Frugivore diversity increases evenness in the seed rain on deforested tropical landscapes

Abstract

The diversity of tropical forests is strongly shaped by mutualistic interactions involving plants and frugivores that disperse their seeds. However, it is little known how decreases in the diversity of frugivores can affect seed dispersal patterns, plant community composition and species' coexistence in tropical forest landscapes. Here, we investigated the effects of bird frugivore diversity on seed dispersal of rare plant species and on the magnitude of equalizing effects on the seed rain in open areas within 12 fragmented landscapes in the Brazilian Atlantic Forest. We monitored the production of bird-dispersed seeds and bird abundance in forest fragments, and sampled the seed rain and the activity of birds attracted to experimental tree nuclei established in neighboring pastures. The activity of frugivores in tree nuclei was positively correlated with the diversity of birds recorded in nearby forest fragments, and the seed rain diversity increased with frugivore activity. The proportion of seeds dispersed more frequently than expected by chance in tree nuclei increased linearly with the species' richness of birds. The richness and abundance of active frugivores in deforested areas promoted a seed rain with evenness and diversity up to five times greater than the seed pool available in forest fragments due to the proportional increase in the dispersal of rare plant species and a concomitant proportional decrease in the dispersal of dominant fruiting plants. Furthermore, every additional bird species detected in a site was associated with a 10% increase in the equalizing effect on dispersed seeds' relative abundance. Our results show that the aggregated behavior of avian frugivore communities on deforested areas results in higher species richness in the seed rain of plant communities and underscore the urgency to reduce bird species' loss and the simplification of their communities in tropical landscapes.

Bird communities in the tropical peri-urban landscape of Bogor, Indonesia

“…Moreover, built-up area habitats contained the most incredible abundance of frugivorous birds; particularly in the built-up areas of the IPB Campus, many fruit trees surround the location of the buildings. The presence of frugivorous birds is essential as seed dispersers (Camargo et al 2021;Hu et al 2022), contributing to vegetation regrowth.…”

Negative density dependence characterizes mutualistic interactions between birds and fruiting plants across latitudes

“…Although we did not measure how frugivory interactions at the pairwise level translated into effective seed dispersal, all the 141 bird species we studied are known to be able to effectively disperse seeds of many plant species. Thus, we expect that NDD frugivory broadly translates into more equal frugivore-generated seed rain patterns [6,19,20], which in general should lead to more diverse communities of plant recruits during forest regeneration and community assembly processes [6,50]. Studies are needed to disentangle the contributions from different mechanisms that could independently underlie NDD frugivory, such as rare-biased frugivory driven by nutrients and/or secondary metabolites, and the effect of fruit resource saturation.…”

“…We predict that preference hierarchies have important demographic and organizational consequences for plant communities during successional processes. For example, field studies have shown how the differences in attractivity of fruit resource species to frugivore communities translate into differences in the density and richness of dispersed seeds and recruitment patterns [6,20,51].…”

“…Moreover, the evenness and diversity further increased as dispersed seeds transitioned to become early successional plant communities, showing that the ‘equalized’ seed rain templates lead to more even and diverse communities [6]. Subsequent field studies conducted in more diverse ecosystems in Brazil found these patterns to be potentially general, and that the magnitude of the equalizing effect become stronger as the diversity of plants and animals increased [19,20]. However, this NDD mechanism remains hypothetical as the empirical evidence so far has been limited to interpreting patterns found in the communities of seeds already dispersed, and it has not been directly studied in the fruit-selection process of birds.…”

How bird community responds to different ages of reforestation? Implications for restoration of a highly threatened Atlantic Forest phytophysiognomy

“…They are also essential for the transit of forest bird species that disperse diaspores to new areas. Birds assist in the regulation of the new system, in the acceleration of the reproduction process and forest regeneration of the planted areas (Camargo et al 2022).…”