Persistence in the longitudinal distribution of lotic insects in a changing climate: a tale of two rivers.

The longitudinal distribution of many taxa in rivers is influenced by temperature. Here we took advantage of two older datasets on net-spinning caddisflies (Hydropsychidae) from contrasting European rivers to assess changes in species occurrence and relative abundance along the river by resampling the same sites, postulating that an increase in river temperature over the intervening period should have resulted in cool-adapted species retreating into the headwaters and warm adapted species expanding upstream. Distributional changes in the Welsh Usk were slight between 1968/69 and 2010, one rare species appearing at a single headwater site and one warm-adapted species disappearing from the main river. Distributional changes in the French Loire, between 1989-93 and 2005, were similarly modest, with no consistent movement of species up- or downstream. We estimate that the decadal rate of increase in the mean summer daily maximum in the Usk was only 0.1°C at one 'summer cool' headwater site, while a neighbouring 'summer warm' tributary increased by 0.16°C per decade, and the main river by 0.22°C. The Loire is warmer than the Usk and the mean decadal rates of increase, over the period 1989-2005, at three sites along the lower reaches were 0.39, 0.48 and 0.77°C. Increases in stream and river temperature, therefore, were spatially variable and were not associated with consistent upstream movement of species in either of these (very different) rivers. We conclude that either the temperature increases have hitherto been insufficient to affect species distribution or, more speculatively, that it may not be possible for river organisms (that do not respond only to temperature) to move upstream because of a developing spatial mismatch between key habitat characteristics, some of them changing with the climate but others not.