Today’s post is prompted by a conversation I had with a man I sometimes see at the local dog park. Early one morning a few weeks ago, while our pooches played, we discussed species concepts. This man expressed his frustration at his biologist friends’ constant efforts to describe and delimit species. This obsession with naming and renaming was a mystery to him, and he thinks it a waste of effort. To paraphrase him: so mistakes were made in the past, but surely now we can just all agree to accept the status quo and refrain from splitting and merging and reclassifying any more species. And if we discover something new, well, if two groups of individuals can interbreed then they are the same species and if they can’t interbreed then they are different species, right?
Except that it’s not quite that simple.
For a start, the biological species concept (in which the ability to interbreed is the key character that delimits a species) is not particularly useful for classification of organisms that reproduce asexually or that are prone to hybridisation with closely related species. Further, there is no single consensus on what actually constitutes a species, and numerous species concepts and methods for delimiting species have been published. This topic is much too big to tackle in a single post, but I hope to return to this theme during 2015. For now, I want to introduce a few ideas about why understanding speciation and species definitions is so important to biologists, especially in ecology and conservation.
Cryptic species are often unrecognised and unprotected
Genetic studies reveal that cryptic diversity within recognised species is surprisingly common. Such discoveries can lead to the realisation that a single species in fact comprises two or more cryptic species. One example previously discussed by WildlifeSNPits is the recent distinction between the African savanna elephant and the African bush elephant. Cryptic species are often closely related to one another and may have diverged relatively recently from a common ancestor. They may look similar, but are now following independent evolutionary trajectories. If one or more of these newly-recognised species has a restricted range, extremely specialised behaviour or a higher susceptibility to a threatening process, then conservation action may be crucial. But unless we understand cryptic diversity we are unlikely to focus our conservation efforts towards something we think is common or widespread.
Cryptic diversity may not be recognised during biodiversity assessment
Many methods used to assess biodiversity rely on measures of species richness (how many species are found in a region) and abundance (how many individuals of each species are found in the region). It makes sense then, that failure to account for large numbers of cryptic species will lead to the underestimation of biodiversity in many habitats. This in turn could have implications for decisions about which areas to protect in nature reserves, where to allow development to occur and how to allocate conservation resources. Conservation organisations and jurisdictions worldwide use species as a unit by which to rank taxa by conservation status and use biodiversity measures to guide distribution of conservation funding. If we get the species definitions wrong, we may get these other decisions wrong as well. In a recent paper, which uncovered significant cryptic diversity in an Australian freshwater fish, Adams et al conclude that future assessments of species diversity should estimate and account for cryptic diversity.
Management of pest species requires a good understanding of those species
Agricultural pests, disease vectors and invasive species are problematic all over the world and vast resources are directed towards preventing their spread and managing their impacts. Taxonomic uncertainty has the potential to hinder successful pest species management. The Anopheles nili group of mosquitoes occurs in sub-Saharan Africa and includes species that are known as important vectors of malaria. A recent study revealed cryptic genetic diversity within this group, which might have implications for understanding the transmission and management of malaria in the region.
In another example, the Australian common brushtail possum has become a major pest in New Zealand. During the 19th century, possums were taken from the south of mainland Australia and from the island of Tasmania, and deliberately released across New Zealand to establish a fur trade. These possums are all currently classified as a single species, Trischosurus vulpecula, but with several recognised subspecies, including T. v. vulpecula (from the southern mainland) and T. v. fuliginosis (from Tasmania). A recent population genetic study of New Zealand possums by Sarre et al (disclaimer, I am a co-author on this paper) has uncovered evidence of a hybrid zone that has formed in New Zealand between possums of Tasmanian origin and possums of mainland origin. Understanding interactions between these two groups of possums, which would never interact in their native ranges, may be important for effective possum management. They may behave differently in response to baiting and there is evidence that susceptibility to the commonly-used poison sodium fluoroacetate is higher in possums of mainland origin.
It’s difficult to study species interactions when you don’t know how many species you’re studying
Ecologists study interactions among individuals, among species and between species and their environments. These interactions include reproduction, predation, competition over resources and habitat use. It makes sense then, that to understand interactions among species, a good understanding of species delimitations is needed. Take the case of the neotropical skipper butterfly, Astraptes fulgerator. This was thought to be a single, widespread, generalist species displaying little morphological diversity among adults but with considerable diversity in caterpillar colouring. Then, in 2004, a paper by Hebert et al used morphological, ecological and genetic studies to demonstrate that this butterfly actually represents a species complex. Caterpillar colour patterns correspond to food plant preferences and genetic variation, suggesting that at least 10 new butterfly species should be described. Such a discovery can completely change perceptions about the ecology and evolution of the species of interest. For these skipper butterflies it is likely that diversification of new species is linked to specialisation on different food plants, an interaction that would have made no sense if these were still classed as a single butterfly species.
I hope that with these example I’ve made my point, that we still have reason to care about understanding species and to get species delimitations right! Of course there are many complicating factors that can hinder our ability to get it right, but more on those another time…