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Impacts of plant invasions become less robust over time

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Impacts of plant invasions become less robust over time Nov 20, 2013 by Julie Cohen
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This photo shows the African invader Melinis minutiflora in Hawaii Volcanoes National Park when first studied (left) and 20 years later. Credit: UCSB

Among the most impressive ecological findings of the past 25 years is the ability of invasive plants to radically change ecosystem function. Yet few if any studies have examined whether ecosystem impacts of invasions persist over time, and what that means for plant communities and ecosystem restoration.

UC Santa Barbara's Carla D'Antonio, Schuyler Professor of Environmental Studies, has conducted one of the only long-term studies of plant invader impacts that spans two decades. Returning to the same grass-invaded field sites in Hawaii Volcanoes National Park that she used in her 1990-1995 studies, D'Antonio, along with postdoctoral scholar Stephanie Yelenik, gathered new data that shed light on mechanisms regulating exotic plant dominance and community change through invasion. The findings are published online today in Nature.

"We were able to take advantage of detailed studies I and others had conducted in the 1990s. We permanently marked sites we had set up and were able to go back and gain insight into how plant invasions changed over time without management," said D'Antonio, who also is a professor in the Department of Ecology, Evolution and Marine Biology. "Such studies are important because managers have little money to control invasive species or to study how impacts might change without management."

"Non-native plants can have very large impacts on ecosystem functioning—including altering groundwater, soil salinity or pH and pollination syndromes," said lead author Yelenik, who earned her doctorate from UCSB's Department of Ecology, Evolution and Marine Biology and now works for the U.S. Geological Survey's Pacific Island Ecosystems Research Center on the island of Hawaii.

When D'Antonio and Yelenik revisited the study sites, they noticed that the invasive exotic perennial grasses (primarily an African invader called Melinis minutiflora) were dying, so they decided to repeat measures of nutrient cycling and plant community change. They found that the grasses' self-reinforcing effects on soil nutrients had disappeared and the percentage of invader coverage had declined.

Data showed that in the past 17 years, nitrogen mineralization rates at the sites dominated by the exotic grasses declined by half, returning them to pre-invasion levels. Nitrogen mineralization is the process by which organic nitrogen is converted to plant-available inorganic forms.

"Measuring mineralization the way we do is extremely time-consuming and expensive, so we did it in snapshots of time (mid-1990s versus 2010-2012)," Yelenik explained. "This is less than ideal because differences between the two study periods could be due to differences in rainfall."

To eliminate rainfall as a factor, the researchers examined long-term rainfall data for the region to determine if a relationship exists between nitrogen mineralization and rainfall during the study years. The data showed that rainfall during the two study periods was similar. In addition, rainfall did not correlate with differences in mineralization between time points. A mineralization assay in the lab, where moisture was kept constant, showed similar patterns to the researchers' most recent field data, gathered in 2011 and 2012.

Taken together, these results suggest that nitrogen mineralization variations between the 1990s and recent years were not due to differences in rainfall.

While the study demonstrates that ecosystem impacts and feedbacks shift over time, it also indicates that this may not necessarily help native species' recovery. Yelenik and D'Antonio conducted a large outplanting experiment to test how a suite of native and exotic woody species responded to shifting ecosystem impacts. They added nitrogen fertilizer to mimic earlier stages of Melinis invasion and reduced Melinis competition to mimic patches during late invasion.

Similar responses occurred in five of the seven outplanted species: Growth rates and survivorship increased due to reduced competition from the exotic grasses as well as nitrogen additions. This indicates that the changing impacts of the grass over time do not alter the seedlings' ability to grow in the ecosystem.

Two nitrogen-fixing trees were exceptions: the native Hawaiian tree Acacia koa and the exotic tree Morella faya (from the Canary Islands but invading Hawaii today). These species did much better in later Melinis invasion conditions, and Morella faya did particularly well.

"The non-native Morella faya did a lot better for various reasons, but primarily because it has a faster growth rate," Yelenik said. "Plus in our sites it is bird-dispersed, which means it gets around and is, in fact, moving into the sites at a frightening rate. By contrast, the native Acacia did reasonably well in the experiment, but it just does not have as robust a growth rate as Morella. It is a very slow disperser and sparse in the region so we are not seeing it entering the sites on its own."

An important lesson here is that even if plant invasions can slow down on their own given enough time, native species may need further assistance in order to make a comeback, the researchers said. Other invaders may be poised to take advantage of reduced competition from the original invader.

"Knowing the mechanisms of how and why invasions alter ecosystems is insightful for predicting what will happen, but without further management we may not get native species back," Yelenik said. "When we see non-native species dying back and getting patchy, that may be the time to plant native species. It might turn out to be the most cost-effective way to get an ecosystem back to a more desirable state."

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Interesting. I have heard about something like this before (probably in a lecture) - you can notice a similar thing here in some weed populations, like Lantana, which is still an invasive and widespread weed, but its negative impacts are less than predicted/less than they were. Where I live you rarely see Lantana except a few places, and further west where it is a big problem its mainly older plants, which are probably going to die soon. Chonky apples (Z. mauritiana) are the same - you mainly see mature ones (but there are still plenty of seedlings/suckers).

Unfortunately, as they pointed out in the article, it may not be a good thing because either way the soil composition changes (possibly permanently) and becomes unfavourable for the weed and consequently other plant species - the weeds are successful usually because they can tolerate or thrive in the same type of soil as the natives prefer, but the native species tend to have more specific requirements (hence why they are only found in certain locations within their native range). Although I dont think it will be seriously bad in the long term as succession will still happen, with pioneer species coming up, changing the soil back to its former condition, dying and allowing longer lived species to return (provided the weeds dont take advantage of the reconditioned soil - but weeds mainly take over places after disturbance of some kind, so somewhere not controlled by fire or regular flattening by storms etc. should eventually go back to near normal (the way it was before the weeds were introduced, but there will probably still be some remaining weeds that can survive but cant take over)

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Yes, I've noticed it with lantana too. Apparently lantana has pretty much reached the limits of its range in Australia as a weed, though I guess this may change as the climate changes. In some areas, the lantana is so dense that I'm not sure if the ecosystem can ever really recover without intervention. Even if the odd native sapling manages to establish itself, all the understory species have been completely overrun. At least some native fauna benefit from the enhanced cover.

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Tripsis, lantana needs light, so as soon as you get decent tree cover it dies back. It's actually quite useful in sealing the edges of new rainforest as it spreads.

I presume the problem is a bit different in sclerophyll where it outcompetes everything except for the trees.

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Interesting read - although it makes a lot of sense when you think about it - in natural systems every organism does seem to eventually strike an equilibrium of sorts, or at least to exhaust what resources are available to it. I think with these invasive species we're only seeing the short-term stuff with the big picture being a bit further out.

When you think that most ecosystems in their natural state with no introduced species have taken millions of years to refine their current balances, it's easy to see how that balance is so quickly toppled with the introduction of a foreign organism. But give it enough time, and I think the forces of natural selection and competition will see things even out again. Like an ecological immune response, sort of?

Or maybe I'm full of shit and the future will be nothing but carp and cane toads and lantana...

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Tripsis, lantana needs light, so as soon as you get decent tree cover it dies back. It's actually quite useful in sealing the edges of new rainforest as it spreads.

I presume the problem is a bit different in sclerophyll where it outcompetes everything except for the trees.

Yes, I'm thinking of sclerophyll areas. The only native vegetation that was left in the worst areas were established trees, though some small saplings battle to make it through the lantana to light.

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