The green thorns of Ulex europaeus play both defensive and photosynthetic roles: consequences for predictions of the enemy release hypothesis

A mechanistic framework of enemy release

“…Effect on host adaptation : Host adaptation caused by enemy release is a phenotypic‐ or genetic‐based response to reduced enemy impact or diversity (Inderjit et al., 2005; Medina‐Villar et al., 2021). How quickly host adaptation occurs will depend on the generation time of the host plant in question, the strength of selection (the degree to which enemy pressure is reduced, Figure 2) and other contexts that affect investment into growth and defence, such as resource availability (see Context ii below; Figure 3f).…”

“…one species has high defence and low growth, while another species has the opposite; Lind et al., 2013; Rotter & Holeski, 2018; Heckman et al., 2019) (but see Chauvin et al., 2018; Hinman et al., 2019), evidence for within‐species trade‐offs is weaker (Hahn et al., 2021; Heckman et al., 2019), yet this is more pertinent as host adaptation requires within‐species variation. However, specific plant organs can show trade‐offs related to defence and growth (Agrawal et al., 2012; Medina‐Villar et al., 2021), and there is evidence that some species can adaptively lower defence and increase growth in response to lower enemy pressure (Coverdale & Agrawal, 2022; Wolfe et al., 2004). Evolutionary changes in growth and defence because of enemy release should only benefit exotics that show strong growth–defence trade‐offs.…”