Bouncing back from drought

The last couple of years in south-eastern Australia has been notable for one very obvious thing - it's started to rain again. And there has been lots of it! This is likely to have had important impacts on many ecological processes. We could probably list the things that we may expect to happen when an ecosystem moves from the dry El Nino-phase of the ENSO cycle to the wetter La Nina phase. A non-exhaustive list might include:

- flowering: many eucalypts basically stop flowering in severe droughts (see MacNally et al. for good evidence of this), so when it rains, flowering ramps up with important consequences for other biota such as honeyeaters (e.g. Wattlebirds) that rely on nectar.

- tree health / vigour: under severe moisture stress, many plants such as eucalypts reduce their canopy and, in extreme cases, die altogether. Rod Fensham's work illustrates that drought is an important determinant of tree mortality in savanna in Queensland. With recent rains, tree canopy health appears to have markedly improved in some places.

- recruitment / breeding: in drought periods, successful recruitment is likely to be limited for many species. Floodplain species like Black Box rely on flooding for regeneration, while the breeding of many woodland birds is tied to resource availability (again, see Mac Nally et al for evidence that).


The ability to 'bounce back' after stress has been termed ecosystem 'resilience'*. Indeed, one definition for resilience is "the speed with which a community returns to its former state after it has been disturbed". Hence, "resilient landscapes" might be considered those that have the capacity to rapidly return their ecological structure and function after disturbance.  But perhaps this is an overly optimistic view of resilience when considering how ecosystems might respond to changing climate.

The Murray River at Mildura

What if ecological stressors such as long-term drought have cumulative impacts on systems? Can a few years of above-average rainfall really return systems to their previous state when decade long-stress may have been operating (negatively) on individuals, populations and communities? Should we expect that the 'bounce back' will be less than if the stress had been of a short duration?

Take, for example, water regulation in the Murray-Darling River Basin. Ralph MacNally recently did a quick calculation of the regulated flows down the Murray River (relative to the natural flow that would have occurred without extraction) and this revealed that there has been a deficit in water flows in every year since 1895. This translates the less than 50% of water having flowed down this river over the last century. While water has flowed into the lower lakes recently, will this be sufficient to recover ecosystems processes that have been under duress for more than a century?

In the same way, the recent above-average rainfall experienced in SE Oz must be viewed in the context that it follows a decade-long drought. While rainfall in 2010 & 2011 was well-above average in many areas of Victoria, record low annual rainfall had been recorded in many districts just prior to this. Indeed, if you plot the 5-yr running average for the meteorological station at Balmoral, an area where I study habitat fragmentation impacts on grassy woodlands, it is clear that natural ecosystems have been undergoing a climate drying trend, and that average annual rainfall since 1980 (600 mm) has been steadily declining (currently around 500 mm per annum). Should we expect grassy woodlands to respond rapidly and positively to recent rains in the context of ongoing climate drying (i.e. show resilience)? If tree health has markedly declined in this fragmented landscape due to habitat deterioration and climate stress, is the drought really over for these trees when two years of above-average rainfall occurs? What about plant populations that have been in decline (see Sutton & Morgan 2009) - is above-average rainfall really going to lead to mass recruitment to replenish populations that have experienced recent losses?

It is clear that climate change will exacerbate the two big threats (stressors) to native ecosystems in southern Australia - habitat loss and degradation. Indeed, for woodland birds, there is already evidence for this occurring (see MacNally et al. again!). I think a couple of things are really needed if we are to understand an ecosystem's resilience to ongoing stress: (i) a good understanding of the factors that affect population processes (i.e. births, deaths, immigrations, emmigrations) and how these are affected by drivers such as climate, as well as (ii) good monitoring and record keeping to put these changes into context. Otherwise, how will we know whether plant communities ever recover from drought?

*  I think it is unfortunate that the term 'resilience' has become a policy goal (see the Australian Biodiversity Conservation Strategy as an example) rather than being seen as a property of a system, much like species diversity, NPP and beta diversity might be considered. In the context I use it here, resilience can be thought of as a 'catch-all' concept or explanation for long-term trends in biota that reflect impacts of climate change, disturbance and habitat quality.