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  • Writer's picturerichelletanner

One plant’s paradise, one population’s problem

Within minutes of arriving on Catalina Island, visitors will either see, smell, or be recruited to pull the invasive plant species, fennel. For those who live and work on the island, fennel has been engrained in their minds as the devil weed. After only three days of training at the Catalina Island Conservancy (CIC) under the Catalina Habitat Improvement and Restoration Program (CHIRP) and the American Conservation Experience (ACE) workers, I have adopted developed the mindset that fennel removal is a necessary evil. However, when my colleagues and I spend hours in the field hiking through poison oak and cacti just to dig fennel out of the hard-packed soil, I often wonder exactly why I feel such animosity toward this plant.

Fennel with taproot. Photo by Richelle Tanner.

Fennel is a highly invasive species to Catalina Island (hereafter referred to as CI), having been introduced in the 1830s by European settlers to feed their grazing animals and landscape homes. It is extremely hearty and can quickly turn a hillside into a monoculture of its own species, outcompeting native plants for sunlight, nutrients, and water. Fennel grows in dense patches and its roots can reach up to ten feet below the ground, not to mention its persistent seed bank – a single plant can produce upwards of 100,000 seeds per year! If fennel is allowed to overtake the species endemic and/or native to the island, it will facilitate a loss of overall plant biodiversity, even if many plants on CI will survive. When species are lost from a habitat, the world biodiversity levels are changed along with those of the affected area because the world biodiversity levels reflect the total number of species species in every habitat. By eradicating fennel from CI, native plants would not be threatened with extinction, and fennel would continue to live in its native habitat in the Mediterranean region.

How did we decide that fennel is an invasive species in the first place? After all, every species (both plant and animal) on CI was introduced at one point in time, since the island was once a bare rock. It is generally agreed upon that all plants existing before European settlements are considered native or endemic to the region. It can be thought of as a distinction for convenience, as it is known that Native Americans brought plants for food and medicine, and animals for food and even as pets when they came to CI.

The CIC and residents of CI alike have done a phenomenal job of making fennel removal a top priority, as demonstrated by the number of Conservancy staff dedicated to invasive plant removal. But despite the efforts, fennel remains a prominent part of the CI ecosystem and continues to infest new areas around the island. Somewhere along my own journey of fennel removal, I have begun to question the notion that fennel can be eradicated. On one hand, the fennel that I helped to remove at the USC Wrigley Institute has not grown back significantly. On the other hand, CHIRP’s extensive work in Middle Canyon still shows fennel regrowth in new areas when we go back to check, years later. While I am not entirely sold in either direction, fennel plays a large part in CI’s ecosystem, whether it is for better or worse.

Methods of removal

There are three main tested ways to remove fennel from an ecosystem, according to our mentors at CHIRP. The most widespread method of removing fennel involves completely digging up the plant, including the taproot. This can be achieved using a shovel, grubber, or a pilaski, all of which require a bit of physical labor. Another method is actually only a temporary fix – mowing or trimming the stalks but not removing the taproot will lead to yet another crop of fennel when it grows back. We have noticed this phenomenon in our experimental fennel plots at the USC Wrigley Institute – in the span of a year, the clipped fennel grew back to match the number of plants in the plots of fennel where we did nothing. The clipping method is often used in tandem with herbicide to open up space for native plants to come in after the fennel is killed. These two methods, while one is more effective than the other, are completely manual and require no inorganic or non-native substances to be introduced into the environment. The last method for fennel removal, or rather fennel killing, is herbicide. A number of chemicals including Glyphosate Pro IV, Rodeo, Garlon 4, Milestone and Fusilade II are used to eradicate fennel without manual treatment.

Rebecca from CHIRP and Casey removing fennel manually and with herbicide. Photo by Richelle Tanner.

A world without regular fennel removal

It is not that I believe invasive species removal is a waste of time in general, but in this case, fennel is very widespread throughout the island and can be easily reintroduced by deer. This easy transport rules out the idea of selective eradication in certain spots, as we have seen that the fences put up to keep deer out have failed. If CHIRP were to continue with its mission, how many years would it take to achieve complete eradication? Missing only a few plants would result in the failure of the project – this is highly likely due to the inaccessible nature of most of the island.

On the other hand, if fennel eradication were to be successful, there are a number of adverse effects that this might have on the ecosystem. Since fennel is so widespread, its complete removal would leave space for other species – which could also be invasive – to cover that area. It could create a completely separate invasive species problem with another species. Fennel could also drain the nutrients from soil in areas they are in monoculture, making it even more difficult for native species to take over the newly cleaned area in the event of complete eradication. Ecosystem rehabilitation would be a long process that the CIC native plant nursery would be charged with, as well as replanting with native and endemic plant species. One common issue that would not be a problem on CI is additional pressure on native plants by existing herbivores. The deer, an invasive species themselves, already prefer native species to the fennel, but even with this being the norm, we have found deer-browsed fennel in the field. If the fennel is removed, there is a chance that the native species would face greater pressure from herbivores.

Another option for removing fennel is to find a way to make it economically advantageous to remove. If it can be used in a commercial product, there would no longer be a need for removal of the plant by private entities such as the CIC – companies would be willing to pay to remove it! While fennel may be recognized as a common culinary ingredient, CI’s fennel is not synonymous with this species – it is largely inedible. Seeing as another commercially viable option has not been discovered yet, CHIRP will continue with their long and arduous mission. However, I think the conservancy would benefit from a cost-benefit analysis of removing this rampant invasive – perhaps they will find fennel can be better managed with other techniques.

Works Cited

Cal IPC, (2011). California invasive plant council symposium 2011. Retrieved from California Invasive Plant Council website:

Erskine Ogden, J. A., & Rejmánek, M. (2005). Recovery of native plant communities after the control of a dominant invasive plant species, foeniculum vulgare: Implications for management. Biological Conservation, 125(4), 427–439. Retrieved from

Pirosko, C. Cal IPC, (2004). Proceedings california invasive plant council symposium volume 8. Berkeley, CA: California Invasive Plant Council.

Reaser, J. K., Meyerson, L. A., Cronk, Q., De Poorter, M., & Eldrege, L. G. (2007). Ecological and socioeconomic impacts of invasive alien species in island ecosystems. Environmental Conservation,34(2), 98-111. Retrieved from

Sahagun, L. (2012, December 24). Catalina island conservancy loses 3 more board members. Los Angeles Times. Retrieved from

Sahagun, L. (2013, February 19). Allegations made against catalina conservancy exec. Los Angeles Times. Retrieved from

Zavaleta, E. S., Hobbs, R. J., & Mooney, H. A. (2001). Viewing invasive species removal in a whole-ecosystem context. Trends in Ecology and Evolution, 16(6), 454–459. Retrieved from

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