COS 287-3 - Adding insult to injury: Light competition and allelochemical weapons interact to facilitate grass invasion

Abstract: Biological invasions are a leading ecological issue of the 21st century because of their worldwide contributions to biodiversity loss and degradation of ecosystem services. Answering fundamental questions about the mechanisms facilitating the spread of successful invasives is key to understanding how to manage them moving forward. The success of non-native plants is often attributed to superior competitive ability or to their release from natural enemies that constrained them in their native range. Non-native plants can also successfully invade and establish in new areas by releasing allelochemical compounds into the environment that are toxic to the native flora. The interactive effects of allelopathy and competition remain underexplored despite evidence that both mechanisms often operate simultaneously. We evaluated the mechanism of invasion by Guinea grass (Megathyrsus maximus: Poaceae), a globally distributed tallgrass from tropical and subtropical Africa with known allelopathic effects associated with 2-hydroxyphenylacetic acid (2HPAA). We asked if allelopathy and light availability interact to give Guinea grass a competitive advantage during seedling establishment in Texas, a region where it was introduced and proceeded to establish significant monodominant populations. We used a fully factorial greenhouse experiment in which allelochemical concentrations and light availability treatments were based on empirical measurements of these variables taken at an invaded site in South Texas. Seedling recruitment and growth were assessed for three native species representing different phylogenetic clades with distinct functional properties, and for Guinea grass itself. We also described the metabolome of an invasive grass for the first time to facilitate a comparison of the effect of the known allelochemical 2HPAA with that of the whole-plant chemical extract. Shading and allelochemistry each reduced recruitment and growth by themselves, and a significant interaction of these stressors exacerbated the negative effects in the shade, resulting in short plants, low biomass, and ultimately decreased seedling recruitment. The whole-plant metabolomic extract had significantly stronger effects than pure 2HPAA, and these negative effects were intensified in the shade. The invasion mechanism showcased here demonstrated that resource competition and biochemical interference interacted to facilitate the spread of a globally distributed invasive species. These findings advance our understanding of how natural enemy release, increased competitive ability, and chemical weapons influence grass invasion dynamics in North America. We end this presentation with a discussion about how these conclusions inform the emerging Guinea grass biocontrol program that seeks to leverage coevolved specialist herbivores from its native range.