Research led by Danai Kontou as part of her PhD thesis while in the Ecosystems and Global Change group at Cambridge, has found that one of North America’s most ecologically and economically damaging invaders, the spiny water flea, may have been in North America for centuries longer than believed. She discusses the implications for how governments and conservation organizations approach invasive species management in this article written with Dr Andrew Tanentzap for 'The Conversation'.
One of Canada’s worst invasive species may have arrived naturally
By Andrew J Tanentzap (Trent University and University of Cambridge) and Danai Kontou (University of Leeds)
Most invasive species are introduced into the areas where they become damaging. That's part of why they tend to be so successful in taking over new environments and causing billions of dollars in damage.
But what if species that we thought were introduced by humans arrived naturally?
In a recent research paper, the authors made a startling discovery about one of North America's most ecologically and economically damaging invaders.
The spiny water flea, also known by its scientific names Bythotrephes longimanus and Bythotrephes cederströmii, may have been in North America for centuries longer than believed.
This finding has implications for how governments and conservation organizations approach invasive species management. Millions of dollars are spent to control the spread of invasive species, but for native species that benefit from environmental change, this strategy may misplace limited funds.
Rewriting history
The widely accepted history of the spiny water flea is that it was introduced unintentionally from Europe into Lake Ontario by trans-Atlantic cargo ships in the early 1980s.
In the study, researchers found reports of the spiny water flea in more than 300 lakes and rivers across Canada and the United States over the past four decades. This number is likely a major underestimate because most lakes are not monitored.
Identification of the spiny water flea relies on physically detecting it in water with nets or inside fish stomachs. Several million water fleas may need to be present in a lake before a single individual is caught in a net. This means only the densest populations may be detected.
In 2019, co-authors of the study made an unexpected finding. While examining mud at the bottom of Three Mile Lake in Muskoka, Ontario, they discovered jaws of the spiny water flea.
Over time, particles and dead organisms settle to the bottom of lakes. New material accumulates on top of older layers, creating a natural archive of a lake's history. These particles can be dated by measuring naturally occurring radioactivity, which decreases over time.
In Three Mile Lake, the jaws were found next to mud that was more than 350 years old. At that time, Sainte-Marie, the first European settlement in what would become Ontario, had just been established.
New tools
Sediments are living environments. Fossils can mix through layers because of waves and the activity of organisms. The best way to estimate the age of a fossil is to measure it directly.
Until recently, dating fossils of tiny animals like the spiny water flea had been impossible. Because of their small size, hundreds of individuals are needed to estimate an age accurately.
The chemical makeup of some animals, including the spiny water flea, can also interfere with the dating process. Using new approaches, the researchers solved this problem.
Using radiocarbon dating, they found that spiny water flea fossils in Muskoka pre-dated the 1950s. To strengthen the findings, they used another new tool based on DNA sequencing.
All organisms shed DNA into the environment. In lakes, DNA accumulates in lake-bed sediments, where it can survive for centuries in cold, dark conditions.
By measuring environmental DNA, researchers can estimate population sizes of species that are difficult to count using traditional approaches.
The study found that the abundance of spiny water flea DNA and fossils in lake sediments correlated almost perfectly. Spiny water flea DNA was found in mud layers dating back to 1867, the year Canada was founded.
How did the flea get here?
How the spiny water flea arrived in the small lakes of central Ontario in the early twentieth century remains a mystery. There were no canal or road networks capable of transporting them inland.
Before the St. Lawrence Seaway was completed in 1959, trans-Atlantic ships could not travel beyond Montréal. Live transport of the spiny water flea is unlikely because the animals die when handled, and their eggs cannot survive more than four hours outside water.
One possibility is that a spiny water flea hitched a ride to North America on a migrating bird. Millions of aquatic birds move between North America and Eurasia annually. Although the odds of such dispersal are low, a single individual can establish a viable population. Over thousands of years, such rare events cannot be ruled out.
What is clear is that the spiny water flea was present for decades at low densities in the Great Lakes region before being detected. Understanding why it recently exploded in numbers may be more important than determining the exact date of introduction.
The study suggests that climate warming may have helped spiny water flea populations grow. Previous work in Europe, where the species is native, indicates that variable summer temperatures and longer growing seasons are beneficial.
Where to from here?
The authors emphasize that invasive species like the spiny water flea still need to be managed. The species can be invasive in some North American lakes even if it arrived naturally. When moved into new areas, native species can become overabundant, damaging ecosystems and threatening other species.
Rather than treating the symptom, the invader itself, the authors argue that management should address the underlying causes of invasion. These causes are often ecosystem-based rather than species-specific. Examples include climatic changes rather than traits unique to a species. This strategy may be particularly effective once an invasive species becomes established.
Determining when a species was introduced depends on proving a negative: showing that it did not exist in a place previously. As the saying goes, "absence of evidence is not evidence of absence."
The study suggests that, in invasion biology, researchers sometimes need to dig a little deeper.
Authors: Andrew J Tanentzap, Professor and Canada Research Chair, Trent University and University of Cambridge; Danai Kontou, Postdoctoral Research Fellow in Invasion Ecology, University of Leeds.
Image: An adult spiny water flea under the microscope. Credit: NOAA Great Lakes Environmental Research Laboratory.
This article was originally published by The Conversation under a Creative Commons licence.