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

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.

Fruit size, crop mass, and plant height explain differential fruit choice of primates and birds.

Seed dispersal by animals is an important ecological process shaping plant regeneration. In general, seed dispersers are highly variable and often opportunistic in their fruit choice. Despite much research, the factors that can explain patterns of fruit consumption among different animal groups remain contentious. Here, we analysed the interactions between 81 animal species feeding on the fruits of 30 plant species in Kakamega Forest, Kenya, during 840 h of observations. Our aim was to determine whether plant characteristics, fruit morphology, fruit colours and/or fruit compounds such as water, sugar, phenols and tannins explained the relative importance of fruit consumption by the two most important consumer groups, primates and birds. We found significant differences in fruit choice between both groups. Primates fed on larger fruits and on higher trees that had larger fruit crops, whereas birds were observed feeding on smaller fruits and on smaller plants producing fewer fruits. Fruit colours did not differ between fruits consumed by primates and those consumed by birds. However, differences in the fruit choice among frugivorous birds were associated with differences in fruit colours. Smaller plants with smaller fruits produced red fruits which contrasted strongly with the background; these fruits were dispersed by a distinct set of bird species. The contents of water, sugar, phenols and tannins did not differ between fruits eaten by primates and those eaten by birds. Some phylogenetic patterns were apparent; primates fed preferentially on a phylogenetically restricted subsample of large plants with large fruits of the subclass Rosidae. We discuss why the observed primate dispersal syndrome is most likely explained by a process of ecological fitting.

Reliable cues and signals of fruit quality are contingent on the habitat in black elder (Sambucus nigra).

Communication mediates interactions between organisms and can be based on signals or cues. Signals are selected for their signaling function, whereas cues evolve for reasons other than signaling. To be evolutionarily stable, communication needs to be reliable on average, but the mechanisms that enforce reliability are hotly debated in light of strong environmental influence on signals and cues. While fruit quality in black elder (Sambucus nigra) is unrelated to fruit color, it is indicated by alternative pedicel phenotypes. Information on fruit quality has thus been transferred from the fruit to the developmentally associated pedicels, which are environmentally determined cues. Within each phenotype, color variation indicates fruit quality. Communication by black elder is thus reliable, but the proximate mechanisms enforcing reliability are habitat specific. High irradiance increases both the contrasts of the visual cue and fruit quality in the anthocyanin-based red pedicel phenotype, while shaded plants of the chlorophyll-based green phenotype apparently use signals by forgoing photosynthesis. This is because lower chlorophyll content in green pedicels creates contrasting pedicels, and higher contrasts indicate higher sugar content in the fruits of green pedicels. Because anthocyanins are light-induced, plants use cues when exposed to high irradiance, whereas they apparently use costly signals in the shade by reducing chlorophyll content in the pedicels. In behavioral field and laboratory experiments we document that avian seed dispersers select among pedicel phenotypes that indicate different fruit quality. Plants can thus increase their reproductive success by sending highly informative cues. Our results indicate how reliable information transfer can be maintained both in cues and signals in spite of substantial environmental influence on visual traits.