Why we should embrace a broad, simplified definition of traits

Blog written by Manuela González-Suárez. Read the full article here.

If you are interested in nature, you probably pay attention to how organisms look, behave, and where they prefer to live. These characteristics have helped us classify life on Earth and are commonly described in nature and identification guides. Natural history has always paid attention to species’ traits, but more recently, traits have taken a central role in the wider ecological research. This is because we have realised that beyond taxonomic boundaries, traits can help us better understand how communities form, species interact, and biodiversity changes. In addition, because traits can be linked to ecosystem functions and services, they offer a tool to develop Nature Based Solutions more efficiently. Traits can help us answer questions such as: What tree features are best to maximize CO2 absorption? Or Which herbivore characteristics are most important to manage vegetation and reduce the impact of wildfires?

White rhinoceros (Ceratotherium simum) grazing at Hluhluwe iMfolozi Park in South Africa. They act as natural “lawn mowers” keeping grass short and reducing fire risk (photo credit Manuela González-Suárez)

We can all probably agree that trait and functional trait approaches have revolutionised ecology. But what the literature shows is that we all cannot agree on what we mean by traits or functional traits. The Cambridge dictionary defines a biological trait as “a characteristic of an organism that is passed from parent to child”. But many ecologists have argued for more refined definitions, limited to only some types of characteristics that describe morphology, physiology, or phenology, and that must be measurable at the individual level. And yet we are not, the way in which we understand traits when conducting and sharing research seem broader, but just how broad?

To answer this question, we decided to gather information in two ways. First, we asked researchers for their opinions using an anonymous survey (thanks if you were one of the 486 people who answered it!). Second, we reviewed the recent literature considering 712 papers that talked about traits and functional traits. And what we found was that those of us working with traits and functional traits do not necessarily mean the same thing when we use these terms. In fact, even the authors of this paper did not totally agree to start!

Researchers do agree about some things. For example, morphological features like body or leaf size are consistently used and understood as traits. But other information about organisms, such as their geographic range or group size are considered traits by some but not all. Researchers also agree features measurable at the individual level are traits, but whether information obtained from parts of an individual like its organs, or from broader organizational levels, like clonal lines or populations, was more disputed. Similarly, researchers disagree on what makes a trait a functional trait, other than agreeing that some condition needs to be met (but what condition was a matter for debate).

Collecting trait data from various organisms. Left: first author Samantha Dawson measuring fruiting bodies in fungi (photo credit Mari Jönsson). Top right: collecting butterfly data as part of a citizen science coordinated by co-author Yolanda Melero (Nil Redón is holding a speckled wood butterfly, Pararge aegeria, photo credit Xavi Rendón). Bottom right: studying within-species variation in size of the European rabbit Oryctolagus cuniculus by co-author Manuela González-Suárez (photo credit Hector Garrido).

We had expected disagreement (as mentioned the authors themselves disagree with each other), but had also predicted disagreement would be linked to field of study and taxonomic focus. We anticipated more disagreement among people working with plants vs. animals for example. Alas, that is not what we found. Disagreement was not clearly associated to study subject or area, or to personal or professional backgrounds. Discrepancies are not great, but there is some good news there. Different subdisciplines or fields are not using different strongly-hold definitions (definitions consistently used in one field but not accepted in another). And that would have been much worse in regards to facilitating transferability of findings and multidisciplinary research. Still, because usage and definitions do vary it is clear that when communicating with others we need to be clear about what we mean by traits and functional traits. And yet, we are not.

Fewer than 5% of the papers on traits and functional traits that we reviewed included a definition of these terms. Not providing a definition for a term presumes its meaning is clear to all, but that is not the case here. We are behaving like there is consensus in what we mean by traits, failing to recognize we are often talking about different things. So, What do we suggest we do?

First, we should accept that there are differences and acknowledge we are unlikely to all agree with a single specific definition. My favourite definition is not going to be “the one” for everyone. So, to avoid misunderstandings and facilitate communication and comparison across studies we recommend explicitly including a definition of traits or functional traits in all research.

Second, more generally we propose embracing differences and accepting a general, broad definition of trait, that could be then refined explicitly in each study as needed.

A trait is a measurable characteristic (morphological, phenological,

physiological, behavioural, or cultural) of an individual organism that is

measured at either the individual or other relevant level of organization.

Finally, our study revealed that the term functional trait has become a fuzzy concept, probably because functionality is hard to measure and not a black or white category, rather than functional vs non-functional, functionality exists along a gradient. So, we suggest limiting its use to when traits are explicitly linked to the functional fitness of organisms (ideally measuring both trait and function).

Ultimately, trait approaches are a way to understand diversity and nature, and they will become most useful when we accept traits are at least as diverse as trait ecologists.

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