Contribution by vertebrates to seed dispersal effectiveness in the Galápagos Islands: a community‐wide approach

Abstract

Seed dispersal and seedling recruitment are crucial phases in the life cycle of all spermatophyte plants. The net contribution of seed dispersers to plant establishment is known as seed dispersal effectiveness (SDE) and is defined as the product of a quantitative (number of seeds dispersed) and a qualitative (probability of recruitment) component. In Galápagos, we studied the direct contribution to SDE (number of seeds dispersed and effect on seedling emergence) provided by the five island groups of frugivores (giant tortoises, lizards, medium-sized passerine birds, small non-finch passerine birds, and finches) in the two main habitats in this archipelago: the lowland and the highland zones, and found 16 vertebrate species dispersing 58 plant species. Data on frequency of occurrence of seeds in droppings and number of seeds dispersed per unit area produced contrasting patterns of seed dispersal. Based on the former, giant tortoises and medium-sized passerines were the most important seed dispersers. However, based on the latter, small non-finch passerines were the most important dispersers, followed by finches and medium-sized passerines. The effect of disperser gut passage on seedling emergence varied greatly depending on both the disperser and the plant species. Although the contribution to SDE provided by different disperser guilds changed across plant species, medium-sized passerines (e.g., mockingbirds) provided a higher contribution to SDE than lava lizards in 10 out of 16 plant species analysed, whereas lava lizards provided a higher contribution to SDE than birds in five plant species. While both the quantitative and qualitative components addressed are important, our data suggests that the former is a better predictor of SDE in the Galápagos archipelago.

Functional outcomes of mutualistic network interactions: A community‐scale study of frugivore gut passage on germination

“…Second, by separating effects of deinhibition and scarification, we found that comprehensive experimental designs may not be important for capturing most of the other variations in gut passage effects. In this system, plant species varied substantially in the effect of gut passage on germination, but a great majority of the effect size was explained by pulp removal, with smaller and inconsistent effects of gut passage by the different frugivores, consistent with previous studies (Kelly et al., ; Nogales et al., ; Robertson et al., ). Researchers focused on dynamics at the community scale may more quickly and cost‐effectively describe plant species’ dependence on gut passage by conducting experiments using only whole fruits and manually depulped seeds, or with gut passage trials using a subset of frugivores.…”

“…First, by assessing the sources of variation in gut passage effects, we show that lumping frugivores into functional groups appears appropriate when considering effects of gut passage on germination, as there were not large or consistent differences across disperser species in their effects on germination. So long as the distinction between seed dispersers and seed predators was in place (which in this case involved complete seed predation, which may often not be the case; Shiels, ), we would have reached appropriate inferences by grouping the dispersers in this system a priori as “passerines/fruit dove,” “small birds/large birds,” or more simply “birds.” The grouping of species into functional groups is already widely used in seed dispersal research, especially for considering differences in effectiveness of different groups of dispersers (González‐Castro et al., ; Nogales et al., ). Our results emphasize that functional groupings for gut passage effects should be based on digestive morphology and diet rather than loosely on phylogeny and body size, which would have grouped the Alopecoenas ground dove and Ptilinopus fruit dove in this study.…”

“…This is a major empirical hurdle especially for plants, in which the outcome of a pollination or seed dispersal interaction can play out over months or years (Howe, 2016;Schupp et al, 2017;Vazquez, Ramos Jiliberto, Urbani, & Valdovinos, 2015). Therefore, existing studies on functional outcomes of mutualisms have focused on one or several interacting pairs rather than entire networks (Genrich, Mello, Silveira, Bronstein, & Paglia, 2016;Nogales et al, 2017;Schupp et al, 2017). Lacking community-scale information on functional outcomes, mutualistic network models typically assume that all pairwise interactions have equal functional outcomes when predicting coextinction following mutualism disruption (Kaiser-Bunbury, Muff, Memmott, Müller, & Caflisch, 2010;Memmott, Waser, & Price, 2004;Pocock, Evans, & Memmott, 2012) and assessing the links between network structure and stability (Bastolla et al, 2009;Gao, Barzel, & Barabási, 2016;Rohr, Saavedra, & Bascompte, 2014).…”

The timing of frugivore‐mediated seed dispersal effectiveness

“…Research on seed dispersal effectiveness has largely focused both on gut passage effects and on the spatial patterns of seed deposition generated by different disperser species, with consequences for recruitment (Jordano & Schupp, ; Schupp, ; Schupp et al., ). Gut passage effects on germination can vary among groups of seed dispersers (Nogales et al., ; Traveset, ), while the fates of seeds and seedlings often differ between microhabitats and habitat types due to spatial variation in biotic and abiotic factors, such as seed predator activity, irradiance, soil humidity or intra‐ and interspecific competition (Gómez‐Aparicio, ; González‐Varo, Nora, & Aparicio, ; Rey & Alcántara, ; Schupp, ). The latter explains why space has been a major factor when considering the quality of seed dispersal services provided by different animal partners (Calviño‐Cancela & Martín‐Herrero, ; Escribano‐Ávila et al., ; Rother et al., ; Schupp et al., ).…”

“…Gut passage effects on germination can vary among groups of seed dispersers (Nogales et al, 2017;Traveset, 1998), while the fates of seeds and seedlings often differ between microhabitats and habitat types due to spatial variation in biotic and abiotic factors, such as seed predator activity, irradiance, soil humidity or intra-and interspecific competition (Gómez-Aparicio, 2008;González-Varo, Nora, & Aparicio, 2012;Rey & Alcántara, 2014;Schupp, 1995). The latter explains why space has been a major factor when considering the quality of seed dispersal services provided by different animal partners (Calviño-Cancela & Martín-Herrero, 2009;Escribano-Ávila et al, 2014;Rother et al, 2016;Schupp et al, 2010).…”