There is growing awareness that predicting biological invasions will require the development of conceptual models for specific taxa at appropriate scales. Salmonids are ideal taxa for testing factors that influence invasions, because large numbers have been introduced worldwide for long periods and their ecology is well known. We evaluated the hypothesis that, among regions with suitable water temperatures, environmental resistance from flood disturbances that wash away trout fry strongly influence invasion success of rainbow trout (Oncorhynchus mykiss), the most widely introduced fish species. We predicted that flow regimes in regions where rainbow trout invasions are successful would match those in their native range and would differ from those in regions where invasions are moderately successful or failed. We tested six specific predictions about how timing, predictability, frequency, duration, and annual variability of floods, as well as timing of low flows, will differ relative to timing of rainbow trout fry emergence among five Holarctic regions. Analysis of hydrologic regimes for eight rivers each in the native range (Pacific Coast) and four regions where rainbow trout invasions varied from highly successful (Southern Appalachians) to moderate (Colorado, USA, and Hokkaido Island, Japan) or failed (Honshu Island) showed that winter flooding and summer low flows in Pacific Coast rivers that favor spring emergence of rainbow trout were closely matched by Southern Appalachian flow regimes. In contrast, the other three regions had spring or summer flooding that hampered rainbow trout recruitment to different degrees, and winter low flows. Rainbow trout invasion success was best explained by a match between timing of fry emergence and months of low flood probability. Alternatively, cold water temperatures, which hamper reproduction, and biotic interactions with brown trout (Salmo trutta) and whirling disease parasites may account for low invasion success in European regions. However, differences in genetic makeup of donor stocks and propagule pressure are unlikely mechanisms to explain invasions. Understanding how abiotic disturbances interact with timing of critical life history events to limit nonnative species will help ecologists develop more robust theories to predict invasion success.

Transfer of energy from more productive donor habitats is frequently significant for the maintenance of consumers in the recipient habitats. Nevertheless, the connection between the distribution of consumers and allochthonous inputs has not been directly examined by field experiment. We present experimental evidence that terrestrial invertebrate inputs directly influence the distribution of stream fishes in a forested headwater stream. When terrestrial invertebrate input to the stream was experimentally reduced by using greenhouse covers, fish biomass also decreased dramatically. Despite the greater amounts of input in the control than in the reduced treatments, terrestrial invertebrates made up ∼90% of stomach contents in both treatments during the study period. Moreover, no difference in fish growth rates was evident between the treatments. These results provide experimental evidence that the flow of such allochthonous resources in the donor‐controlled system can explain the distribution and dynamics of the consumer populations. Corresponding Editor: C. R. Hupp

The occupation of adjacent, nonoverlapping positions along environmental gradients by closely related and ecologically similar species has drawn considerable attention from many ecologists over the past decades. Condition‐specific competition, wherein competitive superiority varies with the abiotic environmental gradient, has been proposed as the major structuring force behind such distributions. However, few studies have elucidated the underlying mechanisms, such as behavioral and demographic processes. We conducted laboratory experiments to examine the effects of temperature on interspecific competition between two stream salmonid fishes, Salvelinus malma and S. leucomaenis. The two species have a largely allopatric altitudinal distribution on Hokkaido Island, Japan, proposed to be the result of temperature‐mediated competition. We tested predictions that at a higher temperature (12°C), S. leucomaenis would dominate over S. malma in aggressive interactions, foraging performance, growth, and survival, but become subordinate at a lower temperature (6°C). Indeed, S. leucomaenis initiated a greater number of aggressive acts, attained greater food intake and greater growth, and finally excluded S. malma at the higher temperature. Although the two species initiated a similar number of aggressive acts and foraged equally well at the lower temperature, S. leucomaenis achieved a higher growth rate than S. malma; however, the latter eventually became numerically dominant. Clear competitive release in allopatry occurred for S. malma only at the higher temperature, providing direct evidence of condition‐specific asymmetric competition. The lower distribution boundary of S. malma in Hokkaido streams may therefore be determined by temperature‐mediated condition‐specific competition. However, mechanisms determining the upper distribution boundary of S. leucomaenis could not be fully explained by the competitive results at lower temperature, but required an understanding of how effects of competition interacted with species‐specific physiological traits. Thus, species distributions along an environmental gradient cannot be solely explained by a simple model of condition‐specific competition without considering mechanistic linkages among behavioral and physiological responses to the environment, resource use, and demographic processes.

Competitive ability changed across a range of 3-26°C among three fish species that show longitudinal replacement in Rocky Mountain streams: brook trout (Salvelinus fontinalis) at high elevations, brown trout (Salmo trutta) at middle elevations, and creek chub (Semotilus atromaculatus) at low elevations. Competitive ability was measured by food consumption and aggression in a stream tank. At 20°C, the trout species were competitively equal, and both were competitively superior to creek chub. Creek chub began to have competitive success against brook trout at 22°C and brown trout at 24°C, temperatures stressful but not lethal for the trout. Creek chub became competitively dominant over brook trout at 24°C and brown trout at 26°C, temperatures lethal to a portion of each trout species. We examined whether reduced food consumption was due to appetite loss or the presence of other species. For brook trout, interactions influenced feeding behavior at 22°C, but appetite loss became important at 24°C. For brown trout, interactions influenced feeding behavior at 24°C, but appetite loss became important at 26°C. For creek chub, there was an interaction between behavioral interactions and appetite in determining food consumption. Field data support a transition from trout to non-trout fishes at 22-25°C.

The important contribution of terrestrial invertebrates to the energy budget of drift‐foraging fishes has been well documented in many forested headwater streams. However, relatively little attention has been focused on the behavioral mechanisms behind such intensive exploitation. We tested for the hypothesis that active prey selection by fishes would be an important determinant of terrestrial invertebrates contribution to fish diets in a forested headwater stream in northern Japan. Rainbow trout, Oncorhynchus mykiss, were estimated to consume 57.12 mg m−2 day−1 (dry mass) terrestrial invertebrates, 77% of their total input (73.89 mg m−2 day−1), there being high selectivity for the former from stream drift. Both the falling input and drift of terrestrial invertebrates peaked at around dusk, decreasing dramatically toward midnight. In contrast, both aquatic insect adults and benthic invertebrates showed pronounced nocturnal drift. Because the prey consumption rates of rainbow trout were high at dawn and dusk, decreasing around midnight, the greater contribution of terrestrial invertebrates to trout diet was regarded as being partly influenced by the difference in diel periodicity of availability among prey categories. In addition, selectivity also depended upon differences in individual prey size among aquatic insect adults, and benthic and terrestrial invertebrates, the last category being largest in both the stream drift and the trout diets. We concluded that differences in both the timing of supplies and prey size among the three prey categories were the primary factors behind the selective foraging on terrestrial invertebrates by rainbow trout.