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.

Ecosystem Physical Structure: Part 2, Case Studies

Megafauna in Africa:

A prime example of a remaining ‘natural’ megafauna regime, consisting diverse bottom-up- (nutrient-limited) and top-down-regulated communities in which herbivores suppress the accumulation of woody vegetation (reducing woody species cover by between 15-95%), thereby opening habitat for grasslands and forbs is, as the avid reader would have guessed from previous posts introducing megafaunal prehistory and contemporary megafaunal abundance, Africa (reduce woody species cover by 15-95%) which gives (as mentioned in the previous post re African elephants) about as good an indication of how megafauna affect ecosystem physical structure as is available. As such many studies analysing such effects are based here, yielding some interesting results.

Asner et al2015 compared the effects of a variety of environmental and biotic factors on treefall rates and patterns in woody plant canopies in Kruger National Park using Light Detection and Ranging – linked a mean biennial treefall rate of 8 (or 12% of total) per hectare strongly to Elephant density, matched only by spatial variation in elevation and soil. This is a pretty astounding result, with megafauna effectively proving as strong a shaper of an ecosystem as their abiotic counterparts.

 Keesing & Young 2014 exhibited a modern example of the effects of megafaunal loss with direct negative impacts for humans in their study of the Kenya Long-Term Exclosure Experiment, where the effects of removal of different combinations of domestic (eg cattle) and native grazers was compared read megaherbivores). Removal of large grazing mammals led to increased abundance in a small grazing mammal, the pouched mouse, attracting venomous snakes (to prey upon them) and fleas (and thus transmission of flea-borne pathogens), and leading to the decimation acacia seedlings – all with potentially substantial and undesirable consequences for humans.

Megafauna in Australia:

Rule et al2012 used data derived from sediment cores dating back 130kya concerning pollen, Sporormiella and charcoal to reconstruct the ecological consequences of Australia’s megafaunal loss. Vegetation, fire, and climate conditions are reconstructed via the proxies of pollen and charcoal, herbivore activity from Sporormiella found in their dung (chiefly associated with large herbivores, thus good proxy for large-herbivore biomass). Analysis of these cores not only suggests human arrival as the cause of Australian megafaunal extinction (Sporomiella decreasing drastically shortly after the arrival of humans ca 40k years ago whereas levels at other substantial climate driven shifts showed no significant variation) but also that this loss triggered the replacement of mixed rainforests with sclerophyll vegetation through relaxed herbivory and increased incidence of fire (note: these factors are by no means independent, megaherbivores often work in competition with or in congress with fire regimes, to great effect) – together consisting an ecosystem shift as large as any ecological effect of climate change over the last glacial cycle in the region, exhibiting the magnitude of megafaunal extinction implications on ecosystem physical structure.

Johnson &Rule et al 2015 later revisited the above discussion in greater detail, arguing that such megafaunal extinctions of large herbivores in fact have effects of varying magnitudes on ecosystem physical structure, as demonstrated by contrasting how a different site reacted to the Australian megafaunal extinction event upon which the previous paper’s conclusions had been drawn. In stark contrast to the warmer, more humid site studied in 2012, another site showed no evidence that the decline of megafauna triggered a change in vegetation or increased fire.

The conclusion that the magnitude of ecological responses to Pleistocene megafaunal extinction varied geographically per regional differences in climate seems an immediately evident, but is well demonstrated here, with a proposed hypothesis that the lack of change was because climatic constraints of cold and low atmospheric CO2 placing severe limits on plant growth, prevented the vegetation from responding to relaxation of herbivory in a substantially measurable fashion. Furthermore, it crucially demonstrates the complexity of the problem of untangling the effects of megafaunal loss, just as with the complexity of untangling the causes of megafaunal loss (although these Sporormiella based studies do seem quite conclusively to point towards a human perpetrator of Australian megafaunal collapse).

These are but a selection of case studies evincing the roles megafauna play in shaping their physical surroundings and the implications of their removal, such effects have been shown all around the world and will continue to be revealed in further detail and clarity, likely even unfolding before our eyes in cases where extant megafauna are under threat.


Ecosystem Physical Structure: Part 1

Ecosystem Physical Structure

Megafauna are capable of significantly shaping and altering landscape structure and functioning of ecosystems, consuming vegetation at higher rates than it can be replenished (as was alluded to as a mechanism for indirect megafaunal effects on ecosystem trophic structure in the past few posts – a la Elk in Yellowstone) and/or damaging/destroying it. Their impact on ecosystem physical structure is so significant that loss of megafauna is thought to have been responsible for past regime shifts in the world’s biomes – further loss could have equally drastic consequences in the future.

Woody plant abundance:

Megafauna can have substantial impacts on woody plant abundance, dependent on body size (greater body size ~ greater ability to consume destroy vegetation), feeding mode (browsers (eg Elk in YNP) directly inhibit abundance via consumption v grazers (eg Elephants) only occasionally feed on woody plants but suppress species abundance through trampling and breakage) and environmental factors controlling the abundance and ability of megafauna to access the plants.

A striking example of megafaunal dominion over woody plant abundance is outlined in a recent study from Bakker et al 2015, comparing Late-Quaternary megafaunal extinctions to analogues in modern exclosure experiments (where megafauna are artificially kept out of areas to simulate their removal from environments). African elephants’ (great in stature and strength ~ great in effect) propensity to pull out / push over shrubs and trees was found to account for >80% woody plant loss – exclusion of elephants resulted in an area with 42% more trees (as strikingly outlined in the following figure – infrared colour indicating higher concentrations of woody vegetation inside the exclosure, intensity of red indicating higher primary productivity)

Species composition effects:

Of course, megafaunal effects on plant abundance are much more nuanced than just equal destruction of all plant species in a given area. Whilst overall trends of more browsing-tolerant shrubs (more palatable plants being eaten in their stead) and more light-demanding woody species (vegetation openness created) are generally found, more complex diverse mosaics of spatially heterogeneous landscapes result from various interactive factors.

Woody plants can persist by defending themselves from browsing/grazing or associating with defended species – this often leads to a cyclic succession: woody plants survive by associating with poisonous / thorny shrubs, which they eventually outgrow and outcompete -> herbivory then cuts them back -> spatial and temporal distributions of this cycle results in a variety of grasslands, shrubs, and different size clumps of trees.

Plants persist by growing in inaccessible areas / areas with high predator activity, resulting in heterogeneously distributed ‘landscapes of fear’ avoided by megaherbivores, thus creating spatial variability in herbivory pressure and allowing local increases in woody plant abundance (eg back to the wolves’ effects on Elk in YNP)

Heterogeneity of landscapes also results from variation of behaviour within megafauna. For example, adult megaherbivores of sufficient stature become functionally immune to predation pressure and consequently frequent woody areas for greater forage while younger individuals, not yet large enough to avoid predation pressure, stay in grasslands where ambush is less likely.

Brief Conclusions

The result of these and other interacting factors are heterogeneous, diverse and robust ecosystems, with positive implications for the environment and for environmental services. 

Stay tuned next week for some interesting case studies in Africa, Australia and North America outlining more detail and nuance!