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Identifying Potential Tipping Points Using EPT Taxa
Nah Eun Kim, Kimberly D. Robinson, and Bryan C. Pijanowski
Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47906
Numerous research studies have shown that freshwater ecosystems are impacted by land use and cover change, often reflective of human dominated landscapes (Kim 2012; Keatley et. al. 2004; Booth and Jackson 1997). The Laurentian Great Lakes region of the U.S. contains some of the highest concentrations of agricultural and industrial land-uses in the world. (U.S. EPA 2011; Pijanowski and Robinson 2011). As a result, both point and non-point source pollutants, nutrient loading, and soil erosion/loss have resulted in ecosystem degradation, at times quite severe/sometimes quite severely/, effecting water quality, aquatic habitats, and freshwater aquatic communities in the region’s rivers, lakes, and streams (U.S. EPA 2011).
The main objective of the research study described below was to investigate how increasing human dominated landscapes (i.e., urban and agricultural lands) within river/stream catchments and eco-regions affects the integrity of the surrounding aquatic freshwater systems. In particular, the responses of EPT taxa (i.e., sensitive aquatic macro-invertebrate insects) communities to upsurges in both urban and agricultural lands were utilized to identify potential ecosystem tipping points within the Wisconsin, Illinois, and Michigan rivers and streams. Here, the term ‘tipping point’ is used to denote a position (or values) within an ecosystem where additional changes/shifts (often as a result of human induced stressors) to the system will cause the ecosystem to shift into a new regime (i.e. a state in which the ecosystem’s structure, function, and feedbacks may be different than previously experienced) (Kim 2012). Often, once a tipping point has been crossed, resulting changes to an ecosystem are unfavorable and reverting back to the old regime (or ecosystem structure, function, and feedbacks) is difficult if not impossible.
Identifying tipping points using EPT taxa
Identifying exact tipping points before they have been reached can be a difficult, if not impossible, process. By in large tipping points can be seen once they have been reached or crossed, but due to variability within environmental conditions, the predictability of exact tipping point measures/values may in some cases be impossible. Researchers therefore have used a number of different techniques to try to predict approximate (instead of exact) tipping point values. One such method is measuring the changes in the variance of species richness (Veraart et al. 2012; Carpenter et. al. 2011; and Scheffer et al. 2009).
Past studies have shown that as the variance in species richness increases (e.g., the variance in the number of different aquatic benthic macro-invertebrate species within a river/stream increases), ecosystems approach potential tipping points (Kim 2012). One such measure of variance employed to measure stream health/quality is calculated using the richness of EPT taxa (i.e., macro-invertebrate insects in the orders of Ephemeroptera (mayflies), Plecoptera (stoneflies), and Tricoptera (caddisflies)) (Plafkin et. al. 1989). EPT taxa species have been shown prefer more pristine environments and possess sensitivity to soil erosion and non-point pollutant run-off from agricultural lands (Foley et. al. 2005; Peters 2009; and Lenat and Resh 2001). In areas where urbanization is prominent, EPT taxa have also been shown to be sensitive to stream flashiness (a stream flow response to storms), substrate instability, non-point and point source pollutants (Reif 2002; USGS 2002).
Frequently referenced land use management guidelines state that no more than 10% and 30% of a watershed’s land area should be composed of impervious surfaces (associated with urban development) and agricultural lands, respectively. Exceeding these percentages is thought to produce readily observable degradation to freshwater systems (Yang et. al. 2010; Riseng et. al. 2010). Certainly these percentages provide a strong starting point (or goal) for land use management, but they are not absolute requirements for land use and their distributions vary widely from watershed to watershed. As such, the following paragraphs describe one study that used changes in EPT taxa communities to validate the 10% impervious surface and 30% agricultural lands management guidelines for land use planning and water resource management within the Great Lakes region.
EPT taxa species abundance at 4,227 sites (Figure 1) within river/stream catchments in the states of Illinois, Michigan, and Wisconsin were analyzed. Data from each of these sites was collected and documented in a stream macro-invertebrate database by Drs. Riseng and Wiley from the University of Michigan. The species abundance data was then analyzed by researchers at Purdue University to study changes in the abundance of EPT taxa in river/stream catchments and riparian buffers (150 m) within two eco-regions (Figure 2) in the Great Lakes states of Illinois, Michigan, and Wisconsin. Note that analysis for catchment riparian buffers were only conducted in Michigan and Wisconsin, while eco-region analyses only included the Michigan and Wisconsin portion of eco-region 50 and the Wisconsin portion of eco-region 52 (See Figure 2). (See the U.S. EPA website for more information and descriptions of all level III eco-regions: www.epa.gov/web/pages/ecoregions/level_iii_iv.htm, last accessed 28 August 2013). Also note that the land use/cover data for the study came from the NLCD 2001 raster dataset, downloadable through the Multi-Resolution Land Characteristics Consortium’s 
website.
Lastly, the standard error of EPT taxa species abundance across sampling sites was substituted for variance, as the standard error metric helps to control for variability in the number of sample sites per land use/cover category.
Study Results
EPT taxa populations within eco-region 52, dominated by agricultural use (i.e., intense row cropping and livestock operations), exhibited two potential tipping points when the percentage of urban and agricultural lands reached 15% and 30% of the entire catchment area. At 15% and 30% total urban and agricultural lands within the catchment, the standard error (the proxy for variance) of the abundance of EPT taxa peaked, indicating that at these percentages, significant changes in the EPT macro-invertebrate populations were evident. In addition, the 30% agricultural and urban lands tipping point within the catchment area was likely a result of the fact that at 30%, more agricultural activities were occurring in the 150m riparian buffer along rivers/streams than in any other part of the catchment area. Riparian buffer areas serve a vital role in slowing the run-off of water from surrounding land and the filtering of water (helping to reduce non-point source pollutants) before they enter rivers/streams.
Within the riparian buffer areas of eco-region 52, possible tipping points were evident at 20% and 40% of the buffer area being composed of urban and agricultural land uses. Unlike the 30% tipping point identified at the entire catchment level, the 40% urban and agricultural lands tipping point within the riparian buffer was most likely reflective of increased agricultural lands outside of the riparian buffer. At this 40% level, more agricultural lands were present outside of the riparian buffer than within the buffer area itself. Higher levels of agricultural and urban lands within these riparian buffer areas 1) may lead to increased non-point source pollution and soil erosion and 2) hinder the buffers from performing much of their vital ecological functions (i.e., does not allow them to slow over-land flow or filter water before entering waterways). It should be noted, however, that in both the entire catchment and at the riparian buffer levels, urban land use exerted a significantly stronger impact on the quality of river/stream water (and thus EPT taxa abundance) per unit area than agricultural lands. These impacts are largely associated with the high concentrations of impervious surfaces within urban areas as opposed to agricultural lands (Kim, 2012; Allan, 2004).
Similar to eco-region 52, analysis of land use/cover and changes in the standard error of EPT taxa population abundance for eco-region 50 showed two potential tipping points. These tipping points occurred at 40% and 60% agricultural and urban land use within the overall river/stream catchment area. When analyzing only the sample sites in eco-region 50 occurring in Michigan, similar catchment tipping points at 35% and 60% were observed. Potential riparian buffer tipping points occurred at 40% and 55% agricultural and urban lands. Such results indicate that tipping points within eco-region 50 occur at higher levels of total urban and agricultural lands, pointing toward the conclusion that eco-region 50 is able to tolerate higher levels of human disturbance than eco-region 52 before EPT taxa populations experience significant levels if impact.
One possible explanation why eco-region 50 can support higher levels of human dominated land use than eco-region 52 without a large impact on EPT taxa, is that eco-region 50’s land use/cover has a higher percentage of forests and natural lands as compared to human dominated lands (i.e., agriculture and urban) than eco-region 52. In addition, eco-region 50 also exhibits a greater amount of topographic relief (difference in elevations) than eco-region 52. The variable topography likely serves to slow down the over-land flow of water, leading to increased water infiltration of water into soils before they are able to enter the waterways.
Discussion and Concluding Remarks
Results from the research presented above show that in rivers and streams within the Great Lakes states of Illinois, Michigan, and Wisconsin:
1) The EPT index standard errors (a proxy for variance) increase near distinct tipping
points. These increases are linked to a rise in developed lands in catchments and stream buffers.
2) Exact tipping point values are dependent upon the location being studied and the
spatial scale examined.
3) It is possible for multiple tipping points to exist within a given system.
Potential tipping points, associated with the total percentage of agricultural and urbanized lands, within eco-region catchments and river/stream riparian buffers do exist within the Great Lakes region. These tipping points correspond to significant changes in the standard error of the abundance of EPT taxa (i.e., aquatic macro-invertebrate) populations present in rivers/streams. However, difference in approximate tipping points occurs as a result of geographic location and spatial scale being examined.
Within eco-region 52, potential tipping points occurred when the total percentage of agricultural and urbanized lands within the catchment reached 15% to 30%. For eco-region 50, potential tipping points shifted, meaning that approximate tipping points did not occur until the total percentage of agricultural and urbanized lands within the watershed reached 35% and 60%. Within riparian buffer areas, at 20% and 40% (in eco-region 52) and at 40% and 55% (in eco-region 50) agricultural and urbanized lands, significant increases in the standard error of the abundance of EPT taxa species was seen. Note that difference between whole catchment and riparian buffer approximate tipping points within an eco-region are fairly small, reflecting the interconnectivity of catchment and buffer land uses between spatial scales. As each tipping point is passed, the ecosystem shifts from one regime another, each consecutive regime representing larger and larger amounts of less ‘natural’ and more ‘humanly’ altered (or dominated) landscapes.
In locations with large amounts of wetlands and forests (especially within riparian buffer areas), the effect of agricultural and urbanized lands may be dampened in part by the ability of the wetlands and forests to slow the velocity of and filter water run-off before it enters rivers and streams. Furthermore, the large amounts of overland flows within catchments and watersheds (often associated with higher levels of human dominated land uses/covers), may lead to increased impacts of human dominated lands on EPT taxa population abundance. In addition, while EPT taxa are known to be reliable indicators of changes in land use/cover surrounding rivers and streams, they (like other macro-invertebrates) may not respond to all impacts associated with land use/cover change (Kim, 2012; Plafkin et. al., 1989). If changes in EPT taxa abundance alone are used to measure ecosystem quality (i.e., integrity) some key tipping points may be missed.
While approximate tipping points may be identified, ecosystems are both complex and dynamic in nature. Land use planners need to be mindful of observable environmental and societal impacts and feedbacks (both positive and negative) resulting from planning decisions and employ adaptive management techniques to avoid (and/or address) unfavorable results (Kim, 2012).
* For more details on the research study described in the previous paragraphs, please see Kim,
2012.
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