Bigger is better, according to neighbour disputes written in stone
Two million years is a mere moment in the history of life, yet this is the longest time span over which scientists have succeeded in studying competition among species.
Lee Hsiang Liow is an Associate Professor at the Museum of Natural History and CEES (Centre for Ecological and Evolutionary Synthesis) at the Department of Biosciences, University of Oslo. She and her colleagues want to find out how competition influences evolution.
They hence needed a system where signs of competition is visible, even a long time after death.They found an ideal system in an obscure, but commonly occurring group of organisms, the bryozoans, also known as moss animals.
Bryozoans are a group of small, colonial, mostly sessile animals. It is an ancient group of which there exists a lot of fossils. After a brief phase as swimming larva, they fasten themselves to a solid surface and stay there for the rest of their lives.
Most living bryozoans build a layer of chalk (calcium carbonate) around themselves for protection. In death, this can become fossilized. You might encounter them as crusting seaweeds washed up on the shore, but some bryozoans also thrive at much deeper depths.
When the bryozoans have attached themselves, the fight for space and food begins in earnest. The neighbour has now become the archenemy, and it’s all about gaining most space as quickly as possible. More than two million years later, Lee Hsiang Liow and her colleagues can trace the bryozoans' neighbour disputes in New Zealand.
– When moving north-east on the North Island along the Wanganui Basin, the rock layers become older and older. It’s just like travelling backwards in time. This makes it possible to study the bryozoans over a longer time span, specifically a little more than two million years, explains Liow.
Big data science
Along the shores of New Zealand, Liow has picked 5000 shells from which her team has documented more than 15 000 interactions among individual colonies, but she plan on collecting more data.
– This is a rather species-rich group with 80-100 species. So if we want to compare each species and their competition with all the other different species in the system, we need a lot of data. In March this year (2017), we collected 2-3 times more shells than last time (january 2014). The geologists who supported our field world in New Zealand thought we were on the brink of insanity, she smiles.
Their motive for this work is an old hypothesis which has shown hard to prove. It is known as the Red Queen Hypothesis – named after the queen in Alice in Wonderland.
Survival of the biggest
The Red Queen Hypothesis proposes that species change in order to keep up with their enemies, which are also contantly evolving. It’s like an arms race where both parties are equally innovative and brisk at developing a new defence or a new weapon.
– Testing the Red Queen Hypothesis empirically has proven difficult because there are no time series of ecological interactions longer than a few decades.
Their work on the bryozoans is crucial for the understanding of competition, but a lot of work remains.
– We still need to understand population dynamics, meaning the growth and decline of populations of bryozoans, and extinction rates, meaning how fast a species disappear.
The researchers have already been surprised.
- What surprised us, was how stable the interactions seem to be for most of the species – over two million years, that's much longer than Homo sapiens have existed! We were also surprised that one single trait was quite good at predicting who will win - and that is size.
Strategies towards the neighbour
The results of their examination show that species with the biggest Zooids (think of a zooid as a bedsit in an apartment building) have the best chance of winning the competition with their neighbours. In other words: Bigger is better.
– But why haven't the Zooids grown monstrously big?
– Our data from commonly occurring species in the Wanganui Basin show that zooid size of a given species varied over time, but there are hard limits to how large they can get. They have no circulatory system like we do, so they rely on diffusion for oxygen, so there are physical limits to how large each zooid can get, explains Liow.
They are all as big as they can be while they still benefit from it, but different species have chosen slightly different evolutionary strategies.
For instance, a species may have colonies that grow quickly, reproduce rapidly, but that are outcompeted by another species that reproduces at a much lower rate, but that protect their resources by being a good competitor and survivor. Yet both species can do well in the long run.
– Size is not all that matters in life. One shall also have children, and they must survive. It probably helps to do things slightly different from your neighbour.
The bryozoans can be compared to two neighbours who both own a Ferrari. One car is the biggest, but the other one is the fastest. Who owns the best car? Both neighbours have focused on different advantages to beat the other. Just as the bryozoans do.
Liow, L. H., Di Martino, E., Krzeminska, M., Ramsfjell, M., Rust, S., Taylor, P. D. and Voje, K. L. (2017), Relative size predicts competitive outcome through 2 million years. Ecol Lett. doi:10.1111/ele.12795
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