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36 results for “Top-down effects”
MCR LTER: Coral Reef: Priority effects in coral-macroalgae interactions can drive alternate community paths in the absence of top-down control, data for Adam 2022 Ecology
These data were generated in support of the manuscript: Adam TC, Holbrook SJ, Burkepile DE, Speare KE, Brooks AJ, Ladd MC, Shantz AA, Thurber RLV, and Schmitt RJ, Ecology The outcomes of species interactions can vary greatly in time and space with the outcomes of some interactions determined by priority effects. On coral reefs, benthic algae rapidly colonize the disturbed substrate. In the absence of top-down control from herbivorous fishes, these algae can inhibit the recruitment of reef-building corals, leading to a persistent phase shift to a macroalgae-dominated state. Yet, corals may also inhibit colonization by macroalgae, and thus the effects of herbivores on algal communities may be strongest following disturbances that reduce coral cover. Here, we report results from experiments conducted on the fore reef of Moorea, French Polynesia, where we: 1) tested the ability of macroalgae to invade coral-dominated and coral-depauperate communities under different levels of herbivory, 2) explored the ability of juvenile corals (Pocillopora spp.) to suppress macroalgae, and 3) quantified the direct and indirect effects of fish herbivores and corallivores on juvenile corals. We found that macroalgae proliferated when herbivory was low but only in recently disturbed communities where coral cover was also low. When coral cover was < 10%, macroalgae increased 20-fold within one year under reduced herbivory conditions relative to high herbivory controls. Yet, when coral cover was high (50%), macroalgae were suppressed irrespective of the level of herbivory despite ample space for algal colonization. Once established in communities with low herbivory and low coral cover, macroalgae suppressed recruitment of coral larvae, reducing the capacity for coral replenishment. However, when we experimentally established small juvenile corals (2 cm diameter) following a disturbance, juvenile corals inhibited macroalgae from invading local neighborhoods, even in the absence of herbivore
Variation in the relative effects of top-down and bottom-up forces on herbivores and herbivory along an elevational gradient in the southern Appalachian mountains in 2001
It is a well established fact that top-down (predation), bottom-up (resource availability) and lateral (interference) interactions are the dominant biotic forces in terrestrial ecosystems in addition to a host of other interactions like mutualism and symbiosis. The primary emphasis has always been to look for a singular mechanistic explanation in determining community dynamics. The ecological literature is replete with controversy on the subject of whether top-down or bottom-up forces predominate in ecosystems and their role in dynamics of ecological communities. The emerging consensus is that both top-down and bottom-up forces act in concert and the impetus is shifting towards the elucidation of the context, biotic and abiotic, under which these forces come into play. There are very few studies that have addressed this question and looked at the simultaneous interaction of these forces. I propose to study the effects of spatial heterogeneity in biotic and abiotic factors along an elevation gradient on the relative impacts of top-down and bottom-up forces and the result of their interactive effects on folivory. Specifically, I shall address the effects of spatial variation in plant quality and predation pressure and their interactive impacts on insect herbivore biomass and consumption. I shall also address the effects of complexity within/among trophic levels on the impact of these forces. The study aims to bring about a greater understanding of the role played by abiotic factors and complexity in community dynamics.
Training and test data, plus saved models for the paper "Top-down effects in an early visual cortex inspired hierarchical Variational Autoencoder" submitted to the SVRHM 2022 Workshop @ NeurIPS
<p>Each .pkl file contains a training or test dataset in the form of a Python dictionary (generated with Python 3.8.5) with the following fields:</p><ul><li>'train_images': 640,000 float32 images used for model training. 20px images contain 400 pixel intensities, 40px images contain 1600 pixel intensities each.</li><li>'train_labels': float32 labels for each image in 'train_images'. All natural images are labeled with 0.0. Texture images are labeled with 0.0, 1,0, 2.0, 3.0, or 4.0, according to their texture family.</li><li>'test_images': 64,000 float32 images used for model testing. 20px images contain 400 pixel intensities, 40px images contain 1600 pixel intensities each.</li><li>'test_labels': float32 labels for each image in 'test_images'. All natural images are labeled with 0.0. Texture images are labeled with 0.0, 1,0, 2.0, 3.0, or 4.0, according to their texture family.</li></ul><p>Each .zip file contains a saved model. Details on these are coming soon.</p><p>For more details, see the paper "Top-down effects in an early visual cortex inspired hierarchical Variational Autoencoder" published at the SVRHM 2022 Workshop @ NeurIPS (<a href="https://openreview.net/forum?id=8dfboOQfYt3">link</a>).</p>
Parasites as ecosystem modulators: Foliar pathogens suppress top-down effects of large herbivores
<p><span>Parasites can catalyze or inhibit interactions between their hosts and other species, but the ecosystem-level effects of such interaction modifications are poorly understood. We conducted a large-scale field experiment in temperate grasslands of northeastern China to understand how foliar fungal pathogens (e.g., rusts and leaf</span><span> spot</span><span>s</span><span>) influenced top-down effects of large herbivores (cattle; <em>Bos</em> <em>taurus</em>) on plant diversity and productivity. In the absence of foliar pathogens, cattle grazing strongly suppressed biomass of the dominant grass, <em>Leymus</em> <em>chinensis</em>, (↓56%), generating competitive release that significantly increased plant species richness (↑75%) and evenness (↑15%). In the absence of grazing, pathogen attack on <em>L. chinensis</em> had no measurable effect on host biomass. However, pathogen infection greatly weakened top-down effects of herbivory by inhibiting cattle grazing on <em>L. chinensis</em> and negating grazing effects on plant biomass and species richness. Mechanistically, foliar pathogens were linked to increased alkaloid (↑45%) and reduced nitrogen levels (↓19%) in leaf tissue that appeared to deter cattle grazing on<em> L. chinensis</em>. Our results demonstrate that foliar pathogens can suppress top-down effects of large herbivores on grassland community structure and function by modifying the strength of its host's interactions with the dominant consumer. We propose that parasites may function primarily as modulators of ecosystem function when their direct effects on host density (often dampened by evolution) are overshadowed by influences on host traits that modify host interactions with strong competitors, herbivores, or predators.</span></p>
Warming alters the top-down effect of a common mesopredator in an aquatic food web
<p class="MsoNormal"><span>Global warming is altering the composition and functioning of both terrestrial and aquatic ecosystems. Most research has focussed on direct effects of warming on species and their population dynamics, while less is known about indirect effects through species interactions. Trophic cascades are especially likely to influence the effects of warming given that trophic levels differ in their response to warming because of differences in metabolism and vital rates such as growth and reproduction. We investigated if the top-down effect of a common mesopredator, the threespine stickleback, alters the impact of warming on a Baltic Sea food web. We manipulated both temperature and the presence of stickleback in mesocosms, and found the stickleback to alter the impact of warming on lower trophic levels. This was through increased consumption of herbivores that exceeded the rate at which the herbivore population could grow under higher temperature, which in turn increased algae biomass compared to a scenario without stickleback, but not compared to conditions under normal temperature. Thus, the mesopredator increased the transfer of biomass to higher trophic levels of the food web when temperature was increased. This stresses the importance of considering the impact of warming on multiple trophic levels and their interactions. Failing to consider differences in responses among trophic levels and the impact this has on their interactions can result in faulty conclusions about the impact of warming on ecosystems.</span></p>
Data from: Numerical top-down effects on red deer (Cervus elaphus) are mainly shaped by humans rather than large carnivores across Europe
<p>Terrestrial ecosystems are shaped by interacting top-down and bottom-up processes, with the magnitude of top-down control by large carnivores largely depending on environmental productivity. While carnivore-induced numerical effects on ungulate prey populations have been demonstrated in large, relatively undisturbed ecosystems, whether large carnivores can play a similar role in more human-dominated systems is a clear knowledge gap. As humans influence both predator and prey in a variety of ways, the ecological impacts of large carnivores can be largely modified. We quantified the interactive effects of human activities and large carnivore presence on red deer (<em>Cervus elaphus</em>) population density and how their impacts interacted and varied with environmental productivity</p> <p>Data on red deer density were collected based on a literature survey encompassing 492 study sites across 28 European countries. Variation in density across study sites was analysed using a generalised additive model in which productivity, carnivore presence (grey wolf, European lynx, Brown bear), human activities (hunting, intensity of human land-use activity), site protection status and climatic variables served as predictors. </p> <p>The results showed that a reduction in deer density only occurred when wolf, lynx and bear co-occurred within the same site. In the absence of large carnivores, red deer density varied along a productivity gradient without a clear pattern. Although a linear relationship with productivity in the presence of all three large carnivore species was found, this was not statistically significant. Moreover, hunting by humans had a stronger effect than the presence of all large carnivores in reducing red deer density and red deer density increased with increasing intensity of human land-use, with stronger large carnivore effects (all three carnivore species present) at sites with low human land-use activities. </p> <p><em>Synthesis and applications</em>: This study provides evidence for the dominant role played by humans (i.e. hunting, land-use activities) relative to large carnivores in reducing red deer density across European human-dominated landscapes. These findings suggest that when we would like large carnivores to exert numeric effects, we should focus on minimizing human impacts to allow the ecological impacts of large carnivores on ecosystem functioning.</p>
Data from: Numerical top-down effects on red deer (Cervus elaphus) are mainly shaped by humans rather than large carnivores across Europe
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Warming alters the top-down effect of a common mesopredator in an aquatic food web
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Parasites as ecosystem modulators: Foliar pathogens suppress top-down effects of large herbivores
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Data from: Synchrony is more than its top-down and climatic parts: interacting Moran effects on phytoplankton in British seas
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Bottom-up and top-down drivers influence urbanization effects on insect herbivory in oaks
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Date from: Top-down effects from parasitoids may mediate plant defense and plant fitness
<ol> <li>Plants face many environmental stresses that can impact the<span>ir </span><span><span>survival</span></span><span>, developm</span>ent, and fitness. Insects are the most diverse, abundant and threatening herbivores in nature. As a consequence, plants produce direct chemical and physical defenses to reduce herbivory. They also release volatiles to recruit natural enemies that indirectly protect them from herbivory. The recruitment of parasitic wasps can benefit plant fitness because they ultimately kill their insect hosts.</li> <li>Recently, studies showed that parasitoids can indirectly mediate plant defenses by modulating herbivore oral secretions. In addition to the direct benefits of parasitoids in terms of reducing herbivore survival, we tested if the reduction in induced defenses by parasitized caterpillars compared to non-parasitized caterpillars may reduce the costs associated with defense expression.</li> <li>We provide evidence that tomato plants treated with saliva from parasitized caterpillars have significantly higher fitness parameters including increased flower numbers (16.3%) and heavier fruit weight (15%), compared to plants treated with saliva from non-parasitized caterpillars. Since plants were grown without actual herbivores, the higher values for these fitness parameters were due to lower costs of induced defenses and not due to reduced herbivory by parasitized caterpillars. Furthermore, the resulting seed germination time was shorter and the germination rate was higher when the maternal plants were previously exposed to parasitized herbivore treatment compared to control (non-treated) plants.</li> <li>Overall, application of saliva did not result in transgenerational priming of offspring defense responses. However, offspring of parents exposed to caterpillar saliva had lower constitutive levels and higher induced levels of trypsin inhibitor than offspring from unexposed parents.</li> <li>This study shows that the saliva of parasitized caterpillars can modulate plant defenses and further demonstrates that the lower induction of plant defenses is associated with elevated plant fitnes<span>s </span><span><span>in the absence of herbivore feeding</span></span><span>, sugge</span>sting that induced plant defenses are costly. </li> </ol>
Data from: Bottom-up effects of host-plant species diversity and top-down effects of ants interactively increase plant performance
While plant diversity is well known to increase primary productivity, whether these bottom-up effects are enhanced by reciprocal top-down effects from the third trophic level is unknown. We studied whether pine tree species diversity, aphid-tending ants and their interaction determined plant performance and arthropod community structure. Plant diversity had a positive effect on aphids, but only in the presence of mutualistic ants, leading to threefold greater number of both groups in the tri-specific cultures than in monocultures. Plant diversity increased ant abundance not only by increasing aphid number, but also by increasing ant recruitment per aphid. The positive effect of diversity on ants in turn cascaded down to increase plant performance; diversity increased plant growth (but not biomass), and this effect was stronger in the presence of ants. Consequently, bottom-up effects of diversity within the same genus and guild of plants and top-down effects from the third trophic level (predatory ants) interactively increased plant performance.
Top-down effects override climate forcing on reproductive success in a declining sea duck
Population performance is predicted to be more strongly influenced by detrimental species interactions such as predation under benign climatic conditions, and by climate forcing under harsh conditions, reflected in geographical gradients in biotic interaction strength. Less appreciated is the potential for site-specific changes in drivers with the advent of anthropogenic alteration of predator-prey relationships, including apex predator restoration and spread of invasive predators. Particularly interesting is the relative impact of climate and biotic interactions on population performance when these conflict. In this 31-year study (1990-2020), we revisit a common eider (Somateria mollissima) population from SW Finland, Baltic Sea, fifteen years on from an earlier study showing that climate warming positively affected reproductive parameters and performance. However, the population is simultaneously exposed to increasing predation by the rapidly recovering native apex predator and invasive mammals. Based on the current population trend, we predicted (i) a weakening of the previously documented positive effects of a warming climate on vital rates, (ii) intensified predation, and (iii) increasing top-down control of vital rates and accompanying population decline. Five out of seven breeding parameters (annual spread in female body condition, breeding phenology and synchrony, interval between arrival and breeding, fledgling production) were best explained by predation indices, whereas climate signals (winter NAO, Baltic Sea maximum ice cover) on breeding parameters have weakened. Particularly intriguing is that the previous positive association between mild ice winters and subsequent reproductive output has disappeared during the past 15 years, highlighting the non-linear nature of climate change responses. Indirect predation effects (selective disappearance, changed reproductive strategies, nest-site selection and population age distribution) can potentially explain also the remaining breeding parameters (annual mean body condition and clutch size). The observed regime shift in predation risk appears to prevent this now endangered population from reaping the potential benefits of a warming climate.
Top-down cascading effects of seed-feeding beetles and their parasitoids on plants and leaf herbivores
<p><span>When feeding on a plant, herbivorous insects alter the quality of the plant as a food source. This affects other organisms interacting with the same plant. These so-called 'plant-mediated interactions' can be altered by parasitoids that attack the herbivores. So far, this research area has mainly focused on interactions at the leaf level, and very little is known about plant-mediated interactions via seeds. </span><span>It is still poorly understood if seeds that survive insect damage have fewer resources to allocate to plant growth and defence against leaf herbivores, and whether parasitoids that kill seed-feeding insects mitigate such negative effects.</span></p> <p><span>Using seeds of wild lima bean plants (<em>Phaseolus lunatus</em>) we studied the effect of the intensity of infestation by seed beetles (<em>Zabrotes subfasciatus</em>) and their parasitoids (<em>Stenocorse bruchivora</em>) on the following parameters under lab conditions: seed mass and germination, plant growth and defensive compounds (cyanogenic glycosides and flavonoids) and performance of a leaf herbivore species (<em>Spodoptera latifascia</em>). In addition, we performed a field experiment using seeds with or without insect damage to investigate the consequences on plant performance and fitness in the wild.</span></p> <p><span>Seed beetle infestation had an overall negative impact on seed germination. Lab experiments revealed that damaged seeds produced plants with slower growth and reduced concentration of defensive compounds, which increased the performance of the leaf herbivores. Effects of seed-feeding on seed germination and plant growth were attenuated by parasitism, resulting in a net increase of the number of viable offspring. In the field, we observed that seed damage impaired germination, delayed flowering time and increased leaf herbivory.</span></p> <p><span>Our results show that plant-mediated interactions between insect herbivores are not limited to leaf herbivores but extend to seed herbivores. In our study system, parasitoids had no apparent effect on these interactions, despite their strong beneficial effects on germination and plant performance. These findings confirm the long-lasting consequences of indirect plant-mediated interactions in a community-wide ecological context. Furthermore, they contribute to a better understanding of the important but understudied effects of parasitoids on plant fitness.</span></p>
Data from: Keystone mutualism strengthens top-down effects by recruiting large-bodied ants
Determining the impacts of mutualistic interactions and predator diversity on food webs are two important goals in community ecology. In this study, we examined how predator community variation mediates the strength of top–down effects in the presence and absence of mutualistic interactions. We examined the impacts of predatory ant species that simultaneously prey on leaf-chewing herbivores (Lepidoptera) and engage in food-for-protection mutualisms with sap-feeding herbivores (Hemiptera) in the lower canopy of Connecticut forests. In this 2-year study, we examined three hypothetical mechanisms by which mutualisms can alter the top–down effects of ants: (1) sap feeders increase ant abundance, thus strengthening predatory effects; (2) sap feeders increase the relative abundance of a species that has stronger predatory effects; and (3) changes to predator diversity (species richness) are caused by sap feeders mediating top–down effects of the ant community. Experiments revealed that host plants occupied by sap feeders favored large-bodied ant species in the genus Camponotus, but there were no changes to community-wide ant abundance or ant species richness. Fitting predictions of predation strength based on the functional trait of body size, large-bodied Camponotus suppressed caterpillars and reduced leaf herbivory. This work shows that the ant–hemipteran mutualism, which has been characterized as a keystone interaction, can generate strong top–down effects on leaf-chewing herbivores and herbivory via increasing the relative abundance of species with functional traits relevant to predation, such as body size. Therefore, the emergence of specific ants as keystone predators in a community can be contingent upon their mutualism with sap-feeding Hemiptera.
Top-down effects on biological soil crust function
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Top-down effects override climate forcing on reproductive success in a declining sea duck
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Top-down cascading effects of seed-feeding beetles and their parasitoids on plants and leaf herbivores
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Data from: Bottom-up effects of host-plant species diversity and top-down effects of ants interactively increase plant performance
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