Why Should We Care about Species? (2024)

Most people have a basic idea of what species are, even if they are not sure of the best way to define the word species. Quite simply, species are kinds, or types, of organisms. For example, humans all belong to one species (the scientific name of our species is hom*o sapiens), and we differ from other species, such as gorillas or dogs or dandelions. But defining, identifying, and distinguishing between species really isn't that simple. In fact, it is often a complex and difficult process-especially in cases of new or previously unknown species. Biologists frequently disagree about species, and even argue over how best to define the word species. This disagreement is so well known, and so much discussed, that it is sometimes referred to by biologists as the "species problem" (Hey 2001).

This article explores the idea of species, including both the meaning of the word species, and how biologists think species can be identified in nature. It also examines why an understanding of species is important, both for the study of biology and for our society.

The central difficulty when studying species is that, even though all species are kinds of organisms, all kinds of organisms are not species. For example, birds are a kind of organism, but birds are not a species --there are many thousands of species of birds. For scientific purposes, it is not enough to identify a kind of organism. As a biologist you must also determine what level or rank of kind to assign to an organism. If you discover a new kind of organism then you must decide if it should be called a new species, or if it falls within an already described species. For example, the common chimpanzee species, Pan troglodytes, appears to include several slightly different kinds of chimpanzees. Each of these have been given the rank of sub-species. Alternatively, a newly discovered kind of organism might be so different from other known species that it receives not only a designation as a new species but also a ranking as a new genus.

To help understand the confusion and uncertainty over species, let's look at the most basic idea of Darwin's theory of evolution by natural selection. Darwin figured out a process by which species could change over time, and he believed that evolution was a slow and gradual process that played out over eons of time. So if species are changing slowly, and if new species are formed at the slow pace of evolution, then we absolutely expect there to be cases where we struggle to decide whether two kinds of organisms should be grouped as separate species or as a single species. In his book, On the Origin of Species, Darwin famously wrote, "I was much struck how entirely vague and arbitrary is the distinction between species and varieties." (Darwin 1859). In other words, Darwin did not believe that there was a definite point at which a species came into existence. Finally, because the large majority of species come into existence gradually, it is not surprising that we have difficulty deciding when to identify new species or what the best way to do so should be.

Imagine you're a biologist on a research expedition to look for previously undiscovered kinds of butterflies. If you find some butterflies that seem different from those species that are already known (Figure 1), then you are faced with two questions. The first question is "what are species?", or to put it another way, "what makes a kind of butterfly an actual species of butterfly, rather than a sub-species or a genus?" The second question is "how should a species be detected?" The answer to this second question depends on the answer to the first question (i.e. "what are species?") but the two questions are not the same.

This business of having separate "what" and "how" questions aboutspecies is part of the confusion around species (de Queiroz 2005). Inthe past most biologists thought that knowing what species are (i.e.,having an answer to the "what" question) was basically the same asknowing how to discover species (i.e., having an answer to the "how"question). The two questions are closely connected, but they are notreally the same thing. For example, biologists might have a theoreticalidea of what species are as well as a practical procedure foridentifying species. The theoretical idea addresses the "what" question,while the practical procedure addresses the "how" question. It is possible that different scientists sharethe same theoretical idea about species, but actually rely upon differentpractical procedures for identifying them. The situation is similar to what a physicist faces when trying to detectunseen particles. The physicist mayknow what an electron is, but that is not the same as knowing how to detect anelectron. The theoretical idea of theelectron, when it is put into practical applications, has yielded multiple waysto detect electrons. In the same waythere are multiple ways to detect species, and some may be better than othersdepending on circ*mstances.

For the most part, biologists agree that speciesare made up of organisms that are evolving together. And they also agree thatfor sexual organisms, shared reproduction within species and the evolution ofreproductive barriers between species, are major factors that cause species toexist. Where biologists tend to disagreeis how these general theoretical ideas should translate into methods fordetecting and identifying species. Inother words, biologists agree that these ideas help us answer the question of what species are, but they are not in agreement about how much theseideas help answer the question of how to identify species.

For Ernst Mayr the answer to thequestion of how species are identified also boiled down to reproduction. In other words, Mayr was using the idea of reproductiveseparation of species to answer both the "what" and the "how" questions aboutidentifying species (Mayr 1957). To Mayr, the key to identifying species is determining whether there isshared reproduction within a population of organisms and whether there arebarriers to reproduction with other organism. Mayr called this idea of defining species on the basis of reproductionthe Biological Species Concept, orBSC.

The BSC has been very widely discussed and debated,and many biologists think that Mayr is essentially correct about whatspeciesare and how they should be identified. Forexample, while the western meadowlark and eastern Meadowlark of theU.S. (Figure 2) are very similar in appearance and have overlappinggeographic ranges, theirdistinctly different songs prevent them from interbreeding. Under BSCrules theyare classified as two different species. In this case, using the BSC todecide whether you have one or more thanone species is very straightforward. However,in many BSC cases, the decision is not straightforward at all. This isparticularly true if you're trying to determine whether two separatepopulations in different geographic locations belong to the samespecies. When geographical separation is involved, theindividuals in the different populations have no chance to reproducewith eachother. If the populations cannot interact under natural conditions youcannot knowfor certain if they would reproduce. Artificialconditions such as zoos and laboratories are not a valid way todeterminewhether individuals will reproduce in the wild, since members of a widevarietyof species will reproduce with those of other species in a zoo, but notintheir natural habitat.

Because of the limitations of using the BSC to make decisions about species, many biologists have proposed other ways to identify species. One popular approach is called the Phylogenetic Species Concept, or the PSC (Rosen 1979; Cracraft 1983; Donoghue 1985). There are actually several versions of the PSC, but they all agree that species can be identified on the basis of shared traits. A group of organisms that all share one or more features pointing to a unique common ancestor, which in turn is not shared by members of other species, would meet the criterion of species under the PSC. The traits used under the PSC are wide ranging and include color or shape, or behavior. For example, species of plants could be distinguished on the basis of the color and shape of their flowers.

Using traits under the Phylogenetic Species Concept is a very different way of identifying species than using shared reproduction under the Biological Species Concept. But like the BSC, using the PSC accurately can still be challenging. Consider a group of organisms that all appear to be in the same population (e.g., maybe they are interbreeding with one another) and yet some individuals in that population are of a different color than the others. When we find different distinct organismal types within the same population, and those types are not uncommon, it is called a polymorphism. Yet under the PSC polymorphism might be interpreted as the presence of multiple species.

Decisions about species and uncertainty about identifying species are not just issues for scientists. Everybody needs to be able to think about and talk about kinds of organisms. A fisherman, a hunter, a birdwatcher, a gardener, or even a person in the grocery store who shops for fruits and vegetables, is depending on being able to distinguish among kinds of organisms. So too must doctors and other medical professionals be able to identify different kinds of parasites, including bacteria and viruses; and farmers must be able to tell the difference between crop plants and weeds.

It is in the preservation of endangered species that many people are most likely to feel the impact of species questions. Many organisms of many kinds are affected by the societies and economies of human populations, often for the worse. For legal, ethical and economic reasons it is a big, and sometimes difficult, decision to conclude that a species is endangered. This is because in the United States, a species with endangered status receives protection that often affects the lives of the people in areas where protected species live.

Consider the case of the North Pacific Right Whale, which not very long ago was recognized as a distinct species based on genetic evidence and the Phylogenetic Species Concept (Rosenbaum et al., 2000). The genetic data--including mitochondrial DNA sequences--indicated that the North Pacific Right Whale has not been exchanging genes with other populations for a very long time. Because this whale population is so small, its new species status meant that the Endangered Species Act could be invoked. In 2008 the species was officially listed as endangered (Federal Register, 2008). However, there are thought to be only a few hundred of these whales still living and little has yet been done to protect their habitat. Habitat protection has wide-ranging governmental and political repercussions, including limits on shipping, fishing and oil-drilling activities in a part of the northern Pacific Ocean. In other words, it could be expensive to save the North Pacific Right Whale. Yet saving the Right Whale is important for ocean ecology, and thus for the thousands of other species that share the food web of the Pacific Ocean with the Right Whale. Saving the Right Whale is also important for strengthening peoples' connections to the beauty and wonder of the world's oceans and the life they contain.