One of best known patterns in ecology is the latitudinal gradient in biodiversity. Near the poles there tend to be fewer species than in the tropics. Here are two examples from my own travels.
First, a moist lowland forest on the Pacific slope of Costa Rica (9°N latitude). In a hectare of forest you can easily find one or two hundred species.
Second, a montane forest in central Norway (63°N latitude). Here a hectare of forest may only have one or two species.
So why this striking difference? This pattern underlies so much of the earth’s biodiversity, and has rightly fascinated ecologists for a century. But satisfying explanations have been lacking. Part of the trouble is that there are too many possible explanations, because most of the options (for example, warmer temperatures, available energy, longer time for evolution) correlate strongly with each other. This makes it hard to falsify any given hypothesis.
I recently had a paper come out that tries to find a way forward. It was just published in PNAS, co-authored with Christine Lamanna, Cyrille Violle, and many other scientists from the BIEN eco-informatics initiative. In the paper, we argue that focusing on species diversity is less useful than focusing on functional diversity. Rather than counting numbers of species, we should be counting the number of ecological strategies available to species. We argue that this shift is useful, because many theories of biodiversities can be naturally expressed in terms of functional variation. So we recast several major theories in terms of functional traits, which are properties of species that reflect their ecological strategies. Then we collected or compiled data for hundreds of forests throughout the New World – surveys of species and of traits. Here are a few photographs from some fieldwork at a site in Costa Rica.
In the end, what we showed is that none of the major biodiversity theories make predictions that are consistent with observed data. This is a big challenge to our current understandings of the pattern. The biggest issue seems to be understanding why functional diversity peaks where it does. You might imagine that tropical regions can support more species because there are more available strategies – whereas closer to the poles, environments are so extreme that there are fewer viable strategies available to plants. It turns out this isn’t true! We find that temperate regions support more strategies that tropical regions, even though tropical regions have more species. In the below movie, you can see the three-dimensional overlap between temperate and tropical functional traits (made with the hypervolume R package). The temperate region is larger – and you can confirm this quantitatively in Figure 3 of the paper).
We don’t understand why the world works this way yet – but it’s hard to argue with data. I hope explaining this diversity mismatch will help us get just a bit closer to understanding the fundamental and elusive latitudinal diversity gradient.