General patterns in the ontogeny of plant defense: A temporally explicit view of plant-herbivore interactions

Background/Question/Methods

Spatial and temporal variation in plant-herbivore interactions underpin much of the complexity in the evolutionary and biogeographical patterns of plant defense. While considerable progress has been made via spatially explicit research, there remain gaps in our understanding about how herbivory and defense vary across time, and what these patterns mean for the evolutionary ecology of plants.  Sources of temporal variation in herbivory and defense can be both internal (i.e. plant ontogeny) and external (i.e. seasonality).  In recent years, considerable evidence has accumulated that plant defense traits vary tremendously as plants develop, showing both increases and decreases as plants progress through ontogenetic stages. Less attention has been paid to variation in plant defense traits during leaf ontogeny and across the growing season. We have quantified general patterns in several plant defense traits (secondary chemicals, trichomes and prickles, leaf toughness) using meta-analysis, separately testing variation during leaf development and during seasonality.  

Results/Conclusions

Strong patterns were detected during leaf ontogeny. Although classes of plant secondary chemicals differ, there is a general tendency for them to decrease during leaf ontogeny, making young leaves more toxic than fully expanded mature leaves. Trichome and prickle densities consistently decrease during leaf ontogeny due to dilution effects because they differentiate early during leaf development.  Leaf toughness varies in the opposite direction, increasing during leaf ontogeny.  In contrast, there is no general pattern across seasons due to considerable variation among secondary chemical classes and among species. In neither leaf ontogeny nor seasonality, did we detect correspondence between defense patterns and herbivory, suggesting that although herbivory may select for high levels of defense, particularly secondary compounds, in young leaves or early in the growing season, herbivores nonetheless prefer young foliage. Other traits, most likely leaf toughness in mature leaves, as well as nutrients which are more concentrated in young leaves, must play a stronger role in determining patterns of herbivory among leaf age classes.  This research highlights how a temporally explicit framework can provide new insights into plant defense and herbivory, and reveals how temporal patterns occur at different scales, at the levels of leaves, whole plants, and across seasons.