MSU researchers examined how freshwater wildlife navigate waterways in the face of climate change and humans altering the landscape.
Like many others, you may have taken a vacation this summer. If you drove, chances are you’ve traveled the main roads. If you flew, you probably traveled through a large airport, quite possibly a major airline hub. This is because highways and hubs tend to include the most efficient long-distance routes to popular destinations.
As it turns out, the same is largely true of wildlife. Under a changing climate, wildlife generally tries to move to cooler locations further north or to higher elevations, known as “range shifts”. Many species also migrate long distances to breed, including many backyard birds and fish like salmon.
One of the biggest challenges for wildlife, however, is getting to their destinations, given how humans have modified landscapes and waterways. Perhaps ironically, the infrastructure that humans use to get around, such as roads, railroads, and airports, as well as cities, fences, and other man-made structures, effectively create obstacles for wildlife.
Although most large freshwater networks (i.e., connected networks of rivers, streams, and lakes) in the US are heavily dammed, hub lakes represent central points within the networks and are therefore priorities important for network restoration and freshwater wildlife conservation. Shown are A) Hoover Dam on the Colorado River (Arizona/Nevada), one of the largest freshwater systems in the US, and B) Slate Lake, a central lake in the Superior National Forest, Minnesota. Photo credits: A) Katelyn King, B) Ian McCullough.
One type of obstacle that can be especially difficult for wildlife to remove is the tens of thousands of dams in the lower 48 states of the United States. Although dams help control water supplies and generate electricity, dams make it difficult for fish and other freshwater animals to swim to other areas, which can be important for spawning or escaping to cooler places in response to climate change.
Before dams were built, the US was home to hundreds of vast networks of connected streams, rivers and lakes. Structurally, freshwater networks are like highway networks or flights. They often contain numerous branches that diverge in various directions from centralized centers. Without these hubs, networks can easily break.
“What would happen if Delta Airlines, for example, suddenly lost metro Detroit airport? Its route network would collapse,” said lead author Ian McCullough, a research scientist at Michigan State University. “We’re seeing something similar in freshwater networks. Dams can really disrupt the natural connectivity between different rivers, streams and lakes if we put them in the wrong place.”
In an article recently published in the magazine ecosphere, McCullough and a team of researchers assessed the current capacity of freshwater networks in the lower 48 states to act as natural highways for freshwater wildlife. They assigned each network a “connectivity score” based on a combination of network size, north-south distance, dam prevalence, and susceptibility to network fragmentation.
Perhaps unsurprisingly, 68% of the 385 networks evaluated received poor connectivity scores. In contrast, only 3% of networks received high connectivity scores, indicating that only a few networks are capable of facilitating long-distance migrations and species range shifts under climate change.
Overall, the study found that most networks are small, spanning an average of 5-6 km from north to south and containing an average of 3 lakes. On the other hand, larger networks are relatively rare: only 10% of networks contain at least 50 lakes and only 8% span more than 100 km from north to south.
“The few networks that are large enough to support long-distance migrations and range changes are often the most affected,” said co-author Katelyn King, Ph.D. graduate from MSU and now a postdoctoral researcher at the University of Michigan.
For example, the largest and highest-rated network in the Midwest is the Red River Network, which is located in Minnesota, North Dakota, and parts of Canada. In the US, this network contains 754 lakes and spans 380 km from north to south, but also has 250 dams. Among all networks, the Colorado River network received the highest network connectivity score. Although this network spans 1,330 km from north to south and contains 2,027 lakes, it also has 954 dams. The Mississippi River network is so large (nearly 33,000 lakes and 25,000 dams) that it was considered its own category.
Since removing dams or rerouting waterways across the country is impractical, the study also looked for hubs of freshwater networks, essentially lakes that act as centralized points within larger networks. In all, the study identified 2,080 central lakes in the lower 48 states, including 131 in Michigan. Although these core lakes make up only 2% of all network lakes in the Lower 48, they are disproportionately important in keeping freshwater networks intact and thus facilitating wildlife movements. Therefore, keeping the centers within freshwater networks could be really useful for wildlife conservation.
The study found, however, that core lakes were no more likely to be found in protected areas than any given lake. This may also not be surprising given that most protected areas (eg national parks) in the US are designed for terrestrial species and habitats. However, focusing on increasing natural connectivity in these core lakes, such as with fish ladders or modifying or removing dams, could represent an efficient strategy for restoring freshwater networks and wildlife populations, especially considering the limited resources for restoration and conservation.
An important next step is to monitor wildlife in hub lakes to study the extent to which they act as natural hubs within larger freshwater networks.
“There are really only so many possible routes for wildlife, especially when looking to travel long distances,” McCullough added. “So it would be surprising if species, including invasive species, didn’t use web hubs. Where else would they go?”
The open access article can be downloaded for free here: