Effects of native omnivores on invertebrate biodiversity and function: an experimental approach (APSF 09-4)

APSF 09-4 | Amount: $ 43,100 | Project Leader: H Gibb | Project Period: Jul 2009 - Jul 2012

A  project undertaken by La Trobe University, Melbourne, and the Australian Wildlife Conservancy, and supervised by Heloise Gibb and Matthew Hayward

In Australia, populations of mammals have been severely affected by European settlement, with twenty-two species having become extinct over the past 200 years. Many highly threatened native mammals, such as bilbies, bandicoots and numbats, are insectivorous or omnivorous, with invertebrates comprising a significant portion of their diet. Additionally, such mammals cause significant disturbance to the soil through burrowing. The loss of this trophic level is thus likely to have had considerable impacts on ground-dwelling invertebrates, both through predation and effects on ground-level habitat structure. This is the first study to investigate these impacts.

Figure 1.  Mallee with Triodia understorey, the dominant vegetation type at Scotia Sanctuary ©AWC.
Figure 2. A greater bilby, Macrotis lagotis, from Scotia Sanctuary ©AWC
Figure 3. A scorpion collected in a dry pitfall trap at Scotia Sanctuary ©AWC

The study is being conducted in the Australian Wildlife Conservancy’s Scotia Sanctuary, which provides a unique opportunity to experimentally test the effects of reintroductions of threatened omnivores on invertebrate assemblages and ecosystem functions performed by invertebrates. We are using an exclusion experiment within the native mammal enclosures to address the following questions:

  1. How does the presence of native omnivores affect the diversity and morphology of the assemblage of invertebrates that constitute their prey?
  2. How does the presence of native omnivores affect ecosystem functions driven by insects, including decomposition of wood, seed dispersal and scavenging?

Mammal disturbance significantly altered the ground layer structure and evidence from scats suggests that these species consume a range of invertebrates.  Our research suggests that reintroductions alter assemblages of invertebrates, including predators, such as scorpions and spiders and detritivores, such as termites.  The abundance of the inland robust scorpion, Urodacus yaschenkoi, was lower in the presence of insectivorous digging mammals, possibly due to predation.  We also observed increases in the abundance of wolf spiders (Lycosidae) that may be due to a reduction in predation by scorpions, suggesting that effects may cascade through arachnid assemblages.  Termite abundance was lower in the presence of reintroduced mammals.  Further, mammal pits supported larger numbers of invertebrate burrows than control areas, suggesting that increased heterogeneity due to their foraging may increase habitat availability for some species.

Although we found evidence for effects on invertebrate species, effects on insect-driven ecosystem functions were less clear.  Removal of threatened omnivores had minimal effects on seed dispersal and scavenging by ants.  Nor did it appear to affect the biomass of root-feeding beetles (Scarabaeidae: Melolonthinae) produced, suggesting that omnivores are not exhausting their food supply.  However, mammal reintroductions were associated with declines in termite-driven decomposition, suggesting that some ecosystem functions are altered in their presence.  Findings to date suggest that ‘ecological extinction’ of omnivorous mammal species may have altered both the composition of invertebrate species and the rate at which they perform some ecosystem functions.

Coggan, N.V., Hayward, M.W., Gibb, H. (2016) Termite activity and decomposition are influenced by digging mammal reintroductions along an aridity gradient. Journal of Arid Environments.133: 85-93
Verdon, S.J., Gibb, H., Leonard, S.W.J. (2016) Net effects of soil disturbance and herbivory on vegetation by a re-established digging mammal assemblage in arid zone Australia. Journal of Arid Environments. 133: 29-36
Hayward, M., Poh, A., Cathcart, J., Churcher, C., Bentley, J., Herman, K., Kemp, L., Rissen, N., Scully, P., Diong, C., Carter, A., Friend, A., Gibb, H. (2015) Numbat nirvana: the conservation ecology of the endangered numbat Myrmecobius fasciatus (Marsupialia: Myrmecobiidae) reintroduced to Scotia and Yookamurra Sanctuaries, Australia. Australian Journal of Zoology. 63: 258-269.
Silvey, C.J., Hayward, M.W. & Gibb, H. (2015) Effects of reconstruction of a pre-European vertebrate assemblage on ground-dwelling arachnids in arid Australia. Oecologia. 178: 497-509.
Gibb, H. (2012) How might terrestrial arthropod assemblages have changed after the ecological extinction of critical weight range (CWR) mammals in Australia? Reintroductions at Scotia Sanctuary as a model for pre-European ecosystems. Proceedings of the Linnean Society of NSW. 134: A19-A26.