Open access publications in the SLU publication database

Abstract

Plant traits can mediate the interactions between plant feeding trophic omnivores and
their herbivore prey via density effects and by altering the omnivore’s trophic behavior
(plant vs. prey feeding). These bottom-up effects can be important for our mechanistic
understanding of omnivory as a stabilizing feature of food-webs, but can also be
applied in management for conservation biological control.

This thesis investigates how plant nutrient status influence heteropteran omnivore
population dynamics and trophic behavior and explores management solutions for
conservation biological control that can reduce the risk of leaf beetle outbreaks in
willow short rotation coppice. The results provide novel empirical support for the
established assumption that plant feeding can decouple omnivores from fluctuations in
their prey populations. Plant feeding stabilizes omnivore population dynamics, which
may explain why omnivore populations show no numeric response to fluctuations in
leaf beetle population densities. The potentially strong omnivore-plant coupling
suggests that omnivores can function effectively at low prey densities (contrary to
specialist predators) to provide what has been referred to as ‘background level’ control
of insect pests.

The applied part of the thesis demonstrates that retaining willow refuges to reduce
omnivore mortality and stabilize population densities across harvests increase rather
than decrease the risk of leaf beetle outbreaks. The results also reveal that willow
stands surrounded by landscapes with high proportion open land cover are less likely to
experience leaf beetle outbreaks. This outcome was expected partly because of the
recorded high and stable densities of heteropteran omnivores on high nutrient status
host plants in agriculture dominated landscapes. In addition, the results illustrate that
landscape-moderated recolonization after disturbance can change over time and that
considering the temporal dynamics of populations may be crucial when designing and
evaluating studies at landscape level. In conclusion, this thesis highlights the
importance of basic ecological knowledge of predator trophic behavior for developing
successful conservation biological control.