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.