nutrient cycling in tropical forest ecosystems remains poorly understoo... more The role of herbivores in nutrient cycling in tropical forest ecosystems remains poorly understood. This study investigates several aspects of nutrient cycling along a gradient in herbivore (Alouatta seniculus, red howler monkey) density among small landbridge islands in Lago Guri, Venezuela. Specifically, two contrasting hypotheses were addressed: (1) herbivores increase the availability of soil nutrients and subsequently primary productivity, and (2) herbivores decrease nutrient availability and primary productivity because they increase the dominance of non-preferred, nutrient-poor, tree species. Although C:N increased with herbivore density, the annual increase in basal area (an indicator of aboveground productivity) increased with herbivore density. According to an analysis of the tree communities on the study islands, herbivory may also be causing a shift in the tree community toward non-preferred species, and thus, over a longer time scale, both nutrient availability and productivity are expected to decline. The influence of herbivores on nutrient cycling and plant productivity has important implications for conservation and rates of carbon sequestration in tropical forests.
Habitat fragmentation can alter herbivore abundances, potentially causing changes in the plant co... more Habitat fragmentation can alter herbivore abundances, potentially causing changes in the plant community that can propagate through the food web and eventually influence other important taxonomic groups such as birds. Here we test the relationship between the density of red howler monkeys (Alouatta seniculus) and bird species richness on a large set of recently isolated land-bridge islands in Lago Guri, Venezuela (n ϭ 29 islands). Several of these islands host relict populations of howler monkeys at densities up to more than 30 times greater than those on the mainland. These ''hyperabundant'' herbivores previously have been shown to have a strong positive influence on aboveground plant productivity. We predicted that this should lead to a positive, indirect effect of howler monkey density on bird species richness. After accounting for passive sampling (the tendency for species richness to be positively associated with island area, regardless of differences in habitat quality) we found a significant positive correlation between howler monkey density and bird species richness. A path analysis incorporating data on tree growth rates from a subset of islands (n ϭ 9) supported the hypothesis that the effect of howler monkeys on the resident bird communities is indirect and is mediated through changes in plant productivity and habitat quality. These results highlight the potential for disparate taxonomic groups to be related through indirect interactions and trophic cascades.
The responses of tropical forests to global anthropogenic disturbances remain poorly understood. ... more The responses of tropical forests to global anthropogenic disturbances remain poorly understood. Aboveground woody biomass in some tropical forest plots has increased over the past several decades, potentially reflecting a widespread response to increased resource availability, for example, due to elevated atmospheric CO 2 and/or nutrient deposition. However, previous studies of biomass dynamics have not accounted for natural patterns of disturbance and gap phase regeneration, making it difficult to quantify the importance of environmental changes. Using spatially explicit census data from large (50 ha) inventory plots, we investigated the influence of gap phase processes on the biomass dynamics of four 'old-growth' tropical forests (Barro Colorado Island (BCI ), Panama; Pasoh and Lambir, Malaysia; and Huai Kha Khaeng (HKK), Thailand). We show that biomass increases were gradual and concentrated in earlier-phase forest patches, while biomass losses were generally of greater magnitude but concentrated in rarer later-phase patches. We then estimate the rate of biomass change at each site independent of gap phase dynamics using reduced major axis regressions and ANCOVA tests. Above-ground woody biomass increased significantly at Pasoh (C0.72% yr K1 ) and decreased at HKK (K0.56% yr K1 ) independent of changes in gap phase but remained stable at both BCI and Lambir. We conclude that gap phase processes play an important role in the biomass dynamics of tropical forests, and that quantifying the role of gap phase processes will help improve our understanding of the factors driving changes in forest biomass as well as their place in the global carbon budget.
It is critical to understand the responses of tropical tree species to ongoing anthropogenic dist... more It is critical to understand the responses of tropical tree species to ongoing anthropogenic disturbances. Given the longevity of large trees and the scarcity of appropriately long-term demographic data, standing size distributions are a potential tool for predicting species' responses to disturbances and resultant changes in population structure. Here we test the utility of several different measures of size distribution for predicting subsequent population changes at the intraspecific level using demographic records from two subsampled 50-ha tree plots in Malaysia (Pasoh and Lambir). Most measures of size distribution failed to successfully predict population change better than random; however, the 'coefficient of skewness' (a measure of the relative proportion of small vs. large stems in a population) was able to correctly predict the direction of population change for approximately three-quarters of species at both sites. At Pasoh, the magnitude of this relationship decreased with adult stature and rate of turnover, but was unrelated to sapling growth rates at either site. Finally, using data for species common at both forests, we found that size distributions were generally uninformative of subsequent differences in population change between sites (only median dbh correctly predicted the direction of change for more species than random). Based on these results we conclude that some measures of intraspecific differences in size distribution are potentially informative of population trends within forests but have limited utility across broader spatial scales.
Climate change is expected to cause shifts in the composition of tropical montane forests towards... more Climate change is expected to cause shifts in the composition of tropical montane forests towards increased relative abundances of species whose ranges were previously centered at lower, hotter elevations. To investigate this process of " thermophilization, " we analyzed patterns of compositional change over the last decade using recensus data from a network of 16 adult and juvenile tree plots in the tropical forests of northern Andes Mountains and adjacent low-lands in northwestern Colombia. Analyses show evidence that tree species composition is strongly linked to temperature and that composition is changing directionally through time, potentially in response to climate change and increasing temperatures. Mean rates of thermophilization [thermal migration rate (TMR), °C·y −1 ] across all censuses were 0.011 °C·y −1 (95% confidence interval = 0.002– 0.022 °C·y −1) for adult trees and 0.027 °C·y −1 (95% confidence interval = 0.009–0.050 °C·y −1) for juvenile trees. The fact that thermophi-lization is occurring in both the adult and juvenile trees and at rates consistent with concurrent warming supports the hypothesis that the observed compositional changes are part of a long-term process, such as global warming, and are not a response to any single episodic event. The observed changes in composition were driven primarily by patterns of tree mortality, indicating that the changes in composition are mostly via range retractions, rather than range shifts or expansions. These results all indicate that tropical forests are being strongly affected by climate change and suggest that many species will be at elevated risk for extinction as warming continues. climate change | conservation biogeography | dispersal modes | species migrations | thermal niches
3 Current affiliation: Center for Tropical Forest Science Harvard University–Arnold Arboretum Cam... more 3 Current affiliation: Center for Tropical Forest Science Harvard University–Arnold Arboretum Cambridge, Massachusetts 02138 USA Key words: forest fragmentation, forest dynamics, herbivory, Lago Guri, predation, predator removal, trophic cascade, vegetation dynamics, Venezuela
Aim To test relationships between the richness and composition of vascular plants and birds and a... more Aim To test relationships between the richness and composition of vascular plants and birds and attributes of habitat fragments using a model land-bridge island system, and to investigate whether the effects of fragmentation differ depending on species natural history traits.
Efforts to anticipate threats to biodiversity take the form of species richness predictions (SRPs... more Efforts to anticipate threats to biodiversity take the form of species richness predictions (SRPs) based on simple correlations with current climate and habitat area. We review the major approaches that have been used for SRP, species-area curves and climate envelopes, and suggest that alternative research efforts may provide more understanding and guidance for management. Extinction prediction suffers from a number of limitations related to data and the novelty of future environments.
Few studies have focused explicitly on the later stages of the fragmentation process, or “late-st... more Few studies have focused explicitly on the later stages of the fragmentation process, or “late-stage fragmentation”, during which habitat area and patch number decrease simultaneously. This lack of attention is despite the fact that many of the anthropogenically fragmented habitats around the world are, or soon will be, in late-stage fragmentation. Understanding the ecological processes and patterns that occur in late-stage fragmentation is critical to protect the species richness in these fragments.
Abstract: In the face of ongoing and future climate change, species must acclimate, adapt or shif... more Abstract: In the face of ongoing and future climate change, species must acclimate, adapt or shift their geographic distributions (ie," migrate") in order to avoid habitat loss and eventual extinction. Perhaps nowhere are the challenges posed by climate change more poignant and daunting than in tropical forests, which harbor the majority of Earth's species and are facing especially rapid rates of climate change relative to current spatial or temporal variability.
Pine wilt disease is a severe threat to the native pine forests in East Asia. Understanding the n... more Pine wilt disease is a severe threat to the native pine forests in East Asia. Understanding the natural regeneration of the forests disturbed by pine wilt disease is thus critical for the conservation of biodiversity in this realm. We studied the dynamics of composition and structure within different plant functional types (PFTs) in Masson pine forests affected by pine wilt disease (PWD).
Abstract The net primary productivity, carbon (C) stocks and turnover rates (ie C dynamics) of tr... more Abstract The net primary productivity, carbon (C) stocks and turnover rates (ie C dynamics) of tropical forests are an important aspect of the global C cycle. These variables have been investigated in lowland tropical forests, but they have rarely been studied in tropical montane forests (TMFs). This study examines spatial patterns of above-and belowground C dynamics along a transect ranging from lowland Amazonia to the high Andes in SE Peru.
Abstract Land-bridge islands formed by dam construction are considered to be “experimental” syste... more Abstract Land-bridge islands formed by dam construction are considered to be “experimental” systems for studying the effects of habitat loss and fragmentation, offering many distinct advantages over terrestrial fragments. The Thousand Island Lake in Southeast China is one such land-bridge system with more than 1000 islands.
Abstract Following habitat fragmentation, species are predicted to go locally extinct from remnan... more Abstract Following habitat fragmentation, species are predicted to go locally extinct from remnant patches in a predictable order due to differential extinction vulnerabilities. This selective species loss will result in nested distributions of species such that species found in depauperate patches will also tend to be found in larger, more speciose patches.
Abstract Tropical forests are threatened by many human disturbances-two of the most important of ... more Abstract Tropical forests are threatened by many human disturbances-two of the most important of which are deforestation and climate change. In order to mitigate the impacts of these disturbances, it is important to understand their potential effects on the distributions of species. In the tropics, such understanding has been hindered by poor knowledge of the current distributions and range limits of most species.
1 We tested the 'green world' hypothesis of Hairston, Smith and Slobodkin by monitoring vegetatio... more 1 We tested the 'green world' hypothesis of Hairston, Smith and Slobodkin by monitoring vegetation change on recently created predator-free land-bridge islands in a huge hydroelectric impoundment, Lago Guri, in the State of Bolivar, Venezuela. 2 Our results affirm the green world hypothesis and expose the operation of a strong top-down trophic cascade that negatively impacted nearly every plant species present, implying that community stability is maintained through the action of predators. 3 To test the hypothesis, we monitored vegetation on nine predator-free islands and compared demographic parameters to those observed at control sites supporting complete or nearly complete suites of predators. 4 Herbivore abundance was high on 'small' ( ≥ 0.5, < 2 ha) islands, moderate on 'medium' islands (> 3, < 15 ha) and low on the 'large' landmasses that served for reference. 5 Small sapling densities on small islands were only 37% of controls in 1997 (after 11 years of isolation), and when recensused in 2002, had fallen to 25% of controls. High mortality and, especially, low recruitment contributed to the decline in sapling cohorts. 6 Sapling decline occurred earlier on small islands, although recruitment failure had become equally pronounced on medium islands by the end of the monitoring period. 7 Several mechanisms could potentially account for suppressed sapling recruitment, but the weight of evidence points to herbivory on seedlings and small saplings by leaf-cutter ants ( Atta spp. and Acromyrmex sp.). Exposure to prevailing trade winds (windward vs. leeward slopes of islands) had no detectable effect on the density or diversity of seedlings or saplings.
The impacts of global change on tropical forests remain poorly understood. We examined changes in... more The impacts of global change on tropical forests remain poorly understood. We examined changes in tree growth rates over the past two decades for all species occurring in large (50-ha) forest dynamics plots in Panama and Malaysia. Stem growth rates declined significantly at both forests regardless of initial size or organizational level (species, community or stand). Decreasing growth rates were widespread, occurring in 24-71% of species at Barro Colorado Island, Panama (BCI) and in 58-95% of species at Pasoh, Malaysia (depending on the sizes of stems included). Changes in growth were not consistently associated with initial growth rate, adult stature, or wood density. Changes in growth were significantly associated with regional climate changes: at both sites growth was negatively correlated with annual mean daily minimum temperatures, and at BCI growth was positively correlated with annual precipitation and number of rainfree days (a measure of relative insolation). While the underlying cause(s) of decelerating growth is still unresolved, these patterns strongly contradict the hypothesized pan tropical increase in tree growth rates caused by carbon fertilization. Decelerating tree growth will have important economic and environmental implications.
This study analyzed the distribution of resident, forest-interior bird species nesting on islands... more This study analyzed the distribution of resident, forest-interior bird species nesting on islands in Lake Guri, Venezuela using several different community assembly rule models. The models that were tested included Diamond's Assembly Rules, Size Structure, Guild Proportionality, Favored States, and Nestedness. It was determined that the species composition of the study communities was only weakly influenced by competition, but that competition did appear to limit the size similarity which is permissible for co-occurring species. There was no tendency for the relative proportion of species within guilds (i.e. insectivore, omnivore, nectivore and frugivore) to remain stable among the islands. When only the insectivorous and omnivorous species were analyzed (using feeding strata as the functional groups) there was some support for the guild proportionality hypothesis. This study found no support for Fox's Favored State hypothesis, possibly due to the overrepresentation of insectivores and omnivores in the species pool. The island communities exhibit a highly nested structure. This high degree of nestedness supports the hypothesis that the assemblages are more strongly determined by differential extinction vulnerability and selective species loss than by interspecific or inter-guild competition. Understanding patterns of community assembly and their underlying forces has important implications for conservation ecology and reserve design.
In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and ... more In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO 2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16-52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (þ0.24 MgC ha À1 y À1 , 95% confidence intervals [0.07, 0.39] MgC ha À1 y À1 ), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (þ0.33 [0.09, 0.55] % y À1 ) compared with the tree community as a whole (þ0.15 % y À1 ); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (þ0.21 [0.02, 0.37] % y À1 ), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests.
The role of herbivores in nutrient cycling in tropical forest ecosystems remains poorly understoo... more The role of herbivores in nutrient cycling in tropical forest ecosystems remains poorly understood. This study investigates several aspects of nutrient cycling along a gradient in herbivore (Alouatta seniculus, red howler monkey) density among small landbridge islands in Lago Guri, Venezuela. Specifically, two contrasting hypotheses were addressed: (1) herbivores increase the availability of soil nutrients and subsequently primary productivity, and (2) herbivores decrease nutrient availability and primary productivity because they increase the dominance of non-preferred, nutrient-poor, tree species. Although C:N increased with herbivore density, the annual increase in basal area (an indicator of aboveground productivity) increased with herbivore density. According to an analysis of the tree communities on the study islands, herbivory may also be causing a shift in the tree community toward non-preferred species, and thus, over a longer time scale, both nutrient availability and productivity are expected to decline. The influence of herbivores on nutrient cycling and plant productivity has important implications for conservation and rates of carbon sequestration in tropical forests.
Habitat fragmentation can alter herbivore abundances, potentially causing changes in the plant co... more Habitat fragmentation can alter herbivore abundances, potentially causing changes in the plant community that can propagate through the food web and eventually influence other important taxonomic groups such as birds. Here we test the relationship between the density of red howler monkeys (Alouatta seniculus) and bird species richness on a large set of recently isolated land-bridge islands in Lago Guri, Venezuela (n ϭ 29 islands). Several of these islands host relict populations of howler monkeys at densities up to more than 30 times greater than those on the mainland. These ''hyperabundant'' herbivores previously have been shown to have a strong positive influence on aboveground plant productivity. We predicted that this should lead to a positive, indirect effect of howler monkey density on bird species richness. After accounting for passive sampling (the tendency for species richness to be positively associated with island area, regardless of differences in habitat quality) we found a significant positive correlation between howler monkey density and bird species richness. A path analysis incorporating data on tree growth rates from a subset of islands (n ϭ 9) supported the hypothesis that the effect of howler monkeys on the resident bird communities is indirect and is mediated through changes in plant productivity and habitat quality. These results highlight the potential for disparate taxonomic groups to be related through indirect interactions and trophic cascades.
The responses of tropical forests to global anthropogenic disturbances remain poorly understood. ... more The responses of tropical forests to global anthropogenic disturbances remain poorly understood. Aboveground woody biomass in some tropical forest plots has increased over the past several decades, potentially reflecting a widespread response to increased resource availability, for example, due to elevated atmospheric CO 2 and/or nutrient deposition. However, previous studies of biomass dynamics have not accounted for natural patterns of disturbance and gap phase regeneration, making it difficult to quantify the importance of environmental changes. Using spatially explicit census data from large (50 ha) inventory plots, we investigated the influence of gap phase processes on the biomass dynamics of four 'old-growth' tropical forests (Barro Colorado Island (BCI ), Panama; Pasoh and Lambir, Malaysia; and Huai Kha Khaeng (HKK), Thailand). We show that biomass increases were gradual and concentrated in earlier-phase forest patches, while biomass losses were generally of greater magnitude but concentrated in rarer later-phase patches. We then estimate the rate of biomass change at each site independent of gap phase dynamics using reduced major axis regressions and ANCOVA tests. Above-ground woody biomass increased significantly at Pasoh (C0.72% yr K1 ) and decreased at HKK (K0.56% yr K1 ) independent of changes in gap phase but remained stable at both BCI and Lambir. We conclude that gap phase processes play an important role in the biomass dynamics of tropical forests, and that quantifying the role of gap phase processes will help improve our understanding of the factors driving changes in forest biomass as well as their place in the global carbon budget.
It is critical to understand the responses of tropical tree species to ongoing anthropogenic dist... more It is critical to understand the responses of tropical tree species to ongoing anthropogenic disturbances. Given the longevity of large trees and the scarcity of appropriately long-term demographic data, standing size distributions are a potential tool for predicting species' responses to disturbances and resultant changes in population structure. Here we test the utility of several different measures of size distribution for predicting subsequent population changes at the intraspecific level using demographic records from two subsampled 50-ha tree plots in Malaysia (Pasoh and Lambir). Most measures of size distribution failed to successfully predict population change better than random; however, the 'coefficient of skewness' (a measure of the relative proportion of small vs. large stems in a population) was able to correctly predict the direction of population change for approximately three-quarters of species at both sites. At Pasoh, the magnitude of this relationship decreased with adult stature and rate of turnover, but was unrelated to sapling growth rates at either site. Finally, using data for species common at both forests, we found that size distributions were generally uninformative of subsequent differences in population change between sites (only median dbh correctly predicted the direction of change for more species than random). Based on these results we conclude that some measures of intraspecific differences in size distribution are potentially informative of population trends within forests but have limited utility across broader spatial scales.
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Papers by Kenneth Feeley