The transformation of natural landscapes by human activities (Martínez-Fernández et al. 2015) has modified over half of the Earth’s land surface (Foley et al. 2005; Hooke et al. 2012). Land use and land cover change (LULCC), including extensive clearing, the creation of pastures, croplands and urban settlements (Fahrig 2003; McKinney 2002) are the main drivers impacting ecosystems and causing drastic global and regional biodiversity collapse (Ellis et al. 2021; Foley et al. 2005; Sun et al. 2021; Turner et al. 2007; Zhang et al. 2007). For certain species, these transformed landscapes can create new niches and provide wild animals with other resources (Dale and Polasky, 2007; Eriksson 2013; Fleming and Bateman 2018).
In anthropized landscapes, the possible access to cultivated zones, gardens, kitchens and garbage has provided new opportunities for wildlife (Hulme-Beaman et al. 2016; Oro et al. 2013). Certain species have adapted or acclimated to these transformed areas, exploiting anthropogenic resources (Hulme-Beaman et al. 2016; Seoraj-Pillai and Pillay 2016; Webber 2017). For instance, wild boar (Sus scrofa) and vervet monkeys (Cercopithecus aethiops) are known to exploit croplands (Cancelliere et al. 2018; Lee and Lee 2019), carnivores such as San Joaquin kit foxes (Vulpes macrotis mutica) or coyotes (Canis latrans) exploit urban resources (e.g. garbage or compost; Newsome et al. 2010; Sugden et al. 2021) and commensal behaviour, notably as household and agricultural pests, has been observed in black rats (Rattus rattus) across different geographical areas on several occasions (Aplin et al. 2011). These changes cause direct modifications in wildlife diet, from the consumption of purely natural resources to the integration of more human-originated food (Murray et al. 2015). This can be a benefit to species, providing year-round food provisioning, reducing food and water deprivation during droughts or winters, or generally contributing to better individual condition. For example, a population of urban kit foxes presented higher cholesterol levels, a good indicator of energy intake (Harder and Kirk-patrick 1994), as well as reduced nutritional deprivation than rural ones in California (Cypher and Frost 1999). The integration of human resources into the diet of storks during periods of constrained resource availability has also been shown to lead to higher reproduction rates (Evans and Gawlik 2020). As another example, crop raiding behaviour in male elephants resulted in larger body size in adulthood (Chiyo et al. 2011), an important trait for this species during mating (Chelliah and Sukumar 2013). Despite certain benefits however, the consumption of human food has also been shown to have a negative impact on wildlife health. For instance, urban coyotes generally showed poorer average body condition as well as higher parasitic infection rates than rural coyotes in a study by Sugden et al. (2020). Reduced reproductive rates of birds (e.g. blue tits) are also associated with urban areas compared to populations in rural zones, linked to a lack of suitable diets for nestlings (Pollock et al. 2017; Seress et al. 2020).
The impacts of human resources on non-human primates appear to a lesser extent in the literature, even though primates often consume human resources (Strum 2010). It has been shown that wild primates obtain higher levels of essential nutrients relative to body weight than humans do through their diets (Milton 1999), but also that varying benefits can follow their consumption of anthropic food resources depending on the type of LULCC (Maibeche et al. 2015; Marty et al. 2020). An interesting primate species to better understand diet change in the presence of human resources is the chacma baboon (Papio ursinus), due to its adaptive capacity (Fischer et al. 2019). Baboons are opportunistic, omnivorous feeders that will eat anything from fruits, grasses, leaves or roots to invertebrates and even occasionally animal matter usually resulting from opportunistic hunting (Allan et al. 2022; Hoffman and O’Riain 2011; Schreier et al. 2019) and are highly adaptable to a wide range of habitats and environments (Johnson et al. 2015). They can be found across deserts, savannahs, forests and on the outskirts or within urban zones (Hoffman and O’Riain 2011, 2012; Johnson et al. 2015; Mormile and Hill 2017). With the encroachment of urban areas on natural habitats, encounters between humans and baboons have become prevalent in some areas, such as in the Western Cape of South Africa (Bracken et al. 2022a, b, 2023; Chowdhury et al. 2020; Lee and Priston 2005). In the Western Cape, as well as across South Africa, chacma baboons frequently raid croplands as well as people’s gardens, bins or houses (Hoffman and O’Riain 2011; Strum 2010; Walton et al. 2021), consuming resources of human origin. When the availability of rubbish is reduced, this affects the baboon’s raiding tendencies: Mazué et al. (2023) showed that when the rubbish resources were removed on regular foraging sites in a peri-urban area on the George Campus of the Nelson Mandela University (NMU), chacma baboons spent more time foraging for natural resources and less time in these more urban areas. The easy access to fruits, vegetables, meats, but also processed foods, refined sugars, added fats and oils may result in significant variations in their nutrient consumption and have unknown effects on their metabolisms. In this context, the objective of this present study was to evaluate whether human land use, characterized by high human modified food availability, has an impact on chacma baboon diet.
Several methods exist to study diets in ecology, including direct observation or fecal analysis. These techniques yield accurate results but they can vary according to what is consumed and the digestibility of the latter (Matthews et al. 2020): some resources will appear in higher proportions in fecal samples than others depending on how easy they are to digest. Direct observation also often requires long periods of time (Jordan 2005), whereas non-invasive methods based on fecal samples, such as DNA metabarcoding and/or isotopic analyses, can counter this problem (Ando et al. 2020; Crowley et al. 2016; De Barba et al. 2014; O’Brien 2015). In the case of metabarcoding however, sample analysis can be costly and a broad list of plant/animal DNA sequences that could be consumed in the region would be necessary (Taberlet et al. 2018). Analysing isotopic ratios is therefore an interesting alternative, both less expensive and less time-consuming than other options, which explains why it is a recurrent method used in feeding ecology studies (Boecklen et al. 2011; Martínez del Rio et al. 2009; Taki et al. 2017). Common isotopes used for diet studies include nitrogen (15N/14N) and carbon (13C/12C). Nitrogen isotopes are a potential biomarker of protein source because tissue nitrogen derives almost entirely from dietary protein (for a review of isotopes as biomarkers see O’Brien 2015). Due to a stepwise increase with the trophic chain, variations of the δ15N isotopic ratios observed within feces may reflect consumption of dietary items with higher protein content within the different LULCC based on what resources the baboons have access to. The carbon in plants derives from atmospheric CO2 which is fixed during the process of photosynthesis. Differences in stable carbon isotope ratios therefore reflect the type of plants consumed based on their photosynthetic pathways (C3, C4 or CAM) due to differential fractionation of carbon isotopes during photosynthesis that leads to distinct δ13C values (Ehleringer and Cerling 2002; Smith and Epsten 1971). Examining carbon and nitrogen ratios may therefore be a suitable method of assessing the potential contribution of human-derived foods to animal diet, comparing protein intake and the type of plants consumed between natural areas and those with anthropic resources (Penick et al. 2015).
To address the objective of this study, we obtained stable isotope ratios from fecal samples of chacma baboons, which would provide quantitative data on the types of plants and animal protein they consume. Baboons may be considered as opportunistic feeders (Allan et al. 2022; Schreier et al. 2019) and therefore the potential access to high protein sources such as meat and legumes in more anthropized areas (with crops, garbage or kitchens) may result in higher proportions of protein in their diet. We could expect that if the anthropized landscapes provide higher protein contents than the natural resources consumed by chacma baboons, the stable nitrogen isotope values would tend to be higher in these transformed landscapes. More positive δ13C values for samples from anthropized areas may be associated with a higher consumption of C4 plants that pervade human-derived foods (e.g. maize, sugarcane) as well as lipids when these types of resources are included in chacma baboon diets (Post et al. 2007). We could then expect to observe more positive δ13C ratios from samples collected in more urbanized environments compared to natural landscapes. To evaluate whether transformed landscapes have an influence on chacma baboon diet, we collected fecal samples from chacma baboons along a gradient of land use in the Garden Route, South Africa, and studied stable isotopic ratio data, incorporating the surrounding landscape characteristics.