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The the concept of nested species assemblages and its utility for understanding effects of habitat fragmentation

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The concept of nestedness represents a null model that measures the order in presence-absence matrices of species in different assemblages (Patterson & Atmar 1986, 2000). Based on the assumptions of the model, consequences of fragmentation and the vulnerability of species to habitat change can be quantified. Different species assemblages are considered to be nested perfectly, if subsets of one or more species are lost at a time as communities decline in species number and none of the species lost from the richer communities reappears in one of the species poor communities once it was lost. Nestedness is most prevalent in habitat fragments (islands) derived from a once continuous system with a common species pool that became isolated. The presence-absence matrix of species at given sites can be ordered in a way to maximize regularity. Then, the order of sites and species reflects habitat suitability of the sites and the vulnerability of a given species to fragmentation.

This concept of nestedness is illustrated and used

1. to derive estimates of minimum viable populations of lemurs in forest fragments of Madagascar;

2. to learn, at which size a forest fagment is perceived as “forest” by Malagasy reptiles;

3. to test whether the system of protected areas within the city of Hamburg represents a network for the local avifauna with species exchange or whether the single protected areas represent islands in the urban matrix;

4. to link life history traits to the vulnerability of species to the urban environment and to develop hypotheses on the causes underlying the evolution of nested patterns.

The analysis of the urban system revealed that the protected areas within the city limits of Hamburg are perceived by birds as islands in an urban matrix. “Green” links between the areas do not act as corridors that allow free movement of all species. The vulnerability of any given bird species to fragmentation of woodland habitat within the city depends on its ability to use the urban matrix between the protected areas and seems to be related to the investments in egg-shell material. Species with high requirements for egg-shells disappear earlier from the community. Analyses of nested patterns can provide insights in biodiversity processes and valuable recommendations for decision making where longterm data are not available. The main values of the analyses of nested patterns, however, is to develop hypotheses that can then be tested with more detailed studies.
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Keywords: Nested pattern; birds; fragmentation; lemurs; minimum viable populations; phylogenetic contrasts; reptiles; urban ecology

Document Type: Original Article

Affiliations: Zoologisches Institut und Zoologisches Museum, Hamburg University, Hamburg, Germany

Publication date: 01 March 2001

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