Species: The Cornerstone of Biodiversity

Source: Food and Agriculture Organization of the United Nations, 2013, Kathryn Pintus, IUCN , "The Youth Guide to Biodiversity" 1st Edition (Chapter 4) Youth and United Nations global Alliance. Reproduced with permission.

Chapter 4. Verbatim.

So far, we’ve had a look at genetic diversity, and we’ve learned that genes are responsible for the wide variety of species that exist on Earth. But what exactly is a species?
A species is a basic biological unit, describing organisms which are able to breed together and produce fertile offspring (offspring that are able to produce young). The above statement is a fairly widely accepted definition, and in some cases it is easy enough to determine whether two organisms are separate species simply by looking at them; the mighty blue whale is clearly not the same species as the fly agaric mushroom.

Redwood National Park, California, USA.© Anthony Avellano (age 14)

However, the situation is not always quite so straightforward. The science of describing and classifying organisms is called taxonomy, and this provides us with a common language that we can all use to communicate about species, but it can get rather complicated! 

Biology is split into several fields, including botany, zoology, ecology, genetics and behavioural science, and scientists from each of these branches of biology will have slightly differing definitions for what constitutes a species, depending on the focus of their specialty. 

For instance, some definitions will be based on morphology (what it looks like), others on ecology (how and where it lives), and others still on phylogenetics (using molecular genetics to look at evolutionary relatedness). For this reason, when considering two organisms which on the surface may look almost identical, scientists sometimes disagree as to how to classify them. Are they individuals of the same species or are they two completely separate species? Or are they perhaps subspecies? Having said that, taxonomy can be very useful (see the box: “How Does Taxonomy Help Biodiversity?” for more details).

To complicate matters further, some individuals of the same species may look considerably different from one another, perhaps due to their sex or as a result of their geographical distribution. The trait whereby males and females look different from one another is known as sexual dimorphism, and can be seen in many species, particularly in birds. 

Scientific disagreements aside, there are about 1.78 million described species on Earth, with millions more out there that we don’t even know about yet. That’s an incredible amount of biodiversity, but unfortunately much of it is being lost, and it is possible that we are losing some species before we even have the chance to discover them.