Seasonality of spiders (Araneae) in Mediterranean ecosystems and its implications in the optimum sampling period

Abstract.  1. Fields such as ecology, macroecology, and conservation biology rely on accurate and comparable data. This is especially important for mostly unknown and megadiverse taxa such as spiders and regions such as the Mediterranean. Short-term sampling programmes are increasingly seen as the best option for sampling spiders. Comparability of results, however, demands standard procedures both in methodology and in sampling period. Cost-efficiency dictates that this period should be the most species rich.
2. Pitfall trapping was conducted in 23 sites from north to south Portugal, comprising three large-scale environmental zones and many different habitat types, during 10 months in each site. The annual richness pattern, differences in this pattern between areas and habitats, the complementarity between sampling periods and possible environmental correlates of richness were studied.
3. May and June present the optimal time for collecting spiders in Mediterranean areas. Northern areas have a later peak in richness and dense tree-cover sites offer more flexibility for sampling, with a higher proportion of species present at each period throughout the year.
4. Day length is the environmental factor most correlated with species richness. Maximum daily temperature may reduce richness, especially in southernmost areas, where summer temperatures can be extremely harsh.
5. It is recommended that short-term sampling programmes, intended to give a reasonable picture of spider communities in Portugal and in the Iberian Peninsula (and possibly extending to all the Mediterranean), should be conducted during May or June, with variable flexibility according to area and habitat. The proposed suggestions should appeal to everyone working in the field, given the cost-efficiency and comparability of results by adopting a common standardised approach.     

Contrasting responses of arable spiders to the landscape matrix at different spatial scales

Aim Animal communities can be influenced by the composition of the surrounding landscape through immigration. Depending on habitat preferences, however, the effect of the landscape matrix can be positive or negative and can vary with scale. We tested this idea with arable spiders and tried to infer dispersal distances from relationships between local density and landscape composition at different spatial scales. Location Thirty‐eight landscapes around the cities of Göttingen and Giessen, Germany. Methods Spiders were captured with pitfall traps in one field of winter wheat in each landscape. Landscape composition around the fields was characterized at 11 scales from 95 m to 3 km radius by land‐use mapping and subsequent GIS analysis. Correlation tests were performed between landscape composition and local densities or species richness. Results In both study regions, local species richness was enhanced by non‐crop habitats on a landscape scale. The overall densities of wolf spiders (Lycosidae), long‐jawed spiders (Tetragnathidae), crab spiders (Thomisidae), and dwarf sheet spiders (Hahniidae) increased significantly in landscapes with high percentages of non‐crop habitats. Out of the 40 species tested, 19 responded positively to the percentage of non‐crop habitats in the surrounding landscape, and five responded negatively. Depending on the species, the spatial scales with the highest explanatory power ranged from 95 m to 3 km radius around the study fields, potentially reflecting dispersal distances. Main conclusions Arable spider species showed contrasting responses to the landscape context with respect both to the direction and to the spatial scale of the relationship. The variation in landscape requirements among species ensures high spider densities in a wide range of situations, which contributes to ecosystem resilience. However, species richness of arable spiders depends on heterogeneous landscapes with high percentages of non‐crop habitats.     

Water limitation prevails over energy in European diversity gradients of sheetweb spiders (Araneae: Linyphiidae)

Across large spatial scales, species richness in many taxa is mainly determined by climatic variables. However, some of the mechanisms behind large-scale patterns of species richness and abundance are expected to act on the community level, and on a smaller scale than the resolution of the data commonly used for deriving these patterns. We studied the distribution of sheetweb spiders (Araneae: Linyphiidae) across Europe using both standardised samples from local habitats and large-scale inventories. In the first approach, we analysed species and individual numbers from standardised pitfall traps from 17 locations distributed from northern Sweden to Spain and Greece. We also calculated Simpson indices to correct for different activity densities. Additionally, we analysed whether diversity of sheetweb spiders is sensitive to habitat type. In the second approach, we investigated the known sheetweb spider species richness of European countries and large islands. In both datasets, species richness of sheetweb spiders reached a maximum at about 55°N and declined towards the Mediterranean and the Arctic. In addition, species richness decreased from western (maritime) to eastern (continental) regions. Also Simpson diversity showed a hump-shaped relation to latitude, and was the only variable influenced by habitat type. In contrast to species richness and diversity, activity density increased monotonously with latitude. Towards the north, sheetweb spiders are likely limited by energy availability, towards the south, water limitation as well as bottom-up and top-down community interactions may reduce sheetweb spiders. Accordingly, their diversity does not follow the pattern of vertebrates, large insects and aquatic organisms, which depend more strongly on energy availability.     

Relative efficiency of anuran sampling methods in a restinga habitat (Jurubatiba, Rio de Janeiro, Brazil).

Studies on anurans in restinga habitats are few and, as a result, there is little information on which methods are more efficient for sampling them in this environment. Ten methods are usually used for sampling anuran communities in tropical and sub-tropical areas. In this study we evaluate which methods are more appropriate for this purpose in the restinga environment of Parque Nacional da Restinga de Jurubatiba. We analyzed six methods among those usually used for anuran samplings. For each method, we recorded the total amount of time spent (in min.), the number of researchers involved, and the number of species captured. We calculated a capture efficiency index (time necessary for a researcher to capture an individual frog) in order to make comparable the data obtained. Of the methods analyzed, the species inventory (9.7 min/searcher /ind.- MSI; richness = 6; abundance = 23) and the breeding site survey (9.5 MSI; richness = 4; abundance = 22) were the most efficient. The visual encounter inventory (45.0 MSI) and patch sampling (65.0 MSI) methods were of comparatively lower efficiency restinga, whereas the plot sampling and the pit-fall traps with drift-fence methods resulted in no frog capture. We conclude that there is a considerable difference in efficiency of methods used in the restinga environment and that the complete species inventory method is highly efficient for sampling frogs in the restinga studied and may be so in other restinga environments. Methods that are usually efficient in forested areas seem to be of little value in open restinga habitats.     

The net result: evaluating species richness extrapolation techniques for littoral pond invertebrates

1. Total species richness for an assemblage or site is a valuable measure in conservation monitoring and assessment, but protocols for sampling and species richness determination in wetland habitats such as ponds, bogs or mires remain largely unrefined. 2. Techniques for estimation of total richness of an assemblage based upon replicated sampling offer the opportunity to derive useful estimates of total richness based upon small numbers of samples, and limit sampling‐derived disturbance which can be particularly problematic in small aquatic habitats. 3. We quantified the performance of eight of the most commonly encountered estimators of species richness for a variety of littoral zone macrofauna from ponds, comparing estimated richness to maximum richness derived from sampling. 4. Estimates using non‐parametric techniques based on species incidence provided the most accurate and precise estimates. The estimators Chao 2 and incidence‐based coverage estimator (ICE) from this category were reliable and consistent slight over‐estimators; the abundance‐based estimator Chao1 also performed well. 5. Species inventory based on relatively small numbers of samples might be significantly improved by use of non‐parametric estimators for quantification of species richness. 6. Use of non‐parametric estimators of species richness can assist biodiversity inventory by preventing erroneous rankings of habitat richness based upon observed species numbers from limited sampling.     

Diversity of spiders and orthopterans respond to intra-seasonal and spatial environmental changes

Our understanding of arthropod responses to environmental pressures is limited, especially for the poorly studied Mediterranean region. In the light of likely further environmental change and the need for protocols for rapid biodiversity assessment, we measured how the abundance and species richness of two taxa, ground spiders and Orthoptera, belonging to different functional groups, fluctuates intra- seasonally (early-mid-late summer) and across habitat types (grasslands, maquis, forests). We also tested their surrogate value. Spiders were found to have higher species richness and abundance almost throughout the investigation. Orthoptera had lower species richness and abundance in forests compared to grasslands and maquis, while no significant difference between habitats was revealed for spiders. Early-summer was the richest period for spiders while mid-summer was the richest for Orthoptera. Canopy cover was found to significantly influence community composition of both groups, while herb height and cover of stones was a determinant factor for Orthoptera only. There was a significant congruence between the two groups and Orthoptera provided the best complementary network. Our results show that diversity patterns of both spiders and Orthoptera are sensitive to environmental changes even over short time-scales (e.g. within the summer period) and space (e.g. across different habitat types), suggesting that small inexpensive experimental designs may still reveal community dynamics. For conservation purposes, we advise a focus on variables regulating habitat heterogeneity and microhabitat characteristics. We provide a list of the most influential species and propose the most effective network for obtaining information on the local fauna.     

Decadal abundance patterns in an isolated urban reptile assemblage: Monitoring under a changing climate

Determine seasonal, annual, and decadal patterns of abundance in reptile species and assemblages occupying central Bold Park (~338 ha), an isolated urban bushland remnant in Perth, Southwestern Australia. Fenced pitfall trapping in four sampling sites, representing different habitats and fire history, over the primary reptile activity period for 35 consecutive years with over 17,000 individuals captured during 3300 days of sampling; the trapping regime was modified for the last 28 years. Sampling occurred in one of 35 global biodiversity hotspots that has a Mediterranean climate experiencing a 15% decline from the century average rainfall over the last 50 years. Twenty‐nine species were recorded, with 16 captured in 32 or more years and accounting for nearly 97% of all captures; the six most common for 81%. Three taxa became locally extinct. Activity predominates in warmer and dryer months (October to April), peaking in November–December. Species richness remained relatively constant between years with around 73% of known taxa captured annually. Assemblages did not change when analyzing the presence/absence data but moved through five statistically significant assemblages analyzing relative abundance data. Over the last 28 years, relative abundance was significantly and positively correlated with annual rainfall residuals, uniquely for the 4 years preceding annual sampling, resulting in significant changes in total assemblages and significantly similar patterns in four sample sites; the presence/absence data indicated only minor assemblage changes across sites. The number of species recorded annually remained relatively constant, but relative abundance illustrated significant temporal changes in assemblages over decades. The modeled relationship between relative abundance and annual rainfall residuals for 4 years preceding annual sampling is supported by known ecological responses and reptile demographics within this Mediterranean climate. Maintenance of urban biodiversity should consider impacts of a significantly drying climate exacerbating the extinction debt already inherent in isolated bushland populations experiencing limited immigration.     

Management intensity and vegetation complexity affect web-building spiders and their prey

Agricultural management and vegetation complexity affect arthropod diversity and may alter trophic interactions between predators and their prey. Web-building spiders are abundant generalist predators and important natural enemies of pests. We analyzed how management intensity (tillage, cutting of the vegetation, grazing by cattle, and synthetic and organic inputs) and vegetation complexity (plant species richness, vegetation height, coverage, and density) affect rarefied richness and composition of web-building spiders and their prey with respect to prey availability and aphid predation in 12 habitats, ranging from an uncut fallow to a conventionally managed maize field. Spiders and prey from webs were collected manually and the potential prey were quantified using sticky traps. The species richness of web-building spiders and the order richness of prey increased with plant diversity and vegetation coverage. Prey order richness was lower at tilled compared to no-till sites. Hemipterans (primarily aphids) were overrepresented, while dipterans, hymenopterans, and thysanopterans were underrepresented in webs compared to sticky traps. The per spider capture efficiency for aphids was higher at tilled than at no-till sites and decreased with vegetation complexity. After accounting for local densities, 1.8 times more aphids were captured at uncut compared to cut sites. Our results emphasize the functional role of web-building spiders in aphid predation, but suggest negative effects of cutting or harvesting. We conclude that reduced management intensity and increased vegetation complexity help to conserve local invertebrate diversity, and that web-building spiders at sites under low management intensity (e.g., semi-natural habitats) contribute to aphid suppression at the landscape scale.     

Diversity of predaceous arthropods in the almond tree canopy in Northeastern Portugal: A methodological approach

The almond tree is an economically important crop in Mediterranean regions. However, knowledge about the biodiversity of natural enemies that may be useful as biocontrol agents is scarce. The objectives of this work were: (i) to study the diversity of predaceous arthropods; and (ii) to establish a suitable sampling protocol for arthropods of the almond tree canopy. Between April and October of 2007 and 2008, 25 randomly selected trees were sampled in an organic almond grove located in the north‐east of Portugal using the beating technique. The specimens collected were counted and identified and the sampling protocol was established by using the accumulation curves and the seasonal richness peaks of the most abundant groups of natural enemies. A total of 1856 and 1301 arthropods were captured, respectively, in 2007 and 2008, where Araneae, Coccinellidae and Formicidae were the most abundant groups. A total of 14 families and 29 species of spiders were identified as Linyphiidae, Philodromidae, Thomisidae, Araneidae and Oxyopidae, the five most abundant families in both years. In the Coccinellidae and Formicidae communities 15 and 13 species were identified, respectively. According to taxa accumulation curves, the minimum sampling effort that provided a reliable picture of the biodiversity was established in 11 samples. Moreover, considering the seasonal richness distribution, it would be advisable to concentrate the sampling period from the beginning of July to the harvesting of almonds. This protocol might generate accurate replicate samples to estimate species richness when the effect of agricultural management is studied.     

Butterfly communities along an elevational gradient in the Tons valley,Western Himalayas: Implications of rapid assessment for insect conservation

As time and money is limited, explicit, cost-effective, quick, and appropriate methods are needed to assist conservation planners and managers for making quick decisions. Butterflies promise to be a good model for rapid assessment and habitat monitoring studies because they are widespread, conspicuous, and easily recognizable and they are effective indicators of forest health. We conducted a rapid assessment of butterflies at five disturbance gradient sites that varied in elevation from 900 m a.s.l. to 3500 m a.s.l. for 20 days during March–April 2010 and recorded 79 butterfly species and 1504 individuals in the Tons valley in Western Himalayas. We were able to sample approximately 77% (123 species) of the estimated species richness on continuing the sampling until July 2010. Species richness at the study site is estimated to be 159 (95% CI: 145–210) species. Diversity was highest in heterogeneous habitats and decreased towards homogeneous habitats. Unique species were highly restricted to lowest disturbed sites. Using Pearson's correlation analysis, the strongest vegetative predictors of butterfly richness were plant species richness, canopy cover, and herb and shrub density. Butterfly species richness and abundance were highly correlated with altitude, temperature, relative humidity, fire signs, and livestock abundance. We also found positive cross-taxon correlation among butterflies, moths, and beetles across sites, indicating that butterflies can be used as surrogate or indicator taxa for insect conservation. Short sampling periods providing comprehensive estimates of species richness were reliable for identifying habitats and sites with the most conservation value in the Tons valley landscape.     

甘肃子午岭天然次生林区三种植被类型中土壤蜘蛛群落的组成及多样性

2004年4—10月,在甘肃子午岭天然次生林区采用巴氏诱罐法采集土壤蜘蛛2164头,隶属于19科43种。研究表明辽东栎林(Quercusliaotungensis)、油松林(Pinustabulaeformis)和杨树林(Populusdavidiana)3种森林植被类型中土壤蜘蛛群落组成有明显的差异;不同生境蜘蛛群落的组成成分和多样性指数各异;从各生境中捕获的蜘蛛个体数量分析,皿蛛科、平腹蛛科和科狼蛛科的数量最多,优势类群的组成相似;各生境类型中蜘蛛功能集团的组成及多样性也有明显差异。同时,分析了影响蜘蛛群落组成和多样性的主要因子是生境的植被类型和生境异质性。     

王朗自然保护区地栖脊椎动物群落结构和生境类型的关系

2006年8—11月,采用围栏陷阱法对四川省王朗自然保护区地栖脊椎动物的群落结构进行了研究,结合森林砍伐、旅游和植被恢复方式等干扰因子所推动的动物栖息生境变化,探讨了动物群落结构和生境类型的关系。监测历时75 d,8个样地共捕获3纲6目17科21种778只地栖脊椎动物,物种组成包括3种两栖类、1种鸟类和17种哺乳类。每百陷阱日的捕获率为32.4%;以普通鼩鼱、高山姬鼠、纹背鼩鼱、高原林蛙等4种为优势种,占全部捕获数的76.9%;其余17种的个体数均较少,物种多度和其分布区域呈显著线性相关(P0.001)。动物群落多样性指数,以干扰相对弱的原生针叶林物种多样性指数和均匀度指数为最高,而优势度指数最低;其次为旅游干扰的原生林灌丛和自然更新的次生林灌丛;人工种植的次生针叶林多样性指数和均匀度指数最低,而优势度指数最高。以动物群落相似性指数进行系统聚类,8个样地的动物群落聚为3类:即干扰相对弱的原生针叶林动物群落、严重干扰的人工针叶林动物群落、部分干扰的原生林灌丛和次生林灌丛动物群落。     

A comparison of flooded forest and floating meadow fish assemblages in an upper Amazon floodplain

Matched sets of gillnets of different mesh-sizes were used to evaluate the degree to which contiguous and connected flooded forest and floating meadow habitats are characterized by distinct fish faunas during the flooding season in the Peruvian Amazon. For fishes between 38–740 mm standard length ( L _S) (the size range captured by the gear), an overriding pattern of faunal similarity emerged between these two habitats. The mean species richness, diversity, abundance, fish mass, mean and maximum L _S, and maximum mass did not differ significantly between flooded forest and floating meadows. Species abundances followed a log-normal distribution in which three species accounted for 60–70% of the total abundance in each habitat. Despite these similarities, multivariate analyses demonstrated subtle differences in species composition between flooded forest and adjacent floating macrophytes. In addition, the absolute number of species was higher in flooded forest, reflecting a higher percentage of rare species. The day–night species turnover was found to be greater in flooded forests than floating meadows. Further, nocturnal samples had higher abundances and greater species richness than diurnal samples in both habitats. Differences in habitat structural complexity between flooded forest and floating meadows may result in a higher abundance and species richness of day-active species in floating meadows.     

How to catch foliage-dwelling spiders (Araneae) in maize fields and their margins: a comparison of two sampling methods

The foliage‐dwelling spider fauna was collected in maize fields and on stinging nettles in adjacent margins in Bavaria, South Germany. Two different sampling methods were evaluated: drop cloth sampling and suction sampling. The overall catch was dominated by juvenile spiders, web‐building spiders, and spiders of the families Theridiidae, Linyphiidae, Tetragnathidae and Araneidae (in decreasing order). Field margins harboured more species than maize fields, whereas the total spider abundance was higher in the maize crop. Web‐building spiders such as Theridiidae and Linyphiidae were prominent in maize by individual numbers. Suction sampling with a small suction device proved to be a more efficient and consistent sampling method for foliage‐dwelling spiders than drop cloth sampling. Density and species richness of foliage‐dwelling spiders in maize was shown to be fairly high, implying that spiders of higher strata may play a more important role in biological control than suspected up to now.     

Do spider diversity and assemblages change in different contiguous habitats? A case study in the protected habitats of the humid chaco ecoregion, northeast Argentina

Spiders are a megadiverse group that can be useful indicators of the overall species richness and health of biotic communities. The spider diversity in subtropical forests of the Neotropical region are not yet well known, especially in Argentinean subtropical forests where systematic fieldwork has not been done until recently. The Great Chaco is very important as the unique dry subtropical forest of the earth, but it is suffering increasing degradation by the advance of agriculture. Spider communities have been shown to be more directly influenced by vegetation architecture than vegetation species composition. In this study, we aim to assess whether spider diversity and assemblages change in adjacent habitats with different types of vegetation. We compare the diversity and spider assemblages in two different contiguous protected habitats (hygrophilous woodland and savannah parkland) of the Mburucuyá National Park, (Humid Chaco ecoregion). Seasonal samples were obtained using three types of sampling methods: pitfall trapping, beating, and manual litter extraction. The spider assemblages were different in the studied areas, and the abundance, diversity, evenness, and species richness were higher in the hygrophilous woodland than the savannah parkland. These differences in spider diversity and assemblages indicate that both types of habitats are important if the biodiversity is to be conserved in the Chaco ecoregion, where different types of habitat are shown as a patchy distribution.     

Diversity of spiders after spring sowing – influence of farming system and habitat type

Spiders contribute considerably to diversity in agroecosystems and are important components of natural pest control. Farming system and adjacent habitats may influence spider diversity. In this study, diversity of the spider families Lycosidae and Linyphiidae was studied after spring sowing until the time when a common pest (Rhopalosiphum padi) colonizes cereal fields. The spiders were collected with pitfall traps at eight organically or conventionally managed farms around Uppsala, Sweden, in three different habitats at each site: field margin, crop field and the edge between the two. The effects of farming system and habitat type on diversity of lycosids and linyphiids were considered using three different measures (activity density, species richness and composition). The most dominant species of each spider family, Pardosa agrestis (Lycosidae) and Oedothorax apicatus (Linyphiidae), had higher activity density at organic sites, and farming systems also contained different species compositions of both lycosid and linyphiid spiders. Also, linyphiid species richness was higher on conventional sites and linyphiid species composition was influenced by habitat type, in contrast with lycosids. Activity density and species richness of lycosid spiders were, on the other hand, more associated with field margins than linyphiid spiders.     

Factors influencing the appearance of spider (Araneae) and beetle (Coleoptera) assemblages in nests of great reed warbler Acrocephalus arundinaceus

The appearance of spider (Araneae) and beetle (Coleoptera) assemblages found in nests of great reed warbler Acrocephalus arundinaceus was studied, firstly to investigate breeding success and the amount of precipitation as potential factors which might affect the abundance and species richness of both groups. In addition, we compared the diversity of spider and beetle assemblages between nests found in different reed habitats, and considered the position of nests (above water or dry ground). In this study we selected five different randomly chosen reed habitats: two mining ponds, two small canals and one large canal. Great Reed Warbler nests were collected either shortly after fledging, or after the clutch had failed. Altogether, 12 species of spider and 19 species of beetle were collected. In both groups there was no significant difference in abundance between successful, lost and cuckoo-parasitized nests; however, there was a significant difference in species richness between the three nest categories in spider assemblages, which was not the case in beetle assemblages. The amount of precipitation did not affect beetle or spider abundance; only the species richness of spiders showed significant growth. Furthermore, we found no significant relationship between vegetation cover and the species richness and abundance of spiders and beetles. The diversity of both groups differed significantly according to reed habitat: beetle assemblages were most diverse by the large canal and spiders at the mining ponds.     

Immediate and longer-term effects of managed flooding on floodplain invertebrate assemblages in south-eastern Australia: generation and maintenance of a mosaic landscape

1. We compared assemblages of ground‐active, terrestrial beetles and spiders from different areas of river red gum Eucalyptus camaldulensis floodplain forest in subhumid, south‐eastern Australia before and for 2 years following a managed flood to determine whether the Flood Pulse Concept is an appropriate ecological model for this regulated, lowland river‐floodplain system. 2. Immediately following flooding, the abundance, species richness and biomass of beetles were greatest at sites that had been inundated for the longest period (approximately 4 months). The abundance, species richness and biomass of spiders were not reduced at sites that were flooded for 4 months compared with unflooded or briefly flooded areas. Sites recently flooded for several months had high densities of predatory, hygrophilic beetles (Carabidae) and spiders (Lycosidae). 3. Over the 2 years following the flood, beetles generally were more abundant at sites that had been inundated for longer. At all sampling times, the species richness of beetles at sites increased with the length of time sites were inundated, even before the flood. Neither the abundance nor species richness of spiders was related to duration of flooding. 4. The structure of beetle and spider assemblages at sites that were flooded for different lengths of time did not appear to converge monotonically over the 2 years after the flood. 5. Managed flooding promotes diversity of beetles and spiders both by providing conditions that create a ‘pulse’ in populations of hygrophilic specialists in the short term, and by creating subtle, persistent changes in forest‐floor conditions. Despite its monotypic canopy, river red gum floodplain forest is a habitat mosaic generated by differing inundation histories.     

Spider communities in urban green patches and their relation to local and landscape traits

Urbanization and urban landscape characteristics greatly alter plant and animal species richness and abundances in negative and positive directions. Spiders are top predators, often considered to be sensitive to habitat alteration. Studies in urban environments frequently focus on ground-dwelling spiders or on spiders in built structures, leaving aside foliage spiders. Effects of habitat, landscape type and structure and local characteristics on spider species composition, richness and relative abundance were evaluated in urban green patches in a temperate city of South America. We also assess whether Salticidae could be an indicator group for the broader spider community in the urban environment. Spiders were sampled with a G-VAC (aspirator) in urban green patches in Córdoba city, Argentina, in urban, suburban and exurban habitats (18 sites; six per habitat) and local and landscape traits were assessed. Overall, the exurban was richer than the urban habitat, however, at the site level Salticidae richness and abundance (but not the total spider assemblage) were significantly lower in urban sites. Species composition moderately differed between urban and exurban sites. Results indicate that on urban green spaces a low impervious surface cover, a coverage of trees, herbaceous vegetation and a vertical structure of vegetation at least up to 1 m in height contribute to higher richness and abundance of spiders, Salticidae being more sensitive than the overall spider community to local effects. In addition, Salticidae richness can predict 74% of the total spider richness recorded and may be used as spider diversity bio-indicators in this climatic region.     

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

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.