CO-Horts Blog

Thursday, June 6, 2019

Noxious weeds can reduce ecosystem function

By Irene Shonle, Gilpin County

Noxious weeds pose a growing threat to our general ecosystem.  These non-native plants  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.
Oxeye daisy monoculture

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 fairly host-specific and may not recognize unfamiliar exotic plants as food.  Many mammals also may hesitate at eating new plants. When weeds crowd out the familiar native plants, the entire food chain is weakened.
This hummingbird moth caterpillar is eating native fireweed

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.
Large Canada thistle infestation

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 in your area (here is a link to all the Noxious weeds in the State:  Then, take care of them on your property, and if you see weeds on public property, notify the appropriate land manager. This is an easy way we can all contribute to the health of our ecosystems.


  1. If we could only get smooth brome declared a noxious weed ...

  2. Thank you for the well researched education Irene. May it spread like WILDFIRE, and may master gardeners help ignite the spark! Molly

  3. Irene: thanks for this very timely article and its research support. Not only master gardeners but native plant advocates could benefit from this information, along with control strategies.

  4. Great article!!! We teach classes on Native plants vs introduced plants. We also present a class on attracting Native pollinators to the backyard. This is a different thought on how noxious weeds plays a part in the stressors of native pollinators and the food chain. Presenting this information could be tricky as it would need to be done in a responsible manner to avoid an individual going out and annihilating an area of noxious weeds thinking they are doing good when in fact their reaction to this info could have adverse affects also... a follow-up to this topic would be "Now what are some of the best ways to eradicate these noxious weeds in an environmentally friendly, safe manner". At our garden center we educate our guests about using pesticides and herbicides with the hopes that they will apply these products wisely. As I said, maybe a follow up to this topic of how now to remove the noxious weed in the best manner would needed. Hmm... I see a new class in our future????