Breaking and remaking a seed and seed predator interaction in the introduced range of Scotch Broom (Cytisus scoparius) in New Zealand

Abstract

Summary Introduced plants may initially experience enemy release, but some of those interactions may be reinstated through biological control. These cases provide opportunities to explore the dynamics of broken and remade consumer–resource interactions. The European shrub broom (Cytisus scoparius) was introduced to New Zealand without a specialist seed predator (Bruchidius villosus) until a biological control programme reinstated this interaction in 1988. Broom produces substantially larger seeds throughout its non‐native range, and there are differences in seedling establishment beneath broom canopies between the native and introduced ranges. We hypothesized that large broom seeds produce seedlings with a survival advantage in shaded conditions, consistent with establishment under broom canopies in the introduced range. We also predicted that the seed‐feeding beetle would experience increased fitness in larger seeds. We found that seedling survival was correlated with seed weight in shaded conditions, consistent with the observed establishment of seedlings beneath broom stands in the non‐native range. Beetle size was positively correlated with seed weight, and female fecundity increased with body size. Comparison of the size distributions of beetle populations before and after hibernation provided indirect evidence that overwintering survival of beetles is also size‐dependent. These findings are consistent with observed higher levels of abundance of B. villosus in New Zealand, compared to native habitats. Selection gradients for seed size‐dependent seedling survival on broom occupancy were explored using the Rees–Paynter population model, demonstrating that a fitness benefit to the plant of large seed size through enhanced seedling survival beneath the parental canopy is only likely under low‐disturbance scenarios and moderate fecundity levels. Synthesis. The dynamics of biocontrol systems provide an underexploited opportunity to examine the impact of removal and reinstatement of species interactions. Differences in seed size between native and non‐native ranges may result from varying intensities of conflicting selection pressures acting on seed size. It remains to be seen whether a decline in seed size towards that seen in the native range will result from the biocontrol agent introduction, as we have demonstrated that selection pressures on seed size vary with disturbance‐mediated population turnover and fecundity.