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Past and potential future effects of habitat fragmentation on structure and stability of plant–pollinator and host–parasitoid networks

Abstract

Habitat fragmentation is a primary threat to biodiversity, but how it affects the structure and stability of ecological networks is poorly understood. Here, we studied plant–pollinator and host–parasitoid networks on 32 calcareous grassland fragments covering a size gradient of several orders of magnitude and with amounts of additional habitat availability in the surrounding landscape that varied independent of fragment size. We find that additive and interactive effects of habitat fragmentation at local (fragment size) and landscape scales (1,750 m radius) directly shape species communities by altering the number of interacting species and, indirectly, their body size composition. These, in turn, affect plant–pollinator, but not host–parasitoid, network structure: the nestedness and modularity of plant–pollinator networks increase with pollinator body size. Moreover, pollinator richness increases modularity. In contrast, the modularity of host–parasitoid networks decreases with host richness, whereas neither parasitoid richness nor body size affects network structure. Simulating species coextinctions also reveals that the structure–stability relationship depends on species’ sensitivity to coextinctions and their capacity for adaptive partner switches, which differ between mutualistic and antagonistic interaction partners. While plant–pollinator communities may cope with future habitat fragmentation by responding to species loss with opportunistic partner switches, past effects of fragmentation on the current structure of host–parasitoid networks may strongly affect their robustness to coextinctions under future habitat fragmentation.

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Acknowledgements

We thank J. Albrecht for helpful comments, and E. Topp and K. Udy for linguistic revision. I.G. and T.T. acknowledge support from DFG Research Training Group 1644 ‘Scaling Problems in Statistics’. Field work was funded by the European Union Framework Programme 6 Integrated Project ALARM (Assessing LArge scale environmental Risks for biodiversity with tested Methods; Pollinator Module GOCECT-2003-506675).

Author information

I.G., B.J., I.S.-D., T.T. and F.J. conceived the study. I.S.-D. obtained the funding and designed the field study. B.J. and F.J. conducted the field work and compiled the data. I.G. analysed the data and prepared the manuscript. All authors discussed the results and contributed to revisions of the manuscript.

Correspondence to Ingo Grass.

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Supplementary Information

Supplementary Tables 1–3, Supplementary Figures 1–8

Reporting Summary

Supplementary Data 1

Order of plants and host species in extinction sequences used for the coextinction

Supplementary Data 2

Site data and network metrics for the 32 plant-pollinator and 32 host-parasitoid networks

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Further reading

Fig. 1: Map of the study area, and examples of study landscapes and associated plant–pollinator and host–parasitoid interaction networks.
Fig. 2: SEMs of the effects of habitat fragmentation on the structure and stability of plant–pollinator and host–parasitoid interaction networks.
Fig. 3: Interactive effects of fragment size and the proportion of additional semi-natural habitats within a 1,750 m radius on the species richness of pollinators and hosts.
Fig. 4: The effects of current network structure on network robustness to simulated future species extinctions depend on species sensitivity to coextinction and rewiring capacity, and differ between plant–pollinator and host–parasitoid networks.