Predator Density Research Articles

Growth, developmental, and size structure responses in tadpole prey under increasing threat from gape-limited newts

Size variability within a cohort can have profound effects on community ecology and evolution. Although competition for resources generally increases size variability, the effect of (non-consumptive) predation on this demographic trait remains relatively poorly understood. Existing models suggest a positive correlation between growth rate (mediated by resource level) and expression of size variability (as measured by the coefficient of variation) in prey cohorts. We tested this prediction by exposing the tadpoles of the Japanese Forest Green Treefrog (Rhacophorus arboreus (Okada and Kawano, 1924) = Zhangixalus arboreus (Okada and Kawano, 1924)) to the non-lethal presence of gape-limited Japanese Fire-bellied Newts (Cynops pyrrhogaster (Boie, 1826)) at low and high predator densities in an outdoor mesocosm experiment. Tadpole growth rates and periphyton biomass increased with newt density. But in contrast to prediction, elevated growth rates did not increase but, reversely, decreased cohort size variability in the tadpoles. We discuss two potential mechanisms behind this outcome. First, increased resource availability mediated by predator feeding may have reduced the strength of competition, ultimately leading to more evenly distributed resource gains among individuals; second, if smaller individuals grew relatively faster than larger individuals, as to quicken entry to a size refuge against the gape-limited predator, then inter-individual size differences could diminish over time.

Spatial and temporal patterns of a pulsed resource dynamically drive the distribution of specialist herbivores

Patterns and drivers of the spatio-temporal distribution of herbivores are key elements of their ecological and evolutionary impacts on plant populations. Herbivore spatial distributions may be influenced by increased (RCH: resource concentration hypothesis) or decreased (RDH: resource dilution hypothesis) resource densities, but the effect of temporal variations in resource densities on such distributions remains poorly documented. We used a survey of a masting tree species and its seed predators in Southeastern France to address the effect of a host’s pulsed resource on the spatio-temporal distributions of highly specialized insect herbivores feeding on seeds. Variations in both resource and seed predator densities were assessed by estimating seed production and seed infestation rates in focus trees during 10 consecutive years. We found increasing seed infestation rates with decreasing host tree densities in years of low seed production, indicating a RDH pattern of seed predators. However, such pattern was not persistent in years of high seed production during which seed infestation rates did not depend on host tree densities. We showed that temporal variations in resource density can lead to transience of seed predator spatial distribution. This study highlights how predictions of plant-herbivore interactions in natural ecosystems may rely on temporal components underlying RCH and RDH hypotheses.

Differences in perceived predation risk associated with variation in relative size of extra-pair and within-pair offspring.

Extra-pair paternity (EPP) is a widespread phenomenon in birds. Researchers have long hypothesized that EPP must confer a fitness advantage to extra-pair offspring (EPO), but empirical support for this hypothesis is definitively mixed. This could be because genetic benefits of EPP only exist in a subset of environmental contexts to which a population is exposed. From 2013 to 2015, we manipulated perceived predator density in a population of tree swallows (Tachycineta bicolor) breeding in New York to see whether fitness outcomes of extra-pair and within-pair offspring (WPO) varied with predation risk. In nests that had been exposed to predators, EPO were larger, longer-winged and heavier than WPO. In nonpredator nests, WPO tended to be larger, longer-winged and heavier than EPO, though the effect was nonsignificant. We found no differences in age, morphology or stress physiology between extra-pair and within-pair sires from the same nest, suggesting that additive genetic benefits cannot fully explain the differences in nestling size that we observed. The lack of an effect of predator exposure on survival or glucocorticoid stress physiology of EPO and WPO further suggests that observed size differences do not reflect more general variation in intrinsic genetic quality. Instead, we suggest that size differences may have arisen through differential investment into EPO and WPO by females, perhaps because EPO and WPO represent different reproductive strategies, with each type of nestling conferring a fitness advantage in specific ecological contexts.

The enemy of my enemy: multiple interacting selection pressures lead to unexpected anti-predator responses.

Despite being a major selective force, predation can induce puzzling variability in anti-predator responses-from lack of predator aversion to lifelong predator-induced fear. This variability is hypothesised to result from variation in the trade-offs associated with avoiding predators. But critical information on fitness outcomes of these trade-offs associated with anti-predator behaviours is lacking. We tested this trade-off hypothesis in Aedes aegypti, by examining oviposition site selection decisions in response towards larval predation risk and comprehensively measuring the fitness implications of trade-offs of avoiding larval predators, using three fitness measures: larval survival, development time and size. In a field study, we find that adult females show a surprisingly variable response to predators, ranging from attraction to avoidance. This variation is explained by fitness outcomes of oviposition along a predation-risk gradient that we measured in the laboratory. We show that ovipositing females could gain fitness benefits from ovipositing in pools with a low density of predators, rather than in predator-free pools, as predators provide a release from negative density effects of conspecific larvae that might co-occur in a pool. Interacting selection pressures may thus explain diverse prey responses. We suggest other systems in which similarly unexpected prey behaviour is likely to occur.

A Comparison of Zooplankton Diel Vertical Distribution during Transparent and Turbid Conditions in a Natural Southern Appalachian Montane Lake

According to the transparency-regulator hypothesis (TRH), water transparency is the main driver of zooplankton diel vertical migration (DVM) because it influences ultraviolet radiation (UV) exposure and visual predation, which vary greatly over a diel period. I tested the TRH by examining zooplankton DVM and Secchi depth over 2 summers in Mountain Lake, VA, a unique natural lake where transparency and water level fluctuate substantially. Daytime mean depth and Secchi depth were positively correlated for Daphnia parvula, but this was not the case for other taxa. Zooplankton exhibited diurnal deficits (DDs; higher night densities compared to day), indicating that DVM does not account for all diel movement. DDs are sometimes attributed to light-mediated sampler avoidance or diel horizontal migration (DHM), but DDs did not increase with Secchi depth and DDs in littoral zones do not indicate DHM. Overall, these results suggest that factors other than transparency—perhaps predator densities—more strongly influence DVM in Mountain Lake.

Relationships between predatory aquatic insects and mosquito larvae in residential areas in northern Thailand.

In order to elucidate the poorly understood relationships between mosquito larvae and their predatory aquatic insects in urban and suburban areas of tropical Southeast Asia, where vector-borne diseases are prevalent, aquatic insects were sampled from 14 aquatic habitats in residential areas of Chiang Mai, northern Thailand, during the rainy season (July to November) in 2016. Correlations among biological variables, densities of major predatory aquatic insect groups (i.e., Odonata, Coleoptera, and Hemiptera: OCH group) in wetlands and artificial lentic habitats, and the density of mosquito larvae were analyzed. Among the sampled mosquito larvae, Culex spp. were the most abundant, and both OCH density and water quality were major determinants of Culex spp. density (rs = -0.302 and -0.396, respectively). Logistic regression analyses indicated that the probability of Culex spp. occurrence was significantly and negatively correlated with OCH density. Furthermore, high macrophyte abundance was associated with higher predator density, potentially reducing mosquito density. Hemipteran predators were most negatively correlated with Culex spp. density, regardless of whether macrophyte abundance was high or low (rs = -0.547 and -0.533, respectively). Therefore, hemipteran predators were the most important aquatic insect predators in the urban and suburban residential areas of Chiang Mai, Thailand, and OCH species, such as the hemipteran Micronecta scutellaris, could be used as biological control agents against mosquitoes in the region.

Lifetime Sex‐Specific Moose Mortality During an Intentional Population Reduction

ABSTRACTWhere elevated harvest of ungulates is a priority, managers benefit by understanding how various sources of mortality affect the age and sex structure and trend of ungulate populations. Prior studies reported a long period (1997–2014) of moose (Alces alces gigas) nutritional stress from overabundance in our study area, an intentional 31% reduction in moose numbers using liberal harvests of females (2004–2012), and low bear (Ursus spp.) predation and high moose harvest densities relative to other largely roadless systems with moose, bears, and wolves (Canis lupus). In this paper, we detailed management findings after describing causes and rates of mortality from 226 female and 164 male moose radio‐collared at 9 months of age (1997–2008) and followed through life (1997–2019) and throughout the population reduction. We listened for mortality signals on radio‐collars 1–2 times/month when snow cover was complete and 2–4 times/month when snow cover was incomplete. Upon hearing a mortality signal, we investigated mortality sites usually within 24 hours via helicopter. Excluding hunter‐caused mortality, we estimated 28% annual mortality for male yearlings versus 17% for female yearlings, then low annual mortality rates (0–4%) to 84 months of age for males and 96 months of age for females, and gradually increasing annual mortality rates thereafter. Most (83%) male moose ≥24 months of age died from hunters; minor causes included wolves (8%), malnutrition or disease (5%), grizzly bears (U. arctos; 2%), and accidents (2%). Most female moose ≥24 months of age died from wolves (37%) or hunters (33%); minor causes included malnutrition or disease (15%), grizzly bears (10%), and accidents (5%). The proportion of radio‐collared females killed by hunters varied depending on numbers of permits issued to hunters; the kill rate of females ≥24 months of age was 58% during the initial 4 years of the 9‐year reduction, moderated at 29% during the final 5 years of the reduction, and was only 7% for all other study years. We attributed 32% of hunter kills to illegal harvest and unrecovered hunter kills. Hunters played a key role in the intentional population reduction by harvesting prime‐age and near prime‐age male and female moose that rarely died from other sources of mortality compared with calf, yearling, and older moose. Restricting general season hunters to primarily harvesting prime‐age and older male moose with antler spreads ≥127 cm did not appreciably reduce harvest of adult males. Male moose 2.0–5.3 years of age rarely died from non‐hunter causes and were largely harvested at older, prime ages (5.3–8.3 yr of age). Yearling moose of both sexes died primarily from wolves, with wolves selecting more for males. By using liberal harvests of female moose to reduce the population, managers improved moose nutrition and reproduction, met mandates for elevated harvests, and may have avoided a reoccurrence of a previous precipitous decline in moose numbers that was initiated by overabundance and extreme snow depths. © 2019 The Wildlife Society.

Dynamic behaviors of a Lotka-Volterra type predator-prey system with Allee effect on the predator species and density dependent birth rate on the prey species

Abstract A Lotka-Volterra type predator-prey system with Allee effect on the predator species and density dependent birth rate on the prey species is proposed and studied. For non-delay case, such topics as the persistent of the system, the local stability property of the equilibria, the global stability of the positive equilibrium are investigated. For the system with infinite delay, by using the iterative method, a set of sufficient conditions which ensure the global attractivity of the positive equilibrium is obtained. By introducing the density dependent birth rate, the dynamic behaviors of the system becomes complicated. The system maybe collapse in the sense that both the species will be driven to extinction, or the two species could be coexist in a stable state. Numeric simulations are carried out to show the feasibility of the main results.

Functional response of adult Cheyletus malaccensis (Acari: Cheyletidae) to different developmental stages of Aleuroglyphus ovatus (Acari: Acaridae)

Cheyletus malaccensis is an important natural enemy of stored pests. In the study, the functional response of C. malaccensis against Aleuroglyphus ovatus was investigated under five constant temperatures of 16, 20, 24, 28 and 32 °C. The results showed that the functional response of adult C. malaccensis to different stages of A. ovatus was conformed to Holling II. Under the temperature between 16 and 32 °C, C. malaccensis females consumed more larvae and nymphs, followed by male and females of A. ovatus. While male C. malaccensis merely consumed larvae and nymphs of prey. Adult C. malaccensis ate few eggs of A. ovatus. The daily consumption and instantaneous attack rate (a/Th) increased with the temperature increasing from 16 °C to 32 °C while the peak was at 28 °C. The maximum daily consumption for larva and a/Th of female C. malaccensis are 135.1 and 110.7, respectively. Cheyletus malaccensis showed positive preference to larvae and nymphs, while negative preference to adults and eggs of A. ovatus. Because of the competition and interference among predators, the predation efficiency decreased with the density of predator increasing, and the interference coefficient (m) was 0.485 at 28 °C. These results could be helpful to provide theoretical basis and practical instructions for the biological control of A. ovatus by C. malaccensis.

Density Dependent Delayed Migration for Rosenzweig-Macaurther Model with Holling Type II Predator Functional Response

The model describing the interaction between the predator and prey species is referred to as a predator-prey model. The migration of these species from one patch to another may not be instantaneous. This may be due to barriers such as a swollen river or a busy infrastructure through the natural habitat. Recent predator-prey models have either incorporated a logistic growth for the prey population or a time delay in migration of the two species. Predator-prey models with logistic growth that integrate time delays in density-dependent migration of both species have been given little attention. A Rosenzweig-MacAurther model with density-dependent migration and time delay in the migration of both species is developed and analyzed in this study. The Analysis of the model when the prey migration rate is greater than or equal to the prey growth rate, the two species will coexist, otherwise, at least one species will become extinct. A longer delay slows down the rate at which the predator and prey population increase or decrease, thus aecting the population density of these species. The prey migration due to the predator density does not greatly affect the prey density and existence compared to the other factors that cause the prey to migrate. These factors include human activities in the natural habitats like logging and natural causes like bad climatic conditions, limited food resources and overpopulation of the prey species in a patch among others.

Ecological determinants of community structure across the trophic levels of freshwater food webs: a test using bromeliad phytotelmata

Understanding the relative importance of habitat and biotic drivers on community assembly across food web components is an important step towards predicting the consequences of environmental changes. Because documenting entire food webs is often impractical, this question has been only partially investigated. Here, we partitioned variation in species assemblages of the major components of tank bromeliad food webs (bacteria, algae, protozoans, detritivorous and predatory invertebrates) into habitat and biotic determinants and examined the influence of habitat variables and predator or prey abundance on all taxonomic assemblages. Ecological determinism of assemblage structure ranged from weak in bacteria (< 10% of the explained variance) to strong in predatory invertebrates (90%). Habitat features and canopy openness significantly influenced species assemblages; however, prey or predator density had far and away the most significant structuring effects. If biotic forces are at least as important as the abiotic forces while the importance of stochasticity declines towards upper trophic levels, then trophic levels could respond differently to natural or anthropogenic disturbance and to shifts in species distributions. The effects of such differential responses on food web reconfiguration, however, remain to be elucidated.

Prey size and predator density modify impacts by natural enemies towards mosquitoes

1. Interactions between multiple predators can modify prey risk and profoundly alter ecological community dynamics. Further, ontogenic prey size changes are known to mediate prey risk through refuge effects. Understandings of these biotic factors is important for robust quantifications of natural enemy effects on target species, yet their combined influence lacks investigation.2. Functional responses were used to quantify the predatory impacts ofNotonecta glauca(water boatman;Ng) andGammarus pulex(river shrimp;Gp) towards four different larval instars ofCulex pipiensin container‐style habitats. Using conspecific pairs of predators, multiple predator effects (MPEs) of both predator species were examined across larval prey sizes, and prey preference tests were applied to examine prey selectivity across predator–prey body size ratios.3. Both predators were able to feed onC. pipiensacross their larval ontogeny; however,Ngconsumed significantly more larvae thanGp. Functional responses ofNgwere typically Type IIs, whereasGptrended towards sigmoidal Type IIIs. Predation by pairs ofNgandGpshowed independent MPEs towards first‐, third‐, and fourth‐instar stages (except predation byGpat higher densities of fourth‐instar) stages, whereas, for second‐instar stages,Ngshowed synergistic MPEs andGpshowed antagonistic MPEs. Both predators preferred late instar mosquitoes (Ng: fourth instar;Gp: third instar). These preferences reflected predator:prey weight–length ratios, showing that relative sizes of predators and prey are important factors in prey selectivity.4. The results obtained in the present study demonstrate that MPEs, combined with intraspecific prey preferences, may modulate trophic interactions within ecosystems. Therefore, such effects should be increasingly considered to further the understanding of agent efficacies.

Physiology and behaviour of juvenile snowshoe hares at the start of the 10-year cycle

Snowshoe hares, Lepus americanus, are the dominant prey species in the boreal forest and they undergo regular, predator-driven population cycles every 8–11 years. Previous research has found that hare stress physiology is highly sensitive to both within-year (seasonal/litter group) and across-year (cycle phase) variation in predation risk and that maternal–offspring stress profiles are tightly correlated. Thus, both pre- and postnatal environments may interact to shape offspring physiological phenotype. If changes in the function and reactivity of the maternal stress axis are associated with variation in offspring behaviours that increase survival during periods of high predation risk, then the maternal effects on offspring physiology seen previously could represent a mechanistic route of adaptive maternal programming. To examine the relationship between physiology and behaviour in snowshoe hares, we monitored free-ranging adult and juvenile stress hormone levels in the first and second litters of the breeding season and assessed open field exploratory behaviour in weaning-age and independent juveniles in the southwestern Yukon (Canada) during 2013–2015, when predator density was low but increasing. Thus, our study spanned the late low phase (2013) and the early increase phase (2014–2015) of the hare population cycle. We found that increased concentrations of faecal cortisol metabolites (FCMs) were associated with aspects of risk avoidance in weaning-age hares. Juveniles with higher stress hormone levels spent more time under cover and were less active during open field trials, highlighting a potential mechanistic route to allow individuals to sensitively cope with a changing environment. Although average FCM levels of breeding females and juveniles were not correlated with one another and litter-based differences in physiology and behaviour were not present during these low-risk phases of the cycle, the association between stress hormone levels and behaviour sets the stage for adaptive maternal effects on offspring behaviour and survival as the cycle progresses and predation risk intensifies.

Multiple stressors influence benthic macroinvertebrate communities in central Appalachian coalfield streams

Headwater streams impacted by surface coal mining in the central Appalachian region of the eastern USA have characteristics not shared by reference-quality streams. These include elevated salinity, often measured using specific conductance (SC) and cited as a primary stressor of benthic macroinvertebrate communities. The study objective was to assess influence by mining-origin stressors on benthic macroinvertebrate community structure in headwater streams. Stream habitat characteristics were measured and benthic macroinvertebrates were sampled from 12 central Appalachian streams, 9 of which were influenced by mining. Multiple benthic macroinvertebrate community metrics, including Ephemeroptera density, richness, and composition were correlated negatively with watershed mining extent and with SC. Predator density and scraper richness were correlated negatively with watershed mining, stream-water selenium, and SC. Clinger richness was correlated positively with stream substrate characteristics including large cobble-to-fines ratios and relative bed stability, and was correlated negatively with watershed mining and SC. Relationships of predator density and scraper richness with selenium concentrations, and relationships of clinger richness with stream substrate characteristics, are consistent with stress mechanisms revealed by prior studies. Improved understanding of how habitat features are altered by mining and influence community structure in headwater streams can inform water resource management in mining areas.

Placement, survival and predator identity of Eurasian Curlew Numenius arquata nests on lowland grass-heath

ABSTRACTCapsuleWithin the UK’s largest lowland Eurasian Curlew Numenius arquata population, curlew preferentially nested on physically disturbed (treated) than undisturbed (control) grassland, and low nest survival rates were primarily attributable to predation by Red Fox Vulpes vulpes.AimsTo inform conservation interventions for Curlew within semi-natural lowland dry-grassland landscapes.MethodsAcross a 3700 ha lowland dry-grassland landscape, over two years, effects of ground-disturbance management on Curlew nest placement (n = 41) were examined using generalized linear models controlling for vegetation strata. The effects of site and management on nest survival (n = 44) were also examined, controlling for lay date and year. Nest predator identity was investigated using temperature sensors (n = 28) and nest cameras (n = 10).ResultsCurlews were five times more likely to nest on physically disturbed than undisturbed grassland. Nest survival (overall mean ± se = 0.24 ± 0.07) was not influenced by year or ground-disturbance but declined with lay date and differed markedly between the two sites, consistent with predator control. Predation accounted for 29/32 failed nests and was predominantly at night (17/23 cases where timing was known), consistent with mammalian predators. Cameras indicated Red Foxes to be the main predator (4/5 cases). Overall breeding productivity was 0.16 ± 0.01 (SE) chicks per nesting attempt.ConclusionCurlew suffered from unsustainably high rates of nest predation primarily attributable to Red Foxes. A combination of perimeter fencing and lethal predator control appeared to improve nest success at one site. Ground-disturbance treatment could encourage nesting attempts in areas managed to minimize predator density.

Regional differences in kelp forest interaction chains are influenced by both diffuse and localized stressors

AbstractOn temperate rocky reefs, overexploitation of high‐trophic‐level omnivores can result in the decimation of kelp forest habitats by releasing sea urchin population networks from top‐down control. However, the local dynamics of the resulting trophic cascades are context‐dependent. Here, we investigate the community‐wide patterns associated with both diffuse stressors and localized protection of high‐trophic‐level omnivores in kelp forest ecosystems by comparing communities among marine reserves and fished areas in two contrasting regions in terms of fishing intensity and land‐based stressors, Fiordland and the Marlborough Sounds, New Zealand. We find higher densities of the potential sea urchin predators, red rock lobsters (Jasus edwardsii) and banded wrasse (Notolabrus fucicola), in the Fiordland region, and larger effect sizes of fishing on the exploited fish communities in the Marlborough Sounds. Patterns in sea urchin density were consistent with the idea that high‐trophic‐level species, such as large fish and rock lobsters, regulate sea urchin population density, with lower densities of Evechinus chloroticus observed inside marine reserves, in both regions. Nevertheless, densities of E. chloroticus were generally high (&gt;3 m2) in the Marlborough Sounds, likely above a grazing threshold in both fished and reserve sites. The proportion of habitat where sea urchins were absent was 29% in Marlborough Sounds and 90% in Fiordland. Consequently, we observe 49% barren habitat in Fiordland vs. 70%, and a larger effect of fishing on kelp community structure, in the Marlborough Sounds, where fishing effect sizes and land‐based stressors were more severe. We propose that a combination of diffuse stressors including regional overexploitation of important sea urchin predators, sedimentation, and warming of coastal waters likely contributed to regional differences in the responses of trophic interaction chains to localized reductions in fishing within marine reserves. The present study highlights how physiologically stressed and modified kelp forest ecosystems are more susceptible to detrimental phase shifts at a regional spatial scale.

Long‐term survey data reveal large predator and temperature effects on population growth of multiple zooplankton species

AbstractPredators can strongly affect prey communities, but their influence may be difficult to distinguish from bottom‐up and other environmental effects. The problem of assessing predator impact is especially difficult in large systems that do not allow for comparisons across multiple units (e.g., small lakes) that have varying predator density. For instance, the invasion of the predatory zooplankter, Bythotrephes longimanus, into the Laurentian Great Lakes contributed to the nearly complete disappearance of several zooplankton species, but current effects on extant zooplankton are not well understood. We used generalized additive models (GAMs) applied to long‐term data time series (1994–2012) to examine B. longimanus effects on zooplankton species in Lake Michigan. Because B. longimanus abundance varied over time, our approach allowed assessment of predator effects from field data while accounting for other factors, including food resources, temperature, and seasonality. Results suggest that B. longimanus substantially reduces some zooplankton population growth rates, with the largest effects on species that B. longimanus affected more strongly in experiments. For example, at maximum B. longimanus abundance, Daphnia mendotae, Bosmina longirostris, and Diacyclops thomasi population growth rates were estimated to be reduced by 17%, 30%, and 21%, respectively, compared to no effect on calanoid copepods. Results further indicated positive temperature effects on population growth that differed by species. Our study thus provides field‐based evidence for ongoing impacts of invasive species and temperature on zooplankton production and composition, with potential consequences for planktivorous fish, and exemplifies how GAMs can be used to determine predator effects from time series data.

Temporal partitioning of activity: rising and falling top‐predator abundance triggers community‐wide shifts in diel activity

Top predators cause avoidance behaviours in competitors and prey, which can lead to niche partitioning and facilitate coexistence. We investigate changes in partitioning of the temporal niche in a mammalian community in response to both the rapid decline in abundance of a top predator and its rapid increase, produced by two concurrent natural experiments: 1) the severe decline of the Tasmanian devil due to a transmissible cancer, and 2) the introduction of Tasmanian devils to an island, with subsequent population increase. We focus on devils, two mesopredators and three prey species, allowing us to examine niche partitioning in the context of intra‐ and inter‐specific competition, and predator–prey interactions. The most consistent shift in temporal activity occurred in devils themselves, which were active earlier in the night at high densities, presumably because of heightened intraspecific competition. When devils were rare, their closest competitor, the spotted‐tailed quoll, increased activity in the early part of the night, resulting in increased overlap with the devil's temporal niche and suggesting release from interference competition. The invasive feral cat, another mesopredator, did not shift its temporal activity in response to either decreasing or increasing devil densities. Shifts in temporal activity of the major prey species of devils were stronger in response to rising than to falling devil densities. We infer that the costs associated with not avoiding predators when their density is rising (i.e. death) are higher than the costs of continuing to adopt avoidance behaviours as predator densities fall (i.e. loss of foraging opportunity), so rising predator densities may trigger more rapid shifts. The rapid changes in devil abundance provide a unique framework to test how the non‐lethal effects of top predators affect community‐wide partitioning of temporal niches, revealing that this top predator has an important but varied influence on the diel activity of other species.

Field exclusion of large soil predators impacts lower trophic levels and decreases leaf-litter decomposition in dry forests.

Shifts in densities of apex predators may indirectly affect fundamental ecosystem processes, such as decomposition, by altering patterns of cascading effects propagating through lower trophic levels. These top-down effects may interact with anthropogenic impacts, such as climate change, in largely unknown ways. We investigated how changes in densities of large predatory arthropods in forest leaf-litter communities altered lower trophic levels and litter decomposition. We conducted our experiment in soil communities that had experienced different levels of long-term average precipitation. We hypothesized that altering abundances of apex predators would have stronger effects on soil communities inhabiting dry forests, due to lower secondary productivity and greater resource overexploitation by lower trophic levels compared to wet forests. We experimentally manipulated abundances of the largest arthropod predators (apex predators) in field mesocosms replicated in the leaf-litter community of Iberian beech forests that differed in long-term mean annual precipitation by 25% (three dry forests with MAP<1,250mm and four wet forests with MAP>1,400mm). After one year, we assessed abundances of soil fauna in lower trophic levels and indirect impacts on leaf-litter decomposition using litter of understorey hazel, Corylus avellana. Reducing densities of large predators had a consistently negative effect on final abundances of the different trophic groups and several taxa within each group. Moreover, large predatory arthropods strongly impacted litter decomposition, and their effect interacted with the long-term annual rainfall experienced by the soil community. In the dry forests, a 50% reduction in the densities of apex predators was associated with a 50% reduction in decomposition. In wet forests, the same reduction in densities of apex soil predators did not alter the rate of litter decomposition. Our results suggest that predators may facilitate lower trophic levels by indirectly reducing competition and resource overexploitation, cascading effects that may be more pronounced in drier forests where conditions have selected for greater competitive ability and more rapid resource utilization. These findings thus provide insights into the functioning of soil invertebrate communities and their role in decomposition, as well as potential consequences of soil community responses to climate change.

Grazing halos on coral reefs: predation risk, herbivore density, and habitat size influence grazing patterns that are visible from space

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 627:71-81 (2019) - DOI: https://doi.org/10.3354/meps13074 Grazing halos on coral reefs: predation risk, herbivore density, and habitat size influence grazing patterns that are visible from space Bartholomew P. DiFiore1,2,3,*, Simon A. Queenborough2, Elizabeth M. P. Madin4,5, Valerie J. Paul3, Mary Beth Decker6, Adrian C. Stier1 1Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 93106, USA 2Yale School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06511, USA 3Smithsonian Marine Station at Fort Pierce, Fort Pierce, Florida 34949, USA 4Hawai’i Institute of Marine Biology, University of Hawai’i, Kane’ohe, Hawai’i 96744, USA 5Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia 6Department of Ecology and Evolution, Yale University, New Haven, Connecticut 06511, USA *Corresponding author: bdifiore@ucsb.edu ABSTRACT: The behavior of organisms at local scales can have dramatic effects on the distribution of habitat and ecosystem processes at large spatial scales. Grazing halos—barren areas of sand surrounding coral reefs—are readily visible in remotely sensed imagery, and are formed by herbivores preferentially grazing close to the safety of the reef. Grazing halo size can vary by more than an order of magnitude, yet we lack an understanding of what underlies this variation. Changes in grazing intensity and the distance herbivores travel may be influenced by predation risk, resource availability, and structural attributes of the reef. Here, we tested how predator density, herbivore density, and patch reef area are correlated with halo size and grazing patterns by linking satellite imagery with mensurative experiments on patch reefs in the Caribbean. Chronic predation risk suppressed the overall foraging intensity of herbivorous fishes; a doubling in predator density led to a 29% decrease in grazing. Variation in herbivore density, which was tightly correlated with patch reef area, was linked to the distance herbivores foraged from the reef. Furthermore, herbivore density and/or reef area was the best predictor of halo width at field sites, and reef area explained a significant proportion of the variation in halo size across the landscape. Our results contribute to a growing body of research suggesting that halos may function as footprints of ecological processes, such as herbivory, that can be quantified using remote sensing, and offer a potential avenue to improve monitoring of coral reef management. KEY WORDS: Predator-prey interaction · Foraging ecology · Herbivory · Predation risk · Remote sensing · Seagrass · Landscape ecology Full text in pdf format Supplementary material PreviousNextCite this article as: DiFiore BP, Queenborough SA, Madin EMP, Paul VJ, Decker MB, Stier AC (2019) Grazing halos on coral reefs: predation risk, herbivore density, and habitat size influence grazing patterns that are visible from space. Mar Ecol Prog Ser 627:71-81. https://doi.org/10.3354/meps13074 Export citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 627. Online publication date: September 26, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.