Ecological tracers reveal resource convergence among prey fish species in a large lake ecosystem

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Non‐lethal predator effects on the performance of a native and an exotic crayfish species

1. I tested the hypothesis that the potential for non‐lethal effects of predators are more important for overall performance of the fast‐growing exotic signal crayfish (Pacifastacus leniusculus Dana) than for the slower growing native noble crayfish (Astacus astacus L.). I further tested if omnivorous crayfish switched to feed on less risky food sources in the presence of predators, a behaviour that could reduce the feeding costs associated with predator avoidance. 2. In a 2 month long outdoor pool experiment, I measured behaviour, survival, cheliped loss, growth, and food consumption in juvenile noble or signal crayfish in pools with either a caged predatory dragonfly larvae (Aeshna sp.), a planktivorous fish that do not feed on crayfish (sunbleak, Leucaspius delineatus Heckel), or predator‐free controls. Crayfish had access to multiple food sources: live zooplankton, detritus and periphyton. Frozen chironomid larvae were also supplied ad libitum outside crayfish refuges, simulating food in a risky habitat. 3. Crayfish were mainly active during hours of darkness, with signal crayfish spending significantly more time outside refuges than noble crayfish. The proportion of crayfish outside refuges varied between crayfish species, time and predator treatment, with signal crayfish spending more time in refuges at night in the presence of fish. 4. Survival in noble crayfish was higher than in signal crayfish, and signal crayfish had a higher frequency of lost chelipeds, indicating a high level of intraspecific interactions. Crayfish survival was not affected by the presence of predators. 5. Gut‐contents analysis and stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) indicated that the two crayfish species had similar food preferences, and that crayfish received most of their energy from feeding on invertebrates (e.g. chironomid larvae), although detritus was the most frequent food item in their guts. Signal crayfish guts were more full than those of noble crayfish, but signal crayfish in pools with fish contained significantly less food and fewer had consumed chironomids compared with predator‐free controls. Length increase of signal crayfish (35%) was significantly higher than of noble crayfish (20%), but signal crayfish in pools with fish grew less than in control pools. 6. This short‐term study indicates that fish species that do not pose a lethal threat to an organism may indirectly cause reductions in growth by affecting behaviour and feeding. This may occur even though prey are omnivorous and have access to and consume multiple food sources. These non‐lethal effects of predators are expected to be particularly important in exotic crayfish species that show a general response to fish, have high individual growth rates, and when their feeding on the most profitable food source is reduced.     

Polymorphism in lake trout in Great Bear Lake: intra‐lake morphological diversification at two spatial scales

Great Bear Lake is the most northerly lake of its size and provides unique opportunities for intraspecific diversification. Despite increasing attention to intraspecific polymorphism, several knowledge gaps remain (e.g. determining the extent of intraspecific diversification in large relatively pristine lakes and at which spatial scale it can occur). We focused on geographical patterns of morphological differentiation within lake trout (Salvelinus namaycush) to describe two levels of intralake diversification in Great Bear Lake. We used a combination of geometric and traditional linear measurements to quantify differences in body shape, head shape, and fin and body lengths among 910 adult lake trout from the five distinct arms of Great Bear Lake. Although head and fin linear measurements discriminated the three common morphotypes at the whole‐lake level, inter‐arm variation in body shape was observed within each morphotype. A comparison of genetic and morphological distance matrices revealed the lack of an association between the two sets of data, although both comparisons revealed an association in the inter‐arm variation patterns among morphotypes, suggesting a phenotypically plastic response to distinct environments. The whole‐lake and inter‐arm morphological variation observed within lake trout demonstrates the importance of considering scale, especially across large lakes that exhibit marked complexity and a variety of freshwater habitats. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114 , 109–125.     

Effects of irrigation system alterations on the trophic position of a threatened top predator in rice‐field ecosystems

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Food‐web structure and mercury dynamics in a large subarctic lake following multiple species introductions

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Long‐term variation in the littoral food web of an acidified mountain lake

Evidence of chemical recovery from acidification in European freshwaters has emerged in recent years, with many previously damaged systems responding to decades of reduced acidifying emissions. Biological recovery, however, has often lagged behind, and this has been ascribed to several possible mechanisms, including inertia in the food web. We examined two decades of change in hindcasted food webs for Lochnagar, a Scottish mountain lake, to make inferences about the potential dynamical stability of the system and to assess the prospects for future biological recovery. Although community composition tracked temporal changes in acidity, this was neither sustained nor directional, and mainly manifested as shifts in relative abundances rather than the establishment of more acid‐sensitive species. The food web was highly interconnected and reticulate, especially in years when species richness was low, and subsidized by external inputs of detritus. Among the primary consumers, generalist herbivore–detritivores maintained feeding links with the scant algal resources, which appeared insufficient to support viable populations of specialist grazers. Together, these characteristics, which are shared with many other acidified freshwaters, are likely to make the community dynamically stable and resistant to invasions of potential new colonists, thereby slowing the pace of future biological recovery.     

Effects of an invasive crayfish on trophic relationships in north‐temperate lake food webs

1. The introduction of invasive species is one of the main threats to global biodiversity, ecosystem structure and ecosystem processes. In freshwaters, invasive crayfish alter macroinvertebrate community structure and destroy macrophyte beds. There is limited knowledge on how such invasive species‐driven changes affect consumers at higher trophic levels. 2. In this study, we explore how the invasive rusty crayfish Orconectes rusticus, a benthic omnivore, affects benthic macroinvertebrates, as well as the broader consequences for ecosystem‐level trophic flows in terms of fish benthivory and trophic position (TP). We expected crayfish to decrease abundance of benthic macroinvertebrates, making most fish species less reliant on benthic resources. We expected crayfish specialists (e.g. Lepomis sp. and Micropterus sp.) to increase their benthic dependence. 3. In 10 northern Wisconsin lakes, we measured rusty crayfish relative abundance (catch per unit effort, CPUE), macroinvertebrate abundance, and C and N stable isotope ratios of 11 littoral fish species. We used stable isotope data and mixing models to characterise the trophic pathways supporting each fish species, and related trophic structure to crayfish relative abundance, fish body size and abiotic predictors using hierarchical Bayesian models. 4. Benthic invertebrate abundance was negatively correlated with rusty crayfish relative abundance. Fish benthivory increased with crayfish CPUE for all 11 fish species; posterior probabilities of a positive effect were >95%. TP also increased slightly with crayfish CPUE for some species, particularly smallmouth bass, largemouth bass, rock bass and Johnny darter. Moreover, both fish body size and lake abiotic variables explained variation in TP, while their effects on benthivory were small. 5. Rusty crayfish abundance explained relatively little of the overall variation in fish benthivory and TP. Although rusty crayfish appear to have strong effects on abundances of benthic macroinvertebrates, energy flow pathways and trophic niches of lentic fishes were not strongly influenced by invasive rusty crayfish.     

Long‐term increases in intrinsic water‐use efficiency do not lead to increased stem growth in a tropical monsoon forest in western Thailand

Rising atmospheric carbon dioxide [CO₂] can accelerate tree growth by stimulating photosynthesis and increasing intrinsic water‐use efficiency (iWUE). Little evidence exists, however, for the long‐term growth and gas‐exchange responses of mature trees in tropical forests to the combined effects of rising [CO₂] and other global changes such as warming. Using tree rings and stable isotopes of carbon and oxygen, we investigated long‐term trends in the iWUE and stem growth (basal area increment, BAI) of three canopy tree species in a tropical monsoon forest in western Thailand (Chukrasia tabularis, Melia azedarach, and Toona ciliata). To do this, we modelled the contribution of ontogenetic effects (tree diameter or age) and calendar year to variation in iWUE, oxygen isotopes, and BAI using mixed‐effects models. Although iWUE increased significantly with both tree diameter and calendar year in all species, BAI at a given tree diameter was lower in more recent years. For one species, C. tabularis, differences in crown dominance significantly influence stable isotopes and growth. Tree ring Δ¹⁸O increased with calendar year in all species, suggesting that increasing iWUE may have been driven by relatively greater reductions in stomatal conductance – leading to enrichment in Δ¹⁸O – than increases in photosynthetic capacity. Plausible explanations for the observed declines in growth include water stress resulting from rising temperatures and El Niño events, increased respiration, changes in allocation, or more likely, a combination of these factors.     

Long‐term ecological changes in marine mammals driven by recent warming in northwestern Alaska

Carbon and nitrogen isotopes analyses were performed on marine mammal bone collagen from three archaeological sites (ad 1170–1813) on Cape Espenberg (Kotzebue Sound, northwestern Alaska) as well as modern animals harvested from the same area to examine long‐term trends in foraging ecology and sea ice productivity. We observed significant and dramatic changes in ringed seal stable isotope values between the early 19th and early 21st centuries, likely due to changing sea ice productivity and reduced delivery of organic matter to the benthos driven by recent warming in the Arctic. These data highlight the importance of the archaeological record for providing a long‐term perspective on environmental variation and interpreting recent changes driven by anthropogenic processes.     

Among‐individual diet variation within a lake trout ecotype: Lack of stability of niche use

In a polyphenic species, differences in resource use are expected among ecotypes, and homogeneity in resource use is expected within an ecotype. Yet, using a broad resource spectrum has been identified as a strategy for fishes living in unproductive northern environments, where food is patchily distributed and ephemeral. We investigated whether specialization of trophic resources by individuals occurred within the generalist piscivore ecotype of lake trout from Great Bear Lake, Canada, reflective of a form of diversity. Four distinct dietary patterns of resource use within this lake trout ecotype were detected from fatty acid composition, with some variation linked to spatial patterns within Great Bear Lake. Feeding habits of different groups within the ecotype were not associated with detectable morphological or genetic differentiation, suggesting that behavioral plasticity caused the trophic differences. A low level of genetic differentiation was detected between exceptionally large‐sized individuals and other piscivore individuals. We demonstrated that individual trophic specialization can occur within an ecotype inhabiting a geologically young system (8,000–10,000 yr BP), a lake that sustains high levels of phenotypic diversity of lake trout overall. The characterization of niche use among individuals, as done in this study, is necessary to understand the role that individual variation can play at the beginning of differentiation processes.     

Long‐term dynamics of submerged macrophytes and algae in a small and shallow,eutrophic lake: implications for the stability of macrophyte‐dominance

1. Submerged macrophyte and phytoplankton components of eutrophic, shallow lakes have frequently undergone dynamic changes in composition and abundance with important consequences for lake functioning and stability. However, because of a paucity of long‐term survey data, we know little regarding the nature, direction and sequencing of such changes over decadal–centennial or longer timescales. 2. To circumvent this problem, we analysed multiple (n = 5) chronologically correlated sediment cores for plant macro‐remains and a single core for pollen and diatoms from one small, shallow, English lake (Felbrigg Hall Lake, Norfolk, U.K.), documenting 250 years of change to macrophyte and algal communities. 3. All five cores showed broadly similar stratigraphic changes in macrophyte remains with three distinct phases of macrophyte development: Myriophyllum–Chara–Potamogeton (c. pre‐1900), to Ceratophyllum–Chara–Potamogeton (c. 1900–1960) and finally to Zannichellia–Potamogeton (c. post‐1960). Macrophyte species richness declined from at least 10 species pre‐1900 to just four species at the present day. Additionally, in the final Zannichellia–Potamogeton phase, a directional shift between epi‐benthic and phytoplankton‐based primary production was indicated by the diatom data. 4. Based on macrophyte–seasonality relationships established for the region, concomitant with the final shift to Zannichellia–Potamogeton, we infer a reduction in the seasonal duration of plant dominance (plant‐covered period). Furthermore, we hypothesise that this change in species composition resulted in a situation whereby macrophyte populations were seasonally ‘sandwiched’ between two phytoplankton peaks in spring and late summer as observed in the contemporary lake. 5. We suggest that eutrophication‐induced reductions in macrophyte species richness, especially if the number of plant‐seasonal strategies is reduced, may constrict the plant growing season. In turn, this may render a shallow lake increasingly vulnerable to seasonal invasions of phytoplankton resulting in further species losses in the plant community. Thus, as part of a slow (over perhaps 10–100s of years) and self‐perpetuating process, macrophytes may be gradually pushed out by phytoplankton without the need for a perturbation as required in the alternative stable states model of plant loss.     

Long‐term changes in the impacts of global warming on leaf phenology of four temperate tree species

Contrary to the generally advanced spring leaf unfolding under global warming, the effects of the climate warming on autumn leaf senescence are highly variable with advanced, delayed, and unchanged patterns being all reported. Using one million records of leaf phenology from four dominant temperate species in Europe, we investigated the temperature sensitivities of spring leaf unfolding and autumn leaf senescence (S_T, advanced or delayed days per degree Celsius). The S_T of spring phenology in all of the four examined species showed an increase and decrease during 1951–1980 and 1981–2013, respectively. The decrease in the S_T during 1981–2013 appears to be caused by reduced accumulation of chilling units. As with spring phenology, the S_T of leaf senescence of early successional and exotic species started to decline since 1980. In contrast, for late successional species, the S_T of autumn senescence showed an increase for the entire study period from 1951 to 2013. Moreover, the impacts of rising temperature associated with global warming on spring leaf unfolding were stronger than those on autumn leaf senescence. The timing of leaf senescence was positively correlated with the timing of leaf unfolding during 1951–1980. However, as climate warming continued, the differences in the responses between spring and autumn phenology gradually increased, so that the correlation was no more significant during 1981–2013. Our results further suggest that since 2000, due to the decreased temperature sensitivity of leaf unfolding the length of the growing season has not increased any more. These finding needs to be addressed in vegetation models used for assessing the effects of climate change.     

Identification of the maternal source of young‐of‐the‐year Arctic charr in Lake Hazen,Canada

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No universal scale-dependent impacts of invasive species on native plant species richness

A growing number of studies seeking generalizations about the impact of plant invasions compare heavily invaded sites to uninvaded sites. But does this approach warrant any generalizations? Using two large datasets from forests, grasslands and desert ecosystems across the conterminous United States, we show that (i) a continuum of invasion impacts exists in many biomes and (ii) many possible species–area relationships may emerge reflecting a wide range of patterns of co-occurrence of native and alien plant species. Our results contradict a smaller recent study by Powell et al. 2013 (Science 339, 316–318. (doi:10.1126/science.1226817)), who compared heavily invaded and uninvaded sites in three biomes and concluded that plant communities invaded by non-native plant species generally have lower local richness (intercepts of log species richness–log area regression lines) but steeper species accumulation with increasing area (slopes of the regression lines) than do uninvaded communities. We conclude that the impacts of plant invasions on plant species richness are not universal.     

Long‐term decline of native freshwater mussel assemblages in a federally protected river

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Reconstructing long‐term changes (150 years) in the carbon cycle of a clear‐water lake based on the stable carbon isotope composition (δ13C) of chironomid and cladoceran subfossil remains

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Long‐term climate impacts on breeding bird phenology in Pennsylvania,USA

Climate change is influencing bird phenology worldwide, but we still lack information on how many species are responding over long temporal periods. We assessed how climate affected passerine reproductive timing and productivity at a constant effort mist‐netting station in western Pennsylvania using a model comparison approach. Several lines of evidence point to the sensitivity of 21 breeding passerines to climate change over five decades. The trends for temperature and precipitation over 53 years were slightly positive due to intraseasonal variation, with the greatest temperature increases and precipitation declines in early spring. Regardless of broodedness, migration distance, or breeding season, 13 species hatched young earlier over time with most advancing >3 days per decade. Warm springs were associated with earlier captures of juveniles for 14 species, ranging from 1‐ to 3‐day advancement for every 1 °C increase. This timing was less likely to be influenced by spring precipitation; nevertheless, higher rainfall was usually associated with later appearance of juveniles and breeding condition in females. Temperature and precipitation were positively related to productivity for seven and eleven species, respectively, with negative relations evident for six and eight species. We found that birds fledged young earlier with increasing spring temperatures, potentially benefiting some multibrooded species. Indeed, some extended the duration of breeding in these warm years. Yet, a few species fledged fewer juveniles in warmer and wetter seasons, indicating that expected future increases could be detrimental to locally breeding populations. Although there were no clear relationships between life history traits and breeding phenology, species‐specific responses to climate found in our study provide novel insights into phenological flexibility in songbirds. Our research underscores the value of long‐term monitoring studies and the importance of continuing constant effort sampling in the face of climate change.     

Long‐term effect of biochar on the stabilization of recent carbon: soils with historical inputs of charcoal

This study was set up to identify the long‐term effect of biochar on soil C sequestration of recent carbon inputs. Arable fields (n = 5) were found in Belgium with charcoal‐enriched black spots (>50 m²; n = 14) dating >150 years ago from historical charcoal production mound kilns. Topsoils from these ‘black spots’ had a higher organic C concentration [3.6 ± 0.9% organic carbon (OC)] than adjacent soils outside these black spots (2.1 ± 0.2% OC). The soils had been cropped with maize for at least 12 years which provided a continuous input of C with a C isotope signature (δ¹³C) ?13.1, distinct from the δ¹³C of soil organic carbon (?27.4 ‰) and charcoal (?25.7 ‰) collected in the surrounding area. The isotope signatures in the soil revealed that maize‐derived C concentration was significantly higher in charcoal‐amended samples (‘black spots’) than in adjacent unamended ones (0.44% vs. 0.31%; P = 0.02). Topsoils were subsequently collected as a gradient across two ‘black spots’ along with corresponding adjacent soils outside these black spots and soil respiration, and physical soil fractionation was conducted. Total soil respiration (130 days) was unaffected by charcoal, but the maize‐derived C respiration per unit maize‐derived OC in soil significantly decreased about half (P < 0.02) with increasing charcoal‐derived C in soil. Maize‐derived C was proportionally present more in protected soil aggregates in the presence of charcoal. The lower specific mineralization and increased C sequestration of recent C with charcoal are attributed to a combination of physical protection, C saturation of microbial communities and, potentially, slightly higher annual primary production. Overall, this study provides evidence of the capacity of biochar to enhance C sequestration in soils through reduced C turnover on the long term.     

Multi‐faceted impacts of native and invasive alien decapod species on freshwater biodiversity and ecosystem functioning

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