African Savannas Introduction
Although my dissertation chapters will each be published separately, I wanted to share the Introduction I wrote for my thesis here.
My thesis title was: "The Impacts of Climate Change and Veterinary Fencing on Savanna Ungulate Populations, Communities, and Behaviors"
Although my dissertation chapters will each be published separately, I wanted to share the Introduction I wrote for my thesis here.
My thesis title was: "The Impacts of Climate Change and Veterinary Fencing on Savanna Ungulate Populations, Communities, and Behaviors"
There is a crucial need to establish relationships between herbivore movements and their changing environments, especially in Africa where most of the world's large herbivore diversity resides. . . . Without [this knowledge], we are likely to see a progressive loss of this legacy, as protected areas lose their effectiveness under shifts in climate, atmospheric carbon dioxide, and human activities plus infrastructure. For the ~2.5 million years since the genus Homo evolved in Africa, the fates of savanna biomes, large herbivores, and Earth's climate have been intertwined with human activity. One of the largest consequences of this relationship were the near-synchronous late Pleistocene extinctions (~50,000–10,000 years before present) of 65% of large mammal genera across the globe (Barnosky et al 2004), driven largely by human hunting and exacerbated in some regions by climatic changes (Faith et al 2009, Koch et al 2006, Sandom et al 2014). Today, the spread of urbanization, agriculture, and organized poaching, as well as the intensifying burden of anthropogenic climate change on the biosphere, threaten many of the planet's remaining large herbivore species. Despite vulnerability to these threats and African savannas' high biodiversity, large African herbivores are understudied compared to their North American, European, and Asian counterparts. The chapters that follow explore how changing climate, landscapes, and human infrastructure affect the movements and behaviors of large African savanna herbivores. 1 African savannas The history and present of large African mammals, including modern humans (Homo sapiens sapiens), are inextricable from the formation of the savanna biome. Savannas are defined by a unique codominance of trees and grasses (<50% tree cover), and form in regions where feedbacks between fire, herbivory, and rainfall prevent tree growth from dominating the herbaceous layer (Scheiter and Higgins 2009, Staver et al. 2011). Savannas are considered disturbance-driven ecosystems, where periodic consumption of the vegetative layer, either through burning or herbivory, maintains an otherwise unstable ecosystem state. The dominance of savannas and grasslands that we see today (over 33% of the Earth’s land surface) was made possible in the late Miocene (about 8 Ma) by the almost- synchronous convergent evolution of the C4 photosynthetic pathway in grasses (Beerling and Osborne 2006, Osborne and Beerling 2006). A major weakness in ancestral C3 photosynthesis occurs when a key photosynthetic enzyme, RuBisCO, reacts with oxygen instead of carbon dioxide at the leaf’s surface. The C4 pathway physically sepa- rates the carbon-gain and sugar-production steps of photosynthesis, moving RuBisCO deep within the leaf where it can be saturated with carbon transported from the leaf’s surface in four-C chains (hence, "C4", Osborne and Beerling 2006). This avoidance of photorespiratory waste provided an energetic advantage for C4 grasses in the low-CO2 and high-aridity environments of the late Miocene (Osborne and Beerling 2006). The spread of C4 grasses enhanced the fire regime of early savannas, as grasses build up more flammable fuel in the wet season. In addition, full forests can encourage their own microclimate of high rainfall to form; replacing forests with savannas therefore interrupted this high-rainfall feedback and allowed understory grasses to flourish (Beerling and Osborne 2006). Due to their disturbance-driven structure and highly seasonal climate variation, savannas have a highly irregular distribution of nutrients and other resources. This landscape heterogeneity of African savannas, through an abundance of ecosystem types and interfaces within small areas, promotes the coexistence of a high diversity of flora and fauna (du Toit 2003). Large herbivores are particularly well-supported by landscape heterogeneity, as their size often demands larger ranges that cover a greater diversity of vegetative biomass (Katayama et al. 2014). This effect of heterogeneity crosses many scales. At the feeding patch scale, variation in leaf to stem ratios or plant height can result in spatial separation of herbivores by diet; therefore, vegetatively diverse patches often support more diverse assemblages of ungulates (du Toit 2003). And, at the habitat scale, a mosaic of habitats allows for more opportunities for speciation to occur, as terrestrial mammals are often habitat specific (du Toit 2003, Vrba 1992). These features of the savanna biome have led to its support, in Africa, of 90% of the world’s diversity in large herbivores (Owen-Smith et al. 2020). 2 Herbivores and their movements As with the mass mammalian extinctions of the late Pleistocene, large mammals today (n = 74 with mean adult body mass >100kg) are disproportionately threatened: 60% of the world’s large herbivores are currently threatened with extinction, compared to 27% of all mammals (IUCN 2023, Ripple et al. 2015). Large mammals are more susceptible to hunting (Venter et al. 2020). Range area generally increases with body size (Harestad and Bunnel 1979, Reiss 1988, Tucker et al. 2014) and, with human encroachment on wild spaces accelerating each year, the space available to free-ranging mammals shrinks. Finally, large mammals are more likely than small mammals to pose a threat to human lives and livelihoods, with instances of wildlife-human conflict increasing with body size (Mukeka et al. 2019). African large mammals in particular are over-threatened and under-studied. 50% of the largest African mammals (17 of 32 species) have decreasing populations or are extinct in the wild (IUCN 2023, Ripple et al. 2015). Despite these population trends and the high biodiversity of large herbivores in Africa, the amount of research in the region (245 mean articles per species) falls far behind that in North America (1,354), Europe (1,045), and Asia (1,183). Additionally, most African research focuses on only four species: African savanna elephant (Loxodonta africana), Cape buffalo (Syncerus caffer), white rhinoceros (Ceratotherium simum) and black rhinoceros (Diceros bicornis); Ripple et al. 2015). The movement of free-ranging African herbivores across the landscape is critical both to herbivore survival and to the maintenance of savanna functioning. African herbivores are often called ecosystem engineers, as their movements significantly shape savanna productivity (Geremia et al. 2019, Holdo et al. 2007), structure (Bakker et al. 2016, Holdo et al. 2009, O’Connor et al. 2020, Staver et al. 2009), and nutrient cycling (McNaughton 1976, Melis et al. 2007, Subalusky et al. 2017). Long-distance migrations allow large herbivores to follow the flush of fresh plant matter that follows the onset of the rainy season (Bischof et al. 2012, Geremia et al. 2019, McNaughton 1976), while local herbivore movements respond to acute climatic conditions such as a higher heat index or local droughts (Boyers et al. 2019). 3 Human-caused changes for large herbivores In the late Pleistocene to early Holocene (50,000-10,000 years before present), a combination of human over-hunting, climate shifts, and the cascading effects of large herbivore loss brought about the extinction of over 60% of then-extant mammalian species (Barnosky et al. 2004, Koch and Barnosky 2006, Prescott et al. 2012). Megafauna (animals with average adult body mass >1,000kg) were completely extirpated in North America, Australia, and Europe, with only a few species of elephant, rhinoceros, hippopotamus, and giraffe remaining in Africa and southern Asia (Owen-Smith 1987). Human impacts on herbivores in the Anthropocene adds climate and landscape change to the extractive declines of the Pleistocene. Anthropogenic climate change will cause southern Africa to warm and dry faster than other subtropical biomes (Engelbrecht et al. 2015). Increasing local temperatures will have differential effects on mammalian species depending on their size and thermoregulatory strategy, as larger mammals have more difficulty shedding heat (Bell 1971, Jarman 1974, Owen-Smith 1989). In addition to climatic changes, human infrastructure dominates the African landscape. As human populations in Africa are expected to reach 2 billion by 2040 (FAO 2023), these features will only increase in density. Veterinary fencing is the largest and fastest-growing linear feature on the Earth’s surface (Jakes et al. 2018), and its fragementation of the landscape can have outsized effects on large mammals that need access to large, heterogeneous landscapes. Landscape fragmentation can lower biodiversity (Fahrig 2003) as fragmented landscapes support fewer species (He and Legendre 1996, McIntyre 1995) and increase extinction rates (Wilcox and Murphy 1985). In addition, the physical barriers creating this fragmentation can prevent herbivore seasonal migration into critical habitats (Kauffman et al. 2021). Finally, the aridization and heating of African savannas will make water access scarcer, yet ever more important, for large African mammals. Ephemeral surface water sources in southern Africa may dry more quickly (Bates et al. 2008, Nkemelang et al. 2018), leading to a greater dependence on permanent rivers and human-supplied waterholes. This aggregation around scarce water may increase risks associated with higher herbivore density, such as competition for forage access (Owen-Smith 1998) and increased predation (Cozzi et al. 2012, Fuller 2016). 4 Dissertation Chapter Summaries Chapter 1 combined thirty years of aerial ungulate census data from Kruger NP for the first time, describing long-term population trends in fourteen species and applying a Bayesian probabilistic joint species distribution model. The GJAM analysis found that, while individual species traits had little impact on their population responses to drought, rare antelope responded similarly to environmental parameters and their enhancement by drought incidence. Particularly, rare antelope populations increased far from waterholes during droughts, and were less sensitive to river locations than other ungulates. This finding challenges black-and-white opinions from the 1960s that artificial waterholes were universally good for rare populations, and from the 1990s that artificial waterholes were solely responsible for species declines. Instead, rare antelope seem to depend on waterholes during drought; perhaps waterhole provision in moderation is a balanced approach to rare antelope management. In addition, rare antelope can be further subdivided by their sensitivities to environmental change: roan and eland are generally more sensitive than eland and sable to changes in surface water and soil substrate. Chapters 2 and 3 investigated how climate change and human infrastructure intersect with antelope thermoregulatory strategies to alter movement behaviors. Chapter 2 found that antelope thermoregulatory strategies (reliance on surface water, cooler microclimates, and changing activity budgets) was stronger at higher temperatures and in the dry season. This shows that, while ungulate behavior is driven by many other factors (e.g., dietary needs and predation risk) that may change with a warming climate, thermoregulation is still important in determining their movement patterns. Chapter 3 found that roan, which greatly expand their ranges in the wet season, frequently encounter and cross the eastern boundary fence, while gemsbok, which maintain small, steady ranges year-round, do not cross the fence. These findings were unique among recent fence-crossing and encounter literature, breaking the neatly drawn lines between migration status, sex, and group size with fence crossing incidences. References
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