No-analog ecosystems

Lianas in rainforest
Lianas have been increasing in the rainforest, possibly due to increasing carbon dioxide levels. Lianas are parasites, increasing tree mortality and decreasing the amount of carbon stored. Climate change is also altering rainforest ecosystems.

When the climate changes, naturally or not, species move. It is already known that species are moving at different speeds, that ecosystems are not traveling together. New reports indicate that the ecosystems of the future may not simply be the loss of high latitude and altitude ecosystems, and their replacement by other ecosystems; these may look very different in 2100.

The May 11 Science (subscription required) describes no-analog ecosystems, such as forests of spruce, sedge, oak, ash, and hophornbeam, that once existed but do not today — there is no analog today. A sizable percentage of the Earth may be covered by these no-analog ecosystems in 2100.

First, much of the Earth will have combinations of summer and winter temperatures and precipitation patterns that we don’t see anywhere today. The tropics and subtropics are most likely to see these novel climates. On top of that, climates we do recognize will require too much travel by plant species to end up with today’s distribution. Connecting reserves to facilitate species shift won’t be enough.

Limiting the analysis to four variables — summer and winter mean temperature and precipitation — and two scenarios (B1, or low, and A2, or high) produced dramatic changes:

[B]y 2100, depending on which climate scenario and model they use, 4% to 39% of the world’s land area will experience combinations of climate variables that do not currently exist anywhere on the globe. Areas with these novel climates are likely to develop no-analog ecosystems.

A 2005 study included more variables, such as soil type, and found more profound changes.

Assuming that ecosystems can migrate at most 500 km, 300 miles, in a century, the recent study found 14 – 85% of the Earth will be covered by no-analog ecosystems.

A number of variables, such as extremes in temperature and fire frequency, were ignored; we can assume estimates are conservative.

The prospect of novel climates has people rethinking traditional goals such as maintaining native ecosystems. “That’s probably going to be impossible,” says Nathan Stephenson, a research ecologist at the USGS Western Ecological Research Center in Three Rivers, California. “But what you can still do, even if you can’t maintain native communities, is potentially maintain regional biodiversity and ecosystem functions.”

This too will be challenging.

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