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Background

Beating drought: Resilience and resistance

How do aquatic plant and animals manage to survive the deprivations of drought?

There is progressive loss of aquatic habitat as flow reduces and then stops and the remnant water evaporates, and it puts aquatic life under stress. Add to that the fact that droughts occur over large slabs of the landscape means they can threaten the survival not only of individual organisms, but whole regional populations or even entire species.

Fortunately, native species in our aquatic ecosystems, exposed to countless droughts over the millennia, usually possess adaptations which allow them to survive the drought by 'sitting it out' (resistance traits), and to recolonise and recruit (breed up) in affected areas after the drought breaks (resilience traits).

Biota that 'sit it out' manage this feat either thanks to desiccation-resistant life-history stages (that is, those resistant to drying out) or by making use of remnant habitats offering less harsh conditions in the otherwise drought-affected environment. These important places are known as refuges (or refugia) and appear to be critical to the survival of many species in our drought-prone rivers.

Resilient species, the ones that 'bounce back', have evolved traits that let them to recolonise after drought. They have well developed mechanisms allowing widespread and rapid dispersal between suitable patches of habitat, and capacity for rapid breeding once they get there.

To illustrate these survival strategies we can look at Australia's arid-zone rivers. Many species in arid-zone river systems have traits that are conducive to survival during droughts and to population recovery after droughts. For example, they:

  • are widely distributed and often found in large numbers,
  • can recolonise and recruit in a wide array of habitat types,
  • use a broad range of food types,
  • have the capacity to move over large distances,
  • continue to produce offspring even in isolated waterholes, and
  • take advantage of flooding on floodplains for growth and recruitment.

The Lake Eyre golden perch or yellow belly (Macquaria sp.) appears to possess these traits. Species such as this can take advantage of the fundamental characteristics of the natural boom and bust hydro-ecological cycles, where they have not been modified by water abstraction or by dams and diversion of water to other catchments.

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Human influences

Human activities in the catchments and bank areas of flowing waters, now and in the past, are making drought bite deeper into freshwater ecosystems, in spite of our native biota's resilience and resistance traits. Only a few individuals remain in a number of species, and/or they are found in relatively few places. As a consequence, the inherent ability of many species and communities to recover from drought is now greatly impaired.

However, management [link to Sustaining aquatic ecosystems through drought] can alleviate some of this, either through proactive planning and action between droughts, or by reactive responses during drought.

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Desirable ecological conditions that work with resilience and resistance to support recovery from drought

  • Reliable refuges that will retain enough water throughout the drought.
  • Water quality in refuges which is refreshed with environmental flows or opportunistic flows whenever possible to support the freshwater biota living there.
  • No physical disturbance (natural, artificial, present, future).
  • Access (no barriers) between refuges and nearby freshwater habitats (wetlands, streams), so the refugees can recolonise the main habitats when the drought breaks.
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Repopulating after severe drought

By and large, repopulating after drought in wetlands and rivers is achieved by a combination of recruitment and recolonisation or immigration.

Immigration occurs when resilient species move back to an area where local refuges were absent. It is a common way of recolonisation for fauna that are wholly dependent on free water, such as fish. However, for immigration to occur, there has to be a suitable connection between the refuge and a species' normal or desirable habitat. Humans, unwittingly, have often broken such connections.

Recruitment of vegetation and invertebrates can happen on the spot, very soon after drought breaks, via the germination and hatching-out of desiccation-resistant seeds, spores or eggs. These germination or hatching-out strategies are common for many plants, from algae to large river plants, and for many invertebrates, including rotifers and crustaceans (Brock et al. 2003). Recruitment is also linked to immigration and recolonisation. With the breaking of a drought, many species, especially fish and crustaceans, reproduce rapidly following the initial wave of immigration (Adams et al. 2005). Flying insects also readily recolonise restored water bodies and commence breeding.

The extent and rate of repopulation by a species is governed by many factors, including the duration, severity and geographic extent of the drought, the availability and distribution of refuges in the landscape, and the way that the drought breaks, whether by flooding rains or a steadily increasing flow. Rainfall runoff from the land into standing waters replenishes them, but also brings in nutrients that can trigger algal growth, detrimental to fish.

Recovery from drought may be incomplete, in that some species may not return at all (depending on the time frame in question) or may not return to their pre-drought abundance. In some cases, species hitherto rare may briefly boom after drought, to be replaced by the 'normal' inhabitants over time.

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Useful references:

Keywords:

australiaView Frequently Asked Questions     View Bibliography
flowView Frequently Asked Questions     View Bibliography
refugeView Frequently Asked Questions     View Bibliography
life-historyView Frequently Asked Questions     View Bibliography
water qualityView Frequently Asked Questions     View Bibliography
barriersView Frequently Asked Questions     View Bibliography
fishView Frequently Asked Questions     View Bibliography
vegetationView Frequently Asked Questions     View Bibliography
invertebratesView Frequently Asked Questions     View Bibliography
rainfallView Frequently Asked Questions     View Bibliography
algalView Frequently Asked Questions     View Bibliography

Topics:

Recovery from droughtView Frequently Asked Questions     View Bibliography
Principels for protecting aquatic biodiversityView Frequently Asked Questions     View Bibliography
Drought impactsView Frequently Asked Questions     View Bibliography
Ecological responsesView Frequently Asked Questions     View Bibliography
Refuge habitatsView Frequently Asked Questions     View Bibliography
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