Sub-lethal effects of disturbance on a predatory net-spinning caddisfly

1. The sub-lethal effects of hydrologic disturbances on stream invertebrates are poorly understood, but integral to some models of how disturbances influence population and community dynamics. Carnivorous larvae of a net-spinning caddisfly, Plectrocnemia conspersa , have a strong predatory impact in some streams. Their silken nets, however, are vulnerable to high flow disturbance and the consequent destruction of nets could reduce predatory impacts and have life history consequences.
2. In a laboratory experiment, we manipulated the frequency of disturbances that destroyed the nets of P. conspersa , in the presence and absence of potential prey. Animals were housed individually and each trial lasted 8 days. We estimated net size, cumulative mass of silk produced, net allocation (net mass expressed as a proportion of body mass), per capita prey consumption and growth or mass loss of larvae.
3. In the absence of prey, increased disturbance frequency was accompanied by increased loss of body mass, a reduction of net size and an increase in the cumulative mass of silk produced. At the highest disturbance frequency, larvae eventually gave up producing nets. The ratio of net mass to body mass decreased with increasing disturbance, suggesting a trade-off in the allocation of resources, with a decreasing proportion of resources available for foraging. In the presence of prey, increased disturbance frequency was accompanied by a reduction in per capita prey consumption. Although foraging success offset the costs of silk production, growth rate decreased with increasing disturbance and could eventually lead to reduced body size and fecundity of adults.
4. These sub-lethal effects suggest that hydrologic disturbances could impose metabolic costs and reduce foraging efficiency of this predator. Thus, disturbances may reduce predator impact on prey populations and reduce predator population size without any direct mortality or loss of individuals.     

Microcrustacean prey and macroinvertebrate predators in a stream food web

1. The consumption of microcrustacea by two polyphagous predators, larvae of the caddisfly Plectrocnemia conspersa (Curtis) and the alderfly Sialis fuliginosa Pictet, was investigated in an English stream with a well-known macro- and microinvertebrate fauna. Benthic samples were collected in August, November, December and April, and the gut contents of all individuals of both predators were examined. 2. All the microcrustacean groups (Cyclopoida, Harpacticoida, Chydoridae and Ostracoda) were identified in gut contents. Of the ten taxa present in the benthos, all occurred in the diet of P. conspersa; nine were found in S. fuliginosa. 3. Ontogenetic shifts in the diets of both predators were found, and microcrustacea were consumed more frequently by small than large instars. 4. There was little evidence of selective feeding by P. conspersa, whereas ostracods were over-represented in the diet of S. fuliginosa, compared with benthic relative densities. The Chydoridae were under-represented in the diet of both predators. 5. The food web of Broadstone Stream is perhaps the most detailed web available for any running water habitat. Increased taxonomic resolution produced marked changes in values of connectance and predator-prey ratios. Linkage density remained fairly constant at different levels of resolution and were high, indicative of a web of generalist species. Omnivory was pronounced and may be characteristic of donor-controlled systems where organic detritus is the primary energy base.     

The effects of disturbance on a herbivore-epilithon interaction in an upland stream

Two experiments were performed in an upland stream to determine the effects of the frequency of physical disturbance on the relationship between an abundant glossosomatid trichopteran (Agapetus monticolus; Banks) and the epilithon upon which it feeds. Artificial cobbles with an established epilithic community were tumbled either every 1, 2 or 4 weeks. The first experiment failed to detect any significant effects of rock tumbling on the abundance of A. monticolus or the epilithon: a result due to several spates. The first experiment did reveal that disturbances may disrupt the ability of A. monticolus to locate patches of abundant food. The second experiment found that although the abundance of A. monticolus was not affected by the disturbances, periphyton abundance was significantly reduced. Increasing the frequency of disturbance did not magnify this effect. Comparisons of these results with other studies of disturbance in streams indicate that the effects of disturbance on herbivory may be highly variable. A variety of factors, such as the relative resistances of the herbivores and the epilithon, need to be examined before the effects of disturbances on lotic herbivorous interactions can be completely understood.     

Effects of flooding on avian top-predators and their invertebrate prey in a monsoonal Taiwan stream

1. The effects of flooding on top predators are poorly understood globally, but particularly in monsoonal streams. We therefore attempted to assess how inter-annual and intra-annual variations in flood magnitude affected an obligate riverine predator, the brown dipper ( Cinclus pallasii ), and its invertebrate prey, in the mountain Tachia River, Taiwan. Major flooding in one of the study years (2005) allowed an insight into the effects of abnormally large flows.
2. The abundance and biomass of insects, and the abundance of dippers, decreased steadily from 2003 to 2005 as flood magnitude grew, but then increased in 2006 when more typical discharge returned. Dipper abundance, insect abundance and insect biomass were all strongly positively inter-related, but negatively related to discharge. Insect biomass, rather than abundance, was more useful in predicting brown dipper abundance.
3. Aquatic insect composition fluctuated among sampling years, revealed by non-metric multidimensional scaling, and these fluctuations were also related to discharge. In turn, dipper abundance and the mean body size of aquatic insects declined with the shift in insect composition as flow increased.
4. These data illustrate how discharge fluctuations can have pronounced effects on top predators in streams, mediated in this case by fluctuating prey abundance. While contributions from bird movement, breeding performance and mortality were not clearly differentiated, our data reveal how dippers have strategies to accommodate varying discharge in river systems. We suggest that the effects of floods on dippers should be taken into account when using this group as indicators of river quality.     

Spatial and temporal trends in stream macroinvertebrate communities: the influence of catchment disturbance

Macroinvertebrate communities of five headwater streams in catchments disturbed by wildfire were compared with five similar streams with no catchment disturbance. Over the five years of observation, communities in disturbed streams were more similar to one another than they were to reference streams. Communities in disturbed streams exhibited more year-to-year variation than reference streams, although some indication of decreasing variation was evident through time, and species richness was greater in reference streams than disturbed streams. No increasing trend in richness over time was observed in disturbed streams. Stability of the relative abundance structure and persistence of dominant taxa through time may be characteristic of temperate streams over moderate time intervals. Local effects of catchment-wide disturbance have persistent effects that alter these trends.     

Parallels and contrasts in the effects of drought on stream macroinvertebrate assemblages

• 1 It is axiomatic that unusually long dry periods (droughts) adversely affect aquatic biota. Recovery after drought is rapid by macroinvertebrates that possess strategies to survive drying or are highly mobile but other taxa take longer to recolonise depending on the timing, intensity, and duration of the dry phase.
• 2 Although drought acts as a sustained ‘ramp’ disturbance, impacts may be disproportionately severe when certain critical thresholds are exceeded. For example, ecological changes may be gradual while a riffle dries but cessation of flow causes abrupt loss of a specific habitat, alteration of physicochemical conditions in pools downstream, and fragmentation of the river ecosystem. Many ecological responses to drought within these habitats apparently depend on the timing and rapidity of hydrological transitions across these thresholds, exhibiting a ‘stepped’ response alternating between gradual change while a threshold is approached followed by a swift transition when a habitat disappears or is fragmented.
• 3 In two Australian intermittent streams, drought conditions eliminated or decimated several groups of macroinvertebrates, including atyid shrimps, stoneflies and free‐living caddisflies. These taxa persisted during the early stages of the drought but did not recruit successfully the following year, despite a return to higher‐than‐baseflow conditions. This ‘lag effect’ in response to drought emphasises the value of long‐term survey data. Although changes in faunal composition were inconsistent among sites, marked shifts in taxa richness, abundance and trophic organisation after the riffle habitat dried provide evidence for a stepped response.
• 4 Responses by macroinvertebrate assemblages to droughts of differing severity in English chalk streams were variable. The prolonged 1988–92 drought had a greater impact than shorter droughts in the early 1970s but recovery over the next 3 years was swift. Effects of the 1995 summer drought were buffered by sustained groundwater discharge from the previous winter. These droughts tended to reduce available riverine habitats, especially via siltation, but few taxa were eliminated because they could recolonise from perennial sections of the chalk streams.
• 5 In the contrasting environments of the intermittent streams studied in England and Australia, there are parallels in the rapid rates of recolonisation. However, recruitment by taxa that lack desiccation‐resistant stages or have limited mobility is delayed. Currently, long‐term data on these systems may be insufficient to indicate persistent effects of droughts or predict the impacts of excessive surface or groundwater abstraction or the increased frequency and duration of droughts expected with global climate change.

     

Observation of epigeic predators and predation on artificial prey in a cereal field

Predation activity of different epigeic predators on artificially supplied prey was studied in a cereal field during several 24 h periods from spring to early summer 1993. Carabids were the most frequently observed predators. Among themPoecilus cupreus was the most voracious species during day-time, andAgonum muelleri during night-time. Predation rates were highest between 10.00 and 18.00 h and lowest between 02.00 and 10.00 h. Linear regressions showed significantly positive relations between temperature and predation rates, and between predation and activity density of epigeic predators. The latter relation was stronger when only activity density of predators which were observed to consume baits was included. Quadrat sampling and pitfall sampling showed that with the former only a small proportion of bait-consuming predators were caught whereas with the latter method this proportion was high.     

Long‐term effects of climatic–hydrological drivers on macroinvertebrate richness and composition in two Mediterranean streams

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Evaluating the effects of large marine predators on mobile prey behavior across subtropical reef ecosystems

The indirect effect of predators on prey behavior, recruitment, and spatial relationships continues to attract considerable attention. However, top predators like sharks or large, mobile teleosts, which can have substantial top–down effects in ecosystems, are often difficult to study due to their large size and mobility. This has created a knowledge gap in understanding how they affect their prey through nonconsumptive effects. Here, we investigated how different functional groups of predators affected potential prey fish populations across various habitats within Biscayne Bay, FL. Using baited remote underwater videos (BRUVs), we quantified predator abundance and activity as a rough proxy for predation risk and analyzed key prey behaviors across coral reef, sea fan, seagrass, and sandy habitats. Both predator abundance and prey arrival times to the bait were strongly influenced by habitat type, with open homogenous habitats receiving faster arrival times by prey. Other prey behaviors, such as residency and risk‐associated behaviors, were potentially driven by predator interaction. Our data suggest that small predators across functional groups do not have large controlling effects on prey behavior or stress responses over short temporal scales; however, habitats where predators are more unpredictable in their occurrence (i.e., open areas) may trigger risk‐associated behaviors such as avoidance and vigilance. Our data shed new light on the importance of habitat and context for understanding how marine predators may influence prey behaviors in marine ecosystems.     

The impact of flash floods on microbial distribution and biogeochemistry in the parafluvial zone of a desert stream

1. The impact of flash flooding on microbial distribution and biogeochemistry was investigated in the parafluvial zone (the part of the active channel lateral to the surface stream) of Sycamore Creek, a Sonoran Desert stream in central Arizona. 2. It was hypothesized that subsurface bacteria were dependent on the import of algal-derived organic matter from the surface stream, and it was therefore predicted that microbial numbers and rates of microbially mediated processes would be highest at locations of surface to subsurface hydrologic exchange and at times when algal biomass was high. 3. Prior to a flash flood on 19 July 1994, chlorophyll a was high (≈ 400 mg m^–2) in the surface stream and microbial numbers were highest at the stream–parafluvial interface and declined along parafluvial flowpaths, supporting the hypothesized algal–bacterial linkage. Immediately following the flash flood, chlorophyll a was low (≈ 7 mg m^–2), and microbial numbers were reduced at the stream–parafluvial interface. 4. Counter to expectations, parafluvial functioning (in terms of nitrate production and dissolved oxygen decline along flowpaths) re-established immediately after the flood receded. Therefore, material other than algal exudates supported parafluvial metabolism immediately postflood, and terrestrially derived dissolved organic matter is the likely source. 5. Algae in the surface stream recovered quickly following flooding, but recovery of parafluvial bacteria lagged somewhat behind. These results highlight the importance of surface–subsurface interaction to stream ecosystem functioning and show that the nature of these interactions changes substantially in successional time.     

Impact of flooding on the densities of selected aquatic insects

Data from a four-year study of five aquatic insect species,Hydropsyche betteni, H. morosa, H. bronta, Isonychia bicolor, andEphoron leucon, were utilized to evaluate the impact of a 60-year flood and a few lesser floods. The survey began in August, 1984 and was terminated in October, 1987 with the 60-year flood occurring in November, 1985. Four sampling sites were established on the South River and six quantitative samples were taken each month from each site. Gauging stations on the South River provided accurate discharge data for the sampling sites and useful historical data. Densities for the five species were utilized in the evaluation of the floods. The importance of timing is pointed out, that is, floods that occur very close together or near the end of the life cycle of an insect make it difficult to evaluate floods as disturbances. The importance of life history traits, such as behavior and egg diapause, are discussed in respect to floods. Densities were reduced to less than 50% of their average values immediately after the 60-year flood for the threeHydropsyche spp. and at three sites forI. bicolor. Ephoron leucon showed no response to the 60-year flood. Densities of the four impacted species returned to previous levels in the following generation. The 60-year flood was considered a disturbance in the near term but not for more than one generation.     

Effects of hydrological variation and planktivorous competition on macroinvertebrate community structure in experimental billabongs

1. To study two factors which are predicted as causing changes to community structure in cut-off meanders (colloquially known in Australia as billabongs, a term of aboriginal origin), 16 experimental billabongs were constructed. These were designed to test two hypotheses: (a) that the structure of macrophyte and invertebrate communities within billabongs is altered by changing the pattern of flooding; and (b) that the presence of small planktivorous fish alters invertebrate community structure and diversity within billabongs.
2. An increase in the duration of flooding seems to favour animals better adapted to a greater availability of macrophyte habitat. Changing the seasonality of flooding resulted in prolonging of the time water was available over the summer months.
3. The presence of a planktivorous fish appears to affect macroinvertebrate communities through competition with other planktivores. Variable top-down pressure may create differing successional patterns and ultimately different communities at lower trophic levels.     

Utilization of autochthonous macroinvertebrate drift by a pool fish community in a woodland stream

Qualitative and quantitative drift data were collected simultaneously above and below a pool both before and after the pool was heavily electrofished. These data revealed no significant difference between drift organism densities (#/m³) above or below the pool before or after fish collection. Qualitative and quantitative fish gut analyses suggest that the sunfishes Lepomis megalotis, L. macrochirus and L. cyanellus may be feeding on drifting invertebrates as these organisms comprised 58%, 37% and 35% gut volume, respectively. A design for a long-term drift net apparatus is presented.     

Emergent effects of multiple predators on prey survival: the importance of depletion and the functional response

The combined effects of multiple predators often cannot be predicted from their independent effects. Emergent multiple predator effects (MPEs) include risk enhancement, where combined predators kill more prey than predicted by their individual effects, and risk reduction, where fewer prey are killed than predicted. Current methods for detecting MPEs are biased because they assume linear functional responses and/or no prey depletion. As a result, past studies overestimated the occurrence of risk enhancement for additive designs, and tended to overestimate the occurrence of risk reduction for substitutive designs. Characterising the predators' functional responses and accounting for prey depletion reduces biases in detection, estimation, interpretation and generalisation of the emergent effects of predator diversity on prey survival. These findings have implications beyond MPE's and should be considered in all studies aimed at understanding how multiple factors combine when demographic rates are density dependent.     

Dodging the moon: The moon effect on activity allocation of prey in the presence of predators

Animals can adapt their activity patterns throughout the circadian cycle. Prey may use moonlight as a predation risk cue and allocate their activity to lower risk periods. Here, we assessed moon transit influence on the activity allocation of nocturnal mammalian prey, in the presence of a predator (pumas, Puma concolor), during different moon phases, through camera trapping in Central Amazon. Thirty camera traps were installed 2 km apart from each other in each of our three study sites. Prey record distributions were assessed across the moon cycle, and their daily activity patterns were described in each moon phase. The record distributions of pacas (Cuniculus paca) (N = 262) and armadillos (Dasypus sp.) (N = 244) were concentrated in darker nights, contrasting with red brocket deer (Mazama americana) (N = 123) and pumas (N = 31), whose records were evenly distributed through the moon cycle. Yet, every prey was found to avoid the brighter times of the night regardless of the moon phase. These findings suggest prey can shift the temporal distribution of their activities under different moon phases when predators are present, perhaps in response to predation risk variations.     

Productivity, dispersal and the coexistence of intraguild predators and prey

A great deal is known about the influence of dispersal on species that interact via competition or predation, but very little is known about the influence of dispersal on species that interact via both competition and predation. Here, I investigate the influence of dispersal on the coexistence and abundance-productivity relationships of species that engage in intraguild predation (IGP: competing species that prey on each other). I report two key findings. First, dispersal enhances coexistence when a trade-off between resource competition and IGP is strong and/or when the Intraguild Prey has an overall advantage, and impedes coexistence when the trade-off is weak and/or when the Intraguild Predator has an overall advantage. Second, the Intraguild Prey's abundance-productivity relationship depends crucially on the dispersal rate of the Intraguild Predator, but the Intraguild Predator's abundance-productivity relationship is unaffected by its own dispersal rate or that of the Intraguild Prey. This difference arises because the two species engage in both a competitive interaction as well as an antagonistic (predator-prey) interaction. The Intraguild Prey, being the intermediate consumer, has to balance the conflicting demands of resource acquisition and predator avoidance, while the Intraguild Predator has to contend only with resource acquisition. Thus, the Intraguild Predator's abundance increases monotonically with resource productivity regardless of either species' dispersal rate, while the Intraguild Prey's abundance-productivity relationship can increase, decrease, or become hump-shaped with increasing productivity depending on the Intraguild Predator's dispersal rate. The important implication is that a species' trophic position determines the effectiveness of dispersal in sampling spatial environmental heterogeneity. The dispersal behavior of a top predator is likely to have a stronger effect on coexistence and spatial patterns of abundance than the dispersal behavior of an intermediate consumer.     

Plant diversity and identity effects on predatory nematodes and their prey

There is considerable evidence that both plant diversity and plant identity can influence the level of predation and predator abundance aboveground. However, how the level of predation in the soil and the abundance of predatory soil fauna are related to plant diversity and identity remains largely unknown. In a biodiversity field experiment, we examined the effects of plant diversity and identity on the infectivity of entomopathogenic nematodes (EPNs, Heterorhabditis and Steinernema spp.), which prey on soil arthropods, and abundance of carnivorous non‐EPNs, which are predators of other nematode groups. To obtain a comprehensive view of the potential prey/food availability, we also quantified the abundance of soil insects and nonpredatory nematodes and the root biomass in the experimental plots. We used structural equation modeling (SEM) to investigate possible pathways by which plant diversity and identity may affect EPN infectivity and the abundance of carnivorous non‐EPNs. Heterorhabditis spp. infectivity and the abundance of carnivorous non‐EPNs were not directly related to plant diversity or the proportion of legumes, grasses and forbs in the plant community. However, Steinernema spp. infectivity was higher in monocultures of Festuca rubra and Trifolium pratense than in monocultures of the other six plant species. SEM revealed that legumes positively affected Steinernema infectivity, whereas plant diversity indirectly affected the infectivity of Heterorhabditis EPNs via effects on the abundance of soil insects. The abundance of prey (soil insects and root‐feeding, bacterivorous, and fungivorous nematodes) increased with higher plant diversity. The abundance of prey nematodes was also positively affected by legumes. These plant community effects could not be explained by changes in root biomass. Our results show that plant diversity and identity effects on belowground biota (particularly soil nematode community) can differ between organisms that belong to the same feeding guild and that generalizations about plant diversity effects on soil organisms should be made with great caution.     

Temperature dependence of predation depends on the relative performance of predators and prey

The temperature dependence of predation rates is a key issue for understanding and predicting the responses of ecosystems to climate change. Using a simple mechanistic model, we demonstrate that differences in the relative performances of predator and prey can cause strong threshold effects in the temperature dependence of attack rates. Empirical data on the attack rate of northern pike (Esox lucius) feeding on brown trout (Salmo trutta) confirm this result. Attack rates fell sharply below a threshold temperature of +11°C, which corresponded to a shift in relative performance of pike and brown trout with respect to maximum attack and escape swimming speeds. The average attack speed of pike was an order of magnitude lower than the escape speed of brown trout at 5°C, but approximately equal at temperatures above 11°C. Thresholds in the temperature dependence of ecological rates can create tipping points in the responses of ecosystems to increasing temperatures. Thus, identifying thresholds is crucial when predicting future effects of climate warming.     

Encounters between aphids and their predators: the relative frequencies of disturbance and consumption

Ecologists may wish to evaluate the potential for predators to suppress prey populations through the costs of induced defensive behaviors as well as through consumption. In this paper, we measure the ratio of non‐consumptive, defense‐inducing encounters relative to consumptive encounters (henceforth the ‘disturbed : consumed ratio’) for two species of aphids and propose that these disturbed : consumed ratios can help evaluate the potential for behaviorally mediated prey suppression. For the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae), the ratio of induced disturbances to consumption events was high, 30 : 1. For the cotton aphid, Aphis gossypii (Glover) (Homoptera: Aphididae), the ratio of induced disturbances to consumption events was low, approximately 1 : 14. These results indicate that the potential for predators to suppress pea aphid populations through induction of defensive behaviors is high, whereas the potential for predators to suppress cotton aphid populations through induced behaviors is low. In measuring the disturbed : consumed ratios of two prey species, this paper makes two novel points: it highlights the variability of the disturbed : consumed ratio, and it offers a simple statistic to help ecologists draw connections between predator–prey behaviors and predator–prey population dynamics.     

Quantifying the direct and indirect effects of flow‐related disturbance on stream fish assemblages

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