Dams like this change the flow regimes of rivers, and prevent some species from accessing their spawning grounds, lowering population viability. But is removing them completely danger-free? (Image Credit: Notorious4Life, CC0 1.0, Image Cropped)

Anybody who has ever studied freshwater ecosystems will end up having to study dams at some point. And they’ll no doubt learn that dams are the enemy. They fragment ecosystems. They cut fish off from their spawning grounds. They change flow regimes. So it makes sense that the recent trend of dam removal across Europe and the world in general would please ecologists. But there’s a problem with dam removal, and it comes in the form of invasive species.

In this series, we’ve always spent a section focussing on the ways species enter new environments. Being able to predict introduction pathways of a potential invader is an enormous advantage when trying to preserve an ecosystem. Past studies have taught us a lot about how certain species have invaded new regions, which allows conservationists to take preventative measures to keeping invaders out.

One of the more recent pathways of introduction has been climate change. I wrote about this in more detail recently, but for now let’s just say that with the climate growing warmer, new species have been able to invade regions that were previously too cold for them, be they further north (our south in the Southern Hemisphere) or at higher altitudes. These species enter new ecosystems and some are now classified in Norway as “locally invasive” by Artsdatabanken, a local institution which monitors invasive and other novel species.

This is of particular concern in freshwater ecosystems, where the spreading of invasive fish, coupled with the warming of the local climate, has seen local extinctions of native species in a large number of lakes. These extinctions have been driven by the presence of the invaders, and restriction of the native species’ habitat as some sections of a lake, or entire lakes, become too warm for them. In Scandinavia, species like the Arctic charr and brown trout have suffered from this over the last few decades, whilst the European perch and northern pike have been moving further up into catchments, aided by introductions by local anglers, eager to fish their favourite species.

Being able to spread into other lakes and maintain a metapopulation is of enormous benefit to the species threatened by the invaders. However, this ability is often significantly impeded by the presence of dams, bridges, and other man-made constructions dotted throughout freshwater ecosystems worldwide. Additionally, some species need to migrate to inland waters to reproduce, and dams cut them off from these grounds. And whilst in some parts of the world, dams seem to be going up no matter how damaging they are (pun definitely intended), there have been movements in Europe and the USA to remove dams and restore the connectivity of freshwater ecosystems and flow regimes.

This seems like a great idea. But species like the pike and perch mean that removing dams could lead to consequences even worse than the restriction of their populations, as dams often prevent the dispersal of invasive fish species through freshwater ecosystems. Species like Arctic charr and brown trout will need to maintain populations through regular dispersal between lakes, so dam removal could benefit them, but the introduction of the pike and perch could outweigh any benefits. Likewise, in the Great Lakes in Canada, there have been calls for removal of dams to allow the local Walleye species to flourish, however this could lead to increased numbers of Sea Lamprey, a devastating invader which forced local authorities to work for decades to reduce to their numbers to manageable levels.

It’s a rock/hard place problem, which institutes worldwide are looking to find the answer to. The AMBER and Odysseus projects are two international collaborations which are trying to find solutions as to how to go about the process with as little damage as possible to the local ecosystem, but as always with science, there’s a high degree of uncertainty, and event the recent advent of models which can predict changes in communities given both climate change and reductions in connectivity can’t account for chance. There’s also the very real possibility that local anglers will keep introducing invasive species illegally. With some lakes very isolated, it’s virtually impossible to catch and fine these types.

This isn’t always the case, of course. Sometimes dams are the very things allowing invasive species to flourish. Often dams will have built in fish-ladders, which mean invasive species pass them without hindrance. However the fact remains that simply removing a dam is not an insta-fix solution to freshwater sustainability.

But there are potential solutions moving forward. Some river networks have built in natural barriers to fish dispersal, such as waterfalls, or slopes too steep for fish to traverse. Dams in these areas can therefore be eliminated safely. And by working with local communities, it is possible can we can help local fishermen understand the impact of introducing novel species, and increase the cultural value of the local species.

But how can you help? I work on the Odysseus project myself, and the aim of the project is to provide tangible management tools to not just local authorities, but to conservation groups and national environmental agencies worldwide. But until then, try and fish for locally occurring species, and don’t use non-native fish as bait. Ecosystems in Scandinavia are now changing under the introduction of the European Minnow, often used as bait by anglers. You can also graffiti large freshwater ecosystem/dam-based puns on your local dam. But don’t tell them I suggested it.

Updates and more information on both the AMBER and Odysseus projects can be found on their websites.