Big Ideas in Conservation - Taking a Census of Nature

Mike McManus serves as The Nature Conservancy in Florida's primary expert on aquatic ecology, emphasizing the spatial relationships between landscapes and watersheds. He collaborates with Conservancy scientists on data analysis, data management, GIS and spatially balanced probabilistic surveys. Mike received his Ph.D. in biological science from Florida State University.

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"Data from spatially balanced sampling could be the environmental science equivalent to census data for demographers — providing us a clearer picture of the species, habitats and landscapes we strive to protect."

—Mike McManus, aquatic ecology specialist for The Nature Conservancy in Florida

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By Mike McManus

Why am I excited about a technique called "spatially balanced sampling" for conservation? Because I want better answers to questions as varied as these:

Are phosphorus and nitrogen at such levels in Florida's Nassau estuary that they could lead to eutrophication and harmful algal blooms that could impact oysters in the estuary?
What proportion of stream kilometers in the Garcia River watershed in California contain the woody debris and gravel conditions needed by salmon for spawning?

Spatially balanced sampling offers a new — and often better — way to answer such questions. Data from spatially balanced sampling could be the environmental science equivalent to census data for demographers — providing us a clearer picture of the species, habitats and landscapes we strive to protect.

These surveys are already happening — and conservationists should be taking advantage of their data so we can assess the status of species, habitats and landscapes and evaluate the effectiveness of our conservation efforts.

What Does 'Spatially Balanced' Mean, Anyway?

The kinds of surveys most people are familiar with — such as political polls and economic surveys — are probabilistic. They draw their survey samples at random and make estimates from those sample measurements, with those estimates containing a measure of spread in the data known as a margin of error.

Spatially balanced surveys are also probabilistic, but their samples are drawn in such a manner as to provide an even or balanced coverage of the sample points over the geographic area of interest. Such even spatial coverage is desirable — it avoids samples being clustered together, as can occur with samples from a simple random survey.

How Can Conservation Use These Surveys?

The 2006 Wadeable Streams Assessment (WSA) provides an excellent example of the value of such surveys to conservation organizations. The WSA was done by the United States Environmental Protection Agency (USEPA) and state natural resource agencies to assess the biological condition of small streams in the contiguous United States.

One way an organization such as the Conservancy can use such a survey is to take the WSA data and combine it with geographic information systems (GIS) data of protected areas. That lets us ask the question: Within ecoregions, do sites that were sampled by the WSA in protected areas have better physical habitat, water chemistry and biodiversity than sites not in protected areas?

Answering that question would let conservationists know if the plants, animals and natural habitats are responding in the manner we expect them to in areas under conservation management versus areas not in conservation.

Does the Conservancy Implement Such Surveys?

A number of spatially balanced surveys have been done by federal and state agencies, and the data are free and readily available for use. But to design, implement and analyze data from a spatially balanced survey isn't easy: It requires careful planning, scheduling and a degree of statistical sophistication.

In 2006, some Conservancy scientists received an introduction in the design and analysis of these surveys at a workshop led by Dr. Anthony Olsen of the USEPA. At that workshop, we learned that the margin of error for estimates from data from spatially balanced surveys is typically smaller than estimates from traditional survey designs and analysis.

What does this mean? That if we use spatially balanced survey design, we can reach our conclusions taking fewer samples than if we use a simple random survey. (The statistical details for spatially balanced surveys are described in this 2004 paper by Drs. Stevens and Olsen.)

Conservancy scientists are just starting to design, implement and analyze data from spatially balanced probabilistic surveys:

In Florida, we have implemented such a survey for vegetation monitoring at the Conservancy's Tiger Creek Preserve. We want to know if the proportion of wetland and upland plant species might change as a result of new land-use activities proposed near the preserve.
Also in Florida, a spatially balanced survey is being designed for the Yellow River watershed, where we want to estimate the proportion of stream banks needing restoration so as to reduce erosion into the Yellow River.
A watershed assessment using a spatially balanced survey is also being done in the Garcia River watershed in California.

These surveys can provide on-the-ground and in-the-water measurements that can complement data from GIS and remote sensing — so we can better answer questions about our efforts to conserve species, habitats and landscapes.

(March 2009)

Fast-Forward to the Future of Conservation!

Photo credits (top to bottom, left to right): © Surveying the Garcia River watershed (CJ Hudlow/TNC); courtesy of Mike McManus