Tradeoffs, correlated traits, and functional diversity

This post was originally published on BioDiverse Perspectives – a research blog aimed at fostering communication about biodiversity.

Tradeoffs are everywhere, and never has this been more apparent to me, than in graduate school. With only so many hours in a day, and an ever-growing to-do list, how I choose to allocate my time becomes an increasingly important decision. Do I work in the greenhouse, resample my field experiment, analyze data, catch up on reading…? Every day, when I wake up, I’m faced with these decisions.  And every day, these decisions are constrained by the same two major factors: What can I do? e.g., how much time do I have in the day; and what have I already done?

Well, I’m glad to say that I know I’m not alone. In fact, most organisms face nearly the exact same constraints. And these two constraints don’t just occur within a single organism’s lifetime. They can often be reflected over evolutionary time, too. In this case, “what can I do?” becomes a question of physiological constraints on phenotypes (some combinations of traits are physiologically impossible), and “what have I done?” becomes a question of evolutionary history, or natural selection (some combinations of traits would confer fitness disadvantages).

As studies of diversity move from descriptions of species numbers (e.g., taxonomic diversity) to descriptions of species physiology (e.g., functional diversity) or evolutionary history (e.g., phylogenetic diversity), it’s becoming more important that we pay attention to the tradeoffs that underlie those patterns. Although not the first to suggest this, Wright et. al’s description of the worldwide leaf economics spectrum demonstrated the universality of tradeoffs between natural selection and physiological constraints, and argued that plants tend to fall on a continuum between two fundamental strategies for dealing with these tradeoffs, termed the slow- to quick-return continuum.

This spectrum runs from species with potential for quick returns on investments of nutrients and dry mass in leaves to species with a slower potential rate of return. At the quick-return end are species with high leaf nutrient concentrations, high rates of photosynthesis and respiration, short leaf lifetimes and low dry-mass investment per leaf area. At the slow-return end are species with long leaf lifetimes, expensive high-LMA leaf construction, low nutrient concentrations, and low rates of photosynthesis and respiration.

For example, imagine you are a small grass seedling, your goal in life is reproduction, and you only have a finite number of resources to allocate in order to reach that goal. Do you allocate them to growth and additional resource acquisition, or do you conserve the resources that you have so that you can allocate them all to reproduction? What Wright et al. suggest is that you are fundamentally asking, “Should I be a quick-return plant or a slow return plant?” Physiologically, it is not possible to exhibit both high rates of photosynthesis (future resource acquisition) as well as low leaf nitrogen concentrations and long lived leaves (resource conservation). In addition, if you choose to allocate few resources to photosynthesis, jack up your leaf nitrogen concentrations, and rapidly shed leaves, you are unlikely to successfully reproduce. As a result, your phenotype is constrained physiologically and evolutionarily, and Wright et al. argue that this tradeoff is fundamental across all plants and results in a predictable spectrum of phenotypes.

I was introduced to this paper very early in my graduate career, long before I had a flicker of an inkling of a speck of a thought about the multiple dimensions of biodiversity. But when I start to think about patterns of functional diversity through the lens of the leaf economics spectrum, I start to wonder. If groups of physiological traits covary and are constrained by physiological processes and evolutionary history, how might this influence the inferences that we draw from patterns of functional diversity? When measuring functional diversity, how important are the specific traits that we consider? And if groups of physiological traits are strongly correlated along a phylogeny, how useful are measures of phylogenetic diversity at inferring patterns of functional diversity?