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

Abstract

Trophic relationships have inherent spatial dimensions associated with the sites where species interactions, or their delayed effects, occur. Trophic networks among interacting species may thus be coupled with spatial networks linking species and habitats whereby animals connect patches across the landscape thanks to their high mobility. This trophic and spatial duality is especially inherent in processes like seed dispersal by animals, where frugivores consume fruit species and deposit seeds across habitats. We analysed the frugivore–plant interactions and seed deposition patterns of a diverse assemblage of frugivores in a heterogeneous landscape in order to determine whether the roles of frugivores in network topology are correlated across trophic and spatial networks of seed dispersal. We recorded fruit consumption and seed deposition by birds and mammals during 2 years in the Cantabrian Range (N Spain). We then constructed two networks of trophic (i.e. frugivore–plant) and spatial (i.e. frugivore–seed deposition habitat) interactions and estimated the contributions of each frugivore species to the network structure in terms of nestedness, modularity and complementary specialization. We tested whether the structural role of frugivore species was correlated across the trophic and spatial networks, and evaluated the influence of each frugivore abundance and body mass in that relationship. Both the trophic and the spatial networks were modular and specialized. Trophic modules matched medium‐sized birds with fleshy‐fruited trees, and small bird and mammals with small‐fruit trees and shrubs. Spatial modules associated birds with woody canopies, and mammals with open habitats. Frugivore species maintained their structural role across the trophic and spatial networks of seed dispersal, even after accounting for frugivore abundance and body mass. The modularity found in our system points to complementarity between birds and mammals in the seed dispersal process, a fact that may trigger landscape‐scale secondary succession. Our results open up the possibility of predicting the consumption pattern of a diverse frugivore community, and its ecological consequences, from the uneven distribution of fleshy‐fruit resources in the landscape.

Functional complementarity of seed dispersal services provided by birds and mammals in an alpine ecosystem

“…The relevance of mammals as seed dispersers in open habitats has been confirmed in previous studies conducted in other temperate regions (e.g. Rumeu et al, 2020).…”

“…The heterogeneity in the contributions of birds and mammals to the total bilberry seed dispersal demonstrate the high complexity and the functional complementarity of the frugivore assemblage in our study area. Functional differences in seed dispersal services between avian and mammalian frugivores, a common feature in temperate regions holding diverse disperser communities (Rumeu et al, 2020;, guarantee seed arrival to different habitats and microhabitats and across the entire fruiting season, which may enhance the chances of seedling recruitment. We believe that this is essential for a better understanding of the reproductive strategies of clonal species such as the bilberry.…”

“…Most plant species, especially in temperate and boreal regions, are dispersed by birds and mammals (Albrecht et al, 2013;. Frugivore species within a community may complement their seed dispersal services not only by specializing on different fleshy-fruited plant species (González-Castro et al, 2015;González-Varo et al, 2014), but also by contributing differently to seed arrival to different microhabitats and by dispersing seeds over short or long distances within the dispersal kernel (Jordano et al, 2007;McConkey & Brockelman, 2011;Rumeu et al, 2020). In addition, sympatric frugivore species may contribute differently to the seed rain across the fruiting season (González-Varo et al, 2019).…”

“…types of substrate, canopy cover) and the timing of seed arrival (Eriksson & Fröborg, 1996;González-Varo et al, 2014. Additionally, habitat preferences and animal movement, generally constrained by body size (Santini et al, 2013;Westcott & Graham, 2000), are crucial to understand the fate of the dispersed seeds (Jordano et al, 2007;Rumeu et al, 2020) and determine the quality of the seed dispersal service provided by each species (Schupp, 1993).…”

Introduced species shape insular mutualistic networks

“…As a result from ecological fitting, the observed interaction strength between two species reflects both the encounter rate (considered as a neutral force) and their trait complementarity (often described as an interaction kernel function; dubbed a niche-based force) (1). It is, therefore, possible to predict the structure of a seed dispersal network from the heterogeneous fleshy fruit resources in the landscape (5). It also means that we can use the trait dispersion of residing species to assess the invasibility of an ecological network and the invasiveness of prospective nonnative species (6).…”