IT’S NOT JUST ABOUT POLLINATORS

Eurasian blackcap eating a rowan berry (Photo: Joan Egert)BY RICHARD CONNIFF

At dinner last night, I had an avocado and jalapeño salad.  (Great salad. Details in the comment section.) I was aware of my debt to nature: The avocado tree needs pollinators to fertilize its flowers. So do the jalapeños. Almost all flowering plants, including three-quarters of our agricultural crops, depend on pollination to produce seed and set fruit. It’s essential to their continued survival, and ours. But pollinators are only part of the story.

Half of all plants also need animals for seed dispersal—that is, to eat their fruits and unwittingly disperse the seeds in their wastes. For avocados, in their original range in the Central American highlands, those animals were multi-ton ground sloths and elephant-like gomphotheres, neither well suited for gathering fruit from treetops. 

Happily, avocado trees had evolved fruits that could sustain a fall from 40 feet up, and then ripen on the ground to attract foragers. The ground sloths and gomphotheres gorged themselves, and wandered off, passing along the big, indigestible, almost un-compostable avocado pits in their scattered dung heaps. Rodents, mainly agoutis, scavenged these pits to hide in their underground stashes.

The animals got the obvious benefit of eating delicious avocados (or delicious pits), and the avocado benefited because a few buried and forgotten pits survived to produce new avocado trees at a comfortable distance from the parent.  Humans only moved in as substitute seed dispersers for avocados about 5000 years ago, after we had pushed the ground sloths and gomphotheres to extinction. 

Seed dispersal has lately become a big topic in conservation circles because it’s not just the ground sloths and gomphotheres that have disappeared. Other animal populations, including birds, mammals, and insects, have experienced catastrophic declines, especially over the past 50 years.