Availability Of Alternative Prey Research Articles

When can we expect natural habitats to enhance pest control by generalist predators? Insights from a simple, simulated agricultural landscape

Many field studies have reported that natural habitats in the agricultural landscape are associated with increased predator abundance and/or decreased crop pest load. However, recently published meta-analyses have found mixed results, with natural habitats being associated with increased pest loads as often as decreased loads. We investigate possible reasons for these conflicting results using a mechanistic model that simulates the dynamics of a generalist, omnivorous predatory bug and a plant bug pest in a landscape consisting of a 1 km2 annual field crop monoculture bisected by a 60 m wide natural habitat strip. The model simulates the movement of adult predators in the landscape via both trivial movement and long-distance dispersal, along with the movement of energy and egg reserves carried by those predators. We simulate multiple scenarios varying the presence or absence of floral resources and alternate prey in the natural habitat and crop, and the presence or absence of overwintering refuges for the predator and pest insect in the natural habitat. Food resources provided to the predator by the natural habitat did not enhance natural control in adjacent crops. When the natural habitat served as an overwintering refuge for predators, natural control of pests was enhanced if food resources were available in the crop for those predators. Availability of alternate prey was more important to the predator than floral resources. Early-season crop pest loads were always increased when pest insects overwintered in the natural habitat, however early-season availability of pest insects enhanced natural control across the whole-season. Overall, crop pest loads were highly variable across simulation scenarios, reproducing the conflicting results from field research.

Estimating direct and indirect effects of habitat structure on nesting field sparrows (Spizella pusilla) using structural equation models

Due to consistent population declines across the continent, grassland birds have become a guild of high conservation and management interest. Despite a large number of studies investigating grassland bird habitat associations, we know relatively little about the mechanisms through which habitat characteristics may impact grassland birds, as these mechanisms are often assumed rather than directly tested. For this study, we estimated whether the effects of habitat structure on breeding Field Sparrows are mediated through changes in predator (snake and raccoon) abundance, alternative prey availability, or arthropod biomass using structural equation models. We found no evidence of nest survival or nest density of Field Sparrows being directly influenced by nest predator abundance, alternative prey, or arthropod biomass, although habitat characteristics associated with increased nest survival were also associated with greater arthropod biomass and reduced predator abundance. We suggest that habitat structure in our study area primarily impacts breeding Field Sparrows through direct means, such as influencing nest concealment or foraging efficiency. Our results also suggest that nest success and nest density are decoupled in our study area, so Field Sparrows may be preferentially selecting nest sites with structural characteristics that do not increase nest survival. Ultimately, our findings from this study indicate that while predator avoidance and food provisioning likely play an important role in determining nest survival for grassland birds, predator abundance and arthropod biomass may not necessarily predict predation risk and foraging efficiency to the extent that is often assumed.

Association between arthropod densities suggests dominance of top-down control of predator-prey food-webs on pear trees during winter

Although winter in the temperate zone is considered to be the period of arthropod quiescence, some pests and their natural enemies remain active and interact in simplified food-webs. Limited information exists about the relative importance of top-down and bottom-up processes regulating arthropod food-webs and their spatio-temporal dynamics during winter. This information is essential for the development of effective conservation biocontrol methods. We investigated how the pest management of pear orchards (integrated [IPM] vs organic) and tree location within orchards (margin vs centre) influence the association between densities of winter-active spiders, insect herbivores and winter-inactive predators (spiders, insects) from October to April. We installed carboard bands on trunks and branches of pear trees in four organic and four IPM orchards to collect bark-dwelling arthropods. We then modelled relationships between the densities of four functional arthropod groups, namely winter-active spiders, winter-inactive spiders, winter-inactive insect predators, and herbivores. In early winter, we found a hump-shaped relationship between the densities of winter-active spiders and herbivores. This agrees with the top-down model of the ecosystem exploitation hypothesis, predicting that predators are first bottom-up limited, then accumulate with prey densities, and exert top-down control when they become sufficiently abundant. The densities of herbivores strongly declined during winter, a phenomenon which may be partly, along with other causes of natural mortality, attributable to predation by winter active spiders. The association between winter-active spiders and winter-inactive arthropod predators switched from positive to none in organic orchards or even to a negative association in IPM orchards. Negative intraguild interactions seem to be intensified due to the declining availability of alternative prey. Overall, the investigated food-webs seem to be mostly top-down regulated during winter, and IPM seems to intensify intraguild interactions.

Response of cavity nesting birds to Lymantria dispar (Lepidoptera) and aerial spraying – An experimental approach

Eruptive population dynamics of forest insect species regularly attract the interest of ecologists but also often evoke debates among stakeholders concerning impacts that outbreaks or pest control activities have on forest communities including insectivorous birds. Lymantria dispar is the most serious native defoliator in mixed oak-broadleaf forests in Central Europe. To investigate the treatment effects of L. dispar density, aerial application of the insecticide Mimic (tebufenozide) and nest box accessibility for L. dispar caterpillars on nest box success of cavity nesting birds, we deployed 352 nest boxes in a full factorial experimental design with 11 replicates. L. dispar density was predicted by egg mass counts made in the year before the study. The accessibility of nest boxes for L. dispar caterpillars was manipulated in all study sites by placing nest boxes on trunks for easy access to caterpillars and on branches for more difficult access. Successful nest boxes (binary metric that combines failed breeding attempt and empty nest box) and nest productivity (number of fledged chicks in successful boxes) were monitored in the peak year of a L. dispar outbreak, during which half of the plots were treated with Mimic, as well as in the following year when L. dispar populations had widely collapsed, and no further treatments were applied. L. dispar and non-target caterpillar biomass was sampled by canopy fogging to quantify prey abundance. Caterpillar biomass was significantly reduced by Mimic in the treatment year and remained low in the post-treatment year. Despite the reduction of caterpillars, our zero-inflation model found no effect of any tested variables on nest box success of the early brood. However, in the late broods the number of successful nest boxes was reduced in Mimic treated plots in the first year by 42%, while unaffected by initial L. dispar density. The number of successful nest boxes in the second year was lower in areas with initially high L. dispar densities and there was a marginal carry-over effect of insecticide treatment. L. dispar caterpillars entered nest boxes on the trunk in higher numbers than boxes on the branch, but nest box position had no effect on nest box success of birds.We conclude that despite its strong negative impact on caterpillar prey, aerial application of Mimic targeting L. dispar caused an increase in the number of failed boxes in late broods in sprayed plots, particularly in the year of application and to some extent in the year after. We presume this delayed effect of Mimic only on late broods to be caused by Mimic-induced caterpillar shortages during first broods leading to an increased parental effort and a subsequent reduced number of second broods. The generally limited impact of Mimic might be explained by the overall extensive availability of alternate prey in oak forests in May during first broods and the ability of tits to increase their parental effort in times of caterpillar shortage. In unsprayed sites, L. dispar densities can create legacies of impact in following years, but caterpillars do not directly affect birds nest box success despite entering cavities.

Incorporating geographic context into coyote and wolf livestock depredation research

AbstractApplying research results to new locations and situations can be confounded by differences in the geographic context between the original and the applied study sites. Replication studies and meta‐analyses may be similarly hindered. We investigated how often canid management research reports (e.g., journal articles, conference proceedings) included information on historical/current lethal control, alternative prey availability, landscape features, and seasonal and settlement characteristics. We included experimental research published between 1970 and 2018, focusing on livestock depredations by wolves and coyotes in North America. Reporting on contextual factors was highly variable; seasonal context was included in 83% of research findings; human settlement characteristics were reported in only 8%. Contextual information was more common in journal versus grey literature, and in reports with academic versus government‐affiliated primary authors. Discussions of the effects of contextual factors on livestock depredation mitigation strategies were underdeveloped. Yet, geographic context of research is vital; it can alter animal behaviour and reduce the efficacy of applied mitigation. We suggest reporting guidelines to improve comparisons and meta‐analysis opportunities, which may enhance comparisons and management decision making.

Uncertainty in foraging success and its consequences on fitness

Optimal foraging models are commonly used to determine the strategy maximizing the proxies for fitness, such as foraging success. The strategies maximizing the proxy for fitness and fitness are assumed to be the same. However, this study shows that this assumption can be invalid when the relationship between the proxy for fitness and fitness is nonlinear and the foraging success is uncertain. A well-known prey choice model that uses long-term energy intake rate as the proxy for fitness was used as an example. This model considers a situation where predators predate on two types of prey that differ in quality, that is, one (primary prey) has higher quality than the other (alternative prey). A strategy can be represented by the probability of attacking alternative prey upon encounter while always attacking primary prey. The probability of attacking alternative prey that maximizes the expected rate of energy intake is either 0 or 1 depending on the density of primary prey (known as the zero–one rule). Meanwhile, the density of alternative prey has no influence on the optimal strategy. A simulation model was used to characterize the stochastic outcomes in the rate of energy intake in a finite foraging duration. The results revealed that foraging strategy influences the expected rate of energy intake and the uncertainty around the expectation. Consequently, the strategies maximizing the rate of energy intake and fitness may not be the same when the relationship between the rate of energy intake and fitness is nonlinear due to Jensen’s inequality. Previous results such as the zero–one rule and the independence of the optimal strategy on the availability of alternative prey are no longer valid when fitness, rather than the proxy for fitness, is explicitly considered in a finite foraging duration.

Foraging behaviour of sympatrically breeding macaroni (Eudyptes chrysolophus) and chinstrap (Pygoscelis antarcticus) penguins at Bouvetøya, Southern Ocean

Species with similar ecological requirements that overlap in range tend to segregate their niches to minimize competition for resources. However, the niche segregation possibilities for centrally foraging predators that breed on isolated Subantarctic islands may be reduced by spatial constraints and limitations in the availability of alternative prey. In this study we examined spatial and trophic aspects of the foraging niches of two sympatrically breeding penguin species, macaroni (Eudyptes chrysolophus; MAC) and chinstrap (Pygoscelis antarcticus; CHIN) penguins, at Bouvetøya over two breeding seasons. To measure at-sea movements and diving behaviour, 90 MACs and 49 CHINs were equipped with GPS loggers and dive recorders during two austral summer breeding seasons (2014/15 and 2017/18). In addition, blood samples from tracked birds were analysed for stable isotopes to obtain dietary information. CHINs displayed marked interannual variation in foraging behaviour, diving deeper, utilizing a larger foraging area and displaying enriched values of δ15N in 2014/15 compared to the 2017/18 breeding season. In contrast, MACs dove to similar depths and showed similar δ15N values, while consistently utilizing larger foraging areas compared to CHINs. We suggest that low krill abundances in the waters around Bouvetøya during the 2014/15 season resulted in CHINs shifting toward a diet that increased their niche overlap with MACs. Our findings may partly explain the decreasing number of breeding CHINs at the world’s most remote island, Bouvetøya, while also highlighting the importance of characterizing niche overlap of species using multi-season data sets.

The Effect of Predator Population Dynamics on Batesian Mimicry Complexes.

AbstractUnderstanding Batesian mimicry is a classic problem in evolutionary biology. In Batesian mimicry, a defended species (the model) is mimicked by an undefended species (the mimic). Prior theories have emphasized the role of predator behavior and learning as well as evolution in model-mimic complexes but have not examined the role of population dynamics in potentially governing the relative abundances and even persistence of model-mimic systems. Here, we examined the effect of the population dynamics of predators and alternative prey on the prevalence of warning-signaling prey composed of models and mimics. Using optimal foraging theory and signal detection theory, we found that the inclusion of predator and alternative prey population dynamics could reverse traditional theoretical predictions: as alternative prey increase in numbers, mimics suffer because larger populations of predators are maintained, resulting in apparent competition. Under some circumstances, apparent competition affects model populations as well, although not as severely as it affects mimics. Our results bear on the intriguing puzzle that in nature warning signals are relatively scarce, yet experiments suggest that such signals can be highly advantageous. The availability of alternative prey and numerical responses by predators can overwhelm advantages observed in experiments to keep warning signals in model-mimic systems relatively scarce.

Arctic fox winter dietary response to damped lemming cycles estimated from fecal DNA

Abstract Climate-caused changes in prey abundance may alter predator–prey dynamics in the Arctic food web. Lemmings (Dicrostonyx spp.) are important prey for Arctic foxes (Vulpes lagopus) and their annual population fluctuations drive fox reproduction, creating strongly linked predator–prey population cycles. Winter diet directly impacts Arctic fox reproductive success, but winter prey diversity on the tundra is low. Strategies such as using the marine environment to scavenge seals have allowed Arctic foxes to persist during years of low lemming abundance. However, warming winters have decreased snowpack quality, preventing lemmings from reaching their previous high abundances, which may reduce their impact on predator dynamics. We investigated Arctic fox dietary response to lemming abundance by reconstructing Arctic fox winter diet in the low Arctic. Next-generation sequencing of fecal DNA, from samples (n = 627) collected at dens in winters of 2011–2018, identified prey both from terrestrial and marine environments. Despite lemming cycle damping, Arctic foxes still increased lemming consumption during years of higher lemming abundance. Alternative prey such as marine resources were consumed more during years of low lemming abundance, with up to 45% of samples containing marine resources in low lemming years. In addition, Arctic foxes consumed high proportions of meadow voles (Microtus pennsylvanicus), which may represent a new alternative prey, suggesting climate change may be creating new foraging opportunities. Changes in prey abundance illustrate how climate-caused disturbances are altering Arctic food-web dynamics. Dietary flexibility and availability of alternative prey may become increasingly important for Arctic predators as the Arctic continues to change.

Evolution of Mimicry Rings as a Window into Community Dynamics

Mimicry rings are communities of mimetic organisms that are excellent models for ecological and evolutionary studies because the community composition, the nature of the species interactions, the phenotypes under selection, and the selective agents are well characterized. Here, we review how regional and ecological filtering, density- and frequency-dependent selection, toxicity of prey, and age of mimicry rings shape their assembly. We synthesize findings from theoretical and empirical studies to generate the following hypotheses: ( a) the degree of unpalatability and age of mimicry rings increase mimicry ring size and ( b) the degree of unpalatability, generalization of the aposematic signal, and availability of alternative prey are positively related to the breadth of the protection umbrella for an aposematic signal and negatively related to the degree of mimetic resemblance. We also provide a phylogenetic framework in which key aspects of mimicry ring diversification may be studied.

A Study on the Culex quinquefasciatus Say, 1823 Control Potentiality of Colisa fasciata (Bloch & Schneider, 1801) in Laboratory Condition

Colisa fasciata consumed significantly more Chironomus ramosus larvae as compared to Culex quinquefasciatus larvae and pupae when those were offered separately. In paired experiment it consumed significantly more C. ramosus larvae than C. quinquefasciatus larvae. When habitat was modified by incorporating sand and gravels on the floor of the aquarium, C. fasciata consumed significantly more C. quinquefasciatus larvae as compared to C. ramosus larvae. Chesson’s food preference index also confirmed these findings. The larvicidal efficiency therefore not only depends on the availability of alternative prey but also on the micro-habitat condition.

Characterizing juvenile salmon predation risk during early marine residence.

Predation mortality can influence the distribution and abundance of fish populations. While predation is often assessed using direct observations of prey consumption, potential predation can be predicted from co-occurring predator and prey densities under varying environmental conditions. Juvenile Pacific salmon Oncorhynchus spp. (i.e., smolts) from the Columbia River Basin experience elevated mortality during the transition from estuarine to ocean habitat, but a thorough understanding of the role of predation remains incomplete. We used a Holling type II functional response to estimate smolt predation risk based on observations of piscivorous seabirds (sooty shearwater [Ardenna griseus] and common murre [Uria aalge]) and local densities of alternative prey fish including northern anchovy (Engraulis mordax) in Oregon and Washington coastal waters during May and June 2010–2012. We evaluated predation risk relative to the availability of alternative prey and physical factors including turbidity and Columbia River plume area, and compared risk to returns of adult salmon. Seabirds and smolts consistently co-occurred at sampling stations throughout most of the study area (mean = 0.79 ± 0.41, SD), indicating that juvenile salmon are regularly exposed to avian predators during early marine residence. Predation risk for juvenile coho (Oncorhynchus kisutch), yearling Chinook salmon (O. tshawytscha), and subyearling Chinook salmon was on average 70% lower when alternative prey were present. Predation risk was greater in turbid waters, and decreased as water clarity increased. Juvenile coho and yearling Chinook salmon predation risk was lower when river plume surface areas were greater than 15,000 km2, while the opposite was estimated for subyearling Chinook salmon. These results suggest that plume area, turbidity, and forage fish abundance near the mouth of the Columbia River, all of which are influenced by river discharge, are useful indicators of potential juvenile salmon mortality that could inform salmonid management.

Numbers, neighbors, and hungry predators: What makes chemically defended aposematic prey susceptible to predation?

Many chemically defended aposematic species are characterized by relatively low toxin levels, which enables predators to include them in their diets under certain circumstances. Knowledge of the conditions governing the survival of such prey animals—especially in the context of the co‐occurrence of similar but undefended prey, which may result in mimicry‐like interactions—is crucial for understanding the initial evolution of aposematism. In a one‐month outdoor experiment using fish (the common carp Cyprinus carpio) as predators, we examined the survival of moderately defended aposematic tadpole prey (the European common toad Bufo bufo) with varying absolute densities in single‐species prey systems or varying relative densities in two‐species prey systems containing morphologically similar but undefended prey (the European common frog Rana temporaria). The density effects were investigated in conjunction with the hunger levels of the predator, which were manipulated by means of the addition of alternative (nontadpole) food. The survival of the B. bufo tadpoles was promoted by increasing their absolute density in the single‐species prey systems, increasing their relative density in the two‐species prey systems, and providing ample alternative food for the predator. Hungry predators eliminated all R. temporaria individuals regardless of their proportion in the prey community; in treatments with ample alternative food, high relative B. bufo density supported R. temporaria survival. The results demonstrated that moderately defended prey did benefit from high population densities (both absolute and relative), even under long‐term predation pressure. However, the physiological state of the predator was a crucial factor in the survival of moderately defended prey. While the availability of alternative prey in general should promote the spread and maintenance of aposematism, the results indicated that the resemblance between the co‐occurring defended and undefended prey may impose mortality costs on the defended model species, even in the absence of actual mimicry.

The resilience of weed seedbank regulation by carabid beetles, at continental scales, to alternative prey

Carabids are generalist predators that contribute to the agricultural ecosystem service of seedbank regulation via weed seed predation. To facilitate adoption of this ecosystem services by farmers, knowledge of weed seed predation and the resilience of seedbank regulation with co-varying availability of alternative prey is crucial. Using assessments of the seedbank and predation on seed cards in 57 cereal fields across Europe, we demonstrate a regulatory effect on the soil seedbank, at a continental scale, by groups formed of omnivore, seed-eating (granivore + omnivore) and all species of carabids just prior to the crop-harvest. Regulation was associated with a positive relationship between the activity-density of carabids and seed predation, as measured on seed cards. We found that per capita seed consumption on the cards co-varied negatively with the biomass of alternative prey, i.e. Aphididae, Collembola and total alternative prey biomass. Our results underline the importance of weed seedbank regulation by carabids, across geographically significant scales, and indicate that the effectiveness of this biocontrol may depend on the availability of alternative prey that disrupt the weed seed predation.

Preventive insecticide use affects arthropod decomposers and decomposition in field crops

Insecticides use in field crops has grown increasingly prophylactic, which poses a potentially unnecessary risk to arthropod decomposers. These decomposers — including millipedes (Diplopoda), soil mites (Acari, mostly Oribatida), and Collembola — act as alternative prey for generalist predators, facilitate crop-residue breakdown, and can influence nutrient cycling. However, the abundance and activity of arthropod decomposers may be reduced by extensive prophylactic pesticide use. We conducted a three-year field experiment in no-till corn and soybean fields to assess how neonicotinoid seed coatings or broadcast applications of a pyrethroid insecticide affect arthropod decomposers and litter decomposition. Both insecticides reduced densities of arthropod decomposers and reduced decomposition of plant litter by over 10%. Neonicotinoid seed coatings reduced collembolan densities by 34% and millipede densities by 52%, while the pyrethroid significantly reduced oribatid mite densities by 24% and millipede densities by 82%. Lower mite and millipede densities correlated with slower litter loss, and decomposition was slower under neonicotinoid seed coatings and broadcast pyrethroid applications. These results indicate that perennial, widespread use of prophylactic insecticides is likely to have concerning negative effects on the availability of alternative prey, plant residue breakdown, and nutrient cycling in field crops.

Investigating factors driving phytoplankton growth and grazing loss rates in waters around Peninsular Malaysia

In tropical waters where temperatures are relatively stable, we investigated whether the relationship between phytoplankton growth and grazing loss rate across different habitats around Peninsular Malaysia can be attributed to its eutrophication states. We measured phytoplankton growth (μ) and grazing loss (g) rates in waters off Bachok Marine Research Station (BMRS), located northeast of Peninsular Malaysia. Chlorophyll-a (chl-a) concentration ranged from 2.90 to 15.78 µg/L and was dominated by nano-and micro-phytoplankton (>2 µm in size). Using the Landry and Hassett dilution method, μ at BMRS ranged from 1.02 to 1.58 per day whereas g varied from 0.07 to 0.88 per day. Grazing accounted for 35% of the primary production at BMRS. A systematic review of available data in waters around Peninsular Malaysia, revealed how μ fluctuated over a wide range (0.01–1.80 per day) and correlated with chl a distribution (R2=0.181- P<0.001). However, the relationship was only significant at <9 µg/L chl a for mesotrophic waters and <16 µg/L chl a for eutrophic waters. In contrast, g ranged from 0.00 to 1.01 per day, and correlated with μ at all locations. The g/μ slope ranged from 19% to 84%, and was generally similar for waters around Peninsular Malaysia. However, all the g/μ slopes had a positive y-intercept except for BMRS, and this seemed to suggest the availability of alternative prey supporting grazing at the other stations.

Adaptations to prey base in the hypercarnivorous leopard cat Prionailurus bengalensis

Investigating biogeographical variations in diet composition can help understand the adaptability and generalism of species. Although the dietary adaptability of omnivorous mesocarnivores is well established, far less work has explored how more specialist hypercarnivores optimise their diets. By reviewing 11 studies of the leopard cat (Prionailurus bengalensis), we quantitatively examined how dietary composition varies over the wide range of biomes they occupy in Asia. Specifically, we contrasted the diet of the Iriomote Island sub-species (south-western Japan), where native rodents are absent, with that of the mainland. Leopard cat diet typically comprised mammals, birds, amphibians, reptiles, and invertebrates. In Iriomote Island, however, the low relative frequency of occurrence of small mammals (only introduced rats) was compensated by higher frequencies of reptiles and amphibians compared to the mainland. Consequently, trophic diversity and dietary niche breadth were higher for leopard cats in Iriomote Island than for the mainland. This shows that even hypercarnivorous species can use trophic plasticity to adapt to local prey availability. Given that rodent numbers often fluctuate substantially over time, the availability of alternative prey, such as herptiles, may be vital for the conservation of the leopard cat, and especially the critically endangered Iriomote cat. More generally, the trophic versatility of hypercarnivores must be considered when assessing their vulnerability to environmental change.

Subsidy type and quality determine direction and strength of trophic cascades in arthropod food webs in agroecosystems

Abstract The subsidy hypothesis states that communities receiving nutrient subsidies will demonstrate top‐down trophic cascades where predators indirectly increase plant biomass. This has been both confirmed and refuted, which might depend on whether the subsidy has mainly targeted the plant or the detrital food web compartment, and on the subsidy quality. This is particularly poorly understood for terrestrial communities such as heavily subsidized agroecosystems. Using cages covering 4 m2 of ground in a long‐term agricultural fertilization experiment, we tested whether subsidies targeting the detrital soil mesofauna compartment with organic fertilizers, or the plants with mineral fertilizer, impacted the direction and strength of trophic cascades in an arthropod–plant food web. We expected top‐down controls of generalist arthropod predators (spiders, ground and rove beetles) on aphid densities to be stronger in organically fertilized plots due to enhanced alternative prey availability in the soil. Bottom‐up control from barley quality on aphids was anticipated to be stronger in the mineral treatments. We examined how the quality (decomposability) of the organic subsidy governed the cascades by comparing treatments with labile (manure) and recalcitrant (hay) organic matter. Top‐down forces dominated in food webs receiving organic subsidies, while bottom‐up forces dominated under mineral fertilization. A high‐quality, easily degradable organic subsidy propagated faster through the food chain, leading to a top‐down trophic cascade with generalist predators having a positive effect on plant biomass in the labile but not in the recalcitrant organic treatment. Synthesis and applications. Management of agricultural soils that bolster the soil mesofauna, for example adding organic fertilizers, has potential to increase top‐down biological control by naturally occurring generalist arthropod predators. Our research demonstrates how barley biomass can be enhanced in manure treatment in the presence of arthropod predators to a level comparable to that of mineral fertilizer.

Predation risk is a function of alternative prey availability rather than predator abundance in a tropical savanna woodland ecosystem

Typically, factors influencing predation risk are viewed only from the perspective of predators or prey populations but few studies have examined predation risk in the context of a food web. We tested two competing hypotheses regarding predation: (1) predation risk is dependent on predator density; and (2) predation risk is dependent on the availability of alternative prey sources. We use an empirical, multi-level, tropical food web (birds–lizards–invertebrates) and a mensurative experiment (seasonal fluctuations in abundance and artificial lizards to estimate predation risk) to test these hypotheses. Birds were responsible for the majority of attacks on artificial lizards and were more abundant in the wet season. Artificial lizards were attacked more frequently in the dry than the wet season despite a greater abundance of birds in the wet season. Lizard and invertebrate (alternative prey) abundances showed opposing trends; lizards were more abundant in the dry while invertebrates were more abundant in the wet season. Predatory birds attacked fewer lizards when invertebrate prey abundance was highest, and switched to lizard prey when invertebrate abundance reduced, and lizard abundance was greatest. Our study suggests predation risk is not predator density-dependent, but rather dependent on the abundance of invertebrate prey, supporting the alternative prey hypothesis.

Frequency dependence shapes the adaptive landscape of imperfect Batesian mimicry.

Despite more than a century of biological research on the evolution and maintenance of mimetic signals, the relative frequencies of models and mimics necessary to establish and maintain Batesian mimicry in natural populations remain understudied. Here we investigate the frequency-dependent dynamics of imperfect Batesian mimicry, using predation experiments involving artificial butterfly models. We use two geographically distinct populations of Adelpha butterflies that vary in their relative frequencies of a putatively defended model (Adelpha iphiclus) and Batesian mimic (Adelpha serpa). We found that in Costa Rica, where both species share similar abundances, Batesian mimicry breaks down, and predators more readily attack artificial butterfly models of the presumed mimic, A. serpa By contrast, in Ecuador, where A. iphiclus (model) is significantly more abundant than A. serpa (mimic), both species are equally protected from predation. Our results provide compelling experimental evidence that imperfect Batesian mimicry is frequency-dependent on the relative abundance of models and mimics in natural populations, and contribute to the growing body of evidence that complex dynamics, such as seasonality or the availability of alternative prey, influence the evolution of mimetic traits.