Amphibians (mostly frogs and toads) are intimately connected to water, literally able to breathe underwater through their skin.  With this close association with water, they are especially sensitive to changes in wetland habitats.  A community of wetland breeding frogs can indicate that a wetland is healthy and functioning well.

Protecting Frog Habitats

It's not enough just to protect coastal wetlands - we need to know if animals are responding to improved habitats.  Our objective for amphibians across Great Lakes coastal wetlands is:

Objective: Maintain or restore Great Lakes coastal wetlands to achieve an 'Improving' trend for the amphibian metric in each Great Lake by 2030, as measured in the  State of the Great Lakes (SOGL) reports.  Specific objectives by lake are as follows:

Lake Superior: Maintain 'Fair' status

Lake Huron: Maintain 'Fair' status

Lake Ontario: Achieve 'Fair' status

Lake Michigan: Achieve 'Fair' status

Lake Erie: Achieve 'Fair' status

Metric: Mean Index of Ecological Condition (IEC), an objective indicator that relates breeding frog populations to wetland condition as reported in SOGL 2017.

What is the Current Status?

  • In the Highlights report for SOGL 2019, the trend in amphibian index was not determined for Lake Superior, but was Unchanging for Lakes Michigan, Huron and Erie.  Lake Ontario experienced an Improving trend.
  • In the Highlights report for SOGL 2019, the status of Amphibians was ranked 'Good' for Lakes Superior, Michigan and Huron, 'Poor' for Lake Erie, and 'Fair' for Lake Ontario.
  • In SOGL 2017, the trend was reported as 'Unchanging' for all Lakes, as measured from 2011-2014 and assessed for statistical significance.
  • The status of the amphibian IEC was assessed as 'Poor' for Lakes Michigan, Erie, and Ontario, and 'Fair' for Lakes Superior and Huron. 

 

Coastal Wetland Health as Indicated by Amphibians

How do frogs relate to ecosystem health?

  • Coastal wetland breeding frogs are sensitive to many known water pollutants, especially nitrates (Rouse et al. 1999).
  • They are also influenced by wetland size, quality of habitat and cover nearby, and pollution such as sediment runoff (see sources in SOGL 2017)
  • For this analysis, seven species of frogs were chosen that are widespread across the region.
  • Some frogs are more likely to be found in healthier wetlands, and vice versa.  This tendency was analyzed and quantified as an 'index of ecological condition' or IEC.
  • The resulting IEC values, divided into categories of "Good", "Fair" or "Poor", represent the relative condition of wetlands as indicated by frogs that were sampled around each Great Lake.

This objective addresses our goal because...

Amphibians are an important indicator of a healthy community of native plants, fish, and insects in coastal wetlands.  Objective 2 tracks the amphibian IEC, which quantifies the relationship between wetland ecosystem health and an assemblage of coastal wetland amphibians.

Further Details on this Goal

The Assembly chose Goal 1 to align with shared commitments outlined in the Great Lakes Water Quality Agreement (between U.S. and Canada) and the Great Lakes Restoration Initiative Action Plan III (U.S. only)

For example, in the GLWQA, key commitments of the U.S. and Canada include "Facilitate binational collaborative actions to reduce the loss of native species and habitat, recover populations of native species at risk, and restore degraded habitat" and "Increase awareness of native species and habitat and the methods to protect, conserve, maintain, restore and enhance their resilience".  Also in the GLWQA, Objective 5 and Annex 7 call for supporting resilient populations of native species and their habitats. 

In the GLRI Action Plan III:Objective 4.2 calls for increasing the resiliency of species through a comprehensive approach, and commits to supporting populations of both federally threatened and endangered species as well as Great Lakes native species.

Amphibians are vanishing worldwide for a host of reasons.  But in coastal wetlands of the Great Lakes, their populations can indicate how well their habitat is functioning.   Keeping track of the status of coastal wetland breeding frogs in the Great Lakes allows for more insight into how well restoration efforts are working for a host of species.

Using ecological indicators that quantify the relationship of several species to the ecological condition of coastal wetlands provides a better picture of overall wetland function.  It is critical to our objectives that conservation work supports the preservation and establishment of resilient landscapes that are able to sustain adequate populations of native species.

This metric (the Index of Ecological Condition or IEC) identifies variability in response of an assemblage of frog species to wetland conditions and provides a relative measure to compare the general conditions among coastal wetlands of the Great Lakes.

In 2014, the mean Index of Ecological Condition (IEC) based on breeding frogs for each lake showed that Lakes Michigan, Erie, and Ontario ranked "Poor", while Lakes Superior and Huron ranked "Fair".  Mean IEC numbers were as follows:

Lake                       Mean IEC 2014           

Erie                        5.4

Ontario                 5.6

Michigan              5.4

Huron                   5.9

Superior               5.9

To categorize the Status of wetland condition based on the IEC, definitions of Good, Fair, and Poor were calculated based on IECs from all years and all wetlands (n = 4,804).  The 66th percentile and above of scores were assigned to the 'Good' category.  The middle third were assigned to the 'Fair' category, and scores less than the 33rd percentile were assigned to the 'Poor' category, as follows:

  • Good: IEC > 6.1
  • Fair: 5.7 ≤ IEC≤ 6.1
  • Poor: IEC < 5.7

The trend from 2011-2014 was "Unchanging" for all lakes, as defined by no statistically significant short-term increase or decrease in IEC.  To achieve Objective 1, all lakes should show an 'Improving' trend in the next 5- year period.

Trends were assigned based on statistically significant changes (geometric mean rates of change) during the period from 2011-2014.  Trends were defined as follows:

  • Improving: increase in IEC from 2011-2014
  • Unchanging: no significant change in IEC from 2011-2014
  • Deteriorating: decrease in IEC from 2011-2014

Additional Information in the SOGL 2017 report

In addition to publishing the average IEC values for 2014, the 2017 SOGL report provided the range of IEC values that were found from 1995-2014.  This range indicates that the IEC can vary considerably when measured over several years. 

Range and means of IEC values by Lake
LakeMean IEC 2014Range 1995-2014
Erie5.45.0 to 6.4
Ontario5.65.1 to 6.2
Michigan5.42.8 to 7.6
Huron5.95.6 to 6.4
Superior5.93.9 to 7.1

Mean IEC for each lake from 2014 and Range of IEC values reported from 1995-2014.  Source: State of the Great Lakes report, 2017.  

The index of ecological condition (IEC) is an objective biotic indicator introduced by Howe et al (2007), and improved by Gnass Giese et al (2015).  An IEC establishes a connection between environmental stressors and biotic variables by calculating an index that includes the following:

  1. 'Reference' gradients of environmental condition – for example, in the SOGL 2017 report, these conditions included agricultural intensity, non-agricultural landscape development, and wetland area
  2. Sensitivity of species to environmental stress (a biotic response (BR) function).  In the SOGL 2017 report, data from the Coastal Wetlands Monitoring Program were used to build these functions due to the availability of site-specific stressor data.
  3. Establish a specific assemblage of species to include.  In SOGL 2017, seven species (or groups) of frogs were identified based in part on their presence at all of the sample points (Table 1).

A detailed description of IEC methodology is available at http://www.uwgb.edu/BIODIVERSITY/forest-index/iec.asp.

Species of frogs used to calculate IEC for Great Lakes coastal wetlands
No.Common nameScientific name
1American ToadAnaxyrus americanus
2BullfrogRana catesbeiana
3Chorus frog (both boreal and western)Pseudacris maculata / Pseudacris triseriata
4Treefrog (both Gray and Cope's Gray)Hyla versicolor / Hyla chrysoscelis
5Green FrogRana clamitans
6Northern Leopard FrogRana pipiens
7Spring PeeperPseudacris crucifer

Table 1. Wetland breeding frog species or groups of species (n = 7) used to generate biotic response functions for calculating indices of wetland health for Great Lakes coastal wetlands.  Source: Great Lakes Coastal Wetland Monitoring Program.

  • Missing information from healthy wetlands, so future collection from more healthy wetlands would give the appearance of improvement.
  • The IEC factored in only 3 stressors, and left out other factors such as dominance of invasive plant species.
  • Variability was much less between wetlands, lake basins, and over time than for the bird indicator, which may limit the scale of changes that can be detected.
  • Surveys for frogs are relatively costly due to the need for three nocturnal sampling surveys.

All data are as reported in the State of the Great Lakes report – 2017

In the SOGL report, data were used from:

References:

Howe, R. W., R. R. Regal, J. Hanowski, G. J. Niemi, N. P. Danz, and C. R. Smith. 2007.  An index of ecological condition based on bird assemblages in Great Lakes coastal wetlands. Journal of Great Lakes Research 33:93-105.

 

Gnass Geise, E.E., R. W. Howe, A. T. Wolf, N. A. Miller, and N. G Walton. 2015. Sensitivity of breeding birds to the 'human footprint' in western Great Lakes forest landscapes. Ecosphere 6(6):90. http://dx.doi.org/10.1890/ES14-00414.1

 

Rouse, J.D., Bishop, C.A., and Struger, J. Nitrogen pollution: an assessment of its threat to amphibian survival. Environ. Health Perspect. 107, 799-803.

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