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en:hier-divisive [2019/05/23 09:59]
David Zelený
en:hier-divisive [2020/03/29 11:59] (current)
David Zelený [TWINSPAN (hierarchical divisive classification)]
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 Because the concept of indicator species is working with species presences and absences, the whole TWINSPAN algorithm is using only presence-absence data. To include also quantitative information about species abundances, species with higher abundances are multiplicated in the matrix by being converted into pseudospecies (i.e. the more abundant is the species in the original data, the more pseudospecies represent it in the pre-processed data); the conversion is done using pre-defined cut-levels. Result of TWINSPAN is a hierarchical dendrogram showing the relationship between individual subdivisions,​ a list of (one or several) indicator species for each split, and also the optional two-way ordered table, where both sites and species are ordered; sites are ordered according to the splits of the hierarchical classification,​ while species are ordered to form the blocks within the groups (<imgref twinspan-table>​). Because the concept of indicator species is working with species presences and absences, the whole TWINSPAN algorithm is using only presence-absence data. To include also quantitative information about species abundances, species with higher abundances are multiplicated in the matrix by being converted into pseudospecies (i.e. the more abundant is the species in the original data, the more pseudospecies represent it in the pre-processed data); the conversion is done using pre-defined cut-levels. Result of TWINSPAN is a hierarchical dendrogram showing the relationship between individual subdivisions,​ a list of (one or several) indicator species for each split, and also the optional two-way ordered table, where both sites and species are ordered; sites are ordered according to the splits of the hierarchical classification,​ while species are ordered to form the blocks within the groups (<imgref twinspan-table>​).
  
-TWINSPAN is often criticized ​by statisticians ​for being a not-elegant sequence of arbitrary and not fully documented steps but is favoured by ecologists ​for often returning ​ecologically intuitive ​resultsEspecially vegetation ​ecologists have a long tradition in using TWINSPAN, ​which is justified by the fact that the author (M. O. Hill) is himself a vegetation ecologist and he designed the method to closely resemble the traditional Braun-Blanquet approach of classifying vegetation (e.g. because it produces the two-way ordered table and emphasized the use of indicator species with high fidelity to individual groups, <imgref twinspan-table>​).+TWINSPAN is often criticized for being a not-elegant sequence of arbitrary and not fully documented steps. On the other side, ecologists often love it, since it does return results which are ecologically ​quite intuitive. ​Vegetation ​ecologists ​especially ​have a long tradition in using TWINSPAN, ​mainly because ​the author (M. O. Hill) is himself a vegetation ecologist and he designed the method to closely resemble the traditional Braun-Blanquet approach of classifying vegetation (e.g. because it produces the two-way ordered table and emphasized the use of indicator species with high fidelity to individual groups, <imgref twinspan-table>​).
  
 <​imgcaption twinspan-table|Two-way ordered table resulting from TWINSPAN algorithm (this example is based on Vltava dataset, which has been reduced to contain only species occurring in at least 30 samples (out of 97) to contain fewer species; the calculation was done in twinspanR library.>​{{:​obrazky:​twinspan-table-vltava.jpg?​direct|}}</​imgcaption>​ <​imgcaption twinspan-table|Two-way ordered table resulting from TWINSPAN algorithm (this example is based on Vltava dataset, which has been reduced to contain only species occurring in at least 30 samples (out of 97) to contain fewer species; the calculation was done in twinspanR library.>​{{:​obrazky:​twinspan-table-vltava.jpg?​direct|}}</​imgcaption>​
en/hier-divisive.txt · Last modified: 2020/03/29 11:59 by David Zelený