A short-lived semiarid plant allocates resources provided by detritivores to improve seed quality over quantity, regardless of florivory

Florivory poses one of the greatest threats to the sexual reproductive success of plants, having both quantitative and qualitative effects by reducing seed yield and seed quality. Plants can resist herbivory, tolerate damage, or employ a mix of these strategies to mitigate fitness costs. Organisms such as detritivores can increase nutrient availability in the rhizosphere, although it remains uncertain how plants invest these additional resources in contexts with and without herbivory, which may be particularly relevant in resource-limited ecosystems. In this study, we investigated the combined effects of detritivore beetle larvae and specialist florivorous pierid caterpillars on a semiarid Brassicaceae herb in a field setting, manipulating the abundance of both detritivores and floral herbivores across a full factorial density gradient. Our findings revealed that, despite promoting tolerance (surplus production of reproductive tissue) and increasing resistance (increased glucosinolate production), a high density of floral herbivores negatively impacted seed production. However, this mixed plant response led to a non-linear relationship between herbivore density and damage, demonstrating its effectiveness against herbivory. Increasing detritivore density did not affect seed production, nor did it enable plants to better resist or compensate for florivore damage. Instead, the high density of detritivores had a transgenerational effect by increasing the emergence rate of seedlings, thereby favouring seed quality over quantity, which may be an adjustment of the reproductive strategy that prioritises seedling establishment in harsh and unpredictable environments at the cost of fewer offspring. This finding underscores the necessity of accounting for transgenerational effects in plant defence theory.