How Form Defines Function
Macroevolutionary convergence connects morphological form to ecological function in birds (2020) Pigot et al, Nature Ecology & Evolution, https://doi.org/10.1038/s41559-019-1070-4
There are an astounding amount of different forms that the animals on our planet take. Likewise, there are a multitude of diverse functions that animals serve in the environment, such as that of a herbivore, a predator, or scavenger. In some cases it’s a clear link between the form of a given animal and its function in the environment, like that of the beak of a hummingbird that allows it to feed on nectar and their role as a pollinator. But whether or not there is a reliable way to predict the function of an animal based off of its form is has been the subject of considerable controversy.
Deciding on how many morphological traits to use to predict ecological function is a difficult prospect. One could argue that it’s impossible to pick a finite number of traits, as there are infinite possible niches that organisms can fill so there’s no way that a set of traits could fill those infinite possible niches. Mapping animal form to function has major implications for quantifying and and conserving biodiversity, and the authors of today’s paper wanted to to determine just how many traits are needed to do that.
What They Did
Matching morphological form to ecological function requires a large amount of data on a large amount of animals. To accomplish this the authors used birds, a group of animals that has been extensively studied for understanding niches and the link between ecological function and morphological forms. Specifically, the authors used nine morphological measurements that can be linked to the locomotion, trophic position, and associated niche of a given bird. To generalize their methods, measurements of the beak provided information on what and how the bird would eat, while measurements of the wings, tails, heads, and legs provide information on how the bird moves through the environment and forages.
The authors wanted to match all of these morphological measurements (form) to the role that a given bird served in nature (function). Because of this, all of the birds measured for the study (almost 10,000 individual birds!!!) were assigned a trophic position based off their diet. These categories included broad classifications (herbivore), but they also provided nuance (terrestrial vs. aquatic herbivore).
Did You Know: Niches
A niche (you may pronounce it “nitch”, others say “neesh”, I tend to switch between the two) is a concept widely used in ecology to describe the space that a given organism occupies in an environment. This can refer to the physical space that it occupies, such as birds nesting in trees vs. in the ground. But a niche can also be temporal, meaning that the same physical space can be occupied at different times. Think an owl hunting at night, while a hawk would do so during the day.
Although they are a human construct, the concept of a niche is useful when trying to categorize the natural world, especially for management and conservation purposes. Some animals may be functionally redundant (i.e. two species serving the same role in the environment), but if you want to preserve a specific organism that serves to make vital connections between different parts of a food web, it helps to understand the niche it occupies such that you can preserve its entire niche and not just the organisms itself.
What They Found
Using analysis of principle components (a way to measure the two or more variables at the same time) the authors found that body size explained the vast majority of the variation among birds, with more subtle differences explained by beak shape. Meaning that body size alone was not enough to predict what function a bird served in nature.
Though body size alone was not enough, the average number of traits needed to predict the trophic position of a given bird was three. This means that not only are niches inherently multidimensional (i.e. you can’t predict an organisms niche with a single trait), but that they are also somewhat limited despite the incredible diversity of animal life.
This study represents the result of an absolute TON of work (measuring almost 10,000 birds is no simple task). However, the authors use of birds for their models can be somewhat problematic due to the fact that birds are a fairly unique group of organisms. Birds have very specialized structures that can be associated with their role in the environment, but it isn’t that easy to assign ecological roles to another group. Additionally, birds (broadly speaking) have the same body parts, you couldn’t make these types of broad assignments with other reptiles (yes, birds are reptiles). For example, it would be harder to assign a terrestrial turtle and terrestrial snake their ecological roles based off of their shell depth, because the snake doesn’t have one.
The predicted association between form and function is nothing new in biology, but doing it on such a large scale like the authors did in this study represents empirical evidence for how accurate it can be. Additionally, their determination of an average of three traits needed to predict the function and niche of an organism goes against the idea of an infinite amount of niches (called an “n-dimensional hypervolume”, sounds cool right?) This study showed that the amazing diversity of animal life (bird life, in this case) represents the many ways the evolution steered different animals towards a set amount of outcomes.
Adam Hasik an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.