Network structure in wet versus dry seasons within each forest in
relation to null models
We predicted that increased seasonality in the rainforest and decreased
seasonality in the dry forest, would lead to different impacts on the
structure of mutualistic networks. In the dry forest, we found that
while the wet season during the El Niño year showed no changes in
network structure in relation to the null models, the network in the dry
season showed several metrics that deviated from expectations, and that
these occurred in different directions. On the other hand, two network
metrics during the wet season of the normal year (weighted NODF and
weighted connectance) showed similar trends to the dry season and were
lower than expected by chance. Explanations for these observations are
likely to reflect changes in fruit availability. Notably, in the dry
forest, most of the consumption of fleshy fruits occurs at the beginning
of the wet season (Vieira & Aldicir, 2006). Although there was a big
decrease in the rainfall in the wet season during this study, the second
month of the wet season (June) had rainfall within the historical
standard deviation that may have restored some of the normal conditions
of the forest in terms of fruit availability. This may explain why
nestedness (weighted NODF) was low during the dry season and why the
network did not show any significant deviation in structure in the wet
season. Another possibility is that the drought could have also led to
an increase in fruit availability during the beginning of the wet
season, which usually happens after a period of drought and leaf fall
(Detto, Wright, Calderón, & Muller-Landau, 2018).
During the normal year, the low values of nestedness and connectance are
likely explained by the low niche overlap between bat species during
this season. The low niche overlap is likely linked with the low number
of plant species in the diet of each bat species that was showed by the
low value of weighted connectance in relation with the null model. The
low niche overlap can promote a decrease in nestedness due to a low
overlap in the diet of generalists and specialists. Even though the
values of niche overlap were lower than expected by chance during the
wet seaon of the normal year, it was higher than the values of the wet
season of the El Niño year according to our null model comparison. This
suggests that even though our null models did not detect a significant
difference during one season only, there was a decrease in relation to
what would be expected during a normal year.
In the rainforest, the changes across the seasons were also not
consistent, which might also reflect changes in food availability across
the year. The lower nestedness and higher modularity of the network
during the dry season might be reflecting the bat niche specialization
during this season due to a reduction in food availability. On the other
hand, the absence of any significance difference in relation to the null
models during the wet season might be happening due to an increase in
fruit production where bats are exploiting more common resources.