Streams in mediterranean-type climate regions are shaped by predictable seasonal events of flooding and drying over an annual cycle, but also present a strong interannual flow variation.The Guadiana River is one of the most important rivers in the Iberian Peninsula. The fish fauna presents 11 native freshwater species, including eight with high conservation status. Several exotic species are present, the most important being the American centrarchids pumpkinseed fish and largemouth bass. As a typical mediterranean-type river, the Guadiana has an irregular hydrological regime with severe drought periods and floods; the interannual variation of discharge presents a ratio of c. 100 to 1.From 1980 to 1995 several dry years were observed, culminating in the drought of 1991/92-1994/95. Analysing the variation of the fish assemblage structure during this period, exotic species (mostly pumpkinseed) progressively increased, strongly dominating in 1995. Indigenous populations dramatically decreased and a previously common endemic cyprinid (Anaecypris hispanica Steindachner) became endangered and one of the most threatened fishes of Europe. However, the following years presented above-average flows with several flood events and an inverse process occurred, with native species increasing their contribution in a short period.The importance of floods as a disturbance factor in the control of lentic or slow flowing water exotics is discussed. Native species apparently possess adaptive responses to high flows which exotics lack. This hypothesis is consistent with probabilityof-use curves and preflood-postflood surveys. Results emphasize the importance of floods in the environmental flows of mediterranean-type rivers. In the absence of flooding disturbance, exotic fish populations predictably grow, increasing the pressure on native species; abiotic disturbance may supersede deterministic outcomes of predation or competition and influence community structure by reducing populations of the exotic species.

Temporary streams and rivers, also referred to as intermittent, are defined as waterways that cease to flow at some points in space and time along their course. They are shaped by alternating wet and dry periods over annual and inter‐annual cycles, making them one of the most dynamic freshwater ecosystems. These distinctive systems represent a substantial proportion of the total number, length and discharge of the global river network and are expected to become more widespread and face increasing pressures in many regions as a result of human activities and climate change. This collection of papers arose from the Conference on ‘Ecohydrology and Ecological Quality in Temporary Rivers’ held at the University of Evora, Portugal, 12–14 September 2012. The primary objectives of the meeting were to bring together researchers and expertise from a range of temporary ecosystems, from small temporary Mediterranean and Californian streams to intermittent rivers from semi‐arid and arid zones. A common thread through the discussions was the assessment of ecological conditions within these dynamic ecosystems. To undertake a ’condition assessment’ is challenging because of the need to differentiate between biotic responses to anthropogenic pressures and natural variability associated with the hydro‐climatology and disturbance regime typical of these systems, that is, drought, drying and flooding. The management of degraded intermittent streams and rivers presents many new challenges, not least how to reconcile ecological and societal goals for emerging ‘novel’ ecosystems. Reconciliation ecology presents a realistic way of managing novel ecosystems given their value and desirable ecological services to society, rather than trying to restore to an original ecological state that may not be sustainable or economically possible. Copyright © 2014 John Wiley & Sons, Ltd.

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Key-words:Pacifastacus leniusculus, Procambarus clarkii, dispersal rate, crayfish coexistence NE Portugal is one of the most isolated and low populated regions of Western Europe. Recently, two American crayfish species reached this area: signal crayfish Pacifastacus leniusculus coming from Spain and red swamp crayfish Procambarus clarkii in its northward expansion. The dispersion of both species was followed throughout the last decade in river Maçãs (Douro catchment). Our results support the hypothesis of faster downstream colonisation. The mean spread rate of signal crayfish was faster for the downstream expansion, 2.8 km·yr −1 , while the upstream rate was 1.7 km·yr −1 . Exceptionally, in one period, the rate of spread reached 6.7 km·yr −1 , which was apparently caused by downstream washing in extreme conditions of flow. The mean rate of spread is higher than the observed in other studies which may also be related to the low current velocity of late spring and summer that provides favourable conditions for a fast upstream colonisation. Coexistence of signal and red swamp crayfish was detected in a large river sector. Signal crayfish strongly dominates in the upper and intermediate sectors which are the colder part of the river and in the lower sectors, species abundances are not significantly different. Winter temperature in the higher altitude reaches seems to moderate the competitive abilities of P. clarkii. plus rapide en aval, soit 2,8 km par an, comparé à 1,7 km par an en amont. Exceptionnellement, sur une période, le niveau de colonisation a atteint 6,7 km par an car les écrevisses on été apparemment emportées en aval à cause des conditions extrêmes d'écoulement. La propagation moyenne est plus élevée que ce qui est observé dans d'autres études, ce qui peut être mis en relation avec la faible vitesse du courant en fin de printemps et en été, favorisant une colonisation rapide en amont. La coexistence de l'écrevisse signal et de l'écrevisse rouge a été observée dans une grande partie de la rivière. L'écrevisse signal domine dans la partie haute et plus froide de la rivière, alors que dans le secteur le plus bas il n'y a pas de différence significative entre les deux espèces. La température hivernale, observée en plus haute altitude, semble modérer les capacités de compétition de P. clarkii. RÉSUMÉ

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