Megafaunal Seed Dispersal

Megafaunal Seed Dispersal

Although less immediate than their effects on physical / trophic ecosystem structure, megafaunal seed dispersal can nonetheless play a vital role in shaping ecosystems. Many megafaunal herbivores are keen consumers of fruits and seeds – providing a vital service:

1. Transporting and spreading seeds of too great a size to be effectively dispersed by smaller animals or other transport system.
2.  By ingesting far more seeds per feeding even than smaller mammals are capable of, transporting small seeds in far greater numbers and far greater distances than they could otherwise be transported, promoting seed germination success and the spatial distribution of the adult plants.

Although lack of megafaunal seed dispersal is  not yet shown to have resulted in any tree extinctions (partially because seed dispersal can occur to a lesser extent via other means) their ability to spread seeds across long distances could have positive effects on genetic diversity of plants and their distribution, crucial for their continued success (variance in net positivity of this factor dependant on geographical factors and counterbalance with negative effects on some species of destruction mentioned in previous post).

Pires MM, et al. (2014) by performing addition and removal simulations of extinct Pleistocene mammals, humans, and livestock in one of the most diverse seed-dispersal networks recorded to date, which includes species from major taxonomic groups of seed dispersers and the plants they interact with , explored how ecosystems have been reconfigured in the wake of megafaunal extinctions. The episode of megafaunal extinction and the arrival of humans changed how seed were distributed, throwing the original system off balance. Although the recent introduction of livestock into the seed-dispersal system partially restored the original organization (and the introduction of surrogate seed dispersers has been suggested as a management tool to locally restore ecological and evolutionary processes in those areas where large vertebrates died out), the ongoing extinction of key large vertebrates will lead to a variety of context-dependent rearranged seed-dispersal networks, most certainly affecting ecological and evolutionary processes. For example, because large mammals ingest more seeds per feeding event than smaller mammals, seeds are more likely to be dispersed in clumps, promoting seed germination success and the spatial distribution of the adult plants.

Difference between major seed dispersers now and then

Guimares etal 2008 explored the intricacies of the first mentioned seed dispersal role, defining such seeds as those of ‘megafaunal fruits’, defining them as either 4-10 cm in diameter with up to 5 seeds or being seeds of fruit >10 cm diameter with numerous small seeds. Out of 1361 samples studied for these criteria, a substantial 103 species of seed were identified as such. By relying on large megafaunal frugivores able to disperse enormous seed loads over long distances, megafaunal fruits have been able to circumvent the trade-off most fruits contend with between seed size and dispersal – the higher the seed size the higher seedling vigor and survival prospects. An illustration of this tradeoff is how larger seeds can survive partial consumption by seed predators and reach germination while smaller seeds would just be fully consumed – but because of this they are highly reliant on megafauna to spread them and guarantee maintenance of genetic variation and decreased among population structuring. Some species, such as large bats, toucans, and scatter-hoarding rodents can still disperse seeds over great distances but in much lesser quantities and rarely as far – although such species, as well as introduced livestock and abiotic forces such as runoff, flooding, gravity, and human-mediated dispersal allowed survival of megafauna-dependent fruit species after extinction of the major seed dispersers.

Megafaunal Fruits!

Megafauna extinction had several potential consequences, such as a scale shift reducing the seed dispersal distances, increasingly clumped spatial patterns, reduced geographic ranges and limited genetic variation and increased among-population structuring of megafaunal and smaller fruits that they transported in larger quantities than can be transported by other species. Extant megafaunal fruits show trait combinations reflecting ancestry, not present-day adaptations to modern seed dispersers and as such could quickly be in trouble if the few remaining surrogate megafauna were to disappear. These effects could be extended to other plant species dispersed by large vertebrates in present-day, defaunated communities as they became more and more reliant on the remaining species to pick up the slack left by the megafauna with potentially drastic consequences.

At this point at long last a positive role for humans, as opposed to just mitigating the damage we’ve already caused and trying to turn back a bunch of wheels, presents itself. With the extinction of Late-Pleistocene megafauna, we have an important role in the maintenance and dispersal of megafaunal seed species, particularly the especially large-fruited, multi-seeded taxa which have probably been more amenable to human use by yielding larger pulp loads/fruit relative to their small seeds (the juicy ones), which we have for generations already been spreading through our own activities and which we must now continue spread.