Cascading Effects of Elephant-Human Interactions in a Savanna
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bioRxiv preprint doi: https://doi.org/10.1101/2021.08.18.456886; this version posted August 18, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 1 Cascading effects of elephant-human interactions in a savanna 2 ecosystem and the implications for ecology and conservation 3 4 5 6 David Western1¶ and Victor N. Mose1¶* 7 8 9 10 1African Conservation Centre, Amboseli Conservation Program, Nairobi, Kenya. 11 12 13 14 *Corresponding author 15 Email: [email protected] (VNM) 16 17 18 ¶These authors contributed equally to this work. 19 20 21 22 23 24 25 1 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.18.456886; this version posted August 18, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 26 Abstract 27 28 Our study monitored the changes in elephant numbers, distribution and ecological impact 29 over a fifty-year period as the free-ranging intermingled movements of wildlife and 30 traditional subsistence pastoralists across the Amboseli ecosystem were disrupted by a 31 national park, livestock ranches, farms, settlements and changing lifestyles and 32 economies. 33 34 Elephants compressed into the national park by poaching and settlement turned 35 woodlands to grassland and shrublands and swamps into short grazing lawns, causing a 36 reduction of plant and herbivore diversity and resilience to extreme events. The results 37 echo the ecological findings of high-density elephant populations in protected areas 38 across eastern and southern Africa. The impact has led to the view of elephants in parks 39 being incompatible with biodiversity and to population control measures. 40 41 In contrast to Amboseli National Park, we found woody vegetation grew and plant 42 diversity fell in areas abandoned by elephants. We therefore used naturalist and 43 exclosure experiments to determine the density-dependent response of vegetation to 44 elephants. We found plant richness to peak at the park boundary where elephants and 45 livestock jostled spatially, setting up a creative browsing-grazing tension and a patchwork 46 of habitats explaining the plant richness. 47 2 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.18.456886; this version posted August 18, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 48 A review of prehistorical and historical literature lends support to the Amboseli findings 49 that elephants and people, the two dominant keystone species in the savannas, have 50 been intimately entangled prior to the global ivory trade and colonialism. The findings 51 point to the inadequacy of parks for conserving mega herbivores and as ecological 52 baselines. 53 54 The Amboseli study underlines the significance of space and mobility in expressing the 55 keystone role of elephants, and to community-based conservation as a way to win space 56 and mobility for elephants, alleviate the ecological disruption of compressed populations 57 and minimize population management. 58 59 Introduction 60 61 After decades of physical forces being viewed as bottom-up drivers of ecosystem 62 processes (1–3) the role carnivores and herbivores in governing community dynamics 63 from the top-down has also been well established in both ecological theory (4–8) and 64 conservation policy and management (9–11). The extermination, reintroduction, 65 displacement and compression of keystone large herbivores and carnivores can all have 66 long-term repercussions on ecosystems and landscapes (12,13) 67 68 Given the growing impact humans have on wildlife (14), population disruptions offer 69 naturalistic experiments for testing the role of megamammals as keystone species 70 precipitating trophic cascade through ecosystems (7). The impact of megafaunal 3 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.18.456886; this version posted August 18, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 71 disruptions will, however, differ between biogeographic regions depending on their 72 histories. The heavily depleted faunal assemblages of North America during the 73 Pleistocene (15) and Madagascar during the Holocene (16), for instance, will respond 74 differently to East Africa’s megafauna which has remained relatively intact since the late 75 Pleistocene (10,17). Whereas the reintroduction of the wolf into Yellowstone’s 76 Pleistocene-depleted predator fauna caused large ripple effects on the plant and animal 77 community (11), the recolonization of wild dogs on the Laikipia Plateau in Kenya (18) in 78 contrast, caused little discernable impact due to the present of other large predators, 79 including lions and hyenas. How much greater then would be impact of reintroducing 80 megaherbivores such as elephants and rhinos into North America biomes after 11,000 81 years of faunal depletion and weakened plant herbivore defenses than the reintroduction 82 of the wolf into Yellowstone? 83 84 Africa offers an indication of the far greater biotic impact of large herbivores than 85 carnivores on ecosystems. The ecological impact of the continent’s suite of 86 megaherbivores on biomes ranging from forests to savanna has been well-documented 87 (19–22). Most studies have, however, been conducted in protected areas where the 88 impact of elephants especially has been intensified by human activity causing 89 compressed populations and curtailed seasonal movements. Wall et al. (2021) showed 90 the distribution of elephants to occupy only 17% of their potential range due to the level 91 of the human disturbance and safety in protected areas. Normalized for rainfall, censuses 92 of 34 populations showed elephant densities in East Africa to be five-times higher in 93 protected than non-protected areas (24). 4 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.18.456886; this version posted August 18, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 94 Discussions on the keystone role of elephants have centered on their impact on woody 95 vegetation and whether the findings support equilibrium, non-equilibrium or alternative 96 multi-state theories of ecosystem dynamics (25–27). Here again, using protected areas 97 as templates exaggerates the deleterious impact of elephants on biodiversity and 98 obscures the keystone role they play in free-ranging interactions with humans. The lack 99 of such studies prior to the establishment of protected areas masks the complex and 100 shifting interplay which likely shaped African ecosystems for millennia (28). Prehistorical 101 and historical reviews of the interlinked ecologies help reveal the role elephants play in 102 ecosystems when free-ranging rather than in compressed and fragmented populations. 103 104 Elephant-human interactions 105 106 Paleo records and historical accounts point to the ancient links and changing relationship 107 between hominins and elephants since the Lower and Middle Stone Age. The earliest 108 evidence of human interactions with elephants comes from the fossil bones in 109 Olorgesailie formation 150 km east of Amboseli in the Rift Valley. Here at 1 million BP, 110 the fossilized remains of Elephas recki, a species twice the weight of Loxodonta africana, 111 is surrounded by discarded stone tools and shows multiple cut marks on the bones (29). 112 It is unclear if the disappearance of Elephas recki, which coincided with the 75% turnover 113 of the large mammal fauna in the transition from the Lower to Middle Stone Age 500,000 114 to 350,000 BP, was due to climatic changes or the emergence of more sophisticated 115 weaponry and hunting techniques or both (30). The eruption of taller grasslands following 116 the loss of the largest species across a range of herbivore taxa at a time the climate was 5 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.18.456886; this version posted August 18, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 117 drying is, however, consistent with Owen-Smith's (1987) herbivore release hypothesis 118 explaining the growth of coarse vegetation following the extinction of megaherbivores. 119 The presence of sophisticated spears found among wild horse fossils preserved in a peat 120 bog in Germany at 350,000 BP (32) adds further evidence of projectile weapons and 121 group hunting as plausible explanations for the Olorgesailie faunal turnover. The 122 transition marks the emergence of Homo sapiens, Loxodonta africana and the modern 123 large mammal fauna (29) in which the keystone roles of elephants and humans have 124 been linked over hundreds of millennia. 125 The Venus Hohle Fels figurines dating from 35,000 to 40,000 BP testify to the role of 126 mammoth ivory in prehistoric cultural practices following the arrival of modern sapiens in 127 Europe (33) and most likely among Neanderthals far earlier. Historical accounts document 128 a flourishing North African ivory trade supplying the Greeks and Romans from as early as 129 2,500 BP, and the extirpation of elephants from their north African ranges during the 130 classical times (34).