Eco-Evo Evo-Eco: Are eco-evolutionary dynamics stronger in the tropics?

Biodiversity is higher in the tropics. Terrestrial productivity is higher in the tropics. The pace of life is faster in the tropics. Mountain passes are higher in the tropics. The tropics are just bigger, faster, and stronger. So what about eco-evolutionary dynamics? Are they stronger in the tropics? A recent trip to Panama provided the motivation to speculate on this possibility.

Coati

Early in the trip, I visited the famous research site of Barro Colorado Island (BCI), where I was able – with my family – to see howler monkeys and all sorts of other wonders. Back at the town of Gamboa a few days later, I was called on to give a lecture to the “tropical boot camp” class that included graduate students from the McGill-STRI NEO program, the STRI-Indiana IGERT program, and Arizona State University. I decided to give my boiler-plate talk outlining a conceptual framework for eco-evolutionary dynamics because I figured most of the students would be ecologists and it would perhaps be worthwhile to encourage them to include an evolutionary perspective into their ecological thinking.

A leaf cutter ant in the clutches of an ant lion (zoom in for a better view of the lion).

During the lecture, I set up several key questions facing the study of eco-evolutionary dynamics, one being the importance of evolution (e.g., changes in phenotypic traits) relative to other non-evolutionary ecological forces (e.g., precipitation, temperature, flooding) in shaping ecological dynamics at the population, community, and ecosystem levels. The standard work addressing this question at the population level is that comparing the effects of phenotypic traits (e.g., body mass) on the population growth rate of ungulates in Canada and Scotland in comparison to climate variables (e.g., rainfall, Pacific Decadal Oscillation). Remarkably, effects of the two causal forces (evolution versus ecology) are roughly the same in each study population, suggesting that phenotypic change is incredibly important to ecological dynamics. Halfway through explaining all of this, it began to strike me as silly to use an example from a temperature vertebrate while lecturing in a building situated on the borders of a verdant tropical rainforest. Why not use a tropical example – even if just for hypothetical illustration.

Lounging capybaras

For some reason, my mind hit on howler monkeys. What effect, I posed as an example, would evolutionary changes in the phenotypes of howler monkeys have on the productivity or diversity of the BCI forest relative to the amount of rainfall. I had no idea of the answer, of course, which got me to wondering. Would eco-evolutionary dynamics be stronger or weaker in the tropics? It seems like an opportune time for some speculation.

Millipede delight

Several properties might increase the strength of eco-evolutionary dynamics. (1) Faster rates of phenotypic change. Perhaps the tropics have faster rates owing to the more rapid pace of life, or perhaps not given their more stable environment. (2) When the species causing the ecological effects have large effects as individuals, such as in the case of keystone species. Perhaps the tropics have more of these (elephants!), or perhaps not given that many more species are present and so the effect of any single species (besides elephants) might be weaker. (3) When the species causing the ecological effects are very numerous (bacteria, viruses, and some insects and plants). The tropics likely have more such organisms given the overall greater productivity, or perhaps not given that so many species are present the effects of any one species – however numerous – might be swamped by all the other numerous species. (4) When feedbacks between ecology and evolution are stronger, such as when trait changes causes an ecological change that promotes (through selection) further changes in that trait. Perhaps the tropics have more feedbacks of this sort because the environment is not reset each year by winter and because so many cool mutualisms are present, or perhaps not because the system can be reset by dry and wet seasons and because mutualisms in temperate regions might have stronger effects given the relative paucity of other species. So would we expect eco-evolutionary dynamics to be stronger or weaker in the tropics than in temperature regions given that effects seem to point in both directions in each case?

Yummy dung

I suggest that evolutionary dynamics on the part of single species (i.e., effects of the evolution of a focal species on aggregate ecological variables) might be weaker in the tropics – simply because the countless other species dilute the effects of any one species. However, I also suggest that eco-evolutionary dynamics in aggregate (i.e., across all species) will be stronger in the tropics – because there are so many other species and interactions, because they have been for around longer, and because the environment is somewhat more stable. I also suggest that eco-evolutionary dynamics associated with phenotypic CHANGE might weaker in the tropics given that organisms have had more time to stabilize their adaptations and so might be less subject to contemporary phenotypic change. However, I also suggest that eco-evolutionary dynamics based on phenotypic STABILITY might be stronger in the tropics. By this I mean that the very stability seen in (some) tropical ecosystems is likely the result of continual ongoing evolutionary change. That is, so many interacting species are present that extinction and extirpation would be common were it not for constant, ongoing eco-evolutionary dynamics that maintain and improve adaptations and thereby stabilize population sizes.

Spooning is universal

Of course, this is all speculation provided in fun and on the fly but how can one not be motivated to think about the uniqueness of the tropics when watching your children feed leaf cutter ants to ant lions, go all warm and fuzzy over a howler monkey mom and baby spooning, marvel at a massive tarantula, try to get close to a capybara, watch dung beetles fight over a prime piece of monkey stink, sneak up on a group of foraging coatis, chase a praying mantis around and around a tree, and try to find toads that look like leaves.