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Past and potential future effects of habitat  fragmentation on structure and stability of   plant–pollinator and host–parasitoid networks 
Ingo Grass, Birgit Jauker, Ingolf Steffan-Dewenter, Teja Tscharntke and Frank Jauker 

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 frag- mentation on the current structure of host–parasitoid networks may strongly affect their robustness to coextinctions under  future habitat fragmentation.

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Map of the study area, and examples of study landscapes and associated plant–pollinator and host–parasitoid interaction networks. Interaction networks were studied on 32 calcareous grassland fragments (fragment size: 314–51,395 m2). Fragments were selected so that the amount of additional habitat in the surrounding landscape varied independent of fragment size, ranging from complex landscapes with a high proportion of semi-natural habitats to simple landscapes dominated by arable fields and forest. The example landscapes shown (calcareous grasslands in white, with a landscape buffer of 500 m radius) include: a large fragment in a complex landscape (site 2), a large fragment in a simple landscape (site 6), a small fragment in a complex landscape (site 23) and a small fragment in a simple landscape (site 28). Interaction networks are shown with plants and hosts scaled in proportion to their interactions with pollinators and parasitoids. Lines connecting trophic levels indicate pairwise interactions, with the line width proportional to the interaction frequency. Green, plants; blue, pollinators and hosts; red, parasitoids.

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https://www.nature.com/articles/s41559-018-0631-2.epdf?shared_access_token=xr32XDnTbph79QeW5N4kudRgN0jAjWel9jnR3ZoTv0MRwkWDCuPA3BtIbdizpKWxiB54VWcsaBqu9xtOXw51zehvLPTrGNuwdddGpz76W4cZeUedXZ_kzIwGTWChd5iM-LTjGsnJ9raGc5QEKGOZ110IH6Ls3eHBEL1zOyYNvgw%3D

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