Top navigation


Invasive Species: Some Science and Some Questions

by Jono Neiger

“Man is part of nature, and his war against nature is, inevitably, a war against himself.” Rachel Carson, Silent Spring, 1962

Invasive species, the new species on the block, have been taking heat for ecosystem changes for several decades. And yet, many of our commonly held assumptions about invasive species are not entirely correct. The science backing up heated claims is increasingly coming into question. As ecological designers, land managers and landscapers we rely on the information we get to inform and direct our work.

Many of us spend large amounts of time getting plants established, maintaining plants, or managing woodlands, edges, and meadows. This is hard work and not made any easier by the tough plants that may already be occupying that space. Observation of invasive species where we live and work seems to confirm our suspicions that they are rapidly taking over.

Early Control Strategies

I first took up the cause of fighting invasive species while restoring riparian forest for The Nature Conservancy on the Sacramento River in California in the early 1990s. Establishing forests in flood prone agricultural fields was difficult but rewarding. Like farming crops, establishing riparian species required controlling the weeds. We took that task seriously and used mowing and hand removal, but mostly we used an ATV spray rig to apply Roundup along the rows of plants. These weren’t just any weeds; we had star thistle, bane of the west; Johnson grass with 6’-long rhizomes; and numerous others. After several years I had been inoculated with the fervor of fighting the battle to get our plants in, and the bad ones out.

The Nature Conservancy restoration project on the Sacramento River 18 years later.

These projects were very successful. I recently had the chance to visit two of the fields we planted about 18 years ago. Our plantings of seeds, starts, and cuttings are now a forest. The stately valley oaks are now big enough that my 13-year-old son could climb them. We saw the massive ten feet tall elderberry bushes and towering willow and cottonwood trees over 30 feet tall. I was moved to see the results of our sweat and toil. A deer ran from us as we climbed over fallen wood and worked through brush and limbs. I found drip line still there, though the rows were now hard to make out through the new growth established among the intentional plantings.

But the strategies we used were very expensive and energy intensive. They relied on a complete domination of the land the same as the farming practices we mimicked. Since that time much of the project focus has been to buy up floodplain land and allow natural systems and succession to establish floodplain forest.

In the years since planting that forest, I have spent many hours trying to understand the feelings that grew in me while we fought the weeds and established “our” plants. When I left the project, I had a passion and fervor for native plants that remains with me. I also had adopted a missionary zeal fueled with anger. In my desire to help natural systems, I found myself at war with a part of it. In the war to heal nature, I felt that invasive plants were the problem.

Reassessing and Questioning

I lost this zeal and anger several years later when a teacher kindly and persistently kept asking what those plants had done to me personally. I was ripping out Scotch broom from a planting we had completed to diversify an orchard into a forest garden. (My teacher kept replanting them….) The proverbial light bulb went off as I saw myself angrily pulling those plants out, believing I knew better how to heal the disrupted landscape we worked in. Suddenly, I felt less sure of myself and full of questions.

The Nature Conservancy project revisted.

Those questions sent me looking to better understand restoration work, the ecology of disturbed landscapes, the ecology of invasive plants, and the reason these newly arrived species are so different. As a pioneer species, the Scotch broom in the forest garden we planted was fixing nitrogen and stabilizing the poor, clayey soils. How was this process different than succession in the woods nearby? Why didn’t it belong there? Was it a problem or a solution? Did succession still operate here and what about disturbance ecology? Were other species helped or harmed by the Scotch broom?

I have since come to have more questions than answers about invasive species. Questions like: How do we separate invasive species effects from climate change, habitat loss, development, sprawl and human population impacts? What objective ecological criteria identify “alien species” or “invaders”? What are the temporal and spatial scales included in the definitions? What protocols will determine the conservation value of new species populations that have moved outside their “historical ranges”? Should we consider the mere existence of an allegedly nonnative species sufficient evidence for control or extermination? On what factual basis can we take this position?

Close observation and good research is required to understand the ecological implications of the changed ecosystems around us. I’ve also looked closely at the emotional reactions of myself and others to the changes in species and ecosystems. The changes we humans are witnessing (and causing) are vast. There is no doubt that we are losing species and habitat; that the climate is changing, getting more extreme; that soils are eroded, compacted, and made toxic; that waterways are polluted; that wells are drawing precious groundwater up faster than it can be replenished; that species are being moved far from where they originated; and that the ground itself is covered over, sealed in a casing of asphalt and concrete.

Defining the Battle Against Invasive Species

It’s change on a scale we can’t imagine. And of course we want to find a way to stop it. And find who or what is responsible. Enter the battle against Invasive Species.

Charles Elton wrote The Ecology of Invasions by Animals and Plants in 1958, using colorful language and metaphor. He essentially started the field of Invasion Biology writing this book. Elton conceived the introduction and spread of non-native species as ecologically distinct from native species dispersal and colonization. After defining invasion biology this way, the field has retained the concepts of invasive species as different and of the invasion process as different from succession ecology and disturbance ecology. Forty-three years later, Davis, Thompson, and Grime looked at research citation practices and concluded that the field of Invasion Biology had dissociated itself from succession ecology and other ecological sub-disciplines (Davis et al. 2001). This distinction between native colonizers and introduced invaders is likely spurious and a hindrance to further understanding the processes of introduced species.

There is no disputing that newly arrived species can, and in many cases do, bring dramatic and alarming changes to ecosystems. Islands like Guam and Hawaii have been hard hit from the brown tree snakes and goats, pigs, rats, and cows respectively. Thousands of species have been moved long distances and are changing ecosystems in known and unknown ways. Many scientists are, however, finding the ecosystem picture far from black and white. Early observations and conclusions are not proving out. Not all interactions are negative and many are positive.

As an example, zebra mussels colonized the great lakes and Hudson River estuary in the early 1990s. Predictions were dire and indeed they have cost industry money to clear intake pipes and outfalls. But the ecological impacts are harder to gauge and continue to unfold over time. Numerous studies show the mussels have dramatically changed water quality parameters, reducing chlorophyll and phosphorous levels and increasing transparency in these highly polluted waters (for instance Fahnenstiel 1995). A cascade of changes in the aquatic food web has ensued. Though phytoplankton levels are down, many reduced fish stocks have returned. In Long Point Bay, Ontario researchers found the introduced mussels are providing food for three duck species increasing waterfowl activity 10-fold (Strayer 2006). These aquatic systems were in a state of decline from decades of eutrophication, water contamination, and species loss. Some species have rebounded, but others still struggle. The ecological effects of the zebra mussel are not all bad. Ecological systems are profoundly dynamic and in many places newly introduced species are adding to the change in ways that can be hard to understand.

The Extinction Quandary

One commonly held (and perpetuated) claim about invasive species is highly questionable. The claim is that “Invasive species are the second leading cause of extinctions”. This declaration was made in an assessment of data sets and personal interviews by David Wilcove and others and published in a 1998 paper titled “Quantifying Threats to Imperiled Species in the United States”. The assessment had serious limitations including partial available data; use of anecdotal accounts; and inclusion of Hawaii, whose distinct conditions greatly skewed the data. The authors themselves stated the limitations to their data. But the paper is widely cited in other scientific papers (over 700 times in the ten years following publication), research proposals, and college classes. In the media the claim of invasive species as the second leading cause of extinctions is presented as fact (sometimes attributing invasive species to extinctions globally), and often the link to where it came from is left off completely. Several authors have reassessed the data and found the link between invasive species and species extinction lacking:

“The link between species invasions and the extinction of natives is widely accepted by scientists as well as conservationists, but available data supporting invasion as a cause of extinctions are, in many cases, anecdotal, speculative and based upon limited observation.” (Gurevitch and Padilla 2004)

A study in Canada analyzing extinction threats found introduced species to be the least important of the six categories analyzed: habitat loss, over-exploitation, pollution, native-species interaction, introduced species, and natural causes such as stochastic events like storms and limited dispersal ability (Venter 2006).

Many questions about invasive species need to be answered. Are naturalized species the drivers or passengers of change? Are the long-term effects negative? Do human-caused invasions differ significantly than “natural” invasions? How do we measure harm and how is harm different from change? How do we separate invasive species effects from climate change, habitat loss, development, sprawl, and human population impacts? Scientists are grappling with these questions. Notably, Dr. James Brown, Distinguished Professor of Biology at the University of New Mexico and Dr. Dov Sax, Assistant Professor at the University of Georgia’s Institute for Ecology, have wrestled with the relationship of invasive species and ecosystems.

“We don’t know how serious the threats of alien invaders are to our native flora and fauna; these are scientific questions. And, as is often the case in science, the answer is less clear than reports in the popular press about the devastating impacts of gypsy moth, zebra mussels, and purple loosestrife might imply…. Although we often accuse invasive species of damage to the structure and function of ecosystems, there is usually little hard scientific evidence of such negative impacts.” (Brown et al. 2007)

An Ongoing Discussion

At the ELA Conference on March 3rd, I will participate in a panel discussion that allows time to delve into more of the research and differing perspectives on invasive species as well as what land management approaches we might take. I hope to engage people in discussion that will bring more consideration to this difficult topic. I know that many people have strong feelings about invasive species. Some feel certain in their approach and others are searching for more understanding. There are no easy answers.

Undoubtedly, we are responsible for the cataclysmic changes we’ve wrought, and we can’t place the blame anywhere else. In the years since I planted trees on the Sacramento River I have learned that we can be a positive part of this process. We need clear thinking, solid research with quality peer review to help direct our management activities. Anything less is wasting our time and efforts and imperiling fragile ecosystems.

Copyright © 2011, Jono Neiger

About the Author

Jono Neiger worked as a restoration biologist with the Nature Conservancy for several years, establishing riparian forest on the floodplain of the Sacramento River in California. Jono is an ecologist/land manager/landscape designer, teaches permaculture design at the Conway School of Landscape Design, and operates a design and implementation firm, Regenerative Design Group in Greenfield, Massachusetts.

Jono, William Cullina, and Peter Del Tredici will engage in a panel discussion entitled “Native, Introduced, Invasive, and Endangered Plants in the Landscape: Untangling the Roots of the Problem” on March 3 at the ELA Conference & Eco-Marketplace. Jono may be found at


Brown, J. H., D. F. Sax, D. Simberloff, M. Sagoff 2007. “Aliens Among Us.” Conservation Magazine, (8)2:14-21.

Colautti, R.I., and H.J. MacIsaac 2004. “A neutral terminology to define ‘invasive’ species.” Diversity and Distributions, (10):135-141.

Davis, M., K. Thompson, and P. Grime 2001. “Charles S. Elton and the dissociation of invasion ecology from the rest of ecology.” Diversity and Distributions, (7):97-102

Davis, M. 2009. “Invasion Biology” Oxford University Press.

Fahnenstiel et al. 1995. “Effects of zebra mussel (Dreissena polymorpha) colonization on water quality parameters in Saginaw Bay, Lake Huron.” Journal of Great Lakes Research, (4):21

Gurevitch, J. and G. Padilla 2004. “Are invasive species a major cause of extinctions?” Trends in Ecology and Evolution, (19):9

Strayer et al. 2006 “Understanding the long-term effects of species invasions”, Trends in Ecology and Evolution, (21):11

Venter, B. et al. 2006. “Threats to endangered species in Canada.” Bioscience, (56):903-910

Wilcove et al. 1998. “Quantifying threats to imperiled species in the United States: Assessing the relative importance of habitat destruction, alien species, pollution, overexploitation, and disease.” BioScience, (48):8. 607-615

Zavaleta et al. 2001 “Viewing invasive species removal in a whole-ecosystem context.” Trends in Ecology and Evolution, (16):454-459