In short, the experiments and data synthesis reveal that native plant recovery following invader control is a realistic outcome—but only under certain conditions, i.e., where water clarity and propagule availability are sufficient to foster native plant recovery. If lake management is to restore native macrophytes, it must target the factors that are limiting native species recovery, and researchers now show that invasive species are one of multiple limiting factors in Minnesota lakes.
Controlling dominant invasive aquatic plants is a common goal of many stakeholders around the state. These invader-reduction efforts are often motivated as ways to promote the health or recovery of native plant communities—but the potential for these efforts to actually meet those goals is uncertain. Researchers hypothesized that, in addition to potential competitive effects of invasive species, insufficient water clarity and native plant recolonization can also be “rate-limiting” components of restoring lake vegetation. If so, these limitations must be addressed and invader control alone will be inadequate for restoration. Researchers addressed this issue in two ways: (1) By evaluating responses of native plants to actual, on-the-ground management efforts in invaded lakes in MN through synthesis and analysis of monitoring data. This can tell us how management is working across the state at scales relevant to lake managers. (2) The team compared those conclusions to results of field experiments designed to untangle how invaders, light limitation, and reproduction can hinder native plant recovery. Overall, the work resulted in the aggregation of more than 4,000 surveys that will be used to evaluate responses of native plants to curlyleaf pondweed, Eurasian watermilfoil, and the management of each of these AIS. The funding supported the completion of all experimental fieldwork, bringing four years of work to a conclusion. In short, the experiments and data synthesis reveal that native plant recovery following invader control is a realistic outcome—but only under certain conditions, i.e., where water clarity and propagule availability are sufficient to foster native plant recovery. In addition, the results show that Eurasian watermilfoil exerts a stronger negative effect on native plants than curlyleaf pondweed. Thus, control of Eurasian watermilfoil is more likely to foster native recovery than is control of curlyleaf pondweed. If lake management is to restore native macrophytes, it must target the factors that are limiting native species recovery, and researchers show that invasive species are one of multiple limiting factors in Minnesota lakes.
Phase II: Impacts of invader removal on native vegetation recovery
To evaluate the ability of invasive aquatic plant control efforts to yield recovery of native aquatic plant communities, this project will:
- Perform in-lake invader control experiments using a small-scale in-lake experiment to evaluate how native plants respond to invader removal, seed-addition, light availability, and the combination of these factors
- Analyze monitoring data from hundreds of lake management projects previously conducted in Minnesota over twenty years
- Develop a publicly available statewide plant monitoring and management database
Why? Because each year, lake managers, lake associations, municipalities, and the Minnesota Department of Natural Resources manage Eurasian watermilfoil and curly-leaf pondweed in hundreds of Minnesota lakes, principally through herbicide applications. These control efforts are motivated in part by the expectation that reducing invader abundance will restore native macrophyte communities. This “spray-and-pray” approach hinges on the little-tested assumption that it is competition from invaders that has displaced native species. Because aquatic invasive plant management often yields little recovery of native vegetation, we must revisit this assumption. It is crucial to understand when invader reductions do foster recovery of native plants vs. when other limitations may need to be overcome to restore lost diversity.
Using applied research on three high-priority aquatic plant species that are invasive or potentially invasive in Minnesota lakes, this research aims to address the need for more guidance and options for controlling these nuisance plants. Species of research include:
- Nitellopsis obtusa (Starry stonewort), a new invasive alga in Minnesota. Researchers will assess the risk of spread through modeling and will also test how long starry stonewort can remain viable out of water to better understand how best to prevent its spread. Additionally, researchers will conduct laboratory experiments to test the efficacy and selectivity of different herbicides and algaecides. This information is urgently needed during this window of opportunity to minimize impacts of starry stonewort to Minnesota lakes.
- Myriophyllum spicatum (Eurasian watermilfoil) and Potamogeton crispus (curly-leaf pondweed), both of which have been in Minnesota for several decades and are now found in hundreds of lakes throughout the state. Although both of these species have been a focus of management and research for several years, there are still limits in our ability to effectively control these species and to support the post-control recovery of native plant species. Researchers will analyze existing datasets, perform new field work, and develop a citizen-science monitoring program to improve understanding of factors that drive invasion of these species and that influence the effectiveness of management efforts.
Collectively, this research will aim to create biologically and economically sound solutions to prevent and control these invasive plants and to disseminate scientific information that assists the DNR, watershed districts, lake associations, and citizen groups around the state with management strategies. Aquatic invasive plants are a major threat to Minnesota’s lakes, rivers, and wetlands. Aquatic invasive plants can form dense mats on the water’s surface, reducing space and light available to other plant species. This can lower native plant diversity, reduce habitat quality for fish and other animals, and change the way lakes function. They can also interfere with boating, recreation, and other human uses.
Phase I findings
We conducted research to predict invasion risk, assess ecological impacts, evaluate control efficacy, and investigate factors limiting post-control recovery of native aquatic plants. This work was applied to three target species at different stages of invasion: (1) Nitellopsis obtusa (starry stonewort), first found in Minnesota in 2015 and now known in 14 lakes; (2) Myriophyllum spicatum (Eurasian watermilfoil), found in 1987 and established in >300 lakes; and (3) Potamogeton crispus (curly-leaf pondweed), here for >100 years and in >750 lakes. For starry stonewort, we developed models to predict risk of further spread and prioritize search locations for statewide volunteer search efforts, experiments to determine how long starry stonewort remains can survive out of water (i.e., remain transportable by boaters), and field and lab-based control experiments to guide management. For Eurasian watermilfoil and curly-leaf pondweed, we investigated relationships with native plant biodiversity, finding that they displace native species, an effect compounded by lower water clarity, and contribute to “biotic homogenization”—loss of ecological distinctiveness. We are investigating how to better control these invasive species and foster recovery of native vegetation by synthesizing thousands of aquatic plant surveys and management records collected in Minnesota and by conducting in-lake removal and restoration experiments. This work will continue under Phase II, above.