Fossilized battlegrounds reveal that bigger really is better
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In a fierce competition for space, the big guys win. That is the conclusion of a new study investigating fossil battlegrounds of species of tiny marine creatures called bryozoans. These encrusting animals grow in colonies on surfaces such as rocks, shells and kelp, and are found in oceans worldwide. When two such animals grow towards each other, the stronger one can overgrow and smother the other (see image). The effect of such competition, well-known in studies of contemporary organisms, is poorer survival and lower reproductive levels. But does competition change over evolutionary time; say in the course of hundreds of thousands or even millions of years? And what traits are most efficient in the fight for space? By analyzing bryozoan battles captured in fossil beds of New Zealand spanning 2 million years, Lee Hsiang Liow (University of Oslo) and colleagues were able to address such questions. The research team detected that bryozoans with larger building blocks (zooids) tend to overgrow smaller ones consistently over evolutionary time. Also, the general ability for one species to compete seems to be stable over time: a good competitor 2 million years ago is still a good competitor today. But despite losing battles, poor competitors still thrive in the oceans, hinting that competition isn’t everything.
Abstract from the paper:
Competition is an important biotic interaction that influences survival and reproduction. While competition on ecological timescales has received great attention, little is known about competition on evolutionary timescales. Do competitive abilities change over hundreds of thousands to millions of years? Can we predict competitive outcomes using phenotypic traits? How much do traits that confer competitive advantage and competitive outcomes change? Here we show, using communities of encrusting marine bryozoans spanning more than 2 million years, that size is a significant determinant of overgrowth outcomes: colonies with larger zooids tend to overgrow colonies with smaller zooids. We also detected temporally coordinated changes in average zooid sizes, suggesting that different species responded to a common external driver. Although species-specific average zooid sizes change over evolutionary timescales, species-specific competitive abilities seem relatively stable, suggesting that traits other than zooid size also control overgrowth outcomes and/or that evolutionary constraints are involved.