Australian eruptions

Red foxes were introduced to Australia in the mid-19th century to provide hunting opportunities for the English colonists.  Since then, the fox population has exploded and spread throughout the continent, creating happy hunters, but probably leading to the decline of many native mammal species.  As a result, conservation ecologists have attempted to reduce the fox population, primarily using a program of poison baits, but also by reintroducing other predators, such as dingos, that might be able to outcompete the foxes. 

A red fox on the hunt

There are many challenges associated with eliminating foxes and restoring endangered native mammal populations.  As it turns out, red foxes are primarily nocturnal so they are not so easy to hunt, and some of their prey have either gone extinct or are reduced to very small remnant populations.  In cases where fox prey have been reduced but not eliminated, conservation ecologists want to understand what the numerical response of a recovering population will look like.  Will the recovering population increase immediately until another factor, such as food availability, becomes limiting, and then level out at a stable equilibrium?  Or might the recovering population show boom-bust dynamics, increase rapidly until it overeats its resources, then decline sharply from food deprivation, which allows food availability to recover, at which point the recovering population may go through another boom phase? If these boom-bust dynamics (also known as eruptive dynamics) are predictable, that knowledge would allow conservation ecologists to know how long they need to monitor a recovering population to establish its long-term trajectory.

Richard Duncan and his colleagues used data from two types of sources.  Most of the data were population abundance time series of at least seven years (mean = 17 years) from 169 populations of 20 native Australian mammalian species that were recovering following fox control programs.  In addition, the researchers compared their Australian data with time series from seven populations of ungulates that were translocated to northern or mountain regions of Canada, Alaska and Europe.

More than half of the populations were either unchanged or continued to decline following fox removal.  Duncan and his colleagues suggest that fox control might have been inadequate in these cases, or that some other factor continued to limit the native mammalian populations. But 46% of the populations did increase following fox control.  Some of these species, for example the brushtail possum, increased to much higher abundance, and then leveled off. 

The brushtail possum in Austin’s Ferry, Tasmania. Credit: J. J. Harrison.
Abundance of the brushtail possum following intensive red fox control that began in 2003 in Boodoree National Park in eastern Australia. The black-dotted, blue-dashed and solid red curves (in this graph and the graph below) were generated by three different models that were fitted to the data. In this case, the blue dashed curve was the best fit.

But most (56%) of the populations that increased following red fox control showed evidence of eruptive dynamics. For example, the long-nosed bandicoot increased sharply following intensive fox control for about two years, but then crashed sharply, approaching pre-removal levels after another three years.

The long-nosed bandicoot at Craters Lake National Park in Queensland, Australia. Credit: Joseph C. Boone.
Abundance of the long-nosed bandicoot following intensive red fox control that began in 2004 in Boodoree National Park in eastern Australia. In this case the solid red curve was the best fit.

Species with higher maximum rates of increase tended to reach their eruptive peak more rapidly.  In addition, larger species, such as the translocated ungulates, took longer to reach their eruptive peaks.

Time until a mammal population reaches its eruptive peak in relation to the maximum annual rate of population growth (top graph), and the mean body mass of each species (bottom graph). The solid line is the best fit generated by the model, while the dashed lines represent 95% confidence intervals. Both axes use logarithmic scales.

These findings demonstrate that it is not sufficient to show that a threatened population is recovering following removal of an invasive species such as the red fox.  It is possible that even with continued fox control, the recovery will be short-lived, only to be followed by a population crash. Density dependent factors – factors that become more important at high population densities – are probably responsible for many of the observed population crashes.  We have already discussed food availability dynamics; other density dependent factors could include predators being attracted to large prey populations, or disease being more easily transmitted when populations reach a certain level. Because they take longer to reach their eruptive peak and then crash, larger species need to be monitored by conservation ecologists for a longer period of time than do smaller species. Conservation managers need to anticipate these eruptive dynamics as they create their species recovery plans following predator removal.

note: the paper that describes this research is from the journal Ecology. The reference is Duncan, R. P.,  Dexter, N.,  Wayne, A., and  Hone, J.  2020.  Eruptive dynamics are common in managed mammal populations. Ecology  101( 12):e03175. 10.1002/ecy.3175  Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

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