As part of nature's evolutionary arms race, animals have evolved an arsenal of different defense mechanisms, including chemical weapons, such as poisons or irritants, camouflage, warning coloration and mimicry. The ways these mechanisms deter predators have been well studied, but little is known about how they might impact evolutionary processes such as speciation and extinction.
In a large-scale empirical test in amphibians that use toxins to protect themselves against predators, scientists at the University of Liverpool examined how rates of extinction and speciation varied across different defensive traits in amphibians. They found that animals that use chemical defense show higher rates of speciation, but also higher rates of extinction, compared to those without, leading to a net reduction in species diversification (the interplay of speciation and extinction). In contrast, the use of warning coloration and mimicry was associated with higher rates of speciation, but unchanged rates of extinction.
There are a number of plausible reasons why the use of chemical defense might lead to higher extinction rates. For example, it could be that there is trade off which leaves prey vulnerable to other kinds of enemies, such as infectious diseases, but we don't yet understand what drives the relationship.
The results of the study are only partially consistent with a long-held hypothesis, called 'escape-and-radiate', which predicts that effective defenses let prey escape from predators and diversify into many different species in the process.
We've shown that this hypothesis, which is widely cited and used, requires some revision because of its failure to account for the effects of extinction rates. We propose that 'escape-and-radiate' should be seen as just one part of a more general hypothesis for the macroevolutionary effects of antipredator defense that includes both speciation and extinction. In addition, our findings could help support the conservation of endangered species by allowing some predictability of extinction risk from knowledge of antipredator defenses. Amphibians are a key example of this as they have suffered population declines worldwide, including many of the iconic poison-dart frogs.