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At high densities kangaroo grazing can reduce biodiversity

Thursday, 1 January 1970  | 

Over-grazing by herbivores can simplify the structure, composition and function of vegetation communities by reducing vegetation cover and diversity, increasing soil degradation and driving biodiversity loss. European colonisation has created ideal conditions for the eastern grey kangaroo in south-eastern Australia, thanks to dingo eradication, new permanent watering points, protection from hunting and increased pasture availability. Consequently, in some areas, kangaroo population densities have risen to levels where impacts on other native species are occurring. At high densities, kangaroo grazing can: (1) reduce the occurrence, height and seeding rates of some native grasses; (2) reduce the diversity and cover of native shrubs following fire; (3) reduce the abundance and diversity of beetles; (4) reduce the overall abundance and diversity of reptiles; (5) reduce the quality of habitat for some species of legless lizards and the three-toe earless skink; (6) reduce the occurrence of the vulnerable striped legless lizard; (7) reduce the quality of habitat for many species of bird; and (8) reduce the quality of habitat for endangered eastern barred bandicoots, threatening a reintroduction program. In particular, the striped legless lizard appears very sensitive to high grazing. One recent study suggested that detection of this species more than halved with each doubling of kangaroo density. The impacts of high density kangaroo grazing on biodiversity in Australia are consistent with studies on native herbivores overseas, where predation and hunting that regulated herbivore density have diminished. Managing grazing pressure is often required to prevent biodiversity loss and ecosystem degradation.

Research Entries

Title
Aims
Results
Neave, H. M. and M. T. Tanton (1989). "The Effects of Grazing by Kangaroos and Rabbits on the Vegetation and the Habitat of Other Fauna in the Tidbinbilla Nature Reserve, Australian-Capital-Territory." Australian Wildlife Research 16(3): 337-351.
Examine the present status of the grassland vegetation in three areas in which exclosures have provided protection from grazing since 1979 and compare the status with that in 1982-83.
Some plants declined, others increased under high kangaroo grazing. Height and seeding rates declined for many species, with implications for habitat quality for other wildlife.
Meers, B. T. and R. Adams (2003). "The impact of grazing by Eastern Grey Kangaroos (Macropus giganteus) on vegetation recovery after fire at Reef Hills Regional Park, Victoria." Ecological Management & Restoration 4(2): 126-132.
(i) determine the grazing patterns of the Eastern Grey Kangaroo after fire at Reef Hills Regional Park, and (ii) determine the impacts that grazing might have on post fire woody shrub recovery.
Preferential grazing by Eastern Grey Kangaroos occurred on small burnt plots compared to adjacent unburnt areas as determined by faecal pellet counts. On burnt areas, there was a significant reduction in shrub diversity on grazed plots compared to ungrazed plots.
Barton, P. S., A. D. Manning, H. Gibb, J. T. Wood, D. B. Lindenmayer and S. A. Cunningham (2011). "Experimental reduction of native vertebrate grazing and addition of logs benefit beetle diversity at multiple scales." Journal of Applied Ecology 48(4): 943-951.
(i)Do differences in vertebrate grazing affect the trophic structure of beetle assemblages?(ii) Does the addition of logs interact with grazing level to affect beetle diversity at the hectare scale? (iii) Does microhabitat structure provided by logs interact with grazing level to affect beetle diversity at small scales because of a localized ‘refuge’ effect?
A reduction in grazing level had benefits for the abundance and species richness of beetles at the site scale. Further benefits were achieved at both site and microhabitat scales when logs are used in combination with exclosure fencing.
Manning, A. D., R. B. Cunningham and D. B. Lindenmayer (2013). "Bringing forward the benefits of coarse woody debris in ecosystem recovery under different levels of grazing and vegetation density." Biological Conservation 157(0): 204-214.
Examined the effect of experimentally adding Course Woody Debris, in four different treatments, on reptile abundance in temperate woodlands in south-eastern Australia – one of the most highly degraded vegetation types on the continent. They investigated the influence that varying grazing pressure and vegetation density had on those effects.
Reduction of grazing was the most effective way of increasing small skink abundance in high density vegetation.
Howland, B., D. Stojanovic, I. J. Gordon, A. D. Manning, D. Fletcher and D. B. Lindenmayer (2014). "Eaten out of house and home: impacts of grazing on ground-dwelling reptiles in Australian grasslands and grassy woodlands." PLoS ONE 9(12): e105966.
Investigated the relationship between (1) density of native eastern grey kangaroos, Macropus giganteus, and grass structure, and (2) grass structure and reptiles (i.e. abundance, richness, diversity and occurrence)
Reptile abundance, species richness and diversity were highest where grazing intensity was low. No species of reptile was more likely to occur at high grazing intensities. Legless lizards (Delma impar, D. inornata) were more likely to be detected in areas subject to moderate grazing intensity, whereas one species (Hemiergis talbingoensis) was less likely to be detected in areas subject to intense grazing.
Howland, B. W. A., D. Stojanovic, I. J. Gordon, J. Radford, A. D. Manning and D. B. Lindenmayer (2016). "Birds of a feather flock together: Using trait-groups to understand the effect of macropod grazing on birds in grassy habitats." Biological Conservation 194: 89-99.
Investigated the potential of a trait-based approach to predict the preference of birds utilizing the grassy layers for different levels of grazing by eastern grey kangaroos within grassy habitats in south-eastern Australia.
Birds that utilized the grassy layer showed a varied response to differences in grazing intensity. Ground foragers, species with elevated nests, and birds that rely on early detection of predators were more likely to utilize the grassy layer under high grazing intensities. In contrast, small aerial insectivores, and a single ground-nesting species (that relies on concealment to avoid predators) were more likely to utilize grassy layer under lower grazing intensities.
Howland, B. W. A., D. Stojanovic, I. J. Gordon, D. Fletcher, M. Snape, I. A. Stirnemann and D. B. Lindenmayer (2016). "Habitat preference of the striped legless lizard: Implications of grazing by native herbivores and livestock for conservation of grassland biota." Austral Ecology 41(4): 455-464.
Investigated habitat preferences of Delma impar at multiple spatial scales
The occurrence of Delma impar was not affected by the size of grassland remnants, but was negatively related to the density of native grazers. This result was likely a consequence of the negative effect of high grazing intensity on grass structural complexity, as the probability of encountering a Delma impar was positively related to grass structural complexity at the fine scale (1m2).
Brown, K., Paczkowska, G., & Gibson, N. (2016). "Mitigating impacts of weeds and kangaroo grazing following prescribed fire in a Banksia woodland." Ecological Management & Restoration 17(2): 133-139.
Investigated (i) how native and weed species richness and cover changed following autumn prescribed fire and (ii) effectiveness of management techniques at reducing the impacts of grazing by western grey kangaroos.
Fencing significantly increased cover of native shrubs and grasses through prevention of kangaroo grazing. However, kangaroos also appeared to play a role in suppressing weedy annual grasses post-fire in these woodlands.
Winnard, A. L., & Coulson, G. (2008). "Sixteen years of Eastern Barred Bandicoot Perameles gunnii reintroductions in Victoria: a review". Pacific Conservation Biology 14(1): 34-53.
Examined characteristics affecting the success of Eastern Barred Bandicoot reintroductions, including competition from herbivores.
A combination of drought and grazing pressure from kangaroos and rabbits reduced the amount of available habitat and possibly lead to population decline of Eastern Barred Bandicoots at one reintroduction site in Victoria.
Mysterud, A. (2006). "The concept of overgrazing and its role in management of large herbivores." Wildlife Biology 12(2): 129-141.
Detailed discussion of what overgrazing means under different land uses and impacts of overgrazing in conservation and production landscapes.
There are several different ways of defining overgrazing. Native and exotic herbivores can result in overgrazing. Overgrazing can lead to new stable ecosystems states and these new states may be irreversible.