“As one grows older one should grow more expert at finding beauty in unexpected places, in deserts and even in towns, in ordinary human faces and among wild weeds.” — C. C. Vyvyan

“But what attracted me to weeds was not their beauty, but their resilience. I mean, despite being so widely despised, so unloved, killed with every chance we get, they are so pervasive, so seemingly invincible.” — Carol Vorvain, A Fool in Istanbul: Adventures of a Self-Denying Workaholic 

As I searched for quotes to create a backdrop, a tenor for this essay, I was struck by the unexpected overwhelming positivity that came across with respect to weeds. The problem with defining and understanding weeds is that there is such a broad definition of what constitutes a weed. The most inclusive definition could be “any organism that has become established outside of its native range.” That definition could include us, humans, across much of the earth’s surface. Being a “weed” in and of itself should not engender a desire to eradicate species from regions where they have not evolved. In many respects, weeds could and should be admired for their tenacious ability to survive in new lands. That tenacity is largely a result of the fact that the “weeds” left their predators and parasites back in their homelands where they evolved, not from some inherent drive or ability to dominate their colonized lands. Back in their homelands they lived integrated into an ecosystem of coevolved species, their populations checked and balanced by their interactions with other native species. In their new, colonized lands, sans predators and parasites, their populations can sometimes grow unchecked.

Every year, and likely every day, we (the human species) are redistributing species around the world, sometimes purposefully. But more often than not the vast majority never become established in their new lands. Early European colonists brought with them earthworms, honeybees, and some had the idea of bringing with them the birds of their homelands whose songs would provide a pleasant reminder of whence they came. Some of those European colonists sought to introduce birds mentioned in Shakespeare’s plays and sonnets into New York’s Central Park. Shakespeare’s works mention choughs, wrens, cormorants, owls, nightingales, larks, and some 60 other species, but just two successfully became established, starlings and house sparrows. These species’ success was based on their finding an “empty niche” to exploit upon arrival in America – farmlands, suburbia (European constructs), and more recently, fast-food restaurants.

While there have been negative effects on native birds (starlings usurp holes in dead trees otherwise desired by bluebirds, wrens, and tree and violet-green swallows) those negative effects are mostly confined to lands already converted to farmlands and suburbia. In natural areas the native birds still thrive. Similarly, while honeybees have been a boon for pollinating farmer’s crops, they have had negative impacts on native bees. But so far that negative impact is mostly focused on human altered landscapes. Another example of exploitation of empty niches are Mediterranean house geckos, now inhabiting the walls of houses in every warm climate city of the world, a "new" niche rarely exploited by native species. Despite the ongoing introductions of new species, relatively few have become established. Unfortunately, those few have had large impacts.

Weeds deserve a bad reputation where and when they alter ecosystem processes and in doing so diminish native biodiversity. A non-native species finding an empty niche, or fitting into an existing niche alongside of the native species is of little concern. Concerns arise when those weedy species reduce natural biodiversity. I have heard some folks argue that we should accept an inevitable future of cosmopolitan species dominating even otherwise natural landscapes. Admire their tenacity, rather than condemn their homogenization of the earth’s biodiversity. Such arguments fail to appreciate the importance of biodiversity for powering the well-being of our planet.

Examples of weeds that have and are in the process of reducing biodiversity include kudzu (pictured above, photo by Sendy Hernández Barrows). Also called Japanese arrowroot or Chinese arrowroot, kudzu refers to a group of climbing, coiling, and trailing deciduous perennial vines native to much of East Asia, Southeast Asia, and some Pacific islands. In the southeastern portions of North America, kudzu was introduced by people as an ornamental plant and to control erosion along riverbanks. Without any predators or parasites kudzu escaped managed landscapes and literally created a green “blanket” overwhelming and smothering native vegetation. Without access to sunlight, native plants whither and die. No birds use kudzu and few if any native insects, reptiles, or mammals use kudzu as habitat. The result is a green wasteland, a monoculture, bereft of biodiversity.

In the Sonoran Desert the worst weed culprit is buffelgrass, introduced from Africa with the objective of providing feed for open range livestock. Buffelgrass is adapted to a summer rainfall pattern, grows like crazy, and leaves no space for wildflowers, native shrubs, or seedling cacti. Buffelgrass is highly flammable and thrives in a regime of frequent fire while the native desert plants, including even the giant saguaro cacti, cannot tolerate wildfire. Fire was never an ecosystem process of the North American deserts and so the plants here have no adaptations to survive such fires. Frequent fires fueled by buffelgrass result in a buffelgrass monoculture. Hence, the Sonoran Desert’s rich biodiversity is eliminated, along with the important carbon-sequestering function of desert trees and shrubs such as creosote bush, ironwood, and palo verde. No birds, no lizards, no tortoises, no snakes, and none of the myriad of insect pollinators that characterize the rich desert landscape.

The same problem exists in the Mojave Desert, although the fuels that feed the wildfires are different. Cheatgrass, red brome, and Mediterranean split grass are all non-native annual grasses, introduced from the Mediterranean region, again to support open range livestock. These grasses became an ecosystem changing problem as a result of nitrogen deposition from smog generated in the Los Angeles basin. The nitrogen fertilizes the Mojave Desert giving these weedy grasses the boost they needed to fill in the spaces between the desert shrubs, crowding out the annual wildflowers. By summer, when annual wildflowers would have long-since dried, dropped their seeds and blown away, the dried stalks of non-native grasses persist, carpeting the desert floor. If the monsoons arrive with scattered lightning strikes, the grasses provide a continuous fuel that allows wildfires to spread across thousands of acres, killing Joshua trees, creosote, and blackbrush. No nurse plants survive to provide protection for young Joshua trees, no downed Joshua tree branches persist to provide habitat for desert night lizards. Deserts are not adapted to wildfires and can take decades and likely centuries to recover, and then if and only if the weedy grasses are gone. However, the weedy grasses evolved with fire and so thrive with a repeating fire cycle.

In the drier-hotter Colorado Desert, wildfires are mostly restricted to the slopes of its sky islands, with non-native fountain grass fueling fires lower and lower down the mountain sides. Although not necessarily a fire-fueling weed, Sahara mustard is changing ecosystems on the desert flats. Sahara mustard was inadvertently introduced to the Coachella Valley with the first date palms brought in the early 1900s. The mustard spreads across sandy dune areas when there are particularly strong early winter storms;  sprouting first and then stealing moisture from the later-sprouting native wildflowers. The fast-growing mustard shades out any natives that try to compete with it, blocking the sun and pollinating insects from reaching the native plants’ flowers. The mustard produces abundant seeds, and so for seed-eating creatures it might seem like a resource bonanza. It turns out that at least for seed-eating insects like harvester ants, mustard seeds are inedible. Adding further insult, even after they complete their life cycle and wither and die, the dry mustard stalks persist. They block the wind and stabilize the sand dunes, altering habitat quality for the fringe-toed lizards and flat-tailed horned lizards, and reducing the abundance of one of their critical food sources, harvester ants.

Not all invasive species are equal in their impacts, and so grouping them all together as noxious pests or admirable competitors is not justified. Each species should be evaluated as to its positive, benign, or negative impacts to natural biodiversity. People bring these species with them as we colonize and exploit new lands. Those that are brought across vast oceans, far away from their population controlling predators and parasites, and those that are adapted to exploiting existing natural habitats have the greatest chance of damaging biodiversity and altering ecosystems.