Not many people have likely heard of Beaver Island, a large, isolated island located far off shore at the northern end of Lake Michigan. Home to roughly 600 permanent residents and accessible only by ferry or small plane, Beaver Island is a well-kept secret of remote Great Lakes shores and dense forests. It is also home to Central Michigan University’s Biological Station (CMUBS), which has served students and researchers alike since 1959. This June, I made the flight to there from Charlevoix, Michigan to work at the biological station with CMU professor Kevin Pangle and his graduate student, Mael Glon. With my work at Shedd Aquarium focusing on invasive species and the ongoing management and conservation of the Great Lakes, Kevin, Mael and I have a shared interest in answering an important question: how are freshwater invasive species interacting with, and potentially benefitting, each other?
Invasional Meltdown: Does One Invasive Species Pave the Way for Others?
Ecologists have used the term “invasional meltdown” to refer to the possibility that exotic species might facilitate each other’s invasions, by improving the survival or worsening the impacts of subsequent invaders. The frequency and importance of invasional meltdowns has been debated, but the invasion of the Great Lakes by quagga and zebra mussels is often cited as one of the best examples of this phenomenon. Introduced from Eastern Europe through commercial shipping traffic, these invasive mussels have reengineered the Great Lakes by intercepting, retaining and recycling nutrients in nearshore habitats, depriving open water habitats and their native species of the foundation to their food web. In their place, a suite of lake bottom (or “benthic”) species – including many non-natives – have subsequently thrived.
Two examples of invasives that have done well in the aftermath of quagga and zebra mussel invasions include the round goby and rusty crayfish, my focus at Shedd Aquarium’s Daniel P. Haerther Center for Conservation and Research. The round goby is a small benthic fish that shares an Eastern Europe native range with the mussels and similarly followed them west in the ballast water of ships, whereas the rusty crayfish was introduced from farther south in the United States, likely through releases of unused live bait by anglers. Both round gobies and rusty crayfish now occur throughout the Great Lakes, often in high abundances and sharing the same rocky near-shore habitats that have been colonized by mussels. These species also share some undesirable impacts on native Great Lakes fishes, especially predation on eggs. Pangle says that “Our native fishes are really pinched between having their food resources diverted to the mussels, while having their eggs preyed upon by these mussel-associated invaders.”
Our research asks: are quagga and zebra mussels really helping round gobies and rusty crayfish coexist, particularly at such high abundances? We posit that the dramatic transformation of benthic nearshore habitats by the mussels, including the increased production of algae and other food resources associated with the mussels’ water filtering, facilitates higher abundances of both gobies and crayfish together. However, it’s hard to find Great Lakes habitats without mussels, gobies or crayfish, which would be necessary to tease apart how these organisms perform in isolation from each other. As a result, we’re using more controlled experiments to evaluate what happens to round gobies and crayfish when the mussels are taken away.
Lake Michigan in Miniature: Mesocosms as Tools in Ecology and Conservation
Central Michigan University’s Biological Station and Institute for Great Lake Research recently built a new facility for controlled, replicated freshwater experiments. We call these “mesocosms” because they’re obviously smaller than Lake Michigan itself (which is what we really want to study), but larger than the “microcosms” that are often used to study even smaller organisms than fish, mussels, or crayfish. The mesocosm facility at CMUBS includes twelve 800 liter tanks that can receive a constant flow of water directly from Lake Michigan. In addition, although Beaver Island is isolated, it hasn’t been spared from the range expansion and impacts of major Great Lakes invaders. Our study organisms, from quagga mussels to round gobies to rusty crayfish, can be collected just off shore from the facility.
Glon notes that “The great thing about these mesocosms is that we can really match conditions to Lake Michigan, from the water source, to the substrate, to the organisms we’re studying. We start from a close match to Lake Michigan, but then can change conditions, like adding or subtracting organisms to get a sense of how they’re interacting with each other.”
Mesocosms are just one tool that ecologists apply to understand how organisms affect, and are affected by, their environment. Mesocosms have occasionally been criticized for the way that the generally short duration and smaller spatial scales of these types of experiments may not reflect what happens out in an actual community or ecosystem. However, mesocosms do provide us with complimentary information to other approaches in ecology, like mathematical modeling or field surveys and monitoring. In the case of Glon’s graduate research, the hope is that the mesocosms will connect to a field survey of sites throughout Lake Michigan, which also seeks to determine if gobies and crayfish occur in higher numbers at places with more mussels.
Although we’ll run a number of experiments over the course of the upcoming summer, our first question is simply: how do rusty crayfish and round gobies perform in the presence and absence of the mussels? We’ve replicated treatments with and without quagga mussels, and are following crayfish and goby behavior, growth and survival under the lake’s current status quo, and in a case where the mussels aren’t present. We don’t yet know if one or both invaders really does better in the presence of the game-changing mussel invaders, but answering this question may identify how management or control approaches could affect this entire community of invaders.
Invaders Are Rarely Alone: The Need for Multi-Species Management
Ecologists occasionally have a bad habit of studying species, and especially invasive species, in isolation. We may work on invasive mussels, or crayfish, or fish, but we rarely put novel pieces of communities together to see how they interact with each other. Yet the majority of ecosystems are invaded by multiple, and often many, invaders. As Great Lakes managers discuss ways to control invaders like quagga and zebra mussels, these conversations may benefit from knowing how other invaders will react to eradication efforts. If we have fewer quagga or zebra mussels, will we also have fewer round gobies and rusty crayfish? Or do these benthic invaders thrive even in the absence of mussels? Knowing how entire communities of invaders respond to management actions may let us better quantify the benefits associated with the costs and challenges of Great Lakes invasive species management.
As a postdoctoral research associate for the aquarium’s Daniel P. Haerther Center for Conservation and Research, Dr. Eric Larson focuses on the conservation and management of crayfish, including invasive species like the notorious rusty crayfish, in the Great Lakes. Working in partnership with the University of Notre Dame Environmental Change Initiative, Dr. Larson investigates habitat associations of native and introduced crayfishes, models the effects of climate change on these species and conducts studies investigating their role in freshwater food webs.