SYDNEY: How many species share our planet? According to a recalculation by an international research team, the number is significantly lower than we thought – only around 5.5 million.
While that may sound like a lot, this number pales in comparison to previous species estimates, which varied widely from 30 million to over 100 million species.
In fact, the new study, appearing in the journal The American Naturalist, shows that there is less than 0.001% chance that the frequently cited previous estimate of 30 million could be true.
The new estimate, which is particularly timely in the International Year of Biodiversity, takes into account plants and animals but, like previous studies, it excludes bacteria – a group that has been notoriously difficult to quantify.
“Everyone keeps redoing the information and coming up with different answers,” said Andrew Hamilton of Melbourne University’s School of Land and Environment and lead author of the study.
“So we came at it from a different perspective. Rather than saying ‘there are so many species,’ we’ve included how certain or uncertain we are in our estimates.”
Previous studies that used averages or point estimates made it impossible to determine the precision of a particular estimate, said Hamilton.
But by using probability-modelling techniques, the ecologists were able to come up with estimates that factor in the probability of the original data being accurate.
This type of model is widely used in financial risk assessments, but has rarely been applied to ecology.
The model used by Hamilton and his team focusses on the numbers of tropical arthropods – the group that includes animals such as insects, arachnids and crustaceans.
Anthropods are thought to be the most diverse group on the planet. They are also the group that is believed to have the most yet-unidentified species.
By looking at all of the beetles that live on a single tree species in Papua New Guinea, the researchers were able to extrapolate their numbers to a global scale.
The team decided to focus on beetles as they are the most described group of insects.
While it may seem unlikely that a single tree of beetles can reveal the total number of species on the planet, the model takes into account many factors – including:
- the relationship between beetles and other arthropods
- the number of tropical tree species in New Guinea compared with the rest of the world
- the probability that some beetles species will not be specialists, only found on one or a few tree species.
According to Hamilton, New Guinea is a good place to start the estimate because it has the most robust dataset of this kind and is home to about 1/3 of the world’s tropical tree species.
He also noted that numbers of beetles found on trees in New Guinea had similar figures to those found in tropical forests across the world.
The computations found a 90% likelihood that there are between 2 and 7 million tropical arthropod species, but that this number is most likely around 3.7 million.
Using this data they then added on best estimates for other groups of animals to this total, including 50,000 vertebrates, 400,000 plants and possibly 1.3 million other organisms, mostly non-bacteria microorganisms.
Positive first step
Arthropod researcher Dieter Hochuli from the University of Sydney, admits that these findings will be hugely important to the field.
“It has really quantified the uncertainty,” he said. “It’s helped us to work out how many [species] we really ought to have.”
There’s an enormous amount of work we still have to do. There are a huge number of species out there and we don’t know what their names are or what they look like.”
According to the paper, there are still around 70% of arthropod species still remaining to be described.
Biodiversity even more valuable
“They’ve done the field a service by having a far more justifiable ball park. The number of species on our planet is such a simple question that every biologist should be able to answer, but that we’ve had trouble answering in the past.”
Hamilton says that millions of species, whether it be 5 or 30, are all really large and hard to conceptualise.
“I hope that people don’t take that as, ‘there’s not such much biodiversity on earth, so we shouldn’t care.’ Hopefully they will look at it the other way – ‘there’s not so many species out there, so we have to be careful about how we treat them.’”