Calvaria major and the dodo hypothesis

A once-abundant tree is now rare, and a bird that lived on the same island has disappeared. Could the loss of the dodo explain the decline of Calvaria major? That is Stanley Temple's hypothesis. The connection sounds possible, but we need to know how the bird could affect the tree's ability to reproduce.

Temple found only thirteen trees, all old and dying. Foresters estimated their ages at more than 300 years. That timing seemed to line up with the dodo's extinction. The trees still produced apparently fertile fruit, yet Temple found no younger trees. He inferred that the seeds had stopped germinating. Notice the leap: failing to find young trees is evidence to investigate, not proof that no young trees exist.

The second paragraph supplies his proposed mechanism. Dodos ate the fruits, so the pits evolved thick walls that could survive the abrasive forces in the birds' digestive systems. But the protective wall supposedly became so thick that a seed could not germinate unless passage through a dodo wore it down. The same feature that once protected the seed would trap it after the bird disappeared. That explains how losing a bird could threaten a tree, rather than merely noting that two events happened around the same time.

Temple then tries to support the mechanism. He estimates the forces in a dodo's gizzard using studies of other birds and tests how much crushing Calvaria major pits can withstand. He also feeds pits to turkeys. Many are destroyed, but ten come through abraded and intact, and three of those sprout.

That gives his idea some appeal. A bird can wear down a pit without always destroying it, and some treated seeds can grow. But showing that this route can work is different from showing that it is the only route. The author's phrase “a semblance of rigor” signals that the results create an appearance of scientific support without settling the explanation.

The final paragraph supplies the challenge. Wendy Strahm finds hundreds of trees, including many younger than three centuries. Those younger trees matter more than the larger count alone: they show that reproduction continued after the dodo disappeared. Temple's assumption that no new trees were growing was wrong.

Anthony Speke's research attacks the proposed dependence even more directly. Some unabraded seeds germinate. They are only a minority, but they do not need to be a majority to keep the species going. Think of an animal that produces many offspring even though relatively few survive to adulthood. What matters for continued survival is whether enough succeed, not whether every attempt succeeds. The same point applies to seeds, and Speke thinks the number that germinate is probably sufficient to prevent extinction.

This does not show that the dodo could never have helped germination, or that thick pit walls have no effect. It shows that dodos and abrasion are not necessary for all reproduction. The author also offers other possible reasons for the genuine population decline, including disease and damage from introduced animals. Those are possibilities, not a newly proved single cause.

So our main-point prediction should reflect more than a neutral report of an interesting idea. Temple supplied an appealing causal account, but the later evidence undercuts its central assumptions. The tree really has declined; the supposed complete loss of reproduction has not occurred, and the dodo's disappearance is probably not the explanation for the scarcity.