Influence of interactive effects on long‐term population trajectories in multispecies reintroductions

Abstract

Reintroduced populations are typically considered to progress through establishment, growth and regulatory phases. However, most reintroduction programs don't monitor intensively enough to test this conceptual model. We studied population indices derived from track activity of four IUCN‐listed species (greater bilby, burrowing bettong, greater stick‐nest rat and Shark Bay bandicoot) over 23 years after multiple reintroductions of each species in arid Australia. We compared population trajectories between species, and investigated the effect of time and environmental variables. All species bred immediately after release and the growth phase lasted between 3 and 16 years, varying markedly between but not within species. The end of the growth phase was characterised by an obvious peak in population density followed by either a catastrophic decline and sustained low density (bettongs), a slow decline to extinction after 20 years (stick‐nest rat), or a slight decline followed by irregular fluctuations in density (bilby and bandicoot). Minor fluctuations were related to environmental variables such as 12 month cumulative rainfall and lagged summer maximum temperatures. Three of the four species did not reach a regulation phase, even after 23 years, possibly due to interspecific competition and trophic cascades triggered by predator removal and multi‐species reintroductions. Bilbies and bandicoots exhibited a second growth phase 18 years after reintroduction, likely caused by high rainfall and increased resources following the population crash of overabundant bettongs. Our results suggest that assemblages within multi‐species reintroductions demonstrate high variability in population trajectories due to interactive effects. Intensive monitoring to assess population viability may require decades, particularly where multiple species are reintroduced, release sites are confined and where the climate is unpredictable. Intensive monitoring also allows for adaptive management to prevent precipitous population declines. Practitioners should not assume reintroduced species pass through predictable post‐release population phases or that viability is assured after a certain time period.This article is protected by copyright. All rights reserved

Density of a cryptic Australian small mammal: The threatened Julia Creek dunnart (Sminthopsis douglasi)

“…This suggests that the population abundance of S. douglasi at Bladensburg National Park fluctuates dramatically (more than tenfold), and this may be due to changes in rainfall, vegetation cover, food availability and formation of cracks for refuge (Mifsud, 1999 ). Fluctuating small mammal populations are not unusual in Australia (Moseby et al., 2023 ; Predavec & Dickman, 1994 ; Smith, 2015 ). However, if exacerbated by threats (e.g., climate change and feral predators), such dramatic fluctuations may be of conservation concern.…”

High survivorship and rapid population growth of the greater bilby (Macrotis lagotis) reintroduced to a feral predator exclosure

“…The inability of founders to develop effective behavioural responses to the release environment can rapidly lead to reintroduction failures (Moseby et al 2016;West et al 2018;Ross et al 2019). Post-release monitoring is therefore critical in the early stages of a reintroduction project, to determine whether founders have successfully adapted to their new environment, while also providing invaluable information for future reintroductions (Armstrong and Seddon 2008;Hayward et al 2015;Watkins et al 2018;Berger-Tal et al 2020;West et al 2020;Moseby et al 2023).…”

“…Evidence from previous reintroductions suggests that bilbies can adapt to a range of environments and successfully establish populations, particularly in the absence of feral predators (Moseby and O'Donnell 2003;Berris et al 2020a;Moseby et al 2023). Post-release monitoring of bilbies reintroduced to an exclosure in arid South Australia showed that founder mortality rates were low and reproductive output was high, despite prior landscape degradation by domestic stock and feral species (Moseby and O'Donnell 2003).…”