Ecological rules of thumb and the 10% impervious rule for watersheds

Conservation planners love general rules of thumb. Rules of thumb are important because they allow us to make decisions without repeating exhaustive ecological studies in every location. By learning from studies conducted in similar ecological systems, we can apply good science while saving time and money. Increasingly, these rules of thumb come in the form of “thresholds”, or points along an environmental gradient (usually anthropogenic or human influenced) where biotic communities shift significantly or experience marked declines (as in diversity or biotic integrity).

One rule of thumb that is well established within the freshwater conservation community is the rule of <10% impervious surface. Several studies in the 1990s and early 2000s showed that biological communities generally began to significantly degrade as a watershed reached approximately 10% impervious. [Note: Impervious means land cover types that do not allow rainfall to infiltrate into the ground, like parking lots, buildings or roads. Such surfaces lead to increased runoff and lower groundwater contributions, leading to increases in both flood and drought conditions.] River conservation groups quickly latched on to these studies to demonstrate the importance of maintaining watershed land cover at <10% impervious or for limiting additional impervious land cover within watersheds that already exceeded 10%.

I believe the 10% rule has sometimes been misapplied in watersheds with low development (much less than 10% impervious). In these watersheds, we should try to provide a buffer (as much as is practical) to avoid significant biotic degradation. This is particularly important when a watershed represents the highest quality example of a particular stream type. But in general, the 10% rule has proven to be a powerful, scientifically-based rule of thumb.

But several recent studies have begun to punch significant holes in the 10% rule. Most recently, a paper by Robert Hilderbrand and colleagues looked at losses of benthic macroinvertebrate taxa with impervious surface percentage in Coastal Plain and Piedmont streams in the Chesapeake Bay watershed in Maryland. They found that about 10% of taxa were already lost from Coastal Plain streams by the time 10% of the watershed was impervious (~4% lost with 5% impervious). However, in Piedmont streams, nearly 40% of taxa were already lost with 10% impervious (nearly 20% lost with 5% impervious). This would indicate that the 10% rule would be entirely inadequate for conserving Piedmont streams. Even for Coastal Plain streams, it doesn’t appear to be ideal.

It is likely that the 10% rule is sufficient in many watershed types, but this and other recent studies provide reason for caution. In particular, we should find studies in watersheds with similar physical and chemical conditions, before applying the 10% rule. Additional studies should be conducted comparing across watershed types. Perhaps for some watersheds, 15% is adequate. But whenever possible, we should look for opportunities for maintaining impervious conditions well below 10%. This will provide a buffer for quality biotic communities to persist in the face of future perturbations—say, if the climate was to become significantly warmer and rainfall patterns were to change significantly over an unusually short period of time.

High density of impervious surface along the Potomac River in Washington D.C. across from beautiful Arlington Cemetary. 

RELEVANT LITERATURE
Booth, D. B. and R. Jackson. 1997. Urbanization of aquatic systems: Degradation thresholds, stormwater detection, and the limits of mitigation. Journal of the American Water Resources Association 33: 1077–1090.

Hilderbrand, R.H., R.M. Utz, S.A. Stranko, and R.L. Raesly. 2010. Applying thresholds to forecast potential biodiversity loss from human development. Journal of the North American Benthological Society 29:1009-1016. http://www.bioone.org/doi/abs/10.1899/09-138.1

Schiff, R., AND G. Benoit. 2007. Effects of impervious cover at multiple spatial scales on coastal watershed streams. Journal of the American Water Resources Association 43:712–730.

Schueler, T.R., L. Fraley-McNeal, AND K. Cappiella. 2009. Is impervious cover still important? Review of recent research. Journal of Hydrologic Engineering 14:309–315.

Wang, L. Z., J. Lyons, and P. Kanehl. 2003. Impacts of urban land cover on trout streams in Wisconsin and Minnesota. Transactions of the American Fisheries Society 132: 825–839

Press vs. pulse disturbance in ecology as applied to the recent oil spills

Ecologists generally classify ecological disturbances as either “press” or “pulse” disturbances. Press disturbances are long-term disturbances that have long-term impacts on an ecosystem, often resulting from changes to the physical structure of an ecosystem. Pulse disturbances are temporary disturbances that, while influential, are recoverable because of the temporary nature of the problem. Press and pulse disturbances can be natural or human-caused (anthropogenic).

The recent oil spill in the Gulf, as well as the oil spill in the Kalamazoo River in Michigan (>1 million gallons in a small tributary) have me thinking a lot about press vs. pulse disturbances. The gulf spill dominated the national news for months. Rarely does the health of an ecosystem get this kind of attention. And when it does, it’s always due to a catastrophe.

These oil spills are certainly nothing to minimize. Oil spills can have huge impacts on ecosystem processes and wildlife (as we’ve all seen). And there are significant human health concerns that go along with these sorts of spills. However, the gulf and Kalamazoo River oil spills are temporary, anthropogenic pulse disturbances. Ecosystems are resilient, as people have begun to point out in reference to the gulf oil spill—the oil is gradually breaking down. But long after the oil is gone, these systems will continue to be impacted by chronic (press) anthropogenic disturbances that are much more significant in the long-term, and the media will hardly even notice (certainly not in any way like the oil spill).

Chronic (or press) disturbances in the gulf include loss of coastal wetlands, a huge dead zone resulting from excessive nutrients delivered from the Mississippi River basin, overfishing, and climate change. Chronic disturbances in the Kalamazoo River include high phosphorus loads from incompatible agricultural and urban-development practices, fragmentation from dams, altered hydrology from increased watershed impervious surfaces, and the introduction and spread of aquatic invasive species. These impacts have been cumulatively impacting these ecosystems (and systems like them) for decades. And there are substantial socio-economic costs that go along with these impacts as well. But the impacts are cumulative and gradual, so we generally overlook them. Rather than killing birds and fish in a gooey mess, their habitat is gradually degraded or destroyed so that they can no longer reproduce. Long after there is any sign of oil within either of these systems, these press disturbances will go on with insufficient action to effectively minimize their impacts. Don’t get me wrong; there are valiant efforts by conservation organizations, natural resource agencies, and dedicated citizens to conserve these systems--and these projects are critical (see brief example from The Nature Conservancy below). But these efforts rarely approach the conservation endpoint of “effectively conserved” against these anthropogenic impacts because the problems are so systemic and the media attention span isn’t sufficient to attain public awareness of the issues.

My fear is that as the oil (and alarm) dissipates, the message from the oil spill(s) will be that ecosystems are resilient and anthropogenic disturbances are always blown out of proportion. This message could ultimately cause more damage than the oil spills themselves. A better message, from the start, would have been that these ecosystems are already under major stress from a variety of chronic anthropogenic stresses, and the last thing they need is an oil spill to further degrade things. We need more attention on the chronic anthropogenic disturbances to the ecosystems upon which we depend. Hopefully the media will stay around for a while.

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The Nature Conservancy works with partners to address both press and pulse anthropogenic disturbances to conserve biodiversity, including work in the Gulf. A new report, Gulf 20/20: Case for Long-Term Restoration, highlights the need to balance short-term, reactive responses with long-term restoration.

Why This Blog?

I'm an aquatic conservation ecologist interested in the conservation of biodiversity (e.g., ecosystems, communities, populations).  I work for one of the largest conservation organizations in the world, The Nature Conservancy.  However, this is a personal blog and the views expressed here are my own and do not necessarily reflect those of my employer.  I live in Michigan and generally focus on conservation of Great Lakes ecosystems.  However, much of my life has been spent in the Upper Mississippi River Basin, which is also near and dear to my heart. 

After spending the last 18 months on Twitter, as Etheostomatt, I've decided to start a blog to allow me to comment on some issue with more than 140 characters.