Differential ant exclusion from canopies shows contrasting top-down effects on community structure

Mestre L., Piñol J., Barrientos J.A., Espadaler X. (2016) Differential ant exclusion from canopies shows contrasting top-down effects on community structure. Oecologia. 180: 193-203.
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Doi: 10.1007/s00442-015-3442-z

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Predators have far-reaching effects on communities by triggering top-down trophic cascades that influence ecosystem functioning. Omnivory and intraguild interactions between predators give rise to reticulate food webs and may either strengthen or dampen trophic cascades depending on context. Disentangling the effects of multiple predator species is therefore crucial for predicting the influence of predators on community structure. We focused on ants as dominant generalist predators in arthropod communities and set up a differential ant exclusion from canopies to examine its effects on assemblage species composition and densities of five arthropod groups (psocopterans, aphids, spiders, heteropterans and beetles). We coupled a glue band with tubes allowing only the ant Lasius grandis to reach the canopies to isolate its effect from the rest of crawling predators (ants, earwigs) and compared it against a full exclusion and a control. L. grandis alone had widespread effects on assemblage species composition, with contrasting species-specific responses within groups, where some species affected by L. grandis presence were not further affected by the presence of the whole crawling predator assemblage, and vice versa. Overall, L. grandis caused two- to threefold decreases of generalist predators and a threefold increase of aphids. However, it lacked further top-down effects on primary consumers, which only emerged when all crawling predators were present. This differential exclusion demonstrates the distinctive and widespread intraguild effects on community structure of a single ant species that contrast with the top-down effects exerted by the whole crawling predator assemblage. © 2015, Springer-Verlag Berlin Heidelberg.

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Management implications of earwigs' overwintering sites in a Mediterranean citrus grove

Romeu-Dalmau C., Espadaler X., Piñol J. (2016) Management implications of earwigs' overwintering sites in a Mediterranean citrus grove. International Journal of Pest Management. : 1-6.
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Doi: 10.1080/09670874.2015.1129079

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To promote earwigs as natural enemies of pests, or to control their populations if they damage crops, earwigs can be managed during their overwintering period on the ground. Here, we obtained more than a ton of soil to study earwigs' overwintering sites in a citrus grove. We found four species of earwigs: Forficula pubescens, Euborellia annulipes, Euborellia moesta, and Nala lividipes. Surprisingly, and although the European earwig Forficula auricularia is abundant in the citrus canopies the rest of the year, we did not find any F. auricularia, indicating that this species spends the winter outside the citrus grove. Therefore, farmers willing to manage European earwig populations in citrus orchards need to consider the possibility that earwigs may spend the winter outside the field. Earwigs that were overwintering in the citrus grove were more abundant at the south side beneath the canopies than at the north side or between rows, indicating that management practices such as soil tillage can impact overwintering earwigs only beneath the canopies, but not between citrus rows. Overall, our results provide insights into how earwig populations can be successfully managed during winter in citrus orchards. © 2016 Taylor & Francis

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Evaluating the importance of trophobiosis in a Mediterranean ant community: a stable isotope analysis

Brewitt K., Pinol J., Werner C., Beyschlag W., Espadaler X., Perez Hidalgo N., Platner C. (2014) Evaluating the importance of trophobiosis in a Mediterranean ant community: a stable isotope analysis. Insectes Sociaux. : 0-0.
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Doi: 10.1007/s00040-014-0375-1

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Trophobiosis between aphids (Aphididae, Hemiptera) and ants (Formicidae, Hymenoptera) is considered to provide an important source of nutrition for ants by aphid honeydew and aphids themselves used as prey. However, little is known about nutrient fluxes and the relative importance of trophobiosis for different ant species. Combining direct contact observations between ants and aphids with stable isotope analyses of distinct multitrophic sample sets (soil, plant, aphid, and ant), we aimed at disentangling the importance of trophobiosis in a Mediterranean food web and possible feedbacks on the functional diversity of ants in a species-rich organic Citrus plantation. We analyzed δ13C- and δ15N-values of sample sets for fertilized and natural soil, using the fertilizer as natural isotope label. The results showed trophic relationships between 18 host plant species, 22 aphid species, and 7 ant species. Direct observation revealed at least 40 different plant–aphid combinations and 25 aphid–ant combinations with a marked range of δ15N-values. However, the δ13C and δ15N isotope ratios still reflected the trophic levels. A significant correlation occurred between the isotope ratios of aphids and their host plants. However, no relationship was found between aphids and ants or between plants and ants revealing that many ant species do not exhibit a close relationship with their trophobiotic partners. Isotopic data allowed us to separate ant species into trophic functional groups and showed the relevance of other food resources. The applied fertilizer shifted the isotopic baseline for the whole trophic system. By combining the stable isotope analysis with the exact origin of the samples, we avoided a misleading interpretation of the high isotopic range of species. Thus, we emphasize the importance of considering a baseline in stable isotope food web studies.

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Bird predation affects diurnal and nocturnal web-building spiders in a Mediterranean citrus grove

Mestre L., Garcia N., Barrientos J.A., Espadaler X., Piñol J. (2013) Bird predation affects diurnal and nocturnal web-building spiders in a Mediterranean citrus grove. Acta Oecologica. 47: 74-80.
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Doi: 10.1016/j.actao.2013.01.001

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Spiders and birds can greatly decrease insect populations, but birds also limit spider densities in some habitats. Bird predation is thought to be one of the causes behind nocturnal activity in spiders, so night-active spiders that hide in retreats during the day should be less affected by bird foraging than day-active spiders. However, this hypothesis has not yet been tested. We investigated the importance of bird predation on the spider community of a Mediterranean organic citrus grove. We excluded birds by placing net cages over the trees and we conducted visual searches in the canopies to sample web-building spiders. As there are many nocturnal species in the family Araneidae, we conducted searches both by day and by night to compare the abundance of active araneids in these two time periods. We sampled the tree trunks with cardboard bands to collect hunting spiders. In bird-excluded canopies there were more spiders of the families Araneidae and Theridiidae. There were higher numbers of active Araneidae at night, but these were just as negatively affected by bird predation as day-active Araneidae, so there was no evidence of nocturnal activity serving as an anti-predator strategy. We did not find any negative impact of birds on hunting spiders. Our results contrast with other studies reporting a negative effect of birds on hunting but not on web-building spiders. © 2013.

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Ant exclusion in citrus over an 8-year period reveals a pervasive yet changing effect of ants on a Mediterranean spider assemblage

Mestre L., Pinol J., Barrientos J.A., Espadaler X. (2013) Ant exclusion in citrus over an 8-year period reveals a pervasive yet changing effect of ants on a Mediterranean spider assemblage. Oecologia. 173: 239-248.
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Doi: 10.1007/s00442-013-2594-y

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Ants and spiders are ubiquitous generalist predators that exert top-down control on herbivore populations. Research shows that intraguild interactions between ants and spiders can negatively affect spider populations, but there is a lack of long-term research documenting the strength of such interactions and the potentially different effects of ants on the diverse array of species in a spider assemblage. Similarly, the suitability of family-level surrogates for finding patterns revealed by species-level data (taxonomic sufficiency) has almost never been tested in spider assemblages. We present a long-term study in which we tested the impact of ants on the spider assemblage of a Mediterranean citrus grove by performing sequential 1-year experimental exclusions on tree canopies for 8 years. We found that ants had a widespread influence on the spider assemblage, although the effect was only evident in the last 5 years of the study. During those years, ants negatively affected many spiders, and effects were especially strong for sedentary spiders. Analyses at the family level also detected assemblage differences between treatments, but they concealed the different responses to ant exclusion shown by some related spider species. Our findings show that the effects of experimental manipulations in ecology can vary greatly over time and highlight the need for long-term studies to document species interactions. © 2013 Springer-Verlag Berlin Heidelberg.

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Trophic structure of the spider community of a Mediterranean citrus grove: A stable isotope analysis

Mestre L., Pinol J., Barrientos J.A., Espadaler X., Brewitt K., Werner C., Platner C. (2013) Trophic structure of the spider community of a Mediterranean citrus grove: A stable isotope analysis. Basic and Applied Ecology. 14: 413-422.
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Doi: 10.1016/j.baae.2013.05.001

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Spiders are dominant terrestrial predators that consume a large variety of prey and engage in intraguild predation. Although the feeding habits of certain species are well known, the trophic structure of spider assemblages still needs to be investigated. Stable isotope analysis enables characterisation of trophic relationships between organisms because it tracks the energy flow in food webs and indicates the average number of trophic transfers between a given species and the base of the web, thus being a useful tool to estimate the magnitude of intraguild predation in food webs. Using this technique, we studied the trophic groups of spiders and their links within the arthropod food web of a Mediterranean organic citrus grove. We assessed the trophic positions of the 25 most common spider species relative to other arthropod predators and potential prey in the four seasons of the year, both in the canopy and on the ground. The analyses showed great seasonal variation in the isotopic signatures of some arthropod species, as well as the existence of various trophic groups and a wide range of trophic levels among spiders, even in species belonging to the same family. Differences in δ15N between spiders and the most abundant prey in the grove usually spanned two trophic levels or more. Our findings provide field evidence of widespread intraguild predation in the food web and caution against using spider families or guilds instead of individual species when studying spider trophic interactions. © 2013 Gesellschaft für Ökologie.

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Effects of ant competition and bird predation on the spider assemblage of a citrus grove

Mestre L., Piñol J., Barrientos J.A., Cama A., Espadaler X. (2012) Effects of ant competition and bird predation on the spider assemblage of a citrus grove. Basic and Applied Ecology. 13: 355-362.
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Doi: 10.1016/j.baae.2012.04.002

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Characterizing intraguild interactions is key to improving understanding of food webs because they are major forces in the structuring of communities. Spiders are generalist predators with intermediate positions in the food web that establish intraguild interactions with ants and birds, which respectively compete with and prey on them. Research has also found interactions between birds and ants, potentially resulting in non-additive effects of both groups on arthropod assemblages, although studies of their combined impacts with tests for multiple-predator effects are scarce. We thus aimed to discern the relative effect of ants and birds on the spider assemblage of a citrus grove. We used a split-plot design to factorially exclude these groups over 2. years, preventing ants reaching the canopies by placing sticky bands around tree trunks, and birds by enclosing groups of trees in cages. We sampled spiders from the canopies (beating) and the ground (pitfalls) every 3. months, and we identified them to species. We found a strong influence of ants on the canopy spider assemblage, mainly through a negative effect on the families Araneidae and Theridiidae. Since spiders' weights from ant-excluded and control trees were similar, these results suggest interference competition of ants on spiders rather than competitive exploitation. Bird exclusion did not affect the spider assemblage, contrasting with other studies reporting a marked predatory pressure of birds on spiders; nor were there any non-additive effects of ants and birds. Our findings show that spider assemblages are not uniformly affected by intraguild competitors. © 2012 Gesellschaft für ökologie.

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Eight years of ant-exclusion from citrus canopies: Effects on the arthropod assemblage and on fruit yield

Piñol J., Espadaler X., Cañellas N. (2012) Eight years of ant-exclusion from citrus canopies: Effects on the arthropod assemblage and on fruit yield. Agricultural and Forest Entomology. 14: 49-57.
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Doi: 10.1111/j.1461-9563.2011.00542.x

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1 Ants are important generalist predators in most terrestrial ecosystems. However, because many ant species are also hemipteran mutualists, their role in agriculture has generally been considered to be negative for plants. 2 In the present study, we report an experiment in ant-exclusion from tree canopies in an organic citrus grove with two main objectives: (i) to examine whether the absence of ants increased the abundance of some beneficial arthropods and reduced the attack of some pests such as aphids and (ii) to examine whether ant-exclusion increased the fruit yield of citrus trees. 3 The exclusion of ants from tree canopies had positive effects on the arthropod assemblage and on fruit yield. However, the 8-year duration of the experiment can be divided into two periods with contrasting results. In the first period, the arthropod assemblage was only slightly affected, except for a greater density of aphids in ant-excluded trees; in addition, fruit yield was higher in ant-excluded trees than in the control ones. In the second period, ant-exclusion increased the abundance of most arthropod groups, although the previous positive effect on fruit yield was no longer observed. 4 There are two main conclusions of the present study. First, from an applied perspective, ant-exclusion from tree canopies is not a sound management alternative in citrus plantations in the Mediterranean. Second, the 8-year duration of the experiment highlighted the importance of long-term experiments in community ecology and biological control because the effects observed in the first 4 years of the experiment were very different from what occurred subsequently. © 2011 The Authors. Agricultural and Forest Entomology © 2011 The Royal Entomological Society.

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Long-term changes and ant-exclusion effects on the true bugs (Hemiptera: Heteroptera) of an organic citrus grove

Piñol J., Ribes E., Ribes J., Espadaler X. (2012) Long-term changes and ant-exclusion effects on the true bugs (Hemiptera: Heteroptera) of an organic citrus grove. Agriculture, Ecosystems and Environment. 158: 127-131.
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Doi: 10.1016/j.agee.2012.06.004

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The Heteroptera assemblage of a citrus grove and how it was affected by ant-exclusion was examined during transformation from conventional to organic agriculture. The results showed that the Heteroptera assemblage changed dramatically over the eight years of the study: at first, it mainly consisted of herbivorous lygaeids and predatory anthocorids but became dominated by predatory mirids in 2008-2009. The predator/herbivore ratio increased steadily over the eight years of the study. Ants can form mutualistic relationships with heteropteran pests. However, exclusion of ants from canopies did not affect the Heteroptera assemblage at the beginning of the study, but had a profound effect later on. In particular, ant-exclusion increased the abundance of most predatory Heteroptera, except for the myrmecomorphic mirid Pilophorus perplexus, which was approximately five times more abundant in control than in ant-excluded trees; the analyses showed that the only mimicked ant species was Lasius grandis. © 2012 Elsevier B.V.

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Trophic diversity in a Mediterranean food web-Stable isotope analysis of an ant community of an organic citrus grove

Platner C., Piñol J., Sanders D., Espadaler X. (2012) Trophic diversity in a Mediterranean food web-Stable isotope analysis of an ant community of an organic citrus grove. Basic and Applied Ecology. 13: 587-596.
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Doi: 10.1016/j.baae.2012.09.006

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Ants as generalist predators and mutualists of herbivores can play an important role in relative stable agroecosystems like plantations. The categorization of the diverse life strategies and traits into ecological groups like trophic levels is essential for a better understanding of food web structures and a better prediction of changes in communities. Stable isotope technology provides simultaneously detection of trophic levels and the ultimate C source of many species.We studied a highly diverse Mediterranean ant community in an organic citrus grove in Tarragona, NE Spain, and analyzed stable isotope contents of 17 species of ants together with dominating plants and important spider and aphid species to establish trophic guilds and detect seasonal changes. The results revealed significant differences between species spanning over a huge range in δ15N-values of at least 10.67‰ which is only comparable to a Peruvian tropical forest with a much higher species diversity. The trophic levels of ants reflected most of previous knowledge on predaceous vs. plant feeding habits. Messor harvester ants and Camponotus species had the lowest δ15N-values. Aphids, smaller spider species, and most other ant genera, including the dominating species Formica rufibarbis and Lasius grandis, had intermediate δ15N-levels. The large spider Dysdera crocata and the typical Mediterranean ant Pheidole pallidula had higher δ15N-values, but two specialized predatory ants with very tiny workers had the highest trophic level. We found unexpectedly high δ13C-values with a high seasonality for several ground-living ant species. The possible role of soil fauna as a second main food resource besides the most commonly analyzed green food chain is discussed. Our results support the hypothesis that the strong seasonality intrinsic to Mediterranean climate and the high heterogeneity of different plant resources and microclimatic conditions in the organically managed plantation are reflected by a notably high trophic diversity of the ant community. © 2012 Gesellschaft für Ökologie.

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