I learned about invasive species at a very early age- my mom would pay my sister and I a quarter a bucket to pull up garlic mustard that was spreading through the woods behind our house. She explained to us that the mustard didn’t belong there, it was an alien plant trying to take over our woods from the pretty native trillium and others. The logic was easy- remove the plant that doesn’t belong- it’s not pretty, and it makes your hands smell funny.
Dealing with an invasive species isn’t always that straightforward. Exotic palms provide the only shade in Death Valley National Park. Invasive tamarisk trees use so much water they are sucking desert canyons dry, but they provide habitat for endangered birds along the Colorado River. Even more complicating- what if the invasive species is actually a native? At first, the idea seems to go against the very definition of invasive species. But rapid population expansion of native species can threaten ecosystems and economics just as much as exotic species.
This complex subject is tackled by a new paper in the September issue of Frontiers in Ecology and the Environment, titled Native invaders – challenges for science, management, policy, and society. (Read the paper ) Written by Seattle-based fisheries scientists, the paper focuses on native fish species that threaten the pacific salmon populations. They argue that native species can become invaders, rapidly expanding populations and threaten other species’ access to key resources when human activities alter habitats.
Salmon live complicated lives, born in rivers, migrating downstream to spend their adult lives in the ocean, and then returning to the rivers of their birth to spawn and die. Human activities in these northwestern rivers, from dams to stocking popular sport fish, have increased the populations of fish species that eat baby salmon. Several species of salmon are listed under the Endangered Species Act, so dramatic population increases in predator species can be interpreted as a native invasion that threatens the endangered salmon populations.
One example in the paper is that Northern Pikeminnow populations have grown dramatically, taking advantage of the habitat created by dams. The expanding population puts a lot of predatory pressure on the young salmon in the Columbia River Basin. From a management perspective, reducing the overpopulation of a native species has a different set of challenges from eradicating an exotic invader. To try and bring Northern Pikeminnow populations back down, a reward program was established that paid fisherman between $4-8 for each Pikeminnow caught. Over nearly two decades, the program resulted in the harvest of 2.2 million of the invasive fish.
That program sounds pretty positive, but as this paper points out, fisheries are so complex that who is invading whom can get complicated:
Within the Mountain West, one species – O mykiss – is simultaneously a stocked native invader (rainbow), a threatened species (steelhead, the anadromous life history of O mykiss), and a stocked non-native, partially responsible for decreasing populations of a native species (west slope cutthroat), all depending on location. This example illustrates the difficulties of conducting research on native invaders where studies and findings may only apply to distinct locations within a relatively small geographic region.
Beyond fish, this paper raises questions about how to handle any ecosystem or food web where one native species gets an unexpected edge and population growth knocks things out of balance. Example that come to mind would be suburban annoyances like deer and geese that now have huge populations because we’ve mostly eliminated their native predators from our habitat, leading to overgrazing impacts on young trees (deer) or nutrient imbalance in lakes (geese). Or the pine bark beetle that’s taking down forests throughout the mountain west because it can thrive now that climate changes have produced slightly warmer, shorter winters.
However, framework for thinking about what turns native species invasive rests on a difficult key question- how to tell the difference between natural ecosystem shifts and significant human-induced impacts? Ecosystems change. What’s an inevitable adaption to changing climate versus a problem that we caused and need to mitigate? This paper isn’t offering up answers to that big question yet, just strong examples that as humans continue to alter natural habitats, we need to expand our understanding of “invasive species” to include native populations that suddenly experience a population boom and begin to threaten other species’ survival.