Defining Species Abundance

Early in the work to determine how to make the science of Natural Communities more accessible for practitioners in the field, it became apparent that the plethora of numbers describing each community was a problem. While necessary for the science, practitioners had difficulty determining the implications of different number combinations. The following Categories were designed to assist in intuition, the Categories also allow clarity for appropriate restoration actions.

Creating Categories for Species Abundance

The work to define Natural Communities is data heavy. While necessary for the scientific exercise, the numbers and statistics are daunting for practitioners. A key focus of this effort has been to create simple but powerful descriptors that translate the Natural Community data into information that is intuitive and readily usable in the field.

To accomplish this objective, we grouped species of plants into five categories-- Dominant, Common, Sparse, Restricted and Rare. This determination combines how commonly the species is found and how much land area it covers in aggregate. The categories are useful for visualization in the field, but also directly relates to guidance on what to plant in restoration settings.

Simple definitions For Each Category

Dominant: This species will be found in relatively large quantities and will be found nearly everywhere. Usually consisting of tree and shrub species, these species set the overall characteristic of the Natural Community.

Common: These species will also be found frequently and in noticeable quantities. It should be easy to find these species in a Natural Community as they generally will cover more than 2% of the surface area of their forest layer

Sparse: These species are found regularly in the Community, but their quantities and sizes are generally small. They may live in small clumps or as individuals spread throughout the landscape.

Restricted: These species are not regularly found in the Community, but their quantities when they are found are noticeable. Many times, these species are finding small microclimates where they can exist successfully. These species are also referred to as "Locally Prevalent"

Rare: This Category is for species that show up only infrequently, and when they do, they show up in small quantities. The largest number of species in a Community will generally fall into this category. This website argues that for restoration purposes, practitioners should generally ignore this category of species as not likely to yield high results for the effort required.

Definitions for Abundance Categories

The categories are defined based on summary statistics about the plots that form the data source for each natural community. In the vast majority of the cases, the plots defined by the DNH are 400 square meters, which is approximately the same as 1/10 acre. Two concepts are essential to understand:

Constancy means the proportion of the time that a species is found within the plots that were surveyed. It is a measure of the frequency of finding a species in a tenth of an acre of forest. When the constancy is 60, that means (roughly) that in 60% of the 1/10 acre sections in a forest, one would find this species.

Cover means the amount of area in the plot that the species occupies. In practice, there are many layers in a forest, so each measure of "cover" roughly corresponds to how much of a layer or layers a species occupies. In two dimensions, cover is comparable to the shadow that the species would throw if the light were directly overhead per layer.

Mean Cover when Present (or Found) is the average cover across all of the plots in which a species was found. Plots in which the species are not found are excluded from the calculation.

The table describes the definitions of each of the Categories of species.

Additional Detail on Constancy

Unfortunately, common approaches to probability calculation do not really apply if one wanted to find the probability of finding a species a different sized area (in an acre, for example). While the plots collected by the DNH were essentially random from each other, the adjacent plots in an area of specific forest would not be randomly related.

Additional Detail on Cover

The definition of Mean Cover when Present is different than the definition of Mean Cover used throughout the DNH data sets. For the DNH, Mean Cover was meant to be a single measure of prevalence of each species in a community, encompassing the concepts of constancy and cover. Mathematically, it averaged Cover scores for a species across the entire plot set, irrespective of whether that species had been found in a plot.

Given the categorization of the data into quadrants, it was more appropriate to use Mean Cover when Present in this analysis. The inclusion of Constancy as a different axis allowed it to be separated out of the Mean Cover calculation as provided by the DNH.

The impact of selecting Mean Cover When Present was to show much more clearly the species that are uncommon, but when they do appear they show up in larger quantities. The following slide shows the impact on the change of definition for one community.

Sources for the Categorization Approach

Please see the Sources and Methods page for the full reference for the Avolio article