Megafauna extinction shortened dispersal distances of large seeds

Research warns of possible jeopardy for forest regeneration and plant species balance if large mammals existing today become extinct.

By Peter Moon  |  Agência FAPESP – Mastodons, giant sloths and armadillos the size of a VW Beetle. Megafauna extinction at the end of the last ice age was a biological tragedy whose aftermath is still seen in the ecology of South America over 10,000 years later. 

The absence of giant mammals in the continent’s ecosystems can be felt in the dispersal dynamics of the largest seeds, such as the pequi or pekea (Caryocar brasiliense), for example. Few frugivores in existence today are able to swallow such large seeds and carry them in their gut to disperse them in the environment. However, for the giant sloth or the gomphothere, an elephant-like proboscidean, this was no trouble at all. 

The largest seeds are not the only ones to have lost their means of transportation. Megafauna extinction also reduced the distances over which seeds are dispersed by the largest mammals alive today, such as the tapir. 

A new study has calculated the distances covered by giant sloths (Megatheriidae) or mastodons (Mammutidae) with seeds in their digestive tract before excreting them into the environment. “We managed to give numbers to the verbal arguments about the importance of these large animals,” said Mathias Mistretta Pires, a scientist at the University of Campinas’s Biology Institute (IB-UNICAMP) in São Paulo State, Brazil. Pires is first author of the study quantifying megafauna seed dispersal distances.

The other authors include Mauro Galetti, a professor at São Paulo State University (UNESP) in Rio Claro, and Paulo Roberto Guimarães, affiliated with the University of São Paulo’s Bioscience Institute (IB-USP). Published in the journal Ecography, the study is part of the Thematic Project “Ecological consequences of defaunation in the Atlantic Rainforest”, for which Galetti is principal investigator.

The largest frugivores that inhabit South America are the tapir (Tapirus spp.), guanaco (Lama guanicoe), alpaca (Vicugna pacos), collared peccary (Pecari tajacu) and red brocket (Mazama americana). However, even the largest of these – the tapir, weighing approximately 200 kg – is only about one-tenth the size of a giant sloth and one-thirtieth the size of a gomphothere.

The seed dispersal distances observed for the largest living frugivores rarely exceed 3.5 km. The study concluded that megafauna seed dispersal distances were far greater, possibly exceeding 6 km in the case of sloths and gomphotheres.

“Our goal was to create a model that could quantify the role of these extinct animals in seed dispersal,” Pires said. “We built a mathematical model to simulate the various stages of the seed dispersal process and generate quantitative predictions of how this dispersal service functioned in the past.”

To estimate megafauna seed dispersal capacity, they first established three basic datasets for the largest seed disperser species in terms of food consumption, food retention time in the gut, and distance covered to elimination.

“These three attributes correlate with body mass. We have the data for elephants, tapirs, brocket deer and peccaries,” Pires said. A tapir can retain food in its gut for more than 30 hours before excreting. “In the case of elephants, food retention in the gut lasts over 40 hours. In other species, it can exceed 50 hours or even 100 hours.”

The next step was to extrapolate estimates of each of the three attributes (food consumption, retention time, and distance covered) for some of the extinct megafauna species that inhabited South America during the late Pleistocene, the period that began approximately 2.6 million years ago and lasted until approximately 11,700 years ago.

The dataset used for extrapolation referred to estimated body mass. In the case of gomphotheres, for example, the estimate was 5-6 metric tons (mt) depending on the species, while the largest sloths weighed between 3.5 mt in the case of Eremotherium and over 6 mt in the case of Megatherium.

“We deduced the amount of food a ground sloth would have eaten, the time it was retained in the gut, and the distance traveled by the animal,” Pires said.

The three attributes were also estimated for gomphotheres, Paleolama (a genus of giant llamas), large ungulate quadrupeds called Macrauchenia, and Cervidae. They selected only folivores and frugivores. Grazers that fed mainly on grass, such as the sturdy Toxodon, were not included in the study. 

As a result of the simulations, the seed dispersal range for gomphotheres was estimated at between 500 m and 3.5 km, stretching to over 6 km in 5% of the simulations.

In the case of ground sloths, it was 300 m-2.5 km, again reaching 6 km in some simulations, while in the case of tapirs, it was 200 m-1.4 km.

“Today, a peccary carries food for about 800 m on average, according to our simulations,” Pires said. “Seed dispersal potential has shrunk a lot. Distances now considered long were relatively short in the past. The largest megafauna not only retained food for 60% longer but also dispersed seeds over much larger distances. Seed dispersal distances very rarely exceed 1 km today.”

Genetic diversity

In this study, the researchers compared the values generated by the simulations to the known data for animals in the Pantanal, a richly biodiverse and relatively well-preserved ecosystem in Mato Grosso State. “We found that seed dispersal capacity had fallen to a third of what it was in the Pleistocene,” Pires said.

The decrease in seed dispersal distances in the past 10,000 years has various consequences for the formation and diversity of plant cover as well as for the genetic diversity of animal species. Megafauna seed dispersal distances were large enough to keep plant species distributed over a wide area.

Without those dispersers, plant populations no longer exchange genetic material, and separation between individuals of the same species results in low genetic variability, potentially reducing the probability of their long-term survival.

Extinction of the giant frugivores reduced the likelihood of dispersal of plant species with large seeds, like the avocado. As a result, the seeds that fall from such plants are less likely to germinate and grow. If they cannot be swallowed and transported intact, the seeds stay on the ground at the mercy of seed predators such as rodents, which chew them and kill the embryo. At the same time, seeds that remain on the ground for a long time are less likely to prosper there because young plants compete for sunlight, water and soil nutrients with the mother plant.

“The plant species that lost most from the megafauna extinction were once used intensely by Paleoindians but aren’t used so much now,” Galetti said.

Is there a way to increase seed dispersal distances in South America’s biomes today? “One proposal might be to reintroduce tapirs, large primates and other frugivores into defaunated areas,” Galetti said. “The problem is that the cause of the extinction hasn’t been resolved. Many areas still have strong hunting pressure.”

He added that while the loss of megafauna reduced seed dispersal possibilities, introduction of the human factor served as a counterbalance in some cases.

“The plants dispersed by megafauna were ‘lucky’ to have other seed dispersers, such as humans, agoutis, and water in the case of the Pantanal. But with the simplification of ecosystems, many more plants became orphans,” he said.

“While on the one hand, we lost those large dispersers, on the other hand, invasive species like feral pigs and pig-boar hybrids are dispersing some of these megaseeds. We don’t know whether these invasive species will play the same role as tapirs and monkeys, but at least for some fruit species, we think they will.”

According to Galetti, who specializes in studying defaunation – the loss of animal species from modern ecosystems – the disappearance of the megafauna caused greater losses on South America than any other continent in terms of seed dispersal.

“North America lost several mammals and some fruit species that were dispersed by megafauna, but it can’t be compared with South America, where the diversity of the savanna-like Cerrado and tropical forest biomes is very high,” he said. “That isn’t the case in African savannas, where plant diversity is low and most megafauna fruit species are in the forests of Gabon. On the other hand, extinction of the elephants that live in African forests will lead to the same pattern of seed dispersal loss we propose in our article.”

The extinct megafauna were fundamental for forest regeneration and maintaining a balance among the various plant species. “The big animals that performed this dispersal service have been lost. Our study is the first to quantify that loss,” Pires said.

“We now have mathematical models and computer programs that enable us to estimate what the seed dispersal process was like.”

The article “Pleistocene megafaunal extinctions and the functional loss of long-distance seed-dispersal services” (doi: 10.1111/ecog.03163) by Mathias M. Pires, Paulo R. Guimarães, Mauro Galetti and Pedro Jordano can be read at: onlinelibrary.wiley.com/doi/10.1111/ecog.03163/full.