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).