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…
WildlifeSNPits 
It seems very specialised to an outsider but having begun to discover the amazing diversity of plants I can see how important this is.
Yes, I expect most people think they know what “species” means, but then when you get a group of people together and ask each of them to define “species”, you are unlikely to end up with a single clear answer! I know the complexity takes a lot of biology students by surprise when we discuss this.
Really interesting. I was was giggling about the description of the “Varied Sitella” in my bird book the other day – are such varied species sometimes cryptic species? Or is there no necessary connection?
Thanks for your interest. I’ve read about numerous “varied” species that do harbour cryptic, or sometimes not-so-cryptic variation. I don’t know the varied sitella well, but I just had a quick read and it has similar traits to some of the other examples I’m familiar with – a widespread species with morphological variation across its range. In other cases these traits can be seen after a large range expansion but before different lineages have fully diverged. These lineages may become partly differentiated, but then hybridise again. This might be what has happened with this species and why it is still classed as a single species? Apparently it used to be described as several species, then was merged into a single species, with five subspecies currently recognised. This isn’t a comprehensive literature search, but these were of interest in my quick look just now:
http://www.publish.csiro.au/paper/ZO9830499.htm
http://bie.ala.org.au/species/Daphoenositta%20chrysoptera#tab_classification
http://www.birdsaustralia.com.au/bird-profile/varied-sittella
I can’t help but feel like, for both ecology and conservation, this is not a central issue.
From an ecological perspective, a species is a discrete definition applied to a continuum. True, genetic studies reveal that some species are in fact multiple cryptic species. But those cryptic species themselves will also contain genetic variation. Are we therefore wrong to classify them as individual species? Where does the splitting end? Any attempt to impose discrete groups on continuous data will inevitably cause perverse outcomes. The appropriate group definition depends on the problem we’re trying to solve. Your example of brushtail possums is a good one. However, if analysis revealed them to be a single species which nevertheless had differential responses to 1080 baiting, we’d still face the same problem you outline: intra-class variation.
From a conservation perspective, the issue of cryptic species is even less important. Like it or not, conservation funding decisions are not made on the basis of species richness or uniqueness, but on the basis of charisma and anthropomorphism. Funding in the US, for example, predominantly flows to subspecies. Of the ten species that receive 50% of federal conservation dollars, 6 are actually subspecies (Northern spotted owl, Florida scrub jay, Florida panther, Grizzly bear, least Bell’s vireo, American peregrine falcon). Most cryptic species are found in poorly-studied, uncharismatic taxa who wouldn’t (and arguably should not) receive large amounts of funding even if those groups were twice as diverse. Split or lump them all you want – they’ll still be ignored by funders and NGOs. Any limited dollars that find their way to insects or plants should be immediately put into direct conservation actions, regardless of whether they are one species or two look-alikes. Spending that money on splitting the lesser green phasmid into southern and northern cryptic species isn’t going to change conservation outcomes, unless the southern group happens to look a lot like humpback whales.
Thanks for the comment, great to get your perspective. I’m hoping to further explore some of what you’ve mentioned in future posts. You are exactly right that much of this debate comes down ultimately to semantics, trying to place a discrete divide at the “perfect” point within a continuum. But I still do believe that understanding “species” or some other unit of biodiversity (subspecies, evolutionarily significant units (ESUs), management units (MUs)…) is important to both ecology and conservation. The appropriate level to define might vary with the question, but if nothing else social and legislative perceptions seem to demand some way to classify wildlife and funding is often linked to these. With the case of the possums in New Zealand, it probably doesn’t matter whether we call them different species or subspecies, the key point is that until now they have been generally considered and managed as a single entity, but now we know that there is variation among these possums and that may need to be taken into account by managers.
You’re also right that many people may not like the way conservation funding is allocated, but surely the advice given to the decision-makers should still be based on the best available science? I’m less familiar with the US system, but in Australia legislation allows for management of threatened species at species level, but also at subspecies or ESU level if there is good evidence to support the need for separate management. So perhaps defining “species” is less important, but understanding cryptic variation does contribute to management outcomes. In some circumstances the presence of an endangered species at a site can have serious implications for planned development activities (mining, building, airport construction etc.) at that site. This might include requirements for the companies involved to contribute money to on-site conservation, or conservation of alternative habitat or restoration of other sites. In that case, understanding cryptic variation, even in something as “uncharismatic” as a skink or a moth, might indeed influence the availability of conservation funding.
Ultimately I think that a lot of the difficulty we have in communicating science can be attributed to language barriers – this is a difficulty I face a lot as a geneticist. Many of my non-scientist friends are fairly confident that they understand what “species” means, but in fact they have a very simplistic definition. I’ve seen some people consequently experience great confusion trying to understand why some things cannot be easily classified or why some decisions are made based on species, but others are based on subspecies or some other classification they’ve never heard of. So perhaps if nothing else this conversation will introduce a few people to the difficulty of understanding species in the first place…