Well-timed meals: how the impact of herbivory on plants changes depending on when it takes place

Herbivory impacts plants in a variety of ways, including in how likely they are to be reproductively successful. However, the exact effects of herbivory on plant reproduction are variable and dependent on combinations of other factors that may also influence reproductive success. One such factor is when the herbivory actually occurs, which is less understood in terms of how it impacts plant productivity and eventual reproductive success compared to other factors. Even less is known about how this works in real environmental settings, as many studies use artificial defoliation to mimic herbivory. To fill in these gaps in our knowledge, Quiroz-Pacheco and colleagues in Mexico investigate how timing of herbivory influences reproductive traits in populations of the tropical dry forest tree Casearia nitida in a recent paper in Annals of Botany.

Quiroz-Pacheco and colleagues surveyed Casearia nitida trees three times over two consecutive years. In each survey, they quantified the degree of foliar herbivory that had occurred naturally rather than artificially mimicking herbivory themselves. They also quantified across the experimental time period various male and female reproductive traits. Quiroz-Pacheco and colleagues first find that herbivory negatively impacts different measurements of female reproductive fitness in the next season to differing degrees, but negatively impacts inflorescence number the most (i.e. the number of complete flower-bearing structures rather than the number of individual flowers). The authors proffer an explanation for this – Casearia nitida is a proleptic species. This means that it holds meristems in a dormant state, rather than them instantly developing into organs. High foliar damage during the previous season may therefore reduce the resources available to activation of dormant meristems at the start of the next season, reducing inflorescence number.

Quiroz-Pacheco and colleagues also find that plants highly damaged by herbivory in the first year of measurement received greater damage in the subsequent year. This supports previous studies in Casearia nitida finding that herbivory damage reduces the amount of plant defence compounds, pre-disposing plants to greater attack the next season. This is reinforced when we consider that the first developing leaves of a season in Casearia nitida trees are the most attractive to herbivores, and so are the most defended. So damage to herbivory in the previous season, and suppression of defence mechanisms into the next season, is double- trouble as the first leaves of the next season are the most attractive to herbivores.

Therefore, both herbivory prior to and during plant reproduction can affect plant reproductive traits in varying ways. This highlights that studies must account for both the past and present herbivory conditions when assessing its impact on plant reproductive traits, and possibly on other plant traits too. As Quiroz-Pacheco and colleagues conclude: ‘This kind evidence calls for attention and highlights the relevance of visualizing the relationships among different reproductive variables to understand how they are influenced by the seasonality of herbivore damage’.