Summary
Subsidy‐stress responses can decouple density‐dependent relationships between taxa within a community by increasing resources for some taxa while removing sensitive species. In this study, we ...examined the effects of a triad of subsidy‐stress responses on predator‐prey relationships in a benthic macroinvertebrate community under the action of insecticides, nutrients and predation pressure all at environmentally relevant doses.
We stocked 72 outdoor artificial streams with benthic invertebrates to establish a fully factorial experiment to investigate the interactive effects of two anthropogenic factors: (i) three levels of nutrient enrichment (low, moderate and high); and (ii) three doses of imidacloprid, a common agricultural insecticide, at control, low (LOEC) and lethal doses (LC50). These factors were crossed with (iii) another, biotic stressor by stocking half of the streams with a predaceous perlid stonefly at near‐natural densities.
Fifty percent of the community‐level responses were driven by the combined effect of nutrients and the insecticide. An additional 27% of the community variation was best explained by predation pressure, which we estimated as the gape width × density of Agnetina capitata at the end of the experiment.
No negative effect of imidacloprid on A. capitata was detected (P = 0.62). By the end of the 3‐week experiment, the overall gape width and predation pressure of A. capitata had more than doubled (2.1–3.7x) in all moderate nutrient streams (P < 0.01). Increased size and predation pressure of A. capitata covaried with the depressed density of all macroinvertebrate taxa as well as EPT (Ephemeroptera, Plecoptera, Trichoptera) taxa and the mayfly Baetis in particular.
Structural equation models (SEM) highlighted that nutrient enrichment was the main driver of individual and community responses in the artificial streams. Additionally, SEM showed that density but not body size of Baetis was severely hampered by A. capitata predation, suggesting that these stoneflies selectively foraged on other species in the first 10 days of the experiment.
Our findings highlight the difficulties of assessing the risk of contaminants in aquatic communities where interacting species respond differently to subsidies and stresses.
The effects of different types of predators may combine in complex ways to impact prey populations. In a series of microcosm experiments I empirically derived the functional response curves for two ...common types of stream predators, sculpins (Cottus bairdi) and stonefly larvae (Agnetina capitata), on two behaviorally and morphologically distinct types of mayfly prey (Baetis tricaudatus and Ephemerella subvaria). Data from these separate trials were combined using simple additive models to generate predicted combined functional responses for both types of predator. For both types of prey, actual combined functional response curves (both types of predators together in a treatment) differed significantly from predicted combined functional response for some ranges of prey densities. When Baetis was used as prey, significant departures from the predicted values occurred at moderate and high prey densities and were in a negative direction (interference between predators). When Ephemerella was the prey, significant departures occurred at low and moderate densities and were in a positive direction (facilitation between predators). Additive models for predicting combined predator effects cannot account for non—linearities in combined functional response introduced by non—trophic (behavioral) interactions that occur between fish and stoneflies, and between these predators and their prey. Other aquatic and terrestrial food webs contain similar linkages between vertebrate and invertebrate predators and their prey. Inclusion of behavioral interaction terms seems likely to be a necessary part of any general predicting combined effects of predators on prey populations in these systems.
In addition to the recent revision of the genus Agnetina Klapálek (Sivec et al., 2005), a list of the examined material (males, females, and larvae), including the type material is given. The main ...diagnostic characters for each species are also mentioned.
Agnetina den, a new species of perlid stonefly (Plecoptera: Perlidae) from Vietnam Cao, Thi Kim Thu (Seoul Women's University, Seoul, Republic of Korea); Bae, Y.J. (Seoul Women's University, Seoul, Republic of Korea), E-mail: yjbae@swu.ac.kr
Entomological research,
(Mar 2006), 2006-03, March 2006, 2006-03-00, Letnik:
36, Številka:
1
Journal Article
A new species of perlid stonefly, Agnetina den, is described from Vietnam. Male adults of this species can be easily distinguished by their black body color pattern, submedian patches of sensilla ...basiconica on hemitergites 78, and clusters of long dense fine setae on the inner sides the abdominal tergites. Illustrations of habitat and diagnostic characters are presented.
A series of experiments examined rates of prey consumption by mottled sculpins (Cottus bairdi) and perlid stoneflies (Agnetina capitata) separately, and when they were together in artificial streams. ...Two types of Ephemeroptera prey (Ephemerella subvaria and Baetis tricaudatus) were used in separate experiments. When both types of predators were present consumption of Baetis larvae was significantly less than model predictions of combined prey consumption, indicating a negative interaction between predators. Consumption of Ephemerella was significantly higher than predicted by the model indicating a positive interaction between predators. Analysis of gut contents indicated that Agnetina captured fewer Baetis larvae in the presence of sculpins, and sculpins captured significantly more Ephemerella larvae in the presence of stoneflies. Negative effects of sculpins on the number of Baetis larvae captured by Agnetina were found even when the sculpins were unable to feed. Positive effects of Agnetina on the number of Ephemerella captured by sculpins were still found even when Agnetina density was reduced by more than half. Understanding the complexity of interaction between fish and stoneflies may provide explanations for the contradiction between observed and expected results of fish removal experiments in streams.
In a series of laboratory experiments we examined the hypothesis that larvae of stream mayflies would respond to the presence of two different types of predators in such a way as to minimize their ...risk of being consumed by each. Positioning of larvae (whether they frequent the top, sides, or bottom of stones) of Baetis tricaudatus and Ephemerella subvaria was altered by the presence of predaceous stoneflies (Agnetina capitata) with a larger proportion of the population occurring on the upper surfaces, where the probability of encountering the predator was lowest. The presence of a benthivorous fish (Cottus bairdi) had no significant effects on positioning of the mayfly larvae. Lack of fish effects may reflect an inability of the mayflies to detect or respond to sculpins, or alternately may indicate that sculpins do not normally present a important predation risk for these mayflies. Failure of mayfly prey to account for fish predators when responding to the presence of stoneflies appears to explain facilitation previously observed between stoneflies and sculpins.
The nymph of Pteronarcys dorsata Say, a large and primitive stonefly, exhibits a range of behavioural defenses. We studied predator-prey interactions between stonefly nymphs and co-existing pelagic ...(trout) and benthic predators (sculpins, suckers and crayfish) to determine whether the defenses used by P. dorsata were predator specific. Nymphs of P. dorsata distinguished between different predator types and applied an appropriate defense mechanism to effectively avoid predation. To benthic and pelagic fish predators, P. dorsata responded primarily by freezing and thanatosis (feigning death). Survival was lower in the stonefly species Agnetina capitata which did not display thanatosis after attacks by trout. Nymphs of P. dorsata responded to benthic tactile crayfish predators by retreating and reflex bleeding (autohemorrhaging) which allowed for escape in all observed encounters. Reflex bleeding has never before been demonstrated as a defense mechanism for insects in aquatic environments. Risk of predation increased in nymphs deprived of their behavioural defenses. Stonefly nymphs were aggressive to conspecifics and experiments suggested that this behaviour may be a mechanism to avoid predation through spacing. Complex anti-predator defenses are generally unknown in aquatic insects; their discovery in such a primitive insect indicates that such behaviours may be evolutionarily ancient.
The nearctic Agnetina (=Phasganophora) are reviewed and three valid species are recognized. A. annulipes (Hagen), n. comb. and A. flavescens (Walsh), n. comb. are removed from the A. capitata ...(Pictet) synonymy and the three species are redescribed. Keys for imagoes and nymphs are provided and a lectotype is designated for A. flavescens.
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