Article and photo by Chris Helzer, Eastern Nebraska Program Director – The Nature Conservancy
Do you think we are being too hard on invasives?
Stop worrying, and just "embrace the change"?
A program director for The Nature Conservancy
gives us his take on the issue.
This is a continuation of the series that began in the May/June issue of the Journal, in response to a call from some biologists that we "embrace the change" that invasive, non-native plants represent. Chris Helzer brings to the discussion the perspective of someone who manages 5,000 acres of reconstructed prairie land in Nebraska, for The Nature Conservancy.
The World of plants is changing.
We need to decide what's worth fighting for.
We live in a changing and confusing world. Non-native species are becoming increasingly abundant members of North American ecological communities, and it doesn't look like the influx is going to slow down. What are conscientious ecologists to do? Do we give up our conception of what "native" communities are, and let in all comers? The answer is clearly...it depends. I do think we need to relax our purist stance toward ecological communities a little, and accept that many, possibly all communities, will include significant numbers of non-native species. On the other hand, I certainly don't think it's smart to tear down our figurative fences, and allow every conceivable species entry. Since it's not possible to prevent all non-native species from invading our natural communities, we have to develop criteria for determining which species are worth fighting.
In order that a species be designated as invasive, most definitions include some requirement that the species cause "harm" to ecological or agricultural systems, or to human health. The problem is that we have trouble defining what "harm" means, and which species inflict it. It's tempting to say that any non-native species is causing harm to an ecological system just by entering it, but that's no longer a sufficient argument. The approach most useful to me is to evaluate the impact of a new species on the ecological resilience of an ecosystem or natural community.
C. S. Holling, one of the conceptual founders of the Society of Ecological Economics, in 1973, defined ecological resilience as the amount of disturbance that an ecosystem can withstand without experiencing changes in self-organizing processes and structures. In other words, how far can you push an ecosystem before it becomes something else? The more resilient an ecosystem, is the more stress it can withstand without losing its integrity. Once it's pushed past a threshold, however, the ecosystem leaves one stability domain (stable state) and transitions into a new one – and it's tremendously hard, or even impossible – to go back. An ecosystem's ability to withstand stress and to resist slipping into a new stable state is largely determined by its diversity and complexity. For instance, a forest with a large number of insect-eating bird species may have a greater ability to counter an outbreak of a pest insect than a forest with fewer species because of the higher total number of birds available to feed on the pests. A prairie with a high diversity of wildflowers is more likely to sustain steady pollination services across the prairie than one with fewer species, because, at any one time, pollinating insects have multiple species and abundant flowers to feed from.
Since ecological resilience relies on complexity, diversity, and redundancy, it seems to me that the designation of a species as invasive or not can often hinge upon the answer to a single question: "Does the new species simplify the ecological community/ecosystem or add to its diversity?" For example, a species that enters a plant community without displacing existing community members is likely adding to the diversity – and thus the ecological resilience – of that plant community. While we might define it as "non-native," it seems foolish to try to stave it off unless we have the spare time and resources to do so. In fact, the species may help bolster scarce resources for pollinators, or provide an additional source of seeds for over-wintering small mammals, and thus increase the chances of survival for other species in the ecosystem.
An example of a species that fits this definition for me is the common dandelion (Taraxacum officinale). I don't include common dandelion in the seed mixtures I use for prairie restoration projects, but I fully expect to see it in both restored and remnant prairies – and I can live with that. It doesn't appear to reduce the overall plant diversity of the prairie around it, and it's one of the most heavily used flowers by native bees in the early part of the growing season.
On the other hand, there are plenty of species that I work hard to suppress and eradicate from my prairies. I know from experience and from others' research that they displace native species and reduce the diversity of the plant community. In other words, they simplify the ecological system as they invade. Species such as crown vetch (Securigaria varia, formerly Coronaria varia) and smooth brome (Bromus inermis) are easily identified as true invasive species in most areas. Crown vetch forms dense monocultural patches, within which few, if any, other plant species can survive – and is very difficult to eradicate once it's established.
Smooth brome can also form monocultural patches, but more often, it infiltrates prairie communities, and squeezes out other species as its own density increases. We work diligently to prevent new patches of crown vetch from establishing, and are vigilant about repeating herbicide treatments on existing patches until we're sure they're gone for good. With smooth brome, we take a suppression approach, and do our best to reduce its vigor frequently enough that other plant species have a fighting chance for survival.
It's not always easy to determine what the long-term impacts will be of an introduced species on the surrounding ecological community. Often, by the time we discover that the new plant or animal is becoming dominant at the expense of other species, it's too late to reel it back in. To further complicate matters, a species may be invasive in some places and elsewhere a relatively benign member of an ecosystem. For example, common mullein (Verbascum thapsis) acts exactly like a large dandelion in my prairies. It increases in abundance during drought years or under intensive grazing, but quickly gives way to native perennial grasses and forbs when moisture returns or livestock grazes elsewhere. However, colleagues to the west of my area of eastern Nebraska, see mullein act very differently. Its abundance increases when the surrounding plant community is stressed, but its presence does not appear to diminish markedly when the stress is relieved. The result is a simplified plant community that is likely to be increasingly dominated by mullein. My colleagues are justified in designating it an invasive species and taking appropriate action to control it.
Unfortunately, the difficulty in determining which species are invasive and which are simply new does not release us from our obligation to do our best to make those determinations early, and respond accordingly. Yes, the world is changing, and that creates confusion, but that process of change makes it even more important for us to build and maintain ecological resilience in natural areas. It's inevitable that new species will continue to be purposely and accidentally introduced to our natural areas. Our responsibility is to make sure those new species do not simplify ecosystems that need complexity to survive.
Return to Invasive Plants page.