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).