Many species of large wildlife have declined drastically worldwide. These reductions often lead t... more Many species of large wildlife have declined drastically worldwide. These reductions often lead to profound shifts in the ecology of entire communities and ecosystems. However, the effects of these large-wildlife declines on other taxa likely hinge upon both underlying abiotic properties of these systems and on the types of secondary anthropogenic changes associated with wildlife loss, making impacts difficult to predict. To better understand how these important contextual factors determine the consequences of large-wildlife declines on other animals in a community, we examined the effects of three common forms of large-wildlife loss (removal without replacement [using fences], removal followed by replacement with domestic stock, and removal accompanied by crop agricultural use) on small-mammal abundance, diversity, and community composition, in landscapes that varied in several abiotic attributes (rainfall, soil fertility, land-use intensity) in central Kenya. We found that small- mammal communities were indeed heavily impacted by all forms of large-wildlife decline, showing, on average: (1) higher densities, (2) lower species richness per site, and (3) different species assemblages in sites from which large wildlife were removed. However, the nature and magnitude of these effects were strongly context dependent. Rainfall, type of land-use change, and the interaction of these two factors were key predictors of both the magnitude and type of responses of small mammals. The strongest effects, particularly abundance responses, tended to be observed in low-rainfall areas. Whereas isolated wildlife removal primarily led to increased small-mammal abundance, wildlife removal associated with secondary uses (agriculture, domestic stock) had much more variable effects on abundance and stronger impacts on diversity and composition. Collectively, these results (1) highlight the importance of context in determining the impacts of large-wildlife decline on small-mammal communities, (2) emphasize the challenges in extrapolating results from controlled experimental studies to predict the effects of wildlife declines that are accompanied by secondary land-uses, and (3) suggest that, because of the context-dependent nature of the responses to large-wildlife decline, large-wildlife status alone cannot be reliably used to predict small-mammal community changes
The number and scale of island invasive species eradications is growing, but quantitative evidenc... more The number and scale of island invasive species eradications is growing, but quantitative evidence of the conservation efficacy of passive recovery is limited. We compare relative abundances of breeding birds on Hawadax Island (formerly named Rat island), Aleutian Archipelago, Alaska, pre-and post-rat eradication to examine short-term (\1 year post-eradication) changes due to rodenticide application, and medium-term (5 years post-eradication) changes due to the absence of invasive rats. In the short term, Bald Eagle (Haliaeetus leucocephalus) numbers decreased from 24 individuals pre-eradication to two individuals \1 year post-eradication, but recovered to 10 individuals (42 % of pre-eradication) 5 years post-eradication, with all individuals nesting (63 % of the pre-eradication nesting). Five years posteradication relative abundances of most terrestrial birds surveyed using point counts either significantly increased [Gray-crowned Rosy Finch (Leucosticte tephrocotis), Lapland Longspur (Calcarius lapponicus), Snow Bunting (Plectrophenax nivalis), Song Sparrow (Melospiza melodia)] or did not differ [Pacific Wren (Troglodytes troglodytes)]. Shorebirds also increased 5 years post-eradication with Black Oystercatchers (Haematopus palliates) increasing fivefold, and Rock Sandpiper (Calidris ptilocnemis) nesting increasing from one to five nests. We confirmed two species of ground nesting seabirds [Tufted Puffin (Fratercula cirrhata) and Leach's Storm-petrel (Oceanodroma leucohoa)] as nesting (puffin) or engaged in courtship behavior (Stormpetrel) 5 years post-eradication. Our results indicate that despite the short-term impact on Bald Eagles, and without further human intervention, most terrestrial and marine birds have newly-colonized, re-colonized, or increased in abundance following the eradication of invasive rats.
Background/Question/Methods Seabirds are keystone species linking marine and terrestrial systems ... more Background/Question/Methods Seabirds are keystone species linking marine and terrestrial systems via their role as nutrient vehicles. Their role in each of these systems differs substantially; their primary role in marine environments is as top predators, while in terrestrial and near shore systems it is predominantly as providers of allochthonous resources. Seabird derived resource subsidies lead to both direct and indirect impacts throughout food webs. These impacts can be complex, and have not been widely studied, and so their universalities are unknown. Notably, the nature of seabird direct and impact impacts may depend on community assemblage at a given site, which is tightly tied to regional location (‘island identity’). Furthermore, the introduction of non-native organisms, particularly mammalian predators, often leads to seabird decline and extinction, which further alters food web dynamics. We review studies documenting the impacts of seabirds on terrestrial and near shore ...
Question: Cocos nucifera, the coconut palm, has a pantropical distribution and reaches near monod... more Question: Cocos nucifera, the coconut palm, has a pantropical distribution and reaches near monodominance in many atolls, low lying islands and coastal regions. This paper examines the ecological correlation between C. nucifera abundance and changes in forest structure, floristic diversity and forest soil characteristics. Location: Palmyra Atoll NWR (USA), Central Pacific Ocean. Methods: Plant surveys were conducted on 83 transects (each 100 m 2 ). All plants ! 5 cm in height were identified and counted; large plants were also measured and ground cover was surveyed. Major macronutrients, pH, macro-elements/base cations, micronutrients and pedogenic elements, and thermodynamic stability levels were quantified from soil samples taken at each transect. Results: Even in a low diversity atoll environment, we found that high abundances of C. nucifera corresponded with pronounced differences in forest communities including: lower diversity of established trees and regenerating understorey; higher stem density and stand basal area; lower abundance of major macronutrients; and differences in macro and trace elements and energy content of soil organic matter. Historical natural experiments document that the expansion of C. nucifera was the likely causative agent of these changes. Discussion: Cumulatively, these data show that C. nucifera has important impacts on floristic, structural and soil characteristics of forests where it becomes dominant. Given the high proportion of tropical coastal areas in which C. nucifera is now naturalized and abundant, this likely has important implications for coastal forest diversity and structure.
The American journal of tropical medicine and hygiene, Jan 23, 2015
Understanding the effects of land-use change on zoonotic disease risk is a pressing global health... more Understanding the effects of land-use change on zoonotic disease risk is a pressing global health concern. Here, we compare prevalence of Yersinia pestis, the etiologic agent of plague, in rodents across two land-use types-agricultural and conserved-in northern Tanzania. Estimated abundance of seropositive rodents nearly doubled in agricultural sites compared with conserved sites. This relationship between land-use type and abundance of seropositive rodents is likely mediated by changes in rodent and flea community composition, particularly via an increase in the abundance of the commensal species, Mastomys natalensis, in agricultural habitats. There was mixed support for rodent species diversity negatively impacting Y. pestis seroprevalence. Together, these results suggest that land-use change could affect the risk of local transmission of plague, and raise critical questions about transmission dynamics at the interface of conserved and agricultural habitats. These findings emphasi...
We live amid a global wave of anthropogenically driven biodiversity loss: species and population ... more We live amid a global wave of anthropogenically driven biodiversity loss: species and population extirpations and, critically, declines in local species abundance. Particularly, human impacts on animal biodiversity are an under-recognized form of global environmental change. Among terrestrial vertebrates, 322 species have become extinct since 1500, and populations of the remaining species show 25% average decline in abundance. Invertebrate patterns are equally dire: 67% of monitored populations show 45% mean abundance decline. Such animal declines will cascade onto ecosystem functioning and human well-being. Much remains unknown about this "Anthropocene defaunation"; these knowledge gaps hinder our capacity to predict and limit defaunation impacts. Clearly, however, defaunation is both a pervasive component of the planet's sixth mass extinction and also a major driver of global ecological change.
The relative importance of both environmental factors and host factors in explaining variation in... more The relative importance of both environmental factors and host factors in explaining variation in prevalence and intensity of flea parasitism in small mammal communities is poorly established. We examine these relationships in an East African savanna landscape, considering multiple host levels: across individuals within a population, across populations within species, and across species within a landscape. We sampled fleas from 2672 small mammals of 27 species. This included a total of 8283 fleas, with 5 genera, and 13 species identified. Across individual hosts within a site, both rodent body mass and season affected total intensity of flea infestation, although the explanatory power of these factors was generally modest (<10%). Across host populations in a landscape we found consistently positive effects of host density and negative effects of vegetation cover on the intensity of flea infestation. Other factors explored (host diversity, annual rainfall, anthropogenic disturbance, and soil properties) tended to have lower and less consistent explanatory power. Across host species in the same landscape, we found that host body mass was strongly positively correlated with both prevalence and intensity of flea parasitism. The average robustness of a host species to disturbance was not correlated with flea parasitism. Cumulatively, these results provide insight into the complex roles of both intrinsic (host) and extrinsic (environmental) factors in explaining complex patterns of flea parasitism across a landscape.
Bulletin of the Ecological Society of America, 2015
Many species of large wildlife have declined drastically in all parts of the world. These reducti... more Many species of large wildlife have declined drastically in all parts of the world. These reductions often lead to profound shifts in the ecology of entire communities and ecosystems. However, the effects of these large-wildlife declines on other taxa hinge upon both underlying abiotic properties of these systems and on the types of secondary anthropopredict. To better understand how these important contextual factors determine the consequences of large-wildlife declines, we examined the effects of three common forms of large-wildlife loss on small-mammal abundance, diversity, and community composition across a series of sites that varied in several abiotic attributes (rainfall, soil fertility, and land-use intensity) in central Kenya. Our results (1) highlight the importance of context in determining the impacts of large-wildlife decline on smallmammal communities, (2) emphasize the challenges in extrapolating results from controlled experimental studies to predict the effects of wildlife declines that are accompanied by secondary uses, and (3) suggest that, because of the context-dependent nature of the responses to large-wildlife decline, large-wildlife activity itself cannot be reliably used to predict the small-mammal community characteristics.
ABSTRACT Competition for fresh water between native and introduced plants is one important challe... more ABSTRACT Competition for fresh water between native and introduced plants is one important challenge facing native forests as rainfall variability increases. Competition can be especially acute for vegetation on Pacific atolls, which depend upon consistent rainfall to replenish shallow groundwater stores. Patterns of sap flow, water use, and diameter growth of Pisonia grandis trees were investigated on Sand Islet, Palmyra Atoll, Line Islands, during a period of low rainfall. Sap flow in the outer sapwood was reduced by 53% for P. grandis trees growing within coconut palm (Cocos nucifera) stands (n = 9) versus away from coconut palm (n = 9). This suggested that water uptake was being limited by coconut palm. Radial patterns of sap flow into the sapwood of P. grandis also differed between stands with and without coconut palm, such that individual tree water use for P. grandis ranged from 14–67 L day−1, averaging 47.8 L day−1 without coconut palm and 23.6 L day−1 with coconut palm. Diameter growth of P. grandis was measured from nine islets. In contrast to sap flow, competition with coconut palm increased diameter growth by 89%, equating to an individual tree basal area increment of 5.4 versus 10.3 mm2 day−1. Greater diameter growth countered by lower rates of water use by P. grandis trees growing in competition with coconut palm suggest that stem swell may be associated with water storage when positioned in the understory of coconut palm, and may facilitate survival when water becomes limiting until too much shading overwhelms P. grandis. This article is protected by copyright. All rights reserved.
Species that are strong interactors play disproportionately important roles in the dynamics of na... more Species that are strong interactors play disproportionately important roles in the dynamics of natural ecosystems. It has been proposed that their presence is necessary for positively shaping the structure and functioning of ecosystems. We evaluated this hypothesis using the case of the world's largest parrotfish (Bolbometopon muricatum), a globally imperiled species. We used direct observation, animal tracking, and computer simulations to examine the diverse routes through which B. muricatum affects the diversity, dispersal, relative abundance, and survival of the corals that comprise the foundation of reef ecosystems. Our results suggest that this species can influence reef building corals in both positive and negative ways. Field observation and simulation outputs indicated that B. muricatum reduced the abundance of macroalgae that can outcompete corals, but they also feed directly on corals, decreasing coral abundance, diversity, and colony size. B. muricatum appeared to facilitate coral advancement by mechanically dispersing coral fragments and opening up bare space for coral settlement, but they also damaged adult corals and remobilized a large volume of potentially stressful carbonate sediment. The impacts this species has on reefs appears to be regulated in part by its abundance-the effects of B. muricatum were more intense in simulation scenarios populated with high densities of these fish. Observations conducted in regions with high and low predator (e.g., sharks) abundance generated results that are consistent with the hypothesis that these predators of B. muricatum may play a role in governing their abundance; thus, predation may modulate the intensity of the effects they have on reef dynamics. Overall our results illustrate that functionally unique and threatened species may not have universally positive impacts on ecosystems and that it may be necessary for environmental managers to consider the diverse effects of such species and the forces that mediate the strength of their influence.
used to assay the recent energetic importance of lagoons to M. alfredi; high-resolution tracking ... more used to assay the recent energetic importance of lagoons to M. alfredi; high-resolution tracking data provided information about how M. alfredi utilised lagoonal habitats over long and short time periods; acoustic cameras logged patterns of animal entrances and departures from lagoons; and photo identification/laser photogrammetry provided some insight into why they may be using this habitat. M. alfredi showed strong evidence of energetic dependence on lagoon resources during the course of the study and spent long periods of residence within lagoons or frequently transited into them from elsewhere. While within lagoons, they demonstrated affinities for particular structural features within this habitat and showed evidence of temporal patterning in habitat utilization. This work sheds light on how and why M. alfredi uses lagoons and raises questions about how this use may be altered in disturbed settings. More generally, these observations contribute to our knowledge of how to assess the ecological importance of particular habitats situated within the broader home range of mobile consumers.
Human impact on biodiversity usually is measured by reduction in species abundance or richness. J... more Human impact on biodiversity usually is measured by reduction in species abundance or richness. Just as important, but much more difficult to discern, is the anthropogenic elimination of ecological interactions. Here we report on the persistence of a long ecological interaction chain linking diverse food webs and habitats in the near-pristine portions of a remote Pacific atoll. Using biogeochemical assays, animal tracking, and field surveys we show that seabirds roosting on native trees fertilize soils, increasing coastal nutrients and the abundance of plankton, thus attracting manta rays to native forest coastlines. Partnered observations conducted in regions of this atoll where native trees have been replaced by human propagated palms reveal that this complex interaction chain linking trees to mantas readily breaks down. Taken together these findings provide a compelling example of how anthropogenic disturbance may be contributing to widespread reductions in ecological interaction chain length, thereby isolating and simplifying ecosystems.
Despite many biotic, physical, and political challenges for place-based conservation in open ocea... more Despite many biotic, physical, and political challenges for place-based conservation in open ocean environments, conservation of discrete oceanic regions by designating pelagic marine protected areas (PMPAs) has gained considerable traction. In the oligotrophic central Pacific, a patchy and dynamic ecosystem, a robust network of PMPAs has recently been established. However, evaluations of PMPA efficacy in providing appropriate habitat coverage for pelagic species are lacking, particularly in the tropics. Here, we used high resolution GPS tracking and home range analyses of tropical boobies to determine the distribution and foraging habitat use of three sympatric species (Sula sula, Sula dactylatra, and Sula leucogaster) in two PMPAs that varied substantially in size and shape. At each site we characterized the extent and use of foraging habitat inside and outside the PMPA that surrounded each breeding colony. We found profound variation within and among species in foraging behavior and habitat use across the two sites; this was partially explained by variation in bathymetry. Yet, despite variation both in PMPA size and shape, and in foraging behavior of the birds, we determined that the PMPAs each encapsulated more than 85% (n = 216 trips) of foraging habitat for each species, indicating that these PMPAs provided important habitat coverage for highly mobile tropical species. While this study highlights the challenges in effectively designing PMPAs even for relatively well-studied, central place foragers, given strong variation in foraging ecologies across sites, it also suggests that PMPAs do provide meaningful habitat coverage for at least some pelagic species.
... contents Introduction xi rodolfo dirzo, hillary s. young, harold a ... Dry Tropical Forests i... more ... contents Introduction xi rodolfo dirzo, hillary s. young, harold a ... Dry Tropical Forests in South America 23 sofia caetano and yamama naciri Chapter 3 Extent and Drivers of Change of Neotropical Seasonally Dry Tropical Forests 45 g. arturo s??nchez-azofeifa and carlos ...
Many species of large wildlife have declined drastically worldwide. These reductions often lead t... more Many species of large wildlife have declined drastically worldwide. These reductions often lead to profound shifts in the ecology of entire communities and ecosystems. However, the effects of these large-wildlife declines on other taxa likely hinge upon both underlying abiotic properties of these systems and on the types of secondary anthropogenic changes associated with wildlife loss, making impacts difficult to predict. To better understand how these important contextual factors determine the consequences of large-wildlife declines on other animals in a community, we examined the effects of three common forms of large-wildlife loss (removal without replacement [using fences], removal followed by replacement with domestic stock, and removal accompanied by crop agricultural use) on small-mammal abundance, diversity, and community composition, in landscapes that varied in several abiotic attributes (rainfall, soil fertility, land-use intensity) in central Kenya. We found that small- mammal communities were indeed heavily impacted by all forms of large-wildlife decline, showing, on average: (1) higher densities, (2) lower species richness per site, and (3) different species assemblages in sites from which large wildlife were removed. However, the nature and magnitude of these effects were strongly context dependent. Rainfall, type of land-use change, and the interaction of these two factors were key predictors of both the magnitude and type of responses of small mammals. The strongest effects, particularly abundance responses, tended to be observed in low-rainfall areas. Whereas isolated wildlife removal primarily led to increased small-mammal abundance, wildlife removal associated with secondary uses (agriculture, domestic stock) had much more variable effects on abundance and stronger impacts on diversity and composition. Collectively, these results (1) highlight the importance of context in determining the impacts of large-wildlife decline on small-mammal communities, (2) emphasize the challenges in extrapolating results from controlled experimental studies to predict the effects of wildlife declines that are accompanied by secondary land-uses, and (3) suggest that, because of the context-dependent nature of the responses to large-wildlife decline, large-wildlife status alone cannot be reliably used to predict small-mammal community changes
The number and scale of island invasive species eradications is growing, but quantitative evidenc... more The number and scale of island invasive species eradications is growing, but quantitative evidence of the conservation efficacy of passive recovery is limited. We compare relative abundances of breeding birds on Hawadax Island (formerly named Rat island), Aleutian Archipelago, Alaska, pre-and post-rat eradication to examine short-term (\1 year post-eradication) changes due to rodenticide application, and medium-term (5 years post-eradication) changes due to the absence of invasive rats. In the short term, Bald Eagle (Haliaeetus leucocephalus) numbers decreased from 24 individuals pre-eradication to two individuals \1 year post-eradication, but recovered to 10 individuals (42 % of pre-eradication) 5 years post-eradication, with all individuals nesting (63 % of the pre-eradication nesting). Five years posteradication relative abundances of most terrestrial birds surveyed using point counts either significantly increased [Gray-crowned Rosy Finch (Leucosticte tephrocotis), Lapland Longspur (Calcarius lapponicus), Snow Bunting (Plectrophenax nivalis), Song Sparrow (Melospiza melodia)] or did not differ [Pacific Wren (Troglodytes troglodytes)]. Shorebirds also increased 5 years post-eradication with Black Oystercatchers (Haematopus palliates) increasing fivefold, and Rock Sandpiper (Calidris ptilocnemis) nesting increasing from one to five nests. We confirmed two species of ground nesting seabirds [Tufted Puffin (Fratercula cirrhata) and Leach's Storm-petrel (Oceanodroma leucohoa)] as nesting (puffin) or engaged in courtship behavior (Stormpetrel) 5 years post-eradication. Our results indicate that despite the short-term impact on Bald Eagles, and without further human intervention, most terrestrial and marine birds have newly-colonized, re-colonized, or increased in abundance following the eradication of invasive rats.
Background/Question/Methods Seabirds are keystone species linking marine and terrestrial systems ... more Background/Question/Methods Seabirds are keystone species linking marine and terrestrial systems via their role as nutrient vehicles. Their role in each of these systems differs substantially; their primary role in marine environments is as top predators, while in terrestrial and near shore systems it is predominantly as providers of allochthonous resources. Seabird derived resource subsidies lead to both direct and indirect impacts throughout food webs. These impacts can be complex, and have not been widely studied, and so their universalities are unknown. Notably, the nature of seabird direct and impact impacts may depend on community assemblage at a given site, which is tightly tied to regional location (‘island identity’). Furthermore, the introduction of non-native organisms, particularly mammalian predators, often leads to seabird decline and extinction, which further alters food web dynamics. We review studies documenting the impacts of seabirds on terrestrial and near shore ...
Question: Cocos nucifera, the coconut palm, has a pantropical distribution and reaches near monod... more Question: Cocos nucifera, the coconut palm, has a pantropical distribution and reaches near monodominance in many atolls, low lying islands and coastal regions. This paper examines the ecological correlation between C. nucifera abundance and changes in forest structure, floristic diversity and forest soil characteristics. Location: Palmyra Atoll NWR (USA), Central Pacific Ocean. Methods: Plant surveys were conducted on 83 transects (each 100 m 2 ). All plants ! 5 cm in height were identified and counted; large plants were also measured and ground cover was surveyed. Major macronutrients, pH, macro-elements/base cations, micronutrients and pedogenic elements, and thermodynamic stability levels were quantified from soil samples taken at each transect. Results: Even in a low diversity atoll environment, we found that high abundances of C. nucifera corresponded with pronounced differences in forest communities including: lower diversity of established trees and regenerating understorey; higher stem density and stand basal area; lower abundance of major macronutrients; and differences in macro and trace elements and energy content of soil organic matter. Historical natural experiments document that the expansion of C. nucifera was the likely causative agent of these changes. Discussion: Cumulatively, these data show that C. nucifera has important impacts on floristic, structural and soil characteristics of forests where it becomes dominant. Given the high proportion of tropical coastal areas in which C. nucifera is now naturalized and abundant, this likely has important implications for coastal forest diversity and structure.
The American journal of tropical medicine and hygiene, Jan 23, 2015
Understanding the effects of land-use change on zoonotic disease risk is a pressing global health... more Understanding the effects of land-use change on zoonotic disease risk is a pressing global health concern. Here, we compare prevalence of Yersinia pestis, the etiologic agent of plague, in rodents across two land-use types-agricultural and conserved-in northern Tanzania. Estimated abundance of seropositive rodents nearly doubled in agricultural sites compared with conserved sites. This relationship between land-use type and abundance of seropositive rodents is likely mediated by changes in rodent and flea community composition, particularly via an increase in the abundance of the commensal species, Mastomys natalensis, in agricultural habitats. There was mixed support for rodent species diversity negatively impacting Y. pestis seroprevalence. Together, these results suggest that land-use change could affect the risk of local transmission of plague, and raise critical questions about transmission dynamics at the interface of conserved and agricultural habitats. These findings emphasi...
We live amid a global wave of anthropogenically driven biodiversity loss: species and population ... more We live amid a global wave of anthropogenically driven biodiversity loss: species and population extirpations and, critically, declines in local species abundance. Particularly, human impacts on animal biodiversity are an under-recognized form of global environmental change. Among terrestrial vertebrates, 322 species have become extinct since 1500, and populations of the remaining species show 25% average decline in abundance. Invertebrate patterns are equally dire: 67% of monitored populations show 45% mean abundance decline. Such animal declines will cascade onto ecosystem functioning and human well-being. Much remains unknown about this "Anthropocene defaunation"; these knowledge gaps hinder our capacity to predict and limit defaunation impacts. Clearly, however, defaunation is both a pervasive component of the planet's sixth mass extinction and also a major driver of global ecological change.
The relative importance of both environmental factors and host factors in explaining variation in... more The relative importance of both environmental factors and host factors in explaining variation in prevalence and intensity of flea parasitism in small mammal communities is poorly established. We examine these relationships in an East African savanna landscape, considering multiple host levels: across individuals within a population, across populations within species, and across species within a landscape. We sampled fleas from 2672 small mammals of 27 species. This included a total of 8283 fleas, with 5 genera, and 13 species identified. Across individual hosts within a site, both rodent body mass and season affected total intensity of flea infestation, although the explanatory power of these factors was generally modest (<10%). Across host populations in a landscape we found consistently positive effects of host density and negative effects of vegetation cover on the intensity of flea infestation. Other factors explored (host diversity, annual rainfall, anthropogenic disturbance, and soil properties) tended to have lower and less consistent explanatory power. Across host species in the same landscape, we found that host body mass was strongly positively correlated with both prevalence and intensity of flea parasitism. The average robustness of a host species to disturbance was not correlated with flea parasitism. Cumulatively, these results provide insight into the complex roles of both intrinsic (host) and extrinsic (environmental) factors in explaining complex patterns of flea parasitism across a landscape.
Bulletin of the Ecological Society of America, 2015
Many species of large wildlife have declined drastically in all parts of the world. These reducti... more Many species of large wildlife have declined drastically in all parts of the world. These reductions often lead to profound shifts in the ecology of entire communities and ecosystems. However, the effects of these large-wildlife declines on other taxa hinge upon both underlying abiotic properties of these systems and on the types of secondary anthropopredict. To better understand how these important contextual factors determine the consequences of large-wildlife declines, we examined the effects of three common forms of large-wildlife loss on small-mammal abundance, diversity, and community composition across a series of sites that varied in several abiotic attributes (rainfall, soil fertility, and land-use intensity) in central Kenya. Our results (1) highlight the importance of context in determining the impacts of large-wildlife decline on smallmammal communities, (2) emphasize the challenges in extrapolating results from controlled experimental studies to predict the effects of wildlife declines that are accompanied by secondary uses, and (3) suggest that, because of the context-dependent nature of the responses to large-wildlife decline, large-wildlife activity itself cannot be reliably used to predict the small-mammal community characteristics.
ABSTRACT Competition for fresh water between native and introduced plants is one important challe... more ABSTRACT Competition for fresh water between native and introduced plants is one important challenge facing native forests as rainfall variability increases. Competition can be especially acute for vegetation on Pacific atolls, which depend upon consistent rainfall to replenish shallow groundwater stores. Patterns of sap flow, water use, and diameter growth of Pisonia grandis trees were investigated on Sand Islet, Palmyra Atoll, Line Islands, during a period of low rainfall. Sap flow in the outer sapwood was reduced by 53% for P. grandis trees growing within coconut palm (Cocos nucifera) stands (n = 9) versus away from coconut palm (n = 9). This suggested that water uptake was being limited by coconut palm. Radial patterns of sap flow into the sapwood of P. grandis also differed between stands with and without coconut palm, such that individual tree water use for P. grandis ranged from 14–67 L day−1, averaging 47.8 L day−1 without coconut palm and 23.6 L day−1 with coconut palm. Diameter growth of P. grandis was measured from nine islets. In contrast to sap flow, competition with coconut palm increased diameter growth by 89%, equating to an individual tree basal area increment of 5.4 versus 10.3 mm2 day−1. Greater diameter growth countered by lower rates of water use by P. grandis trees growing in competition with coconut palm suggest that stem swell may be associated with water storage when positioned in the understory of coconut palm, and may facilitate survival when water becomes limiting until too much shading overwhelms P. grandis. This article is protected by copyright. All rights reserved.
Species that are strong interactors play disproportionately important roles in the dynamics of na... more Species that are strong interactors play disproportionately important roles in the dynamics of natural ecosystems. It has been proposed that their presence is necessary for positively shaping the structure and functioning of ecosystems. We evaluated this hypothesis using the case of the world's largest parrotfish (Bolbometopon muricatum), a globally imperiled species. We used direct observation, animal tracking, and computer simulations to examine the diverse routes through which B. muricatum affects the diversity, dispersal, relative abundance, and survival of the corals that comprise the foundation of reef ecosystems. Our results suggest that this species can influence reef building corals in both positive and negative ways. Field observation and simulation outputs indicated that B. muricatum reduced the abundance of macroalgae that can outcompete corals, but they also feed directly on corals, decreasing coral abundance, diversity, and colony size. B. muricatum appeared to facilitate coral advancement by mechanically dispersing coral fragments and opening up bare space for coral settlement, but they also damaged adult corals and remobilized a large volume of potentially stressful carbonate sediment. The impacts this species has on reefs appears to be regulated in part by its abundance-the effects of B. muricatum were more intense in simulation scenarios populated with high densities of these fish. Observations conducted in regions with high and low predator (e.g., sharks) abundance generated results that are consistent with the hypothesis that these predators of B. muricatum may play a role in governing their abundance; thus, predation may modulate the intensity of the effects they have on reef dynamics. Overall our results illustrate that functionally unique and threatened species may not have universally positive impacts on ecosystems and that it may be necessary for environmental managers to consider the diverse effects of such species and the forces that mediate the strength of their influence.
used to assay the recent energetic importance of lagoons to M. alfredi; high-resolution tracking ... more used to assay the recent energetic importance of lagoons to M. alfredi; high-resolution tracking data provided information about how M. alfredi utilised lagoonal habitats over long and short time periods; acoustic cameras logged patterns of animal entrances and departures from lagoons; and photo identification/laser photogrammetry provided some insight into why they may be using this habitat. M. alfredi showed strong evidence of energetic dependence on lagoon resources during the course of the study and spent long periods of residence within lagoons or frequently transited into them from elsewhere. While within lagoons, they demonstrated affinities for particular structural features within this habitat and showed evidence of temporal patterning in habitat utilization. This work sheds light on how and why M. alfredi uses lagoons and raises questions about how this use may be altered in disturbed settings. More generally, these observations contribute to our knowledge of how to assess the ecological importance of particular habitats situated within the broader home range of mobile consumers.
Human impact on biodiversity usually is measured by reduction in species abundance or richness. J... more Human impact on biodiversity usually is measured by reduction in species abundance or richness. Just as important, but much more difficult to discern, is the anthropogenic elimination of ecological interactions. Here we report on the persistence of a long ecological interaction chain linking diverse food webs and habitats in the near-pristine portions of a remote Pacific atoll. Using biogeochemical assays, animal tracking, and field surveys we show that seabirds roosting on native trees fertilize soils, increasing coastal nutrients and the abundance of plankton, thus attracting manta rays to native forest coastlines. Partnered observations conducted in regions of this atoll where native trees have been replaced by human propagated palms reveal that this complex interaction chain linking trees to mantas readily breaks down. Taken together these findings provide a compelling example of how anthropogenic disturbance may be contributing to widespread reductions in ecological interaction chain length, thereby isolating and simplifying ecosystems.
Despite many biotic, physical, and political challenges for place-based conservation in open ocea... more Despite many biotic, physical, and political challenges for place-based conservation in open ocean environments, conservation of discrete oceanic regions by designating pelagic marine protected areas (PMPAs) has gained considerable traction. In the oligotrophic central Pacific, a patchy and dynamic ecosystem, a robust network of PMPAs has recently been established. However, evaluations of PMPA efficacy in providing appropriate habitat coverage for pelagic species are lacking, particularly in the tropics. Here, we used high resolution GPS tracking and home range analyses of tropical boobies to determine the distribution and foraging habitat use of three sympatric species (Sula sula, Sula dactylatra, and Sula leucogaster) in two PMPAs that varied substantially in size and shape. At each site we characterized the extent and use of foraging habitat inside and outside the PMPA that surrounded each breeding colony. We found profound variation within and among species in foraging behavior and habitat use across the two sites; this was partially explained by variation in bathymetry. Yet, despite variation both in PMPA size and shape, and in foraging behavior of the birds, we determined that the PMPAs each encapsulated more than 85% (n = 216 trips) of foraging habitat for each species, indicating that these PMPAs provided important habitat coverage for highly mobile tropical species. While this study highlights the challenges in effectively designing PMPAs even for relatively well-studied, central place foragers, given strong variation in foraging ecologies across sites, it also suggests that PMPAs do provide meaningful habitat coverage for at least some pelagic species.
... contents Introduction xi rodolfo dirzo, hillary s. young, harold a ... Dry Tropical Forests i... more ... contents Introduction xi rodolfo dirzo, hillary s. young, harold a ... Dry Tropical Forests in South America 23 sofia caetano and yamama naciri Chapter 3 Extent and Drivers of Change of Neotropical Seasonally Dry Tropical Forests 45 g. arturo s??nchez-azofeifa and carlos ...
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Papers by Hillary Young
with domestic stock, and removal accompanied by crop agricultural use) on small-mammal abundance, diversity, and community composition, in landscapes that varied in several abiotic attributes (rainfall, soil fertility, land-use intensity) in central Kenya. We found that small- mammal communities were indeed heavily impacted by all forms of large-wildlife decline, showing, on average: (1) higher densities, (2) lower species richness per site, and (3) different species assemblages in sites from which large wildlife were removed. However, the nature and magnitude of these effects were strongly context dependent. Rainfall, type of land-use change, and the interaction of these two factors were key predictors of both the magnitude and type of responses of small mammals. The strongest effects, particularly abundance responses, tended to be observed in low-rainfall areas. Whereas isolated wildlife removal primarily led to increased small-mammal abundance, wildlife removal associated with secondary uses (agriculture, domestic stock) had much more variable effects on abundance and stronger impacts on diversity and composition. Collectively, these results (1) highlight the importance of context in determining the impacts of large-wildlife decline on small-mammal communities, (2) emphasize the challenges in extrapolating results from controlled experimental studies to predict the effects of wildlife declines that are accompanied by secondary land-uses, and (3)
suggest that, because of the context-dependent nature of the responses to large-wildlife decline, large-wildlife status alone cannot be reliably used to predict small-mammal community
changes
with domestic stock, and removal accompanied by crop agricultural use) on small-mammal abundance, diversity, and community composition, in landscapes that varied in several abiotic attributes (rainfall, soil fertility, land-use intensity) in central Kenya. We found that small- mammal communities were indeed heavily impacted by all forms of large-wildlife decline, showing, on average: (1) higher densities, (2) lower species richness per site, and (3) different species assemblages in sites from which large wildlife were removed. However, the nature and magnitude of these effects were strongly context dependent. Rainfall, type of land-use change, and the interaction of these two factors were key predictors of both the magnitude and type of responses of small mammals. The strongest effects, particularly abundance responses, tended to be observed in low-rainfall areas. Whereas isolated wildlife removal primarily led to increased small-mammal abundance, wildlife removal associated with secondary uses (agriculture, domestic stock) had much more variable effects on abundance and stronger impacts on diversity and composition. Collectively, these results (1) highlight the importance of context in determining the impacts of large-wildlife decline on small-mammal communities, (2) emphasize the challenges in extrapolating results from controlled experimental studies to predict the effects of wildlife declines that are accompanied by secondary land-uses, and (3)
suggest that, because of the context-dependent nature of the responses to large-wildlife decline, large-wildlife status alone cannot be reliably used to predict small-mammal community
changes