Shrub Encroachment as a Legacy of Native Mammal Decline

Shrub Encroachment as a Legacy of Native Mammal Decline

Invasive native scrub cover in arid Australia has increased dramatically over the past century coincident with declines of native mammal species in the critical weight range. Hypotheses to explain the increase in shrub cover and associated land degradation range from increased CO2 emissions to overgrazing but none adequately explains all elements of invasive native scrub encroachment. This project considers the role of declining native mammals in shrub encroachment, as recent experiments in our lab have suggested that these mammals may play an important role in inhibiting growth and spread of invasive native scrub species.

We have study sites throughout arid and semi-arid NSW and SA and are running field trips throughout the year. If you would like to know more about this project or to volunteer please contact Charlotte Mills.

Research Program: 
Threatened Species
Research Themes: 
Terrestrial Ecosystems

Publications for this project

Author Date Title Description PDF
Mills and Letnic 2018 Reversing functional extinction of mammals prompts a rethink of paradigms about seed fate in arid Australia


Functional extinction of once abundant species has frequently preceded understanding of their ecological roles. Consequently, our understanding of ecosystems is prone to shifting baselines because it often relies on observations made on depauperate species assemblages. In Australian deserts, current paradigms are that ants are the dominant granivores, mammals are unimportant seed predators and that myrmecochory in many Australian shrubs is an adaptation to increase dispersal distance and direct seeds to favourable germination sites. Here, we ask whether these paradigms could be artefacts of mammal extinction. We take advantage of a predator-proof reserve within which locally extinct native mammals have been reintroduced to compare seed removal by ants and mammals. Using foraging trays that selectively excluded mammals and ants we show that a reintroduced mammal, the woylie (Bettongia penicillata) was at least as important as ants in the removal of seeds of two shrub species (Dodonaea viscosa and Acacia ligulata). Our results provide evidence that the dominance of ants as granivores and current understanding of the adaptive benefit of myrmecochory in arid Australia may be artefacts of the functional extinction of mammals. Our study shows how reversing functional extinction can provide the opportunity to rethink contemporary understanding of ecological processes.

Access the paper here:

PDF icon Mills & Letnic 2018 RSOS.pdf
Mills et al. 2017 Rewilded mammal assemblages reveal the missing ecological functions of granivores

Rewilding is a strategy for ecological restoration that uses reintroductions of animals to re-establish the ecological functions of keystone species. Globally, rewilding efforts have focused primarily on reinstating the ecological functions of charismatic megafauna. In Australia, rewilding efforts have focused on restoring the ecological functions of herbivorous and omnivorous rodents and marsupials weighing between 30-5000g inside of predator-proof exclosures.
In many arid ecosystems, mammals are considered the dominant seed predators. In Australian deserts, ants are considered to be the primary removers and predators of seeds and mammals unimportant removers and predators of seeds. However, most research on granivory in Australian deserts has occurred in areas where native mammals were functionally extinct.
Here, we compare rates of seed removal by mammals and ants on shrub seeds and abundance of shrub seedlings in two rewilded desert ecosystems (Arid Recovery Reserve and Scotia Wildlife Sanctuary) with adjacent areas possessing depauperate mammal faunas. We used foraging trays containing seeds of common native shrubs (Acacia ligulata and Dodonaea viscosa) to examine rates of seed removal by ants and mammals. We quantified the abundance of A. ligulata and D. viscosa seedlings inside and outside of rewilded areas along belt transects.
By excluding ants and mammals from foraging trays, we show that ants removed more seeds than mammals where mammal assemblages were depauperate, but mammals removed far more seeds than ants in rewilded areas. Shrub seedlings were more abundant in areas with depauperate mammal faunas than in rewilded areas.
Our study provides evidence that rewilding of desert mammal assemblages has restored the hitherto unappreciated ecological function of omnivorous rodents and bettongs as seed predators. We hypothesize that the loss of omnivorous mammals may be a factor that has facilitated shrub encroachment in arid Australia.
We contend that rewilding programs aimed at restoring ecological processes should not ignore consumers with relatively lower per capita consumptive effects. This is because consumers with low per capita consumptive effects often occur at high population densities or perform critical ecological functions and thus may have significant population level impacts that can be harnessed for ecological restoration.

See the press release here:
Hear Mike Letnic discuss the paper on the radio:

Access the paper here:

PDF icon Mills et al 2017 Functional Ecology.pdf
Gordon et al. 2017 Shrub encroachment is linked to extirpation of an apex predator

Abstract The abundance of shrubs has increased throughout Earth's arid lands. This ‘shrub encroachment’ has been linked to livestock grazing, fire-suppression and elevated atmospheric CO2 concentrations facilitating shrub recruitment. Apex predators initiate trophic cascades which can influence the abundance of many species across multiple trophic levels within ecosystems. Extirpation of apex predators is linked inextricably to pastoralism, but has not been considered as a factor contributing to shrub encroachment. Here, we ask if trophic cascades triggered by the extirpation of Australia's largest terrestrial predator, the dingo (Canis dingo), could be a driver of shrub encroachment in the Strzelecki Desert, Australia. We use aerial photographs spanning a 51-year period to compare shrub cover between areas where dingoes are historically rare and common. We then quantify contemporary patterns of shrub, shrub seedling and mammal abundances, and use structural equation modelling to compare competing trophic cascade hypotheses to explain how dingoes could influence shrub recruitment. Finally, we track the fate of seedlings of an encroaching shrub, hopbush (Dodonaea viscosa angustissima), during a period optimal for seedling recruitment, and quantify removal rates of hopbush seeds by rodents from enriched seed patches. Shrub cover was 26–48% greater in areas where dingoes were rare than common. Our structural equation modelling supported the hypothesis that dingo removal facilitates shrub encroachment by triggering a four level trophic cascade. According to this model, increased mesopredator abundance in the absence of dingoes results in suppressed abundance of consumers of shrub seeds and seedlings, rodents and rabbits respectively. In turn, suppressed abundances of rodents and rabbits in the absence of dingoes relaxed a recruitment bottleneck for shrubs. The results of our SEM were supported by results showing that rates of hopbush seedling survival and seed removal were 1·7 times greater and 2·1 times lower in areas where dingoes were rare than common. Our study provides evidence linking the suppression of an apex predator to the historic encroachment of shrubs. We contend that trophic cascades induced by apex predator extirpation may be an overlooked driver of shrub encroachment.


Check out the cartoon!

News for this project


Re-introduction of native mammals helps restore arid landscapes

Small native mammals including bettongs are much bigger consumers of plant seeds than had been realised, and their loss due to predation by foxes and feral cats has likely caused significant change


Arid field season off to a roaring start in 2016

The 2016 field season has kicked off with a big start for researchers in the Landscape

Updates for this project

Good kinds of records

In complete contrast to last trip - where we had record-breaking temperatures of 40+ degree days for 10 consecutive days - this trip we had perfect weather and hit a record of 5 fences built in 3.5 days!

Home again, back again

I’m writing this from the relative safety of my desk back at UNSW, although preparing to head back to the field next week.

More than just field work

We survived the hot weather in Scotia to arrive in a soggy Roxby Downs, the town nearby Arid Recovery Reserve.

2016's adventures begin!

Last Tuesday I set off from Sydney for a five week adventure in the red sand at Scotia Wildlife Sanctuary and Arid Recovery Reserve.

November Field Trip - All systems go!

The time has come around again so soon! We were only just out in June!

Go to top