Combined Effects Of Predation Research Articles

The combined effects of predation, fishing, and ocean productivity on salmon species targeted by marine mammals in the northeast Pacific.

Along the northeast Pacific coast, the salmon-eating southern resident killer whale population (SRKW, Orcinus orca) have been at very low levels since the 1970s. Previous research have suggested that reduction in food availability, especially of Chinook salmon (Oncorhynchus tshawytscha), could be the main limiting factor for the SRKW population. Using the ecosystem modelling platform Ecopath with Ecosim (EwE), this study evaluated if the decline of the Pacific salmon populations between 1979 and 2020 may have been impacted by a combination of factors, including marine mammal predation, fishing activities, and climatic patterns. We found that the total mortality of most Chinook salmon populations has been relatively stable for all mature returning fish despite strong reduction in fishing mortality since the 1990s. This mortality pattern was mainly driven by pinnipeds, with increases in predation between 1979 and 2020 mortality ranging by factors of 1.8 to 8.5 across the different Chinook salmon population groups. The predation mortality on fall-run Chinook salmon smolts originating from the Salish Sea increased 4.6 times from 1979 to 2020, whereas the predation mortality on coho salmon (Oncorhynchus kisutch) smolts increased by a factor of 7.3. The model also revealed that the north Pacific gyre oscillation (NPGO) was the most important large-scale climatic index affecting the stock productivity of Chinook salmon populations from California to northern British Columbia. Overall, the model provided evidence that multiple factors may have affected Chinook salmon populations between 1979 and 2020, and suggested that predation mortality by marine mammals could be an important driver of salmon population declines during that time.

Pathogens and predators: examining the separate and combined effects of natural enemies on assemblage structure.

Natural enemy ecology strives to unify predator-prey and host-pathogen interactions under a common framework to gain insights into community- and ecosystem-level processes. To address this goal, ecologists need a greater emphasis on: (1) quantifying pathogen-mediated effects on community structure to enable comparisons with predator-mediated effects and (2) determining the interactive effects of combined natural enemies on communities. We conducted a mesocosm experiment to assess the individual and combined effects of predators (dragonfly larvae and adult water bugs) and a pathogen (ranavirus) on the abundance and composition of a larval amphibian assemblage. We found that our three natural enemies structured victim assemblages in unique ways, producing distinct assemblages. Additionally, we found that in combination treatments, predators mainly drove assemblage structure such that the assemblages most closely resembled their respective predator treatments. We also found that predators reduced infection prevalence in combination treatments, and that the magnitude of this effect was dependent on predator identity. Compared to virus-alone treatments, the presence of dragonflies and water bugs reduced infection prevalence by 79% and 63%, respectively. Additionally, the presence of dragonflies eliminated ranavirus infection in two species, which demonstrates the prominent role of predators in disease dynamics in this system. Overall, this work demonstrates the importance of considering natural enemies in community ecology, as each enemy can elicit a unique structural change. Additionally, this study provides a unique empirical test of the healthy herds hypothesis for multi-species assemblages and underscores the importance of advancing our understanding of multi-enemy interactions within communities.

Predation, parasitism, and drought counteract the benefits of patch-burn grazing for the reproductive success of grassland songbirds

Abstract Intensification of livestock production has reduced heterogeneity in vegetative structure in managed grasslands, which has been linked to widespread declines in grassland songbird populations throughout North America. Patch-burn grazing management aims to restore some of that heterogeneity in vegetative structure by burning discrete pasture sections, so that cattle preferentially graze in recently burned areas. Although patch-burn grazing can increase reproductive success of grassland songbirds, we know little about possible interactions with regional variation in predator communities or brood parasite abundance, or annual variation in weather conditions. Using six years of data from two tallgrass prairie sites in eastern Kansas, USA, we tested effects of patch-burn grazing on the rates of brood parasitism, clutch size, nest survival, and fledging success of three common grassland songbirds, Dickcissels (Spiza americana), Eastern Meadowlarks (Sturnella magna), and Grasshopper Sparrows (Ammodramus savannarum), among pastures managed with patch-burn grazing versus pastures that were annually burned and either grazed or ungrazed. Dickcissel nests experienced lower parasitism (72.8 ± 4.6% SE vs. 89.1 ± 2.2%) and Eastern Meadowlarks had higher nest survival (63.2 ± 20.5% vs. 16.5 ± 3.5%) in annually burned and ungrazed pastures than pastures managed with patch-burn grazing. However, average number of host fledglings per nesting attempt did not differ among management treatments for any species. Annual variation in weather conditions had a large effect on vegetation structure, but not on reproductive success. Probability of brood parasitism was consistently high (25.5‒84.7%) and nest survival was consistently low (9.9–16.9%) for all species pooled across treatments, sites, and years, indicating that combined effects of predation, parasitism and drought can offset potential benefits of patch-burn grazing management previously found in tallgrass prairies. Although differences in reproductive success among management treatments were minimal, patch-burn grazing management could still benefit population dynamics of grassland songbirds in areas where nest predators and brood parasites are locally abundant by providing suitable nesting habitat for bird species that require greater amounts of vegetation cover and litter, generally not present in burned pastures.

Vital economic threat of predatory birds and parasites to cultivated fishes in Egypt

Despite continuing worldwide expansion in production and demand, recent performance in the aquaculture sector in Egypt has been volatile, and yield has reduced. The most significant and visible problems arise from parasitic diseases and predation by birds. In this study, we examine the impact of local parasite fauna and bird predation in relation to economic loss in fish farms, with a focus on the importance of water quality. Water, fish, and bird samples were obtained from private fish farms at El-Abbasa in northern Egypt. Results showed significant (P < 0.05) production losses between 54.08 ± 1.88% and 62.11 ± 2.37%. Eleven predatory bird species were observed, with two species feeding in a variety of ways. The estimated total loss from fish eaten by predatory birds was ~1.59 tons over the six-month production cycle, valued at USD 1, 990. The studied fish had high rates of parasitic infestation: 63.89% in Oreochromis niloticus (Nile tilapia) and 75.85% in Clarias gariepinus (African sharptooth catfish). Zoonotic fish parasites that may constitute a health hazard for those consuming the farmed fish were recovered. Hemato-biochemical indices showed significant decreases in red blood cell count, hemoglobin, and total protein, and significant increases in white blood cell count, activities of alanine aminotransferase and aspartate aminotransferase, and levels of glucose and uric acid in parasitized fish. Water physicochemical factors deviated significantly from norms in the majority of metrics. This study's results could help us gain a better understanding of production losses at fish farms. Overall, the combined effects of predation by birds and parasite infection increased fish loss and resulted in significant negative economic effects at fish farms.

A complex regulating pattern induced by the effects of predation and parasites on root vole (Microtus oeconomus) populations during the breeding season

Abstract Predators and parasites have synergistic effects on the phenotype of the species they share as prey and host. Experimental studies that incorporate the interactions between predation and parasitism are, however, scarce in small-mammal populations. Our previous work has shown that the combined effects of predation and coccidian infection reduce overwinter survival and population density in root voles (Microtus oeconomus). Here, we examined the separate and combined effects of these two drivers on the population growth of root voles during the breeding season. We carried out a two-level factorial experiment, in which we manipulated predator exclusion and the removal of parasites in enclosures and measured survival, fecal corticosterone metabolite (FCM) concentration, recruitment, and population density. An expected synergistic effect of predators and parasites on vole population was not found, due to no effect of parasites on FCM level and recruitment rate during the period of the experiment. Instead, we found phase-related effects of predation on demography. Predation reduced the survival rate of voles in spring, which was intensified by parasite infection. Predation risk reduced recruitment rate in early summer by elevating FCM levels. Consequently, both direct and indirect effects of predation lowered population density during the experimental period. In addition, for populations free of predators, the peak density that occurred in early autumn elevated FCM level of adult voles, which reduced recruitment rates and halted population growth. Moreover, predation, parasites, and density affected the quality of the offspring. Our study suggests that multiple regulation processes influence population fluctuations during the breeding season. We conclude that a population experiencing stress acts as a common interface through which interactions between intrinsic and extrinsic factors can be important determinants of fluctuations. We propose a new hypothesis of integrative stress effects to explain small-mammal population fluctuations.

Combined effects of predation, flow speed, and sub-lethal exposure to insecticide on the feeding behavior of a spionid polychaete

Behavioral responses of infauna to predation have received limited study, especially in varying hydrodynamic conditions above estuarine sediments. Many estuaries also are impacted by chemical pollution, including runoff of terrestrial insecticides such as chlorpyrifos. We tested how feeding behaviors of a tube-dwelling, interface-feeding worm, Polydora cornuta, change due to predation by an omnivorous estuarine fish, Fundulus parvipinnis, in the contexts of hydrodynamic regime and sub-lethal exposure to chlorpyrifos. We performed two experiments in a laboratory flume at two flow speeds (6 or 15 cm s−1). We analyzed feeding behaviors of P. cornuta when F. parvipinnis were either actively feeding on the worms, caged upstream to prevent predation, or absent from the flume. In our first experiment, the presence of actively feeding F. parvipinnis reduced the time P. cornuta spent feeding by half relative to when fish were absent. Fish caged upstream had no effect on worms' feeding activity, indicating that P. cornuta reduced their feeding in response to tactile stimuli or sensing light shadows, rather than chemical alarm cues. Our second experiment tested the effects of actively feeding F. parvipinnis on the feeding behaviors of P. cornuta that had undergone a 96-h exposure to chlorpyrifos. Worms exposed to chlorpyrifos reduced feeding relative to unexposed worms, but only in the absence of predatory fish. Flow speed also influenced worms' behavior, with worms preferentially suspension feeding at the faster flow. In both experiments, individual worms that were observed to be attacked by F. parvipinnis resumed feeding approximately 10–11 min after the attack. The combined results demonstrate that the close proximity of actively feeding fish has a consistently greater effect on the feeding behavior of an infaunal polychaete than does sub-lethal exposure to a toxicant or variation in water flow.

Combined effects of predators and parasites on shoaling behavior of fishes

A review of experimental, field and theoretical papers on several topics related to the study of variability of fish shoaling behavior caused by separate impacts of predators and parasites as well as by combined effects of these factors. First, antipredator functions and changes in fish shoal parameters caused by predation risk are briefly discussed. Then, effects of parasites that have the potential to act as a force that can select for either larger or smaller group size, or even for solitary behavior are reviewed. Predation- and parasitism-induced variations in the shoal size and shape, distance between members of a shoal, position of fish within a shoal, effects of habitat complexity, parasite-assortative shoaling are described. Finally, an interplay between the parasitism and predation risks that could influence protective functions of fish shoaling is discussed. It is emphasized that not only the binary “predator-prey” and “host-parasite” systems, but a three-component system “parasite.-host-predator” which embrace both direct and indirect effects have to be studied.

How do shoot clipping and tuber harvesting combine to affectBolboschoenus maritimusrecovery capacities?

In French Mediterranean wetlands, the combined effects of predation of tubers by wildlife and grazing of aboveground tissue by livestock on the recovery capacities of Bolboschoenus maritimus (L.) Palla are not well known. A container study was conducted that applied tuber harvests at varying levels (20%–90%) and shoot clipping (with or without). Response to harvesting and clipping was recorded as changes in total biomass, number, and mean mass of tubers (calculation of variation indexes). Bolboschoenus maritimus failed to recover from even the lowest tuber harvesting level of 20% and the total number of tubers and biomass decreased. A significant decrease in mean tuber mass over time and approximately no production of new tubers accounted for this absence of compensatory response. The harvesting level had a linear effect on the variation indices of total number of tubers and mean tuber mass. By separating the relative effect of shoot clipping from that of tuber harvesting alone, the results showed that clipping had an additive effect on mean tuber mass, reducing it by about 20%, without any effect on tuber number. The absence of compensatory response under our experimental conditions suggests that clonal plant regrowth partially depends on post-disturbance environmental conditions in the growing season, in our case, dry conditions.

SUPPRESSION OF SOYBEAN APHID BY GENERALIST PREDATORS RESULTS IN A TROPHIC CASCADE IN SOYBEANS

Top-down regulation of herbivores in terrestrial ecosystems is pervasive and can lead to trophic cascades that release plants from herbivory. Due to their relatively simplified food webs, agroecosystems may be particularly prone to trophic cascades, a rationale that underlies biological control. However, theoretical and empirical studies show that, within multiple enemy assemblages, intraguild predation (IGP) may lead to a disruption of top-down control by predators. We conducted a factorial field study to test the separate and combined effects of predators and parasitoids in a system with asymmetric IGP. Specifically we combined ambient levels of generalist predators (mainly Coccinellidae) of the soybean aphid, Aphis glycines Matsumura, with controlled releases of the native parasitoid Lysiphlebus testaceipes (Cresson) and measured their impact on aphid population growth and soybean biomass and yield. We found that generalist predators provided strong, season-long aphid suppression, which resulted in a trophic cascade that doubled soybean biomass and yield. However, contrary to our expectations, L. testaceipes provided minor aphid suppression and only when predators were excluded, which resulted in nonadditive effects when both groups were combined. We found direct and indirect evidence of IGP, but because percentage parasitism did not differ between predator exclusion and ambient predator treatments, we concluded that IGP did not disrupt parasitism during this study. Our results support theoretical predictions that intraguild predators which also provide strong herbivore suppression do not disrupt top-down control of herbivores.

Biodiversity, population regulation, and the stability of coral-reef fish communities.

Unprecedented population declines and extinctions because of human activities, combined with a growing recognition that such losses affect the stability of ecosystems, underscore the need to better understand how populations persist naturally. We provide field experimental evidence that high biodiversity-in particular, the combined effects of predators and competitors-acts in a way that regulates the size of local fish populations within their coral-reef community. These results indicate that complex interactions among multiple species are necessary for the stability of a highly diverse community, and so forewarn that overexploiting such species may have cascading negative consequences for the entire system.

FACILITATION AMONG PISCIVOROUS PREDATORS: EFFECTS OF PREY HABITAT USE

The combined effects of predators on prey may substantially differ from thatof each predator species alone because of alterations in prey behavior. Using enclosures within a pond, we examined experimentally the effects of two piscivorous predators on prey mortality and prey resource levels in two habitats. The two predators use two different foraging modes, which also allowed us to examine the behaviorally induced indirect effects of prey on predator growth and prey food resources. Both perch (Perca fluviatilis) and pike (Esox lucius) caused significant mortality of roach (Rutilus rutilus), and the combined predator mortality was higher than predicted from a multiplicative prey consumption model. Growth rates of perch were similar when enclosures contained only perch and when they contained perch combined with pike. The growth rate of pike was higher when they were together with perch compared to when alone. Growth of roach was similar among treatments. The invertebrate food resources of roach increased by a factor 10 in the open water but remained at similar levels throughout the experiment in the vegetation. Biomass of Daphnia longispina, the dominant zooplankton species in the open water, was strongly correlated with mortality of roach, indicating a density-mediated indirect effect of predators on prey resources. There was no indirect effect on D. longispina in the vegetation caused by habitat restriction of roach and only a weak relationship in the open water. There was a strong indirect effect of pike predation on macroinvertebrates induced by a habitat shift of roach. Our results suggest that there was facilitation between predators caused by conflicting antipredator behavior of roach, which resulted in density-mediated indirect effects on prey resources. The behavioral response of roach to the two predators also induced indirect effects on invertebrate prey.

Effectiveness of Lacewing Larvae in Reducing Russian Wheat Aphid Populations on Susceptible and Resistant Wheat

The use of resistant plants may enhance or reduce the ability of natural enemies to control pest insects. We conducted a series of experiments to compare the effectiveness of lacewing larvae, Chrysoperla plorabunda (Fitch), in suppressing populations of the Russian wheat aphid, Diuraphis noxia (Mordvilko), on susceptible and resistant wheat. Caged plants were inoculated with the same initial density of aphids, and predators were added to half the plants after nearly two aphid generations. Additional plants were used to estimate aphid densities on resistant and susceptible plants when predators were introduced. In one comparison, the effects of plant resistance and predation were synergistic; lacewing larvae caused a greater proportional reduction in aphid density on a tolerant-resistant line (carrying the Dn 4 gene) than on its near-isogenic susceptible parent. Higher predation rates on the resistant line persisted over a wide range of predator/prey ratios. The combined effects of predation and plant resistance were mostly additive in other experiments, including a comparison of an antibiosis-resistant line (possessing the Dn 1 gene) and its near-isogenic parent. In no comparison was lacewing effectiveness lower on the resistant line. Our results suggest that even modest reductions in aphid population growth (as might occur on tolerant-resistant plants) can be sufficient to produce a synergistic level of pest suppression.

THE INFLUENCE OF MULTIPLE INTRODUCED PREDATORS ON A LITTORAL POND COMMUNITY

In a replicated field experiment we studied the effects of natural densities of two exotic consumers, the predatory and herbivorous signal crayfish (Pacifastacus leniusculus) and the predatory rainbow trout (Oncorhynchus mykiss), on multiple trophic levels of a pond community. The goals were to: (1) determine the individual and combined effects of predators on macroinvertebrates, macrophytes, and periphytic algae; (2) evaluate the strength of direct and indirect interactions in a food web influenced by omnivores; and (3) evaluate the relative importance of direct and indirect predator effects on mortality and growth of a native frog species, Rana temporaria. The experiment showed that both signal crayfish and rainbow trout had strong effects on multitrophic levels of a littoral pond community, through direct consumption and indirect effects on lower trophic levels. Crayfish had weak but significant negative effects on the biomass of predatory invertebrates and greatly reduced the biomass of snails, the most abundant invertebrate grazers. Although the number of active herbivorous tadpoles tended to be higher in crayfish cages relative to control cages, the proportion of surviving froglets was lower in crayfish cages than in control cages, possibly due to crayfish predation on injured tadpoles. The size of surviving froglets did not differ from controls, but tadpoles in crayfish cages often suffered tail injuries. Macrophyte coverage decreased as a result of crayfish consumption and nonconsumptive fragmentation. However, the biomass of periphyton increased in crayfish cages relative to controls, probably due to reduced grazing from snails. In contrast, trout had strong negative effects on the biomass of both predatory invertebrates and insect grazers, whereas trout had less effect on snail biomass than did crayfish. Also, in contrast to crayfish cages, the number of active tadpoles in trout cages was lower than in controls, probably due to a combination of trout predation and trout-induced reduced tadpole activity. Trout had a strong negative impact on froglet survival, and froglets in trout cages metamorphosed at a smaller size and had reduced growth rates compared to froglets in crayfish and control cages. As with crayfish, the biomass of periphyton increased in trout cages relative to controls, which may be due to a combination of both density and trait-mediated trout effects on tadpole grazing. In treatments with multiple predators the effects of crayfish and trout on caged communities were independent, and there were few interactions. Mostly effects of combined predators reflected those in single predator cages. Our results demonstrate that noninteracting, introduced multiple predators can have strong direct and indirect effects on multiple trophic levels in pond communities. Trophic cascades may develop in aquatic food webs even with omnivores such as crayfish, and in complex habitats with trout. These strong indirect effects are mediated through both predation on important grazers (i.e., the crayfish-snail-periphyton link) and a combination of density and behavioral responses of grazers to predators (i.e., the trout-tadpole-periphyton link). When two noninteracting predators have strong but different effects on prey survival or activity, their combined effects on intermediate trophic levels reflect responses to the more dangerous predator. (Less)

Effects of fish predation in erosional and depositional habitats in a temperate stream

Combined effects of predation by benthic and drift-foraging fish (prickly sculpin (Cottus asper) and coho salmon (Onchorhynchus kisutch) parr) on benthic invertebrate community and trophic structure were evaluated in Mayfly Creek, a previously fishless stream in the Coast Range Mountains of British Columbia. The role of microhabitat (substrate) in mediating predation effects was assessed by comparing invertebrate community structure on unglazed ceramic tiles and gravel baskets nested within enclosures. The role of macrohabitat was evaluated by placing enclosures in pool and riffle habitats. Effects of fish predation were most pronounced on tile substrate and in riffle habitat and least pronounced on gravel substrate in pool habitat. The presence of fish caused a decrease in abundance of larger-bodied herbivores (primarily the mayflies Ameletus and Baetis) and had positive indirect effects on algae and smaller invertebrates (primarily Orthocladiinae chironomids and nemourid stoneflies), probably through competitive release. In contrast with herbivores, detritivorous invertebrates were less influenced by fish predation and more highly correlated with the abundance of organic detritus. The distribution and abundance of detritivores in Mayfly Creek appear to be primarily influenced by bottom-up forces (implying resource limitation), while grazers in algal-based food chains are more strongly influenced by top-down effects (fish predation).

Non-Additive Predation in Littoral Habitats: Influences of Habitat Complexity

The combined effects of predators on prey in structurally complex habitats may not always be described by additive models. Changes in habitat complexity can affect the rates of consumption by individual predators as well as alter the interactive, combined effects of predators with contrasting foraging styles. We examined the combined consumption of a common prey by two predators across a gradient of three habitat complexities. In microcosm experiments, consumption of larval mayfly prey (Cloeon cognatum) by juvenile bluegill sunfish (Lepomis macrochirus) and libellulid dragonfly larvae (Erythemis simplicicollis) exceeded additivity at low habitat complexity, but were additive at higher levels of complexity. Prey capture by odonates was unaffected by fish presence during both day and night. At low stem density, fish capture more mayfly larvae than expected in the presence of dragonflies than in their absence, while consumption by dragonflies is unchanged in the presence of fish. Both the behavioral attributes of predators and prey as well as structural complexity of their habitat affect encounter rates, and thus their net interaction.