ELUs in Conserving Biodiversity

Conservation Goal
Protection of native fauna and flora is one of the basic goals of most conservation organizations.  This is often accomplished by conservation of sites that contain rare and endangered species and protection of habitats or landscapes that are known to be important for plants and animals.  Over the long term, ecosystems change and this is a natural phenomenon.  Natural causes of change are disturbances such as fire, flood, drought, and wind; diseases and pathogens; and changes brought about by the arrival of new species or the loss of keystone species.

Change Happens Faster Now
Climate change
will exacerbate ecosystem change.  As temperatures increase, weather patterns change, and sea levels rise, the composition of plants and animals in our ecosystems will change as well.  This poses a challenge for conservationists – how do you protect habitats for plants and animals when the composition of the ecological communities will be rapidly changing over the next century? Mark Anderson, the regional scientist for The Nature Conservancy, argues that it is difficult to protect the specific actors (i.e., the plants and animals) because we do not know who they will be as climate change effects occur, but we can protect the stage (i.e., the physical habitat) upon which they will thrive. Therefore, the purpose of this project is to establish a process where the conservation community can assess the potential value of “the stage” to ensure that we be able to maintain rich communities of native plants and animals as climate change effects manifest themselves. Landscapes with a large variety of species tend to be resilient to disturbance and able to deliver important ecosystem services.

Our Audience
Conservationists work at many scales. Our project strives to inform local conservationists who work at the parcel scale (10’s-100’s of acres). Other conservationists work at regional, national, and global scales. All are important. Moreover, other criteria must inform conservation decisions such as the size and connectedness of protected lands. The ELUs of a candidate site are one of many important factors to consider.

Ecological Land Units
Using Ecological Land Units — ELUs — we provide a mechanism to identify properties that will be important in protecting biodiversity now and into the future as different species of plants and animals come and go as climatic conditions change. It is uncertain what particular species will occupy specific habitats when climates are very different, but ELUs help us identify landscapes that will support rich biodiversity in changing climates. To learn about our process, view the paragraphs below outlining:

  • What Drives Biodiversity?
  • What are Ecological Land Units?
  • Making ELUs
  • ELUs as a Planning Tool
  • Case Study: South Kingstown
    • We have used the the Town of South Kingstown and the South Kingstown Land Trust as an example of how ELUs might be used to identify priority conservation areas.

1. What Drives Biodiversity?

Plant and Animal Diversity
Many factors determine which species of plants and animals live in an area.  For animals, the variety and species of plants in an area are important components of “habitat.”  For plants, the physical properties of a site frequently determine the suitability of the area for supporting specific species.  Critical physical properties are elevation, slope, aspect, geology, soils, and hydrology.  For example, some species of plants (e.g., pitch pines) thrive in gravelly, well-drained soils, in dry landscapes; whereas, other species, such as red maple, prefer moist, highly organic soils in poorly-drained locations.  The land use and disturbance history of a site are also very important and can, in some cases, be the most important factors determining the richness of biodiversity. This is most true when land use destroys habitat, alters hydrology, or replaces natural vegetation with impervious surfaces.

Physical Diversity Begets Biodiversity
Research by many scientists have shown that there is a strong positive relationship between the diversity of physical characteristics on the landscape and the variety of plants that occur there.  This relationship occurs at site-level scales (5 acre study plots) and larger landscape scales (tens and hundreds of acres). Recently, scientists from The Nature Conservancy showed the same relationship across states in the eastern United States.  In that study, over 90% of the variation in plant and animal variety could be explained by the diversity of geological and topographic settings in a state.

Physical Diversity and Conservation
The relationship between physical diversity and ecological diversity has important relevance to the conservation community – protected lands that are highly variable with respect to physical properties will likely support diverse communities of plants and animals as climate changes.  It is difficult to know which species will occur at a specific location many decades into the future, but landscapes that contain many different physical characteristics will contain diverse assemblages of plants (and therefore animals) that are best suited for those specific conditions.  Therefore, the identification of areas on the landscape that are physically diverse might be important targets for conservationists because they are likely to support high biodiversity as climactic conditions change.

2. What are Ecological Land Units?

ELU Basics
Conservation ecologists have coined the term Ecological Land Units (ELU) to describe and map the physical properties of landscapes.  Typically, ELUs are defined by the geology, soils, elevation, and landform (hilltop, hillside, valley).  A specific ELU has a unique combination of soils, geology, landform, and elevation.  Specific ELUs are often associated with specific plant communities.  For example, in Rhode Island pine barren habitats are associated with ELUs characterized by excessively drained gravelly sand on flat areas or gentle slopes.  Conversely, red maple swamps are found in ELUs that contain poorly drained silt loam on flat areas.

Mapping ELUs
We have defined and mapped ELUs for Rhode Island.  A single ELU is a unique combination of soil type and landform. The different components of ELUs in Rhode Island are given here. There are 204 unique types of ELUs in Rhode Island, but over 85% of the land area of the state is covered by 20 of the most common ELU types.  We call these the Dominant ELUs since they cover so much of the state.

ELUs and Plant Communities
Different ELUs often support different kinds of plant communities. Using Geographic Information System (GIS) mapping tools, it is possible to overlay ELU boundaries with detailed aerial photography. The correspondence between ELUs and plant types is sometimes very clear as can be seen here.

ELUs and Biodiversity: A Test
Presumably, a conservation area with many different types of ELUs will have many different types of plant communities, thus high biodiversity. We have tested this hypothesis by counting the different kinds of ELUs on 24 Audubon Society of RI refuges where we have excellent plant survey data. After removing the bias associated with the size of the refuge (larger refuges have more ELUs and species of plants just because they are larger), we found a positive relationship between the number of ELUs on a refuge and plant species diversity. Thus, we are confident that areas with a variety of ELUs will typically support large numbers of plant and animal species. This result has been observed in other studies of ELUs.

3. Making and Mapping Ecological Land Units

Components of ELUs
ELUs are derived from soil and elevation data using a GIS. It was important that we used readily available data and we kept the derivation of ELUs as simple as possible. After consulting the published literature and conferring with expert soil scientists and plant ecologists, we focused on two aspects of soils, soil drainage class and soil texture. Soil drainage class is very good at distinguishing wet versus dry habitats. Soil texture (sandy, silty, loamy, etc.) is an important habitat component for plants. Using USDA SSURGO (State Soil Survey Geographic Database) data that is readily available from RIGIS, we created a raster dataset (50 feet cell size) of the different soil drainage classes and another raster dataset of the soil texture classes. There are many properties of soils that are available to use for analyses such as this, for example stoniness, depth to bedrock, etc. The two factors we chose are extremely important soil properties in supporting different plant communities.

Landform represents where a location is with respect to elevation, slope, and aspect (direction a hillside is facing). Landform distinguishes hilltops, hill sides, valley bottoms, etc. We used the RIGIS digital terrain model as our source of elevation data to measure landform. Landform classes were identified using GIS modeling of slope, aspect, and elevation.

Our Method
A graphic description of the technical methods and workflow to identify ELUs is:

The final ELU map is made by adding together the raster datasets for landform, drainage class, and soil texture. Because we were careful with our encoding system, the sum of the three rasters provides us a composite of the individual datasets. For example, a location that is a well-drained (code value 2000) and consists of gravelly sand (code value 100) a sits on a hilltop (code value 21) and would combine to be ELU 2121 (2000+100+21). This process yielded 204 unique ELUs for the state of Rhode Island. Examination of a cumulative distribution function (CDF) of the ELUs showed that most of the ELUs were small and did not occur very often. Conversely, 20 ELUs were quite dominant and encompassed almost 85% of the land area of RI.

4. ELUs as a Planning Tool — Identifying Conservation Priorities

ELUs Assist Conservation Planners
ELUs are a tool to help resource managers and conservationists achieve important goals for land protection: to preserve diversity, and to preserve representative assemblages of plants and animals. It is important to note that ELUs are but one of many factors conservationists might consider in choosing properties to protect. Aesthetic and cultural values, water resource protection, public access, etc. are criteria that might be considered along with ELUs.

ELU Diversity Maps
To preserve diversity, we mapped how many kinds of ELUs there are within 1,500 feet (30 pixels) of every location in RI.  The values range from 1 ELU in the 1,500 foot radius to a maximum diversity of 65 different ELUs.  Large contiguous areas of high ELU variety would be desirable conservation targets because they are likely to support many different kinds of plant communities.  A map of the different ELUs in a portion of South Kingstown is shown here and a map of the variety (diversity) of ELUs in the same area is also shown.

Identifying Unprotected ELUs
Protecting plants and animals that are representative of the region is another important conservation goal.  ELU’s can provide insight into these targets too.  We calculated the total proportion of land occupied by each of the 20 dominant ELUs in Rhode Island.  We also computed the variety of ELUs in lands that have been protected (e.g., by the towns, the state, federal agencies, NGO’s such as the TNC and the Audubon Society, and local land trusts).  By comparing the extent of an ELU in the state by how much is protected, we can identify ELU classes that are not well-represented in our portfolio of protected lands.  These underrepresented ELUs might be good targets for future land acquisitions.  A map of underrepresented ELUs in an area in the Matunuck Hills of South Kingstown is presented here.

How To Use ELUs
So how can conservation planners use ELUs in their assessment of potential lands to conserve? Here are some simple guidelines:

  • The map of the variety of ELUs is an important planning tool. Where you have many different kinds of ELUs in an area, you have the possibility of finding many different plant and animal communities now and in the future. If the conservation goal is to protect biodiversity, these are high value sites. We have aggregated the ELU variety classes into a small number of categories (* see note below) to guide conservation planners. The most diverse areas of RI have 48 or more different ELU types in a 1,500 foot radius. Only 1 percent of the state is in this class. The next most diverse areas of the state (6.6% total land area) have 39 to 47 different ELU types in a 1,500 footradius. This is followed by diverse areas (23.1% of RI land surface) of 29 to 38 kinds of ELUs in a 1,500 foot radius.
  • The map of underrepresented ELUsis a helpful guide if the conservation goal is to make sure that RI’s most common ecosystems are adequately protected. The objective here would be to protected areas that are known to contain underrepresented ELUs.

* These classes are defined by how many standard deviations from the state mean ELU variety they are. The most diverse class is 3 or more SD units greater than the state mean variety (24 kinds of ELUs within 1,500 feet). The next most diverse class is 2 – 3 SD units followed by 1 – 2 SD units. We have chosen this method of aggregating ELU variety classes because it is objective and statistically-based.

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5. A Case Study: The South Kingstown Land Trust

A Practical Tool
ELUs are another arrow in the quiver of characteristics that land trusts and other conservation organizations can use to evaluate potential properties to acquire. There are many potential “values” that can be evaluated and include the presence of rare species, the suitability of habitat for fauna and flora, the presence of wetlands or riparian habitats, cultural features, aesthetics, ground water protection, and potential recreational opportunities such as hiking, paddling, bird-watching, etc. ELUs add another value — the potential of a property to support diverse or representative ecological communities far into the future. We anticipate that large conservation organizations such as the RI Department of Environmental Management and The Nature Conservancy will include ELUs in their suitability assessments to ensure that a property will support diverse fauna and flora as climate change effects manifest themselves. The purpose of this Case Study is to show how ELU data to be relevant at local scales for land trusts. The South Kingstown Land Trust has partnered with us in this exercise.

Patterns of ELUs
The geography of ELUs in South Kingstown is shown in this map based on the dominant (20 most common) ELUs in RI. Some interesting patterns can be seen. Regions that contain many rare and unusual species of plants and animals in South Kingstown, such as the moraine (Matunuck Hills) and the Great Swamp Management Area are not very diverse with respect to different types of ELUs. This is an important point — ELUs do not represent rarity, they represent geophysical settings that can support different plant communities. The area east and southeast of Wordens Pond, the Chickasheen Brook area and the Saugatucket River corridor show very complex patterns of ELUs, thus supporting many different species of plants and animals.

ELU variety is one factor to consider. In this map of the variety of dominant ELUs within 1,500 feet of every location in South Kingstown, we find regions of very high diversity in the central part of the community and the northwest part of South Kingstown. By overlaying the town parcels data, we can identify individual properties with exceptionally high landscape diversity (number of different ELUs within the property). Many parcels stand out as being interesting targets for acquisition.

ELU representation is another factor conservationists might consider. The idea is to ensure that ELUs that are common are protected. In this analysis, we calculated the percent of RI each ELU encompassed. Then we calculated the proportion of conserved lands (obtained from RIGIS database) that each ELU comprised. If an ELU was protected in proportion to its occurrence, the two percentages would be similar. In this map, we show ELUs that are under-represented in RI’s portfolio of protected land by at least 3-5% and more than 5%.

ELUs: One of Many Criteria
When evaluating a property for acquisition, the SKLT might consider including ELU variety and ELU representation in their suite of assessment factors. We do not advocate that ELU characteristics be the only factor to consider, but they should be included in an assessment along with another characteristics of a property.

Who We Are

The technical analyses in deriving and mapping ELUs was done by Kevin Ruddock (TNC), Peter August (URI EDC), Christopher Damon (URI EDC), and Charles LaBash (URI EDC). We thank Mark Anderson (TNC), Mike Bradley (URI EDC), Mark Stolt (URI NRS), and Keith Killingbeck (URI BIO) for their technical assistance.

This project is part of a larger effort by Pam Rubinoff (RI Sea Grant), Donald Robadue (URI CRC), and Amanda Ryan (CRC) to assist coastal communities in adapting to climate change effects. Climate change factors such as increased temperature, sea level rise, and increased precipitation can have implications for land trust properties and conservation goals, such as protecting groundwater and critical habitat. Sea Grant is piloting a project with South Kingstown Land Trust to identify and test tools that incorporate climate change considerations into conservation programs for land acquisition and management.

Joanne Riccitelli and Clarkson Collins of the South Kingstown Land Trust helped develop ELU measures that were relevant to local land trusts and municipal conservation organizations.

James Boyd and Caitlin Chaffee of the RI Coastal Resources Management Council provided helpful guidance of our work.

This project was supported by grants and and support from The Nature Conservancy, NOAA, RI CRMC, RI Sea Grant Program, and the USDA Renewable Resources Extension Act, and the URI Ag Experiment Station and Cooperative Extension Program.

Relevant Scientific Papers:

Ruddock, K., August, P. V., Damon, C., LaBash, C., Rubinoff, P., & Robadue, D. 2013. Conservation in the Context of Climate Change: Practical Guidelines for Land Protection at Local Scales. PloS one8(11), e80874.

Anderson MG, Ferree CE, 2010. Conserving the stage: Climate change and the geophysical underpinnings of species diversity. PLoS ONE 5(7): e11554. doi:10.1371/journal.pone.0011554

Kintsch JA, Urban DL 2002. Focal species, community representation, and physical proxies as conservation strategies: a case study in the Amphibolite Mountains, North Carolina, U.S.A. Conservation Biology 16(4): 936-947.

Nichols, W., K. T. Killingbeck, and P. V. August. 1998. The influence of geomorphological heterogeneity on biodiversity: II. A landscape perspective.  Conservation Biology, 12:371-379.

 Burnett, M. B., P. August, J. Brown, and K. T. Killingbeck.  1998. The influence of geomorphological heterogeneity on biodiversity: I. A patch-scale perspective.  Conservation Biology, 12:363-370.

Anderson, M. 1999. Biophysical Assessment of the Northern Appalachian Ecoregion. Unpublished Ph.D. Dissertation.

Data and Mapping

Click here to use the online mapping utility (ArcGIS.COM application)

Atlas-quality ELU maps


Zoomed-in to a small area (example of using ELUs to assess site suitability for conservation)

Download Data (ESRI rasters in zipped file geodatabase, see README.txt, 26 Mb)