Biologists have long considered the origins and continued coexistence of the immense diversity of species found in our environment. How can we explain the fact that no single species predominates?
A generally accepted hypothesis is that there are trade-offs, which means that no organism can do best in all conditions.
“Every trait of a species have advantages and disadvantages. They can be good at one thing and then they will be worse at something else. This is what generates biodiversity – by organismal traits having advantages and disadvantages – or trade-offs.”
This is explained by Professor Thomas Kiørboe, DTU Aqua. He has just published a new large study in the scientific journal, PNAS. The study looked into a classic trade-off that is commonly assumed; that between gleaner organisms —which are able to acquire and consume more food than other species when resources are scarce— and exploiters, which rapidly consume large quantities of the same resources when they are in abundance.
“It has been a fundamental principle in ecology, and lots of models are based on that assumption. But surprisingly, no one has ever examined this with data “, says Thomas Kiørboe.
Same trend for more than 500 species
Together with a colleague from the Université de Genève (UNIGE), he analyzed data from more than 500 widely different species, both aquatic and terrestrial, and from single-celled organisms and water fleas to insects, birds and mammals. Thomas Kiørboe and the Center for Ocean Life have data for organisms in the sea, and they had access to an external database with a large number of other organisms – both terrestrial and aquatic.
They found that it was not a question of either/or. Rather both / and. The organisms that are efficient when food is abundant, are also best when food resources are scarce.
“One would expect a negative correlation – that is, if they are good at one thing, then they are bad at the other, and the other way around. However, we found the opposite – a positive correlation. The better they are at one thing – the better they are at the other. That is inconsistent with a very fundamental assumption in ecology. This means that some of the things that have been modelled on the basis of that assumption must be revised,” says Thomas Kiørboe.
To eat or to be eaten
So biodiversity cannot be explained as a trade-off between gleaners and exploiters. However, the researchers’ interpretation does not call the concept of trade-offs into question.
“Without trade-offs, it is very hard to maintain diversity. Our research does not explain biodiversity, but it does overturn an existing theory about precisely why we have biodiversity,” says Mridul Thomas, senior research in the Department F.-A. Forel for Environmental and Aquatic Sciences in UNIGE’s Faculty of Sciences and the study’s second author.
Instead, the idea of risk-taking to obtain food needs to be considered, as explained in the PNAS publication. Thomas Kiørboe explains this other trade-off:
“A balance of risk-taking by seeking food is more likely and would be in line with our results. An organism that is very active in its search for food gets more to eat, but it also runs a greater risk, and vice versa. Organisms are distributed differently on this food search / risk gradient, and this explains the pattern we see. Our study suggests that the food search / risk balance trade-off is very fundamental and applies across species, and that it is a significant driving force of biological diversity,” says Thomas Kiørboe.
The Danish-Swiss study fundamentally changes an important idea about an important source of biodiversity. This idea is taught at universities and has been taken for granted. The study provides significant new knowledge that plays a role in our understanding of ecosystems.
“The models of marine ecosystems we develop at the Center for Ocean Life have the organisms’ (unconscious) balancing of gain and risk in foraging as a key element. Our study has reinforced this basic assumption. But there are many theoretical models and considerations that need to be revised because they are based on a wrong assumption,” says Thomas Kiørboe.