Cycads Evolved To Grow In Groves With Seed Dispersal By Large Frugivores
April Flowers for redOrbit.com – Your Universe Online
Before the age of dinosaurs, the ancient cycad lineage existed. Cycads also co-existed more recently with large herbivorous mammals, such as the Ice Age megafauna that only went extinct a few tens of thousands of years ago. Modern cycads have large, heavy seeds with a fleshy outer coat, suggesting they rely on large bodied fruit-eating animals to disperse their seeds. However, little evidence has been found that modern larger-bodied animals like emus or elephants are eating and dispersing the seeds.
Researchers John Hall and Gimme Walter of the University of Queensland, Australia, questioned how these plants could still be around today if they are adapted for dispersal by a set of animals that has been missing from Earth’s fauna for tens of thousands of years. Their findings, published in the American Journal of Botany, propose the clumped dispersal mechanism these ancient plants most likely relied upon still serves them well today.
Cycad fossils have been recorded from around 280 million years ago – approximately the time the coniferous forests first arose. The ecological distribution pattern suggests the seed dispersal of many living cycads today is limited and ineffectual. Macrozamia miquelii, for example, is a cycad endemic to Australia that is found in highly clumped, dense, numbers, where it dominates the understory. Inexplicably, large areas of seemingly suitable habitat separate colonies from one another. Researchers say these patterns suggest few to none of the seeds are being dispersed at any distance from the parent plants, one of the long-standing tenets of the advantages of seed dispersal.
The researchers wanted to determine whether the seed dispersal and seedling distribution pattern of M. miquelii might indicate it is maladapted to its current dispersers, so they proposed a new twist on the functional significance of the megafaunal dispersal syndrome.
“Naturalists are very comfortable with the idea of animals gaining a biological advantage by choosing to live together in high density ‘colonies’—such as ant nests or seabird rookeries—in certain parts of the landscape,” notes Hall. “But when it comes to plants, there is a bit of a subconscious assumption that the purpose of seed dispersal is to simply spread seeds as far and as evenly as possible across the broadest possible area.”
The researchers investigated whether cycads might be a type of plant that forms such colonies. “The main idea behind our research,” Hall clarifies, “is to ask the question: when it comes to the spatial ecology of plants, could it be useful to think of some plant species as also forming and maintaining ‘colonies’ or ‘groves’ in the wider landscape?
“Australian cycads once co-existed with megafauna that could have dispersed their large, heavy seeds—such as giant ground birds, bigger then present day emus, and Diprotodon, a rhino sized marsupial quadruped,” explains Hall. “The large, heavy and poisonous seeds, surrounded by a fleshy and non-toxic fruit-like layer, seem well adapted to being occasionally swallowed whole en masse by megafauna, which would then pass the many seeds simultaneously at a new location: the genesis of a new grove.”
One or two cones with multiple large seeds are produced by female cycads. Each seed is covered with a thin outer fleshy sarcotesta. The researchers tagged ten large seeds from the single cone of 12 plants with a small steel bolt to track how many of the seeds were removed from the parent plant, and how far they would be dispersed.
Within three months of tagging, nearly all the seeds had their sarcotesta eaten – mostly by brushtailed opposums, which scrape the flesh off and discard the large seeds. The disperser’s identity was confirmed with camera traps at two fruiting females and hair traps baited with seeds. However, the researchers found 97 percent of the tagged seeds had moved less than three feet from their parent plant. Only a few were moved farther out, and in all cases, the seeds were found less than 15 feet from the parent.
The team also found although most of the seeds ended up under the parent cycad, there were almost no seedlings within a five foot radius of the adult cycads. This suggests most seeds close to their parent plant perish.
The researchers say despite the large seed size, the primary dispersers for cycads today are smaller bodied animals. Such animals do not spread the seeds far and wide, nor take them to new habitats that are colonizable. Despite this, the plants seem to be flourishing by sprouting up near the adults and forming mono-dominant stands.
“Since their potential Australian prehistoric megafaunal dispersers became extinct around 45,000 years ago, why haven’t Australian cycads begun to evolve smaller seeds that would be more readily dispersed by flying birds or possums for example, over the interim?” posits Hall.
“We argue that the answer to this question is that cycads are actually disadvantaged by dispersing as lone individuals that may travel long distances, but in so doing so, become isolated from others of their kind,” Hall states.
Hall adds cycad plants are all born either male or female. They rely on host specific insect pollinators, meaning a cycad dispersed alone, a long way from others of its kind, would probably be at a reproductive disadvantage.
If, as scientists believe, cycads evolved to be dispersed by large-bodied frugivores, it is most likely that these animals deposited many cycad seeds in their dung at once. This led to the plants adapted to grow in groves, which plays to their favor today despite the loss of these megafauna dispersal animals.
“There’s no doubt that cycad ancestors were contemporary with herbivorous dinosaurs for many hundreds of millions of years, so it’s plausible that cycad seed dispersal ecology and “colony forming” behavior may be extremely ancient, and echo the ecology of dinosaur-plant interaction” he concludes, “but of course we now enter into the realm of speculation.”
Hall’s research into the spatial ecology of “colony” forming plants continues beyond cycads. He plans to explore these issues in other plants and landscapes, especially in forest understories.