Fine-scale microhabitat niche separation allows coexistence of two invasive species

When invasive tubenose gobies (Proterorhinus semilunaris) and round gobies (Neogobius melanosotmus) occur in the same invaded ecosystem, the first is almost always displaced by the latter. Here, we report on a series of habitat-oriented laboratory experiments and field surveys aimed at revealing mechanisms that allowed coexistence of both species along the River Dyje (Danube basin). The experiments suggested that (i) both gobiids prefer the same habitat type, i.e., interstitial shelters between rocks, (ii) round goby is stronger, more aggressive competitor and (iii) round goby presence results in shift in tubenose goby habitat use. Field sampling indicated that round and tubenose gobies tended to occupy different microhabitats both along the longitudinal and lateral profile. Tubenose goby was found most often in shallow, near-bank rip-rap composed of smaller rocks, while round goby occurred most often in deeper areas with gravel substrates, larger rocks and faster current. Both species avoided the faster-flowing stretch in the middle of the river. Similar complementarity was also noted in temporal activity patterns, with round goby least active during the night and tubenose goby during the day. Our findings suggest that fine-scale niche separation facilitates the coexistence of these two invasive species, despite large 'broad-scale' niche overlap.