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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.

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