How fluctuating competition and phenotypic plasticity mediate species divergence

Area of Science:

• Ecology
• Evolutionary Biology
• Behavioral Ecology

Background:

• Resource competition is a key driver of species divergence, but direct causal evidence is limited.
• Spadefoot toads (Spea spp.) exhibit phenotypic plasticity, with tadpoles developing into omnivorous or carnivorous morphs based on resource availability.
• Spea multiplicata are outcompeted by Spea bombifrons for fairy shrimp, a key resource inducing the carnivorous morph.

Purpose of the Study:

• To investigate if character displacement in spadefoot toads reflects the intensity of resource competition.
• To determine the roles of phenotypic plasticity and genetic adaptation in mediating this divergence.

Main Methods:

• Coupled field observations with controlled laboratory experiments.
• Compared trophic characters of Spea multiplicata across natural ponds with varying frequencies of Spea bombifrons.
• Manipulated species frequencies in laboratory populations to assess competitive interactions and phenotypic responses.

Main Results:

• Spea multiplicata tadpoles became more omnivorous as the abundance of Spea bombifrons increased.
• In laboratory settings, increased relative abundance of one species led to divergence: Spea multiplicata became more omnivorous, while Spea bombifrons became more carnivorous.
• Phenotypic plasticity allowed Spea multiplicata to reduce carnivore development in the presence of S. bombifrons.
• Fine-scale genetic differentiation was observed, with local adaptation to S. bombifrons presence evident over short distances.

Conclusions:

• The degree of character displacement is a reliable indicator of interspecific competition intensity.
• Both phenotypic plasticity and fine-scale genetic differentiation are crucial mechanisms driving resource competition-induced species divergence.
• This study provides causal evidence linking resource competition to evolutionary divergence in spadefoot toads.