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Richard Shine - One of the best experts on this subject based on the ideXlab platform.
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why does vulnerability to toxic invasive cane toads vary among populations of australian freshwater Crocodiles
Animal Conservation, 2013Co-Authors: Ruchira Somaweera, Richard Shine, Jonathan K Webb, Tim Dempster, Mike LetnicAbstract:The ecological impact of an invasive species can be heterogeneous through space and time. One such case in Australia involves native freshwater Crocodiles Crocodylus johnstoni, which are highly sensitive to invasive cane toads Rhinella marina in some areas, whereas other populations experience little or no mortality from ingestion of the toxic toads. We studied the impact of toad invasion on three crocodile populations: one crashed, one showed a minor decrease and one appeared unaffected. We tested three hypotheses for the cause of this spatial variation in impact: differences among populations in toad–crocodile encounter rates (proximity of toads to Crocodiles during spotlight surveys), differences in crocodile feeding responses (trials of prey preference in the laboratory) and differences in crocodile physiology (reduction of swim speed after receiving a dose of toad toxin). We found little divergence among populations in any of these traits: Crocodiles from the three populations all encountered cane toads in the wild, and exhibited similar feeding responses and toxin tolerances. Thus, we cannot confidently identify causation for the impact heterogeneity. Reliance on alternative food resources and an ability to rapidly learn taste aversion may have allowed Crocodiles to deal with toad arrival in Lake Argyle and the Daly River. Future work could usefully evaluate potential explanations for the failure of these adaptive mechanisms in the severely affected (Victoria River) population. We suggest that spatial variation in the availability of alternative prey (and thus the willingness of Crocodiles to attack a novel toxic prey item) may have contributed to that variation in impact.
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the non impact of invasive cane toads on freshwater Crocodiles at lake argyle in tropical australia
Animal Conservation, 2012Co-Authors: Ruchira Somaweera, Richard ShineAbstract:The most substantial (and to date, unexplained) heterogeneity in the impact of toxic cane toads (Rhinella marina) on the native fauna of tropical Australia involves freshwater Crocodiles (Crocodylus johnstoni); some populations have experienced catastrophic mortality whereas others have been unaffected. A trend for higher impact in more arid areas suggests that Western Australian (Kimberley region) crocodile populations may be at high risk. We monitored crocodile densities and body sizes, and the spread of cane toads, at a large water body (Lake Argyle) in the eastern Kimberley. Toads arrived on the lakeshore in early 2009, and spread to cover > 300 km of lakeshore, and colonize all of the larger islands within a 24-month period. Physical removal of > 10 000 toads by a community group depressed toad abundances, but only briefly. Spool-tracking showed that toads moved extensively along the lakeshore (up to 90 m per night), often into floating vegetation in the lake. Crocodiles thus encountered toads (3–15% of Crocodiles were < 2 m from a toad when sighted), and we recorded 36 cases of toads being seized by Crocodiles. Nonetheless, crocodile mortality was rare, and crocodile numbers did not decrease through time, nor differ between toad-infested versus toad-free areas of the lake. Although ingestion of a single adult toad may be fatal to a freshwater crocodile, and the Lake Argyle Crocodiles encounter and consume these toxic anurans, the population-level impact of toad invasion has been trivial. Hence, the Lake Argyle crocodile population may not warrant immediate active management to reduce the impact of cane toads.
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determinants of habitat selection by hatchling australian freshwater Crocodiles
PLOS ONE, 2011Co-Authors: Ruchira Somaweera, Jonathan K Webb, Richard ShineAbstract:Animals almost always use habitats non-randomly, but the costs and benefits of using specific habitat types remain unknown for many types of organisms. In a large lake in northwestern Australia (Lake Argyle), most hatchling (<12-month-old) freshwater Crocodiles (Crocodylus johnstoni) are found in floating vegetation mats or grassy banks rather than the more widely available open banks. Mean body sizes of young Crocodiles did not differ among the three habitat types. We tested four potential explanations for non-random habitat selection: proximity to nesting sites, thermal conditions, food availability, and exposure to predation. The three alternative habitat types did not differ in proximity to nesting sites, or in thermal conditions. Habitats with higher food availability harboured more hatchlings, and feeding rates (obtained by stomach-flushing of recently-captured Crocodiles) were highest in such areas. Predation risk may also differ among habitats: we were twice as likely to capture a crocodile after seeing it in open-bank sites than in the other two habitat types. Thus, habitat selection of hatchling Crocodiles in this system may be driven both by prey availability and by predation risk.
Ruchira Somaweera - One of the best experts on this subject based on the ideXlab platform.
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why does vulnerability to toxic invasive cane toads vary among populations of australian freshwater Crocodiles
Animal Conservation, 2013Co-Authors: Ruchira Somaweera, Richard Shine, Jonathan K Webb, Tim Dempster, Mike LetnicAbstract:The ecological impact of an invasive species can be heterogeneous through space and time. One such case in Australia involves native freshwater Crocodiles Crocodylus johnstoni, which are highly sensitive to invasive cane toads Rhinella marina in some areas, whereas other populations experience little or no mortality from ingestion of the toxic toads. We studied the impact of toad invasion on three crocodile populations: one crashed, one showed a minor decrease and one appeared unaffected. We tested three hypotheses for the cause of this spatial variation in impact: differences among populations in toad–crocodile encounter rates (proximity of toads to Crocodiles during spotlight surveys), differences in crocodile feeding responses (trials of prey preference in the laboratory) and differences in crocodile physiology (reduction of swim speed after receiving a dose of toad toxin). We found little divergence among populations in any of these traits: Crocodiles from the three populations all encountered cane toads in the wild, and exhibited similar feeding responses and toxin tolerances. Thus, we cannot confidently identify causation for the impact heterogeneity. Reliance on alternative food resources and an ability to rapidly learn taste aversion may have allowed Crocodiles to deal with toad arrival in Lake Argyle and the Daly River. Future work could usefully evaluate potential explanations for the failure of these adaptive mechanisms in the severely affected (Victoria River) population. We suggest that spatial variation in the availability of alternative prey (and thus the willingness of Crocodiles to attack a novel toxic prey item) may have contributed to that variation in impact.
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the non impact of invasive cane toads on freshwater Crocodiles at lake argyle in tropical australia
Animal Conservation, 2012Co-Authors: Ruchira Somaweera, Richard ShineAbstract:The most substantial (and to date, unexplained) heterogeneity in the impact of toxic cane toads (Rhinella marina) on the native fauna of tropical Australia involves freshwater Crocodiles (Crocodylus johnstoni); some populations have experienced catastrophic mortality whereas others have been unaffected. A trend for higher impact in more arid areas suggests that Western Australian (Kimberley region) crocodile populations may be at high risk. We monitored crocodile densities and body sizes, and the spread of cane toads, at a large water body (Lake Argyle) in the eastern Kimberley. Toads arrived on the lakeshore in early 2009, and spread to cover > 300 km of lakeshore, and colonize all of the larger islands within a 24-month period. Physical removal of > 10 000 toads by a community group depressed toad abundances, but only briefly. Spool-tracking showed that toads moved extensively along the lakeshore (up to 90 m per night), often into floating vegetation in the lake. Crocodiles thus encountered toads (3–15% of Crocodiles were < 2 m from a toad when sighted), and we recorded 36 cases of toads being seized by Crocodiles. Nonetheless, crocodile mortality was rare, and crocodile numbers did not decrease through time, nor differ between toad-infested versus toad-free areas of the lake. Although ingestion of a single adult toad may be fatal to a freshwater crocodile, and the Lake Argyle Crocodiles encounter and consume these toxic anurans, the population-level impact of toad invasion has been trivial. Hence, the Lake Argyle crocodile population may not warrant immediate active management to reduce the impact of cane toads.
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determinants of habitat selection by hatchling australian freshwater Crocodiles
PLOS ONE, 2011Co-Authors: Ruchira Somaweera, Jonathan K Webb, Richard ShineAbstract:Animals almost always use habitats non-randomly, but the costs and benefits of using specific habitat types remain unknown for many types of organisms. In a large lake in northwestern Australia (Lake Argyle), most hatchling (<12-month-old) freshwater Crocodiles (Crocodylus johnstoni) are found in floating vegetation mats or grassy banks rather than the more widely available open banks. Mean body sizes of young Crocodiles did not differ among the three habitat types. We tested four potential explanations for non-random habitat selection: proximity to nesting sites, thermal conditions, food availability, and exposure to predation. The three alternative habitat types did not differ in proximity to nesting sites, or in thermal conditions. Habitats with higher food availability harboured more hatchlings, and feeding rates (obtained by stomach-flushing of recently-captured Crocodiles) were highest in such areas. Predation risk may also differ among habitats: we were twice as likely to capture a crocodile after seeing it in open-bank sites than in the other two habitat types. Thus, habitat selection of hatchling Crocodiles in this system may be driven both by prey availability and by predation risk.
Kay Zin Than - One of the best experts on this subject based on the ideXlab platform.
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estimating population status and site occupancy of saltwater Crocodiles crocodylus porosus in the ayeyarwady delta myanmar inferences from spatial modeling techniques
Global Ecology and Conservation, 2020Co-Authors: Kay Zin Than, Colin T Strine, Tuanjit Sritongchuay, Alice C HughesAbstract:Abstract Saltwater Crocodiles Crocodylus porosus are listed as critically endangered in Myanmar because they are limited to Meinmahlakyun Wildlife Sanctuary (MKWS) in the Ayeyarwady delta region. Little contemporary data exists on their distribution and population size which hinders effective conservation and management. We conducted standardized spotlight surveys and camera trap surveys along the rivers inside MKWS, and two nearby reserved forests. We used Hierarchical N-mixture models, Spatial Count models, and the relative abundance index to estimate site use by and population sizes of saltwater Crocodiles in the Ayeyarwady delta. To address biases in detectability, we used maximum-likelihood and Bayesian approaches (1) to assess occupancy (site use) and population parameters of saltwater Crocodiles, and (2) to assay abiotic and anthropogenic factors affecting it. Saltwater Crocodiles were more likely to be abundant and occupy in the waterways inside MKWS than the reserved forests, and in the narrow and low salinity waterways than the wide and high salinity ones. Abundance of saltwater Crocodiles was lower in areas with the human settlements than in areas with no settlement. Creeks within MKWS had moderate salinity and no human settlement and therefore it can be regared as the last remaining optimal saltwater crocodile habitat of the Ayeyarwady Delta. We estimated the saltwater crocodile population sizes in MKWS were 75 ± 9.92 individuals as absolute spotlight index, 58 ± 8.02 individuals as the maximum likelihood estimate of the N-mixture models and 68 ± 10.00 individuals as the Bayesian estimate of the spatial count models. Current population estimates of saltwater Crocodiles are lower than the previously reported population size in 1999, and the declining population is now restricted to MKWS. We suggest developing buffer zones in the reserved forests around the wildlife sanctuary to increase habitat areas for saltwater Crocodiles and to improve the outlook for long-term saltwater crocodile survival in Myanmar.
Alice C Hughes - One of the best experts on this subject based on the ideXlab platform.
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estimating population status and site occupancy of saltwater Crocodiles crocodylus porosus in the ayeyarwady delta myanmar inferences from spatial modeling techniques
Global Ecology and Conservation, 2020Co-Authors: Kay Zin Than, Colin T Strine, Tuanjit Sritongchuay, Alice C HughesAbstract:Abstract Saltwater Crocodiles Crocodylus porosus are listed as critically endangered in Myanmar because they are limited to Meinmahlakyun Wildlife Sanctuary (MKWS) in the Ayeyarwady delta region. Little contemporary data exists on their distribution and population size which hinders effective conservation and management. We conducted standardized spotlight surveys and camera trap surveys along the rivers inside MKWS, and two nearby reserved forests. We used Hierarchical N-mixture models, Spatial Count models, and the relative abundance index to estimate site use by and population sizes of saltwater Crocodiles in the Ayeyarwady delta. To address biases in detectability, we used maximum-likelihood and Bayesian approaches (1) to assess occupancy (site use) and population parameters of saltwater Crocodiles, and (2) to assay abiotic and anthropogenic factors affecting it. Saltwater Crocodiles were more likely to be abundant and occupy in the waterways inside MKWS than the reserved forests, and in the narrow and low salinity waterways than the wide and high salinity ones. Abundance of saltwater Crocodiles was lower in areas with the human settlements than in areas with no settlement. Creeks within MKWS had moderate salinity and no human settlement and therefore it can be regared as the last remaining optimal saltwater crocodile habitat of the Ayeyarwady Delta. We estimated the saltwater crocodile population sizes in MKWS were 75 ± 9.92 individuals as absolute spotlight index, 58 ± 8.02 individuals as the maximum likelihood estimate of the N-mixture models and 68 ± 10.00 individuals as the Bayesian estimate of the spatial count models. Current population estimates of saltwater Crocodiles are lower than the previously reported population size in 1999, and the declining population is now restricted to MKWS. We suggest developing buffer zones in the reserved forests around the wildlife sanctuary to increase habitat areas for saltwater Crocodiles and to improve the outlook for long-term saltwater crocodile survival in Myanmar.
Jonathan K Webb - One of the best experts on this subject based on the ideXlab platform.
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why does vulnerability to toxic invasive cane toads vary among populations of australian freshwater Crocodiles
Animal Conservation, 2013Co-Authors: Ruchira Somaweera, Richard Shine, Jonathan K Webb, Tim Dempster, Mike LetnicAbstract:The ecological impact of an invasive species can be heterogeneous through space and time. One such case in Australia involves native freshwater Crocodiles Crocodylus johnstoni, which are highly sensitive to invasive cane toads Rhinella marina in some areas, whereas other populations experience little or no mortality from ingestion of the toxic toads. We studied the impact of toad invasion on three crocodile populations: one crashed, one showed a minor decrease and one appeared unaffected. We tested three hypotheses for the cause of this spatial variation in impact: differences among populations in toad–crocodile encounter rates (proximity of toads to Crocodiles during spotlight surveys), differences in crocodile feeding responses (trials of prey preference in the laboratory) and differences in crocodile physiology (reduction of swim speed after receiving a dose of toad toxin). We found little divergence among populations in any of these traits: Crocodiles from the three populations all encountered cane toads in the wild, and exhibited similar feeding responses and toxin tolerances. Thus, we cannot confidently identify causation for the impact heterogeneity. Reliance on alternative food resources and an ability to rapidly learn taste aversion may have allowed Crocodiles to deal with toad arrival in Lake Argyle and the Daly River. Future work could usefully evaluate potential explanations for the failure of these adaptive mechanisms in the severely affected (Victoria River) population. We suggest that spatial variation in the availability of alternative prey (and thus the willingness of Crocodiles to attack a novel toxic prey item) may have contributed to that variation in impact.
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determinants of habitat selection by hatchling australian freshwater Crocodiles
PLOS ONE, 2011Co-Authors: Ruchira Somaweera, Jonathan K Webb, Richard ShineAbstract:Animals almost always use habitats non-randomly, but the costs and benefits of using specific habitat types remain unknown for many types of organisms. In a large lake in northwestern Australia (Lake Argyle), most hatchling (<12-month-old) freshwater Crocodiles (Crocodylus johnstoni) are found in floating vegetation mats or grassy banks rather than the more widely available open banks. Mean body sizes of young Crocodiles did not differ among the three habitat types. We tested four potential explanations for non-random habitat selection: proximity to nesting sites, thermal conditions, food availability, and exposure to predation. The three alternative habitat types did not differ in proximity to nesting sites, or in thermal conditions. Habitats with higher food availability harboured more hatchlings, and feeding rates (obtained by stomach-flushing of recently-captured Crocodiles) were highest in such areas. Predation risk may also differ among habitats: we were twice as likely to capture a crocodile after seeing it in open-bank sites than in the other two habitat types. Thus, habitat selection of hatchling Crocodiles in this system may be driven both by prey availability and by predation risk.