Fatin Chowdhury ’20

In a recent paper, scientists from the University of Irvine examined the resistance of shrub plants to herbivory, as enabled by plant morphology or the presence of avian species. The scientists studied both direct and indirect defenses in nature and assessed these mechanisms in the context of energetic trade-offs. Characteristics that were already known to directly lower the success rate of herbivory included chemical defenses, while indirect defenses had already been observed to increase the attraction of herbivore predators to plant matter. These two types of defenses were considered to possibly be redundant, with trade-offs existing between them. The researchers’ goal was to detail the presence and nature of such tradeoffs. 

The researchers broadly predicted a negative relationship between direct and indirect plant defenses. A negative correlation between morphological complexity and indirect defenses was further predicted, and it was also postulated that there was a positive correlation between morphological complexity and direct defenses. Nine shrub species from a coastal scrub ecosystem were examined in terms of bird-facilitated defense and physical traits. Defenses were examined via the separation of birds and plant matter using a mesh netting, with the subsequent quantification of arthropod population size and qualification of arthropod trophic level.

Upon bird exclusion, resistance to herbivores fluctuated, which ascertained that bird presence was an example of indirect defense. Bioassays were also completed in order to observe relationships between the quality of plant matter and their predator resistance. The results confirmed that birds supplied indirect defense for the plants by lessening herbivore levels, although the amount of defense varied amongst shrub species. Furthermore, greater morphological complexity was positively related to the proportion of predators to herbivores present. In addition, tradeoffs between resistance methods were identified: high morphological complexity, an example of direct resistance against predators, was observed to be accompanied by a lesser amount of avian-powered indirect plant defense. 

Through the relating of plant morphological complexity with direct and indirect defenses, specific tradeoffs existing within the sphere of herbivory related predator-prey dynamics have been identified. This knowledge may be applied to fields such as agriculture in order to, for example, shield crops from predation through the utilization of natural methods, and may similarly be used in order to engage in more effective conservation efforts. 

Sources:

1. C.S, Nell, et al., Plant structural complexity mediates trade-off in direct and indirect plant defense by birds. Ecology 100, e02853 (2019). doi: 10.1002/ecy.2853.

2. Image retrieved from:  https://unsplash.com/photos/voMFq_EOscA