Two million years of competition prove that bigger is better

16 June 2017

Fossil shell covered in bryozoan colonies

An encrusted fossil shell showing competing bryozoan colonies © Lee Hsiang Liow et al., Proceedings of the Royal Society B, licensed under CC BY 4.0

A study of millions years of competition between a group of marine organisms shows that the bigger species regularly outcompete their smaller rivals for living space.

The research on the coral-like bryozoans examines direct evidence of competition between species, which is rarely recorded in the fossil record.

Museum palaeontologist Dr Paul Taylor, who worked on the study published in Ecology Letters, explains:

'The fossil record is a rich source of information about long-term evolutionary changes but seldom preserves biotic interactions, especially competition. Our study has traced competition through more than two million years of geological time.'

Competition as an evolutionary driver

One of the driving forces in evolution is competition between species for the same resources. For example, if two species are both after the same food source or living space, an evolutionary change may give one species an advantage over the other.

However, it is hard to chart such competitive interactions over evolutionary time, as few direct clues remain in the fossil record to show which species won out in day-to-day struggles.

Colonial concerns

One group of animals that does leave fossil evidence of such competition is the bryozoans.

These small, coral-like aquatic animals live in colonies and sieve food particles out of the water. Most species attach themselves to a solid surface, such as a rock or shell, where they remain fixed for the rest of their lives. They also reproduce asexually, so more members join the colony and it grows.

Each zooid - an individual member of the colony - is small, typically less than one millimetre long. But colonies can grow from one centimetre to over one metre in diameter, depending on species and conditions.

As a bryozoan colony grows along a surface, it might encounter another colony of the same or another species. The resulting competition for space can have three outcomes. Either one colony overgrows and reduces the size of the other, or both colonies overgrow different parts of each other - or there can be a standoff where the growth of both colonies is halted. 

Magnified images of bryozoan colonies

Different competitive outcomes among bryozoans. One colony grows over another (left), both colonies grow over each other (middle) and both colonies come to a standoff (right). © Lee Hsiang Liow et al., Proceedings of the Royal Society B, licensed under CC BY 4.0
 

Because bryozoans have a mineralised skeleton, their remains fossilise well. This means the fossil record contains evidence of direct competition for space between bryozoan species, and which one won by overgrowing the other.

2.3 million years of competition

The researchers examined bryozoans that grow on shells from sedimentary rocks in the North Island of New Zealand. They looked at a succession of fossil bryozoans from five different levels of rock dating back to 2.3 million years ago, as well as bryozoans found on shells living today in the sea nearby.

They then compared the results of almost 7,000 interactions between pairs of bryozoan colonies, to see which species overgrew the other.

The researchers showed that, across the 2.3 million year timespan, bryozoans that had larger zooids were more likely to grow over bryozoans with smaller zooids. So the species with the largest zooids generally won competitions with other species.

A shell covered in bryozoan colonies

Various different bryozoan colonies competing for space on the same shell surface © Lee Hsiang Liow et al., Proceedings of the Royal Society B, licensed under CC BY 4.0

Being bigger isn't everything

Despite bryozoans with larger zooids generally winning the battle over those with smaller zooids, the smaller species have survived and have not gone extinct. Neither has there been an evolutionary increase in size. Why is this?

According to Dr Taylor, though zooid size plays a part, it must be only one factor in the complex interactions between species.

'It is likely that multiple biological traits influence survival, for example the bryozoan's reproductive abilities, colony growth rate and resistance to predation. In the future we plan to study other fossilised traits, such as specialised zooids used for defence against tiny predators.'

  • By James McNish

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