Effects of disturbance on long-grazed semi-arid grasslands

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Botany, School of Life Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Semi-arid grasslands of the Victorian Riverine Plain in south-eastern Australia have been extensively cleared and modified since European occupation. This loss has been recognised through their recent listing as a critically endangered community under the EPBC Act 1990 (as the Natural Grasslands of the Murray Valley Plain). Over the past decade, the area of reservation of this critically endangered ecological community has greatly increased, the conservation estate in Victoria now exceeding 10 000 ha. This thesis addresses the hypothesis that; i) semi-arid grasslands of south-eastern Australia are composed of a flora with an evolutionary exposure to fire that has resulted in a subset of the flora having a pre-adaptation to grazing. ii) this pre-adaptation has resulted in species persistence despite the widespread application of the anthropogenic disturbance of grazing in recent times (less than 200 years). iii) as a result of this filtering of the flora and there now exists a native flora that is stable under the continued application of a domestic stock grazing regime. The recovery and management of these areas is currently being undertaken using the concept of “status quo” management philosophy (i.e. the maintenance of existing management regimes that are thought to have maintained the current composition of these areas). This concept is based on the assumption that grasslands are now stable under current stock grazing regimes and that ongoing degradation due to grazing by domestic stock has ceased. This thesis investigates this concept, and investigates whether the alternative disturbances (removal of grazing and fire) have positive, neutral or negative effects on native species diversity. These disturbances were assessed for their impact on species composition, spatial arrangement (Species-Area Relationships) and the functional signatures (Grime’s CSR plant strategies) in three long-grazed semi-arid grasslands. I found that species composition was little affected by changes in disturbance type, at least in the short term, and that the species composition of grasslands appears to be stable. However, changes do appear to occur in the spatial arrangement of species in this community. Both the action of Grazing, and its removal, in the absence of fire, appear to be homogenising species distribution and as such, reducing β-diversity when compared to areas that have been burnt. The application of disturbance however appears to be important to maintaining α-diversity. Changes in the distribution of species are also reflected in shifts in the functional signature of these grasslands. These shifts are characterised by an increase in the ruderal signature of the community under grazing, and its removal. This shift in functional signature was hypothesised to be a response to short-term climatic fluctuations, in particular drought-driven disturbance, rather than the imposed disturbance types. This shift in functional signature however is not observed under the application of fire. It is proposed that these results support the hypothesis that the semi-arid grasslands of south-eastern Australia are composed of a flora that has an evolutionary exposure to fire. This evolutionary exposure to fire has resulted in a subset of the flora having a preadaptation to grazing that has resulted in species persistence despite the widespread application of the anthropogenic disturbance of grazing in recent times (less than 200 years). Rather than being described as a stable flora, as has previously been hypothesised, it has been demonstrated that this flora is continuing to shift under “status quo” management. The historic decline of this flora is continuing and as such the continued loss or extinction of species at the local and regional scale is believed to be still occurring ie the extinction debt has yet to be realised. The continued loss of species is not expected to halt unless management of protected areas is altered. An understanding of demographic processes under grazing, and other disturbances, is likely to give a mechanistic insight into alternative management regimes. To increase the understanding of the protection and management of these significant areas, further research into the impacts of the ongoing application of fire as a disturbance regime, as well as the influence of climatic fluctuations on species abundance and persistence is required. It is advocated that a protected area adaptive management regime be implemented to aid in the building of this understanding.