Remembering Why We’re Fighting Invasive Plants

Why do we worry about invasive plants?  Most of us aren’t trying to stave off invasives simply because they aren’t part of the native plant community.  Instead, those invasive plants are causing some kind of problem that disrupts or degrades the diversity and function of an ecosystem.  Plants like crown vetch and Caucasian bluestem, for example, can form near or total monocultures, displacing other plant species.  The result is a simplified plant community that also provides much lower quality habitat for most prairie animals (and livestock, where applicable) and is less resilient to drought or other challenges.  Invasion by eastern red cedar and other trees converts prairie habitat into woodland habitat, making it inhospitable to many prairie animals.  At the same time, tree invasion also decreases available forage for livestock, displaces native prairie plants, and facilitates the encroachment of other invasive plants around and beneath trees.

Eastern red cedar trees can shade out large areas of vegetation and transform grassland habitat into woodland habitat.

As land managers, we need to be able to specifically define the desired condition of our prairies and the precise ways in which invasive plants prevent us from achieving those goals.  After all, our ultimate objective isn’t to eradicate any particular invasive species, it is to create or maintain those defined site conditions.  In the heat of battle, however, it can be easy to lose sight of what we’re fighting for.  We can become so focused on eliminating an invasive plant that we ignore the collateral damage and cause negative impacts to our site that can rival the impacts of the invasive plant.

Eye on the Prize

There are a number of ways invasive species control can go awry when our focus is misplaced.  One example occurs when a land manager goes after the biggest patches of an invasive instead of first concentrating on new infestations.  It can be tempting to start with the largest and most dense areas of invasion, but in many cases, those areas should actually be the lowest priority.  If the goal is to prevent the invasive species from damaging the site, the focus should be on the edges of those big patches, and more importantly, any small satellite invasions that are just starting.  By first attacking new patches, we can contain the invasive threat.  It’s easy to think that killing the most enemies is the best strategy, and that it’s easiest to do that in the most invaded areas.  Usually, however, the best strategy for the condition of the overall prairie is to prioritize the portions that are still in good shape before wading into the already damaged patches.

Another big example of misplaced focus happens when herbicides are applied to large swaths of a prairie, instead of through a targeted spot-treatment approach.  In the worst case scenario, this occurs in a battle against a species that like ragweed or others that aren’t really an invasive threat in the first place.  Let’s assume, however, that the targeted plant is a real problem.  There are certainly times when a broadcast application of herbicides across large dense patches is the best tactic (while remembering the lesson from the previous paragraph), but that comes at a tremendous cost.  Even the more selective herbicide options available today still affect non-target species (the label often provides a long list of species that are resistant to the chemical, but that list is a lot smaller than the species list of most prairies).  Reducing the plant diversity of an area through herbicide application runs counter to most prairie managers’ goals and can also make the same area more vulnerable to future invasion.

When Kentucky bluegrass or other invasive plants become ubiquitous, attacking them with broadcast herbicide application can affect the diversity and resilience of the remaining community. It is usually better to attempt to suppress the vigor of the invasives through the use of fire, grazing, mowing, or other targeted strategies that allows the remaining community to better compete with the invasive plants.

Applying herbicide to individual plants or small dense patches is usually much better than spraying a larger area that contains populations of invasive and non-invasive plants.  In some cases, it’s even more effective to dig out or chop individual plants and avoid herbicide altogether.  Even applying herbicide to a single plant at a time can affect surrounding vegetation.  Sometimes this happens as some spray droplets simply miss the target and hit other plants.  Other times, however, it can happen via volatilization – a process in which some herbicides can transform into a gas and travel away from the plant they’ve been sprayed upon.  Unfortunately, many invasive plants require herbicides to kill them because their growth strategies make digging or chopping attacks unsuccessful.  Just remember that even if only a few non-target plants are killed each time an invasive plant is sprayed, that can add up to a huge impact over many years.

Applying herbicide to individual plants is time consuming (and still affects some nearby plants) but can help preserve the diversity of the surrounding prairie much better than broadcast herbicide application.

Of course, approaches that target individual plants instead of larger areas come with tricky trade-offs regarding time and resources.  Dedicating a lot of time and people to a slow and meticulous approach to invasive plant control can drain resources away from other important land management tasks – including work on other invasive threats.  The flip side, though, is that if our quicker, more aggressive approach is degrading the plant diversity of a site over time, what are we really fighting for anyway?  Creative approaches to finding resources (volunteer labor, better equipment, etc.) may really pay off in the long run.  Regardless, the key is to recognize our overall goals and how each attack option will impact those goals.

Finally, it’s important to focus on broader goals when deciding whether to attempt eradication versus suppression of a particular invasive plant species.  In many cases, invasive plants can become so embedded within a plant community that eradication is impossible without serious impacts on the rest of the community.  Often, designing a management strategy aimed at reducing the dominance of those invasives can maintain a diverse plant community despite the presence of a ubiquitous invasive species.  In prairies invaded by Kentucky bluegrass, smooth brome, or other invasive grasses, for example, strategic burning and/or grazing can suppress the vigor of those plants often enough to allow the rest of the community to thrive.

This prescribed fire helped set back Kentucky bluegrass and smooth brome, allowing a more diverse plant community to flourish. It is a short-term effect and needs to be repeated, but can help maintain a diverse prairie despite a heavy dose of invasive plants.

Measuring Success

Whether we are evaluating the success of our invasive species work via observation or hardcore data collection, we need to be sure we’re looking at the right metrics.  It can be tempting to measure success by counting how many plants have been killed, or how much area has been cleaned up.  However, if our broader goal is to sustain plant diversity, forage productivity, or habitat quality, we should really be measuring the status of those attributes. 

Measuring how many acres have been cleared of eastern red cedar or crown vetch doesn’t tell us whether or not plant diversity is high or nesting bird success has increased.  Focusing on the potential impacts of invasive species to our goals will indicate what to measure.  It might be more difficult to measure nesting success or plant diversity than to count invasive plants or acres.  However, focusing the impacts rather than the abundance of invasive plants is both a better indication of success and a way to be sure our approach to attacking invasive species is actually achieving our real goals.

If prairie plant diversity is the attribute we want to protect from invasive plants, we should measure success by quantifying plant diversity rather than invasive plant abundance or the number of individual invasive plants removed.

The Upshot

If you’re fighting invasive species at your prairie, and of course you are, it can be useful to pause now and then to make sure you’re still focusing on your broader goals.  There will always be difficult decisions to make when combating invasive plants and there are many right and wrong approaches to any situation.  The key is to be sure you’re designing strategies and evaluating them based on the long-term success of the prairie, rather than on how many invasive plants you can kill.  Good luck out there!