A bee lands on white sweet clover in Riverdale. The plant is an invasive species. (Joel Krahn/Yukon News)

Invasive species a threat to Yukon environment and economy

‘I worry about zebra mussels, to be honest’

What do sweet clover and rainbow trout have in common? Under the right conditions in the wrong places, they’re both invasive species in the Yukon, says Todd Powell, manager of biodiversity for Yukon fish and wildlife with the environment department.

Rainbow trout — highly prized by Yukon anglers — jump right out in that list. Populations of rainbow trout are found in the Kathleen River system and are stocked for recreational fishing in pothole lakes throughout the Yukon, Powell said, but have been showing up in other lake systems to which they are not native.

“There have been releases, both by previous governments and private individuals — we’re not really sure who — into other places,” Powell said.

“It’s tough to differentiate sometimes between what’s exotic and what’s invasive and rainbow trout are right on the line.”

For a species to be considered invasive, it has to meet one or more of three main criteria, Powell said. It must have a negative impact on the environment to which it is introduced, a negative economic impact, or an associated human health impact. In the case of rainbow trout, its precise impacts are unknown, but it’s possible the fish could displace or draw resources from already-established species.

“They’re not native to those watersheds… Are they displacing native species? What’s their impact? We don’t know, but we’re working to find out,” he said.

This scenario is a good example of a species which might not be considered invasive when it stays where it’s supposed to, Powell said, but can become invasive when introduced to a favourable environment. Sweet clover is considered one of the most invasive plants in the Yukon. Sweet clover was originally introduced by farmers because it aids the accumulation of nitrogen, an essential plant nutrient, in the soil. Like most living things, however, sweet clover doesn’t have much respect for human-imposed boundaries like fields and fences.

The species — which can produce 300,000 seeds per year per plant — quickly spread beyond its intended purpose and became ubiquitous along Yukon’s roadsides. Sweet clover not only displaces native species but creates tempting pockets of edible vegetation for wildlife along traffic corridors, which can result in vehicle collisions with animals. It also creates safety concerns for drivers, as it can grow so tall that it can be difficult to see over, potentially causing traffic accidents.

“What’s beneficial and wanted on one side of the fence can be unwanted and have negative impacts on the other side of the fence,” Powell said.

The potential conflict between introduced beneficial agricultural species and native wild species “is an area of concern” for invasive species management in the Yukon, Powell said, adding that sheep are a good example of this. As the News has previously reported, domestic sheep in more southerly regions have decimated bighorn sheep populations by spreading a variety of pathogens which are harmless in domestic animals but cause pneumonia in wild populations.

The Yukon government is concernedt the same thing could happen in the territory where thinhorn sheep — genetic kissing-cousins to bighorns — populations overlap with agriculturally-introduced sheep populations.

“Sheep are a perfect example … where (a species can) have an impact broader than predicted,” Powell said.

Likewise, sometimes native species can behave like invasive ones, Powell said, either when they move beyond their natural ranges or experience a population boom. Earlier this year, the Yukon experienced an invasion of black army cutworms. The insects are native to the territory but experience natural cyclical population explosions which can cause damage to both wild plants and food crops.

“We always have to consider ‘is this within the normal biological range?’ when we are thinking about invasive species,” Powell said.

So what invasive species keeps Powell up at night?

“I worry about zebra mussels, to be honest,” he said, adding the species could survive here. “We have to make sure we have programs in place to stop that from happening.”

Quagga mussels and their cousins zebra mussels are invasive freshwater bivalves. They gather on stationary surfaces, such as docks, stones and breakwalls, sometimes in such numbers that they clog pipes, causing infrastructure damage. More dramatically, however, they harm environments by altering concentrations and species of algae, changing food webs and water systems. According to the Yukon Invasive Species Council (YISC), each mussel can filter up to one litre of water a day. They consume only brown and green algae while filtering out blue-green algae, resulting in toxic algae blooms.

Once established, the “reproductive cycles of these mussels allow for successful and rapid infestation,” YISC said. Each female can produce up to one million eggs and the larvae, called, veligers, are free-swimming, making them capable of moving up and down water systems to infect new areas. Mussels from both species are exceptionally hardy, capable of surviving out of water for up to five days.

While they are currently not thought to be present in the Yukon, the invaders will be familiar to people who have lived in Ontario and Quebec, where the bivalves were accidentally introduced in the late 1980s by transoceanic shipping.

Quagga mussels are more of a threat to the Yukon than zebra mussels, Powell said, because they are more cold-tolerant and can live in lower-oxygen waters.

Currently the Yukon “really benefits” from prevention programs and data collection in British Columbia, which spends $2 billion annually on check programs to screen boats for potential infestation, said Powell. One of those checkpoints is in Dawson Creek, B.C., which helps to protect the Yukon.

If the mussels did make it to the Yukon’s water systems, they would be “very expensive, damage wise,” he said.

“If (quagga and zebra mussels) get here, the can really clog things up. We need to have programs in place to prevent that,” Powell said.

Currently, there is no overarching invasive species act in the territory, Powell said, but there are federal acts that apply.

“In terms of a big-p invasive species policy, no, we don’t have that,” Powell said. “But there are small-p policies. The Yukon Invasive Species Council is doing a lot of great research.”

The Yukon is “still a relatively small player,” when it comes to invasive species, Powell said.

“It’s a scale-based challenge and an act is a big thing.”

“(Invasive species) are an area in biodiversity we spend a lot of time on,” he said. “Everything is linked.”

Contact Lori Garrison at lori.garrison@yukon-news.com

Environment Yukoninvasive species

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