4.3 Can morphological traits and environmental variables be used as predictors for seed dispersal distances?
Frugivore body mass was a key predictor for seed dispersal distance; both mean and maximum seed dispersal distance are positively correlated to body mass in birds and mammals. Larger species tend to have larger foraging areas and range distributions and so carry seeds further from the parental source (Godinez-Alvarez et al, 2020). Flying species also had larger mean distances than non-flying species, suggesting that bird and bat species are key for long distance seed dispersal events (Medellin & Gaona, 1999). This is further strengthened by maximum seed dispersal distances significantly increasing with an interaction between a species body mass and ability to fly. This indicates that an increase in body mass of a volant species has a greater, positive effect on maximum dispersal distance compared to the same increase for a nonvolant species.
Most studies that calculated seed dispersal distances focussed on birds, which are key for seed dispersal due to their mobility and ability to cross matrices and connect habitat patches (Mueller et al, 2014). Additionally, there is large interspecific diversity amongst the functional traits of bird species, which offers a wider dietary scope and allows them to be relatively flexible to switch to other resources in response to fluctuations in fruit resource availability through seasonal or land use changes (Bender et al, 2017). Amongst the most frequently studied bird species were Eurasian jay (Garrulusglandarius ), Clark’s nutcracker (Nucifragacolumbiana ), white-crowned manakin (Pseudopipra pipra ) and Swainson’s thrush (Catharus ustulatus ) – these species have broad, omnivorous diets, ranging from insects to fruit. Birds with generalist diets are key to seed dispersal in fragmented landscapes and tend move between different habitats and facilitate early forest succession in open areas (Carlo & Morales, 2016; Barros et al, 2019).
Bats species, like birds, can functionally connect fragmented landscapes, and are associated with long-distance seed dispersal (Abedi-Lartey et al, 2016). Although often smaller than birds, the few studies bats tend to disperse seeds farther than birds of similar body mass (Egyptian fruit bat, Rousettusaegyptiacus – 132g, Madagascan flying fox, Pteropusrufus – 361g, Orii’s flying fox, Pteropusdasymallus inopinatus – 435g, Straw-coloured fruit bat,Eidolon helvum – 253g). Flying foxes can travel 70km a night (Abedi-Lartey et al, 2016). This is perhaps possible due to their gap crossing abilities, nocturnal activities, plasticity of habitat use, and foraging strategies (Muscarella & Fleming, 2007; Regoloin et al, 2020; Lourie et al, 2021). Furthermore, bats can defecate during flight, as opposed to when perched like most birds, which increases the likelihood of seeds being deposited in open areas where pioneer plant species can recruit and initiate forest regeneration (Muscarella & Fleming, 2007; Pena-Domene et al, 2014; Regolin et al, 2020). For instance, Phyllostomid bats can disperse seeds of over 300 plant species (Lobova et al, 2009; Voigt et al, 2017) and will regularly commute between foraging areas in natural and degraded landscapes, enabling establishment of early successional plant species (Galindo-Gonzalez et al, 2000; Ripperger et al, 2015). Seed-handling by bats could also increase their effectiveness as long-distance dispersers (Ong et al, 2022), since fruits taken by bats are not limited by mouth/beak gape width, and some bat species carry fruits that exceed their own body mass (Mahandran et al, 2018).
Finally, our models also appear to show a trend in mean dispersal distance for those studies undertaken in protected areas. These areas are likely to offer large tracts of undisturbed, continuous habitat enabling frugivores are able to travel long distances unrestricted by inhospitable landscapes such as intensive farmland or urban areas. In our analysis, animals may have moved out of protected areas into surrounding disturbed habitat during the duration of the study. However, our results still support the idea that dispersal distances are longer in areas with more intact habitat and validates previous evidence that seed dispersal services are likely to reduce in areas of landscape change (Wright & Duber, 2001). Furthermore, protected areas ensure frugivore safety through reducing illegal hunting events. This has a direct, positive effect on frugivore abundance and population levels that can increase seed dispersal services (Wright et al, 2000; Nunez-Iturri & Howe, 2007; Beckman & Muller-Landau, 2007).