Humans have played a key role in moving species to new locations, resulting in an exponential spread of species over the last century. Many of these nonnative species never become invasive – that is, damaging – and a few may even have positive effects on ecology or human economy. However, many, such as Asian carp in North American rivers and Burmese pythons in the Florida Everglades, cause enormous ecological and economic damage.

Damage caused by invasive species has been estimated to cost the United States nearly US$120 billion per year. But even this figure may be low because it quantifies only direct costs associated with invasive species – for example, crops lost to invasive pests, or the expenses and potential ecological impacts associated with invasive population control.

Invasive species also cause other kinds of harm that are difficult to quantify. In a recent study, we sought to capture the full scope of this damage by quantifying the impact of an invasive species on ecosystem services – the benefits humans derive from nature, largely for free.

Our findings were staggering. We estimated that a single invasive species in Wisconsin’s Lake Mendota had caused $140 million in damage (present-day value of a multi-year estimate) to a single ecosystem service – clear water. We calculated the cost of recovering that service to be between $80 million and $160 million ($430-$810 per household in Dane County, Wisconsin). These results provide a strong economic case for investing more in controlling or preventing invasions in order to protect ecosystem services.

The value of clear water

The widely recognized concept of ecosystem services captures the value we receive from nature. By estimating these benefits in dollars, we can quantify the true worth of natural resources that we often describe as “invaluable.”

Surprisingly, however, little attention has been paid to how invasive species harm ecosystem services. We tackled this challenge by designing a case study of the invasive spiny water flea in Lake Mendota, which covers 15 square miles in southeastern Wisconsin adjacent to Madison, the state capital.

Hundreds of thousands of citizens swim, fish and boat on Lake Mendota and care deeply about the lake’s water quality. But few of them know about a tiny, unsung hero called Daphnia pulicaria. Like many U.S. lakes, Lake Mendota is eutrophic, meaning it has been polluted with nutrients that cause large blooms of algae, mainly during warm weather. Daphnia are microscopic aquatic organisms that eat huge amounts of algae – so much algae that their grazing until recently created a “clear water phase” in the lake from mid-April to mid-June each year.

Jake R. Walsh , Author provided

The invasive spiny water flea is a tiny aquatic predator introduced through recreational boating that feeds voraciously on Daphnia. Since the spiny water flea was detected in 2009, Lake Mendota has lost 60 percent of its little grazers. As a result, now algae thrive and Lake Mendota has lost nearly a full meter of visibility. This means that when we lower a Secchi disk (a device used to measure water clarity) into the water, it disappears a meter sooner. In other words, the water is beyond murky.

A murky lake is unattractive. It also costs us money. We wanted to measure the financial impact of losing one meter of clarity in the lake – in essence, the economic “damage” caused by the spiny water flea invasion.

In a 2001 survey, University of Wisconsin researchers asked 500 randomly selected citizens of Dane County, where Madison and Lake Mendota are located, how much they would be willing to pay for a project that would improve water quality in Lake Mendota sufficiently to gain one meter of water clarity. We took their average response, which was an impressive $350 per household (present-day value of a multi-year project), adjusted for inflation to $640 per household, and multiplied it by the current number of households in Dane County (217,000). We found that Dane County values the water clarity lost with the spiny water flea invasion at $140 million (present-day value).

There are no known methods to control or eradicate the spiny water flea, so we looked for a different strategy to fix the damage it has caused and make Lake Mendota clear again.

Much of the land in Lake Mendota’s watershed is farmed. Water that flows off of fields and into the lake contains phosphorus fertilizer. In the lake this phosphorus fertilizes algae growth, just as it fertilizes crops on land. Reducing phosphorus runoff into Lake Mendota would offset the spiny water flea’s impact on water quality.

We built a statistical model that confirmed the effects of these two drivers of water clarity in Lake Mendota: Daphnia are good and phosphorus runoff is bad. Using the model, we predicted how much phosphorus runoff would have to decrease to offset the loss of losing 60 percent of Daphnia in the lake. We found that restoring the water clarity that has been lost due to the spiny water flea invasion would require reducing phosphorus runoff by 71 percent – an enormous challenge.

To see how much this would cost, we used a 2013 report prepared for the Clean Lakes Alliance, a local nonprofit group, that calculated the costs and benefits of best management practices to reduce phosphorus runoff into Lake Mendota. Based on these numbers, we estimated that achieving a 71 percent reduction would cost anywhere from $80 million to $160 million over a 20-year period – a steep price tag, but comparable to our estimate that 1 meter of water clarity was worth $140 million to Dane County residents.

A wider lens

Researchers have estimated that ecosystem services are worth over $100 trillion per year globally. Our study demonstrates that Americans also value these services highly, and are willing to pay to protect them. Understanding how damage from invasive species harms those services can spur us to do more to control invasions.

Invasive species affect ecosystems and the critical services they offer around the globe. Invasion ecologists have studied these impacts extensively in our Great Lakes fishery and found that invasive species damage this fishery alone on the order of $140 million per year. If we expanded this analysis to all ecosystem services in North America, we would likely find the cost of invasive species is much larger than the billions of dollars in direct costs that they generate.

Quantifying the value of ecosystem services like water quality is a huge challenge. Methods like willingness-to-pay surveys allow us to “ballpark” their value, and ecological research such as our model helps us to understand how these services are delivered. Understanding how invasive species affect ecosystem services calls for multi-disciplinary research that combines ecology with economics and sociology. As experts in ecosystem service science have pointed out, bringing this perspective into everyday policy decisions is critical for conservation in the 21st century.