- Species are time-tested units in measuring biodiversity.
- They are not however discrete entities in the continuum of life.
- Life exists in different forms and performs complex functions.
The English language dictionary, Chambers Concise 20th Century Dictionary, provides several meanings for life: state of being alive, existence, conscious existence, continued existence, the sum of the activities of plants and animals, among others. However, to biologists, life is a ‘driving force’ that makes organisms breathe, feed, metabolise, excrete, grow, reproduce, feel, and move. Surprisingly, “life” does not have a precise yet simple meaning or definition for a word in such everyday use.
Vague as life may sound, apparent variations in its form and functions are summed as biological diversity. ‘Biodiversity’ is merely a shortened expression that has become a buzzword after the Convention on Biological Diversity (CBD) came into force in 1993. Soon after CBD emerged as a globally binding law, the United Nations Environment Program (UNEP) launched an ambitious project to assess the earth’s biodiversity. After a mammoth effort that involved several international workshops and the contributions of around 1500 experts, the Global Biodiversity Assessment (GBA) Report was published.
Preparatory workshops for drafting the GBA – a global-level assessment of changes in Earth’s biodiversity that have occurred over a specified period – began in April 1994 at the Natural History Museum in London.
Some of the world’s most renowned taxonomists were present, including Joel Cracraft from the American Museum of Natural History. Joel Cracraft worked on birds and is among the pioneers who advocated using cladistics – sorting into groups based on the last common ancestor – in taxonomy.
Among others, the workshop’s agenda was determining the number of species on earth. Earlier estimates had placed the number between five and fifty million. As there was little consensus among the experts, it was agreed that a ‘working number’ should be arrived at, and it was thirteen million.
Species as units of natural life
Despite its apparent diversity in forms and functions, life is a continuum – a continuous chain that cannot be easily broken up into distinct and separate units. This is why attempts to draw discrete boundaries have led to unresolved debates.
The dichotomous system of classifying life forms as plants and animals, which was common knowledge for thousands of years, has proved inadequate. At the functional level, the biosphere is the largest and placed on top of the biological hierarchy. At the lower end, the biological hierarchy begins with genes, the smallest functional units. It progressively becomes more complex as individuals, populations, species, and ecosystems overlap at different levels between the successive units.
Traditionally, however, the term ‘species’ has been the most widely used biological unit for assessing biodiversity. But what is a ‘species’? Species were first defined as groups of organisms that are physically similar. Carl Linnaeus and his fellow biologists and taxonomists in the 1700s believed that species were stable biological units. Physical features alone could delineate them. Such simplistic definitions led to naming males and females differently in dimorphic (two distinct forms or “morphs”) species and adults and larval forms separately in metamorphosing species.
Charles Darwin’s treatise on the origin of species in 1859, for the first time, underlined the need to see species as biological units that continue to evolve.
The thinking further changed when German-American evolutionary biologist Ernst Mayr proposed the biological species concept in the early 1960s. According to him, populations that potentially interbreed and produce viable offspring, belong to the same species. By his definition, all existing human beings belong to the same species (Homo sapiens), dogs and wolves belong to the same species (Canis lupus), and the thousands of different varieties of rice also belong to the same species (Oryza sativa). The biological species concept accommodates fair amounts of physical variations within a species, a biological trait that inspired Charles Darwin more than a century earlier.
However, the biological species concept was not widely accepted by taxonomists studying organisms that reproduced asexually. This and other reasons have contributed to the emergence of at least twenty alternate species definitions. And with the advent of molecular biological tools, delimiting organisms as species has become murkier.
Modern definitions of species tend to emphasise the unstable nature of species. Thus, a species is an ‘independently evolving meta-population lineage’ to some. And to others, like Joel Cracraft, species are ‘indiscrete and arbitrary’ segments in the evolutionary continuum.
Species form complex functional units
Irrespective of how they are defined and delineated, species play vital roles in the structure and functioning of ecosystems. CBD defines an ecosystem as a dynamic complex of plant, animal, and microorganism communities and their non-living environment interacting as a functional unit. Biological communities are formed when two or more species share the same habitat and function cohesively. As ecosystems locally synchronize with biological communities, their structure and functioning are best understood by how species are organised into communities. The larger the number of species and the more evenly they are organised, the greater the volume of interactions will be. This rationale has placed higher ecological values on species-rich tropical ecosystems such as rainforests, mangroves, and coral reefs.
Each species occupies one or more niches. Therefore, species organisation in an ecosystem can vary in space and time. Robert H. MacArthur, considered one of the founders of ecology, and others developed interesting mathematical models to explain why and how species are organized the way they are. Some species, however, play ‘larger-than-life’ roles in ecosystems. These are called ‘keystone’ species. American zoologist Robert T. Paine coined the term ‘keystone species’ in 1969 based on his study of a starfish and a mussel in a tidal ecosystem, and it has since found wide application in worldwide prioritisation and restoration of ecosystems. The most commonly cited tropical keystone species are fig trees, which include the sacred banyans.
Scope of species mining
Thirty years ago, when GBA was drafted, only 1.75 million species were discovered and named. During the last three decades, thousands of new species have been added. Even if 250,000 new species have been discovered and named in the past 30 years, there would still be another six million species unknown to science. This is a conservative estimate, taking the number of species as eight million, a figure that is in contemporary use. It was also calculated that discovering and naming all the unknown species may take more than 500 years, with the kind of taxonomic expertise and infrastructure available worldwide. While there is little doubt that taxonomic advancements have taken significant strides since GBA was published, the question is whether it is really necessary that all the unknown species be discovered and scientifically named.
Global trends in the discovery of new species indicate that the discoveries are not evenly distributed, nor can they be predicted using the number of species already known in the different taxa of living organisms. Discoveries are disproportionately higher in amphibians, for instance. The explosion in amphibian discoveries has happened only in the last two or three decades. It may concern the ever-changing definition of species or that many biologists see ‘species mining’ as a prospective career option. Further, frogs and toads are vocal, readily betraying their presence, and least mobile among vertebrates, making field collections easier. A recent publication by Wolfgang Wuster and colleagues highlighted the dangers of resorting to shortcuts to delineate species. Are all new species real, or are many just ‘ghosts’ of the past? Only time will tell.
CITATION:
Starr, C (2003) Basic Concepts in Biology (5th edition). Brooks and Cole, Australia.
Heywood, V (1995) Global Biodiversity Assessment. Cambridge University Press-UNEP, Cambridge, UK.
Cracraft, J (1981) Pattern and process in paleobiology: the role of cladistic analysis in systematic paleobiology. Paleobiology 7(4): 456-468 https://www.jstor.org/stable/2400697.
Wuster et al. (2024) How not to describe a species: lessons from a tangle of anaconda (Boidae: Eunectes Wagler 1830). Zoological Journal of the Linnean Society 201(4): https://doi.org/10.1093/zoolinnean/zlae099.
Wren et al. (2024) Amphibian Conservation Action Plan. IUCN-SSC. DOI: https://doi.org/10.2305/QWVH2717.
Aravind et al. (2004) “Croak, croak, croak: Are there more frogs to be discovered in the Western Ghats?” Current Science 86(11): 1471-1472.
Daniels, R J R (2008) Taxonomic vandalism: the case of the giant wrinkled frog. Current Science 94(2): 158-159.
Banner image: The great hornbill. A larger number of species, organized more evenly, leads to more interactions. This has led to higher ecological value being assigned to species-rich ecosystems like rainforests, mangroves, and coral reefs. Image by Kalyan Verma.