Paper authors: Lucie M. Bland, Jessica Rowland, Tracey J. Regan, David A. Keith, Nicholas J. Murray, Rebecca E. Lester, Matt Linn, Jon Paul Rodríguez, Emily Nicholson [Link]
The natural world is changing all around us. Species are going extinct, ecosystems are collapsing, and these losses are only expected to rise. To measure the risks posed to species and ecosystems we use Red Lists, such as the International Union for the Conservation of Nature Red List of Ecosystems (RLE).
The RLE is the global protocol for assessing risk to ecosystems. Risk is measured based on change in ecological indicators through time or across space. Indicators can represent the geographic distribution, biotic features, or abiotic environment that characterise the ecosystem.
To measure these risks, we need to define a clear negative outcome, or end point. The endpoint for species or populations is extinction, when all individuals have died; but for ecosystems, defining the endpoint (i.e. ecosystem collapse) is a little trickier.
What is ecosystem collapse?
An ecosystem moves into a collapsed state once it has lost its defining biotic or abiotic features, or been replaced by different type of ecosystem. For example, the Aral Sea moved into a collapsed state after the water volume fell by 92% due to excessive removal of water. The fish, invertebrates, waterbirds and reedbeds vanished, leaving a desert with saline lakes (Keith et al, 2013).
The Aral Sea before (1985) and after (2011) the ecosystem collapsed.
The difference may be stark between the intact, or initial, and collapsed states of an ecosystem based on change in one or more indicators. Yet it can be challenging to quantitatively define the point at which an ecosystem moves into a collapsed state for each indicator, i.e. a threshold of collapse.
Why do we need to quantitatively define ecosystem collapse?
Effectively managing ecosystems and minimising risk depends on making informed decisions based on reliable information. This is unlikely to occur if we don’t clearly differentiate between intact and collapsed states of an ecosystem, and quantify the point at which an ecosystem changes from one state to another (i.e. the collapse thresholds).
How has collapse been defined in the past?
We wanted to understand how ecosystem collapse is commonly defined, so we examined research in marine pelagic and temperate forest ecosystems around the world.
Many studies failed to describe the biota, abiotic environment, ecological processes and spatial distribution in the initial and collapsed states of the ecosystem, or how an ecosystem may change between states. The quantitative collapse thresholds separating initial and collapsed were informed by field data or predicted based on a model.
Studies of marine pelagic ecosystems tended to focus on the functioning of the system. These studies typically used indicators of biotic and abiotic features to measure change. They also used conceptual models to depict the dynamics of the ecosystem more often than temperate forest studies. On the other hand, temperate forests studies mostly looked at change in the ecosystem area and biotic features, such as dominant tree species.
Photo Credit: USFWS / Kydd Pollock via Flickr
The four steps for defining ecosystem collapse
We created a four-step guide for defining ecosystem collapse to help improve consistency among risk assessments:
(1) Quantitatively defining initial and collapsed states
At the outset, the initial and collapsed states of the ecosystem must be described, of which there may be multiple. This allows us to determine whether there have been any changes over time or across the area of the ecosystem.
(2) Describe collapse and recovery transitions
It is important to describe the ways that an ecosystem can move into or out of collapsed states. This can help highlight useful indicators, collapse thresholds for each indicator, and whether there are intermediate ecosystem states.
(3) Identify and select indicators
It is pivotal that informative and sensitive indicators are used that represent different aspect of an ecosystem. There may be multiple ways in which an ecosystem can collapse, so, using a range of indicators of the geographic distribution, biotic and abiotic features can improve the chances of detecting detrimental change.
(4) Set quantitative thresholds for collapse
Lastly, quantitative thresholds of collapse must be defined for each indicator that provide a meaningful distinction between an intact and collapsed ecosystem. There is often uncertainty in the exact point at which this can occur. To deal with this uncertainty, it is best to include bounded estimates for each threshold.
As the world is rapid changing, understanding what risks are posed to ecosystems allows for timely and suitable actions. Ensuring ecosystem collapse is accurately defined will increase our ability to take effective actions to mitigate changes in the future.
Photo Credit: Tatters via Flickr