Specialization and interaction strength in a tropical plant–frugivore network differ among forest strata

Abstract

The degree of interdependence and potential for shared coevolutionary history of frugivorous animals and fleshy-fruited plants are contentious topics. Recently, network analyses revealed that mutualistic relationships between fleshy-fruited plants and frugivores are mostly built upon generalized associations. However, little is known about the determinants of network structure, especially from tropical forests where plants' dependence on animal seed dispersal is particularly high. Here, we present an in-depth analysis of specialization and interaction strength in a plant-frugivore network from a Kenyan rain forest. We recorded fruit removal from 33 plant species in different forest strata (canopy, midstory, understory) and habitats (primary and secondary forest) with a standardized sampling design (3447 interactions in 924 observation hours). We classified the 88 frugivore species into guilds according to dietary specialization (14 obligate, 28 partial, 46 opportunistic frugivores) and forest dependence (50 forest species, 38 visitors). Overall, complementary specialization was similar to that in other plant-frugivore networks. However, the plant-frugivore interactions in the canopy stratum were less specialized than in the mid- and understory, whereas primary and secondary forest did not differ. Plant specialization on frugivores decreased with plant height, and obligate and partial frugivores were less specialized than opportunistic frugivores. The overall impact of a frugivore increased with the number of visits and the specialization on specific plants. Moreover, interaction strength of frugivores differed among forest strata. Obligate frugivores foraged in the canopy where fruit resources were abundant, whereas partial and opportunistic frugivores were more common on mid- and understory plants, respectively. We conclude that the vertical stratification of the frugivore community into obligate and opportunistic feeding guilds structures this plant-frugivore network. The canopy stratum comprises stronger links and generalized associations, whereas the lower strata are composed of weaker links and more specialized interactions. Our results suggest that seed-dispersal relationships of plants in lower forest strata are more prone to disruption than those of canopy trees.

Stability and generalization in seed dispersal networks: a case study of frugivorous fish in Neotropical wetlands

“…Even large seed dispersal networks composed of multiple frugivore taxa with low levels of specialization and nestedness are robust to the loss of generalized frugivores (e.g. 88 bird and mammal species, 33 plant species, mean d 0 ¼ 0.31, Z-score WNODF ¼ 28.99 [44]; figure 3c). However, robustness to the loss of generalists declines with specialization, which means that networks with few generalists and mostly specialists are highly susceptible to the loss of generalists.…”

Phenology of tropical understory trees: patterns and correlates

“…However, the idiosyncratic nature of specieslevel phenological patterns was obscured at the community-level where we found little temporal or spatial variation in the proportion of fruiting species. This species-level variability and community-level constancy may well contribute to the lack of dietary specialization among frugivores (Schleuning et al 2011) as well as the persistence of large frugivorous guilds in many tropical vertebrate and invertebrate lineages.…”

Biodiversity, Species Interactions and Ecological Networks in a Fragmented World

“…Memmott, 1999) and plant-frugivore/seed disperser networks (e.g. Donatti et al, 2011;Schleuning et al, 2011a), with less familiar forms including plant-ant networks and host-symbiont interactions (e.g. gut microbiomes; Purdy et al, 2010).…”