Basic metrics of food webs. A pitcher plant ( Nepenthes albomarginata ) food web . Nepenthes albomarginata. S = 19 species L max = 19*18/2 = 171 possible links between two species L = 35 realized links between two species Connectance : C = 35/171
Basic metrics of foodwebs
S = 19 species
Lmax = 19*18/2 = 171 possiblelinksbetweentwospecies
L = 35 realizedlinksbetweentwospecies
Connectance: C = 35/171
Ch = 100 totallength of allfoodchains
Li = 40 isthetotalnumber of chains
ChL = 100/40 = 2.5 istheaveragechainlength
L/S = 35/19 = 1.8 isthemeannumber of links per species
N = 28
Fill = 28/80=0.35
Metrics of speciesassociationsinbiogeographicmatrices
TheC-score as a metric of negativeassociations
TheClumping-score as a metric of positiveassociations
TheTogetherness-score as a metric of nicheoverlap
Theadditivenature of theC-score
Numbers of checkerboards for entrieswithinthearea AT are a measure of spatialspeciesturnover.
Numbers of checkerboards for entrieswithinthearea ATC are a measure of turnover independent speciessegregation.
CMixed = CS – CTurn - CSegr.
Therankcorrelation of matrixentriesis a metric of spatialturnover.
R2is a moreliberalmetricthanCturn.
R2 = 0.347
Thecorrelation of ordinationscoresisalso a metric of turnover but even less selective.
Thereare 17 embeddedabsences.
Thenumber of embeddedabsencesis a measure of speciesrangesizecoherence.
Themetricdependsstrongly on theordering of rows and columns
Themeasurement of nestedness
Thedistanceconcept of nestedness.
Sort thematrixrows and olumnsaccording to some gradient.
Define an isoclinethatdividesthematrixinto a perfectlyfilled and an empty part.
Thenormalizedsquared sum of relativedistances of unexpectedabsences and unexpectedpresencesisnow a metric of nestednessis.
Nestedness based on Overlap and Decreasing Fill (NODF)
NODF is a gap basedmetric and moreconservativethantemperature.
The disorder measure of Brualdi and Sanderson
Ho many cellsmust be filledoremptied to achieve a perfectlyorderedmatrix.
TheBrualdiSandersonmeasureis a count of thisnumber
Discrepancyis a gap countingmetric.
How to measurespeciesaggregation?
Nearestneighbouris a presence – absencemetric
A sum of cellentriesaround a focalcellmultiplied by theentry of thefocalcell
Joincountoperates on presence – absence and abundancematrices
Thesemetricshaveverylowpower a moderate to smallmatrixsize and high orlowmatrixfill.
The metric CA is a count of the number of abundance checkerboards in the matrix.
Other 2x2 submatricescatchmatrixpropertiesthathave not welldefinedecologicalmeaning.
Themetricis a sum of allpairsinthematrix (first sortedaccoding to speciesrichnessthensortedaccording to weights), wheretheweightintherow/column of lowerspeciesrichnessissmallerthantheweightintherow/column of higherspeciesrichness
A completetable of methods for co-occurrenceanalysis
Pajek: software for socialnetworkanalysis
Theseprogramsuseclusteranalysis and ordination to sort thematrixaccording to numbers of occurrences. Didstancemetricsarethenused to identifycompartments.
They do not fullyallow for statisticalinference.
KliqueFinder: software for compartmentanalysis