The life of Drosera brevifolia is often short. The USA’s smallest sundew, it grows, sets seed and dies in the summer heat. It’s a carnivorous plant that struggles to compete with other plants when they move in, so the seeds are constantly in search of fresh ground. A study in Plant Ecology by James Krupa and colleagues finds a surprising source of help, the Bluegrass crayfish (Cambarus batchi). By examining how the crayfish can be an ecosystem engineer, the botanists find other potential lines of research into the life cycles of carnivorous plants.
The reason some plants, like sundews, have turned to a predatory lifestyle is that they are seeking nutrients. Carnivory isn’t the simplest way to get these chemicals, but it works when the soil lacks these nutrients. When other plants move into a patch, a carnivorous plant has a problem because plants drop litter. Unfortunately for carnivorous plants, other plants benefit from this litter. The remains of leaves, or broken petals, contain phosphorus and nitrogen that returns to the soil when they break down.
D. brevifolia has a further problem. It’s a small plant. When bigger neighbours arrive, they can shade out the sundew. So D. brevifolia populations are forever in search of fresh soil.
The Bluegrass crayfish is not in search of fresh soil. It’s in search of water, as it’s an aquatic animal. Soil is in the way when they make a burrow down to the water table to breathe. It’s a lot of earth that the crayfish does not need. They also burrow up, producing chimneys. These are stacks of mud excavated from the burrow that form a tube around the entrance. They’re prone to collapse, leaving a mound of bare nutrient-poor soil for plants to colonise.
Krupa and colleagues wanted to see if the crayfish’s work was crucial in maintaining a population of D. brevifolia at Hazeldell Meadow. This location is just one of two places in Kentucky where you can find D. brevifolia, and you can also find bluegrass crayfish. The botanists examined the site to find crayfish chimneys and watched how they developed over sixteen months. They also scraped some bare soil themselves to see if the sundews could colonise those patches. They also examined the chemical content of the chimneys.
They found that as the chimneys eroded, they doubled in diameter – meaning the area of bare soil increased fourfold. The fastest plants colonised these patches, and these were often sundews. They also found the crayfish helped in another way.
When the crayfish brought up soil for their chimneys, they brought up soil with little nutrient value. So not only did they provide fresh soil, they provided soil that the sundew’s competitors struggled to use. This earth provided a refuge for what the authors describe as a ‘fugitive species’, forever escaping their competitors.
But how do they escape? The plants can’t move as adults, but they are mobile as seeds.
“Dwarf sundews in Hazeldell Meadow appear to have a seed bank,” write Krupa and colleagues. “Often after extended hot, dry summer periods when most sundews die, regular autumn rainfall results in water-soaked soil and standing water which leads to extensive seed germination with thousands of young sundews on bare soil including crayfish castles and mounds. How sundew seeds disperse to the bare soil remains a mystery. The seeds are extremely small. It is possible that they disperse by wind, flowing water, being hit by raindrops, or attaching to the legs of small invertebrates. Despite our lack of information on the seed bank and dispersal, it is clear the dwarf sundew can rapidly colonize newly formed bare soil.”
The need for bare soil makes fire a popular trigger for new plant growth, but the botanists argue that researchers should study small-scale animal disturbance. They add that no one has looked at large-scale disturbance by extinct megafauna. They suggest a couple of future directions for research. One is to examine the effect of crayfish elsewhere. They are found in many more states than Kentucky.
Another question is, “what effect did bison have?” Bison ‘wallow’. This is rolling around on dry soil as a dust bath. It follows that a good bison wallow will expose a lot of dry, dusty dirt, and these wallows would have been found in many places in North America. Krupa and colleagues suggest creating artificial wallows to see if carnivorous plants can seize the soil before their competitors.
While the mega-herds of bison may be no more, this research shows that the small crayfish is still an important engineer. Its work means there’s always somewhere a sundew population can move to, to escape other plants.
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