Estimating population parameters of African elephants: a photographic mark-recapture application in a South African protected area

Abstract

Accurate estimates of demographic parameters are instrumental in effective management of animal populations. For species with individually distinctive features, photo-identification (photo-ID) provides a reliable means to gather capture–recapture data for population parameter estimation with considerable precision and accuracy. We use a 3-year photo-ID mark-recapture dataset of African savannah elephants (Loxodonta africana) in Pilanesberg National Park (PNP), South Africa, to model their population size and estimate survival rates. All photographed elephants, irrespective of age, were individually identified based on their unique pattern of facial wrinkles. The population currently numbers 385 elephants (95% CI = 380–401), of which nearly half are grown individuals in a sex ratio of 1 male: 1.23 female. Considerable heterogeneity in capture and recapture probabilities, both within and between sex-age classes suggests some form of individual-specific or herd-specific variability, perhaps behavioural or spatio-behavioural dissimilarity within the PNP population. Estimated annual survival rates are high (0.967–0.996) and do not differ between sex-age classes, a likely expression of an extended parental care, low predation pressure, access to rich food and water resources, and absence of targeted killing/poaching. The lack of detectable difference between sexes in adult survival/mortality sets PNP elephants apart from other known African elephant populations and warrants further research attention. Given previous estimates (aerial counts in the early 2000s), the PNP elephant population has grown ~ 5.7% per annum over a 16-year period. This is similar to what is reported in other conservation areas in South Africa, but considerably lower than previously projected. Natural mortality, even if low as 0.4–3.3%, is not negligible and plays a role in moderating population growth. This realisation must be recognised when considering population management measures. It is, therefore, important to obtain and apply the most up-to-date population-specific demographic parameters when making management decisions. Periodic photo-ID surveys with mark-recapture analyses can generate such demographic indicators with a considerable accuracy and should be adopted as a useful tool to inform management decisions, complimentary to direct aerial counts, especially in small-to-medium size fenced conservation areas.

Appendices

Appendices

See Fig. A1, Appendix 1, and Appendix 2.

Fig. A1

Photo credit: Scott Y.S. Chui

All elephants encountered in Pilanesberg National Park were individually photo-identified regardless of their age, including small calves, based on their unique pattern of facial wrinkles (see Chui and Karczmarski 2022). The image shows an adult male (back) and a female (front).

Appendix 1: Preliminary assessment of reproductive parameters and calf survival rates

Over the course of this study, a total of 56 births (b_t) were recorded in the PNP, with all newborns and young calves individually identified using their unique patterns of facial wrinkles (Chui and Karczmarski 2022). To assess the reproductive parameters and calf survival rates, the age of calves was estimated based on the body proportions and relative size, motor coordination, and the behaviour of both the calf and the mother (adapted and modified from Moss 2001). All calves were assigned a birth-year with an accuracy of ± 3 months, while the birth-month of 15 newborns was known with an accuracy of ± 2 weeks. The sex of most of these calves, however, could not be reliably determined.

Reproductive parameters and calf survival rates of the PNP elephants were estimated based on a 3-year photo-ID mark-recapture dataset summarised in Table A1. While the approach to estimate fecundity differs among field studies of large mammals (e.g., Wells and Scott 1990; Coulson et al. 2000; Wittemyer et al. 2013; Reed et al. 2022), in this study we quantify the fecundity rate at year t (FR_t) as the number of births (both sexes, b_t) per the number of identified grown females (f_t), $FR_{t} = b_{t} /f_{t}$, as in similar studies of large terrestrial herbivores (Coulson et al. 2000; Moss 2001; Forrester and Wittmer 2013). This was the most suitable approach, as the sex of most young calves remained undetermined. The crude birth rate at year t (CBR_t) was calculated as the ratio of the number of births (b_t) to the total number of identified individuals (n_t): $CBR_{t} = b_{t} /n_{t}$; while the recruitment rate at year t (RR_t) was calculated as: $\it RR_{t} = b_{12,t} /\left( {n_{t} - b_{t} } \right)$ (Caughley 1977; Wells and Scott 1990; Chang et al. 2016). The age-specific calf survivorship at year t to the age x (l_x,t) was estimated as follows: $l_{x,t} = b_{x,t} /b_{t}$, where b_x,t is the number of calves surviving to age x; while the age-specific calf survival rate at year t (p_x,t) was calculated as: $p_{x,t} = l_{x,t} /l_{x-1,t}$ (Stolen and Barlow 2003; Moore and Read 2008; Chang et al. 2016). Weighted averages and binomial variances were used to calculate the mean and the standard deviation, respectively (Wells and Scott 1990).

Table A1 Reproductive parameters and calf survival rates of the PNP elephants. All newborns and calves were individually identified based on the unique patterns of facial wrinkles, so the number of births (b_t) is included in the number of identified individuals (n_t)

Despite current limitations due to the relatively short study period (~ 3 years), the individual-ID data of newborns and calves (which are usually non-distinctive, except when using the pattern of facial wrinkles for individual identification, as in this study) offers first clues of reproductive dynamics and calf survivorship of the PNP elephants. Although preliminary at this stage, our estimates indicate a fecundity rate (FR_mean) of 19.4% (SD = 2.5%; Table A1), accounting for calves of both sexes. If we assume that sex ratio at birth is 1:1, the fecundity rate would approximate 9.7% for female calves (i.e., 0.097 births of female calf per grown female per year). This rate of producing female calves appears to be at a higher end of the spectrum among other populations; higher than the rates reported for the Dzanga Bai population of forest elephants in Central African Republic (Turkalo et al. 2018), and comparable to those of Samburu and Amboseli populations of savannah elephants in Kenya (Moss 2001; Wittemyer et al. 2013).

The age-specific survivorship of calves in the PNP appears to be relatively stable across the first two years of age (Table A1). The first 3-months of their life, however, are their most vulnerable and accounted for all cases of calf mortality during our study (survival rate, p_0–3,t = 0.920). Calves older than 3-month had a high chance of further survival (survival rates = 1.000), likely a result of extended parental care in elephants (Foley et al. 2008; Moss and Lee 2011) and low predation pressure in the PNP (see Discussion). These survival rates of calves are comparable to those reported in Kenya reserves (Moss 2001; Wittemyer et al. 2013, 2021) and slightly lower than in some other populations across the continent (e.g., Gough and Kerley 2006; Foley and Faust 2010; Turkalo et al. 2018). The high survival rates of calves older than 6-months correspond with our mark-recapture modelling results which suggest no apparent differences in survival rates between sex-age classes (see Results section ‘CJS models and apparent survival rates’).

As the current assessment of reproductive dynamics is based on ~ 3 years of study, even with fine-detail individual-ID data the mean values of parameters are averaged across at best 2–3 annual estimates, hence susceptible to yearly fluctuations (e.g., the highest fecundity rate estimate is more than double of the lowest). Therefore, the numeric values of these estimates have to be seen as preliminary only. They are illustrative, however, of the type and resolution of demographic information that can be generated with a photo-ID mark-recapture approach. We recommend this approach as a powerful research tool with useful management applications, complimentary to traditional techniques of monitoring elephant populations such as aerial counts, and highly informative in predictive modelling of population trends.

Appendix 2: Heterogeneous capture and recapture probabilities

See Table A2.

Table A2 Estimates of finite mixture parameter (π), capture probabilities (p_closed) and recapture probabilities (c) averaged across the two best-fit closed population candidate models of PNP elephants. π and (1-π) represent the proportion of individuals in mixture group 1 and mixture group 2, respectively