Network dissimilarity across forests and seasons
We observed high interaction dissimilarity between seasons in both
forests. Similar observations have been made for other mutualistic
networks over time (years) where the percentage of retained interactions
was similarly low, ranging from 5% to 31% (Petanidou et al. 2008;
Alarcón, Waser, & Ollerton, 2008; Vázquez et al. 2009). It’s likely
that most variation in interactions between seasons in the dry forest in
our study can be explained by differences in fruit availability, which
tends to show temporal variation (Kushwaha, Tripathi, Tripathi, &
Singh, 2011). The dissimilarity in species composition between the
rainforest and dry forest of our observed networks is in accordance with
what has been observed for the species turnover between lowland dry
forests and rainforests in Costa Rica, where 50%-100% of the flora and
fauna were common to both forests (Janzen, 1986). Mello (2009) suggested
that patterns of temporal species turnover in phyllostomid bats are
related to the abundance of preferred food items with the abundance of
understory and canopy frugivores positively correlated with the rainfall
in the Atlantic rainforest. Therefore, the impact of El Niño is unlikely
to be equally distributed across all bat species. While species that
forage in the canopy tend to be more specialized on tree species that
produce big numbers of fruits for short periods of time, understory bats
feed mainly on plants that produce few fruits over many months of the
year (Mello, 2009).
Although El Niño causes floods in some regions of the world, the overall
trend is to promote droughts in tropical rainforests (Holmgren et al.
2001) with occurrences of high annual rainfall and low seasonality
considered unlikely (Borchert, 1998). This trend is usually reversed
during the year after El Niño due to La Niña (Holmgren et al. 2001).
However, most studies evaluate the effects of droughts, but not floods,
in rainforests (Wright et al. 1999; Harrison, 2000). Dunham, Erhart, &
Wright (2011) worked in Madagascar, and reported one of the few studies
that examined the impact of heavy rains caused by an ENSO event on a
rainforest, where he observed a disruption in the cycle of the primary
fruit food sources, with a consequent increase in infant mortality in
lemurs. Working in semi-deciduous forest in Panama during the strong
ENSO event of 1982-1983, Leigh, Windsor, Rand, & Foster (1990) reported
a drought that led to tree mortality that was 5 times higher than usual,
yet resulted in no clear serious negative consequences for animal
populations. They suggested that the forest might have adapted to
surviving droughts provoked by ENSO events without too much damage.
El Niño seems to be impacting network structure in contradictory ways,
which might promote biodiversity on one hand through increasing
compartmentalization, but might deplete it on the other by increasing
competition and decreasing stability via lower nestedness and
connectance. It is especially interesting that different phenomena
related to El Niño (floods and droughts) induced similar effects in the
structure of mutualistic seed dispersal networks. This has broad
consequences as increasingly strong El Niño events are expected to
impact large portions of Latin America, where phyllostomid bats are
distributed. More studies are required to see if there are similar
responses in mutualistic networks to different stressors such as habitat
fragmentation or other scenarios that potentially boost the effects of
extreme climatic events (Butt et al. 2015).