Predation by Euseius scutalis (Acari: Phytoseiidae) on Tetranychus urticae and Eutetranychus orientalis (Acari: Tetranychidae): effect of prey density and developmental stage

Predation ability is a key component determining the suppression of agricultural pests by natural enemies. The aim of this study is to evaluate the effect of prey density and developmental stage on the predation of Euseius scutalis (Acari: Phytoseiidae) on the two-spotted spider mite, Tetranychus urticae (Acari: Tetranychidae) and the oriental red mite, Eutetranychus orientalis (Acari: Tetranychidae). For this purpose, laboratory functional response experiments were conducted using female adults of E. scutalis. Results indicated that the predator causes an inverse density dependent mortality to larvae, nymphs and adults of the two-spotted spider mite, as well as the oriental red mite. The random predator equation was used to fit the data. E. scutalis handling time increased significantly with T. urticae and E. orientalis developmental stage. The maximum number of T. urticae prey consumed was 48.3 larvae, 28.5 nymphs and 11.0 adults, where the corresponding values for E. orientalis were 48.8, 32.2 and 18.2 larvae, nymphs and adults, respectively. Estimated handling times and attack rates of E. scutalis on larvae, nymphs and adults of T. urticae were 0.4970 h and 0.1058 h-1, 0.8435 h and 0.1395 h-1, and 2.1834 h and 0.0835 h-1, respectively. The corresponding values on larvae, nymphs and adults of E. orientalis were 0.4920 h and 0.1166 h-1, 0.7452 h and 0.1361 h-1, and 1.3186 h and 0.0597 h-1, respectively. The results of this study may be indicative of the predation ability of adults of E. scutalis on these notorious pests, providing a basis for determining release rates appropriate for various pest densities.

On periodic solutions of a discrete Nicholson’s dual system with density-dependent mortality and harvesting terms

In this study, we discuss the existence of positive periodic solutions of a class of discrete density-dependent mortal Nicholson’s dual system with harvesting terms. By means of the continuation coincidence degree theorem, a set of sufficient conditions, which ensure that there exists at least one positive periodic solution, are established. A numerical example with graphical simulation of the model is provided to examine the validity of the main results.

Demographic Processes Allow Echinochloa crus-galli to Compensate Seed Losses by Seed Predation

The effect of weed management that targets the seed stage on subsequent life stages is largely unknown. Post-dispersal seed predation reduces the number of seeds from the soil surface before the seeds contribute to the seedbank. Density-dependent processes can mitigate the effect of seed predation in subsequent life stages. In this study, we tested if (i) targeting the seed stage affects the subsequent seedling stage; (ii) if density-dependent mortality in subsequent life stages partly compensates seedling abundance; and (iii) if the magnitude of final seed production depends on seed predation. We fully parameterized a model for the summer-annual weed Echinochloa crus-galli (L.) P. Beauv.. Field data from three maize fields in north-eastern Germany were obtained, in the presence or absence of seed predation and different population levels of the weed species. Seeds of E. crus-galli were applied in autumn and the number of seedlings, adult plants, and seed production per m2 was determined the following season. Seed predation reduced the number of seedlings. Density-dependent mortality during the seedling stage increased fecundity with decreasing seedling density, and, thus, compensated for lower numbers of seedlings. The final level of seed production per m2 did not depend on seed predation and initial population densities, but differed among fields. We conclude, solely targeting the seed stage can scarcely limit the population growth of E. crus-galli.

Wildlife disturbances as a source of conspecific negative density-dependent mortality in tropical trees

Large vertebrates are rarely considered important drivers of conspecific negative density-dependent mortality (CNDD) in plants because they are generalist consumers. However, disturbances like trampling and nesting also cause plant mortality, and their impact on plant diversity depends on the spatial overlap between wildlife habitat preferences and plant species composition. We studied the impact of native wildlife on a hyperdiverse tree community in Malaysia. Pigs ( Sus scrofa ) are abnormally abundant at the site due to food subsidies in nearby farmland and they construct birthing nests using hundreds of tree saplings. We tagged 34 950 tree saplings in a 25 ha plot during an initial census and assessed the source mortality by recovering tree tags from pig nests ( n = 1672 pig-induced deaths). At the stand scale, pigs nested in flat dry habitats, and at the local neighbourhood scale, they nested within clumps of saplings, both of which are intuitive for safe and efficient nest building. At the stand scale, flat dry habitats contained higher sapling densities and higher proportions of common species, so pig nesting increased the weighted average species evenness across habitats. At the neighbourhood scale, pig-induced sapling mortality was associated with higher heterospecific and especially conspecific sapling densities. Tree species have clumped distributions due to dispersal limitation and habitat filtering, so pig disturbances in sapling clumps indirectly caused CNDD. As a result, Pielou species evenness in 400 m 2 quadrats increased 105% more in areas with pig-induced deaths than areas without disturbances. Wildlife induced CNDD and this supported tree species evenness, but they also drove a 62% decline in sapling densities from 1996 to 2010, which is unsustainable. We suspect pig nesting is an important feature shaping tree composition throughout the region.

Enemies mediate distance- and density-dependent mortality of tree seeds and seedlings: a meta-analysis of fungicide, insecticide and exclosure studies

Conspecific negative distance- and density-dependence is often assumed to be one of the most important mechanisms controlling forest community assembly and species diversity globally. Plant pathogens, and insect and mammalian herbivores, are the most common natural enemy types that have been implicated in this phenomenon, but their general effects at different plant life stages are still unclear. Here, we conduct a meta-analysis of studies that involved robust manipulative experiments, using fungicides, insecticides and exclosures, to assess the contributions of different natural enemy types to distance- and density-dependent effects at seed and seedling stages. We found that distance- and density-dependent mortality caused by natural enemies was most likely at the seedling stage and was greater at higher mean annual temperatures. Conspecific negative distance- and density-dependence at the seedling stage is significantly weakened when fungicides were applied. By contrast, negative conspecific distance- and density-dependence is not a general pattern at the seed stage. High seed mass reduced distance- and density-dependent mortality at the seed stage. Seed studies excluding only large mammals found significant negative conspecific distance-dependent mortality, but exclusion of all mammals resulted in a non-significant effect of conspecifics. Our study suggests that plant pathogens are a major cause of distance- and density-dependent mortality at the seedling stage, while the impacts of herbivores on seedlings have been understudied. At the seed stage, large and small mammals, respectively, weaken and enhance negative conspecific distance-dependent mortality. Future research should identify specific agents of mortality, investigate the interactions among different enemy types and assess how global change may affect natural enemies and thus influence the strength of conspecific distance- and density-dependence.

Cryptic density dependence: effects of spatio-temporal covariation between density and site quality in reef fish

The importance and strength of density dependence continues to engender debate because of its central importance to population dynamics and regulation. Here, we show how confounding effects of site quality can mask strong effects of density dependence. In particular, we explore spatiotemporal variation and covariation among (1) densities of newly settled coral reef fish (Thalassoma hardwicke), (2) environmental characteristics, and (3) the strength of density-dependent mortality. Environmental features of patch reefs were spatially and temporally variable and influenced density-dependent survival. Higher-quality sites (i.e., reefs possessing features that yield greater numbers of recruits at any given settlement level) received greater settlement, and this relationship masked the operation of density dependence when variation in quality among sites (or times) was not distinguished (a common approach in many Observational studies of density dependence). Our work illustrates how spatiotemporal covariation in settlement density and site quality can obscure patterns of density dependence at larger scales, contributing to a phenomenon we call "cryptic density dependence." Acknowledging patterns and consequences of covariance may alter the way we study population dynamics, especially of marine organisms, where the link between processes that affect settlement and post-settlement survival remains relatively poorly understood.