Sunday, January 22, 2012

Restoration, Natives and Invaders by Moderator Scott Weidensaul


[Discussion Topic: Return to Wild America: A Yearlong Search for the Continent's Natural Soul]

Because there's been so much interesting discussion here the past week or two on the subject of what constitutes "wild" and how to approach ecological restoration, I wanted to share a paper that was published last week in the online journal PLOS One. It raises some fascinating questions about human impacts on the natural world, our ability to reverse them, and it ties into a new and sometimes controversial view of natives vs. invasives at the ecological level.


The paper ("All is not loss: Plant biodiversity in the Anthropocene") was written by Erle C. Ellis and Erica C. Antill from the University of Maryland and Holger Kreft from the University of Göttingen in Germany. In it, they try to model the losses in plant diversity from native extinctions and gains from exotic introductions to find what they term "anthropogenic species richness" – what you're left with when humans alter wild plant communities in this, the Anthropocene, the age of humans.

To calculate this, they started by dividing the Earth's ice-free land area into 7,800 square kilometer units and determining the native species diversity of each unit. Then they subtracted species lost through human actions, and finally added in the "anthropogenic species increase," which comprised exotic invasives, agricultural crops and ornamentals.

While acknowledging that this approach oversimplifies reality, they concluded that half the terrestrial biosphere has lost 5 percent of its native plant diversity, and a quarter of the landscape has lost up to 10 percent. On the other hand, the number of exotics added to terrestrial ecosystems, they argue, have more than outweighed the losses, for a median increase of 7 percent in plant diversity in each biome – an increase made up mostly of invasive exotics, not crops or ornamentals.

In all, they say, humans have caused a net increase in plant diversity over two-thirds of the ice-free land - "a vast biotic homogenization of plant communities" marked by "moderate loss of native species coupled with related but larger gains in exotic species, mostly by invasions."

Is this good or bad? Ellis and his colleagues don't make a strong argument either way. They also acknowledge, at least tacitly, a problem with this and other related studies on the impacts of invasives – setting the bar far too low by using local extinction as the criteria for change, when in fact ecological function may vanish long before actual extinction occurs.

Some researchers, however, have tried to make the case for net benefits from invasive exotics. A paper published last year ("Fruit quantity of invasive shrubs predicts the abundance of common native avian frugivores in central Pennsylvania," Gleditsch and Carlo 2011) found a tight correlation between the density of invasive Asian shrub honeysuckles in the Northeast and American robins and gray catbirds.

Noting that both bird species have increased dramatically since shrub honeysuckles were introduced half a century ago, the authors warn that honeysuckle eradication could "negatively impact frugivore abundance, which could have undesirable ecological and economical consequences in both local and distant regions."

What's more, they claim that by attracting more birds, thickets of honeysuckle actually enhance the chances for surviving native fruit-bearing plants to have their berries eaten and their seeds dispersed by birds. These positive interpretations of plant invasions were given an even more approving spin in the press. "Invasive Plants can Cause Positive Ecological Change," the announcement from Penn State trumpeted.

All this struck me (and a lot of other people) as a whopper of a stretch, especially since the honeysuckles have succeeded by largely crowding out native species like spicebush, shrub dogwoods and viburnums that generally have fruit of higher quality for migrant songbirds; the slight edge gained by natives through better seed dispersal isn't much help if there's no room left in which to grow.

But there are further levels of complexity here – which by happy coincidence are explored in a fascinating article in the current issue of Birding magazine.

While acknowledging that some species of birds, including cardinals, catbirds and robins, use bush honeysuckles for food and cover, Rodewald cautions that this is only a small part of the picture. The birds that benefit most from the exotic shrubs are generalists – birds that do well in many habitats. Specialists fare far worse. Acadian flycatchers - which don't eat fruit but which need fairly open understory layers in which to nest - get crowded out of forests overrun with honeysuckle, for example.

Most intriguingly, she has found that the carotenoid pigments in honeysuckle berries may be short-circuiting the means by which male cardinals "advertise" their fitness to potential mates. Normally, brilliant crimson plumage is a sign of a healthy, well-fed male cardinal, whose territory provides lots of high-quality food. But because honeysuckle berries are a rich source of carotenoids but a poor source of nutrition, they beef up a male cardinal's color even when his condition is poor – and the females attracted to such a male may thus find themselves nesting in a marginal or dangerous territory.

All in all, the picture is far more complicated - and the balance sheet far less clear-cut - when you look more closely at the question of invasives, even from a fairly simple perspective of plant-and-avian interactions. The full ecological reality, of course, is orders of magnitude more complex.

Ellis, Antill and Kreck's PLOS One paper on plant diversity makes one thing crystal clear – the impact of humans is utterly pervasive, and in many respects irreversible. We also barely understand the ramifications of such changes, even on the smallest level, as Amanda Rodewald's work makes evident.

Does that mean we shouldn't try to make things better – to rebuild (or, as someone said last week on this blog, "reimagine") ecological systems? Hardly; the pace of habitat loss, climate change and rising population makes it essential. But we do need to be humble before the fact of our profound ignorance.

Last month, Erle Ellis, lead author on the plant diversity study, was one of four coauthors of a New York Times op-ed piece in which they argued that the Anthropocene – this new geological age of global human impact - "does not mean we inhabit an ecological hell."

Instead, he and his coauthors concluded, "It is the stage on which a new, more positive and forward-looking environmentalism can be built. This is the Earth we have created, and we have a duty, as a species, to protect it and manage it with love and intelligence. It is not ruined. It is beautiful still, and can be even more beautiful, if we work together and care for it."

That's a sentiment I think we can all agree with.

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