Global threats to biodiversity result from the accelerating, planet-wide loss of natural landscapes. Aggravating the trend is the fragmentation of remaining habitats and the distributions of plants and animals that occupy them. Species are commonly distributed across fragmented landscapes as "nested subsets," where the species in smaller biotas comprise a proper subset of those in richer assemblages. The pattern is thought to result from the predictable sequence of species undergoing extinction from fragments: species that need vast areas to sustain minimum viable populations will go extinct quickly from each of the patches, while less needy species will find sufficient resources on a larger proportion of fragments.
The mechanism that creates these patterns of ordered extinctions appears to be quite simple: it is the differing minimum sustainable habitat area requirements of the various species that populate an archipelago of islands. As an "island" area shrinks, there will always be one species more at risk of local extinction than any other due to its areal requirements. As island area continues to decrease and the most suspectible species disappears, another species takes its place as the species-most-at-risk. If this pattern comes to be reliably repeated, fragment-to-fragment, then a pattern of nested subsets of species occupation is created.
The Nestedness Calculator measures the extent of the order present in nested presence-absence matrices, as well as provides a risk assessment of the extinction probability of the various species' populations isolated on islands of fragmented habitat.
All physical observations are a mixture of signal and noise — and nestedness is no different. The continuum that exists between the extremes of perfect nestedness and complete randomness can be described in the simplest of thermodynamic terms. A perfectly ordered system (perfectly nested), absent of all randomness, may be described as maximally "cold" (0°). Similarly, a system absent of all order may be labeled as maximally "hot" (100°). A presence-absence matrix that contains both a degree of nestedness order and randomness will register an intermediate "temperature" between these two end-points.
A Windows-based (Windows 3.x, 95, 98 or NT4) calculator has been developed to calculate the "temperature" (signal to noise) of nestedness order within a species presence-absence matrix and is available for immediate downloading. The calculator includes 294 presence-absence matrices taken from the primary ecological literature. Some of the matrices are highly nested, some are not, while others are anti-nested due to factors such as "checkerboarding" and allopatric speciations.
Download the Nestedness Temperature Calculator here. (453K)
- If you are on a PC and using either Netscape Navigator 3.0 or Microsoft Internet Explorer 3.0 (or greater), click on the download link above.
- Save the self-extracting, executable downloaded file, nested, anywhere you wish on your PC. Its residency will only be temporary. Once the Nestedness Calculator has been unzipped, this file will be deleted.
- Double click the nested self-extracting file you just downloaded. Press the UnZip button. Two hundred ninety-five files will be unzipped and a "Nested" folder (subdirectory) will be built on your C: drive. Close the UnZip program.
- Run the program NestCalc (found in the new "Nested" folder) to insure that you have a good copy. Once assured, you may delete the downloaded file, nested, from your PC. You won't have further need of it.
- Congratulations on a successful download. Further instructions on the use of Nestedness Calculator can be found in the calculator itself. A folder is provided as a repository for any additional presence-absence matrices you might wish to enter.
A companion paper explaining the philosophy of the Calculator is available on-line in HTML format by clicking here.
Atmar, W. and B.D. Patterson. 1993. The measure of order and disorder in the distribution of species in fragmented habitat. Oecologia 96:373-382