A common mechanism for establishing new areas for conservation is to remove potential threatening processes and then let nature take over, with the assumption that the ecosystem will revert back to a desirable state. For example, we may put up a fence to exclude undesirable species from a new reserve. We assume (& hope) that removing these species will lead to an increase in native species and ultimately benefit conservation. But is that really the case? Or will our new reserve become overrun with other undesirable species, such as exotic weeds or invasive mammals, and have negative consequences for conservation? We recently published a paper in Biological Invasions that investigated this question in the high country regions of New Zealand.
In New Zealand, much of the high country pastoral land has been leased to sheep and cattle farmers on long-term leases, covering ~27% of the total land area. However, a process of tenure review began in 1998 to evaluate the leasehold tenure of some of these properties. In some cases, areas of land was set aside for conservation purposes and the livestock removed. We were interested in what happens to the vegetation and invasive mammal communities after the removal of livestock. Do we end up with a native-dominated ecosystem that enhances conservation values? Or will these formerly grazed paddocks become weedy and full of invasive mammals that will need significant management and end up being a burden on the already limited conservation dollar? Is the vegetation really “greener” on the other side of the fence?
We know that sheep and cattle affect native vegetation in the high country by browsing or trampling, which can reduce seedling recruitment and increase the abundance of exotic plants. It is often assumed that removing livestock will reverse such processes, leading to the recovery of native biodiversity. However, plant communities are complex and the removal of grazing pressures may result in unexpected changes to community diversity and structure. For instance, a dense sward of exotic grasses may form after the removal of livestock if such grasses are more competitive than native shrubs. Sites retired from grazing may also be more attractive to invasive mammals, requiring more active management to achieve positive conservation outcomes. The variable nature of community responses to livestock removal make it difficult for conservation managers and policymakers to plan for the long-term impacts of a change from pastoral to conservation land. To manage former pastoral lease land for conservation, it is important therefore that we clearly identify the potential responses of native communities to livestock removal, and the mechanisms that drive these changes.
So we set out to investigate the impacts of livestock removal on mid-altitude dry grassland-shrubland communities, by comparing the presence and abundance of plant and invasive mammal species on currently grazed sites with that on conservation sites where pastoralism ceased 10–40 years ago. Areas were chosen on four properties in the eastern South Island of New Zealand where paired pastoral and conservation sites were separated by fences.
Removal of livestock had little impact on the total number of plant species present on either side of the fence. However, the composition and structure of these plant communities differed significantly . Sites on conservation land had higher native biodiversity, with small native herbs, grasses and shrubs more abundant than on the adjacent pastoral sites. Sites on pastoral land were dominated by exotic plants, particularly herbs and grasses. Exotic grasses had a negative impact on native biodiversity on both sides of the fence but the effect was stronger on pastoral land. Exotic hawkweeds (Hieracium/Pilosella) were equally abundant on both pastoral and conservation land, while native shrubs were more abundant than exotic shrubs on conservation land. These changes indicate that the study sites are undergoing successional changes towards a native-shrub-dominated ecosystem after the removal of livestock.
The change in tenure from pastoral to conservation land also had an impact on the invasive mammal communities present. Rabbits and hedgehogs were more abundant on pastoral sites, while possums, hares and mice were more abundant on conservation sites. Rabbits prefer shortgrass habitats, while hedgehogs may be attracted to areas with animal dung containing abundant invertebrates such as fly larvae and earthworms. By comparison, invasive mammals found on conservation land were generalist species, attracted to structurally complex and diverse habitats. It is not clear whether these patterns are driven ‘bottomup’ (i.e. by invasive mammals responding to available resources) or ‘top-down’ (i.e. by invasive mammals effectively engineering suitable habitat for themselves), or a combination of both.
Overall, removal of livestock led to the development of native-dominated plant communities, with a high abundance of shrubs. This has positive implications for conservation, as the low abundance of exotic weeds means there may be little need for active weed management. However, this benefit may be compromised by increases in the relative abundance of some invasive mammal species, potentially leading to negative implications for some species of conservation interest.
For more information about this research, check out our recently published paper in Biological Invasions:
Whitehead, Amy L., Byrom, Andrea E., Clayton, Richard I. & Pech, Roger P. (2013). Removal of livestock alters native plant and invasive mammal communities in a dry grassland–shrubland ecosystem. Biological Invasions DOI: 10.1007/s10530-013-0565-1
An earlier version of this post featured in an issue of Landcare Research‘s Vertebrate Pest Newsletter:
Whitehead, Amy L., Byrom, Andrea E., Clayton, Richard I. & Pech, Roger P (2011). Community responses to livestock removal from drylands. Kararehe Kino, 18, 9-10. [pdf]