The population ecology of the Nile crocodile (Crocodylus niloticus) in the panhandle region of the Okavango Delta, Botswana

[摘要] The Okavango Delta, Botswana, is a unique ecosystem and this is reflected in itsextraordinary biodiversity. The Nile crocodile (Crocodylus niloticus Laurenti) is theapex predator, and performs a number of vital functions in this system, making it akeystone species. The panhandle crocodile population has declined significantly overthe last 80 years and is now threatened as a result of past over-exploitation andpresent human disturbance. In order to effectively conserve this species and in turnthe health of this important region it is imperative to gain an understanding of theirecology and population dynamics.The population status of the Nile crocodile in the panhandle region of the OkavangoDelta, Botswana, was assessed using a combination of capture-mark-recapturesurveys, spotlight surveys and aerial surveys. The capture-mark-recaptureexperiment was conducted continuously from 2002 - 2006. A total of 1717individuals, ranging in size from 136 mm – 2780 mm SVL, were captured, of which224 animals were recaptured. Using a Bayesian technique, the total annualpopulation in the panhandle region of the Okavango Delta was estimated to be 2 570± 151.06 individuals, with an adult population of 649.2 individuals with the numberof breeding females estimated to be 364 individuals. It was concluded that thispopulation cannot sustain the further harvest of breeding animals prior to the increaseand stabilization of the population.Spotlight counts revealed a decline in the encounter-rate of crocodiles on theOkavango River with time, although more long-term data needs to be collected toconfirm this trend. During the low-water season (September - February), 22.34 % ofall crocodiles were observed, while during the flood-season only 13.34 % wereobserved, yielding correction factors for spotlight surveys of 4.46 (low-water) and7.49 (high-water) for all animals in the panhandle.Two aerial surveys, conducted at the low-water and high-water peaks yielded totalestimates of 588 (77.7 % of adults) during the low-water period and 350 (56.7 % ofadults) during the high-water period. Correction factors of 1.28 (low-water) and 1.77(high-water) were calculated for aerial surveys. An annual average of 50 nests was located in the panhandle, representing a 50 - 60 %decrease over the last 20 years. In regions with high human disturbance, breedingfemales situated their nests in hidden locations, away from accessible channels.Hatchlings exhibited elongation of the jaw in order to capture smaller prey items andmorphometric shifts in jaw shape coincided with a dietary change at 400 mm SVL.The jaw became broader and deeper as animals matured, presumably in preparationfor larger mammalian prey. The average growth rate of recaptured yearlings was0.198 ± 0.116 mm.d-1 SVL and was closely correlated to the amount of time anindividual spent in above-average water temperatures. Body condition (RCF) wassignificantly and positively correlated with a rise in water-level and negativelycorrelated to time spent in above-average water and air temperatures. Average RCFvalues were intersected when animals had spent 50 % of their time in above-averagetemperatures and water level.Generally crocodiles in the panhandle showed no significant sex-related differencesin their sizes or the distances they travelled. The majority of recaptures (62.5 %)moved less than 500 m from the initial capture site. Adults in the panhandle occupieddefinite ranges, within which were preferred core areas where the majority of theirtime was spent.The panhandle crocodile population has declined significantly over the last 80 years,and is now threatened as a result of past over-exploitation and present humandisturbance. The management of this population, including both its conservation andsustainable commercial utilisation, requires an adaptive strategy based on accuratemonitoring procedures.

[发布机构] Stellenbosch University