Irene Shonle, CSU Extension Gilpin County. Noxious weeds pose a growing threat to our general ecosystem. They are non-native plants that have a competitive advantage over our native plants because of the lack of insects and diseases that control them in their native countries, and they are spreading rapidly on both private and public lands. 

The reason this is such a problem is that these new invaders do not have an evolutionary history with all of the rest of the plants, insects, birds, and other wildlife that depend on our ecosystem. The introduction and establishment of invasive plants into new habitats in which they have not coevolved with the native organisms have been identified as a major threat to biodiversity and ecosystem structure and function (Bezemer et al. Annual Review of Entomology 2013).

Plants are the bottom of the food chain, with insects and a variety of mammals eating the plants directly, and then other species such as birds and mountain lions eating the insects and mammals. Non-native plants such as noxious weeds don’t provide that bottom rung of the food chain. This is because 90% of herbivorous (plant-eating) insects are quite host-specific and don’t recognize unfamiliar exotic plants as food. Many mammals also hesitate at eating new plants. When weeds crowd out the familiar native plants, the entire food chain is weakened.

Examples of our ecosystems collapsing due to this weakening are coming at an alarming pace. A recent paper (Sánchez-Bayo et al., Biological Conservation 2019) presented a comprehensive review of 73 historical reports of insect declines from across the globe. More than 40% of insect species are declining and a third are endangered, the analysis found. The main drivers of species decline, according to the study, are (in order of importance): “i) habitat loss, ii) pollution, iii) biological factors, including pathogens and introduced species; and iv) climate change.”  The effect of noxious weeds can be seen twice here, once in habitat loss, and once in the introduced species bucket.

Additionally, noxious weeds have the potential to reduce the connectivity of native plant patches by forming large monocultures. Wide swaths of noxious weeds could isolate suitable habitat patches, thereby increasing the local extinction risk of native herbivores, pollinators, and their natural enemies, since many insects fly only short distances and smaller populations are more vulnerable to getting wiped out by natural disasters or other events.

Native pollinators are critical for pollinating native plants (which brings future generations of plants), and invasive weeds can disrupt their pollination. Several large analyses have shown that invasive plants, on average, have a negative effect on the visitation rates of pollinators and reproductive success of the native plants around them (Morales and Traveset, Ecological Letters, 2009 and Montero-Castano and Vila, Journal of Ecology, 2012).

So, these are all reasons why it is so important to keep the populations of noxious weeds to a minimum on both private and public land. Learn how to recognize the noxious weeds that are particularly bad in our area: scentless chamomile, oxeye daisy, Canada thistle, musk thistle, diffuse (and spotted) knapweed. Cheatgrass and yellow toadflax are also right up there. Then, take care of them on your property, and notify the appropriate public land manager of their presence if you see them. This is an easy way we can all contribute to the health of our ecosystems.

The CSU Gilpin County Extension Office is located at the Exhibit Barn, 230 Norton Drive, Black Hawk, CO 80422, 303-582-9106, www.gilpin.extension.colostate.edu. Colorado State University Extension provides unbiased, research-based information about, horticulture, natural resources, and 4-H youth development. Colorado State University Extension is dedicated to serving all people on an equal and nondiscriminatory basis. 

(Originally published in the June 13, 2019, print edition of The Mountain-Ear.)