Occurrence patterns and trends of frogs in coastal wetlands of the Great Lakes call for further habitat restoration

Countless wetlands have been lost and degraded globally making amphibians
the most threatened vertebrate class. However, despite facing extensive
threats and stressors, coastal wetlands of the Laurentian Great Lakes of
North America (lakes Superior, Michigan, Huron, Erie, and Ontario) still
support sizable populations of frogs (order Anura, including toads). We
used data from the Great Lakes Coastal Wetland Monitoring Program to
quantify the first-ever annual occurrence probabilities and trends
(2011–2023) of eight marsh-breeding frog species, or groups of species, at
1,550 point count locations in 747 coastal wetlands throughout the Great
Lakes, and to assess 11 potential drivers of occurrence. Across our study
area, Green Frog (Lithobates clamitans) occurrence increased by 8% per
year, whereas Chorus Frog (Pseudacris maculata, P. triseriata) occurrence
decreased by 14% per year. We found more positive or stable trends among
lakes and species (85%) than negative trends (15%). Occurrence of all
species was negatively associated with one or two indicators of poor water
quality: specific conductance, ammonium nitrogen, nitrate nitrogen, and
urban and agricultural land cover in the surrounding watershed (median
area: 12 km2). Occurrence of multiple species was positively associated
with high lake levels and surrounding wetland (< 250 m) and forest
(< 2.5 km) land cover and negatively associated with surrounding
road density (< 2.5 km). Even though occurrence of most species was
increasing or stable and was relatively high (> 50%), all will
likely benefit from conservation actions. Fifty to 90% of Great Lakes
coastal wetlands have been lost and converted to anthropogenic land uses
leaving frog populations at a fraction of their former, original sizes.
Therefore, extra precaution is critical to help ensure their growth and
persistence. Improving water quality, increasing natural forest and
wetland cover within 2.5 km, and reducing roads within 2.5 km of Great
Lakes coastal wetlands will help conserve these important indicator
species in this globally-recognized but imperiled ecosystem.