Values of modularity are negative when the number of links between taxa in a module is lower than expected by chance.
References:
Almeida-Neto, M., Guimarães, P., Guimarães, P. R., Loyola, R. D., & Ulrich, W. (2008). A consistent metric for nestedness analysis in ecological systems: Reconciling concept and measurement. Oikos, 117(8), 1227–1239. https://doi.org/10.1111/j.0030-1299.2008.16644.x
Newman, M. E. J., & Girvan, M. (2004). Finding and evaluating community structure in networks. Physical Review E - Statistical, Nonlinear, and Soft Matter Physics , 69 (2 2), 1–15. https://doi.org/10.1103/PhysRevE.69.026113
Figure S3: Correlation plots between trophic position characteristics and the metrics whose link with network robustness is investigated. The strength of the correlation is given by Spearman’s correlation coefficients and supported by the level of significance (*** for p-value<0.001, ** for p-value<0.01, * for p-value<0.1). Taxa with the largest number of preys tend to have large values of centrality.