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Effects of Habitat Fragmentation on Abundance, Larval Food and Parasitism of a Spider-Hunting Wasp

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Effects of Habitat Fragmentation on Abundance, Larval Food and Parasitism of a Spider-Hunting Wasp Effects of Habitat Fragmentation on Abundance, Larval Food and Parasitism of a Spider-Hunting Wasp Vale´rie Coudrain1,2,3*, Felix Herzog1, Martin H. Entling3 1 Research Station Agroscope Reckenholz-Ta¨nikon ART, Zu¨rich, Switzerland, 2 Institute of Ecology and Evolution, University of Bern, Bern, Switzerland, 3 Institute for Environmental Sciences, University of Koblenz-Landau, Landau/Pfalz, Germany Abstract Habitat fragmentation strongly affects species distribution and abundance. However, mechanisms underlying fragmentation effects often remain unresolved. Potential mechanisms are (1) reduced dispersal of a species or (2) altered species interactions in fragmented landscapes. We studied if abundance of the spider-hunting and cavity-nesting wasp Trypoxylon figulus Linnaeus (Hymenoptera: Crabronidae) is affected by fragmentation, and then tested for any effect of larval food (bottom up regulation) and parasitism (top down regulation). Trap nests of T. figulus were studied in 30 agricultural landscapes of the Swiss Plateau. The sites varied in the level of isolation from forest (adjacent, in the open landscape but connected, isolated) and in the amount of woody habitat (from 4 % to 74 %). We recorded wasp abundance (number of occupied reed tubes), determined parasitism of brood cells and analysed the diversity and abundance of spiders that were deposited as larval food. Abundances of T. figulus were negatively related to forest cover in the landscape. In addition, T. figulus abundances were highest at forest edges, reduced by 33.1% in connected sites and by 79.4% in isolated sites. The mean number of spiders per brood cell was lowest in isolated sites. Nevertheless, structural equation modelling revealed that this did not directly determine wasp abundance. Parasitism was neither related to the amount of woody habitat nor to isolation and did not change with host density. Therefore, our study showed that the abundance of T. figulus cannot be fully explained by the studied trophic interactions. Further factors, such as dispersal and habitat preference, seem to play a role in the population dynamics of this widespread secondary carnivore in agricultural landscapes. Citation: Coudrain V, Herzog F, Entling MH (2013) Effects of Habitat Fragmentation on Abundance, Larval Food and Parasitism of a Spider-Hunting Wasp. PLoS ONE 8(3): e59286. doi:10.1371/journal.pone.0059286 Editor: Diego Fontaneto, Consiglio Nazionale delle Ricerche (CNR), Italy Received September 19, 2012; Accepted February 13, 2013; Published March 14, 2013 Copyright: ß 2013 Coudrain et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was supported by the Swiss National Science Foundation under grant number 3100A0-114058. http://www.snf.ch. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: [email protected] Introduction Quantifying food availability to a species is difficult owing to factors, such as fluctuating spatial and temporal prey abundance Habitat fragmentation can strongly affect species distribution and the ability of the species to access to the resources. and abundance with consequences for ecosystem functioning and Studying the body condition of a species in different landscapes related ecosystem services [1], [2], [3], [4]. Fragmentation studies can indicate if fragmentation effects are due to altered food need to distinguish between (i) habitat loss, which is the reduction availability [22], [25]. More direct evidence for fragmentation in total amount of habitat within the landscape, and (ii) effects on food availability can be revealed by studying the actual fragmentation per se, which is the breakdown of habitat into diet of a species. Here, trap-nesting Trypoxylon wasps offer an ideal smaller patches that become isolated from one another [5]. These study system because prey items deposited as larval food can be two processes can have different effects on the abundance and collected and determined to species level [26], [27], [28], [29]. diversity of organisms [6], [7], [8], [9], [10]. Whereas this issue is Moreover, trap-nests allow a reliable assessment of parasitism increasingly being recognised, the mechanisms underlying species rate of brood cells. response to fragmentation often remain unresolved. This infor- Habitat loss and fragmentation can destabilize host-parasitoid mation is, however, essential to maintain or restore effective relationships [30], [31]. According to the trophic levels hypothesis, landscape connectivity [11], [12], [13], [14]. The effects of higher trophic levels, such as parasitoids, should suffer more from fragmentation can result from (1) altered migration of a species in fragmentation than their hosts [32], [33]. As a consequence, hosts fragmented habitats, caused e.g. by limited dispersal ability of the may be released from their parasitoids in isolated habitats [26], species [15], [16], or by the hostility of the surrounding matrix and [30], [34]. However, this hypothesis has been challenged [35], reduced availability of shelter and nesting opportunities [17], [18], [36] and the number of studies investigating parasitism in relation [19]. Alternatively, it can result from (2) altered species interac- to habitat loss and isolation is still limited [37]. tions in fragmented habitats, such as reduced food availability We studied the impact of fragmentation on the mud-dauber [20], [21], [22], or increased parasitism [23], [24]. wasp Trypoxylon figulus Linnaeus (Hymenoptera: Crabronidae). The rarity of studies that distinguish between different This wasp nests in cavities in wood or other plant material and mechanisms behind fragmentation effects may be partly due to readily colonizes artificial trap-nests. It hunts spiders as larval food the difficulty to quantify trophic interactions in the field. [38], [39], [40]. Diet and brood parasitism of T. figulus were PLOS ONE | www.plosone.org 1 March 2013 | Volume 8 | Issue 3 | e59286 Fragmentation Effects on Trophic Interactions studied in 30 trap-nest locations that varied (a) in their isolation entrance. The tubes were closed again so that the wasp larvae in from other woody habitats and (b) in the percentage of woody the remaining cells could develop for subsequent species identi- habitats in the surrounding landscape. We predicted that (i) the fication. The number of spiders per cell was counted and the abundance of T. figulus decreases with increasing habitat isolation spiders were determined as close as possible to the species level. and / or decreasing amount of woody habitat. We expected (ii) The fresh weight of the total spider content of 202 cells with higher number and/or mean weight of spiders per brood cell in undamaged spider individuals was measured. Total spider weight sites with higher abundance of T. figulus. We further expected that per cell and the number of spiders per cell were positively (iii) if T. figulus hunts spiders according to their abundance in the correlated (Pearson correlation test: N = 204, r = 0.56, P , 0.001) landscape, its diet will vary as a function of the surrounding and we retained only the number of spiders per cell for further landscape, with higher frequencies of forest-related spider species analyses. Habitat preference of spiders was based on the ecological in habitats adjacent to forest and/or landscapes with large characterisation of Central European spiders by Entling et al. [45]. amounts of woody habitats. Eventually, according to the They identified shading as a main niche dimension and derived trophic-level hypothesis, we predicted lower parasitism rate in niche positions on a gradient from open habitats to closed forest isolated sites (iv). for 590 spider species. We considered spiders with niche position in the upper half of the shading gradient as forest-related species Materials and Methods (Table 1). The two trap nests per site that were not used for prey assessment were collected in October 2010 to determine wasp Ethics statements abundance and parasitism. They were stored at 5uC until All sites were visited with landowner permission, and no permits February 2011. Then, nesting tubes were singly transferred to for sampling were required. Our study did not involve any test tubes and stored in a greenhouse for hatching of T. figulus and endangered or protected species. any parasitoids that have developed on them. Abundance of T. figulus can be measured as the number of colonized nesting tubes Study sites ( = number of nest-building females) or as the number of brood We conducted our study in agricultural landscapes of the Swiss cells that have been built ( = productivity of nest-building females). Plateau between the cities of Bern, Solothurn and Fribourg. Thirty In our study, both measures were strongly correlated (Pearson experimental sites were chosen over an area of 23632 km varying correlation test: N = 30, r = 0.98, P , 0.001), such that we in altitude between 465 and 705 m above sea level [4]. Each site retained only the number of colonized nesting tubes as a measure consisted of an 18-m-long row of seven 6-year-old cherry trees that of wasp abundance. We additionally measured the number of cells had been planted on permanent
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