Tags

, , , , , , , ,

IMG_8035

Western Grey Kangaroos fight it out in one of our revegetation habitat zones

One of the most persistent challenges in our work to revegetate areas of the farm has been managing kangaroos. Despite its previous status as woodland, for decades the farm has been an enforced grassland as hay paddock and pasture, the preferred environment of Western Grey Kangaroos. While early accounts of the region describe the southwestern Fleurieu as “kangaroo country”, land clearing, the elimination of predators such as dingoes, reduced hunting pressure, and in our case, the provision of year-round green pick in the form of a nearby irrigated golf course has contributed to a steady increase of kangaroo numbers.

We’ve observed that the kangaroos follow a seasonal rhythm of converging on our property in numbers during the cooler, wetter months, before dispersing into smaller family groups as the weather warms and dries. During this time, they typically move into the neighbouring golf course, and because of the constant availability of fresh feed it is rare to see a female kangaroo without a joey. While most species of kangaroos typically prefer grass, the Western Grey is also noted as a browser of shrubs and seedlings.

While we’ve theorised about whether kangaroos consciously maintain their preferred grassland environment by devouring every plant that might grow larger than a clump of phalaris, we’ve also worked to protect the plants as best we can. While the common corflute guards are ineffective against kangaroos, we’ve found these useful for maintaining some humidity and sheltering new seedlings through summer. We combine these with “mallee mesh” or other salvaged mesh, to provide protection from kangaroos for the first year or two. However, as soon as the seedlings reach the top of the guard, the kangaroos once again plough into it, necessitating a seemingly endless process of adding additional mesh storeys to the guards. (We’ve recently discovered Rowan Reid’s tree guard concept and will look to trial this as well).

When calculating the impact of kangaroos on pasture, many studies use the measure of “Dry Sheep Equivalent” (DSE), typically used to calculate stocking rates based on how many non-lactating ewes an animal is equivalent. These is much debate about how many sheep a kangaroo is equivalent to, with the most frequently used calculations varying from 0.2-0.7 DSE. With approximately 300 kangaroos grazing on Yarnauwi in 2016 and similar numbers again in 2017, this is the equivalent of anywhere from 60-210 sheep. A suggested maximum stocking rate for Yarnauwi in a good year is around 80-100 sheep. With kangaroos in these numbers, clearly any effort to allow pasture recovery through rotational grazing is undermined by kangaroos maintaining or even increasing the grazing pressure. Likewise, while livestock have been excluded from regeneration areas for five years, the persistent presence of kangaroos has prevented stabilisation of some erosive areas and challenged the regeneration of woodland. Indeed, while Eastern Greys and Red Kangaroos are considered to be grass specialists, the Western Grey Kangaroo’s diet is broader, including browse of trees and shrubs.

While kangaroos obviously perform important ecological functions such as grazing and manuring our fenced habitat zones, there’s a growing body of research documenting the impacts of unmanaged kangaroo populations on the landscape. While much research focuses on the impact of Eastern Grey Kangaroos, it still offers indicators of the possible impact. Research from 2014 found that unmanaged populations of kangaroos can graze grassland environments until it’s “like lawn”, leaving little cover for birds, reptiles or insects. The same research found that reptile numbers decreased with the highest levels of kangaroos grazing. Kangaroo grazing and management seems to be a particularly vexed issue in the ACT, where annual culls occur to try and reduce their impacts. Recent research, summarised by the ACT Government, found that at high populations, kangaroos “may inhibit soil, water and nutrient processes essential for restoration of healthy functioning grassy woodlands” and “heavy grazing from a high density of kangaroos poses a significant barrier to sites undergoing ecological restoration”. Likewise, in his PhD thesis on the impacts of high densities of kangaroo grazing in Melbourne’s water catchment, Philip Alviano found that at high densities, kangaroos had a similar impact on the landscape as domestic stock, with heavy grazing resulting in the change of plant species composition, reduction of groundcover, exposure of soil to water erosion, compaction and the channelisation of kangaroo tracks. More locally, Associate Professor David Paton, from the University of Adelaide recently highlighted the stark comparison between areas grazed by kangaroos and areas from which kangaroos are excluded in the Sandy Creek Conservation Park in the Barossa Valley. Exclusion of Western Grey Kangaroos from certain areas has seen vegetation return to a density and complexity not seen since the 1970s, with an accompanying increase of bird and other species dependent on more complex plant structures for habitat. Observationally, a similar pattern is seen on the Fleurieu Peninsula in the Deep Creek Conservation Park where exclosures demonstrate the regeneration possible when kangaroos are excluded from a landscape: inside: a forest, outside: closely grazed grass with limited species regeneration.

All of these studies are echoed in our own observations at Yarnauwi: the slow pace of shrub and tree growth due to constant kangaroo browsing, the difficulty of stabilising channelised waterways when kangaroos commute along, up and across them in numbers, even the absence of the small birds that call enticingly from the scrub beyond our back boundary. It’s clear to us that the recovery of our landscape is limited by the current high level of kangaroo grazing pressure.

In his essay After the Future, Tim Flannery asserts that through their attentive management of the landscape, Australian Aboriginal people, “the greatest practical environmentalists on the planet”, had become a keystone species in the landscape. Over their millennia of occupation of the Australian continent, “Aborigines acted as a keystone in Australia by carefully burning the vegetation that was once eaten by the megafauna, and by regulating the abundance of the remaining species through hunting. Take a keystone out of an arch and the structure collapses. And so, when the European settlers began to disrupt Aboriginal land management, they removed the human keystone that lay at the heart of Australia’s ecosystems. Environmental collapses can occur on the timescale of decades or centuries, and the consequences of this particular keystone removal are still being played out today.”

Considering the work of Tim Flannery and the long history of Aboriginal management, it’s possible that our landscape requires active and engaged management, it will not thrive through neglect. The dilemma for us has been to consider the best way of managing this process, the time and investment in engineering structures to minimise kangaroo impact on individual trees is costly in both time and money and not sustainable, while the cost of culling is minimal in both resources and time. While the culling of kangaroos is not especially appealing, and the legal use of their meat or hides is highly restricted, the choice appears to be between the short-term reduction of an abundant species compared to the long-term reduction or prevention of regeneration of a range of plant and animal species currently impacted by kangaroo pressure. As noted above, in a pre-colonial system, kangaroo population and movements would have been regulated by pack-hunting predators such as dingoes, human hunting and land-management as well as seasonal fluctuations in feed and water supplies. While the reintroduction of wolves into Yellowstone National Park and their impact on overabundant deer and landscape regeneration has become legendary (and subsequent research suggests that the actual relationships are much more complex than the original elegant, but simplistic, story indicates), clearly the reintroduction of dingoes or other pack-hunting predators onto the Fleurieu Peninsula is not viable. (Interestingly, there is research also emerging in Australia that appears to echo elements of this concept when looking at the role of dingoes in ecosystems, for example, this article suggesting a connection between dingoes and soil health). Likewise, non-lethal “relocation” of kangaroos is not practical due to cost, but also because kangaroo numbers are high everywhere so moving a population simply relocates the impact to another environment likely to already be under pressure from kangaroo grazing. Considering this, in 2016 we began culling kangaroos, with appropriate permits from the Department of Environment, Water and Natural Resources. While it’s not a particularly pleasant experience, the shooters kill cleanly and efficiently and only shoot a fraction of the total number. Following this, the kangaroos temporarily moved on from the farm, dispersing into surrounding pasture.

Tim Flannery writes about the mega-faunal Australian landscape as one in which the grazing of megafauna supercharged nutrient recycling. With their disappearance, nutrient cycling was reduced to the role of microbes which work slowly in dry climates, or fire, which typically results in varying rates of nutrient loss. Following the 2017 cull, we began reading Allan Savory and Jody Butterfield’s Holistic Management: A new framework for decision making. This wide ranging work outlines Savory’s approach to land management, based around using livestock to regenerate landscapes (see video below), and his broader approach to decision-making that plans for quality of life, financial planning and the future health of the landscape, relationships and the community in which you live. This approach is controversial for its challenge of much environmental wisdom, and some documented failures, as well attracting adherents who seem to swear by its success. While not all of Savory’s assertions are appropriate or sufficiently documented for our context, it is, nonetheless, one of those books that both confirms some of our thinking and observations while also challenging many of our assumptions about how landscapes may function. It doesn’t provide a definitive solutions for managing wild herbivores, but it does present questions and lines of inquiry to help us think through what might be best for the landscape in the long-term.

One of Savory’s guiding metaphors for human intervention in an ecosystem is that of the coiled spring, where human interventions push an environment back to an earlier stage of succession, from which the “spring” must expand, with accompanying “bounce” and instability. For example, the maintenance of a paddock as pasture through chemical or mechanical means keeps “recoiling the spring” with unpredictable results should this maintenance end as the “spring expands” again to move through succession to woodland. Savory alludes to the targeted removal of specific species potentially contributing to this short-term instability and fluctuation in a system. While we certainly do not want the complete removal of kangaroos, it’s instructive to us to remain attentive to the impacts of reduced kangaroo numbers and to keep our eyes open to both positive and negative impacts.  

A central holistic management principle is to design systems that optimise the level of grazing pressure and soil disturbance by grazing animals to support landscape regeneration. Savory has observed that in the absence of predator pressure, grazers – including kangaroos – change their grazing behaviour. They typically disperse over a wider area, stay for longer and manure is so dispersed that there is a reduced need to move on. In contrast, when predator pressure is applied, grazers band together, concentrate their impact and move on more quickly. According to Savory’s model, this has a desirable impact on the landscape. Savory goes on to note that in the absence of predation, the population growth of wild grazers continues to grow while home territories shrink resulting in the same area becoming occupied by more and larger groups all of whom return to grazing areas more frequently. In an echo of the research on the impact of unmanaged kangaroo populations, this can result in “a snowballing breakdown of ecosystem processes,” including the loss of many non-herding wildlife species, such as the reptiles, insects and birds noted in recent Australian studies. Another key consideration for the Holistic Management model is to develop approaches that support the co-existence of domestic and wild grazers, and this warrants further, significant consideration for us.

While Aboriginal people became masters of fire management, there are echoes in Tim Flannery’s megafauna grazing hypothesis of large herbivores maintaining ecological health through grazing and defecation of Savory’s Holistic grazing approach. Given that kangaroos seem to have an annual cycle of convergence and dispersion, could there be some way we manage this annual pulse of high density disturbance and grazing to heal the landscape? Is it possible that Aboriginal land management processes, combined with hunting and pack-hunting predators, were also a means through which grazing impact could be concentrated for the health of the landscape?

We’re now well into summer, three months since this year’s cull. Small groups of kangaroos are returning to the farm, but with reduced browsing pressure the trees are able to translate two wet winters into abundant growth, in some cases finally growing beyond kangaroo reach. Areas of bare soil that were previously kangaroo camps experienced a flush of pasture growth and have kept soil cover into summer. In the spirit of Allan Savory’s Holistic Management approach, we watch and test our decisions against the ultimate goal of a healthy landscape, community and economy.  As difficult and unpleasant as it is, at the moment, culling kangaroos feels like the right decision for the health of the landscape, a landscape that we hope in the future provides habitat not just for kangaroos, but a plethora of plants and animals currently without habitat in the open grazing country. We keep observing, keep planting, keep tweaking our approach, keep researching and keep trying to unravel the almost infinite variables that make this landscape function.