Few of the pressures documented in this section occur in isolation. Rather, pressures interact in complex ways, often compounding the threat to biodiversity. For example, land clearing removes natural habitat and typically replaces it with agricultural, urban or industrial development, which may bring additional pressures such as grazing or pollution. In addition, remnant vegetation after land clearing is often fragmented and isolated. Fragmented woodland landscapes have been shown to have more vertebrate pests than intact woodlands (Graham et al. 2012).
Interactions between multiple pressures amplify the threat faced by native mammals across Australian ecosystems. For example, during the past 5 years, evidence has emerged that the greatest impact on mammals in northern Australia comes from a combination of predation by feral cats in recently burned environments. These impacts are synergistic. Specifically, in the absence of cats, native mammals are able to survive fire and continue to find food. In addition, cats forage less effectively in unburned environments (Ziembicki et al. 2014).
Recent experimental evidence supports an association between grazing by introduced herbivores (cattle, horses, donkeys and buffalo) and the magnitude of small mammal decline. Both fire and grazing regimes in northern Australia have intensified substantially over recent decades in ways that could contribute to the contemporary decline in native mammals. These changes to fire and grazing regimes have generally made grass communities less complex and more open. They also create conditions that favour feral cats, probably because their hunting success is improved (McGregor et al. 2014).
Other interactions among pressures include those between invasive plants, particularly high-biomass pasture grasses, and fire. High-biomass grasses fuel intense fires that kill trees, which in turn facilitates the further invasion of grasses, creating a positive feedback loop.
The growing Indigenous estate faces challenges in dealing with the interaction among pressures because of its large size, diversity of management and overall low capacity. One example of proactive distribution of relevant Indigenous ecological knowledge that can be incorporated into management is through the production of seasonal calendars, which highlight the high level of Indigenous understanding of biodiversity interaction linked to human activity.
Interactions of multiple pressures with climate extremes is an area of intense debate. In the past 5 years, Australia has seen record-breaking summer temperatures in some places, particularly in 2012–13. Monitoring of the occurrence and severity of daily fire weather increased, with statistically significant increases at 16 of 38 climate reference sites in Australia from 1973 to 2010, and nonstatistically significant increases at the other sites. As well, extreme fire-weather days became more extreme at 24 of the 38 locations since the 1970s (BoM & CSIRO 2014). The most significant increases occurred in the south-east, and the largest increases occurred inland. Quantifying the cumulative impact of climate extremes interacting with the multiple other pressures affecting biodiversity (invasive species, habitat fragmentation and clearing) is extremely difficult, but this cumulative impact is widely considered to be a key threat across terrestrial, aquatic and marine habitats. The Overview report notes that climate pressure has only recently started to become a significant detectable impact.