The Strength and Drivers of Bird-Mediated Selection on Fruit Crop Size: A Meta-Analysis

Abstract

In seed-dispersal mutualisms, the number of fruit a plant displays is a key trait, as it acts as a signal for seed dispersers that entails fruit removal and exportation of reproductive units (fruit crop size hypothesis). Although this hypothesis has gained general acceptance, forces driving the shape and strength of natural selection exerted by birds on fruit crop size remains an unresolved matter. Here, we propose that ecological filters promoting high functional equivalence of interacting partners (similar functional roles) translate into similar selection pressures on fruit crop size, enhancing selection strength on this trait. We performed a meta-analysis on 50 seed-dispersal systems to test the hypothesis that frugivorous birds exert positive selection pressure on fruit crop size, and to assess whether different factors expected to act as filters (fruit diameter, fruit type, fruiting season length, bird functional groups, and latitude) influence phenotypic selection regimes on this trait. Birds promote larger fruit crop sizes as a general pattern in nature. Short fruiting seasons and a high proportion of species belonging to the same functional group showed higher selection strength on fruit crop size. Also, selection strength on fruit crop size increased for large-fruited species and toward the tropics. Our results support the hypothesis that fruit crop size represents a conspicuous signal advertising the amount of reward to visually driven interacting partners, and that both plant and bird traits, as well as environmental factors, drive selection strength on fruit display traits. Furthermore, our results suggest that the relationship among forces impinged by phenology and frugivore functional roles may be key to understand their evolutionary stability.

Interspecific interactions among functionally diverse frugivores and their outcomes for plant reproduction: A new approach based on camera-trap data and tailored null models

“…Our results revealed that frugivores respond to crop sizes of fruiting plants, by visiting more frequently those plants bearing more fruits. These results are in line with previous studies that have also found a positive relationship between frugivore visitation and plant resource availability [see 69 and references therein]. Interestingly, the positive relationship between crop size and frugivore visitation was mostly driven by two functional groups of frugivores (i.e.…”

Intrinsic and extrinsic drivers of intraspecific variation in seed dispersal are diverse and pervasive

“…With respect to the proportion of seeds dispersed, results to date show no consistent relationship (Table 2). These patterns suggesting a general increase in the number but not the proportion of seeds dispersed with increasing crop size are supported by a meta-analysis that found positive bird-mediated selection on fruit crop sizes as measured by both visitation rate and the quantity of seeds dispersed, but no selection on the proportion of seeds dispersed (Palacio and Ordano 2018).…”

Selection on fruit traits is mediated by the interplay between frugivorous birds, fruit flies, parasitoid wasps and seed‐dispersing ants

“…We developed three competing a priori hypotheses based on our understanding of the relationships between plants, fruit and seed traits, seed removal, fruit infestation and parasitoids ( Figure 2). The initial full SEM hypothesized that both birds (Palacio et al, 2014;Palacio & Ordano, 2018;Siepielski & Benkman, 2007a) and ants (Alcántara et al, 2007;Manzaneda, Rey, & Alcántara, 2009) act as agents of selection on fruit and seed traits, so we connected the two fruit traits (fruit crop size and mean sugar concentration) and mean seed size to bird fruit removal, and mean sugar concentration and mean seed size to ant seed removal ( Figure 2). The number of fruits produced is often a function of plant size and neighbour density (Minor & Kobe, 2019), so we included a path connecting tree height, DBH and the distance to the nearest neighbour to fruit crop size, as well as a correlation between tree height and DBH.…”

“…Given that each fruit encloses only one seed, fruit removal equals seed removal. Fruit removal was used as a surrogate for tree fitness (Jordano, 1995; Palacio & Ordano, 2018; Siepielski & Benkman, 2007a; Sobral, Guitián, Guitián, & Larrinaga, 2013) in the context of selection by bird and ants, under the assumption that trees with a higher number of seeds dispersed would have greater potential for dispersal and germination of their seeds (Primack & Kang, 1989; Siepielski & Benkman, 2007a, 2008a; see Section 4). Although the proportion of fruits removed may be a better surrogate of fitness for plants whose reproductive output is size‐dependent (Jordano, 1995; Siepielski & Benkman, 2007b; but see Palacio & Ordano, 2018), we used the number of fruits removed as a fitness component because tree size and fruit crop size were included as covariates in the statistical models (see below).…”