# Calculates the (normalised) mean number of checkerboard combinations (C-score) in a matrix

### Description

Calculates the C-score for all higher-level species; the C-score represents the average number of checkerboard units for each unique species pair.

### Usage

1 |

### Arguments

`web` |
A matrix with pollinators as columns and plants as rows. Alternatively, when used on e.g. species occurrences across islands, rows are islands. |

`normalise` |
Logical; if |

`FUN` |
Function to use when summarising the C-scores for each pairwise comparison. Defaults to |

`...` |
Options to be passed on to FUN, e.g. na.rm=T for matrices with many zeros and normalise=TRUE. |

### Details

As a first step, any quantitative matrix is converted to a binary matrix of presences and absences.

Then, the formula given in Stone and Roberts (1990) is calculated for all species combinations, by calling `designdist`

from the package vegan. See code for details.

### Value

Returns whatever the FUN produces as output. Default would be a single value, i.e. the mean C-score of the web.

### Note

The normalisation, since Jan. 2015, is by brute force: the 1s and 0s are distributed for each pairwise comparison for maximum checkerboardness. (The previously used approach was incorrect!) As a consequence, large matrices will take some time to compute.

The minimum is set to 0.

### Author(s)

Carsten F. Dormann

### References

Gotelli, N.J. and Rohde, K. (2002) Co-occurrence of ectoparasites of marine fishes: a null model analysis. *Ecology Letters* **5**, 86–94

Stone, L. and Roberts, A. (1990) The checkerboard score and species distributions. *Oecologia* **85**, 74–79

### Examples

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