The ecological effects of animal mass mortality events

HSF19069 | Amount: $61,940 | Project Leader: T Newsome | Project Period: Jul ’19 – Jul ‘22

A project undertaken at The University of Sydney, and supervised by Dr Thomas Newsome.

Animal mass mortality events (MMEs) occur when many individuals within a certain population die within short timeframes, affecting all age classes equally. These can occur naturally or as a result of anthropogenic activity and extreme weather events. The resource pulses resulting from MMEs may provide important nutrients to a range of organisms from insects and vertebrates to microbes, but if scavengers and decomposers do not clear the remains efficiently, MMEs could also release excess nutrients that kill plants, provide hotspots for disease spread, and slow nutrient cycling processes.

In this project we sought to determine how different trophic levels of a scavenging food web are affected by MMEs in Kosciuszko National Park, within the Australian alps. This aim was tested by experimentally placing carcasses in the field in 8m x 8m plots with 10 carcasses each, effectively simulating a small MME in a localised plot, paired with single carcasses. Additionally, we experimentally excluded vertebrate scavengers, and vertebrate and invertebrate scavengers from a subset of plots, measured soil nutrient concentrations and vegetation growth following carcass decomposition. This experimental approach helped to establish causal links between biomass loss, the type of scavenger present and the consequence of their absence on nutrient cycling.

The main vertebrate scavengers identified during the study period were red foxes (Vulpes vulpes), dingoes (Canis dingo), corvids (Corvus spp.), and brushtail possums (Trichosurus spp.), while the main invertebrates were carrion beetles and blowflies. We found that insect numbers were higher at single carcasses with vertebrate exclusion, when compared to mass mortality plots. We found that beetles and flies were 2.3 and 3.6 times higher respectively at single carcasses than mass mortality plots, despite increased resource availability at mass mortalities. At mass mortality plots, there were no observable differences in insect numbers when vertebrate scavengers were excluded. In contrast, vertebrate scavengers visited mass mortality plots 4.9 times more than single carcasses, although they did not spend more time feeding there. These results suggest that both insects and vertebrates play a key functional role as scavengers following MMEs, and that vertebrate scavengers may be adopting behavioural strategies to exploit increased resources while reducing the risk of potential conflicts with other species.

Carcasses were 1.33 times more likely to decompose by Week 4 post-carcass placements if they were in open treatments, where vertebrates and insectd had access, compared to insect suppression treatments. However, there was no difference in decomposition time between vertebrate exclusion and open treatments, which indicates that insect scavengers can effectively consume carcasses in the absence of vertebrates within this ecosystem.

We looked at short-term soil nutrient and vegetation responses to simulated MMEs just after carcasses reached dry decomposition. Inorganic nitrogen (N) increased the most at single carcasses, where concentrations increased as much as 25.7 times higher compared to baseline measurements. Inorganic N at mass mortality plots only increased up to 5.8 times compared to baseline measurements. While the largest nutrient increases were recorded at single carcasses, overall vegetation cover increased the most at mass mortality plots, where values were up to 17.2 times higher compared to baseline measurements. Large decreases in vegetation cover were instead recorded at single carcasses, where the N concentrations were highest. This indicates that carcasses have highly localised impacts on vegetation growth, especially in the absence of vertebrate scavengers and/or when insect numbers are low, however we saw no clear scaling relationship with increased carcass quantities.

Overall, we found that small MMEs (< 500kg) were consumed quickly by the Australian alpine scavenging community, and scavengers adopted strategies to rapidly consume the carcass biomass available. The results contribute to our understanding of trophic interactions following MMEs, providing evidence about how scavengers respond to increased carcass quantities, and building on studies which utilised single carcasses to determine biotic and abiotic factors influencing decomposition. Furthermore, we provide evidence for how MMEs influence a scavenging community and show that invertebrates and vertebrates may be equally as important in carcass biomass consumption. These results provide important information for the management of large herbivore carcasses and scavenger populations in remote areas, providing a foundation for further study on MMEs.