Written by Stacy Endriss is a 2016-2017 Sustainability Leadership Fellow and Ph.D. Candidate in the Department of Bioagricultural Sciences and Pest Management and the Graduate Degree Program in Ecology.
A beautiful landscape:
It’s a morning in late June and I close my eyes, tilting my head into the warmth of the sun just as it peaks over the neighboring foothills. Then I listen.
In less than a minute I hear the beginning crescendo of an apian symphony. First, a persistent high-pitched whine fills my ears, sustained by hundreds of foraging honey bees. I can hear each individual female, not by her sound, but by its absence: her buzzing becomes more staccato, punctuated by brief seconds of silence as she stops briefly to scrounge for food within the newly blooming flowers. Next, a deeper more intermittent hum quickly darts in and out of my hearing. Bumblebees, it seems, are more fickle in how they flit from plant to plant.
My ears tell me this is a thriving, healthy habitat. However, when I open my eyes I am met with a different story. Common mullein overwhelms the landscape, pale green stalks covered in tiny yellow flowers blocking the view of the neighboring creek. Interspersed among these plants are ugly, brown stalks, bleak tombstones of the now-dead plants of last year. Looming above everything else are brief splotches of purple, clumps of musk thistle that haphazardly dot the landscape. Even my feet, I find, are enfolded by waves of gently nodding cheatgrass, the still-green seeds already stuck in the creases of my snakeguards and the eyelets of my boots. I am surrounded by invasive weeds.
Perhaps I should feel disgust for this less than ‘pristine’ habitat. Yet, to me, this is beauty. We often vilify invasive species. But like any good villain they have depth, a complexity that allows for both good and bad.
A story of survival:
If allowed to tell their story, invasive species would undoubtedly be the epic heroes, the unwitting protagonists in a tale of a small population overcoming innumerable barriers to survive and flourish in a foreign land.
Their journey is one of hardship and perseverance. Being brought to a new place is rare, and surviving long enough to reproduce rarer still. As newcomers to their neighborhoods, these plants often have difficulty finding mates, and are more vulnerable to being wiped out by chance events such as floods, lightning strikes, or even the hoof of a passing deer. To make matters worse, thanks to natural selection they are especially equipped to survive in their native habitat, not this drastically different land they now inhabit.
So what is it about these plants that allowed them to overcome the odds? To outcompete the native plants that had successfully survived in these environments for thousands of years?
The key to success:
Invaders often rapidly adapt, quickly changing their looks and personality to better match the unique challenges of their new home. What’s more, these changes are often consistent, predictable regardless of the plant in question. Most invaders grow bigger, produce more but smaller seeds, and reproduce faster than their compatriots back home. Why is it that they often change in the same way?
Similar changes may mean plant invaders face similar challenges. For example, the type of plant-eating insects they must defend against often differs between their old and their new homes. Within a plant’s native habitat they are often eaten by many different insects, but mostly by specialists. Specialists are like the toddlers of the insect world, extremely picky in what they eat, but likely to gorge on what they find good. On the other hand, when plants are brought to a new habitat they lose the specialists that have fed upon them for thousands of years, and are attacked mostly by generalists. If specialists are the toddlers, generalists are the teenagers, game to eat most anything.
Understanding how plants shift their defense in response to specialists and generalists is surprisingly difficult within native populations, as we must first tease apart the separate effect of specialists and generalists, but both are feeding on the same plants at the same time.
However, invasions have offered much needed insight into how plants change when they consistently experience these differences for hundreds of years. Invasions provide two sets plants from the same species that have experienced two very different types of attack: one mostly by specialists and one mostly by generalists. By studying how plants differ between their native and introduced habitats, we can begin to pick out the traits that play an important role in defending against insects, and how flexible they are at adapting to sudden shifts in insect communities.
In addition, plant invaders often must adapt to more than just differences in the insect community. In their new home they often experience new climate, new plant competitors, new pathogens, new pollinators, and many more factors we are only just beginning to understand. In this way, plant invasions are one of nature’s greatest experiments, their differences allowing us to finally understand how plants adapt in response to many, very specific, types of environmental change.
Villains to the rescue
Successful invaders are, for us, often a great source of dismay. They destroy native habitats and overtake farmers’ fields. They increase wildfires, devastate recreational areas, and cost billions of dollars each year in management and lost revenue.
Yet some good may come from better understanding their story. Like invasive species, native plants are engaged in their own epic battle against natural selection. In today’s world of rapid global change, their habitat is changing at an increasingly rapid pace. Climate is shifting. Globalization is increasing. And with these changes come cascading consequences. Changes in climate may alter wildfire or flood regimes. With increasing globalization comes greater development and disturbance of native habitat. As transportation improves, plants and animals often hitch a ride, meaning that native plants must interact with an increasingly novel suite of plants and animals.
All of these new challenges may seem overwhelming. Yet these challenges are the very same barriers already successfully overcome by many invaders. Where many native plants fail, invasions not only succeed, they flourish.
In this way, understanding how plants adapt to their new habitats doesn’t just help manage for future invasions, it can also help protect the native plants we care about. By understanding which traits allow invaders to succeed when they experience a new habitat, we can potentially identify which plants may be flexible enough to adapt in the face of rapid change, and focus on protecting those that are not. Environmental change is inevitable, but least we can be prepared to help our native plants survive and persist.
Standing in the midst of a floral oasis, listening to the cacophony of thousands of beating wings, it is hard for me to feel hate for these invaders. Instead I see survivors, and stand full of admiration. The irony is that we often worry that invaders may be the downfall of native species, but they just may hold the key to their success. Perhaps it is invaders’ more villainous qualities that may actually be their most redeemable. And that if we listen carefully enough to their story, we may just be able to use these qualities to fight for a better, more sustainable future.
Intro photos caption: On the left a honey bee forages for food at a mullein flower, her legs already coated in mullein's orange pollen. On the right is a honey bee look-alike, a two-wing hoverfly stealing some sugary nectar.