Linking structure and functionality in mutualistic networks: do core frugivores disperse more seeds than peripheral species?

Abstract

Seed dispersal networks are often organized in nested structures in a way that a few core species can disproportionally affect the remaining species in a network, in both the ecological and evolutionary sense. Yet, the relative importance of core and peripheral species has not been properly tested in seed dispersal network studies. We determined core species from 10 local seed dispersal networks composed by fleshy-fruited plants and frugivorous birds. Each of those local quantitative networks was characterized with a core quality value, a core score for each species and a threshold value between core and peripheral species. From a total of 52 bird and 69 plant species that interacted in the study area, only 8 and 15, respectively, were identified as core. Each local network had a core that comprised a small number of birds and plants, always lower than 30% of the interacting species. There was no difference in the quantitative component of seed dispersal effectiveness (QC) provided by the frugivorous bird assemblage to plant functional groups clustered according to their growth form and fruit characteristics. Core birds dispersed seeds from each of these plant functional groups with a higher QC than peripheral species. Thus, we empirically demonstrate for the first time that seed dispersal networks at a regional scale have a small core set of fruit-eating birds, upon which heavily rely most fleshy-fruited plants for their seed removal. Hence, the activity of just a few core frugivores could deeply impact the demography of an entire assemblage of fleshy-fruited plants.

Functionally specialised birds respond flexibly to seasonal changes in fruit availability

“…; Ruggera et al . ). According to this concept, functionally specialised species interact with functional characteristics of partners that no or only few other species interact with (Bellwood et al .…”

Functional Equivalence in Seed Dispersal Effectiveness of Podocarpus parlatorei in Andean Fruit-Eating Bird Assemblages

“…Both the frequency of visits and the estimated deposition of seeds depend strongly on bird abundance, and subordinate to it, the outcome of the plant-frugivore interaction is influenced by behavioral and morphological bird traits. Moreover, even pulp mashers, usually considered inefficient dispersers compared to "legitimate" dispersers like fruit gulpers, can be quantitatively important effective dispersers in Andean forests (Ruggera et al, 2011(Ruggera et al, , 2016. Although species of gulpers are those that reach higher values of SDE of P. parlatorei, also some species of mashers can be very effective dispersers of this species with medium-sized seeds (see Loayza and Knight, 2010, for a similar finding).…”

“…This is because there would be no reason to suppose that bird species will substantially modify their main habitat use patterns, in addition to the fact that the species that reach higher values of SDE are mostly abundant, for which fruits represent an important dietary item (Ruggera et al, 2016;Nazaro and Blendinger, 2017). Moreover, given the greater functional equivalence observed in the habitats where most of the recruitment occurs, longterm changes in the abundances of particular bird species could have low effect on P. parlatorei population dynamics.…”

“…Assemblages of fruit-eating birds differ from those of old-growth forests in their greater richness given by the turnover by related species that favor more open environments and the persistence in low density of several species typical of old-growth forests. They include the Elaenia species already mentioned and other noteworthy species like T. rufiventris and T. sayaca, two of the main seed dispersers at lower elevation subtropical Andean forests (Blendinger et al, 2015), but less abundant in upper old-growth cloud forests (Ruggera et al, 2016). In summary, dispersal services depend on an enriched set of dispersers in secondary forests, where the high quantitative component of SDE is largely achieved by the spatial turnover of habitat specialists of closely related and morphologically similar habitat generalists, which are able to overcompensate for the loss of effective seed dispersers typical of mature forests.…”

“…However, the strength of the linkages between a plant species and its mutualistic partners is usually very variable, showing quantitatively strong relationships with only some species of their seed dispersers (Jordano, 1987;Bascompte and Jordano, 2007;Ruggera et al, 2016). These strong links are often established with the most abundant mutualistic partners, whose numerical dominance leads to a higher frequency of visits (Vázquez et al, 2005), and may result in a greater number of removed fruits and dispersed seeds.…”