frontiers of geobiology
East Africa’s iconic Serengeti Plains attract worldwide acclaim in media and tourism. For decades, films and photos have highlighted the largest unaltered animal migration remaining on Earth. Wildlife documentaries portray the iconic dramas of life and death among the huge herds of antelopes and their predators of sunlit grasslands. The complex ecological impacts of over a million migrating ungulates on this African savanna help to maintain one of the most productive ecosystems known: sustaining the largest number of ungulates and highest concentration of dependent predators in the world.
The rich volcanic soils are a primary ecological determinant; their mineralogy testifies to the landscape’s origins in an active rift, cloaked by frequent volcanic eruptions deposited layers of nutrient-rich ash. In the Early Pleistocene, tectonic uplift of the western margin of the Gregory Rift forged a vast plateau, drained by the Grumeti and Mara – the largest rivers the Serengeti plains draining westwards into the Lake Victoria basin. The rich soils and wetlands of this ecosystem have sustained a wealth of biodiversity for over two million years. The upthrown eastern scarp bounding the Serengeti Plains exposes the fossil-rich sediments of Olduvai and Laetoli preserving a rich diversity of ancestral mammals. These include some of our human ancestors.
The modern fauna includes 29 species of ungulates, several of which rank as conservation flagships. Occurring nowhere else on Earth, at least three antelopes are endemic to the Serengeti-Masai Mara: Western White-bearded Gnu (wildebeest), Connochaetes mearnsi, Serengeti Thomson’s Gazelle, Eudorcas nasalis, and Serengeti Topi, Damaliscus jimela. Such Flagship Species underscore conservation values of the ecosystem where they evolved and persist.
A recent taxonomic discovery reveals a new Flagship in the Serengeti ecosystem. Confined within the wetlands of the Grumeti river, it is a killifish – a tiny vertebrate; and the new research highlights the conservation status of this hitherto overlooked endemic species. Attenborough’s Killifish is formally described as the new species Nothobranchius attenboroughi, in the journal, Ichthyological Exploration of Freshwaters. It is one of six new species described from East Africa in the detailed taxonomic study. Attenborough’s Killifish rarely exceeds 4 cm in length. Nevertheless, this small tropical fish is a voracious predator of invertebrates: tiny shrimps and insect larvae mostly. Typical of killifishes, its habitat is small streams and pools. Key stages in the annual life cycle of this small fish underscore the vulnerabilities of the ecological processes, linking wetlands with the enshrouding savanna habitats. Sustaining this ecological glue is critical if all these interdependent species are to persist. Thus, the ecology Attenborough’s Killifish underscores its credentials as a conservation flagship, highlighting the importance of the Serengeti’s aquatic biodiversity.
In the early 1990s, aquatic surveys discovered this isolated population of Nothobranchius attenboroughi; only years later, has DNA sequence data confirmed its distinctiveness. These first collections revealed its restricted range is confined entirely within the catchment of the Grumeti river. This aquatic lifeline sustains the “Western Corridor” of the Serengeti Ecosystem in northwest Tanzania. The ecological linkages of savannas and wetlands along the Grumeti are portrayed in the natural history documentary, The Tides of Kirawira. This film highlights exemplar species linking savanna with wetland through the seasonal cycling of this river.
The strong entraining of life cycles of the biota with the seasons is centred in their timings with the annual rains. The biodiversity dependent on the Grumeti persists through the challenges each species faces, as flood alternates with drought. Especially spectacular is the massing of mammalian biomass in the great Gnu Migration. Trekking north to fresh grazing in the Masai Mara, each year this “Black Tide” of migrating gnus meets the “Brown Tide” of the flooding Grumeti. Within a tight seasonal window, Brown and Black tides intersect at perilous river-crossings, when the gnus suffer high losses: in drownings and the annual feasting of huge crocodiles.
Pertinently, a notable scene in Kirawira portrays salient episodes in the life cycle of Nothobranchius attenboroughi, which illustrates how the adult killifishes live for a few months only; their rapid growth in a short lifespan exploits invertebrates blooming in seasonal, eutrophic pools. Although desiccation of these pools claims the lives of all adult fishes, the killifishes’ eggs are nurtured deep within the dried-out clay. Here, aestivating in moist cracks, these eggs hatch only months after seasonal rains again flood the pool. As with gnus and their predators, killifishes depend on seasonal resources to survive challenges. Perpetuating their life cycle depends on key aquatic habitats.
Tides of Kirawira highlights exemplar processes maintaining the less obvious ecological links between wetland and savanna habitats. And recent research reveals that each gnu who dies crossing the Grumeti bequeaths a critical resource in its skeleton, whose gradual decay feeds minerals essential to the aquatic ecosystem. Enriching the river, the fates of these mammals’ skeletons poignantly illustrate how a key linkage anchors the integrity of foodwebs across savanna and wetland. Thus, mammal migrations drive a flux of minerals across the landscape, northwest from the rich volcanic grasslands of the Eastern Serengeti, and enrich many habitats including the westerly flowing rivers.
Impacts on single species can ramify far and wide through interwoven foodwebs, especially with risks to key wetland resources sustain a diversity of both aquatic and terrestrial species in the habitat mosaic. Their complex interdependencies illustrate why the survival of many species depends on the diversity of resources in riparian habitats. Meeting these challenges underscores the River Continuum as a keystone concept in conservation planning and action. As with all wetlands, the ecological future of the Grumeti and Mara rivers hinges on how well humans maintain the habitat integrity of the entire landscape: from headwaters to Lake Victoria.
Intensifying anthropogenic demands threaten the future of the Serengeti. Both agriculture and tourism are intensifying at key points in the ecosystem – at its wetlands especially. Burgeoning densities of human communities and livestock compete for aquatic resources, especially surface water. Their impacts on fringing vegetation and drinking-points weaken hydrological integrity. Key stages in the life cycles of species are vulnerable where disturbances exceed critical thresholds to aquatic ecosystems, particularly if an impact escalates into unforeseeable cascades through foodwebs. Few aquatic species can escape such escalating impacts.
Each wetland is a keystone seam, which stitches together the ecology of the encompassing landscape. Case studies elsewhere in the world document how impacts disrupting hydrological processes spread widely beyond riparian habitats. The risks of anthropogenic impacts on the Serengeti are magnified in its wetlands. As the authors emphasize, “….sound management of these wetlands is crucial to the conservation of this ecosystem”. Vulnerable to such impacts, the ecology and vulnerability of freshwater fishes highlight how key processes sustain the ecology of the Serengeti as a single integrated landscape mosaic. With its bounds defined by the annual migration it supports, the geological history, biotic wealth and scenic grandeur of the Serengeti-Mara Ecosystem underwrite its World Heritage status. In summary, the complementary and overlapping life cycles of all species – large mammals together with the less obvious aquatic biota – underscore the rationale to conserve the Serengeti plains together with its wetlands.
This discovery of a small tropical fish, endemic to the Serengeti Ecosystem, illustrates the primary role of field-based natural history in species discoveries, in tandem with taxonomy. Together, natural history and taxonomy undergird the conservation sciences. Fittingly, the paper’s authors attribute: “The specific name is given in honour of Sir David F. Attenborough, in recognition of his dedicated efforts to promote Biophilia: raising awareness of the wonders and beauties of nature for so many people worldwide, promoting awareness of the importance of biodiversity conservation, and above all, inspiring so many researchers in the field of natural history, including the authors of this paper.”.
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