We suggest that the ultimate outcome of interactions between native species and invasive species (extinction or coexistence) depends on the number of simultaneous negative interactions (competition and predation), which depends on relative body sizes of the species. Multiple simultaneous interactions may constrain the ability of native species to trade fitness components (i.e., reduced growth for reduced risk of predation) causing a spiral to extinction. We found evidence for five types of interactions between the adults and juveniles of introduced western mosquitofish (Gambusia affinis) and the juveniles of native least chub (Iotichthys phlegethontis). We added ten large (23-28 mm) and seven small (9-13 mm) young-of-the-year (YOY) least chub to replicate enclosures with zero, low, and high densities of mosquitofish in a desert spring ecosystem. Treatments with mosquitofish reduced the average survival of least chub by one-third. No small YOY least chub survived in enclosures with high mosquitofish densities. We also performed two laboratory experiments to determine mortality to predation, aggressiveness, and habitat selection of least chub in the presence of mosquitofish. Mean mortality of least chub due to predation by large mosquitofish was 69.7% over a 3-h trial. Least chub were less aggressive, selected protected habitats (Potamogeton spp.), and were more stationary in the presence of mosquitofish where the dominance hierarchy was large mosquitofish>>large least chub approximately small mosquitofish>>small least chub. Least chub juveniles appear to be figuratively caught in a vice. Rapid growth to a size refuge could reduce the risk of predation, but the simultaneous effects of competition decreased least chub growth and prolonged the period when juveniles were vulnerable to mosquitofish predation.

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Twelve categories/traits were used to classify and rank aquatic invertebrates based on their propensity to drift and importance as a food resource for salmonids. Invertebrate availability was based on their (i) propensity to intentionally drift, (ii) likelihood of being accidentally dislodged by the current, (iii) drift distance, (iv) adult drift, (v) benthic exposure, (vi) body size, and (vii) abundance. This study represents the first attempt to characterize the intentional drift propensity of stream invertebrates. A ranking procedure separated invertebrates into Baetis and three groups decreasing in availability. Predicted ranks were significantly correlated with the actual rank of invertebrates in trout guts taken in three separate studies conducted in the central Rocky Mountains, suggesting that this procedure can effectively rank invertebrates based on their availability as a food resource for salmonids. A cluster analysis separated the 95 taxa into four drift guilds and six availability groups. This study provides criteria for determining when alterations in invertebrate community composition will affect food resources for higher trophic levels by causing a decline in the most available taxa. This research also supports previous findings that floods are important in maintaining invertebrates that represent an important food resource for salmonids.Résumé : Douze catégories ou traits ont été utilisés pour classer les invertébrés aquatiques et leur attribuer un rang d'après leur tendance à dériver et leur importance comme ressource alimentaire pour les salmonidés. La disponibilité des invertébrés a été établie en fonction (i) de leur tendance à dériver intentionnellement, (ii) de la probabilité qu'ils soient emportés accidentellement par le courant, (iii) de la distance de dérive, (iv) de la dérive des adultes, (v) de l'exposition benthique, (vi) de leur visant à caractériser la tendance à dériver intentionnellement des invertébrés de cours d'eau. Une méthode d'attribution de rangs a servi à séparer les invertébrés en Beatis et trois groupes de disponibilité décroissante. Les rangs prévus étaient corrélés de manière statistiquement significative avec les rangs réels occupés par les invertébrés dans les estomacs de truite dans trois études distinctes réalisées dans la région centrale des montagnes Rocheuses, ce qui indiquerait que cette méthode peut effectivement servir à hiérarchiser les invertébrés en fonction de leur disponibilité comme ressource alimentaire pour les salmonidés. Une analyse par grappe a séparé les 95 taxons observés en quatre groupes de dérive et six groupes de disponibilité. Cette étude fournit des critères permettant de déterminer quand les modifications de la composition de la communauté des invertébrés influeront sur les ressources alimentaires des niveaux trophiques plus élevés en entraînant une réduction des taxons offrant la plus grande disponibilité. Ces travaux viennent également appuyer les constatations antérieures voulant que les inondations soient importantes pour maintenir les...

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Most studies of the diel pattern of trout foraging have focused on examining gut fullness at different times of the day. We used radiotelemetry coupled with stomach content and macroinvertebrate drift sampling to compare the diel activity of Colorado River cutthroat trout Oncorhynchus clarki pleuriticus with variation of invertebrate drift and light intensity on six dates during summer 1993 in the North Fork Little Snake River, Wyoming. The percentage of trout active was greater during midday than at night (medians, 100% versus 40%) and active fish had fewer transmitter signal strength fluctuations per minute during crepuscular periods than during the remainder of the day (medians, 8.8 versus 12.3). Light intensity and daytime mac roin vertebrate drift rate were significantly correlated with trout activity but 24-h drift rates were not. Stomach samples collected in the afternoon weighed significantly more than samples collected in the morning. Despite declines in daytime drift rate and stream discharge, the diel movement of trout also declined as summer progressed. Drift and diet significantly overlapped on all six dates, but selection occurred for certain taxa, especially terrestrial insects. We believe that Colorado River cutthroat trout in this stream forage primarily during the day in summer and that low light intensities prevented trout from exploiting the nocturnal increase in drifting invertebrates.

The concept of ecological exchangeability, together with genetic exchangeability, is central to both the Cohesion Species Concept as well as to some definitions of Evolutionarily Significant Units. While there are wellestablished criteria for measuring genetic exchangeability, the concept of ecological exchangeability has generated considerable confusion. We describe a procedure that uses the complementary strengths, while recognising the limitations, of both molecular genetic data and ecological experiments to determine the ecological exchangeability of local populations within a species. This is the first synthesis of a combined approach (experiments and genetics) and the first explicit discussion of testing ecological exchangeability. Although it would be ideal to find functional genes that interact to influence quantitative traits resulting in ecological differences (e.g. growth, size, fecundity), we suggest that our current knowledge of functional markers is too limited for most species to use them to differentiate adaptively different local populations. Thus, we argue that ecological experiments using whole organisms combined with neutral markers that indicate evolutionary divergence, provide the strongest case for detecting adaptive differences among local populations. Both genetic divergence and ecological experiments provide the best information for infering ecological exchangeability. This procedure can be used to decide which local populations should be preserved to maintain intraspecific variation and to determine which populations would enhance captive-breeding programs, augment endangered local populations and could best be used to re-introduce native species into historically occupied areas.

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