© Wikimedia Commons

Biodiversity is an abstract concept without a universally accepted definition (Meinard et al., 2019), with measures often incorporating both processes and entities. These may include: the functional diversity of a system, reflecting its biological complexity; phylogenetic diversity, whereby the distances between evolutionary lineages are summed; or species densities, relative abundances and total richness. Choosing an appropriate measure for conservation can depend on the end goal of the policy maker, the values of the stakeholders or simply the data which are available at the time. Most of these measures, however, rely heavily on how we perceive, define and identify species.

Please note: The points made in this article do not necessarily reflect the author’s views, but are made to encourage discussion of the topic.

What is a species?

For almost three centuries scientists have been categorising life into binomial species. This classification system was first used consistently by Carl Linnaeus (1735) in Systema naturae, and comprises a genus (which may be shared by other species) and a species epithet, e.g. Homo sapiens. The practice of delineating and ‘discovering’ species has since been guided by at least 34 concepts (Zachos, 2018a). Scientists now typically follow derivatives of the Evolutionary Species Concept (Wiley, 1978) as well as various identification criteria. These are primarily concerned with a meta-population’s status as a separately evolving lineage of organisms. New concepts on how to delineate and identify such lineages have been proposed as recently as 2017 (Hill, 2017 and Shanker, 2017).

Zachos (2018b) contends that all of these concepts try to impose a discrete binary (same / different) system on a continuous, evolutionary process. No matter which concept we use, lineages must pass over a subjective line before we consider them a separate species, a grey area that can differ according to the taxonomic group we are observing. Is a species of plant really the same ‘thing’ as a species of insect, bird or bacteria? Are some species more inclusive of populations and individuals than others? These are important questions to ask when deciding what to prioritise in conservation, and where our values lie. Whether we consider a species endemic or threatened is heavily dependent on the inclusiveness of our classification, and therefore so is our identification of biodiversity hotspots.

A species tree of the genus Homo depicting their distribution across Africa and Eurasia. The beginning and end of each ‘species’ is unclear, and it’s unlikely that other taxonomic groups (e.g. birds, plants and bacteria) would be classified using the same concepts. © Wikimedia Commons
Should we prioritise global or local species richness?

The above issues are exacerbated at the global level, as it is more difficult to classify lineages that are allopatric (geographically separated) and do not interact in the wild. This uncertainty undermines conservation targets that prioritise global species richness, i.e. avoiding global species extinctions. If populations are sympatric (occurring in the same area) it is much easier to observe differences between organisms, such as whether they occupy separate niches and are reproductively isolated, and therefore to quantify diversity. These local populations are likely to have different ecological roles that are important for ecosystem functioning, and therefore the persistence of other organisms in the community. The loss of such a population may not be considered a global extinction, but locally it could be highly detrimental, either ecologically, commercially or culturally. From this perspective, the importance of losing a population depends on its local context, not on whether scientists have classified it as a separate species and the last of its kind.

However we define species, there are lineages that are clearly more rare and unique than others, such as those identified in the EDGE Programme. This uses ‘a scientific framework to identify the world’s most Evolutionarily Distinct and Globally Endangered (EDGE) species’, a list of which can be viewed here. These fascinating animals and corals can take the important role of ambassadors for their habitats, which may otherwise be overlooked. However, one must accept that the value we see in the globally rare and unique is just that — a value — and one that likely differs from local or national values. At these scales, what scientists consider common and ordinary might be highly important for ecosystems and people. It may therefore be easier to communicate the value of maintaining overall local or national species richness and abundances rather than just those which fit a global conservation agenda.

The Largetooth Sawfish (Pristis pristis) tops the list of the world’s most Evolutionarily Distinct and Globally Endangered (EDGE) species, alongside Sir David’s long-beaked echidna (Zaglossus attenboroughi). © Wikimedia Commons

Assume that we do accept all currently recognised ‘species’ as real, objective entities, and that the primary goal of conservation (such as that of the Alliance for Zero Extinction) is to prevent as many extinctions as possible. Focusing efforts on species that are the most threatened (at the expense of others) is likely to result in many ‘last populations’ that have a high dependence on conservation. They may also be ecologically or functionally extinct, occurring in such low numbers or artificial environments that they no longer have a significant role in an ecosystem. Such a strategy might provide good publicity for certain conservation organisations, at least those that can afford expensive species recovery programmes (we saved x number of species, and they need us to stay saved). But resources can only stretch so far, and the more species that slip from ‘Least Concern’ or ‘Near-Threatened’ (see IUCN Red List categories), the more overwhelmed these organisations would become by species needing immediate and continued attention. If we prioritise at the ecosystem or landscape level, and local or national species richness, we may have a better chance at stemming global losses in the longer term. In particular, we should endeavor to keep populations and ecosystems that are still abundant and intact (see here), and are more capable of adapting to environmental change. In future, these may act as important species pools for replenishing other areas and populations.

Do species have a right to survive?

It could be asserted that species have a right to survive regardless of their utilitarian or cultural value, and we have a moral obligation to prevent all species extinctions resulting from human activities. But we must remember that species are subjective human categorisations, while individual organisms are not. For example, consider the 22 gibbons remaining on Hainan island, China, which have been classified as the last of the species Nomascus hainanus. Do they have any more ‘right’ to survive than the 1,000-1,300 gibbons living on mainland China, just because those have been classified as the same species (Nomascus concolor) as some populations in Laos and Vietnam? An analogy could be made with dialects or languages, the linguistic classification of which could be considered equally subjective. Should we value the lives of the last 22 people who speak an unrecorded language over 1,000 who speak a related, but common language? For a humanist that equally values individuals regardless of ethnicity or mother tongue, probably not.

It was recently reported that an estimated 3 billion birds (29% of total abundance) have been lost from the continental United States and Canada in the past 50 years, many of which were considered ‘common’ species. According to the IUCN Red List, this region has had zero global extinctions within the same period. Perhaps we should look further than species extinctions and consider the needs of the many as well the few, or soon only the few will remain.

© Wikimedia Commons

Di Marco, M., Ferrier, S., Harwood, T. D., Hoskins, A. J., & Watson, J. E. (2019). Wilderness areas halve the extinction risk of terrestrial biodiversity. Nature, 1-4.

Hill, G. E. (2017). The mitonuclear compatibility species concept. The Auk: Ornithological Advances, 134(2), 393-409.

Mayr, E. (1942). Systematics and the Origin of Species. Columbia Univ. Press, New York.

Meinard, Y., Coq, S., & Schmid, B. (2019). The Vagueness of “Biodiversity” and Its Implications in Conservation Practice. In From Assessing to Conserving Biodiversity (pp. 353-374). Springer, Cham.

Shanker, K., Vijayakumar, S. P., & Ganeshaiah, K. N. (2017). Unpacking the species conundrum: philosophy, practice and a way forward. Journal of genetics, 96(3), 413-430.

Wiley, E. O. (1978). The evolutionary species concept reconsidered. Systematic zoology, 27(1), 17-26.

Zachos, F. E. (2018a). Species concepts, species delimitation and the inherent limitations of taxonomy. Journal of genetics, 97(4), 811-815.

Zachos, F. E. (2018b). Mammals and meaningful taxonomic units: the debate about species concepts and conservation. Mammal review, 48(3), 153-159.