November 16, 2016 - Nick Higgsdeep sea biodiversity

Deep-sea biodiversity - a known unknown?


How does biodiversity in the deep oceans compare to that of shallower seas and terrestrial environments? The answer is fundamental to understanding how many species exist because the deep sea is widely regarded as one of the largest reservoirs of undiscovered biodiversity on Earth. But there is a wide range of estimates of just how many species species there are left to be discovered. To help decide which of these is likely to be correct we must understand the large scale patterns of diversity in the oceans.

One of the most basic properties of biodiversity patterns is the size of species’ geographical range, i.e. their distribution across the face of the Earth. What we know about this feature of deep-sea species is summarised in an excellent review by Craig McClain and Sarah Mincks Hardy. Current evidence suggests that species seem to be broadly distributed across the deep sea in general, compared to their shallow water relatives. If correct, this suggests that species density is probably lower on average than in shallower waters. In our paper Biases in biodiversity: wide-ranging species are discovered first in the deep sea we set out to test whether this prevailing view is representative of reality or whether we might have a biased view of deep sea biodiversity.

During my masters research I examined the relationship between a single dimension of species ranges (depth) and patterns of diversity in deep-sea brittle stars. I soon began to realise that it is quite difficult to know what the true geographical range of a species is. There are all sorts of problems, especially related to sampling. How can you be sure that you have sampled enough to to truly capture the full range of a species? This leads on to bigger questions about how good our knowledge of species distributions really is and how this might affect our picture of biodiversity.

It was several years later that I began thinking about these questions again. I saw a tweet from marine macroecologist Tom Webb, asking if anyone knew of a study demonstrating that wide-ranged species are discovered earlier than narrow-ranged species. While I didn’t immediately have a reference to hand, we had a short exchange where I suggested that you could use geographical data from OBIS to look at this.

This got me thinking about how species range size might have affected species discovery, particularly in the deep-sea. I had recently been working to create a deep-sea subsection of the World Register of Marine Species using depth data from OBIS. I knew that you could extract geographic data on species occurrences from OBIS and the date that species was described from WoRMS. I did just this and the data showed a strong negative correlation between the date that a species is described and its geographic range (actually a rough proxy of range-size).

This raises an interesting paradox because the direction of causality in this relationship can work both ways. Usually, the correlation is taken to show that widespread species are discovered earlier than geographically restricted species, because they are more likely to come up in samples (the ‘encounter hypothesis’). But the relationship could be equally explained if recently described species have simply not been around long enough to be recorded enough times to have the true extent of their ranges determined (the ‘records hypothesis’).

Understanding which of these two mechanisms is the dominant cause of the correlation is important because it changes how we view undiscovered species diversity.

The diagram below illustrates why. It shows trends in the mean range size of newly described species over time. The solid red line shows an apparent decrease in the mean range size of newly discovered species over time as did our data. The encounters hypothesis suggests that the trend will continue into the future (solid grey line) and that more and more of the newly described species will have (on average) smaller ranges.

On the other hand, the records hypothesis implies that the decreasing trend is just an artefact and in the future the range sizes of newly discovered species will be elevated (dashed red line) as they accrue records over time (read the graph from right to left this time). Under this scenario there will always be a dip in the range size of newly described species (dashed grey line), but they will eventually adjust with a time lag, as they accrue records. Under this scenario there isn’t really any bias towards wide-ranging species.

We tried to unpick these two alternatives using the data available by asking two questions:
(1) Does the proportion of singletons (species with a single record in the database) go down over time, as early singletons accrue records?
(2) Is there a correlation between the number of records that the species has in the OBIS database and the date that it was described?

In both cases the answer was no, showing that the accumulation of records over time was not driving the observed trend. This suggests that the encounter hypothesis is an important driver in the observed trend. Furthermore, we can expect that undiscovered species will have narrower ranges than those discovered to date.

So where does all of this leave us? Firstly, it suggests that we have a biased view of deep-sea species ranges. We are overestimating the proportion of deep-sea species that have wide geographical ranges. This is important because the proposition that deep-sea species tend to have wide geographical ranges relative to shallow species is often used to justify lower predictions of total marine species richness. If species have wider ranges then there will be fewer per unit area. While this is probably somewhat true, we don’t really know to what extent this is the case.

Of course, this trend is subject to the caveat of cryptic species; those species that are morphologically indistinguishable but reproductively and genetically separate. Many of the really wide-ranged species in the dataset are likely to be multiple species with smaller ranges. Nevertheless, this still means that true species ranges are narrower than predicted.

So there is still a case to be made that the deep-sea species richness may actually be closer to some of the larger estimates proposed, but ultimately only more detailed analyses that account for these biases will be able to refine these estimates. Until then we cannot really claim to know how many species there are in the ocean.

Higgs ND and Attrill MJ (2015) Biases in biodiversity: wide-ranging species are discovered first in the deep sea. Front. Mar. Sci. 2:61. doi: 10.3389/fmars.2015.00061