Different megafauna vary in their seed dispersal effectiveness of the megafaunal fruit Platymitra macrocarpa (Annonaceae)

Abstract

The world’s largest terrestrial animals (megafauna) can play profound roles in seed dispersal. Yet, the term ‘megafauna’ is often used to encompass a diverse range of body sizes and physiologies of, primarily, herbivorous animals. To determine the extent to which these animals varied in their seed dispersal effectiveness (SDE), we compared the contribution of different megafauna for the large-fruited Platymitra macrocarpa (Annonaceae), in a tropical evergreen forest in Thailand. We quantified ‘seed dispersal effectiveness’ by measuring the quantity and quality contributions of all consumers of P. macrocarpa fruit. Seed dispersal quantity was the proportion of the crop consumed by each species. Quality was defined as the proportion of seeds handled by each animal taxon that survived to produce a 2-month seedling. Megafauna (elephants, sambar deer, bears) dispersed 78% of seeds that produced seedlings, with 21% dispersed by gibbons (a medium-sized frugivore). The main megafaunal consumers displayed different dispersal strategies. Elephants were the most effective dispersers (37% of seedlings) and they achieved this by being high-quality and low-quantity dispersers. Bears displayed a similar strategy but were especially rare visitors to the trees (24% of the total seedlings produced). Sambar were high-quantity dispersers, but most seeds they handled did not survive and they were responsible for only 17% of seedlings. Gibbons displayed a high SDE relative to their body size, but they probably cannot match the role of elephants despite being more regular consumers of the fruit. The low density and poor regeneration of P. macrocarpa in the study site suggest that current dispersal rates by megafauna are insufficient, possibly reflecting reduced or missing megafauna populations. We show that different megafaunal species disperse seeds in different ways and may make unique contributions to the reproductive success of the plant species.

Spitting Seeds From the Cud: A Review of an Endozoochory Exclusive to Ruminants

“…The fate relies on many factors, not only the relative size of seeds and consumers (Forget et al, 2007), but also on consumer satiation, foraging speed, and availability of alternative foods, among others. In addition, direct observation of seed regurgitation by ruminants is very difficult, even in captivity (Prasad et al, 2006;McConkey et al, 2018). Because of this, data about the individual rate of seed spitting often come from captive or semi-captive individuals (e.g., Möhring, 1963).…”

“…Different authors refer to "small piles" or "loose clusters" of clean seeds (e.g., Corlett, 2011), but others mention dense clusters and "very shiny piles" (Brodie et al, 2009a). Piles of five seeds of Platymitra macrocarpa regurgitated by sambar deer were found by McConkey et al (2018) in Thailand. The same deer species, also in Thailand, deposited seeds of Choerospondias axillaris in piles containing "between 14 and 140 seeds, with the exception of one pile which was found to contain 750 seeds;" in the same area muntjak "tended to deposit seeds in smaller piles (of usually <100)" (Chanthorn and Brockelman, 2008).…”

“…Most of the dispersed seeds of Spondias mombin in Costa Rica were killed by bruchid beetles (Janzen, 1985). In Thailand, bruchids attack the dispersed seeds of Platymitra macrocarpa in 6-22 days (McConkey et al, 2018). In Brazil, >50% of seeds of the palm Acrocomia aculeata chewed by cattle were infested by bruchids, and this proportion increased to 99% after 27 days; regurgitated seeds had a significant higher rate of insect predation than seeds of non-chewed fruit (Scariot, 1998).…”

“…Different studies have stated that seeds were intact and in a great proportion alive after being regurgitated from the rumen (e.g., Delibes et al, 2017). Some others indicated that spat seeds germinated under field conditions (e.g., Feer, 1995;Castañeda et al, 2018;McConkey et al, 2018). In his pioneer description of seed dispersal while chewing the cud, Troup (1921) indicated that retention at the rumen improved germination of Acacia nilotica from 7 to 35%, concluding: "The superiority of seed collected from goat and sheep pens is generally recognized and seed so collected is extensively used for artificial sowing."…”

Timing of fruiting rather than topography determines the direction of vertical seed dispersal by mammals and birds

“…Surface areas at lower altitudes are generally larger than those at higher altitudes (13), and thus downhill seed dispersal is likely to occur if frugivores move randomly in the mountains (3). Human activities represented by habitat fragmentation and hunting can dramatically change frugivore composition, often resulting in the local extinction of megafauna, which plays an important role in seed dispersal (14)(15)(16). Therefore, vertical seed dispersal patterns can differ depending on the megafauna that are present/extinct in the mountains.…”