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Colleen T. Downs - One of the best experts on this subject based on the ideXlab platform.
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use of an unmanned aerial vehicle drone to survey Nile Crocodile populations a case study at lake nyamithi ndumo game reserve south africa
Biological Conservation, 2018Co-Authors: Mohamed A Ezat, Camille J Fritsch, Colleen T. DownsAbstract:Abstract Observer bias and inexperience are challenging aspects of Crocodile survey methods for determining population numbers and structure. Aerial surveys with either a helicopter or a fixed winged aircraft are generally preferred methods to ground surveys; however, the high cost of the former is a limiting factor. Recently unmanned aerial vehicles (UAVs) or drones have been proposed for surveys because of their potential of improving over traditional techniques of wildlife monitoring and as they have relatively lower costs. We investigated of the suitability of a UAV to determine numbers and structure of the Nile Crocodile, Crocodylus niloticus, population during winter at Lake Nyamithi, Ndumo Game Reserve in South Africa. We used the UAV for eight flights covering ~132 ha. We also conducted a diurnal ground survey of Crocodiles for comparison. Using the UAV, 287 Crocodiles were identified and body length measured accurately for size class allocation whereas only 211 Crocodiles were counted in the diurnal ground survey. Consequently, the UAV aerial survey recorded 26% more Crocodiles. The potential of using UAVs to estimate Crocodile population size and measure the total length (TL) of individuals accurately and precisely at a relatively low cost should improve management actions, enable monitoring of the Crocodile populations annually and importantly avoid observer bias. Implications of this may facilitate improved crocodilian survey techniques.
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Status of the Nile Crocodile Population in Pongolapoort Dam After River Impoundment
African Zoology, 2017Co-Authors: Garreth Champion, Colleen T. DownsAbstract:The major Nile Crocodile Crocodylus niloticus populations in South Africa are threatened by pollution, habitat alteration/destruction, and poaching. This has highlighted the importance of other minor populations. The Phongola River Nile Crocodile population was previously considered as unsubstantial. Consequently, we investigated the Nile Crocodile population numbers and status and the effects of the impoundment of the Phongola River on this. In 2009–2010 we determined a minimum population number of 281 Nile Crocodiles in Pongolapoort Dam using a combination of survey methods. The population structure was identified as having a minimum of 116 (41.3%) juveNiles ( 2.5 m total length). At the commencement of the breeding season in August, Crocodiles congregated at a major basking site where the main tributary entered the dam. Three major nesting areas were identified, two of which were located on the river inlet to th...
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Nest predation and maternal care in the Nile Crocodile (Crocodylus niloticus) at Lake St Lucia, South Africa.
Behavioural processes, 2016Co-Authors: Xander Combrink, Jonathan K. Warner, Colleen T. DownsAbstract:Abstract Information regarding nest predation, nest abandonment, and maternal care in the Nile Crocodile (Crocodylus niloticus) is largely restricted to anecdotal observations, and has not been studied quantitatively. Consequently, we investigated their nesting biology using camera-traps over four years at Lake St Lucia, South Africa. We obtained 4305 photographs (daylight captures = 90.1%, nocturnal = 9.9%) of 19 nest-guarding females. Of 19 monitored nests, 37% were raided by predators (mean = 12.1 ± 6.2 days subsequent to camera placement). All females returned to their nests following first predation, and on average returned three times between predator raids before nest abandonment. Water monitors (Varanus niloticus) and marsh mongoose (Atilax paludinosus) were the main egg predators. Nesting raids lasted 5.9 ± 1.6 days. Diurnally females were seldom on the nest, except during cool/cloudy weather or rain, preferring to guard from nearby shade. Females defended nests aggressively against non-human intruders. Five Nile Crocodile females were observed liberating their hatchlings from nests. A detailed sequence of a mother excavating and transporting hatchlings revealed 13 excursions between nest and water over 32.5 h. This, after months of continual nest attendance and defence, is illustrative of the high level of maternal care in Nile Crocodiles. Camera-trapping is an effective, non-invasive method for further Crocodile nesting behaviour research.
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morphometrics sex ratio sexual size dimorphism biomass and population size of the Nile Crocodile crocodylus niloticus at its southern range limit in kwazulu natal south africa
Zoomorphology, 2016Co-Authors: Garreth Champion, Xander Combrink, Jonathan K. Warner, Peter M Calverley, Colleen T. DownsAbstract:Animal body size and sex are requisite data for understanding population structure and demography. Little information exists regarding Nile Crocodile, Crocodylus niloticus, morphometrics, sex ratios of wild populations, sexual size dimorphism and standing crop biomass. We captured 322 C. niloticus at Lake St Lucia and Ndumo Game Reserve, South Africa, the two largest Crocodile populations at the southern extent of the species’ range in Africa, and measured a suite of physical characteristics to create predictive models of body length from other morphological attributes and body mass. Our sample included 118 hatchlings, 91 subadults and 113 adults. Strong positive allometric relationships were found between body length metrics (total length and snout-vent length) and other morphometrics. All morphometric regressions were linear, with the exception of the relationship of body length to body mass, which was logarithmic. Among relationships of cranial morphology and body length, we found considerable individual variation among all size classes. The mean head width-to-head length ratio was 1.9 ± 1.6, and mean head length-to-total length ratio was 0.14 ± 0.005. The sex ratios for non-hatchling individuals at both populations were essentially 1:1, but adult sex ratios were male biased. We calculated a total standing crop biomass of 96,867.18 kg (161.45 kg/km) and 52,640.40 kg (1504.01 kg/km) for C. niloticus at Lake St Lucia and Ndumo Game Reserve, respectively, and an estimated 3650 non-hatchling individuals for the province of KwaZulu-Natal. The data presented here will help inform Crocodile management and population surveys in South Africa, where C. niloticus is an important apex predator that partitions aquatic resources and occasionally comes into conflict with human beings.
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Movement and Home Range of Nile Crocodiles in Ndumo Game Reserve, South Africa
Koedoe, 2015Co-Authors: Peter M Calverley, Colleen T. DownsAbstract:The study of movement patterns and home range is fundamental in understanding the spatial requirements of animals and is important in generating information for the conservation and management of threatened species. Ndumo Game Reserve, in north-eastern KwaZulu-Natal, bordering Mozambique, has the third largest Nile Crocodile ( Crocodylus niloticus ) population in South Africa. Movement patterns of 50 Nile Crocodiles with a total length of between 202 cm and 472 cm were followed over a period of 18 months, using mark-resight, radio and satellite telemetry. The duration of radio transmitter attachment (131 ± 11.4 days) was significantly and negatively related to total length and reproductive status. Satellite transmitters failed after an average of 15 ± 12.5 days. Home range was calculated for individuals with 10 or more radio locations, spanning a period of at least 6 months. There was a significant relationship between home range size and total length, with sub-adults (1.5 m – 2.5 m) occupying smaller, more localised home ranges than adults (> 2.5 m). The largest home ranges were for adults (> 2.5 m). Home ranges overlapped extensively, suggesting that territoriality, if present, does not result in spatially discrete home ranges of Nile Crocodiles in Ndumo Game Reserve during the dry season. Larger Crocodiles moved farther and more frequently than smaller Crocodiles. The reserve acts as a winter refuge and spring breeding site for an estimated 846 Crocodiles, which also inhabit the Rio Maputo during the summer months. Nile Crocodile movement out of the reserve and into the Rio Maputo starts in November and Crocodiles return to the reserve as water levels in the floodplain recede in May. Conservation implications: Movement patterns of Nile Crocodiles show the important role the reserve plays in the conservation of Nile Crocodile populations within the greater Ndumo Game Reserve–Rio Maputo area.
Alison J. Leslie - One of the best experts on this subject based on the ideXlab platform.
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Phylogeography, genetic diversity, and population structure of Nile Crocodile populations at the fringes of the southern African distribution
PloS one, 2019Co-Authors: Barbara Van Asch, Ruhan Slabbert, William F. Versfeld, Alison J. Leslie, Pierre Du Preez, Kelvin L. Hull, Timoteus I. Matheus, Petrus Beytell, Clint RhodeAbstract:Nile Crocodiles are apex predators widely distributed in sub-Saharan Africa that have been viewed and managed as a single species. A complex picture of broad and fine-scale phylogeographic patterns that includes the recognition of two species (Crocodylus niloticus and Crocodylus suchus), and the structuring of populations according to river basins has started to emerge. However, previous studies surveyed a limited number of samples and geographical regions, and large areas of the continent remained unstudied. This work aimed at a fine scale portrait of Nile Crocodile populations at the fringes of their geographic distribution in southern Africa. Wild and captive individuals were sampled across four major river systems (Okavango, Lower Kunene, Lower Shire and Limpopo) and the KwaZulu-Natal region. A multi-marker approach was used to infer phylogeographic and genetic diversity patterns, including new and public mitochondrial data, and a panel of 11 nuclear microsatellites. All individuals belonged to a phylogenetic clade previously associated with the C. niloticus species, thus suggesting the absence of C. suchus in southern Africa. The distribution of mitochondrial haplotypes indicated ancestral genetic connectivity across large areas, with loss of diversity along the north-south axis. Genetic variation partitioned the populations primarily into western and eastern regions of southern Africa, and secondarily into the major river systems. Populations were partitioned into five main groups corresponding to the Lower Kunene, the Okavango, the Lower Shire, and the Limpopo rivers, and the KwaZulu-Natal coastal region. All groups show evidence of recent bottlenecks and small effective population sizes. Long-term genetic diversity is likely to be compromised, raising conservation concern. These results emphasize the need for local genetic assessment of wild populations of Nile Crocodiles to inform strategies for management of the species in southern Africa.
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Using a Binomial Mixture Model and Aerial Counts for an Accurate Estimate of Nile Crocodile Abundance and Population Size in the Kunene River, Namibia
African Journal of Wildlife Research, 2016Co-Authors: Arnaud Lyet, Ruhan Slabbert, William F. Versfeld, Alison J. Leslie, Piet Beytell, Pierre Du PreezAbstract:The Nile Crocodile,Crocodylus niloticus, is found throughout sub-Saharan Africa, including Namibia, Botswana and Angola. The species was transferred from CITES Appendix I to Appendix II in 2004, although it is recognized as peripherally endangered in Namibia due to diminishing habitat availability primarily from human encroachment. In 2013, a species management plan was approved in Namibia to assess the management of the Namibian Nile Crocodile populations. During 2012, an aerial survey was conducted to provide an estimate of Nile Crocodile population numbers. A recently developed N-mixture model for estimation of abundance and spatial variation was used. Detection probability correlated to animal size and environmental covariates. Our data also suggest that small Crocodiles are easier to detect during the spring. The abundance for different size classes was influenced by river complexity (vegetation, depth, channels) and the distribution of human settlements. An estimated 806 individuals were counted alo...
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Population size and structure of the Nile Crocodile Crocodylus niloticus in the lower Zambezi valley
Oryx, 2013Co-Authors: Kevin M. Wallace, Alison J. Leslie, Tim Coulson, Audrey S. WallaceAbstract:Concern has been raised about the lack of population data for the Nile Crocodile Crocodylus niloticus in the lower/middle Zambezi valley. This area is important for conservation as well as being a source of Crocodile eggs and adults for the ranching industry. Two spotlight surveys, in 2006 and 2009, were used to estimate population size, structure and trends. A stage-structured matrix model was parameterized from existing literature and the expected predictions were compared to those observed. The survey data suggests a population increase since 2006. Crocodile density was greatest (3.1 km −1 ) in the areas of increased wildlife and habitat protection and lowest (1.4 km −1 ) in areas of increased human presence. The predicted population stage structure differed to that observed, suggestive of a population not at equilibrium. Data on offtakes of Crocodile eggs and adults would be useful for examining why this is the case. Continued monitoring of the wild population is necessary, to evaluate the trend of an increasing Crocodile population, and additional demographic data for modelling purposes would be desirable.
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Estimating demographics of the Nile Crocodile (Crocodylus niloticus Laurenti) in the panhandle region of the Okavango Delta, Botswana
African Journal of Ecology, 2011Co-Authors: Sven L. Bourquin, Alison J. LeslieAbstract:The population status of the Nile Crocodile (Crocodylus niloticus Laurenti) in the panhandle region of the Okavango Delta, Botswana, was assessed using mark–recapture and spotlight survey techniques following a decline because of commercial utilization. A total of 1717 individuals, ranging from 136 to 2780 mm in Snout Vent Length, were captured over a 4-year period. Eighty-one per cent of young juveNiles encountered were successfully captured, representing 59.3% of total captures and 75% of recaptures. A Bayesian technique was used to estimate the number of young juveNiles, and these estimates were then extrapolated for the other size classes. Survival and recapture analyses highlighted individual size-dependent increases in wariness and survival. The total annual population was estimated to be 2570 ± 151.06 individuals, with an adult population of 649.2 individuals, including 364 females. We suggest that the harvest of breeding animals for commercial purposes should be halted until population recovery in this region is established.
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A preliminary disease survey in the wild Nile Crocodile (Crocodylus niloticus) population in the Okavango Delta, Botswana
Journal of the South African Veterinary Association, 2011Co-Authors: Alison J. Leslie, C. J. Lovely, J. M. PittmanAbstract:The objective of this study was to conduct a preliminary survey of diseases that might be present in the wild Nile Crocodile population in the Okavango Delta, Botswana. Blood samples were collected from Crocodiles ranging in size from 34.0 cm to 463.0 cm total length. Samples were examined for blood parasites and underwent a haematological analysis. Before release the Crocodiles were examined for various clinical abnormalities. Of the 144 Crocodiles examined, none were visibly sick or displayed any signs of disease. No antibodies to Mycoplasma crocodyli were detected. Hepatozoon pettiti was present in 55.3 % of blood smears examined, but there was no significant difference in any of the haematological values between the infected and uninfected Crocodiles, and a high prevalence of Hepatozoon infection is not uncommon in other species. Only 7.6 % of the examined Crocodiles were infested with leeches. Further research is required for several of the crocodilian diseases, in particular to elucidate the role of wild crocodilians as reservoirs of infection.
James R. Spotila - One of the best experts on this subject based on the ideXlab platform.
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Alien plant threatens Nile Crocodile (Crocodylus niloticus) breeding in Lake St. Lucia, South Africa.
Biological Conservation, 2001Co-Authors: Alison J. Leslie, James R. SpotilaAbstract:Abstract We observed that the majority of Lake St. Lucia's nesting Nile Crocodiles ( Crocodylus niloticus ) selected open, sunny, sandy areas in which to deposit their eggs. Nests were only found in shaded sites in the Mpate River breeding area and these nests were shaded primarily by an alien plant Chromolaena odorata . Soil temperatures of shaded sites at 25-cm depth, were on average 5.0–6.0°C cooler than in sunny sites at the same depth. They were well below the pivotal temperature for nests of St. Lucia's Nile Crocodiles, i.e. they probably produced a female-biased sex ratio, and may have prevented embryonic development altogether. Many females abandoned nesting sites when they encountered the fibrous root mats of Chromolaena odorata while digging egg chambers. When additional nesting sites were experimentally created, the percent of sites utilized increased, indicating that suitable nesting sites were in short supply. This alien plant is posing a very serious threat to the continued survival of the Nile Crocodile in Greater St. Lucia Wetland Park, and unless immediate action is taken, a female-biased sex ratio will result in eventual extirpation of the species from this recently acclaimed Word Heritage Site.
Ct Downs - One of the best experts on this subject based on the ideXlab platform.
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Nest predation and maternal care in the Nile Crocodile (Crocodylus niloticus) at Lake St Lucia, South Africa
Elsevier, 2016Co-Authors: Combrink X, Warner J, Ct DownsAbstract:Information regarding nest predation, nest abandonment, and maternal care in the Nile Crocodile (Crocodylus niloticus) is largely restricted to anecdotal observations, and has not been studied quantitatively. Consequently, we investigated their nesting biology using camera-traps over four years at Lake St Lucia, South Africa. We obtained 4305 photographs (daylight captures = 90.1%, nocturnal = 9.9%) of 19 nest-guarding females. Of 19 monitored nests, 37% were raided by predators (mean = 12.1 ± 6.2 days subsequent to camera placement). All females returned to their nests following first predation, and on average returned three times between predator raids before nest abandonment. Water monitors (Varanus niloticus) and marsh mongoose (Atilax paludinosus) were the main egg predators. Nesting raids lasted 5.9 ± 1.6 days. Diurnally females were seldom on the nest, except during cool/cloudy weather or rain, preferring to guard from nearby shade. Females defended nests aggressively against non-human intruders. Five Nile Crocodile females were observed liberating their hatchlings from nests. A detailed sequence of a mother excavating and transporting hatchlings revealed 13 excursions between nest and water over 32.5 h. This, after months of continual nest attendance and defence, is illustrative of the high level of maternal care in Nile Crocodiles. Camera-trapping is an effective, non-invasive method for further Crocodile nesting behaviour research.National Research Foundation (South Africa
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Morphometrics, sex ratio, sexual size dimorphism, biomass, and population size of the Nile Crocodile (Crocodylus niloticus) at its southern range limit in KwaZulu-Natal, South Africa
Springer Verlag, 2016Co-Authors: Warner J, Combrink X, Calverley P, Champion G, Ct DownsAbstract:Animal body size and sex are requisite data for understanding population structure and demography. Little information exists regarding Nile Crocodile, Crocodylus niloticus, morphometrics, sex ratios of wild populations, sexual size dimorphism and standing crop biomass. We captured 322 C. niloticus at Lake St Lucia and Ndumo Game Reserve, South Africa, the two largest Crocodile populations at the southern extent of the species’ range in Africa, and measured a suite of physical characteristics to create predictive models of body length from other morphological attributes and body mass. Our sample included 118 hatchlings, 91 subadults and 113 adults. Strong positive allometric relationships were found between body length metrics (total length and snout-vent length) and other morphometrics. All morphometric regressions were linear, with the exception of the relationship of body length to body mass, which was logarithmic. Among relationships of cranial morphology and body length, we found considerable individual variation among all size classes. The mean head width-to-head length ratio was 1.9 ± 1.6, and mean head length-to-total length ratio was 0.14 ± 0.005. The sex ratios for non-hatchling individuals at both populations were essentially 1:1, but adult sex ratios were male biased. We calculated a total standing crop biomass of 96,867.18 kg (161.45 kg/km) and 52,640.40 kg (1504.01 kg/km) for C. niloticus at Lake St Lucia and Ndumo Game Reserve, respectively, and an estimated 3650 non-hatchling individuals for the province of KwaZulu-Natal. The data presented here will help inform Crocodile management and population surveys in South Africa, where C. niloticus is an important apex predator that partitions aquatic resources and occasionally comes into conflict with human beingsNational Research Foundation (South Africa
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Movement and home range of Nile Crocodiles in Ndumo Game Reserve, South Africa
AOSIS, 2015Co-Authors: Calverley P, Ct DownsAbstract:The study of movement patterns and home range is fundamental in understanding the spatial requirements of animals and is important in generating information for the conservation and management of threatened species. Ndumo Game Reserve, in north-eastern KwaZulu-Natal, bordering Mozambique, has the third largest Nile Crocodile (Crocodylus niloticus) population in South Africa. Movement patterns of 50 Nile Crocodiles with a total length of between 202 cm and 472 cm were followed over a period of 18 months, using mark-resight, radio and satellite telemetry. The duration of radio transmitter attachment (131 ± 11.4 days) was significantly and negatively related to total length and reproductive status. Satellite transmitters failed after an average of 15 ± 12.5 days. Home range was calculated for individuals with 10 or more radio locations, spanning a period of at least 6 months. There was a significant relationship between home range size and total length, with sub-adults (1.5 m – 2.5 m) occupying smaller, more localised home ranges than adults (> 2.5 m). The largest home ranges were for adults (> 2.5 m). Home ranges overlapped extensively, suggesting that territoriality, if present, does not result in spatially discrete home ranges of Nile Crocodiles in Ndumo Game Reserve during the dry season. Larger Crocodiles moved farther and more frequently than smaller Crocodiles. The reserve acts as a winter refuge and spring breeding site for an estimated 846 Crocodiles, which also inhabit the Rio Maputo during the summer months. Nile Crocodile movement out of the reserve and into the Rio Maputo starts in November and Crocodiles return to the reserve as water levels in the floodplain recede in May. Conservation implications: Movement patterns of Nile Crocodiles show the important role the reserve plays in the conservation of Nile Crocodile populations within the greater Ndumo Game Reserve–Rio Maputo area.National Research Foundation (South Africa
Clint Rhode - One of the best experts on this subject based on the ideXlab platform.
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Phylogeography, genetic diversity, and population structure of Nile Crocodile populations at the fringes of the southern African distribution
PloS one, 2019Co-Authors: Barbara Van Asch, Ruhan Slabbert, William F. Versfeld, Alison J. Leslie, Pierre Du Preez, Kelvin L. Hull, Timoteus I. Matheus, Petrus Beytell, Clint RhodeAbstract:Nile Crocodiles are apex predators widely distributed in sub-Saharan Africa that have been viewed and managed as a single species. A complex picture of broad and fine-scale phylogeographic patterns that includes the recognition of two species (Crocodylus niloticus and Crocodylus suchus), and the structuring of populations according to river basins has started to emerge. However, previous studies surveyed a limited number of samples and geographical regions, and large areas of the continent remained unstudied. This work aimed at a fine scale portrait of Nile Crocodile populations at the fringes of their geographic distribution in southern Africa. Wild and captive individuals were sampled across four major river systems (Okavango, Lower Kunene, Lower Shire and Limpopo) and the KwaZulu-Natal region. A multi-marker approach was used to infer phylogeographic and genetic diversity patterns, including new and public mitochondrial data, and a panel of 11 nuclear microsatellites. All individuals belonged to a phylogenetic clade previously associated with the C. niloticus species, thus suggesting the absence of C. suchus in southern Africa. The distribution of mitochondrial haplotypes indicated ancestral genetic connectivity across large areas, with loss of diversity along the north-south axis. Genetic variation partitioned the populations primarily into western and eastern regions of southern Africa, and secondarily into the major river systems. Populations were partitioned into five main groups corresponding to the Lower Kunene, the Okavango, the Lower Shire, and the Limpopo rivers, and the KwaZulu-Natal coastal region. All groups show evidence of recent bottlenecks and small effective population sizes. Long-term genetic diversity is likely to be compromised, raising conservation concern. These results emphasize the need for local genetic assessment of wild populations of Nile Crocodiles to inform strategies for management of the species in southern Africa.