Devonian land plants caused ecological havoc (Credit: Walter Myers/SPL)
The BBC Earth Blog has a story about the Devonian mass extinction, which is one of the least understood of the great extinction events in Earth's history. In new research, Matt Algeo has proposed that vascular land plants were the cause. The vascular land plants evolved during the Devonian Period. Their roots penetrated the ground, breaking up rocks. This process would have released minerals and nutrients that eventually made it to the oceans. This could have led to algal blooms, which in turn led to "anoxic dead zones", as we see today with runoff from agriculture.
The Devonian period saw the rise of "vascular" land plants, a group that includes everything from trees to ferns and flowering plants. More primitive "non-vascular" plants, such as moss and lichen, had already evolved before the Devonian began.
The vascular plants had a powerful impact on the land. Their roots penetrated the ground deeply, breaking up rocks and helping to create soil.
That is when things began to go wrong for life in the seas.
By breaking down rocks, the plants released nutrients and minerals. These would have benefited the plants, but they also got washed into rivers and then out into the oceans.
These nutrients were food for microscopic algae in the oceans, says Algeo, and those algae began to multiply. The resulting blooms of algae would have stained vast areas of the sea green.The resulting "dead zones" can span many square kilometres
The algae were then broken down by bacteria, which used up oxygen in the process. "This can entirely deplete the water column of oxygen," says Algeo.
The end result was an "anoxic zone", a region of the ocean where there wasn't enough oxygen dissolved in the water for animals to breathe.
This happens in parts of the ocean today, for example when nutrients from farms run into the sea. The resulting "dead zones" can span many square kilometres.
In the Devonian, the dead zones would have spread over many thousands of years, gradually forcing animals into confined areas. If Algeo is right, the animals in the oceans then began to struggle, and ultimately died out: all thanks to the success of land plants.
