Nestedness is a measure of structure in an
ecological system
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
, usually applied to species-sites systems (describing the distribution of species across locations), or species-species interaction networks (describing the interactions between species, usually as bipartite networks such as hosts-parasites, plants-pollinators, etc.).
A system (usually represented as a matrix) is said to be nested when the elements that have a few items in them (locations with few species, species with few interactions) have a subset of the items of elements with more items. Imagine a series of islands that are ordered by their distance from the mainland. If the mainland has all species, the first island has a subset of mainland's species, the second island has a subset of the first island's species, and so forth, then this system is perfectly nested.
Measures of nestedness
One measurement unit for nestedness is a system's '
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
' offered by Atmar and Patterson in 1993. This measures the order in which species' extinctions would occur in the system (or from the other side - the order of colonizing a system). The 'colder' the system is, the more fixed the order of extinction would be. In a warmer system, extinctions will take a more random order. Temperatures go from 0°, coldest and absolutely fixed, to 100° absolutely random.
For various reasons, the Nestedness Temperature Calculator is not mathematically satisfying (no unique solution, not conservative enough). A software (BINMATNEST) is available from the authors on request and from the Journal of Biogeography to correct these deficits In addition, ANINHADO solves problems of large matrix size and processing of a large number of randomized matrices; in addition it implements several null models to estimate the significance of nestedness.
Bastolla ''et al.'' introduced a simple measure of nestedness based on the number of common neighbours for each pair of nodes. They argue that this can help reduce the effective competition between nodes in certain situations. For instance, two insect species might "help" each other by pollinating the same subset of plants, thereby reducing the extent to which they are harmful to each other. The authors suggest that this effect is behind a correlation between nestedness and diversity in plant-pollinator ecosystems. However, Johnson ''et al.'' have shown that this measure does not, in fact, properly account for the desired effect.
These authors propose a refined version of the measure, and go on to show how certain network properties affect nestedness.
References
Software
Nestedness Temperature Calculator Program{Dead link, date=October 2019 , bot=InternetArchiveBot , fix-attempted=yes
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ttp://hypatia.agricolas.upm.es/WINE/ Weighted Interaction Nestedness Estimator. It works with weighted matrices (i.e. species abundance)br>
NeD - Nestedness for Dummies. A user friendly interface for nestedness analysis
Systems ecology