Learning Lessons from Invaded Forests

In 1946, North American beavers were introduced to the archipelago of Tierra del Fuego at the tip of South America in an attempt to start an industry based on beaver fur. Although this industry has not thrived, beavers have multiplied enormously. By cutting trees and building dams, they have transformed forests into meadows and also fostered the spread of introduced ground cover plants. Now numbering in the tens of thousands in both Chilean and Argentinian parts of the archipelago, beavers are the target of a binational campaign to prevent them from spreading to the mainland of these two nations. — Invasive Species: What Everyone Should Know by Daniel Simberloff

Beavers in South America are just one example of the series of effects a species can have when it is placed in a new environment. Prior to the arrival of beavers, there were no species in the area that were functionally equivalent. Thus, through their felling of trees and damning of streams, the beavers introduced novel disturbances that have, among other things, aided the spread of non-native plant species. Ecologists call this an invasional meltdown, wherein invasion by one organism aids the invasion of another, making restoration that much more difficult.

Complicated interactions like this are explored by David Wardle and Duane Peltzer in a paper published last month in Biological Invasions. Organisms from all walks of life have been introduced to forests around the world, and while many introductions have had no discernable impact, others have had significant effects both above and below ground.

The authors selected forest ecosystems for their investigation because “the imprint of different invaders on long-lived tree species can often be observed directly,” even when the invading organisms are doing their work below ground. Moreover, a greater understanding of “the causes and consequences of invasions is essential for reliably predicting large-scale and long-term changes” in forest ecosystems. Forests do not regenerate quickly, so protecting them from major disturbances is important. Learning how forests respond to invasions can teach us how best to address the situation when it occurs.

The authors begin by introducing the various groups of organisms that invade forests and the potential impacts they can have. This is summarized in the graphic below. One main takeaway is that the effects of introduced species vary dramatically depending on their specific attributes or traits and where they fall within the food chain. If, like the beaver, a novel trait is introduced, “interactions between the various aboveground and belowground components, and ultimately the functioning of the ecosystem” can be significantly altered.

Wardle, D.A., and D.A. Peltzer. Impacts of invasive biota in forest ecosystems in an aboveground-belowground context. Biological Invasions (2017).  doi:10.1007/s10530-017-1372-x

After highlighting some of the impacts that invasive species can have above and below ground, the authors discuss basic tenets of invasion biology as they relate to forest ecosystems. Certain ecosystems are more vulnerable to invasions than others, and it is important to understand why. One hypothesis is that ecosystems with a high level of species diversity are more resistant to invasion than those with low species diversity. This is called biotic resistance.  When it comes to introduced plants, soil properties and other environmental factors come in to play. One species of plant may be highly invasive in one forested ecosystem, but completely unsuccessful in another. The combination of factors that help determine this are worth further exploration.

When it comes to restoring invaded forests, simply eliminating invasive species is not always enough. Because of the ecological impacts they can have above and below ground, “invader legacy effects” may persist. As the authors write, this requires “additional interventions to reduce or remove [an invader’s] legacy.” Care also has to be taken to avoid secondary invasions, because as one invasive species is removed another can take its place.

Nitrogen-fixing plants (which, as the authors explain, “feature disproportionately in invasive floras”) offer a prime example of invader legacy effects. Introducing them to forest ecosystems that lack plants with nitrogen fixing capabilities “leads to substantially greater inputs of nitrogen … and enhanced soil fertility.” Native organisms – decomposer and producer alike – are affected. Simply removing the nitrogen fixing plants does not at once remove the legacy they have left. Examples include Morella faya invasions of forest understories in Hawaii and invasions by Acacia species in South Africa and beyond.

“It has been shown that co-invasion by earthworms enhances the effects that the invasive nitrogen fixing shrub Morella faya has on nitrogen accretion and cycling in a Hawaiian forest, by enhancing burial of nitrogen-rich litter.” – D.A. Wardle and D.A. Peltzer (2017) – photo credit: wikimedia commons

The authors conclude with a list of “unresolved issues” for future research. A common theme among at least a couple of their issues is the need for observing invasive species and invaded environments over a long period. Impacts of invasive species tend to “vary across both time and space,” and it is important to be able to predict “whether impacts are likely to amplify or dampen over time.” In short, “focus should shift from resolving the effects of individual invasive species to a broader consideration of their longer term ecosystem effects.”

This paper does not introduce new findings, but it is a decent overview of invasion biology and is worth reading if you are interested in familiarizing yourself with some of the general concepts and hypotheses. It’s also open access, which is a plus. One thing that is clear after reading this is that despite our growing awareness of the impacts of invasive species, there is still much to be learned, particularly regarding how best to respond to them.

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Alien Plant Invasions and the Extinction Trajectory

One of the concerns about introduced species becoming invasive is that they threaten to reduce the biodiversity of the ecosystems they have invaded. They do this by spreading rampantly, using up resources and space, altering ecosystem functions, and ultimately pushing other species out. In the case of certain invasive animals, species may be eliminated via predation; but plants don’t eat each other (generally), so if one plant species is to snuff out another plant species it must use other means. Presently, we have no evidence that a native plant species has been rendered extinct solely as a result of an invasive plant species. That does not mean, however, that invasive plants are not doing harm.

In a paper published in AoB Plants in August 2016, Paul O. Downey and David M. Richardson argue that, when it comes to plants, focusing our attention on extinctions masks the real impact that invasive species can have. In general, plants go extinct more slowly than animals, and it is difficult to determine that a plant species has truly gone extinct. Some plants are very long-lived, so the march towards extinction can extend across centuries. But the real challenge – after determining that there are no above-ground signs of life – is determining that no viable seeds remain in the soil (i.e. seed bank). Depending on the species, seeds can remain viable for dozens (even hundreds) of years, so when conditions are right, a species thought to be extinct can emerge once again. (Consider the story of the Kankakee mallow.)

On the other hand, there is plenty of evidence that invasive plant species have had significant impacts on certain native plant populations and have placed such species on, what Downey and Richardson call, an extinction trajectory. It is this trajectory that deserves our attention if our goal is to save native plant species from extinction. As described in the paper, the extinction trajectory has six steps – or thresholds – which are defined in the infographic below:

6-threshold-extinction-trajectory

Downey and Richardson spend a portion of the paper summarizing research that demonstrates how invasive plants have driven native plants into thresholds 1-3, thereby placing them on an extinction trajectory. In New Zealand, Lantana camara (introduced from the American tropics) creates dense thickets, outcompeting native plants. Researchers found that species richness of native plants declined once L. camara achieved 75% cover in the test sites. In the U.S., researchers found reduced seed set in three native perennial herbs as a result of sharing space with Lonicera mackii (introduced from Asia), suggesting that the alien species is likely to have a negative impact on the long-term survivability of these native plants. Citing such research, Downy and Richardson conclude that “it is the direction of change that is fundamentally important – the extinction trajectory and the thresholds that have been breached – not whether a native plant species has actually been documented as going extinct due to an alien plant species based on a snapshot view.”

Introduced to New Zealand from the American tropics, largeleaf lantana (Lantana camara) forms dense thickets that can outcompete native plant species. (photo credit: wikimedia commons)

Introduced to New Zealand from the American tropics, largeleaf lantana (Lantana camara) forms dense thickets that can outcompete native plant species. (photo credit: wikimedia commons)

In support of their argument, they also address problems with the way some research is done (“in many instances appropriate data are not collected over sufficiently long periods,” etc.), and they highlight the dearth of data and research (“impacts associated with most invasive alien plants have not been studied or are poorly understood or documented”). With those things in mind, they make recommendations for improving research and they encourage long-term studies and collaboration in order to address the current “lack of meta analyses or global datasets.” A similar recommendation was made in American Journal of Botany in June 2015.

The language in this report makes it clear that the authors are responding to a certain group of people that have questioned whether or not the threat of invasive plants has been overstated and if the measures we are taking to control invasive plants are justified. The following cartoon that appeared along with a summary of the article way oversimplifies the debate:

04_figure2

Boy: There are no studies that show weeds cause native plants to go extinct, thus we should not control them. Plant: If we wait until then, we’ll all be gone!!! Girl: Just because no one has demonstrated it does not mean that extinctions do not occur. The problem is not overstated!

It seems to me that a big part of why we have not linked an invasive plant species to a native plant species extinction (apart from the difficulty of determining with certainty that a plant has gone extinct) is that extinctions are often the result of a number of factors. The authors do eventually say that: “it is rare that one threatening process in isolation leads to the extinction of a species.” So, as much as it is important to fully understand the impacts that invasive plant species are having, it is also important to look at the larger picture. What else is going on that may be contributing to population declines?

Observing invaded plant populations over a long period seems like our best bet in determining the real effects that invasive species are having. In some cases, as Downey and Richardson admit, “decreased effects over time” have been documented, and so “the effects [of invasive species] are dynamic, not static.” And speaking of things that are dynamic, extinction is a dynamic process and one that we generally consider to be wholly negative. But why? What if that isn’t always the case? Extinctions have been a part of life on earth as long as life has been around. Is there anything “good” that can come out of them?