Research note Order and disorder in ectoparasite communities: the case of congeneric gill monogeneans (Dactylogyrus spp.) Andrea S Ï imkova  a,b, * , Milan Gelnar b , Serge Morand c a Faculty of Natural Sciences, Comenius University, Mlynska  dolina B-1, 84215 Bratislava, Slovak Republic b Department of Zoology and Ecology, Faculty of Science, Masaryk University, Kotla Ârska  2, 61137 Brno, Czech Republic c Centre de Biologie et d'Ecologie Tropicale et Mediterrane Âenne, UMR 5555 CNRS, Universite  de Perpignan, Avenue de Villeneuve, 66860 Perpignan Cedex, France Received 10 April 2001; received in revised form 22 May 2001; accepted 29 May 2001 Abstract The measure of order and disorder in the distribution of species in fragmented habitats proposed by Atmar and Patterson (Oecologia, 96 (1993) 373±82) was applied to investigate nested patterns of Dactylogyrus species parasitising the gills of roach. Organisation in dactylo- gyrid assemblages was investigated at three levels: (1) host populations between localities; (2) local host populations over seasons; and (3) individual hosts over one season within a local host population. Dactylogyrid assemblages showed nested patterns when analyses were conducted at the level of localities (among host populations) and at the level of seasons (among host populations within localities). The analysis at the level of hosts (infracommunities of parasites) revealed that nested pattern is not common. We suggest that nestedness may have a variety of causes and does not necessarily imply competition. q 2001 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Order and disorder; Nested pattern; Monogenean assemblages Ecologists question the patterns and processes governing the structure of communities based on the prediction of species±area relationships (Connor and McCoy, 1979; Rick- lefs and Lovette, 1999). An important question is whether assemblages occur at random or whether they are ordered. The answer to this question may be determined by the roles of competition, adaptation and stochastic processes acting on each species. Island biogeography theory has stimulated much debate on the question of the structure of communities, and it appears that the pool of species that inhabit a particular island (or isolated habitat) is not a random sample of the mainland pool of species. The occurrence of nested patterns is one indication that communities are not a pure stochastic organisation (Atmar and Patterson, 1993; Gue Âgan and Hugue Âny, 1994). However, several problems may arise when testing for nested patterns. Firstly, lack of a signi®cant nested pattern need not imply a random organisation (for instance Poulin and Gue Âgan, 2000). Secondly, it is dif®cult to compare indices of nestedness between populations or samples. Atmar and Patterson (1993) proposed the use of a direct measure of order which is based on entropy. As emphasised by Atmar and Patterson (1993): ªstatistical stochasticity is a concept closely related to heat, information, noise, order and disorderº. They proposed the use of a metric that measures the heat of disorder inherent in the historical biogeography of an archipelago (i.e. historical organisation of commu- nities). In what they termed cold systems, the extinction order of species would be perfectly determined, independent of random processes that may occur on each island (or isolated habitat). If the temperature of the system rises, then the extinction order will be less predictable. Hence the tempera- ture of the system is a measure of the order and ranges from 0 (complete order and predictable extinction) to 100 (unpre- dictable extinction and disorder). Parasites are good models in which to investigate the patterns and processes that may govern the structure of communities. However, the results of these studies are often neglected by ecologists working with free-living organisms, even if half of the living organisms on earth are parasites according to Price (1980) and even if parasites International Journal for Parasitology 31 (2001) 1205±1210 0020-7519/01/$20.00 q 2001 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S0020-7519(01)00245-4 www.parasitology-online.com * Corresponding author. Tel.: 1420-5-41129219; fax: 1420-5- 41211214. E-mail address: simkova@sci.muni.cz (A. S Ï imkova Â).