When is a trait not a trait?

Last week I was on a course on using traits for ecological analysis in Coimbra. Verdict: Unimpressed by the course, but impressed by the beauty of the city.

But I digress.

One of the things that kept coming up on the course was what people considered a to be a trait.

Apparently the strict definition that the people running the course was something along the lines of “morpho-physio-phenological traits which impact fitness indirectly via their effects on growth, reproduction and survival, the three components of individual performance” taken from Violle et al 2007.

I agree roughly with this definition.

But there were a few naysayers in our group. Some of them argued that distribution size was a trait or that habitat that a species preferred was a trait. Personally I think these two things are the result of a trait-environment interaction, and are not themselves traits. However,even papers in the Holy Grail of publications, Nature, can get this wrong so I can understand the confusion.

Certain traits may be increase the likelihood of a species found in a region to be found at a particular site
Certain traits may be increase the likelihood of a species found in a region (left) to be found at a particular site (right)

For example, take a regional species pool that is made up of species that vary in traits which impact their fitness. These traits can determine whether a species is present at a particular site.

Trait_filtering 02
Some species may only be found in particular habitats (green patches) and vary in range size compared to others (black line) but we shouldn’t call these traits.


If you add up all the areas that a species is present in you have an idea of a species range and also the habitats they occupy. Therefore it is fairly obvious that these two things are not traits but are rather the products of traits.

This is not to say they aren’t useful in some way. Range size (or more accurately area of occupancy or extent of occurrence) is fundamental to one of conservation biology’s flagship projects – the Red List, and knowing the habitats a species uses can be useful in lots of ways.

It would be helpful if we could all agree what we were talking about when it come to traits, as very few of us seem to have thought about it. When people review a paper that uses the word ‘trait’ they should make sure that the term has some meaning and isn’t just used as a sexier synonym for ‘characteristic’ or ‘feature.’

Ecology is fraught with problems of definition (as I have discussed here and here) and personally I think it is one of the things that holds back our science. If our aim is to form meaningful generalisations about how the natural world works, we can’t do it until we agree what the hell we’re talking about in the first place.

3 thoughts on “When is a trait not a trait?

  1. trait |treɪt, treɪ|
    a distinguishing quality or characteristic, typically one belonging to a person: the traditionally British trait of self-denigration.
    • a genetically determined characteristic. breeders were installing some trait that allowed the crop to thrive.

    Since trait IS the same as a characteristic or a feature, I don’t see any reason why we should reinventing English. In fact, there’s nothing better than plain English in Science. That’s why we can simple add the ‘functional’ word before trait if you want to mean something that has a function.

    1. I generally agree with the point you make. I would refine the definition by saying it should be definable at the level of the individual though.

      Also, regarding functional/non-functional traits this is another area beset with definition problems. What would you say is a function? I know what I think but peoples idea of what a function is varies a lot and generally papers are quite poor in justifying why they consider certain phenomena to be functions.

      1. Again I don’t understand why we must made definitions strict with no reason. Is there any specific reason you think that ‘species’ trait should be measured only at the level of the individual. This is the case if you want to study the variability of the trait among individuals of the same species. When you go to species-level trait: Isn’t geographic range, or let’s say, population genetic structure, a property of the species?

        Regarding the definition of functional, I agree that this definition is highly flexible and depends much on the question you are asking. But I let the burden to define this to the people who thinks that only ‘functional’ traits matter. I’m happy by using trait in my papers simply as synonym of characteristic and seeing how they influence (or predict the outcome of) the response variable.

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