biodiversity varies throughout the range of environmental gradients ( landscape ) , with variations in the richness of species ; some areas are rich , whereas others are poor ( watts 1996 ) . even if there has not been much interest in the study of these types of patterns at a regional or local level , two alternative paradigms have been proposed to explain local community assembly : dispersal - driven assembly (  island paradigm  ) and niche assembly (  trait - environment paradigm  ) ( hubbel 2001 ) . numerous studies have revealed that variations in species assemblages can reflect nested distribution patterns at the landscape level . nestedness in metacommunities is demonstrated when the assemblages of species - poor sites are subsets of those in the successively richer assemblages ( atmar and paterson 1993 ; baker and patterson 2011 ) . the concept of nestedness refers more to a description of an observable situation than an ecological process , which is why it can be considered as a measurement of the ordered composition of biodiversity , in a determined geographical area . in any case , there is no consensus concerning the mechanisms which determine nestedness in nature ( higgins et al . the magdalena basin supports a richness comprising 213 fish species ( maldonado - ocampo et al . however , at the present time , the floodplain is undergoing dramatic transformation and deterioration of habitats , caused by the expansion of agriculture , cattle , gold mining , oil extraction , african palm cultivation , and illicit cocaine plantations . the fishing population is estimated at 35,000 people , with an annual catch of less than 17,000 t year ( a sixth part of that caught 30 years ago ) . as a greater part of the fishes are obtained from the flooded cinagas , these are overexploited owing to the great number of fishermen who have no other means for survival . along the river channel , the fishermen 's catches are concentrated during five months , the three months during the first season of the year ( december  february ) and lastly , in the second season ( july  august ) , taking advantage of species migration . traditionally , the commercial catches include three species : prochilodus magdalenae steindachner , 1879 ( bocachico ) , pseudoplatystoma magdaleniatum buitrago - surez and burr 2007 ( pintadillo ) , and pimelodus blochii valenciennes 1840 ( blanquillo ) ( galvis and mojica 2004 ) , but the decline in catches has caused fishing pressure to fall on species of smaller size , with diminished fertility . out of the 44 species  in danger  included in the red book of fish from colombia , which are in the endangered category , 19 are from the magdalena river valley . two of these which are important to fisherman are in the category of critically endangered ( prochilodus magdalenae and pseudoplatystoma magdaleniatum ) , three are endangered ( ageneiosus pardalis , ichthyoelephas longirostris , and sorubim cuspicaudus ) , eight are vulnerable , and six are near threatened ( mojica et al . 2002 ) . in order that the conservation of an elevated number of species in a region should be effective , the first stage requires a knowledge of the distribution of species , the patterns of richness , and taxonomic composition , as well as their interrelationships through space and time ( margules and pressey 2000 ; sachs et al . for this purpose , methodologies should be applied which offer the greatest reliability in terms of their results and applicability . among the few possible alternatives which fulfill these requisites is that of measuring biogeographic nestedness ( patterson and atmar 1986 ; patterson 1987 ; atmar and patterson 1993 ) . it is within this framework of reference that the present study has been carried out ; in a geographical area which brings an elevated biodiversity of fish , with many endemic species in the hydrographic basin , and an ever more numerous fisherman population , caused by displacement from other zones of the country because of armed conflict and where fishing constitutes the only means of susbsistence . the middle section of the magdalena river valley constitutes an extensive floodplain with many lakes ( cienagas ) marginal to the main channel , covering an estimated area of 22,000 km ( correa 2008 ) . the seasonality of the flooding manifests highly predictable timing , permitting the evolution of adaptive life strategies for the species ( restrepo and jerfve 2000 ) . the fish undertake two kinds of migrations : longitudinal , along the principal water channel of the river and lateral , between the river channel and the floodplain . thirty cienagas of various sizes were selected for sampling along 840 km of the midsection of the magdalena river floodplain between the localities of puerto boyac and barrancabermeja ( 27 connected to the main river and three isolated ; fig . 1 and table 1 ) . samples were taken during the high water season ( november ) . the sampling program began in 2008 ( 10 cienagas ) up to 2010 ( 20 cienagas ) . fish were caught using experimental multifilament gillnets ( 100  3 m ) , with mesh sizes from two to 10 cm between opposite knots , and the order of the panels was originally random ( kukilahti et al . 2008 ) . besides , these kinds of nets are easy to use , low in cost , and appropriate for varying profundities and depths ( winemiller et al . location of the floodplain lakes used in the analyses of nested subsets for fish species in magdalena river . name , geographic area , main river , and hydrological and limnological characteristics of floodplain lakes studied in the magdalena basin . nets remained set for 24 h and were checked every four hours . in all lakes , fishes were collected within structurally different aquatic habitats : littoral with trees , littoral without trees , pelagic , and the connecting channel between cienaga and canal . in all analyses for a lake , only the taxa present at that lake were used . we refer to the number of species present at a lake as the local species richness at that lake . in each habitat , the measurements for environmental variables were analyzed using a one - way anova with the aim of discovering differences between habitats ( bonferoni correction included ) . pearson 's correlation analysis was used to explore the possible correlations between the environmental variables measured . for each cienaga we use the length of the channel that links the cienaga to the main river as a measurement of connectivity ( tockner et al . the matrix for presence  absence in all the cienagas was used for the study of the pattern of nestedness . various systems have been proposed for measuring the adjustment of a determined matrix of presence  absence to the nested subset model : n ( patterson and atmar 1986 ) , t ( atmar and patterson 1993 ) , u ( cutler 1991 ) , and c ( wright and reeves 1992 ) . the most popular measuring device is that of the matrix temperature t , introduced by atmar and patterson ( 1993 ) , with its recent modifications ( rodriguez - girons and santamaria 2006 ; ulrich and gotelli 2007 ; ulrich et al . 2009 ) . nestcalc software was used to sort the dataset from high to low for site diversity ( top to bottom ) and species diversity ( left to right ) ; in our case , the cienagas represent the rows and the species the columns . nestcalc also calculates as statistical test value t of the order ( nestedness ) or disorder ( lack of nestedness ) in the dataset ( atmar and patterson 1993 ) . t ranges from 0 to 100. a temperature near to 0 ( complete order ) would indicate a very nested matrix , whereas one near to 100 ( complete disorder ) would not be nested , but random . the observed t value was then compared to a distribution of values generated by monte carlo simulations . nestcalc was then used to calculate idiosyncratic t values by sites and by species presence or absence leading to specifically higher t values than the complete dataset . such elevated t values may indicate that the species ( or sites ) in question was influenced by a biogeographic event different from that affecting the other species ( or sites ) . nested sites are those dominated by nested species and idiosyncratic sites are those with a majority of idiosyncratic species ( mcabendroth et al . to analyze nested structure among floodplain fish species , we utilized binmatnest , an implementation of atmar and patterson 's ( 1993 ) temperature concept and metric , t , with an improved algorithm for packing matrices ( rodriguez - girons and santamaria 2006 ) . we used binmatnest to assess the significance of t in the observed matrices against three null hypotheses , each used to construct a pool of 1,000 simulated assemblage sets with the observed . the detection of nestedness is strongly determined by the metric and null models used ( heino et al . we also use the aninhado software designed by guimaraes and guimaraes ( 2006 ) , which is based on the algorithms from the nestedness temperature calculator . it is considered to be the best software package and permits choosing between four different null models . in order to determine the correlations of nestedness , we used spearman rank and partial spearman rank correlation tests to find the order of the cienagas in the maximally packed matrix to cienaga area , measured environmental variables , and isolation ranks ( cutler 1994 ) . to estimate the minimum protected area , we constructed species  area model and cumulated by progressively adding the number of new species gained with each increase in lake area . for each model , three kinds of submodels were tested as linear regression ( s / a ) , semilog model ( s / log a ) , and power model ( log s/ log a ) . the optimum models were determined to estimate the minimum protected areas for the total fish species . during the study , a total of 18,237 specimens , distributed among 50 species in 43 genera , 19 families , and six orders , were collected in the 30 lakes . cienagas ranged in surface area between four ( paticos ) and 2.333 has ( simiti ) , and contained from nine ( tortugas ) to 39 species ( simiti ) from a total species pool of 50 . two species ( caquetaia kraussii , and cyphocharax magdalenae ) were widespread across the cienagas archipelago ( 100% of incidence ) , while eight species ( colossoma macropomum , geophagus steindachneri , leporellus vittatus , pseudopimelodus buffonius , spatuloricaria gymnogaster , sturisoma panamense , sturisomatichthy leitoni , and symbranchus marmoratus ) were restricted to a single cienaga each . species with wide distributions comprised the poor assemblages , while richer assemblages contained these species plus a number of additional , more narrowly distributed forms . the resulting dataset showed a strong pattern of nestedness in the distribution of magdalenese fishes , and differed significantly from random species assemblages generated using monte carlo simulations . the reorganized matrix obtained after the original data matrix was maximally nested . the calculated temperature of the whole cienagas was 17 . 36. the expected temperature for the same fish assemblages was 70.47 ( sd = 2.96 ) . the probability of randomly obtaining a matrix colder than the one estimated was 2 . 31  10 . we detected a high degree of nestedness for the entire fauna under the binmatnest and anhidado algorithms ( table 2 ) . the results differ according to kind of cienagas : only connected ( t= 20.59 , fill 41.4% , p t < 20.59= 9.33  10 ) ; only connected and native fish species ( t= 21.15 , fill 45.1% , p t < 21.15= 1.71  10 ) ; only sedentary fish species ( t= 20.81 , fill 39.4% , p t < 20.81= 1.27  10 ) ; all statistically significant and as such not random ( null hypothesis ) . some characteristics of the three metrics for biogeographic temperature obtained in this study : nestedness calculator , bhmatnest , and anhidado . the first positions in the nesting range , both for the cienagas and species , are shown in figure 2 . the temperature method of atmar and patterson ( 1993 ) asserts that the topmost  cienaga  in a packed matrix is the most hospitable , while the leftmost species is most resistant to extinction . spearman rank correlations between species order in the  maximally nested matrix  correlated significantly with its% incidence ( rs = 0.97 ; p 0.001 ) , total abundance ( rs = 0.59 ; p 0.01 ) . out of all the measurements taken in the cienagas , only the size ( area ) and local richness are significantly related to the range of order of nested subset patterns ( r=0.59 and 0.90 , respectively , at p < 0.01 ) . in the same way , significant relationships are revealed if the analysis is carried out with respect to the richness of migratory species ( r=0.71 , p < 0.05 ) and sedentary species ( r=0.77 , p < 0.01 ) . the other variables ( including connectivity ) did not manifest any significance , some of them having strong relationships between each other . in our study , lake area was shown to be a factor significantly correlated with species richness ( r= 0.68 , p < 0.01 ) . maximally ordered species presence  absence matrix for 30 floodplain lakes in the magdalena river , ( a ) packed matrix showing distribution of species ( columns ) and floodplain lakes ( rows ) , ( b ) nested and idiosyncratic lakes ordered with corresponding species richness shown in , ( c ) idiosyncratic species are listed here , whereas all species collected are indicated in table 3 . roughly 13 out of the 50 species caught ( table 3 ) show markedly idiosyncratic distributions ( migrants species : astyanax fasciatus , astyanax magdalenaea , pimelodus blochii , and prochilodus magdalenae ; sedentary species : ctenolucius hujeta , eigenmania humboldti , geophagus steindachneri , hoplias malabaricus , hoplosternum magdalenae , hypostomus hondae , sternopygus macrurus , pterigoplichthys punctatus , and trichogaster pectoralis ) . la india and simit supported the greatest number of idiosyncratic ichtyofauna ( 10 species ) . el encanto , samaria , el dorado , and opn revealed only three idiosyncratic species . it is important to emphasize that palo grande and victoria suffer from very little fishing activity ; el dique , simit , el llanito , and palagua suffer very high fishing and intensity of human activities ( cattle and urban contamination , principally ) . great differences exist between the natural systems where nestedness has traditionally been studied ( e.g. landbridge islands , oceanic islands , and isolated mountains ) and aquatic ecosystems , streams , and lagoons . whereas the first group is characterized by the space and time scales which govern ecological processes , the last are very dynamic systems with high interconnectivity , dependent on the variations in the hydrological regimes ( flooding ) and other environmental factors dependent on seasonal and annual cycles ( godoy et al . 1999 ; taylor and warren 2001 ) . nestedness is an important compositional descriptor that can potentially identify risk - prone species or groups of species . due to the high correlation found between the nested structure and floodplain area , it is possible to predict the species composition on the basis of the cienaga area . in addition , the nested assemblage of some cienagas allows the identification of species at risk of total extinction should habitat area be reduced below that needed by the species . also , certain rare species might be indicators of total species richness , because they would typically occur only in species - rich communities . nestedness analysis is often portrayed as a tool to predict the order in which extinctions are likely to occur at a suite of sites in response to habitat reduction or other types of disturbance ( kerr et al . 2000 ) . differential species extinction and differential species colonization have been proposed as the two main causes producing a nested species assemblage in habitat islands : ( 1 ) differential species colonization is suggested when the reorganized matrix is correlated with some index of isolation ( or connectivity ) of the habitat islands , or with the dispersal ability of the species ( cook and quin , 1995 ; conroy et al . the species with greater capacity for dispersal are able to cover long distances , and colonize more distant sites , than those which are either less mobile or sedentary . this pattern has been observed in freshwater ecosystems ( taylor and warren , 2001 ) . even if the connectivity ( distance ) to the main channel can be considered a prediction variable , this indicates the richness of a cienaga , or at least in relation to migratory species as has been demonstrated in certain amazon floodplain lakes ( granado - lorencio et al . . this may be because of the varying hydrological behavior of the river , or the different capacity for a species to colonize or simply that the range of lengths of the connecting canals was not sufficient . ( 2 ) differential species extinction is suggested as the cause of a nested species assemblage , when the reorganized matrix of species occurring in habitat islands is correlated with island area ( soul et al . the range of nestedness in the cienagas did not manifest significant relationships with the environmental variables , except in terms of the size of the cienaga ( log area ) . however , other unevaluated factors may also affect this : habitat nestedness ( wright et al . 1998 ) . if the diversity of habitat is not uniform between cienagas , or even in the same cienaga throughout the year ( habitat selection hypothesis ; rodrguez and lewis 1990 ) , it may allow a greater number of species to coexist than others , producing a nested pattern . ( it would also be possible if some disturbance factor did not affect all the cienagas equally ; e.g. , contamination and intensity of fishing . ) the most conserved cienagas are simiti and el llanito , besides being those which suffer from least fishing pressure . on the other hand , el dorado , tortugas , el encanto , and represa are those which show greatest environmental deterioration from anthropic causes such as cattle , being situated in oil extraction zones , loss of vegetation cover on the banks , and the effect of urban contamination isolated from the main river . idiosyncratic site temperatures are not usually independent of the idiosyncratic species temperature ; assemblages dominated by idiosyncratic species usually have a high local species turnover ( soinen 2008 ) . ( 2005 ) indicate that the idiosyncratic species occupy fewer sites than the nested species , this may indicate that the idiosyncratic species have less capacity for dispersal than the nested ones . in our study , the four most abundant species ( cyphocharax magdalenae , caquetaia kraussi , trachelyopterus insignis , and roeboides dayi ) are present in more than 86% of cienagas studied and none are idiosyncratic ; however , only the first one of these are migratory species . out of all the idiosyncratic species , four are migratory ( astyanax fasciatus , astyanax magdalenae , pimelodus blochii , and prochilodus magdalenae ) and the rest ( 10 ) have a sedentary lifestyle . this admits the possibility that species - specific , non - biological factors exist , for example , pressure from fishing . ten idiosyncratic migratory species suffer from intense pressure from fishing ( ageneiosus pardalis , curimata mivarti , hypostomus hondae , leporinus muyscorum , pimelodus blochii , pimelodus grosskopfii , plagioscium surinamensis , prochilodus magdalenae , sorubim cuspicaudus , and triportheus magdalenae ) . the analysis of nested subset structure in ecological communities has been linked to the debate surrounding refuge design : sloss ( single large or several small reserves ) . in a low - temperature matrix , where species presence is predictable . therefore , a number of species  island combinations exist for a given temperature , favoring several small reserves . it needs only two simple parameters ( species richness and the size of area ) . a fundamental pattern is the way in which the total number of species increases as the sampled area increases . the increase in number of species comes about for two reasons : first , as more individuals are sampled , the chance of encountering additional species increases , especially if species are not randomly distributed . second , a larger area is likely to be more environmentally heterogeneous , thus containing additional species that differ in their niches . more extensive areas comprise greater habitat diversity than other smaller ones ( gaston and blackburn 2000 ) , and in nested systems , small cienagas contain species with ample distribution and abundance ( patterson 1987 ) . the existing regional richness in magdalena river ( 50 fish species ) is contributed mainly by eight cienagas : simiti , vaquero , la india , el llanito , canaletal , contento , chucur , and samaria ( 6.284 has ) . on the other hand , dealing with the conservation of total fish fauna , we recommend that at least 2.800 has of cienagas should be preserved ( 30 , 3% of the total area studied ) hotspots  while considering fish diversity conservation in the cienagas because they maintain rich assemblages . we propose that commercial fishing in the cienagas of the middle magdalena river should be banned all the year round , and a core protected area , where all human activities are forbidden , should be demarcated . conservation planning and policy often demand that decisions about the fates of specific species and communities be made on the basis of a limited amount of information , gathered in a limited amount of time . the use of nested subset analysis of species occurrence patterns has become an increasingly common tool in community ecology and many have advocated its use in conservation planning , particularly in reserve design and predicting species susceptibility to extinction . these regularities obtained in our study permit introducing ecological aspects into the communities in these world regions for purposes of development and planning strategies for the conservation of biodiversity ( lomolino 2000 ) .
we investigated if fish assemblages in neotropical floodplain lakes ( cienagas ) exhibit nestedness , and thus offer support to the managers of natural resources of the area for their decision making . 
 the location was floodplain lakes of the middle section of the magdalena river , colombia . 
 we applied the nested subset analysis for the series of 30 cienagas ( 27 connected to the main river and three isolated ) . 
 all fish were identified taxonomically in the field and the matrix for presence  absence in all the lakes was used for the study of the pattern of nestedness . 
 the most diverse order was characiformes ( 20 species ) , followed by siluriformes ( 19 species ) . 
 characidae and loricaridae were the richest families . 
 the species found in all the lakes studied were migratory species ( 17 ) , and sedentary species ( 33 ) . 
 two species ( caquetaia kraussii and cyphocharax magdalenae ) were widespread across the cienagas archipelago ( 100% of incidence ) . 
 nestedness analysis showed that the distribution of species over the spatial gradient studied ( 840 km ) is significantly nested . 
 the cienagas deemed the most hospitable were simiti , el llanito , and canaletal . 
 roughly , 13 out of the 50 species caught show markedly idiosyncratic distributions . 
 the resulting dataset showed a strong pattern of nestedness in the distribution of magdalenese fishes , and differed significantly from random species assemblages . 
 out of all the measurements taken in the cienagas , only the size ( area ) and local richness are significantly related to the range of order of nested subset patterns ( r=0.59 and 0.90 , respectively , at p < 0.01 ) . 
 differential species extinction is suggested as the cause of a nested species assemblage , when the reorganized matrix of species occurring in habitat islands is correlated with the island area . 
 our results are consistent with this hypothesis .