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Although parasites represent a substantial part of marine communities' biomass and diversity, their influence on ecosystem functioning, especially via the modification of host behaviour, remains largely unknown. Here, we explored the effects of the bopyrid ectoparasite Gyge branchialis on the engineering activities of the thalassinid crustacean Upogebia pusilla and the cascading effects on intertidal ecosystem processes (e.g. sediment bioturbation) and functions (e.g. nutrient regeneration).Laboratory experiments revealed that the overall activity level of parasitized mud shrimp is reduced by a factor 3.3 due to a decrease in time allocated to burrowing and ventilating activities (by factors 1.9 and 2.9, respectively).Decrease in activity level led to strong reductions of bioturbation rates and biogeochemical fluxes at the sediment–water interface.Given the world‐wide distribution of mud shrimp and their key role in biogeochemical processes, parasite‐mediated alteration of their engineering behaviour has undoubtedly broad ecological impacts on marine coastal systems functioning.Our results illustrate further the need to consider host–parasite interactions (including trait‐mediated indirect effects) when assessing the contribution of species to ecosystem properties, functions and services. [ABSTRACT FROM AUTHOR]

Theory suggests that non‐trophic interactions can be a major mechanism behind community stability and persistence, but community‐level empirical data are scarce, particularly for effects on species interactions mediated through changes in the physical environment.Here, we explored how ecosystem engineering effects can feed back to the engineer, not only modulating the engineer's population density (node modulation) but also affecting its interactions with other species (link modulation).Gall induction can be viewed as ecosystem engineering since galls serve as habitat for other species. In a community‐level field experiment, we generated treatments with reduced or elevated ecosystem engineering by removing or adding post‐emergence galls to different plots of their host plant in the Brazilian Cerrado. We tested the effect of post‐emergence galls on the galler, as well as on the galler–parasitoid and galler—aphid interactions.The manipulation of post‐emergence galls had little effect on the galler—abundance and survivorship were not affected, and gall volume changed only slightly—but modified interactions involving the galler, parasitoid wasps and inquiline aphids. Aphid inquilines negatively affected density‐dependent parasitism rates (interaction modification) likely by killing parasitised galling larvae. Post‐emergence galls interfered with aphid inquilinism—likely by the provision of alternative habitat for aphids—and thus interfered with the negative effect of aphids on parasitism (modification of an interaction modification).This work is one of the few studies to demonstrate experimentally the role played by environment‐mediated interaction modification at a community level in the field. Moreover, by manipulating a species' ecosystem engineering effect (post‐emergence galls) instead of the species itself, we demonstrate the novel result that populations can be regulated by non‐trophic effects initiated by their own activities that alter their interaction with other species. This reveals that indirect interactions mediated via the environment offer new pathways of feedback loops for population regulation. Our results indicate that interaction modification has the potential to be a key regulatory mechanism underlying interaction variation in nature, and play a major role in community structure, dynamics and stability. [ABSTRACT FROM AUTHOR]

Studies on the robustness of ecological communities suggest that the loss or reduction in abundance of individual species can lead to secondary and cascading extinctions. However, most such studies have been simulation-based analyses of the effect of primary extinction on food web structure., In a field experiment we tested the direct and indirect effects of reducing the abundance of a common species, focusing on the diverse and self-contained assemblage of arthropods associated with an abundant Brazilian shrub, Baccharis dracunculifolia D.C. ( Asteraceae)., Over a 5-month period we experimentally reduced the abundance of Baccharopelma dracunculifoliae ( Sternorrhyncha: Psyllidae), the commonest galling species associated with B. dracunculifolia, in 15 replicate plots paired with 15 control plots. We investigated direct effects of the manipulation on parasitoids attacking B. dracunculifoliae, as well as indirect effects (mediated via a third species or through the environment) on 10 other galler species and 50 associated parasitoid species., The experimental manipulation significantly increased parasitism on B. dracunculifoliae in the treatment plots, but did not significantly alter either the species richness or abundance of other galler species. Compared to control plots, food webs in manipulated plots had significantly lower values of weighted connectance, interaction evenness and robustness (measured as simulated tolerance to secondary extinction), even when B. dracunculifoliae was excluded from calculations., Parasitoid species were almost entirely specialized to individual galler species, so the observed effects of the manipulation on food web structure could not have propagated via the documented trophic links. Instead, they must have spread either through trophic links not included in the webs (e.g. shared predators) or non-trophically (e.g. through changes in habitat availability). Our results highlight that the inclusion of both trophic and non-trophic direct and indirect interactions is essential to understand the structure and dynamics of even apparently discrete ecological communities. [ABSTRACT FROM AUTHOR]