Region 2- The Phongola floodplain and associated pans
5.4 RESULTS .1 Nest surveys
Nile crocodile nesting surveys in NGR during the late 1960‟s and late 1980‟s showed that the majority of nesting sites occurred along the old Phongola River course (sub-region 6). The river has subsequently diverged and now runs along the eastern boundary of the reserve (sub- region 8) (Fig. 1).During the course of the current study no nests were found along the old or
new river courses. The eastern boundary fence of the reserve has recently been forcibly removed by the local inhabitants offering unprecedented access to the Phongola floodplain and subsequent disturbance to nest sites. In 2009-2010 at least two historical nesting sites along the new course of the Phongola River were abandoned due to increased anthropogenic disturbance. These sites were not used in the 2011/2012 nesting season. Furthermore, no nests were located along the eastern periphery of the Phongola floodplain where the majority of anthropogenic disturbance was taking place, including Mavilo Hill, a historical nest site. In 2009/2010 prime historical nesting sites along the old course of the Phongola River were not used and it was unlikely that this was because of disturbance as this part of the reserve remains relatively isolated. However, in 2011 the swing bridge which grants field rangers access to this part of the reserve and allows for foot patrols was destroyed. In the past field guides excavated eggs from these areas to show guests which could negatively impact nesting here (pers. comm. G.Bloy). Anthropogenic disturbance in this part of the reserve is therefore higher than previously thought. The lack of crocodiles nesting here could also be attributed to the change in the Phongola River‟s main course altering the nesting landscape (pers.obs).
In January 2010, four nests were found along the western periphery of the Phongola floodplain in Sub-region 6. Although these nests were in historical nest sites, they were significantly further inland (west) due to high water levels. No nests were found in Sub-region 6 in 2011/1012. Two nests were found at historical nesting sites on the south eastern shore of Lake Nyamithi (Sub-region 5) in both 2009/2010 and 2011/2012 surveys. One nest was found along the banks of the main course of the Usuthu River (Sub-region 2) in March 2010. This is the first time a nest has been found along this river course and could be in response to disturbance in more favorable parts of the reserve or a change in the nesting landscape due to the change in the
course of the Usuthu River. Three new nesting sites were found on the new course of the Usuthu River in the region of Diphini Hide (Sub-region 3) in 2011/2012. Nesting sites were not always used in consecutive seasons, and successive nesting surveys result in additional nesting sites being discovered.
A new nesting site was found on the banks of the Balamhlanga stream in 2010 as it enters Lake Nyamithi. The nest site contained 4 active nests and 1 active burrow which was used by at least one female and numerous hatchlings. The stream is narrow and shallow and the burrow provided shelter for the female during the incubation period and shelter for hatchlings.
5.4.2 Nest success
In 1964 Pooley documented the abandonment of 5 Nile Crocodile nesting sites at Lake Nyamithi due to disturbance associated with increased motor vehicle activity (Pooley, 1969).
Pooley listed the Water Monitor Lizard as the primary predator of crocodile eggs during his surveys from 1962-1969 in NGR (Pooley, 1969). Predation pressure during this time usually effected less than 20% of nests (Pooley, 1969). During the 2009/2010 survey two nests found at Lake Nyamithi were abandoned and predated by Water Monitor Lizards. It is not clear if disturbance to the nest sites from the nearby road or natural predation had led to the abandonment of these nests. Nor is it clear whether the nests were first abandoned then predated.
However, it is likely that disturbance increases the frequency of nest predation as the female crocodile is usually driven off the nest allowing predators an opportunity to raid the nest. In 2011/2012 two nests were once again found in this area, however only one was predated. The un-predated nest was closely guarded by a female who refused to move from the nest site, emitting a warning hiss to researchers who approached too closely.
Three of the 4 nests attended by female Nile Crocodiles in the western periphery of the floodplain showed signs of predation but were subsequently submerged by flooding, thus it was not possible to quantify the levels of predation. The fourth nest in this area was considerably closer to the water and was flooded prior to discovery. This nest was not predated after flood waters subsided neither did it hatch. Consequently predation affected 86 % of nests in NGR for the 2009/2010 season. All nests were ultimately abandoned either due to predation (86%), flood damage (71 %) or a combination of the two (100 %). Consequently, there was zero recruitment from the 7 Nile Crocodile nests located in NGR over the 2009/2010 breeding season.
Conversely, none of the 9 nests located in 2011/2012 were flooded and 66 % were predated. Three nests hatched successfully, however it was not possible to estimate how many hatchlings actually emerged. One of the nests that hatched showed signs of predation prior to hatching and the female was not observed at the nesting site. However, a fresh body print was visible on the nest indicating that she had recently been in attendance.
Camera traps located at 4 nest sites during the 2011/2012 season failed to capture images of females attending their nests. Images of potential nest predators captured at the nesting site included the Cape Porcupine (Hystrix africaeaustralis) and the Vervet Monkey (Chlorocebusa ethiops).
5.4.3 Characteristics of nesting sites
Nile Crocodile nests were found in lacustrine and riverine habitats in NGR and in a variety of substrates. Nests located on the south eastern bank of Nyamithi Pan were elevated 2-3 m above water level and 1 - 4 m from water and were laid in hard, red rocky soils in a boulder bed outcrop. Nesting sites located on the banks of the western inlet of Nyamithi in an area known
as the Balamhlanga stream were 1 m above water level and 1– 2 m from the water and in white sandy soil. Nesting sites on the western periphery of the floodplain were found in fine white sand and were 5 – 25 m away from water. These nests were attended by females but were abandoned later in the season possibly due to predation or flooding owing to their low elevation above water (< 2 m). Nests in the same area, but closer to the water, were deposited in a substrate of white sand and alluvial silt/clay.
Considering the fluctuations in water levels throughout the rainy season, measuring height above and distance to water provided incidental data dependent on the date of inquiry, and was of little comparative use. Perhaps the only standardized method of collecting such data would be to accurately record the altitude of nests and calibrate this to water level data for the particular river/lake being investigated.
5.4.4 Nesting effort
We ran a correlation matrix to determine any relationships between date, nesting effort and numbers of nests found in NGR over the last 50 years. When nesting effort (E) was calculated using winter estimations of total population size (N), a significant decrease (p = 0.001) in the nesting effort of Nile Crocodiles in NGR from over 20 % in the 1960‟s to just over 5 % in 2010 and 6 % in 2012 (Fig. 2) was found. This was a function of an increasing Nile Crocodile population size with time, while the number of nests remained constant (p = 0.285). The number of nests found within NGR has stayed relatively constant over the years, while the population has increased threefold (Fig. 3) resulting in a much lower nesting effort with time (Fig. 4). When E was calculated using N during summer E was calculated at 18 – 22 % and no significant difference in E was found from the 1960‟s to the present.