Descriptors of network structure
To determine network structure and resilience from each habitat during a
whole El Niño year, and for each habitat during each season during the
El Niño and normal year, we assessed network structure by measuring six
key metrics. First, we quantified nestedness, which measures the extent
to which the interactions of one species are a sub-set of the
interactions of another species when the matrix of interaction is
organized by decreasing number of links (Dormann, Fründ, Blüthgen, &
Gruber, 2009). We calculated nestedness using the weighted NODF
approach, which is a measure of nestedness that uses overlap and
decreasing fill in the weighted matrix, that has been shown to
outperform other methods for estimating nestedness in binary networks
(Almeida-Neto & Ulrich, 2011). Second, we quantified modularity,
characterized as more interactions within a module than between modules
(Dormann & Strauss, 2014), using the QuanBiMo algorithm that is based
on simulated annealing and is more specifically designed for weighted
bipartite networks (Dormann & Strauss, 2014). Third, we calculated
weighted connectance by dividing linkage density by the number of
species in the network (Tylianakis, Tscharntke, & Lewis, 2007), which
reveals the number of links in the network in relation to the total
number of links (Altena, Hemerik, & Ruiter, 2016). Fourth, we
measured/calculated number of compartments, which are defined as
isolated sub-sets of nodes interacting with each other that do not have
any connections with another compartment in the network (Dormann, Fründ,
Blüthgen, & Gruber, 2009). Fifth, robustness was calculated as the area
below the curve of secondary extinction of bats when primary extinction
of plant species was simulated according to three methods: random
extinction of plant species (random), extinction of most connected to
least connected plant species (degree) and extinction from the least
connected to most connected plant species (abundance). And finally,
niche overlap among bat species was calculated using the Morisita-Horn
index (Horn, 1966). Apart from robustness, all metrics chosen have
little or no biases to sampling completeness and network size (Fründ,
McCann, & Williams, 2015). We used the function network level from the
Bipartite package (Dormann, Gruber, & Fründ, 2008) to determine network
structure and resilience from each habitat during the whole year and for
each habitat during each season and calculated the following network
metrics: weighted NODF, modularity (QuanBiMo), number of compartments,
niche overlap, and robustness.