Illinois makes up a large portion of the American Corn Belt with the highest productivity of corn and soybeans in the nation. Historically, the landscape was quite different. Before 1800, tall grass prairie, forests, and wetlands dominated the area. Approximately 70 to 90% of the natural landscape was replaced with farmland, cities, and roadways in the past 200 years. These dramatic changes in the landscape have greatly affected freshwater organisms and the habitats in which they live (Fig. 1).
Aquatic life and their environments interact with each other to make up freshwater ecosystems. Native American and early European settlers relied on these ecosystems for food, materials (e.g., pearls and buttons from mussel shells), drinking water supplies, transportation, and irrigation. Today, dominant ecosystem services include sport fisheries, recreation, wildlife habitats, biodiversity support, water purification, and nutrient removal. However, the health of most streams and rivers in our state and throughout the Midwest has been compromised to varying extents in terms of biodiversity loss, water pollution, habitat degradation, and overloading of waterbodies with nutrients and fine sediment.
Federal and state environmental agencies need to assess the biological health of individual streams for effective management and restoration. The health of lotic systems is normally assessed based on the extent to which the biological assemblages and ecosystem functions have shifted away from expectations under natural conditions. Natural resource managers and researchers also use these expectations as a guide to establish management goals for stream restoration and conservation plans. A standard approach is to infer expected biological conditions of individual streams based on sampling of a large number of minimally disturbed or nearly pristine sites. However, this approach is difficult to apply in the Midwest where only a small number of streams remain in a relatively natural state.
Fig 2. Predicted natural distribution of a mussel species in Illinois streams
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Fortunately, Illinois Natural History Survey (INHS) scientists have diligently documented the occurrence of plant, animal, and fungal species across the entire state since 1858. These specimens are archived in our various research collections and their label data (identifications, locations, dates of collection) are recorded in databases. For example, Dr. Stephen Forbes, the first chief of INHS, collected fish specimens from hundreds of locations in Illinois in the late 1800s and early 1900s. Similarly, Theodore Frison, Hebert Ross, and Bernard Burks collected millions of aquatic insect specimens from across the state. Great efforts were also made for freshwater mussels. Records of these precious species can be used to reconstruct the pre-European settlement distributions of aquatic species via species distribution modeling. The models can then be used to predict the occurrence of a species in any stream. When combined, the predictions can be used to estimate species richness and assemblage composition at a given site. These predictions provide a historical benchmark for assessing aquatic ecosystem health.
In the past few years, INHS researchers, including ecologists, museum curators, and GIS experts, have worked together to model and predict the distributions of three main aquatic groups: freshwater mussels, stoneflies, and fish. We drew species record data from INHS research collections, other museums, and trusted sources. After using appropriate quality assurance and control measures, species records were linked to specific stream reaches and defined as a confluence-to-confluence stream segment. Large numbers of environmental variables summarized for the watershed and riparian zone of each stream reach by the Great Lake Gap Analysis Program were associated with stream reaches. From these data we built a statistical model for each species to predict the occurrence of species in stream reaches across the state (see Fig. 2 for example for mussel species).
When adding the predictions of all species modeled together for a reach, we obtained estimates of local species richness and assemblage composition (Fig 3a-b). Across a set of recently sampled sites in Illinois, on average, an estimated 46% of the native mussel species were lost, and the number of stream reaches occupied has declined by 35%. This study demonstrated that the Chicago area has lost much of its historical mussel species diversity.
In a separate effort, we predicted the distributions of stonefly species, a group of aquatic insects favoring clean and cool/cold water across streams in the state. Hotspots of stonefly species richness include the Shawnee Hills of southern Illinois, the Middle Fork of the Vermilion River in east-central Illinois, and the Rock River in the northwestern part of the state.
An ongoing study is estimating the diversity loss of three major aquatic insect groups: caddisflies, mayflies, and stoneflies across the state. Our research team has also started to model and reconstruct the distributions of Illinois stream fishes and their diversity based on the rich collection of fish records. In the future, we will expand our efforts to large rivers and other waterbodies. The ultimate goal of the team is to provide site-specific benchmarks of historical species diversity for all major aquatic groups to help rank ecosystem health of streams and rivers and reconstruct the detailed distributions of aquatic species under pre-European settlement conditions to guide the optimization of species conservation and ecological restoration.