As in a crime novel in which detectives must solve a mystery and find a culprit, in natural sciences we scientists also look for clues, present and past, that help us understand what is observed (and what is not) in a ecosystem. One of the great ecological mysteries today is the presence of plant species that produce very large fruits in places where there are no longer large animals that consume them and can disperse their seeds. For plants, seed dispersal is a fundamental process that allows species to survive over time and colonize new places.
The fruits that are attractive to animals are edible and have highly nutritious fleshy tissues. They are usually of a size proportional to the main animals that consume them. For this reason, plants with large fruits and seeds are dispersed by large animals (megafauna), since they are the only ones capable of swallowing the fruits.
In some current ecosystems, even in the presence of plants with fruits apparently adapted for consumption by large animals, it is not possible to find large modern native fauna that consumes them and disperses their seeds. These megafaunic fruits, observed decades ago in Central American ecosystems, are considered an anachronism, and their presence is attributed to the disappearance of large beasts about 10,000 years ago, towards the end of the last ice age, at the end of the Pleistocene.
Many species of plants bear megafaunic fruits in South America. In a recent study, we looked at an ecological scene where an endangered tree that only grows in a small geographic area of the coastal zone in south-central Chile participates.
The fruit of this tree, called queule (Gomortega keule), it is edible and large (20 to 40 grams). It has a seed protected by a very hard woody cover. At the fruiting season, in autumn (April and May in their area of origin) the fruits fall to the ground and there they rot without native animals consuming them in significant quantities, and even less dispersing the seeds. However, in Chile there is fossil evidence of the occurrence of megafauna in the Pleistocene, such as gonfoteria, equidae and cervids.
Unfortunately, it seems almost impossible to find a fossil stomach of these animals with queule seeds inside. We must then look for other evidence that points to the megafaunic character of the queule fruit.
An important observation corresponds to the consumption of queule fruits by large modern animals. As part of our study, mature queule fruits were placed in animal cages at a zoo and also for domestic animals on local farms. Some animals did not go near the fruit, others ate the pulp but discarded the pit, and some consumed the whole fruit.
This evidence makes it possible to ensure that the queule fruits are attractive to large animals and that at least some of these animals swallow the seed and therefore can transport it. But it is also relevant to know if the seed maintains its ability to germinate after passing through the mouth or digestive tract of the animal. For this we carry out germination experiments with the recovered pits, where we observe germination in all cases.
Another important observation, now in the natural environment of the tree, was the presence of cheule pits in manure from pigs and cows. In some areas where the species persists, local inhabitants indicate that cattle feed on the fruits of the queule, which confirms this observation and supports the megafaunic character of the fruit.
However, since there are no seedlings of queule in areas with livestock, these domestic animals are not currently performing the seed dispersal process effectively for this species. Among native animals, where there is very little information, only a small deer has been seen nibbling the fruits, but due to its small body size (less than 10 kilograms), it is not likely to swallow the seed.
Should we help the queule?
Although it seems quite clear that the megafaunic fruit of queule represents an anachronism, there are still many questions that future research must address in order to effectively advance the conservation of this species of tree. The poor survival of its seedlings, the possible role of seed dispersal by animals such as rodents and livestock, and especially the multiple effects of the alteration that agriculture and forestry have produced on the original forest are some of the questions that must be answered.
Intervention with pigs or horses, which could disperse queule seeds, may seem attractive, but the complexity of the system makes it difficult to foresee the negative effects that in the case of pigs have already been observed in other Neotropical ecosystems.
Before considering the introduction of megafauna to reestablish important ecological processes (rewilding) such as seed dispersal, recent experiences must be considered very worrying due to their social and ecological consequences.
The case of queule can also awaken ethical and philosophical reflections, since it is a species with seed dispersal problems, very possibly from times before the planetary changes that we are experiencing today, a situation shared by several other species of plants.