How does avian seed dispersal shape the structure of early successional tropical forests?

Abstract

Frugivores shape plant communities via seed dispersal of fleshy‐fruited plant species. However, the structural characteristics that frugivores impart to plant communities are little understood. Evaluating how frugivores structure plant communities via the nonproportional use of available fruit resources is critical to understand the functioning of ecosystems where fleshy‐fruited plant species are dominant, such as tropical forests. We performed a seed‐addition field experiment to investigate how frugivorous birds shape the composition and richness of forests during early stages of secondary succession in cleared areas in Puerto Rico. The experiment tested whether the bird‐generated seed rain and the subsequent early successional plant communities were proportional representations of the fleshy‐fruited species that dominated the surrounding community. Experimental treatments consisted of patches with (a) seed‐additions by wild birds attracted to experimental patches with pole perches, (b) manual seed‐additions proportional to fruit abundance at the local scale (≤50 m from experimental plots) and (c) manual seed‐additions proportional to fruit abundance at the landscape scale (entire study site). Birds' seed‐additions differed in composition and abundance to expectations based on fruit availability at local and landscape scales. Treatments with seeds added by birds had the highest species richness in both the seed rain and the emergence stages despite how, on average, the monthly richness in the landscape‐scale treatment was double that of birds and the local‐scale treatment. This phenomenon was explained by the highest heterogeneity from the bird seed‐addition treatment across months, and the lowest seed per capita emergence rates in landscape treatments. Rather than reflecting relative fruit abundance, birds biased seed rain and per capita emergence towards a non‐random mixture of both small‐ and large‐seeded species, resulting in richer and distinct plant communities. Because frugivory and seed dispersal patterns depart from random encounters between frugivores and plants in communities, successional forests are characterized by an overrepresentation of proportionally rare plant species, and decreases in the dominance of many common species. Thus, for regenerating tropical forests, frugivory can function as mechanism that promotes persistence of rare plant species and their coexistence with more abundant plants. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13250/suppinfo is available for this article.

Can network metrics predict vulnerability and species roles in bird‐dispersed plant communities? Not without behaviour

“…Finally, even though we expected that resource use selectivity would reflect species’ contribution to evenness (Morán‐López et al, ), the d’ specialisation index (Blüthgen et al ) was uninformative (Figs c and c). Our modified index was able to detect frugivores that increased evenness via rare‐biased fruit choices (Carlo & Morales ; González‐Castro et al ) but it was unable to distinguish those that contributed to evenness thanks to their high abundance or increased mobility (Morales & Vazquez ). Overall, our findings point out that to accurately estimate species’ role on seed rain composition outcomes, information about their behaviour should be integrated in species‐level metrics.…”

Frugivore species maintain their structural role in the trophic and spatial networks of seed dispersal interactions

“…For example, thrushes concentrate their activity on habitat patches rich in fruit resources, resulting in seed deposition being heavily concentrated in these patches (García & Ortiz-Pulido, 2004;Jordano, 1993;Morales et al, 2013). Similarly, In this sense, the influence of frugivorous birds in plant spatial dynamics has been long recognized (González-Castro, Yang, & Carlo, 2019;Howe, 1989;Nathan & Muller-Landau, 2000;Schupp & Fuentes, 1995), including their role in 'habitat shaping', that is the modification and maintenance of the frugivores' own habitats (Herrera, 1985). In accordance with this, we found bird species, In a modular trophic system, species may be functionally redundant within a module but complementary between modules (Mello et al, 2011;Montoya, Yallop, & Memmott, 2015).…”

Searching for keystone plant resources in fruit‐frugivore interaction networks across the Neotropics

“…We focused on identifying the plant taxa that could be considered a potential keystone resources for avian frugivores, because birds figure among the most diverse and important group of seed dispersers across Neotropical ecosystems (Karubian et al., 2012). Besides, many bird species serve as agents of seed dispersal to forest regeneration nuclei in both natural and restoration areas (Cole, Holl, & Zahawi, 2010; González‐Castro, Yang, & Carlo, 2018; Zahawi, Holl, Cole, & Reid, 2013). We developed a new methodological framework to identify keystone plants by using a comprehensive database of fruit–frugivore networks in the Neotropics, a remarkable heterogeneous region, which holds the highest plant diversity in the world (Kier et al., 2005).…”