Host-enemy interactions provide limited biotic resistance for a range-expanding species via reduced apparent competition

Aim: As species’ ranges shift poleward in response to anthropogenic
change, they may lose antagonistic interactions if they move into less
diverse communities, fail to interact with novel populations or species
effectively, or if ancestral interacting populations or species fail to
shift synchronously. We leveraged a poleward range expansion in a
tractable insect host-enemy community to uncover mechanisms by which
altered antagonistic interactions between native and recipient communities
contributed to “high niche opportunities” (limited biotic resistance) for
a range-expanding insect. Location: North America, Pacific Northwest
Methods: We created quantitative insect host-enemy interaction networks by
sampling oak gall wasps on 400 trees of a dominant oak species in the
native and expanded range of a range-expanding gall wasp species. We
compared host-enemy network structure between regions. We measured traits
(phenology, morphology) of galls and interacting parasitoids, predicting
greater trait divergence in the expanded range. We measured function
relating to host control and explored if altered interactions and traits
contributed to reduced function or biotic resistance. Results: Interaction
networks had fewer species in the expanded range and lower complementarity
of parasitoid assemblages among host species. While networks were more
generalized, interactions with the range-expanding species were more
specialized in the expanded range. Specialist enemies effectively tracked
the range-expanding host, and there was reduced apparent competition with
co-occurring hosts by shared generalist enemies. Phenological divergence
of enemy assemblages interacting with the range-expanding and co-occurring
hosts was greater in the expanded range, potentially contributing to weak
apparent competition. Biotic resistance was lower in the expanded range,
where fewer parasitoids emerged from galls of the range-expanding host.
Main conclusions: Changes in interactions with generalist enemies created
high niche opportunities, and limited biotic resistance, suggesting weak
apparent competition may be a mechanism of enemy release for
range-expanding insects embedded within generalist enemy networks.