Bernal V., Espadaler X. (2013) Invasive and socially parasitic ants are good bioindicators of habitat quality in Mediterranean forest remnants in northeast Spain. Ecological Research. 28: 1011-1017.LinkDoi: 10.1007/s11284-013-1083-4
We surveyed ants in 16 forest remnants in the Vallès lowlands north of Barcelona, Spain: eight with invasive exotic ants (IE = Linepithema humile and/or Lasius neglectus) present, seven with native parasitic ants (P = Lasius meridionalis, Lasius carniolicus, Plagiolepis xene, Chalepoxenus muellerianus, and/or Polyergus rufescens) present, and one remnant with both invasive exotic and parasitic ants present. Forest remnants with IE ants were smaller, more isolated, had greater perimeter/area, lower ant species density, and lower ant species richness than remnants with P ants. The community composition was also significantly different, with greater dissimilarity within remnants with alien, invasive species. The presence of some species is bio-indicator of low disturbance, whereas others are indicative of high disturbance. Our findings underscore the value of different types of ants as bio-indicators of fragmentation and habitat quality. © 2013 The Ecological Society of Japan.
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.LinkDoi: 10.1016/j.actao.2013.01.001
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.
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.LinkDoi: 10.1007/s00442-013-2594-y
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.
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.LinkDoi: 10.1016/j.baae.2013.05.001
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.
Trojelsgaard K., Baez M., Espadaler X., Nogales M., Oromi P., Roche F.L., Olesen J.M. (2013) Island biogeography of mutualistic interaction networks. Journal of Biogeography. 40: 2020-2031.LinkDoi: 10.1111/jbi.12165
Aim: The seminal theory of island biogeography, based on changing rates of immigration and extinction, should be seen in a geological context, as an island's maturity influences the richness of its biota. Here, we develop an island biogeography of biotic interactions, recognizing that, besides species richness, biodiversity also encompasses the multitude of interactions among species. By sampling interactions between plants and pollinators across the Canarian archipelago, we illustrate how the local richness, specialization and endemism of biotic interactions vary with island age and area. Location: Canary Islands (27.62° N-29.42° N and 13.33° W-18.17° W). Methods: On five islands, covering the full age range of the archipelago, plant-pollinator interactions were catalogued and their strength estimated. Network parameters (e.g. interaction richness and specialization) and the number of single-island interactions (equivalent to single-island endemics) were estimated from interaction matrices and related to island area and age. Results: Plant species richness, interaction richness and average degree of specialization of pollinator species showed hump-shaped relationships with island age. Pollinator richness varied with island area and plant richness. Plant specialization increased with island age, and the proportion of single-island interactions (pSII) exhibited a U-shaped relationship with age. Main conclusions: The previously reported hump-shaped relationship between species richness and island age, both on the scale of islands and of habitats, was confirmed for plant species in local networks. Both plants and pollinators were more generalized on the youngest island, which may be due to a predominance of generalist colonists. Pollinator specialization peaked on mid-aged islands, whereas plants showed the highest specialization on old islands, potentially reflecting their different life histories. The U-shaped relationship between the proportion of single-island interactions and island age might be explained by (1) young islands having a high proportion of unique interactions, due to interactions between generalists, and (2) old islands having unique interactions due to an accumulation of unique pairwise interactions that have evolved through time. Thus, island age - which not only captures time per se, but also the geomorphological changes of islands - may act as a regional driver of local network structure, and so the contemporary networks we observed across the Canarian archipelago illustrate the development of a network through geological time. © 2013 John Wiley & Sons Ltd.
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