Predator Recruitment Research Articles

Estimates of Predator Densities Using Mobile DIDSON Surveys: Implications for Survival of Central Valley Chinook Salmon

AbstractThe Sacramento–San Joaquin Delta (hereafter, “the Delta”) is one of the estuaries with the most invasive species in the world, and nonnative predators may be a major factor in the observed decline of Central Valley Chinook Salmon Oncorhynchus tshawytscha over recent decades. In order for managers to take actions that might reduce predation‐related mortality for these ecologically, culturally, and economically valuable fish, it is important to understand the factors influencing the distribution and abundance of piscivores in the Delta. In this study, we used a dual‐frequency identification sonar (i.e., DIDSON) to conduct mobile surveys to quantify the abundances of piscivores in the Delta. We then used these data to identify the habitat features that are correlated with the abundance of piscivores. Prior to conducting the surveys, we used DIDSON data from captured fish to develop an algorithm to distinguish piscivores from nonpiscivores with high confidence (98% accuracy). A generalized linear mixed‐effects model fit to these survey data indicated that predator abundances were most associated with areas of increased submerged aquatic vegetation patches, and channels that are straighter, with increased bathymetric complexity. When applied to the entire survey area, this model was successfully able to predict known areas of high predator densities. These results indicate that one approach to reduce predator densities in key locations throughout the Delta, and improve juvenile salmonid outmigration survival, is to reduce the extent of invasive submerged aquatic vegetation. Because experimental predator removals have been largely ineffective in the Delta, efforts to manipulate habitat to discourage nonnative predator recruitment and favor native species recruitment may provide a more effective solution to improve salmonid survival rates.

Variable effects of mycorrhizal fungi on predator-prey dynamics under field conditions.

Interactions between herbivores and their predators are shaped, in part, by plant phenotype. Consequently, ubiquitous symbionts of plants below-ground, such as arbuscular mycorrhizal fungi (AMF), may influence interactions above-ground between predators and their prey by altering plant phenotype. However, the ecological relevance of below-ground organisms on predator-prey interactions under field conditions remains unclear. We assessed how AMF influence herbivore-predator interactions through a field experiment. We planted two milkweed species (Asclepias curassavica and Asclepias incarnata) provided with different amounts of AMF inoculum (zero, medium, and high) in a randomized block design. We added aphids to plants and reduced predator pressure weekly for 5weeks to evaluate effects of AMF on predator recruitment. We then allowed herbivore-predator interactions to re-establish naturally for the remainder of the season to examine whether AMF-mediated variation in predator recruitment influenced the suppression of aphid populations. Arbuscular mycorrhizal fungi availability in soils mediated interactions between predaceous aphid midge flies Aphidoletes aphidimyza and their aphid prey Aphis nerii, but the effects were plant species-specific. On A. curassavica, by mid-season, midges were recruited most strongly on plants under medium AMF availability and least on plants under high AMF availability. In contrast, each midge killed fewer aphids with increasing aphid density on medium AMF plants, but killed more aphids with increasing aphid density on high AMF plants. In combination, aphid mortality rates imposed by midges were greatest on medium AMF plants, followed by high and zero AMF plants. By comparison, on A. incarnata, the recruitment of midges was strongest on high AMF plants and weakest on medium AMF plants. AMF had no effect on the number of aphids killed per midge, relative to aphid density, so mortality rates of aphids imposed by midges mirrored recruitment. Rates of decline in aphid populations following predator recolonization were associated with midge densities, as well as lacewing and syrphid densities, which were unaffected by AMF availability. Therefore, the effects of AMF on aphid population decline were not a simple function of AMF-midge interactions. Our findings demonstrate that the availability of AMF in soils has pervasive, but complex, effects on predator-herbivore dynamics in the field.

Match-mismatch dynamics in the Norwegian-Barents Sea system

A key process affecting variation in the recruitment of fish into fisheries is the spatio-temporal overlap between prey and predator (match-mismatch hypothesis, MMH). The Northeast Arctic cod Gadus morhua and its dominant prey, the copepod Calanus finmarchicus, have long been studied in the Norwegian-Barents Sea system. However, the mechanistic explanation of how cod survival is affected by MMH dynamics remains unclear. Most MMH studies have focused on either the time synchrony or the spatial overlap between trophic levels. Here, we used G. morhua larvae and C. finmarchicus data collected in the Norwegian-Barents Sea via ichthyoplankton surveys from 1959-1992 to assess the effect of the predator-prey relationship on predator recruitment to the fisheries at age 3 (as a measure of survival) and to develop a metric of predator-prey overlap using spatio-temporal statistical models. We then compared the interannual variability of the predator-prey overlap with the predator’s abundance at recruitment to assess how MMH dynamics explain the survival of cod during its early life stages. We found that the amount of overlap between cod larvae (length: 11-15 mm) and their prey explained 29% of cod recruitment variability. Positive correlations between predator-prey overlap and subsequent recruitment were also found for predators of 6-10 and 16-20 mm, but not for 21+ mm. This improved predator-prey overlap metric is thus (1) useful to better understand how predator-prey dynamics at early life stages of fish impact the survival of later stages; and (2) a valuable tool for assessing the state of an ecosystem.

Enhanced recruitment of larger predators in the presence of large prey.

Most carnivores undergo diet shift from smaller to larger prey items during ontogeny. The trophic relationship between a growing carnivore and larger prey is representative of a size-structured predator-prey interaction. The strength of this interaction is, in part, determined by the recruitment of individuals from smaller predatory size classes into larger predatory size classes. Therefore, it is interesting to investigate how larger prey alter the recruitment of smaller predator size classes into larger predator size classes, since this can affect their own future predation risk. Past empirical studies have exclusively documented that large prey reduce predator recruitment by decreasing growth and/or survival of the smaller predators. In this study, we provide empirical evidence of the contrasting pattern: large prey enhance the recruitment of smaller predators into larger predators even though they increase cannibalism mortality of the smaller predators. We have done this here by studying the trophic interaction between predatory salamander larvae Hynobius retardatus and the frog tadpoles Rana pirica that represent their large prey. In a field experiment in which salamander hatchlings were exposed to the presence or absence of large frog tadpoles, we found that more giant salamanders emerged in the presence of frog tadpoles than in their absence. Reassignment of frog tadpoles (to both treatments) in the subsequent experimental period showed that the enhanced emergence of giant salamanders in the presence of frog tadpoles leads to the intensification of salamander predation on the frog tadpoles. In an additional laboratory experiment, to better understand the underlying mechanisms, we manipulated both the presence of frog tadpoles and the occurrence of cannibalism between salamander hatchlings. This experiment revealed that frog tadpoles intensify the cannibalism of salamander larvae during their hatchling stage, thus allowing more salamander larvae to become large-sized predators. Our results suggest that frog tadpoles can inadvertently intensify their own future predation risk by intensifying cannibalistic interactions among predatory salamander hatchlings, thereby enhancing the degree of predator recruitment to a larger size class. Hence, large prey can enhance the recruitment of individuals from small predator size classes into larger predator size classes.

Identifying forage populations of concern: A new perspective based on predator recruitment considerations

Concerns about forage fish taxa declines caused by fishing or environmental change and their impact on predator populations have become an important issue in fisheries management. However, it has proven difficult to identify critical forage taxa, and elusive to establish clear linkages between forage and predator dynamics. We provide a new avenue for identifying forage taxa of concern by focusing on organisms consumed during predator recruitment, the juvenile phase of the predator lifecycle that is most important in determining predator abundance. Using a case study of four coastal predatory fish species on the West Florida Shelf, Gulf of Mexico, USA, we show that consumption of exploited fish and crustacean species is less common during the phase of predator recruitment than in the predator lifecycle as a whole. Many taxa consumed during this critical phase are not systematically monitored. Among the monitored taxa, many showed relative-abundance declines despite not being targets or bycatch of any known fishery, which points towards environmental factors as drivers. We conclude that environmentally-driven declines in forage resources consumed during the critical phase of predator recruitment is an overlooked, but potentially important, “forage fish” concern.

Agroforestry impacts the seasonal and diurnal activity of dominant predatory arthropods in organic vegetable crops

Agroforestry can improve predator recruitment by increasing the diversity of microhabitats and modifying the microclimate. Four treatment plots were defined with three tree-shading levels in a 20 years old agroforestry system combining organic vegetable crops with hybrid walnut trees. Temperature and canopy openness were recorded in each treatment in July and September 2015 and arthropods were sampled using pitfall traps at two dates (2 weeks in July and 2 weeks in September). The daily activity pattern of the main taxa was then estimated using dry pitfall traps for 7 days in July and 8 days in September. Agroforestry significantly limited the daily temperature extremes in the day and night (± 1.5 °C). We observed a significant effect of treatment on species distribution. In July, the main xerophilic species, Pseudoophonus rufipes (Coleoptera, Carabidae), was less abundant in the two most shaded plots (− 25%). Pardosa hortensis (Arachnida, Lycosidae) showed significant differences in activity-density and diurnal activity between treatments. This spider was more active between 10:00 and 14:00 in the two most shaded treatments especially in tomatoes (more than 20% of the daily activity) compared to the control (13%). The activity-density of this species was also higher in the two shaded treatments than in the control (> 20%). Our results highlight that agroforestry, by buffering climate extremes, is likely to modify predatory arthropod activity and possibly the associated services such as biocontrol.

Prey-driven control of predator assemblages: zooplankton abundance drives aquatic beetle colonization.

Trophic interactions are critical determinants of community structure and ecosystem function. In freshwater habitats, top predators are traditionally viewed as drivers of ecosystem structure, shaping populations of consumers and primary producers. The temporary nature of small water bodies makes them dependent on colonization by many organisms, particularly insects that form highly diverse predator assemblages. We conducted mesocosm experiments with naturally colonizing populations of aquatic beetles to assess how prey (zooplankton) abundances influenced colonization and assemblages of natural populations of aquatic beetles. We experimentally demonstrate that zooplankton populations can be proximate regulators of predator populations and assemblages via prey-density-dependent predator recruitment. Our results provide support for the importance of prey populations in structuring predator populations and the role of habitat selection in structuring communities. We indicate that traditional views of predators as drivers of ecosystem structure in many systems may not provide a comprehensive picture, particularly in the context of highly disturbed or ephemeral habitats.

Deadly competition and life-saving predation: the potential for alternative stable states in a stage-structured predator-prey system.

Predators often undergo complete ontogenetic diet shifts, engaging in resource competition with species that become their prey during later developmental stages. Theory posits that this mix of stage-specific competition and predation, termed life-history intraguild predation (LHIGP), can lead to alternative stable states. In one state, prey exclude predators through competition (i.e. juvenile competitive bottleneck), while in the alternative, adult predators control prey density to limit competition and foster coexistence. Nevertheless, the interactions leading to these states have not been demonstrated in an empirical LHIGP system. To address this gap, we manipulated densities of cannibalistic adult cyclopoid copepods (Mesocyclops edax) and their cladoceran prey (Daphnia pulex) in a response-surface design and measured the maturation and survival of juvenile copepods (nauplii). We found that Daphnia reduced and even precluded both nauplii maturation and survival through depletion of a shared food resource. As predicted, adult copepods enhanced nauplii maturation and survival through Daphnia consumption, yet this positive effect was dependent on the relative abundance of Daphnia as well as the absolute density of adult copepods. Adult copepods reduced nauplii survival through cannibalism at low Daphnia densities and at the highest copepod density. This work demonstrates that predation can relax a strong juvenile competitive bottleneck in freshwater zooplankton, though cannibalism can reduce predator recruitment. Thus, our results highlight a key role for cannibalism in LHIGP dynamics and provide evidence for the interactions that drive alternative stable states in such systems.

Optimizing Crops for Biocontrol of Pests and Disease.

Volatile compounds and extrafloral nectar are common defenses of wild plants; however, in crops they bear an as-yet underused potential for biological control of pests and diseases. Odor emission and nectar secretion are multigene traits in wild plants, and thus form difficult targets for breeding. Furthermore, domestication has changed the capacity of crops to express these traits. We propose that breeding crops for an enhanced capacity for tritrophic interactions and volatile-mediated direct resistance to herbivores and pathogens can contribute to environmentally-friendly and sustainable agriculture. Natural plant volatiles with antifungal or repellent properties can serve as direct resistance agents. In addition, volatiles mediating tritrophic interactions can be combined with nectar-based food rewards for carnivores to boost indirect plant defense.

Soybean (Glycine max) plants genetically modified to express resistance to glyphosate: can they modify airborne signals in tritrophic interactions?

Upon herbivory, plants activate complex biochemical pathways that result in an array of defense responses including the emission of a novel blend of volatile organic compounds (VOCs). These compounds mediate the recruitment of predators and parasitoids that exert biological control of the attacking herbivore. Genetic manipulation of a particular trait to improve agricultural plant varieties may affect other traits as a result of possible pleiotropy or insertional mutations, which in turn can affect the interaction of the plant with other organisms. Changes in herbivore-induced VOC emissions are known to occur in transgenic plants engineered to express resistance to insects (mainly Bt-plants), not only as a result of modified insect behavior but also as a result of altered resource allocation. Transgenic glyphosate-resistant plants express a variant of the enzyme EPSPS (5-enolpyruvylshikimate-3-phosphate synthase) that is insensitive to the herbicide glyphosate. This enzyme is essential in metabolic routes that result in the synthesis of amino acids and secondary metabolites. We addressed whether the constitutive and Anticarsia gemmatalis-induced emissions of VOCs from a transgenic soybean line differ from those of the isoline, and whether changes may interfere in the foraging behavior of the predatory bug Podisus nigrispinus. Analyses showed that both herbivory and genotype influenced VOC emissions. In addition, the genotype affected the herbivore-induced VOC emission. Larger emissions were measured in the transgenic line than the non-transgenic line upon herbivory. The bioassays showed that P. nigrispinus significantly discriminated only between the odors of undamaged and damaged plants of the non-transgenic line. No preference was observed for herbivore-damaged plants of any of the two lines over the other. The results from this study suggest that despite a greater emission of volatiles the predators are less able to discriminate between herbivore-damaged and undamaged transgenic plants. This condition does not necessarily increase the preference of the predator for damaged non-transgenic plants over transgenic plants. This study opens possibilities for new studies of chemical ecology in tritrophic systems to assess the effect of transgenic glyphosate-resistant plants.

What happens when crops are turned on? Simulating constitutive volatiles for tritrophic pest suppression across an agricultural landscape

Herbivore-induced plant volatiles, or HIPVs, are increasingly considered as a biocontrol enhancement tool by constitutively emitting these carnivore-attracting chemicals from agricultural fields. While ample data substantiate the olfactory preference of predators for HIPVs in laboratory environments, little is understood about the consequences of 'turning crops on' in the field. To explore the ramifications for arthropod pest management, a spatially explicit predator-prey population model was constructed that simulated a crop field releasing signals to recruit natural enemies from the surrounding landscape. Field size had an overriding influence on model outcome, both isolated as a single factor and interactively shaping responses to other parameters (e.g. habituation, foraging efficiency). Predator recruitment exponentially declined with increasing field size from nearly double the baseline density in small fields (225 individuals m(-2)) to a mere 4% increase (130 individuals m(-2)) in large fields. Correspondingly, HIPVs enhanced pest consumption in small fields (ca 50% fewer prey), while generating virtually no impact in large fields. Collectively, the model suggests that reducing the perimeter/core area ratio will ultimately constrain the utility of predator retention as a pest control tactic in commercial-sized fields and illustrates potential consequences of the widespread commercialization of this technology in agriculture.

Population Dynamics of Leptinotarsa decemlineata (Coleoptera: Chrysomelidae): Measuring the Effects of Methyl Salicylate and Predator Recruitment in Potato

Methyl salicylate is an organic compound produced by potato and other plants in response to insect herbivory. In other studies the compound has been shown to be attractive to numerous predatory arthropods. Experiments were conducted in Virginia to evaluate methyl salicylate lures for attracting natural enemies in potato plots to manage populations of Leptinotarsa decemlineata (Say). Abundance of predatory arthropods and L. decemlineata life stages were measured in plots treated with and without 90 day, 5 g slow-release packets of methyl salicylate (95% methyl salicylate [Predalure™]). Mortality of L. decemlineata eggs and small larvae was estimated by calculating the difference in numbers of individuals recruited to subsequent stages using a stage-specific life table approach. Methyl salicylate treatment had no impact on predator recruitment or mortality of L. decemlineata eggs and small larvae, compared with nontreated plots. Cumulative mortality of L. decemlineata ranged from 87.9 - 89.2% in 2010 and from 81.9 - 94.8% in 2011. The dominant arthropod predators observed on potatoes included Hippodamia convergens Guerin-Meneville, Coccinella septempunctata L., and Perillus bioculatus (F.).

Possible Secondary Population-Level Effects of Selective Harvest of Adult Male Muskoxen

Selective harvest regimes are often focused on males resulting in skewed sex-ratios, and for many ungulate species this strategy is sustainable. However, muskoxen (Ovibos moschatus) are very social and mature bulls (≥4 years old), particularly prime-age bulls (6–10 years old), play important roles in predator defense and recruitment. A year-round social structure incorporating large males into mixed-sex groups could make this species more susceptible to the effects of selective harvest if population composition and sex-ratios influence overall survival and reproductive success. Using detailed data collected on the muskox population occupying the Seward Peninsula, Alaska during 2002–2012, we formulated the hypothesis that the selective harvest of mature bulls may be related to documented changes in population composition and growth rates in this species. In addition, we reviewed existing published information from two other populations in Alaska, the Cape Thompson and Northeastern populations, to compare population growth rates among the three areas under differential harvest rates relative to our hypothesis. We found that on the Seward Peninsula, mature bull:adult cow ratios declined 4–12%/year and short-yearling:adult cow ratios (i.e., recruitment) declined 8–9%/year in the most heavily harvested areas. Growth rates in all 3 populations decreased disproportionately after increases in the number of bulls harvested, and calf:cow ratios declined in the Northeastern population as harvest increased. While lack of appropriate data prevented us from excluding other potential causes such as density dependent effects and changes in predator densities, our results did align with our hypothesis, suggesting that in the interest of conservation, harvest of mature males should be restricted until causal factors can be more definitively identified. If confirmed by additional research, our findings would have important implications for harvest management and conservation of muskoxen and other ungulate species with similar life-histories.

Annual recruitment, predation rates and biocontrol of Linatella caudata (Mollusca: Gastropoda) in suspended enclosure culture of the pearl oyster Pinctada imbricata

We present data on recruitment of the marine gastropod Linatella caudata (Gmeling, 1791) and their predation rates on the pearl oyster Pinctada imbricata in suspended culture using Spanish baskets. The recruitment rate of juveniles of L. caudata was studied every 30days over a year. The bivalve culture enclosures were suspended in the water column at two sites in the Gulf of Cariaco (northeastern Venezuela) characterized by different oceanographic and environmental conditions; Turpialito and an internal area of Laguna Grande del Obispo (LGO). Environmental variables including temperature, chlorophyll a, total (organic and inorganic) seston and wind intensity were recorded. The sea urchin Echinometra lucunter was placed on culture baskets as a predator (biocontrol). Plastic mesh cultivation bags were also tested as an alternative method to prevent or decrease the recruitment of these predators at Turpialito. The results showed a significantly higher number of predators at Turpialito than in the sheltered LGO site in May, June and July only. During the period of intense predator recruitment, principal component analysis (PCA) revealed that chlorophyll a, organic seston and wind strength were directly related to predator recruitment. During the period of less intense predator recruitment when wind intensity was lower, PCA showed that the same variables were directly related to predator recruitment whereas temperature showed an inverse relationship. In contrast, no relationship between recruitment of predators and environmental variables could be established for the LGO site. However, predator recruitment at LGO was significantly lower in the plastic mesh bags than in Spanish baskets, and in the presence of the sea urchin Echinometra lucunter. Due to high rates of predation observed, L. caudata constitutes a serious problem for the aquaculture industry of marine bivalves.

Inbreeding alters volatile signalling phenotypes and influences tri‐trophic interactions in horsenettle (Solanum carolinense L.)

The ecological consequences of inter-individual variation in plant volatile emissions remain largely unexplored. We examined the effects of inbreeding on constitutive and herbivore-induced volatile emissions in horsenettle (Solanum carolinense L.) and on the composition of the insect community attracted to herbivore-damaged and undamaged plants in the field. Inbred plants exhibited higher constitutive emissions, but weaker induction of volatiles following herbivory. Moreover, many individual compounds previously implicated in the recruitment of predators and parasitoids (e.g. terpenes) were induced relatively weakly (or not at all) in inbred plants. In trapping experiments, undamaged inbred plants attracted greater numbers of generalist insect herbivores than undamaged outcrossed plants. But inbred plants recruited fewer herbivore natural enemies (predators and parasitoids) when damaged. Taken together, these findings suggest that inbreeding depression negatively impacts the overall pattern of volatile emissions - increasing the apparency of undamaged plants to herbivores, while reducing the recruitment of predatory insects to herbivore-damaged plants.

The impact of flower-dwelling predators on host plant reproductive success

Flowers attract insects and so are commonly exploited as foraging sites by sit-and-wait predators. Such predators can be costly to their host plant by consuming pollinators. However, sit-and-wait predators are often prey generalists that also consume plant antagonists such as herbivores, nectar robbers and granivores, so may also provide benefits to their host plant. Here we present a simple, but general, model that provides novel predictions about how costs and benefits interact in different ecological circumstances. The model predicts that the ecological conditions in which flower-dwelling predators are found can generate either net benefits to their host plants, net costs to their host plants, or can have no effect on the fitness of their host plants. The net effect is influenced by the relative densities of mutualists and antagonists. The flower-dwelling predator has a strong positive effect on the plant if both the pollinators and the granivores are at high density. Further, the range of density combinations that yield a positive net outcome for the plant increases if the performance of pollinators is negatively density dependent, if the predator is only moderately effective at influencing flower visitor rates by its potential prey, and if pollinators are very effective. If plants of a given species find themselves consistently in conditions where they benefit from the presence of a predator then we predict that natural selection could favour the evolution of plant traits that increase the likelihood of predator recruitment and retention, especially where plants are served by highly effective pollinators.

Effect of Volatile Emissions, Especially -Pinene, From Persimmon Trees Infested by Japanese Wax Scales or Treated With Methyl Jasmonate on Recruitment of Ladybeetle Predators

Persimmon, Diospyros kaki L., is an important fruit tree in northern China. Japanese wax scale, Ceroplastes japonicus Green (Hemiptera: Coccoidea: Coccidae), is the most destructive pest in persimmon orchards and is difficult to control using chemical pesticides. This paper studied the variety of volatile emissions from persimmon trees attacked by wax scale or induced by applications of methyl jasmonate (MeJA), an exogenous signaling material. We also studied the role of volatiles in recruiting the ladybeetle, Chilocorus kuwanae Silvestri (Coleoptera: Coccinellidae), a primary predator of wax scale. The results showed that the ladybeetles displayed strong taxis responses to the odor sources from trees treated with MeJA and wax scale-damaged trees. Within 24 h after the trees were treated with MeJA, the number of ladybeetles responding to the treated trees fluctuated based on the amount of volatiles released over a diel period. Chemical analysis of the volatiles showed that terpenoid compounds, especially alpha-pinene, increased from both the wax scale-damaged and MeJA-treated trees and that this was the reason for the ladybeetle attraction. Therefore, alpha-pinene was used as a single signal source to examine the taxis response of C. kuwanae. The results suggest that alpha-pinene plays a significant role in attracting the ladybeetles. It is therefore concluded that MeJA application might be used to stimulate the defense function of persimmon trees by inducing the release of terpenoids, especially alpha-pinene, and thereby recruiting more ladybeetles to control the scale insects.

Coexistence and stability of predator–prey model with Beddington–DeAngelis functional response and stage structure

We present a predator–prey model of Beddington–DeAngelis type functional response with stage structure on prey. The constant time delay is the time taken from birth to maturity about the prey. By the uniform persistence theories and monotone dynamic theories, sharp threshold conditions which are both necessary and sufficient for the permanence and extinction of the model as well as the sufficient conditions for the global stability of the coexistence equilibria are obtained. Biologically, it is proved that the variation of prey stage structure can affect the permanence of the system and drive the predator into extinction by changing the prey carrying capacity: Our results suggest that the predator coexists with prey permanently if and only if predator's recruitment rate at the peak of prey abundance is larger than its death rate; and that the predator goes extinct if and only if predator's possible highest recruitment rate is less than or equal to its death rate; furthermore, our results also show that a sufficiently large mutual interference by predators can stabilize the system.

A Stage-structured Predator-prey Model of Beddington-DeAngelis Type

We formulate and study a robust stage structured predator-prey model of Beddington-DeAngelis-type functional response. The time delay is the time taken from birth to maturity. The Beddington-DeAngelis functional response is similar to the Holling type 2 functional response but contains an extra term describing mutual interference by predators. First we show that the predator coexists with prey permanently if and only if the predator's recruitment rate at the peak of prey abundance is larger than its death rate. Second, we show that if the system is permanent, then a sufficiently large degree of the predator interference can not only stabilize the system but also guarantee the stability of the system against the increase of the carrying capacity of prey and the increase of birth rate of the adult predator. Third, we show both analytically and numerically that stability switches of interior equilibrium may occur as maturation time delay increases: stability may change from stable to unstable to finally stable, implying that a large delay can be stabilizing.

Physical forcing and the dynamics of the pelagic ecosystem in the eastern tropical Pacific: simulations with ENSO-scale and global-warming climate drivers

We used a model of the pelagic ecosystem in the eastern tropical Pacific Ocean to explore how climate variation at El Niño – Southern Oscillation (ENSO) scales might affect animals at middle and upper trophic levels. We developed two physical-forcing scenarios: (1) physical effects on phytoplankton biomass and (2) simultaneous physical effects on phytoplankton biomass and predator recruitment. We simulated the effects of climate-anomaly pulses, climate cycles, and global warming. Pulses caused oscillations to propagate through the ecosystem; cycles affected the shapes of these oscillations; and warming caused trends. We concluded that biomass trajectories of single populations at middle and upper trophic levels cannot be used to detect bottom-up effects, that direct physical effects on predator recruitment can be the dominant source of interannual variability in pelagic ecosystems, that such direct effects may dampen top-down control by fisheries, and that predictions about the effects of climate change may be misleading if fishing mortality is not considered. Predictions from ecosystem models are sensitive to the relative strengths of indirect and direct physical effects on middle and upper trophic levels.