Invasive Species

The first couple decades of the 21st century saw the rise, and hopefully the fall, of a genre of books, opeds, and articles that sought to minimize or deny altogether the threat posed by invasive species. Opposed to the demonization of invasive species, the writers ironically proceeded to demonize land managers and native plant advocates. I'd like to think my critiques of this misguided genre have contributed to its demise. 

A species becomes invasive when it is freed of nature's accustomed checks and balances. Invasiveness is a behavior, not an inherent trait. As species in a region co-evolve over thousands or millions of years in proximity to one another, a balance between reproduction and predation is struck. Nature achieves balance primarily through consumption. As a species proliferates, it becomes an abundant food source for any predator or pathogen that has evolved to consume it. A classic example is the rabbit and the fox. If the rabbit population increases, the fox population increases due to the abundant food source, which in turn leads to a reduction in the number of rabbits, at which point the fox population drops for lack of food, allowing the rabbit population to rebound. The rabbit, having evolved with the fox, is good enough at eluding capture that the fox is not able to catch all the rabbits, which preserves not only the rabbits but also the fox. No predator can succeed if it eliminates its prey.

The great majority of species that are behaving invasively have been introduced from other continents. They may have been intentionally introduced for commerce or erosion control, or hitchhiked on planes, boats, shoes, or in soil. Only a small portion of introduced species become invasive, but it is difficult to guess which ones will become problematic. Native species have been "vetted" over millenia; a species that is taken out of the region where it evolved and then introduced to a new land has no track record, and oftentimes no predators or prey that have evolved to control its numbers.

A common trait of non-native plants that become invasive is that they are not eaten by the herbivores that normally keep a plant species in check. Herbivores, be they deer or insects, seem extremely slow to change their dietary preferences, and so we have introduced plants like honeysuckles, winged euonymus, stiltgrass, and porcelainberry, some of which were introduced many decades ago and still are rejected by herbivores that could potentially curb their growth.

Invasive behavior can usually be traced back to human impacts that have altered the balance previously achieved by nature over millenia. We impact ecosystems by removing or introducing species, or altering hydrology or fire regimes. Taking responsibility for these impacts means actively managing habitats to restore or compensate for what has been altered.

A native species can become invasive if we remove its predators. Deer proliferate for lack of predators to consume them. Since deer prefer to eat native plants, intense browsing pressure leads to a decimation of native plants in the forest, creating a void that in turn is colonized by invasive plants the deer won't eat. Over time, the forest becomes dominated by inedible plants, and the effective habitat for wildlife shrinks, undermining the goals of open space acquisition. If we wish not to have wolves or other wild predators in our woodlands, then we must play the role of the missing predator, limiting the deer's numbers through hunting and professional culling. In Princeton, 20 years of deer management has led to an impressive rebound of native plant species, such as the spicebush, which in turn improves habitat for birds.

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