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The response of lions (Panthera leo) to changes in prey abundance on an enclosed reserve in South Africa
Article in Acta theriologica · July 2012 DOI: 10.1007/s13364-011-0071-8
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The user has requested enhancement of the downloaded file. The response of lions (Panthera leo) to changes in prey abundance on an enclosed reserve in South Africa
Charlene Bissett, Ric T. F. Bernard & Daniel M. Parker
Acta Theriologica
ISSN 0001-7051 Volume 57 Number 3
Acta Theriol (2012) 57:225-231 DOI 10.1007/s13364-011-0071-8
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Acta Theriol (2012) 57:225–231 DOI 10.1007/s13364-011-0071-8
ORIGINAL PAPER
The response of lions (Panthera leo) to changes in prey abundance on an enclosed reserve in South Africa
Charlene Bissett & Ric T. F. Bernard & Daniel M. Parker
Received: 25 August 2011 /Accepted: 18 December 2011 /Published online: 15 January 2012 # Mammal Research Institute, Polish Academy of Sciences, Białowieża, Poland 2012
Abstract Large mammalian carnivores place significant Keywords Diet switching . Large predators . Predator–prey pressure on their prey populations and this is exacerbated interaction . Prey preferences within the fenced reserves of Africa. However, foraging theory predicts that diet switching by predators may mitigate this pressure. In this study, we use data collected between Introduction 2003 and 2007 from an enclosed system in the Eastern Cape Province of South Africa to examine the response of lions The diet of predators in multiple prey species systems is Panthera leo to changes in the abundance of two important affected by a range of factors including abundance of different prey species — kudu Tragelaphus strepsiceros and warthog prey species and their vulnerability, anti-predator behavior Phacochoerus africanus. As the relative abundance of war- and defence, age and sex, and the presence of and interference thogs increased, the number of kudu kills decreased signif- from other predators (Perry and Pianka 1997; Garrott et al. icantly, whereas warthog kills became significantly more 2007). Vulnerability is not solely a species-specific character- frequent. A similar pattern was observed for lion prey pref- istic and will vary with body condition and therefore with age, erence and the switch from kudu to warthog was also reproductive status and environmental factors such as drought reflected in a significant decrease in the mean prey mass. (Owen-Smith and Mills 2008). Thus diet can vary in space Our results suggest that a diet shift occurs in lions and that and time. The optimal foraging theory (MacArthur and Pianka the change in diet is primarily in response to an increase in 1966) provides a theoretical framework against which feeding warthog numbers. Prey switching may promote the persis- behavior can be understood and predicts that the diet of tence of predator–prey systems, which is particularly impor- predators, such as lions Panthera leo, will vary as the relative tant for fenced systems where natural immigration of prey is abundance of one or more of a range of alternative prey species not possible. However, continued collection and analysis of varies (Pyke et al. 1977). This dietary variation may be sea- long-term observational data from the multipredator, multi- sonal as in some mustelids (Carss et al. 1998;Beggetal.2003) prey systems of Africa is required to facilitate a full under- and Geoffroy’scatLeopardus geoffroyi (Canepuccia et al. standing of predator–prey dynamics. 2007), where seasonal climate variation drives seasonal changes in food abundance. Alternatively, it may coincide with longer population cycles such as that of the snowshoe hare Lepus americanus where lynx Lynx canadensis switch to Communicated by: Dries Kuiper : : preyingonredsquirrelsTamiasciurus hudsonicus during peri- C. Bissett R. T. F. Bernard D. M. Parker ods of least hare abundance (O’Donoghue et al. 1998). Finally, Wildlife and Reserve Management Research Group, Department of Zoology and Entomology, Rhodes University, it may be driven by longer cycles of climate change, such as Grahamstown 6139, South Africa drought, where one species may be more susceptible to drought than others and that species then becomes more vul- * C. Bissett ( ) nerable (Owen-Smith and Mills 2008). Kwandwe Private Game Reserve, Grahamstown 6139, South Africa The influence of generalist mammalian predators on prey e-mail: [email protected] population dynamics has been well studied (Korpimäki and Author's personal copy
226 Acta Theriol (2012) 57:225–231
Krebs 1996;Hanskietal.2001;Dell’Arte et al. 2007). Much 33°09 S, 26°37 E) in the Eastern Cape Province, South of this work has focussed on the effect of predators on the Africa. The reserve has a warm temperate climate with an well-known population cycles of rodents in the relatively average annual rainfall of 410 mm, and the vegetation is a simple systems of the northern hemisphere, and a central mosaic of open savanna-like areas and more thickly wooded theme has been that of predator switching which is thought areas (Mucina and Rutherford 2006). Rainfall data were to be responsible for controlling some population cycles recorded at one site in the center of the reserve on a monthly (Křivan 1996; van Baalen et al. 2001;Maetal.2003;Sundell basis. et al. 2003;Murrell2005). However, evidence from the south- ern Hemisphere (where ecosystems are generally more com- Data collection plex) suggests that other factors (e.g., disease) are also important in shaping the population cycles of prey animals Annual changes in lion numbers (Pech et al. 1992). While many of these studies have used an experimental approach, with invertebrate or small vertebrate The lions were located daily and thus the total number of models, it is the large mammalian predators that may be animals was known. The total number of lions is expressed expected to place the greatest pressure on their prey popula- in Female Equivalent Units (FEQs), where 1FEQ is equiv- tions because of their high daily energy requirements (Wil- alent to the mass of an adult female (Bertram 1973). An liams et al. 2004). However, comparatively little data exists adult male is 1.5FEQs, subadult lions 1FEQ, large cubs for Africa and the southern hemisphere in general (Viljoen (1–2 years) 0.75FEQ and small cubs (less than one year 1993; Höner et al. 2002; Owen-Smith and Mills 2008). This is old) 0.3FEQ (Schaller 1972; van Orsdol 1982; Packer et al. because direct experimentation using large mammalian pred- 1990). We calculated lion FEQs in July and December of ators is difficult and data can typically only be collected each year and used the average value in our analyses. through careful, long-term observation of natural systems (Estes 1994; Radloff and du Toit 2004; Owen-Smith and Mills Prey abundance 2008; Randa et al. 2009). The reintroduction of lions to enclosed conservation areas Annual, helicopter-based game counts were used to estimate in the Eastern Cape Province of South Africa and elsewhere the abundance of all prey species. Aerial counts were con- has created a range of conditions which we have used to study ducted over two to three consecutive days (weather depen- aspects of the feeding behavior of predators including the dent) in July/August of each year (i.e., the period after most response of predators to changes in prey abundance. At one ungulates had given birth) and covered the entire reserve of the sites of lion reintroduction (Kwandwe Private Game (Reilly and Emslie 1998; Reilly 2002; Bothma and du Toit Reserve), two prey species (kudu Tragelaphus strepsiceros 2010). and warthog Phacochoerus africanus) from a total of 21 species killed, comprised more than 55% of all kills each year Carnivore diet (Bissett 2007). No other prey species comprised more than 10% of kills in any year. On Kwandwe, the numbers of We collected data for three small prides of lions (1–4 adults kudu and warthogs changed substantially over a rela- and 0–8 juveniles per pride) at Kwandwe between 2003 and tively short period, and here, we use data from this site 2007. At least one member of each pride was equipped with to examine the response of lions to the changes in the either a very high frequency (VHF) radio collar or an abundance of two principal prey species. We hypothe- implanted VHF transmitter (Africa Wildlife Tracking, Riet- sized that as the abundance of the primary prey species fontein, Gauteng, South Africa) which incorporated (i.e., kudu) declined, so significantly more alternative Telonics high-power transmitters (Telonics, Mesa, AZ, prey (i.e., warthog) would be consumed. In view of USA). All animals were located by radiotelemetry using a the reported relationship between rainfall, prey vulnera- Telonics TR-4 receiver and Telonics RA-2A directional bility and lion diet (Owen-Smith and Mills 2008), we antenna. All capture and immobilization was done by a also investigated the effect of rainfall. qualified veterinarian. Radio collars were not removed at the end of this project to allow ongoing studies of the feeding and spatial ecologies of the animals. Information Methods on the diet was collected in three ways: opportunistic (ad hoc) observations of kills made during the daily location of Study sites the lions; kills recorded during six intensive, 2-week-long periods of continuous observations (Bissett 2007), and kills The study was carried out between 2003 and 2007 on recorded by field guides on their daily game drives, which is Kwandwe Private Game Reserve (Kwandwe; 185 km2; ca. considered a viable method for monitoring the diet of Author's personal copy
Acta Theriol (2012) 57:225–231 227 carnivores (Radloff and Du Toit 2004; Bissett and Bernard Results 2007). For all kills, the identity of the predator, the date, species, sex, and, where possible, age (juvenile, subadult, Annual changes in the abundance of lions, kudu, adult) of the prey were recorded. The corrected mass of each and warthogs and changes in rainfall kill was estimated by accounting for the sex and age of the animal (Radloff and du Toit 2004). Adult body mass was The number of lion FEQs increased through the study taken from Meissner (1982), Bothma (2002), and Skinner (Table 1), and there was a significant correlation between and Chimimba (2005). The mean prey mass for the lions year and FEQs (r00.90; n05; P<0.05). There was no was calculated using the corrected mass of every kill. change in the abundance of subadult lions over the study period (Table 1; r00.44; n05; P>0.05). The abundance of Analysis of prey preference kudu and warthogs changed substantially during the study period (Table 1; Fig. 1A), while the abundance of all prey Jacobs’ index, D) (Jacobs 1974) was calculated for each species killed by lions declined by about 8% (Table 1). The prey species as follows: number of kudu declined by 23% from 2003 to 2005 and r p then increased slightly to 2007. Over the 5 years, the decline D ¼ was not statistically significant (r0-0.60; n05; P>0.05). r þ p 2rp Over the same period, the number of warthogs increased where r is the number of kills of a prey species as a significantly (Fig. 1A; r00.98; n05; P<0.05). The relative proportion of kills made by lions, and p is the proportional abundance of warthogs (Nwarthog/Nkudu) increased signifi- availability of the prey species. Proportional availability was cantly through the study (Table 1; r00.99; n05; P<0.05). based on data from annual game counts and was the number Rainfall varied throughout the study (Fig. 1A)and of a particular species as a proportion of the total number of was above the 10-year average of 410 mm in 2002 and all species preyed upon by the lions. Hayward et al. (2007b) 2006 and below average in the other years. There was further derived an equation for predicting the number of no significant linear trend in rainfall over time (r00.40; kills of a particular species: n05; P>0.05). X Dipi þ pi Ri ¼ K Annual changes in the diet of lions 1 Di þ 2Dipi where Ri is the predicted number of kills of species i Kudu comprised 42% of all lion kills recorded at the begin- when a total of ∑K kills are observed, Di represents the ning of the study, and from 2003 to 2007, there was a Jacob’s index value of species i,andpi represents the significant decrease in kudu kills (Table 1;Fig.1B; proportional abundance of prey species i at a site (Hayward et r0-0.96; n05; P<0.05). Over the same period, there was al. 2007b; Meena et al. 2011). We used this equation to a significant increase in warthog kills (Table 1;Fig.1B; calculate the predicted number of warthog and kudu kills r00.99; n05; P<0.005). Although the observed numb- in each year. ers of warthogs and kudu killed were similar to the predicted values in 2003 and 2004, there were substan- Data analyses tial differences between observed and predicted kills from 2005 to 2007 (Table 1). Significantly fewer kudu Dependent variables were not normally distributed. Thus, were killed between 2003 and 2007 than predicted Spearman’s rank correlations were used to analyze annual (Table 1; χ2036.6; df04; P<0.0001), and significantly more changes in the abundance of lion FEQs, in the numbers of warthogs than predicted were killed over the same period kudu and warthogs, in the relative abundance of warthogs (Table 1; χ2021.4; df04; P<0.0001).
(Nwarthog/Nkudu), in kills of warthogs and kudu as a propor- The switch from kudu to warthog was further reflected in tion of all kills in that year, and in mean annual prey mass. a change in the mean prey mass (Table 2) which decreased Kills were expressed as a proportion of all kills in that year significantly through the study (r0-0.98; n05; P<0.005). to overcome any bias introduced by changes in annual observation effort and total numbers of kills recorded. Chi- The effect of changing prey base on the prey preference square tests were used to compare the observed and pre- of lions dicted kills for warthogs and kudu over the study period. The relationship between total annual rainfall and time was At low relative warthog abundance (0.3–0.50two to three tested using a Spearman’s rank correlation. All statistical times as many kudu as warthogs), the preference of lions for analyses were completed using Statistica (StatSoft, Tulsa, kudu and warthog was similar and stable (D00.2; Fig. 2). OK, USA). As the relative abundance of warthogs increased from 0.5, Author's personal copy
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Table 1 Annual changes in the abundance of lions, warthogs, 2003 2004 2005 2006 2007 and kudu and the number of warthogs and kudu killed by Lion FEQs 8.8 7.2 9.8 11.8 13.7 lions at Kwandwe Private Game Sub-adult lions 01003 Reserve. The total abundance is Kudu 1,602 1,422 1,239 1,314 1,388 for all the prey species consumed by lions and total Warthog 559 731 835 1,141 1,447 observed kills is the total of all Total abundance 4,902 4,311 3,634 4,055 4,476 kills made by lions in that year. Relative abundance (Nwarthog/Nkudu) 0.34 0.51 0.67 0.86 1.04 Predicted warthog and kudu kills Predicted 25 19 39 39 38 were calculated using the equation derived by Observed 27 20 23 17 13 Hayward et al. (2007b) Predicted 8 10 26 33 39 Observed 10 11 34 47 61 Total observed kills 64 49 95 101 104
Jacobs’ index for kudu declined to a negative value indicat- Discussion ing an avoidance of kudu as prey (r0- 0.93; n05; P<0.05). By contrast, the Jacobs’ index for warthogs was greater than The results suggest that as warthog numbers increased, the zero at all times (Fig. 2), and as the relative abundance of lions killed more warthogs, in absolute and relative terms, warthogs increased above 0.6 so did the Jacobs’ index and this was matched by a decline in the number of kudu increase (r00.94; n05; P<0.05). killed. The increase in the proportion of warthogs killed
Fig. 1 Annual changes in total 1800 1800 rainfall (grey bars) and A numbers of kudu (solid circles) 1600 1600 and warthogs (open circles) (A), and kudu and warthog kills 1400 1400 as a proportion of all kills (B) 1200 1200
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800 800 Rainfall (mm) 600 600 Number of animals
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0.4 0.4 kudu kills Warthog kills
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0.1 0.1 2002 2003 2004 2005 2006 2007 Years Author's personal copy
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Table 2 Annual changes in the mean prey mass (kg±SD) of all Years kills made by lions and in the relative abundance of warthogs 2003 2004 2005 2006 2007 on Kwandwe Private Game Reserve Mean prey mass 182±169.6 150±91.9 141±136.7 130±135.2 108±75.5 Relative abundance of warthogs 0.34 0.51 0.67 0.86 1.04 resulted in an increase in prey preference for warthogs and a changes in the abundance or vulnerability of prey species. decline in preference for kudu that was driven primarily by In the Chobe National Park, Botswana, large prey (buffalo the change in warthog numbers. Syncerus caffer and zebra Equus burchelli) comprise the The development of Kwandwe as an ecotourism reserve majority of lion diet but, in the dry season, when they began in 1999. Prior to this, most of the indigenous large migrate out of the area, warthogs become an important prey mammalian fauna had been locally extirpated and surviving, species (Viljoen 1993, 1997). Similarly, in the Ngorongoro resident populations of kudu and warthog were heavily Crater, Tanzania, spotted hyaenas (Crocuta crocuta) includ- hunted. Between 1999 and 2001, over 2,000 ungulates were ed a significantly greater proportion of buffalo in their diet reintroduced, but this did not include kudu or warthogs. In after a significant increase in buffalo numbers (Höner et al. October 2001, two adult male and two adult female lions 2002). This was not only a response to an increase in the were introduced, and in 2002, the first cubs were born abundance of buffalo but also to an increase in the abun- (Hayward et al. 2007a). In addition, reintroductions of other dance of more vulnerable juvenile animals (Höner et al. large carnivores (cheetahs Acinonyx jubatus and African 2002). In the Kruger National Park (KNP), selection for wild dogs Lycaon pictus) occurred. In the early years after alternative prey by lions, including warthogs, is affected reintroduction, the predators all killed kudu (Bissett 2007) by changes in the relative abundance and vulnerability of while only the lions killed small numbers of warthogs. We the three principal prey species (wildebeest Connochaetes suggest that a combination of the initial release from hunting taurinus, zebra and buffalo; Owen-Smith and Mills 2008). pressure and the subsequent predation pressure on kudu and, Selection for buffalo increased after a severe drought which to a much lesser extent, on warthogs resulted in the increase increased their vulnerability, while wildebeest and zebra in warthog numbers reported in this study. On another appeared to be less susceptible to predation under conditions enclosed reserve in South Africa, the impala (Aepyceros of low rainfall (Owen-Smith and Mills 2008). The latter melampus) population responded to the reintroduction of examples clearly illustrate how changes in predator diet lions by increasing in a way that was similar to the response are not simply driven by changes in prey numbers and that of warthogs on Kwandwe (Power 2002). changes in vulnerability, which may result from climatic Dietary flexibility and prey switching have been reported variability or be a characteristic of a particular age or sex, previously for large carnivores and may be driven by play an important role in shaping diet. Interestingly, a recent
Fig. 2 The relationship 0.6 between the relative abundance of warthogs and Jacob’s index for warthogs (open circles) and 0.4 kudu (solid circles)
0.2
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-0.6 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Relative abundance of warthogs Author's personal copy
230 Acta Theriol (2012) 57:225–231 meta-analysis by Hayward (2011) suggested that the lions of abundance (Hunter 1998) and that prey availability has no sub-Saharan Africa tend to select their preferred prey less significant effect on the diet of lions (Hayward and Kerley frequently when the abundance of these species increases. By 2005). However, if lions are opportunistic predators (Schaller contrast, when nonpreferred prey species become more abun- 1972), then it should be expected that their diet will be dant, they are preyed upon more regularly by lions (Hayward influenced by prey abundance. 2011). These findings support those of the present study where True prey switching is likely to promote the persistence the diet shift was due to an increase in the abundance of a of predator–prey systems (van Baalen et al. 2001) as pred- preferred but secondary prey species, the warthog. ator pressure on a declining prey species will decrease, Changes in the diet of large carnivores, as reported in the providing the opportunity for that species to recover present and previous studies, need not be associated with a (O’Donoghue et al. 1998). This will be particularly important change in prey preference. In the present study, the Jacobs’ on relatively small fenced conservation areas where opportu- index for kudu was positive when kudu were relatively nities to escape predation are few and population renewal abundant and negative when they were less common thus through immigration is not possible. Thus, an understanding meeting the criteria for prey switching (Murdoch 1969; of the factors that influence prey selection is important for the Garrott et al. 2007). The results for warthogs were ambigu- management of enclosed conservation areas. However, a full ous in that the Jacobs’ index was always positive but the understanding of the dynamics of multipredator, multiprey pattern, where preference increased with increasing relative systems, such as those that characterize the African savanna, abundance, again supports the occurrence of a prey shift. is dependent on the careful analysis of data from long-term Similar trends were observed for spotted hyaenas in the observational studies. Ngorongoro Crater, Tanzania, where preference for adult buffaloes remained the same despite a significant increase in relative abundance (Höner et al. 2002). Acknowledgments Our sincere thanks go to Angus Sholto-Douglas of Kwandwe Private Game Reserve for providing logistical and finan- There are three possible explanations for the dietary shift cial support for this project. We acknowledge the staff of Kwandwe for observed here. Firstly, it is possible that as the lion population providing assistance and information. All experiments complied with increased during the study, there was an increase in the num- the ethical protocols of Rhodes University and the laws of South ber of subadult lions not yet capable of hunting larger prey Africa. Matt Hayward and two anonymous referees are thanked for constructive comments on earlier drafts of the manuscript. such as kudu (a pride demography response; Hayward et al. 2007b). However, our results indicate that there was no sig- nificant change in subadult lion numbers during the study period, making this explanation unlikely. Secondly, one or References more of the adult lions could have learned to effectively hunt warthogs, allowing the rest of the pride(s) to acquire the Begg CB, Begg KS, du Toit JT, Mills MGL (2003) Sexual and technique, and ultimately resulting in an increase in the num- seasonal variation in the diet and foraging behaviour of a sexually ber of warthogs killed (a learned response). Thirdly, changes dimorphic carnivore, the honey badger (Mellivora capensis). J – in the relative abundance and vulnerability of warthogs and Zool 260:301 316 Bertram BCR (1973) Lion population regulation. E Afr Wildl J kudu may have influenced lion diet (prey density response). 11:215–225 We propose that the mechanism driving the suggested dietary Bissett C (2007) The feeding and spatial ecologies of the large carni- shift was probably a combination of the prey density and vore guild on Kwandwe Private Game Reserve. 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