What are Native Species?

When settlers first made their way to Colorado in the early 1800’s, they brought with them seeds of plants from all over the world. Some were seeds planted in the Old World for food crops, windbreaks, landscaping, erosion control, and livestock forage. Many other seeds arrived accidentally, mixed with crop seeds, animal feed, or even in the ballast of ships. These came westward with shipments of agricultural goods, or they were dispersed along irrigation ditches, railroad tracks, wagon roads, and cattle trails. Today, either by accident or design, the introduction of nonnative plants to Colorado continues. Because of past and present human activities, Colorado’s landscape is replete with “nonnative” species.

But what is a native plant? The contributors and reviewers of this guide spent a great deal of time wrestling with this question. In the end, it was easiest to start by defining which plants were nonnative. Although there are other perspectives, the group decided to use the definition from Colorado's Weed Management Act. The act defines a nonnative plant species as: “. . . a plant species which is not indigenous to the state of Colorado, nor to the native plant community in which it is found.” Title 35, Colorado Revised Statutes: Colorado Weed Management Act.

This latter definition will be used in this guide. Conversely, a native species is defined as: “A plant species which is indigenous to the state of Colorado, or to the natural plant community in which it is found.”

The terms alien, nonnative, exotic, and adventive are all expressions used to describe plants that have been introduced to Colorado. To keep things simple, this guide uses “nonnative” throughout.

 

Why we should be concerned about the presence of nonnative plants in Colorado?

Many, but not all, nonnative plants spread rapidly and outcompete native species for water, light and soil nutrients. Native plant species have evolved with local herbivores and diseases that regulate population numbers. In contrast, nonnative species frequently have no local predators acting to keep population sizes in check. Many nonnative agronomic species were bred in the Old World for rapid growth, prolific reproduction, and ability to tolerate both disease and a wide range of soil, moisture, and light conditions. Nonnative weeds have often had several thousand years evolution in the presence of human disturbances, resulting in enhanced growth, reproduction, and environmental tolerance, similar to what was purposedly bred into agronomic species of the Old World. While these characteristics are valuable for agricultural crops, they enable nonnative species to become aggressive invaders of native ecosystems. If left uncontrolled, these species often form extensive single-species stands where once there were diverse and productive native communities.

Nonnative species cover bare ground with dense greenery and sometimes with showy flowers, but may provide little in the way of habitat values or plant community diversity and structure. For example, a marsh full of purple loosestrife in bloom makes a beautiful impression from a distance. However, closer observation will reveal a lack of other plant species and a dearth of animal life. The same observation can be made about areas where knapweeds, thistles, leafy spurge and other nonnative plants have become established.

Plants such as knapweed and leafy spurge release toxins into the soil which inhibit the germination of native species. In addition, many nonnative species are not palatable to most North American animals. Such characteristics enable nonnative plant species to establish large monotypic stands that deplete the soil moisture and shade the ground, eliminating chances that native plants will germinate and grow."

 

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