Direct and indirect effects of predation on mosquitofish behavior and survival

Predation can have strong direct and indirect effects on the behavior of prey. We investigated whether predation by chain pickerel (Esox niger) caused adult eastern mosquitofish (Gambusia holbrooki) to alter their habitat use and whether pickerel predation influenced survival of adult and neonate mosquitofish. The number of adult mosquitofish using the riskier of three habitats was lowest when two predators occupied the risky habitat, intermediate in the treatment with one predator, and highest when no predators occurred there. More mosquitofish neonates survived high predation treatments than treatments lacking pickerel. We conclude that pickerel predation causes adult mosquitofish to shift to refuge habitats. The pattern of neonate survival suggests that adult habitat use may create a refuge from cannibalism for neonate mosquitofish, resulting in higher neonate survival in treatments with more pickerel. Hence, pickerel predation has a direct effect on adult mosquitofish behavior and a strong indirect effect on neonate survival. Both interspecific and intraspecific predation can effect prey populations and can interact to produce important indirect effects.     

Climate-driven synchrony in seed production of masting deciduous and conifer tree species

Aims Understanding fluctuations in plant reproductive investment can constitute a key challenge in ecology, conservation and management. Masting events of trees (i.e. the intermittent and synchronous production of abundant seeding material) is an extreme example of such fluctuations. Our objective was to establish the degree of spatial and temporal synchrony in common four masting tree species in boreal Finland and account for potential causal drivers of these patterns.Methods We investigated the spatial intraspecific and temporal interspecific fluctuations in annual seed production of four tree species in Finland, silver birch Betula pendula Roth, downy birch Betula pubescens Ehrh., Norway spruce Picea abies (L.) H.Karst. and rowanberry Sorbus aucuparia L. We also tested to see whether variations in seed production were linked to annual weather conditions. Seeding abundance data were derived from tens of stands per species across large spatial scales within Finland during 1979 to 2014 (for rowanberries only 1986 to 2014).Important findings All species showed spatial synchrony in seed production at scales up to 1000 km. Annual estimates of seed production were strongly correlated between species. Spring and summer temperatures explained most variation in crop sizes of tree species with 0-to 2-year time lags, whereas rainfall had relatively little influence. Warm weather during flowering (May temperature) in the flowering year (Year t) and 2 years before (t ?2) were correlated with seed production. However, high May temperatures during the previous year (t^-1) adversely affected seed production. Summer temperatures in Year t^-1 was positively correlated with seed production, likely because this parameter enhances the development of flower primordials, but the effect was negative with a time lag of 2 years. The negative feedback in temperature coefficients is also likely due to patterns of resource allocation, as abundant flowering and seed production in these species is thought to reduce the subsequent initiation of potential new flower buds. Since the most important weather variables also showed spatial correlation up to 1000 km, weather parameters likely explain much of the spatial and temporal synchrony in seed production of these four studied tree species.     

Direct and indirect effects of marsupial browsing on a foundation tree species

Plant‐mediated indirect effects can be important ecological drivers in plant communities, especially in systems where extended genetic effects of foundation species can shape communities and influence ecosystem dynamics. Here we investigate the direct and indirect effects of uncontrolled browsing by marsupial herbivores including the common brushtail possum Trichosurus vulpecula, Bennetts wallaby Macropus rufogriseus and the red‐bellied pademelon Thylogale billardierii, in a Eucalyptus system known to have extended community and ecosystem genetic effects. In a common garden trial containing 525 full‐sib families from an incomplete diallel crossing program located in northeastern Tasmania, Australia, we assessed the genetic basis to herbivore preferences, the impact of a single and repeated marsupial browsing event on tree fitness and morphological traits and the associated indirect plant‐mediated effects on a subsequent herbivore, autumn gum moth Mnesampela privata. Marsupial browsing was not influenced by plant genetics, but spatial components instead affected the pattern of damage across the trial. Marsupial browsing had significant impacts on tree development, morphology and survival, resulting in reductions in survival, height and basal area, an increase proportion in multiple stems, delays in flowering as well as delays in phase change from juvenile to adult foliage. Fitness impacts were minimal in response to a once‐off browsing event, but effects were exacerbated when trees suffered repeated browsing. We demonstrate clear plant‐mediated indirect effects of marsupial browsing on subsequent tree use by an invertebrate herbivore, through induced changes in plant morphology. Such indirect effects have the potential to influence biotic community structure on a foundation species host‐plant, and the evolutionary interactions that occur between organisms and the host‐plant themselves.     

Direct and indirect effects of long-term fertilization on the stability of the persistent seed bank

Plant and Soil - Desert environments are characterized by limited and highly variable rainfall, which is an intermittent source of water critical to the evolution of the structure and functioning...     

Direct and indirect effects of pollinators and seed predators to selection on plant and floral traits

Although flowering traits are often assumed to be under strong selection by pollinators, significant variation in such traits remains the norm for most plant species. Thus, it is likely that the interactions among plants, mutualists, and other selective agents, such as antagonists, ultimately shape the evolution of floral and flowering traits. We examined the importance of pollination vs pre-dispersal seed predation to selection on plant and floral characters via female plant-reproductive success in Castilleja linariaefolia (Scrophulariaceae). C. linariaefolia is pollinated by hummingbirds and experiences high levels of pre-dispersal seed predation by plume moth and fly larvae in the Rocky Mountains of Colorado, USA, where this work was conducted. We first examined whether female reproduction in C. linariaefolia was limited by pollination. Supplemental pollination only marginally increased components of female reproduction, likely because seed predation masked, in part, the beneficial effects of pollen addition. In unmanipulated populations, we measured calyx length, flower production, and plant height and used path analysis combined with structural equation modeling to quantify their importance to relative seed set through pathways involving pollination vs seed predation. We found that the strength of selection on calyx length, flower production, and plant height was greater for seed predation pathways than for pollination pathways, and one character, calyx length, experienced opposing selection via pollination vs seed predation. These results suggest that the remarkable intraspecific variation in plant and floral characters exhibited by some flowering plants is likely the result of selection driven, at least in part, by pollinators in concert with antagonists, such as pre-dispersal seed predators. This work highlights the subtle but complex interactions that shape floral and vegetative design in natural ecosystems.     

Evolutionary jumping and breakthrough in tree masting evolution

Many long-lived plants such as trees show masting or intermittent and synchronized reproduction. In a coupled chaos system describing the dynamics of individual-plant resource budgets, masting occurs when the resource depletion coefficient k (ratio of the reproductive expenditure to the excess resource reserve) is large. Here, we mathematically studied the condition for masting evolution. In an infinitely large population, we obtained a deterministic dynamical system, to which we applied the pairwise invasibility plot and convergence stability of evolutionary singularity analyses. We prove that plants reproducing at the same rate every year are not evolutionarily stable. The resource depletion coefficient k increases, and the system oscillates with a period of 2 years (high and low reproduction) if k<1. Alternatively, k may evolve further and jump to a value >1, resulting in the sudden start of intermittent reproduction. We confirm that a high survivorship of young plants (seedlings) in the light-limited understory favors masting evolution, as previously suggested by computer simulations and field observations. The stochasticity caused by the finiteness of population size also promotes masting evolution.     

Direct and indirect effects of leptin on adipocyte metabolism

Leptin is hypothesized to function as a negative feedback signal in the regulation of energy balance. It is produced primarily by adipose tissue and circulating concentrations correlate with the size of body fat stores. Administration of exogenous leptin to normal weight, leptin responsive animals inhibits food intake and reduces the size of body fat stores whereas mice that are deficient in either leptin or functional leptin receptors are hyperphagic and obese, consistent with a role for leptin in the control of body weight. This review discusses the effect of leptin on adipocyte metabolism. Because adipocytes express leptin receptors there is the potential for leptin to influence adipocyte metabolism directly. Adipocytes also are insulin responsive and receive sympathetic innervation, therefore leptin can also modify adipocyte metabolism indirectly. Studies published to date suggest that direct activation of adipocyte leptin receptors has little effect on cell metabolism in vivo, but that leptin modifies adipocyte sensitivity to insulin to inhibit lipid accumulation. In vivo administration of leptin leads to a suppression of lipogenesis, an increase in triglyceride hydrolysis and an increase in fatty acid and glucose oxidation. Activation of central leptin receptors also contributes to the development of a catabolic state in adipocytes, but this may vary between different fat depots. Leptin reduces the size of white fat depots by inhibiting cell proliferation both through induction of inhibitory circulating factors and by contributing to sympathetic tone which suppresses adipocyte proliferation. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.     

Direct and indirect effects of n-species competition

Summary A measure of net interspecific competition is proposed which incorporates both direct and indirect effects between each species pair in a community. Indirect effects of one species on another may be mediated through other species in the community such that changes in the first species induce changes in other species which in turn directly affect the second species. Even in a competitive community these indirect effects are not necessarily always of a competitive nature and may sometimes actually act to oppose the effects of direct competition. Species pairs which are strong competitors in isolation may exhibit little net competition or even appear mutualistic depending on the other species present in the community.An expression for the net competitive effects between species pairs is derived in terms of the elements of the -matrix representation of the direct competitive effects. Examples calculated from the literature suggest that indirect competitive effects may often act to counterbalance direct competition between otherwise strong competitors in natural communities.     

Direct and indirect effects of winter harshness on the survival of Mallards Anas platyrhynchos in northwest Europe

To understand population dynamics it is necessary to understand vital rates, which may be affected by a wide range of factors including environmental variables such as weather. Weather conditions can affect birds’ vital rates directly through increased mortality due to impaired conditions, or indirectly via changes in body condition and/or behaviour. Most understanding of direct and indirect effects of weather comes from studies of breeding birds, whereas the situation in non‐breeding periods is less clear. Here, we analysed annual survival of non‐breeding Mallard Anas platyrhynchos, the most hunted waterfowl species in Europe, and assessed whether survival is related directly to winter harshness and/or indirectly via changes in winter recovery distributions. Recovery data on Mallards, initially marked in southeast Sweden, were analysed with an information‐theoretic approach using program mark . Over 10 000 Mallards were marked in two time periods, 1964–1982 and 2002–2008, of which 13.3 and 4.7%, respectively, were later recovered. Mallards had lower annual survival in the early trapping period (0.58–0.63) than in the later period (0.69–0.71), with no clear effects of sex, age or year. Within each study period, winter harshness did not directly correlate with survival. However, milder winters may have contributed indirectly to higher survival in the second period, as winter harshness data were correlated with the distances to recovery positions for females, and also because winter recovery areas have shifted northeast during the past decades, possibly indicating a shortened migratory distance. Migration is costly, and there is therefore a likely linkage between migration behaviour and survival of dabbling ducks, in which direct as well as indirect effects of winter harshness may play a role. Other factors, such as hunting pressure, are also likely to have changed in the past decades, and may also have contributed to improved survival of Mallards in northwest Europe.     

Direct and indirect effects of understorey bamboo shape tree regeneration niches in a mixed temperate forest

Plant cover plays a major role in shaping the nature of recruitment microsites through direct (resource mediated) and indirect (consumer mediated) interactions. Understorey plants may differentially affect seedling establishment, thus contributing to regeneration-niche separation among canopy tree species. We examined patterns of early tree seedling survival resulting from interactive effects of understorey bamboo (Chusquea culeou) and resident consumers in a mixed temperate Patagonian forest, Argentina. Newly germinated seedlings of Nothofagus dombeyi and Austrocedrus chilensis were planted in bamboo thickets and non-bamboo patches, with or without small-vertebrate exclosures. We found species-specific patterns of seedling survival in relation to bamboo cover. Nothofagus survival was generally low but increased under bamboo, irrespective of cage treatment. Desiccation stress accounted for most Nothofagus mortality in open, non-bamboo areas. In contrast, Austrocedrus survival was highest in non-bamboo microsites, as most seedlings beneath bamboo were killed by small vertebrates through direct consumption or non-trophic physical damage. There was little evidence for a negative impact of bamboo on tree seedling survival attributable to resource competition. The balance of simultaneous positive and negative interactions implied that bamboo presence facilitated Nothofagus early establishment but inhibited Austrocedrus recruitment via apparent competition. These results illustrate the potential for dominant understorey plants to promote microsite segregation during early stages of recruitment between tree seedlings having different susceptibilities to water stress and herbivory. We recognise, however, that patterns of bamboo–seedling interactions may be conditional on moisture levels and consumer activity during establishment. Hence, both biotic and abiotic heterogeneity in understorey environments should be incorporated into conceptual models of regeneration dynamics and tree coexistence in forest communities.     

Direct and indirect effects of termites on savanna tree-seedling growth

Termites are considered to be ecosystem engineers because they modify their biophysical environments. We tested the effects of soil-nutrient alteration in termite-enriched soils compared with termite-free open savannas. We also tested whether non-nutrient alterations (soil disturbance) induced by termites led to changes in tree-seedling growth. Soil samples taken from termite-enriched soils and adjacent open savanna sites in KwaZulu-Natal, South Africa were analyzed for nitrogen, pH, organic carbon and water-holding capacity. Seeds from three dominant tree species, Acacia sieberiana, Celtis africana and Ziziphus mucronata, were grown in soils taken from termite-enriched soils and adjacent savannas. Overall, organic carbon and nitrogen content were higher in termite-enriched soils than in adjacent savannas. We found that these differences in nutrients did not directly affect seedling growth rates or final height. However, C. africana had increased growth rates in compacted termite-enriched soils, while A. sieberiana seedlings were taller in non-compacted soils. We conclude that the indirect effects of disturbance by termites may be as important as the direct effects of increased nutrients for growth of savanna trees.     

Population and individual level of masting in a fleshy-fruited tree

Masting is usually considered as a population phenomenon but it results from individuals?? reproductive patterns. Studies of individual patterns of seed production and their synchrony are essential to an understanding of the mechanisms of masting. The aim of this study was to find the relationship between population and individual levels of masting. We examined individuals?? contribution to masting, considering their endogenous cycles, interannual variability and associated weather cues, as well as inter-individual synchrony of fruit production. We studied masting of Sorbus aucuparia L., which in Europe is one of the most common trees bearing fleshy fruits and is strongly affected by a specialized seed predator. The data are 11-year measurements of fruit production of 250 individuals distributed on a 27-ha area of subalpine forest in the Western Carpathians (Poland). Population- and individual-level interannual variability of fruit production was moderate. Synchrony among individuals was relatively high for all years, but the trees were much less synchronized in heavy crop years than in years of low fruit production. Weak synchrony among trees for heavy production years suggests that the predator satiation hypothesis does not explain the observed masting behavior. Fruit production, both at individual and at population level, was highly correlated with weather conditions. However, the presence of masting cannot be fully explained by the resource-matching hypothesis either. We suggest that adverse weather conditions effectively limit fruit production, causing high inter-individual synchrony in low crop years, whereas the unsynchronized heavy crop years seem to have been affected by individually available resources.     

Direct and indirect effects of body weight on adult wages

Previous estimates of the association between body weight and wages in the literature have been conditional on education and occupation. In addition to the effect of current body weight status (body mass index (BMI) or obesity) on wages, this paper examines the indirect effect of body weight status in the late-teenage years on wages operating through education and occupation choice. Using the National Longitudinal Survey of Youth 1979 data, for women, we find that a one-unit increase in BMI is directly associated with 1.83% lower hourly wages whereas the indirect BMI wage penalty is not statistically significant. Neither a direct nor an indirect BMI wage penalty is found for men. However, results based on clinical weight classification reveal that the indirect wage penalty occurs to a larger extent at the upper tail of the BMI distribution for both men and women via the pathways of education and occupation outcomes. Late-teen obesity is indirectly associated with 3.5% lower hourly wages for both women and men. These results are important because they imply that the total effect of obesity on wages is significantly larger than has been estimated in previous cross-sectional studies.     

Direct and indirect climate change effects on photosynthesis and transpiration

Climate change affects plants in many different ways. Increasing CO(2) concentration can increase photosynthetic rates. This is especially pronounced for C(3) plants, at high temperatures and under water-limited conditions. Increasing temperature also affects photosynthesis, but plants have a considerable ability to adapt to their growth conditions and can function even at extremely high temperatures, provided adequate water is available. Temperature optima differ between species and growth conditions, and are higher in elevated atmospheric CO(2). With increasing temperature, vapour pressure deficits of the air may increase, with a concomitant increase in the transpiration rate from plant canopies. However, if stomata close in response to increasing CO(2) concentration, or if there is a reduction in the diurnal temperature range, then transpiration rates may even decrease. Soil organic matter decomposition rates are likely to be stimulated by higher temperatures, so that nutrients can be more readily mineralised and made available to plants. This is likely to increase photosynthetic carbon gain in nutrient-limited systems. All the factors listed above interact strongly so that, for different combinations of increases in temperature and CO(2) concentration, and for systems in different climatic regions and primarily affected by water or nutrient limitations, photosynthesis must be expected to respond differently to the same climatic changes.     

Direct and indirect effects of leptin on preadipocyte proliferation and differentiation

Leptin has been shown to reduce body fat in vivo. Adipocytes express the leptin receptor; therefore, it is realistic to expect a direct effect of leptin on adipocyte growth and metabolism. In vitro studies examining the effect of leptin on adipocyte metabolism require supraphysiological doses of the protein to see a decrease in lipogenesis or stimulation of lipolysis, implying an indirect action of leptin. It also is possible that leptin reduces adipose mass by inhibiting preadipocyte proliferation (increase in cell number) and/or differentiation (lipid filling). Thus we determined direct and indirect effects of leptin on preadipocyte proliferation and differentiation in vitro. We tested the effect of leptin (0-500 ng/ml), serum from leptin-infused rats (0.25% by volume), and adipose tissue-conditioned medium from leptin-infused rats (0-30% by volume) on preadipocyte proliferation and differentiation in a primary culture of cells from male Sprague-Dawley rat adipose tissue. Leptin (50 ng/ml) stimulated proliferation of preadipocytes (P<0.05), but 250 and 500 ng leptin/ml inhibited proliferation of both preadipocyte and stromal vascular cell fractions (P<0.01), as measured by [3H]thymidine incorporation. Serum from leptin-infused rats inhibited proliferation of the adipose and stromal vascular fractions (P=0.01), but adipose tissue-conditioned medium had no effect on proliferation of either cell fraction. None of the treatments changed preadipocyte differentiation as measured by sn-glycerophosphate dehydrogenase activity. These results suggest that leptin could inhibit preadipocyte proliferation by modifying release of a factor from tissue other than adipose tissue.