Trends in Ecology & Evolution
ReviewThe Net Effect of Functional Traits on Fitness
Section snippets
Demographic Trade-offs and Population Fitness
The alluring prospect that functional traits (see Glossary) can explain variation in species performance has invigorated comparative functional ecology, yet identifying the traits that determine fitness remains an important empirical challenge [1., 2., 3., 4., 5., 6.]. Inspired by classic evolutionary theory that linked morphology to performance and fitness [7], ecologists have recently intensified their search for relationships between functional traits and vital rates, but have avoided the
Vital Rates Can Be Misleading Proxies for Fitness
Individual vital rates can be misleading proxies for fitness without considering demographic trade-offs. For example, species with fast individual growth rates may exhibit low rates of survival. If the growth–survival trade-off can generate covariation in individual growth rates and survival rates that yield equal fitness, all else being equal (e.g., equal reproduction rates), then individual growth rates tell us little about fitness [32,33] (Figure 1). Similarly, populations with high survival
Quantify Intrinsic Growth Rates
Theory predicts that species are sorted along environmental gradients because species only occur in sites (i) to which they can disperse, (ii) where their traits are adapted to the local conditions, and (iii) where they maintain competitive advantage in multispecies communities [26]. We are focused on the second step in this review, and so our emphasis is on the difficult task of quantifying the fundamental niche. Many statistical approaches test if traits predict species occurrences and
Identify the Traits That Drive Intrinsic Growth Rates
The next step is to model fitness as a function of trait-by-environment interactions [12,55,91]. This tests the dynamic adaptive landscape model to determine how the effects of traits on population fitness across species depends on the environment [92]. The question is not whether population fitness among species varies along environmental gradients; this has been known for centuries (Figure 3A). The question is whether traits explain variation in population fitness (or occurrence) among
Concluding Remarks
Predicting fates of populations and communities using traits has often been called the ‘holy grail’ of ecology [1,102], yet we often lack clear evidence that functional traits live up to the hype [24]. To advance this important research agenda, we encourage studies that link traits directly to intrinsic growth rates to test the generality of traits for predicting species performance. The complexity of population dynamics may have hindered an earlier integration of population demography into
Acknowledgments
This work was funded by a grant to the authors from the National Science Foundation (DEB-1906243).
Glossary
- Community assembly
- process by which species arrive, establish, persist, increase, or decrease in abundance over time, and go extinct within and across environmental gradients.
- Components of fitness
- measures of individual performance including survival, growth, and reproduction; also referred to as vital rates. The integration of fitness components yields an estimate of total fitness and integration of vital rates yields an estimate of population growth rate.
- Demographic trade-offs
- negative
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