Collaborative Research: Effects of top scavenger declines—from microbes to ecosystems

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Unless first consumed by a predator, all animals enter the carrion pool when they die. Scavengers and microorganisms play a critical role in returning carcass-derived nutrients to the soil where they get recycled and used for plant uptake and growth. The impact of carrion inputs on nutrient cycling, food web dynamics, and ecosystem carbon balance remains a mystery. With global declines of many species including scavengers, it is essential to quantify how the quantity and quality of carrion-derived nutrients shapes plant community structure and ecosystem dynamics. Tasmanian devils are an ideal and charismatic species with which to study the effects of scavenging on ecosystem processes. They are one of a few carnivores worldwide that consume bones. By accelerating the cycling of key plant growth-limiting nutrients that would otherwise remain locked in bone material for years, Tasmanian devils provide a critical ecosystem function. In recent years, the emergence of a highly transmissible cancer — devil facial tumor disease, or DFTD — has dramatically reduced devil population sizes in eastern Tasmania and has spread throughout the island, threatening this iconic species with extinction. Researchers will use this tragic situation to test whether devil-scavenging impacts can be detected on an ecosystem scale and how devil population declines result in a shift in the role of other scavenger species. This project will provide training opportunities for US graduate students and post-doctoral scholars in an international setting. Through education and outreach activities, researchers will highlight the linkages that exist between scavengers, biodiversity, and ecosystem resiliency. Few studies have directly investigated the effects of non-plant inputs on energy transfer within food web networks. As a result, there is a considerable knowledge gap in understanding how carrion and scavenger guilds govern biogeochemical cycling at an ecosystem scale. In Tasmania, the east to west spread of DFTD, a lethal, transmissible cancer, threatens devils with extinction and has created a population density gradient across the island (from 0% to 100% of carrying capacity). The resulting 'natural experiment' offers a rare opportunity to study how apex scavenger declines alter energy transfer within food webs and nutrient flows through carrion, soils, and plants. High-frequency monitoring and sampling stations will be established at five study sites that span the DFTD gradient and capture four experimental treatments: (1) full scavenger access (staked prey carcass); (2) devil exclusion (fenced prey carcass allowing access to all scavengers but devils); (3) reconstructed devil latrines to capture nutrient pulses from localized scat; (4) and soil-only controls to monitor background C and nutrient availability. Devil and mesopredator densities at carcasses will be estimated using camera arrays; invertebrate densities will be assessed using appropriate insect traps. Stable isotope tracing will be used to track how carcass-derived nutrients influence microbial community diversity and soil carbon formation. Plant productivity (NPP), leaf quality, seedling recruitment, and fine root biomass will be estimated at each site and across the continental DFTD gradient. Using temporal and spatial variation in the extent and timing of devil population declines, a series of scaled experiments will be used to: (1) determine how variation in scavenger densities affects local soil biogeochemistry; (2) investigate how carrion and scavenging networks induce shifts in the metabolic efficiency of microbial communities and decomposition rates of new plant litter inputs; (3) test the scale at which scavenging by devils is detectable; and (4) integrate findings from field and laboratory experiments to predict how biodiversity loss will affect ecosystem function over the next 50 years. The project will support a team led by an early-career female scientist, involve international collaboration and provide training at the graduate student and postdoctoral levels.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.