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What Makes Invasive Plants Successful?

Posted: February 11, 2014

Invasive species are fairly ubiquitous in our forested landscape. Why do these plants succeed? Why can’t our native species successfully compete with invasives?

By Allyson Muth, Forest Stewardship Program Associate, Penn State Department of Ecosystem Science and Management

We’ve all seen them. Invasive species are fairly ubiquitous in our forested landscape. We don’t always know that they’re invasive. Sometimes all that we see is green and we think that’s a good thing, but in recent years the introduction of invasive species and their effects on forest ecosystem health have become major sources of concern. Not all things green are fully good. We have many invasive plants in our environment. According to the Invasive Plant Atlas of New England, an invasive plant is a plant that has or is likely to spread into native systems and cause economic or environmental harm by developing self-sustaining populations and becoming dominant or disruptive to those systems.

Perhaps you’ve asked yourself why these plants succeed; why can’t our native species successfully compete with invasives? I’ve often wondered. After looking at the literature, I’m not so sure how it all plays out. The reality is that about 1 of every 1000 non-native plants actually becomes an invasive pest. Biologists call it the “ten-ten-ten rule.” One of every ten imported plants appears in the wild. One of those ten introduced plants becomes established in an ecosystem. One of those ten established plants actually becomes a pest which elevates it to our radar screen. The rigor of this idea has recently come into question. But what can be said is that a majority of plants that have been accidentally or intentionally introduced haven’t yet become problematic. There just may be a lag time (depressing thought).

There are many hypotheses about why invasives are successful. Some hypothesized mechanisms are:

•    Reproductive ability –some researchers argued that invasives have better seed production, better dispersal mechanisms, and/or larger seed production than our native species.

•    Vegetative growth – some assumed rapid growth to occupy fully a site, leaf structure that maximizes photosynthesis and allocates more resources to growth or reproduction.

•    Predation – this one was my assumption. I figured that invasive plants were successful because they had biochemistry that repelled herbivores, or that no herbivores had developed a “taste” for these plants. In the absence of predation and increased “grazing” on competing plants (think about what white-tailed deer will eat – not Japanese stiltgrass or hay-scented fern, but oak seedlings trying to get established), the system gives competitive advantage to the invasive. Diseases are another form of predation. We import plants, not the associated pests and diseases, such as fungal, viral, or other biocontrol mechanisms. In the absence of the plant’s native control these plants have the ability to thrive.

•    Adaptation – some have argued that invasive plants have adaptive phenology, meaning they can leaf out earlier, holding leaves longer to maximize the growing season. Others argue for phenologic plasticity meaning that they can vary their phenology to different conditions, perhaps making them habitat generalists – adapting to a wide array of sites or responding better to disturbance.

•    Allelopathy – This has drawn increased study in recent years. The hypothesis is that invasive plants have biochemistry exudates that alter site conditions such that competing plants are either less competitive in the toxic environment or are directly poisoned. Garlic mustard is the poster child for allelopathy. It does have a negative effect on soil mycorrhizae that many plants need to increase nutrient uptake, but then some argue that these plants eventually destroy the site for themselves.

The reality is that non-invasive plants often have similar adaptive traits. It is likely an over simplification to try to point to one thing and say, “This! This is the reason! We solve this and we solve the problem!” To be a successful plant species there are many ways to compete. Some are jacks of all trades, these are the highly flexible generalists; some invasives might not be so plastic, but rather are specialists, masters of specific habitats and can quickly exploit degraded sites; other plants may exhibit aspects of both robustness to a wide array of conditions as well as the ability to exploit favorable conditions when they arise. Same goes for our native plants.

So why are invasive plants bad? Do we just let this happen and accept a homogenized ecosystem? What do we stand to lose? Unfortunately when invasive plants outcompete our native plants, we tend to lose specialist herbivores (and sometimes whole ecosystems) that have co- evolved with given plants: specific biochemistry that is the only thing edible to a specific insect. Some generalist herbivores, like woolly bear caterpillars, have been shown to feed equally well on native and invasive plants. Many animal and insect species are generalists and will eventually find uses for some invasives. In this case I’m thinking about the recent news that white-breasted nuthatches and three species of woodpecker have developed an affinity for emerald ash borer; however, what are we giving up if we accept the loss of specialists adapted to native plants? What insects? What food resource base goes away?

What’s the real root of the problem? Our behaviors! We create corridors that invite these plants in. Think about Japanese stiltgrass seeds embedded in mud stuck on graders, logging equipment, or vehicles. We move those seeds on average 50 times further than they can go on their own, helping them to take advantage of disturbances that are perfect seedbeds. While not a plant, the corridors by which the hemlock woolly adelgid (an insect which feeds on our eastern hemlock, decimating them across their range) entered Delaware Water Gap National Recreation Area directly followed the Appalachian Trail and its side trails. The horticultural industry is responsible for 60% of the plants species that have become invasive in the United States – things like Japanese barberry, burning bush, purple loosestrife. Conservation practices, trying to prevent erosion, feed wildlife, etc., are responsible for 30% of the invasive plant introductions – think about kudzu, honeysuckle, multiflora rose, autumn and Russian olive. Accidental introductions only account for 10% of invasive plants.

What should we do? Our actions have advanced the spread of many species, many of which outcompete our native plant species, which have evolved in place. Then too, we have inadvertently brought insects, diseases, and animals to our shores that further disrupt ecosystems. Maybe at some point in our history it was okay to “let Nature take its course,” and expect that the forest would recover from whatever we threw at it. But perhaps we’ve passed the point where natural resilience accepts continued perturbations. Simply there are too many threats and challenges. If the hope is to keep things as they were, we have an obligation to act; otherwise we lose opportunity, we lose plant diversity and the systems based on them. And we have to decide if we’re willing to accept those losses.