The competitive edge of an invading specialist

The following article, Bøhn, T. & Amundsen, P.-A. (2001). The competitive edge of an invading specialist. Ecology, 82(8), 2150-2163, can be accessed at https://esajournals.onlinelibrary.wiley.com/journal/19399170. Copyright by the Ecological Society of America.Introduced species represent major threats to native and natural biodiversity. On the other hand, biologists may increase the understanding of ecological interactions by following communities during establishment of exotic species. Accordingly, feeding ecology and habitat use were studied in native whitefish (Coregonus lavaretus) and recently invading vendace (C. albula) in two lake localities situated 50 km apart within the subarctic Pasvik River system, northern Norway and Russia. Whitefish originally dominated the native fish communities of both lakes. The recent invasion and successive downstream expansion of vendace allowed comparisons between two sites: one in which the influence of the new potential competitor on the native fish species was weak, and one in which the influence was strong. In the downstream lake vendace was recorded for the first time at the time of the study, and only in small numbers, whereas in the upstream lake vendace had established a high population density and was the dominant fish species in the pelagic zone. No vertical segregation in pelagic habitat use was found between the two fish species in either lake. In the downstream lake both whitefish and vendace fed exclusively on zooplankton and had almost identical diets. In the upstream lake, in contrast, whitefish fed predominantly on zoobenthos and surface insects, while vendace fed mainly on zooplankton. Thus, the strong presence of vendace as a specialized planktivore reduced the availability of zooplankton as prey for the more generalist whitefish. The food segregation between the two fish species in the upstream lake was apparently interactive and caused by a strong asymmetrical competition for zooplankton, vendace being the superior species. The ecological consequences (including reduced zooplankton size and species diversity, alteration of the pelagic food web, and eutrofication as a possible cascading effect on the primary production) of the vendace invasion in the Pasvik watercourse are considerable, even after a few years, and are likely to proceed and intensify in the future

Seasonal variations in the profundal Chironomidae (Diptera) assemblage of a subarctic lake

Zoobenthos samples were collected over a 12-month period in the profundal zone of the subarctic Fjellfrøsvatn, a lake in northern Norway. Chironomid larvae numerically dominated the profundal zoobenthos and their mean density decreased with increasing depth. Procladius (Holotanypus) sp. A, Mesocricotopus thienemanni, Psectrocladius (Mesopsectrocladius) sp. and Heterotrissocladius subpilosus exhibited the highest densities at the depth of 20 m, whereas H. subpilosus dominated the chironomid assemblage at 30–40 m. At 20-m depth, the chironomid density was highest in June, decreased towards autumn, and was relatively stable during the ice-covered period. The peak density in June was mainly related to a strong seasonal pattern in the density of Heterotrissocladius maeaeri and M. thienemanni. At 30–40-m depth, no significant seasonal variations in total density of chironomid larvae were observed, although the density of H. subpilosus exhibited a significant seasonal peak in winter. Larval instar groups of H. subpilosus were identified from a classification based on head capsule width. The profundal dwelling H. subpilosus were hemivoltine, having predominantly a two-year life cycle

Ecosystem type shapes trophic position and omnivory in fishes

This is the peer reviewed version of the following article: Sánchez-Hernandez, J. & Amundsen. P.-A. (2018). Ecosystem type shapes trophic position and omnivory in fishes. Fish and Fisheries, 19(6), 1003-1015, which has been published in final form at https://doi.org/10.1111/faf.12308. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.The identification of patterns in ecological characteristics of organisms is a central challenge in macroecology with a growing research interest. The goal of this study was to establish whether patterns in trophic ecology (trophic position and omnivory) of fishes can be extended to an ecosystem dimension (freshwater vs. marine environments), based on the premise that differences in environmental and ecological conditions of aquatic ecosystems have a large influence on the feeding ecology of fishes. To elucidate any relationship between trophic ecology and ecosystem type, we compiled a database using a global data set for fishes (http://www.fishbase.org). The database included 5,426 species distributed in 53 orders based on three common feeding strategies (herbivory, filter‐feeding and predatory). Trophic position and omnivory increased from freshwater to marine ecosystems in filter‐feeding and predatory species. In herbivore species in contrast, omnivory decreased, whereas no statistically significant trends were found for trophic position, which may reflect a similar diet specialization on primary producers regardless of ecosystem type. These findings suggest that ecosystem type has a marked effect on trophic position and omnivory in fishes, but the impact depends on the type of feeding strategy. Prey availability, inherent feeding traits linked to the phylogenetic relatedness of species, ontogenetic effects, spatial variability (habitat‐related factors) and body size are considered as responsible factors for the observed patterns. Our findings demonstrate consistent patterns in trophic characteristics of organisms linked to ecosystem type and underline the usefulness of fishes as model organisms to test macroecology hypotheses

Prey diversity as a driver of resource partitioning between river‐dwelling fish species

Although food resource partitioning among sympatric species has often been explored in riverine systems, the potential influence of prey diversity on resource partitioning is little known. Using empirical data, we modeled food resource partitioning (assessed as dietary overlap) of coexisting juvenile Atlantic salmon (Salmo salar) and alpine bullhead (Cottus poecilopus). Explanatory variables incorporated into the model were fish abundance, benthic prey diversity and abundance, and several dietary metrics to give a total of seventeen potential explanatory variables. First, a forward stepwise procedure based on the Akaike information criterion was used to select explanatory variables with significant effects on food resource partitioning. Then, linear mixed‐effect models were constructed using the selected explanatory variables and with sampling site as a random factor. Food resource partitioning between salmon and bullhead increased significantly with increasing prey diversity, and the variation in food resource partitioning was best described by the model that included prey diversity as the only explanatory variable. This study provides empirical support for the notion that prey diversity is a key driver of resource partitioning among competing speciesNorwegian Research Council; Xunta de Galicia, Grant/Award Number: Plan I2CS

Phenotype-environment association of the oxygen transport system in trimorphic European whitefish (Coregonus lavaretus) populations

Replicated adaptive radiation events, typified by phenotypic divergence across resource axes, provide important insight into the eco-evolutionary dynamics that lead to the formation of new species. Here, we show that in trimorphic adaptive radiations of European whitefish (Coregonus lavaretus), divergence of the oxygen transport system has occurred across the pelagic/littoral (shallow)—profundal (deep) resource axis, and at multiple biological scales. Profundal whitefish exhibited significantly larger red blood cells (RBCs), a greater proportion of cathodic hemoglobin protein components, and higher hemoglobin transcript abundance in kidney compared to littoral and pelagic morphs. Hemoglobin transcript abundance in brain and gill, but not kidney, and anodic hemoglobin protein component diversity in blood were also linked to variation at an intronic single nucleotide polymorphism (SNP). As the whitefish morphs differ in population genetic structure at this SNP, hemoglobin transcript and protein divergence between profundal and pelagic/littoral morphs is likely being driven by genetic divergence. Our findings, along with our previous work on lake whitefish, highlight the importance of the oxygen transport system to the postglacial colonization of novel lacustrine environments by whitefish throughout the northern hemisphere

Ecological Drivers of Mercury Bioaccumulation in Fish of a Subarctic Watercourse

Mercury (Hg) is a serious concern for aquatic ecosystems because it may biomagnify to harmful concentrations within food webs and consequently end up in humans that eat fish. However, the trophic transfer of mercury through the aquatic food web may be impacted by several factors related to network complexity and the ecology of the species present. The present study addresses the interplay between trophic ecology and mercury contamination in the fish communities of two lakes in a pollution-impacted subarctic watercourse, exploring the role of both horizontal (feeding habitat) and vertical (trophic position) food web characteristics as drivers for the Hg contamination in fish. The lakes are located in the upper and lower parts of the watercourse, with the lower site located closer to, and downstream from, the main pollution source. The lakes have complex fish communities dominated by coregonids (polymorphic whitefish and invasive vendace) and several piscivorous species. Analyses of habitat use, stomach contents, and stable isotope signatures (δ15N, δ13C) revealed similar food web structures in the two lakes except for a few differences chiefly related to ecological effects of the invasive vendace. The piscivores had higher Hg concentrations than invertebrate-feeding fish. Concentrations increased with size and age for the piscivores and vendace, whereas habitat differences were of minor importance. Most fish species showed significant differences in Hg concentrations between the lakes, the highest values typically found in the downstream site where the biomagnification rate also was higher. Mercury levels in piscivorous fish included concentrations that exceed health authorization limits, with possible negative implications for fishing and human consumption. Our findings accentuate the importance of acquiring detailed knowledge of the drivers that can magnify Hg concentrations in fish and how these may vary within and among aquatic systems, to provide a scientific basis for adequate management strategies.publishedVersio

Ectoparasites population dynamics are affected by host body size but not host density or water temperature in a 32-year long time series

Host density, host body size and ambient temperature have all been positively associated with increases in parasite infection. However, the relative importance of these factors in shaping long-term parasite population dynamics in wild host populations is unknown due to the absence of long-term studies. Here, we examine long-term drivers of gill lice (Copepoda) infections in Arctic charr (Salmonidae) over 32 years. We predicted that host density and body size and water temperature would all positively affect parasite population size and population growth rate. Our results show that fish size was the main driver of gill lice infections in Arctic charr. In addition, Arctic charr became infected at smaller sizes and with more parasites in years of higher brown trout population size. Negative intraguild interactions between brown trout and Arctic charr appear to drive smaller Arctic charr to seek refuge in deeper areas of the lake, thus increasing infection risk. There was no effect of host density on the force of infection, and the relationship between Arctic charr density and parasite mean abundance was negative, possibly due to an encounter-dilution effect. The population densities of host and parasite fluctuated independently of one another. Water temperature had negligible effects on the temporal dynamics of the gill lice population. Understanding long-term drivers of parasite population dynamics is key for research and management. In fish farms, artificially high densities of hosts lead to vast increases in the transmission of parasitic copepods. However, in wild fish populations fluctuating at natural densities, the surface area available for copepodid attachment might be more important than the density of available hosts

Prey consumption rates and growth of piscivorous brown trout in a subarctic watercourse

This is the peer reviewed version of the following article: Jensen, H., Amundsen, P.-A., Elliott, J.M., Bøhn, T. & Aspholm, P.E. (2006). Prey consumption rates and growth of piscivorous brown trout in a subarctic watercourse. Journal of Fish Biology, 68(3), 838-848, which has been published in final form at https://doi.org/10.1111/j.0022-1112.2006.00972.x. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.Prey consumption rates of piscivorous brown trout Salmo trutta were studied in the Pasvik watercourse, which forms the border between Norway and Russia. Estimates of food consumption in the field were similar to or slightly less than maximum values from a bioenergetic model. The piscivore diet consisted mainly of vendace Coregonus albula with a smaller number of whitefish Coregonus lavaretus. Individual brown trout had an estimated mean daily intake of c. 1·5 vendace and 0·4 whitefish, and a rapid annual growth increment of 7–8 cm year−1. The total population of brown trout >25 cm total length was estimated as 8445 individuals (0·6 individuals ha−1), giving a mean ± s.e. annual consumption of 1553880 ± 405360 vendace and 439140 ± 287130 whitefish for the whole watercourse. The rapid growth in summer of brown trout >25 cm indicated a high prey consumption rate

Using mathematical modelling to investigate the adaptive divergence of whitefish in Fennoscandia

Modern speciation theory has greatly benefited from a variety of simple mathematical models focusing on the conditions and patterns of speciation and diversification in the presence of gene flow. Unfortunately the application of general theoretical concepts and tools to specific ecological systems remains a challenge. Here we apply modeling tools to better understand adaptive divergence of whitefish during the postglacial period in lakes of northern Fennoscandia. These lakes harbor up to three different morphs associated with the three major lake habitats: littoral, pelagic, and profundal. Using large-scale individual-based simulations, we aim to identify factors required for in situ emergence of the pelagic and profundal morphs in lakes initially colonized by the littoral morph. The importance of some of the factors we identify and study - sufficiently large levels of initial genetic variation, size- and habitat-specific mating, sufficiently large carrying capacity of the new niche - is already well recognized. In addition, our model also points to two other factors that have been largely disregarded in theoretical studies: fitness-dependent dispersal and strong predation in the ancestral niche coupled with the lack of it in the new niche(s). We use our theoretical results to speculate about the process of diversification of whitefish in Fennoscandia and to identify potentially profitable directions for future empirical research.Peer reviewe

Patterns and repeatability of multi-ecotype assemblages of sympatric salmonids

publishedVersio