Bottom-up vs. top-down effects on terrestrial insect herbivores: a meta-analysis

Area of Science:

• Ecology
• Evolutionary Biology
• Entomology

Background:

• Primary consumers face dual pressures from resource availability (bottom-up) and predation/parasitism (top-down).
• The relative strength and interplay of these ecological forces on consumer fitness remain largely unquantified.
• Understanding these dynamics is crucial for predicting population dynamics and community structure.

Purpose of the Study:

• To quantitatively compare the strength of top-down versus bottom-up forces on primary consumer fitness.
• To investigate how factors like diet breadth, feeding guild, and environment modulate these selective pressures.
• To specifically assess these forces within the diverse group of herbivorous insects.

Main Methods:

• Conducted a meta-analysis synthesizing data from existing studies on herbivorous insects.
• Compared the magnitude of top-down and bottom-up effects on insect fitness metrics.
• Analyzed the influence of various ecological moderators including diet, feeding strategy, and habitat.

Main Results:

• Top-down forces were found to be generally stronger than bottom-up forces on insect herbivores.
• This pattern held true across different feeding guilds (chewing, sucking, gall-making) and environments (natural and cultivated).
• Parasitoids and predators exerted similarly strong top-down control on insect herbivore populations.

Conclusions:

• Natural enemies exert a surprisingly potent influence on herbivorous insect populations.
• A tri-trophic perspective, including predators and parasitoids, is essential for studying insect-plant interactions.
• Further research should expand to understudied consumer groups, terrestrial systems, and top-down controls like pathogens.