Megacarcass Legacy Effects
We still know surprisingly little about how individual carcasses, especially the carcasses of megafauna, shape spatiotemporal heterogeneity in ecosystem processes.
Only within the last few decades of ecosystem ecology have the roles of animals as important drivers of ecosystem processes become appreciated. Animals can transform and move nutrients across landscapes, with their aggregations forming biogeochemical hotspots with disproportionately high rates of nutrient and material flux relative to surrounding areas. For example, schools of fish, herds of ungulates, and aggregations of termites create hotspots of nutrients that drive spatiotemporal heterogeneity in primary production and herbivory. More recently, the role of dead animals has been integrated into ecosystem ecology, revealing how mass mortalities leave ecological legacies that can last for decades. However, the ecosystem-level legacies of individual carcasses of very large animals have received comparatively little attention. This gap in our understanding of the role of animals as drivers of ecosystem processes can only be closed through research in ecosystems where megacarcasses still occur. Our research focuses on the role of megacarcasses of the largest terrestrial animal, the African savanna elephant, in generating a spatially and temporally dynamic pattern of nutrient hotspots with legacy effects on the ecosystem.
The backbone of our project is a unique dataset of >400 georeferenced elephant megacarcasses in Kruger National Park, South Africa, based on yearly field observations by park staff. Importantly, these elephant megacarcasses are distributed across a heterogeneous abiotic template. A strong rainfall gradient, from 700 mm/yr in the south to 375 mm/yr in the north, creates a strong gradient in primary production. Orthogonal to the rainfall gradient is a change in geology from lower-fertility, granitic sandy soils in the west to higher-fertility, basaltic clay soils in the east. These soil types differ in physical and nutrient properties, including phosphorous (P), sodium (Na), and calcium (Ca). On both soil types, N and P limit (or co-limit) plant production, suggesting that nutrients from megacarcasses would significantly affect ecosystem processes. The natural factorial design, rainfall crossed with geology, provides a robust experimental layout for testing the ecosystem-level legacy effects of megacarcasses across space and time.
Kruger National Park (20,000 km2) in South Africa (red box) is roughly divided into lower-fertility, sandy granitic soils (orange) in the west and higher-fertility, clayey basaltic soils (blue) in the east. Rainfall decreases from south to north, resulting in a gradient of productivity on both soil types. Elephant carcasses are distributed across the landscape (dots).
Our main hypotheses are:
H1: As megacarcasses age, they transform from being hotspots of soil carbon (C) and N with high levels of microbial activity to being hotspots of soil P with lower levels of microbial activity.
H2: Plant productivity at megacarcass sites is initially suppressed by N toxicity but becomes stimulated as
the soil N:P ratio declines over time with N depletion and prolonged P enrichment.
H3: Increased plant productivity and forage quality at older megacarcass sites attract increased densities
of herbivores and therefore facilitate increased rates of herbivory.
Chronosequence of the fate of nutrients from elephant megacarcasses with age of megacarcass post death increasing from left to right. At younger megacarcasses (0-1 yrs old), we expect short-term pulses of N, leading to soil that is toxic to plant growth, and soil organic carbon (SOC), leading to increased microbial activity (enlarged magnifying glass with microbes). As carcasses age (3-5 yrs old), we expect an increase in P into the soil due to leaching from bones, lowering soil N:P, and declines in soil N and SOC leading to reductions in microbial activity. We expect plant communities to become more productive at this point and increased foliar nutrients to begin attracting herbivores. At older carcasses (5-10 yrs), we expect further increases in P from bones to result in increased primary production and high quality forage to attract significant densities of herbivores.