Gilbert Cabana

Estimating the trophic position of aquatic consumers in river food webs using stable nitrogen isotopes

Abstract Intraspecific and interspecific variation in the trophic position of various consumers is central to many theories of aquatic ecosystem functioning and dynamics. In recent years, such issues have been addressed using stable N isotopes (δ15N) to estimate trophic position of consumers in aquatic ecosystems. However, one needs to correct for variation in baseline δ15N among sites to estimate the trophic position of any aquatic consumer using δ15N. We first discuss the steps needed to select a baseline indicator in river ecosystems, where abundant foodweb isotopic data have been published but limited attention has been paid to identification of baseline organisms. We examined within-site differences in δ15N among primary consumers belonging to different functional and taxonomic groups and found significant differences in δ15N. Collectors were significantly enriched compared to other primary consumers. We propose scrapers as a baseline δ15N indicator because they showed low δ15N values and were more widely distributed than other primary consumers throughout our study sites. Using this baseline δ15N, we calculated continuous estimates of trophic position of consumers (invertebrates and small fish) for our 87 river food webs. Primary consumers had a significantly lower mean trophic position (2.3) than predatory invertebrates (2.9) and fish (3.5), and these results are concordant with estimates based on traditional studies. However, trophic-position estimates of consumers (both invertebrates and fish) were highly variable across sites with standard deviations spanning up to 0.67 trophic levels, which suggested potential omnivory. Comparison of these trophic-position estimates with estimates based on mean δ15N of all primary consumers combined (no targeting of scrapers as baseline indicators) suggested that use of a constrained number of groups and corrections for isotopic differences among groups when estimating baseline δ15N could reduce some biases induced by the use of various functional feeding groups with variable δ15N values.

Incorporating temporally dynamic baselines in isotopic mixing models

Stable isotopes (particularly C and N) are widely used to make inferences regarding food web structure and the phenology of consumer diet shifts, applications that require accurate isotopic characterization of trophic resources to avoid biased inferences of feeding relationships. For example, most isotope mixing models require that endmembers be adequately represented by a single probability distribution; yet, there is mounting evidence that the isotopic composition of aquatic organisms often used as mixing model endmembers can change over periods of weeks to months. A review of the literature indicated that the delta13C values of five aquatic primary consumer taxa, commonly used as proxies of carbon production sources (i.e., trophic baselines), express seasonally dynamic cycles characterized by an oscillation between summer maxima and winter minima. Based on these results, we built a dynamic baseline mixing model that allows a growing consumer to track temporal gradients in the isotopic baselines of a food web. Simulations showed that the ability of a consumer to maintain or approach isotopic equilibrium with its diet over a realistic growth season was strongly affected by both the rate of change of the isotopic baseline and equilibration rate of the consumer. In an empirical application, mixing models of varying complexity were used to estimate the relative contribution of benthic vs. pelagic carbon sources to nine species of juvenile fish in a fluvial lake of the St. Lawrence River system (Québec, Canada). Estimates of p (proportion of carbon derived from benthic sources) derived from a static mixing model indicated broad interspecific variation in trophic niche, ranging from complete benthivory to > 95% reliance on pelagic food webs. Output from the more realistic dynamic baseline mixing model increased estimated benthivory by an average of 36% among species. Taken together, our results demonstrate that failing to identify dynamic baselines when present, and (or) matching consumers with baseline taxa that possess substantially different equilibration rates can seriously bias interpretation of stable isotope data. Additionally, by providing a formalized framework that allows both resources and consumers to shift their isotopic value through time, our model demonstrates a feasible approach for incorporating temporally dynamic isotope conditions in trophic studies of higher consumers.

Sexual Dichromatism in North American Freshwater Fish: Do Parasites Play a Role?

North American freshwater fish were scored for parasites and sexual dichromatism to test the interspecific prediction of the Hamilton-Zuk hypothesis that more sexually dichromatic fish species should harbour more parasites. We used two measures of parasite load, the mean number of helminth species per individual host and the total number of parasite genera reported per host species.Data were collected on 68 species of indigenous, non driadromous North American freshwater fishes

Estimating the feeding range of a mobile consumer in a river–flood plain system using δ13C gradients and parasites

1. The feeding range of an individual is central to food web dynamics as it determines the spatial scale of predator-prey interactions. However, despite recognition of its importance as a driving force in population dynamics, establishing feeding range is seldom done as detailed information on trophic interactions is difficult to obtain. 2. Biological markers are useful to answer this challenge as long as spatial heterogeneity in signal is present within the area investigated. A spatially complex ecosystem, Lake St. Pierre (LSP), a fluvial lake of the St Lawrence River (Québec, Canada), offered a unique opportunity to determine the feeding range of a secondary consumer, yellow perch (Perca flavescens) using isotopic ratios of carbon (δ(13)C). However, because food chains based on phytoplankton have generally more negative δ(13) C than those depending on periphyton, it was essential to determine the contribution of zooplankton in fish diet to correctly interpret spatial patterns of δ(13)C. We used parasites in perch to examine whether their δ(13)C was reflecting local δ(13)C baseline conditions rather than a feeding specialization on zooplankton. 3. δ(13)C of primary consumers was highly variable and exhibited a striking gradient along the shore-channel axis, suggesting that δ(13)C should reflect an individual consumers spatial position in LSP. 4. This strong isotopic gradient allowed us to estimate the spatial scale of the resources used by individual perch following an approach presented by Rasmussen, Trudeau & Morinville (Journal of Animal Ecology, 78, 2009, 674). By comparing the δ(13)C variability in perch to that of primary consumers, we estimated that the adults feeding range was around 2 km along the shore-channel axis. 5. The combined use of isotopic ratios and parasites allowed us to determine that the adult population uses a wide range of habitats between the flood plain and the main channel. However, individually, each perch depended on a limited foodshed.

A comparative study of sexual selection and reproductive investment in the slimy sculpin, Cottus cognatus

Three slimy sculpin (Cottus cognatus) populations in central Ontario, Canada, were extensively surveyed using SCUBA to determine interpopulation variability in reproductive behavior. When compared to a random model (Poisson), the mating patterns varied significantly among populations. In Mountain Lake, sculpins were usually monogamous and males were subject to relatively low potential intensities of sexual selection. In this lake only, there appeared to be a cost to polygyny as suggested by an observed high incidence of zygote mortality in polygynous nests. In Salmon Lake, however, sculpins were exclusively polygynous, and males were subject to very high potential intensities of sexual selection. The majority of males in Salmon Lake were bachelors and only the larger, older males successfully competed for suitable nest sites and mates. In the third population (Blue Chalk Lake), sculpin mating patterns were similar to those predicted by the random model, and the potential intensity of sexual selection on males of this population was intermediate between the two other study populations. Patterns of biomass allocation to gonads varied greatly among populations. Females from all populations, and males from Blue Chalk and Mountain lakes exhibited relatively linear increases with age in the proportion of body weight devoted to gonads (GSI), although the rate of increase varied between populations. Males of the Salmon Lake population, however, exhibited very low GSIs up to the age of 5 or 6 years, after which GSI increased dramatically. We suggest that the pattern of age dependent allocation to testes observed in Salmon Lake sculpins is a direct result of the high potential intensities of sexual selection imposed upon males of this population.

Feeding by black fly (Diptera:Simuliidae) larvae causes downstream losses in phytoplankton, but not bacteria

Abstract Experimental removal of early instar black fly larvae from 3 oligotrophic lake-outlet streams in southern Quebec showed that larvae reduced phytoplankton biomass, but had little effect on bacterioplankton biomass. The rates of decline in bacterioplankton and phytoplankton biomass with distance downstream from 6 lake outlets were determined before the insecticide Bacillus thuringiensis var. israelensis was used to remove black fly larvae from 3 of the streams. The rates of decline in bacterioplankton and phytoplankton biomass were measured again after the larvae were removed, and the differences in the rates of decline between before and after were used as indicators of the rates of biomass ingestion by the black fly larvae. The proportion of bacterioplankton ingested/m stream length was <0.081%/m in all streams, but the proportion of phytoplankton ingested/m stream length was 0.35%/m and 0.24%/m in 2 of the 3 manipulated streams. Downstream declines in both bacterioplankton and phytoplankton persisted in the absence of black fly larvae. Factors other than black fly feeding (such as biofilm adhesion and ingestion by other filter-feeders) were responsible for 50 to 90% of the total downstream loss rate of bacteria, whereas factors other than black fly feeding were responsible for 0 to 55% of the total downstream loss rate of phytoplankton. Apparently, free-living bacteria were not ingested to a great extent by black fly larvae in these oligotrophic lake-outlet streams, and the link between bacteria in the plankton and their potential black fly predators seemed weak. The relative importance of algivory over bacterivory is expected to be greater in eutrophic lake-outlet streams, where the ratio of bacterial C to phytoplankton C is smaller than in oligotrophic lake-outlet streams, and in systems with a greater abundance of later-instar larvae, which are less efficient at capturing bacteria than early instar larvae.

Ruffed Grouse Brood Habitat Use in Mixed Softwood‐Hardwood Nordic‐Temperate Forests, Quebec, Canada

Abstract Adequate cover is a critical component of ruffed grouse (Bonasa umbellus) habitat during the brood-rearing period when chick mortality is high. We assessed habitat use by ruffed grouse during the brood-rearing period by comparing characteristics of tree, shrub, and ground layers at ruffed grouse brood and random locations. We captured and radiomarked 29 females with broods in 2 forest settings of the Réserve faunique de Portneuf, Quebec, Canada. We described grouse habitat using ground surveys and forest maps, and we identified the used habitat characteristics using analysis of variance and logistic regression. Females with broods used mixed and regenerated clearcut stands that were 1.5–7 m tall and 11–20 years old. Compared with random locations, grouse locations had higher lateral obstruction (76% vs. 68%), higher small-stem density (29,085 stems/ha vs. 19,340 stems/ha), and were closer to roads and trails. Percentage of coverage by ground vegetation was not higher at grouse locations as often reported in previous studies. Results from this study will help orient ruffed grouse habitat management on Quebec public land and elsewhere in nordic–temperate mixed hardwood–softwood forests to maintain suitable brood habitat after logging operations. Forest management should promote growth of young mixed stands with high horizontal and vertical cover provided by high small-stem density, which offers protection against aerial and terrestrial predation. Edges such as roadsides are also important in brood habitat as they provide food and cover.

Testing the influence of environmental heterogeneity on fish species richness in two biogeographic provinces.

Environmental homogenization in coastal ecosystems impacted by human activities may be an important factor explaining the observed decline in fish species richness. We used fish community data (>200 species) from extensive surveys conducted in two biogeographic provinces (extent >1,000 km) in North America to quantify the relationship between fish species richness and local (grain <10 km2) environmental heterogeneity. Our analyses are based on samples collected at nearly 800 stations over a period of five years. We demonstrate that fish species richness in coastal ecosystems is associated locally with the spatial heterogeneity of environmental variables but not with their magnitude. The observed effect of heterogeneity on species richness was substantially greater than that generated by simulations from a random placement model of community assembly, indicating that the observed relationship is unlikely to arise from veil or sampling effects. Our results suggest that restoring or actively protecting areas of high habitat heterogeneity may be of great importance for slowing current trends of decreasing biodiversity in coastal ecosystems.