1. We investigated the effects of a wildfire on stream physical, chemical and biological characteristics in a Mediterranean climate, comparing stream community structure and consumer resource use in burned versus unburned catchments in Santa Barbara County, CA, U.S.A. 2. Canopy cover was lower and water temperature was higher in streams draining basins where the riparian vegetation burned than in streams in unburned basins or burned basins where riparian vegetation remained intact. Stream flow and suspended sediment concentrations during large post-fire storms and wet season nutrient levels were higher in burned than unburned catchments, with increased sedimentation after flood peaks. 3. A year after fires, algal levels were highest in streams where riparian vegetation burned and lowest in streams in burned basins where the riparian canopy remained intact. In contrast, streams in burned basins had lower particulate organic matter, detritivore and predator levels than unburned basins, regardless of whether riparian vegetation burned. Where present, southern California steelhead trout (Oncorhynchus mykiss) were extirpated from burned basins. 4. Algivore densities were high in streams with burned riparian vegetation for two post-fire years before declining to unburned stream levels. Shredder densities rebounded in streams in burned basins with intact riparian vegetation, but remained low for 4 years where riparian vegetation burned. Predatory invertebrate densities increased at sites where trout were eliminated by wildfire. 5. Hydrogen stable isotope analysis indicated that the diets of most invertebrate taxa in streams with burned riparian vegetation a year after fires were comprised of a higher proportion of algal material than riparian detritus relative to invertebrates in streams with intact riparian vegetation. 6. Wildfire impacts on stream food webs are determined, in part, by fire severity in the riparian zone. Streams with burned riparian canopies supported algal-based food webs and streams with intact riparian canopies sustained detrital-based food webs. Fire affected basal resources (nutrients, light, allochthonous inputs) with bottom-up effects on primary producers and consumers, but top-down effects were decoupled at the trophic link between invertebrate predators and primary consumers.
Understanding how introduced species succeed and become widely distributed within non‐native areas is critical to reduce the threats posed by them. Our goal was to reconstruct the main invasion routes and invasion dynamics of a global freshwater invader, the red swamp crayfish, Procambarus clarkii, through the analysis of its genetic variability in both native and invasive ranges. We inferred invasion routes and population structure from the analysis of a fragment (608 base pairs) of the mitochondrial marker cytochrome c oxidase subunit I from 1,062 individuals of P. clarkii in addition to 354 GenBank sequences, for a total of 122 populations (22 natives and 100 invaded). Genetic structure was assessed using analysis of molecular variance and non‐metric multidimensional scaling analyses. We analysed haplotype frequencies for the genetic variability in each locality and region. The haplotype network was depicted by using PopART software. A high haplotype diversity was found in the native range (haplotype diversity [Hd]: 0.90), but also in some non‐native areas, such as western U.S.A. (Hd: 0.80), areas of Mexico (Hd: 0.78), and some hotspots in Europe (e.g. southern Spain or Italy), suggesting a complex pattern of multiple introductions. We grouped all localities in five differentiated groups according to biogeographic origin: the native area, west Americas, east U.S.A., Asia, and Europe. Additionally, the identification of 15 haplotypes shared between at least two localities, the phylogenetic network estimation and indices of genetic differentiation among localities allowed us to identify a large genetic admixture in the native range; the two independent invasion routes (i.e. westwards and eastwards) in U.S.A. from the native range (Louisiana and Texas) with translocations within each area; a stepping‐stone introduction from U.S.A. to Japan (involving few individuals) themselves introduced to China afterwards; the entry of P. clarkii from Louisiana (U.S.A.) into southern Spain and their multiple secondary introductions over Europe as well as other possible introductions in central Europe. Our study emphasises the need for unravelling the global invasion routes and the demographic processes underlying the introduction of exotic species (i.e. admixture, bridgehead invasion effect, and propagule pressure) to control the spread of invasive species. Our findings highlight the value of genetic analyses to identify the geographic origin of source populations as well as the variability of invaded areas in order to reconstruct invasion dynamics and facilitate management of invasive species (e.g. through environmental DNA monitoring).
1. The effects of omnivorous exotic species on native communities are often difficult to predict because of the broad diets and behavioural flexibility of the omnivore, and the diverse abiotic and biotic characteristics of invaded systems. We investigated experimentally the effects of a gradient of density of the introduced, omnivorous red swamp crayfish Procambarus clarkii (Decapoda: Cambaridae) on two stream communities in southern California, U.S.A. 2. The Ventura River is a clear, flowing stream with a cobble substratum, with abundant algae but low densities of large invertebrates, small herbivores and snails. The Santa Ynez River at the time of the study consisted of a series of drying pools underlain by sand, with abundant charophytes, large predatory invertebrates and herbivores, including snails. 3. In the Ventura River, periphyton biomass and inorganic sediment decreased with increasing crayfish abundance, but in the Santa Ynez River, periphyton and sediment were unrelated to crayfish densities. 4. In the Ventura River, the biomass and density of all benthic invertebrates combined, chironomids, micropredators, the meiofauna (chydorid cladocerans, copepods and ostracods), and specific predatory and herbivorous taxa, as well as taxon richness, were negatively related to crayfish density. In the Santa Ynez River, the biomass and average body size of benthic invertebrates, predatory invertebrates, herbivores and chironomids, but not total invertebrate density or taxon richness, were negatively related to crayfish density. 5. Fewer large predatory invertebrates and snails (Physella gyrina) in both streams, and baetid mayflies in the Ventura River, were visible at night in channels where crayfish were abundant. Snails responded to crayfish by moving above the water line in the Santa Ynez River, but not in the Ventura River. 6. We suggest that the same omnivore had different effects on these neighbouring streams because of crayfish predation on large invertebrates in the Santa Ynez River and the scarcity of such prey in the Ventura River, leading to increased crayfish grazing on periphyton, and reductions in periphyton-associated invertebrates, in the Ventura River.
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