A new study from the University of California, Santa Cruz suggests that the emergence of birds 150 million years ago played a key role in shrinking the average body size of insects. Emergent birds proved to be superior airborne hunters, competing with large insects and forcing them to become smaller, or die out.
Research has found a link between atmospheric oxygen levels and insect size. Oxygen concentration acts as a control for body size, with high-oxygen conditions necessary to sustain larger insects, which have higher metabolic demands.
The oxygen-rich atmosphere 300 million years ago was capable of supporting large insects with up to 70 cm-long wingspans. Researchers Matthew Clapham and Jered Karr used insect fossil records to show the link between oxygen levels and insect size over hundreds of millions of years.
The pattern falls apart at the end of the Jurassic period, 150 million years ago, when atmospheric oxygen levels increased while insect size decreased. This period coincided with the evolution of birds, which has researchers hypothesize were the evolutionary pressure that forced insects to adopt a uniformly smaller size.
Predatory birds forced insects to adapt, favouring smaller, more manoeuvrable bodies capable of evading the agile birds, and thus biotic factors superseded oxygen as the key force in insect evolution.
Source: Science Daily