Collapsing Ecosystems

Ecological collapse refers to a situation where an ecosystem suffers a drastic, possibly permanent, reduction in carrying capacity for all organisms, often resulting in mass extinction. Usually, an ecological collapse is precipitated by a disastrous event occurring on a short time scale.

The Earth’s biodiversity is under attack. We would need to travel back over 65 million years to find rates of species loss as high as we are witnessing today.

The world’s climate is already changing. Extreme weather events (floods, droughts, and heatwaves) are increasing as global temperatures rise. While we are starting to learn how these changes will affect people and individual species, we don’t yet know how ecosystems are likely to change.

Research published in Nature, using 14 years of NASA satellite data, shows eastern Australia’s drylands are among the most sensitive ecosystems to these extreme events, alongside tropical rainforests and mountains. Central Australia’s desert ecosystems are also vulnerable, but for different reasons.

Tipping Points

Ecological theory tells us that as ecosystems become unhealthy, they approach critical thresholds (also referred to as tipping points). The more unhealthy they become, the quicker they respond to disturbances.

Ecosystems that cross a critical threshold are transformed into new states, often with losses in biodiversity, exotic species invasions, and sudden forest die-off events. For example, over the past 10 years, ecosystems in the western US have experienced large-scale tree deaths and native, black grama grasslands have been transformed to the exotic, South African Lehmann lovegrass.

Farms and crops can be thought of as agricultural ecosystems, and they are highly sensitive to variations in climate. This means they are very challenging to manage for sustainable livestock and crop production under such intensifying conditions of sudden good and bad periods.

As humans we show weakened resistance when we are sick, and we become more susceptible to external conditions. Similarly, slower than normal ecosystem responses to external changes may also be indicative of an unhealthy ecosystem.

Both of these measures, fast and slow, are early warning signs for ecosystem collapse

Conservation often focuses on the big, enigmatic animals – tigers, polar bears, whales. There are many reasons to want to save these species from extinction. But what about the vast majority of life that we barely notice? The bugs and grubs that can appear or vanish from ecosystems without any apparent impact?

Biodiversity increases resilience: more species means each individual species is better able to withstand impacts. Think of decreasing biodiversity as popping out rivets from an aircraft. A few missing rivets here or there will not cause too much harm. But continuing to remove them threatens a collapse in ecosystem functioning. Forests give way to desert. Coral reefs bleach and then die.

New research that I have been involved in suggests that there biodiversity has a value that has been overlooked, but could be vitally important if we are to manage our impacts on ecosystems. shows that crucial information about the overall health or resilience of an ecosystem may be lurking in data about supposedly inconsequential species. In fact, the presence or absence of some of the rarest species may be giving us important clues as to how near an ecosystem is to a potential collapse.

Such rare species we call ecosystem canaries. Like canaries that coal miners used to check for poisonous gasses deep underground, ecosystem canaries are often the first species to disappear from a stressed ecosystem. Their vanishing can be linked to changes in the functioning of ecosystems, which can serve as a warning that a collapse is approaching.

Our study used data collected from lakes in China that showed changes in the abundance of species from algae (diatom) and aquatic midges (chironomid) communities as they compete for resources under environmental pressures. From this data it was possible to identify three types of organism: slowly-replicating but strongly competitive ‘keystone’ species; weakly-competitive but fast-replicating ‘weedy’ species; and slowly-replicating and weakly-competitive ‘canary’ species.

Runoff of fertiliser from surrounding fields has a major environmental impact on these lake ecosystems. As the situation worsens the canary species is the first to suffer.

With continuing degradation affecting all species, this leads to the eventual collapse of the keystone species as they are replaced by the weedy species. The loss of keystones tips the ecosystem into a critical transition – the point at which a system shifts into an alternate state which in lakes is dominated by smothering algae and absence of many plant and animal species. Moving a lake back to a clear water, high biodiversity state can be extremely difficult.Better to avoid the collapse in the first place.

Waiting to observe changes in the keystone species would not allow any intervention because the system would already be spiralling towards collapse. By searching for changes in the structure of populations that includes the ratio of keystone, weedy and canary species, we were able to detect a clear signal of an impending collapse many years, sometimes decades before the actual event. Time enough to put in place changes to farming and other practices.

The ecological theory underpinning this approach should apply to many other aquatic and terrestrial ecosystems. Given the extent and rapidity of human impacts there should not be any shortage of ecosystems to apply our findings to.

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