Looking to the past to ensure the future of the world's oldest living vertebrate: Isotopic evidence for multi‐decadal shifts in trophic ecology of the Australian lungfish

Abstract

Meeting the conservation challenges of long‐lived animal species necessitate long‐term assessments of trophic ecology. The use of dietary proxies, such as ratios of naturally occurring stable isotopes in animal tissues demonstrating progressive growth, has shown considerable promise to reconstruct trophic histories of long‐lived organisms experiencing environmental change. Here, we combine innovative radiocarbon scale‐ageing techniques with stable isotope analysis of carbon and nitrogen from cross sections of scale to reconstruct the trophic ecology of Australian lungfish (Neoceratodus forsteri) across its remaining global distribution. Over a 65‐year period, we found pronounced temporal shifts in the δ13C and δ15N isotopic ratios of lungfish that coincided with a period of hydrological modification by dams and land‐use intensification associated with agriculture and livestock grazing. In the Brisbane and Burnett Rivers, whose hydrology is substantially regulated by large dams, lungfish showed consistent trends of δ13C depletion and δ15N enrichment over time. This may indicate anthropogenic changes in background isotopic levels of basal energy sources and/or that additional seston exported downstream from impoundments represent a carbon source that was previously unavailable, thus shifting lungfish diet from benthic‐dominated primary production typical of unmodified river systems, to pelagic carbon sources. By contrast, δ13C ratios of lungfish in the unregulated Mary River were more stable through time, whereas δ15N ratios increased during a period of dairy industry expansion and increased application of nitrogen fertilization and then subsequently decreased at the same time that rates of pasture development declined and nutrient inputs presumably decreased. In conclusion, we provide evidence for human‐caused alterations in background isotopic levels and potential changes in availability of benthic versus pelagic energy resources supporting Australian lungfish and demonstrate how detectable trophic signals in long‐lived fish scales can reveal long‐term anthropogenic changes in riverine ecosystems.

Integrating multiple aquatic values: Perspectives and a collaborative future for river science

“…In a biophysical context, the biology of long‐lived organisms can provide a valuable historical perspective by giving insights into the effects of past anthropogenic impacts on riverine ecosystems and potentially predict the potential impacts of future threats, including river regulation, land‐use change, and climate change (see Delong & Thoms, ; Thoms & Delong, ). Trophic changes, in the form of changing energy sources for fish production, can be elucidated by stable isotopes of long‐lived fish tissues and basal carbon resources (Olden, Fallon, Roberts, Espinoza, & Kennard, 2019). Olden et al (2019) compared the δ 13 C and δ 15 N ratios of scales from long‐lived Australian lungfish ( Neoceratodus forsteri ) in contrasting rivers over a 65‐year period.…”

“…Trophic changes, in the form of changing energy sources for fish production, can be elucidated by stable isotopes of long‐lived fish tissues and basal carbon resources (Olden, Fallon, Roberts, Espinoza, & Kennard, 2019). Olden et al (2019) compared the δ 13 C and δ 15 N ratios of scales from long‐lived Australian lungfish ( Neoceratodus forsteri ) in contrasting rivers over a 65‐year period. The authors attributed changes in isotopic signatures to hydrological modification and land use intensification (Olden et al, 2019).…”

“…Olden et al (2019) compared the δ 13 C and δ 15 N ratios of scales from long‐lived Australian lungfish ( Neoceratodus forsteri ) in contrasting rivers over a 65‐year period. The authors attributed changes in isotopic signatures to hydrological modification and land use intensification (Olden et al, 2019).…”

“…Olden, Fallon, Roberts, Espinoza, & Kennard, 2019). Olden et al(2019) compared the δ 13 C and δ 15 N ratios of scales from long-lived Australian lungfish (Neoceratodus forsteri) in contrasting rivers over a 65-year period.…”

Tell Us a Story Granddad: Age and Origin of an Iconic Australian Lungfish

“…N. forsteri currently face multiple anthropogenic threats throughout its range (Arthington, 2009), which have elevated risk of population decline and possible extinction, resulting in recent relisting as "endangered" under the IUCN Red List of Threatened Species (Brooks et al, 2019). Concomitantly, numerous studies have focused on N. forsteri, aiming to address key knowledge gaps to inform conservation efforts (Espinoza et al, 2013;Marshall et al, 2014;Roberts et al, 2014;Schmidt et al, 2018;Fallon et al, 2019;Olden et al, 2019;Mayne et al, 2021b;Meyer et al, 2021).…”