Alien plant species can significantly reduce biodiversity but they can also boost an ecosystem's biomass production, on average by more than half, according to a global analysis of scientific literature.
Introduced alien species are usually seen as a threat to biodiversity and hence to the poor who depend on ecosystems for their livelihoods. But boosting the amount of biomass produced is seen by some as more important for alleviating poverty, than conserving biodiversity.
Many of the benefits that the poor get from ecosystems do not depend on high biodiversity, said Craig Leisher, senior social-science advisor at the US-based Nature Conservancy, a non-profit conservation organisation. For example, monocultured tree farms can protect watersheds just as well as biodiverse forests.
"For the rural poor who depend on what nature provides, volume matters more than the variety ... Increasing the volume of biomass nature produces is crucial for alleviating poverty."
Now there is evidence that invasive plant species may help boost the overall plant volume produced in the ecosystems they are introduced into. This was "one of the most striking findings" of a meta-analysis of 199 articles reporting 1,041 field studies on 135 plant taxa from around the world, published in Ecology Letters last month (18 May).
Montserrat Vilà, the study's lead author and a conservation biologist at the Spanish National Research Council (CSIC), said that alien species are often introduced to fill empty niches, reduce soil erosion and increase food production, especially in developing countries. Examples of introduced species that boost biomass production include Eucalyptus and Pinus tree species in South Africa, where the latter are now spreading through the native ecosystems.
Whether it is a good or bad thing that introduced species can start to dominate an ecosystem is a subjective judgement, she said. "Do you want an area with one species only, that is an exotic species, which is reducing diversity and probably changing the soil properties, just because it increases biomass?" she asked.
But Dov Sax, a conservation biologist at Brown University, United States, said that invasive species do not always come to dominate when one looks at large areas of land. "If you add species to a large area then there is often 'room' for more of them to co-exist," he said. "Most exotic species don't become dominant, most just integrate into communities."
Leisher said that we may sometimes have to sacrifice biodiversity to feed a growing population. "It's not pragmatic to try and save all the species on the planet, at any cost. The surest way to provide long-term protection for nature is empowering communities to co-manage their natural areas to sustainably produce the things they value, such as fish, fuel, fodder and clean water," he said.
Focusing on increasing biomass can be the best strategy for badly degraded ecosystems, Leisher added.
In well preserved ecosystems, Vilà suggested working with native species to improve biomass and biodiversity. "The complementarity between species means that you can increase production because the species are using resources more efficiently," she said.
Ecology Letters doi: 10.1111/j.1461-0248.2011.01628.x (2011)