In the Blink of a Dove’s Eye, a Window into an Age-old Theory
Tropical trees often flower during the peak of the dry season. And this time, Planet’s Doves caught a peek.
Over the last few months, Planet Doves captured images of tropical forests that had me grinning. In the Western Amazon, in Colombia, the dry season has been underway, so Planet has had a better shot than normal of seeing through the clouds. While tropical forests ordinarily look like a kind of carpet of broccoli, in January and February, we captured stunning images in which we could distinguish different species of trees. How could we tell them apart? We saw their flowers — first pink, then yellow.
Flowering events like this are common among tropical trees, but they often last only a few days. Planet’s constellation is uniquely equipped to capture a whole series over a given season, and this opens up unique possibilities into conservation, and also a fundamental theory in ecology.
For the former, mapping flowering events would allow the ranges and habitats of trees — and many of the animals that coexist with them — to be understood at larger scale. This would enhance decisions on conservation and land-use in the tropics, where more than half of the world’s species reside.
On the theoretical side, the increasing observations across space and time are a game changer when it comes to Niche Theory — a long-standing body of ecology that explains the distribution of species. For flora and fauna, niche is the place to which they belong. A coconut tree is right at home on a white sand beach with pounding surf and blinding sun. A sloth fits clandestinely in the canopy of a tropical forest, clutching branches with her perfectly sculpted claws. Or — ironically — behind the desk of the DMV in Disney’s Zootopia.
But in the 1950s and 60s, ecologists began to question how so many species could co-exist in places like tropical forests or coral reefs. There didn’t seem to be as many different niches as their were species. Tropical tree species were particularly vexing: in parts of the Amazon basin, an area the size of a soccer field may have several hundred different tree species sharing more or less the same niche. G. Evelyn Hutchinson, one of the giants of ecology at the time, asked quite mater-of-factly in the subtitle of a seminal paper: “Why are there so many kinds of animals?”
And so ecologists set out testing various theories to explain the rich diversity. For tropical trees, one possibility was that even though soil and light conditions looked largely the same, there might actually be enough heterogeneity to maintain as many niches as there were trees. The niches were there, just hard to resolve.
Another option was that animals played an outsized role. Ecologists Joseph Connell and Dan Janzen independently theorized that the animals and pathogens that put pressure on a tree’s ability to reproduce — by eating (or infesting) their seeds and fruits — deter any one species from becoming too abundant. With this pressure to maintain a smaller and more evenly spaced population, many more trees could co-exist.
Most controversially, physicist-turned-ecologist Stephen Hubbell proposed a unique solution. Neutral theory, a counter to the niche concept entirely, suggests that biodiversity may arise at random — that in incredibly diverse tropical forests, the traits of each tree species are not relevant to their success. Hubbell worked to test his theory in a unique way: he established large plots — some bigger than 50 soccer fields combined. Inside he and colleagues monitored the welfare of every tree with a trunk bigger than your pinky finger. In the longest running plot, Hubbell and colleagues have tracked these trees for more than 30 years.
The verdict? It’s a little of both. There is strong empirical evidence for Connell and Janzen’s explanation (especially where pathogens are concerned) — but Hubbell’s theory also has support. Many (even Hubbell) believe that there are probably certain conditions where Neutral Theory operates, and others were it does not. The same for classic Niche Theory. One of the biggest outstanding questions is scale: suppose the experiments and plots so far are too small to get the full picture? Maybe it’s niche processes and pathogen pressures at small scales, but neutral processes at larger scales.
Planet’s ability to monitor vast regions of forest with high frequency may help us learn where those regions are, and how biodiversity is structured differently within them. The pink flowing event that Planet observed in January spanned at least some 2,000 square kilometers, straddling floodplains and mountain ranges. This area is four orders of magnitude larger than Hubbell’s largest monitoring plot. Monitoring the behavior of tree populations over such vast scales has never been accomplished. That’s a potential game changer.
