We compared the abundance of foliage-living spiders among seven widespread plant species comprising a gradient of levels of structural complexity in a tropical savannah-like region in southeastern Brazil. Spider abundance among plant species was positively related to the foliage density of their branches. A field experiment using artificial branches was carried out to isolate foliage density effects on spider abundance, thus controlling both biomass and texture effects. Artificial branches were attached to branches of three plant species with similar foliage density, Baccharis dracunculifolia, Diplusodon virgatus, and Microlicia fasciculata. Two treatments were set up: artificial branches with higher foliage density attracted more spiders than those with lower foliage density. The guild structure of hunting spiders was compared among vegetative branches of three plant species with different levels of foliage density: B. dracunculifolia, D. virgatus, and Bidens gardneri. Stalker, and ambusher spiders were more abundant on branches of B. dracunculifolia, which had the highest foliage density. Foliage-runners constituted the dominant guild on D. virgatus and B. gardneri, which have lower foliage density branches. Our results suggest that branch architecture is the most important factor determining the abundance of plant-dwelling spiders in the study area independently of branch biomass, leaf surface area or texture.

RESUMO

A abundância de aranhas foi comparada entre sete especies de plantas, que formam um gradiente de niveis de complexidade estrutural, descrito atraves do numero de folhas por unidade de volume do ramo, numa regiao de cerrado no sudeste do Brasil. A abundância de aranhas entre as especies de plantas foi positivamente relacionada com a densidade de folhas de seus ramos. Um experimento de campo, usando ramos artificiais, foi conduzido para isolar o efeito da densidade de folhas dos efeitos de biomassa. Ramos artificiais foram amarrados a ramos de tres especies de plantas com densidade foliar semelhante, Baccharis dracunculifolia, Diplusodon virgatus and Microlicia fasciculata. Dois tratamentos foram usados, ramos artificiais com alta densidade de folhas atrairam mais aranhas do que os com baixa densidade de folhas. A distribuicao de guildas de aranhas errantes foi comparada entre ramos de tres especies de plantas com diferentes niveis de densidade de folhas, B. dracunculifolia, D. virgatus e Bidens gardneri. Aranhas saltadoras (Oxyopidae e Salticidae) e de tocaia (Thomisidae) foram mais abundantes nos ramos de B. dracunculifolia, cujos ramos possuiam uma maior densidade de folhas. Aranhas cacadoras noturnas foi a guilda dominante em D. virgatus e B. gardneri. Nossos resultados sugerem que a arquitetura de ramos pode ser o fator mais importante que determina a abundância de aranhas que habitam plantas, independentemente da biomassa, area superficial de folhas e textura dos ramos.

https://www.jstor.org/stable/pdfplus/30043203.pdf

Foliage Density of Branches and Distribution of Plant‐Dwelling Spiders1

Brachypelma, a genus of nine endangered tarantula species in Mexico, is the only group of spiders included in Appendix II of CITES, owing to habitat degradation and illegal trafficking. However, while the majority of the nine species of Brachypelma are thought to be threatened, little is known of their ecology and distribution. Brachypelma klaasi is the rarest species, occurring in a few isolated populations on the Pacific coast of Mexico. We present an analysis of population distribution and micro-habitat requirements of B. klaasi over different spatial scales within the biological reserve at Chamela, Jalisco as part of a wider ecological study of the endangered Brachypelma group. Burrows and dispersing spiders were confined to a southern area of the reserve covering approximately 0.5 km2. Within this area, burrows were not aggregated at lower spatial scales (24–216 m2), unlike other related species. Also, there was no evidence that intra-specific interactions (either positive or negative interactions) influenced the distribution of burrows. Distribution of burrows at low spatial scales was related to low afternoon temperatures and high humidity in mid-summer. These abiotic factors may influence the survival and development of eggs and spiderlings, and appear to be more important in governing the distribution of B. klaasi than are food resources or intra-specific interactions. We discuss how these findings may facilitate the re-introduction of captive-bred individuals of B. klaasi and other Brachypelma species.

Spatial distribution and habitat preference of the endangered tarantula, Brachypelma klaasi (Araneae: Theraphosidae) in Mexico

In a predator–prey system where both intervenients come from the same taxon, one can expect a strong selection on behavioural and morphological traits involved in prey capture. For example, in specialised snake-eating snakes, the predator is unaffetced by the venom of the prey. We predicted that similar adaptations should have evolved in spider-eating (araneophagous) spiders. We investigated potential and actual prey of two Palpimanus spiders (P. gibbulus, P. orientalis) to support the prediction that these are araneophagous predators. Specific behavioural adaptations were investigated using a high-speed camera during staged encounters with prey, while morphological adaptations were investigated using electron microscopy. Both Palpimanus species captured a wide assortment of spider species from various guilds but also a few insect species. Analysis of the potential prey suggested that Palpimanus is a retreat-invading predator that actively searches for spiders that hide in a retreat. Behavioural capture adaptations include a slow, stealthy approach to the prey followed by a very fast attack. Morphological capture adaptations include scopulae on forelegs used in grabbing prey body parts, stout forelegs to hold the prey firmly, and an extremely thick cuticle all over the body preventing injury from a counter bite of the prey. Palpimanus overwhelmed prey that was more than 200% larger than itself. In trials with another araneophagous spider, Cyrba algerina (Salticidae), Palpimanus captured C. algerina in more than 90% of cases independent of the size ratio between the spiders. Evidence indicates that both Palpimanus species possesses remarkable adaptations that increase its efficiency in capturing spider prey.

Armoured spiderman: morphological and behavioural adaptations of a specialised araneophagous predator (Araneae: Palpimanidae).

The influence of the architecture of vegetative branches on the distribution of plant-dwelling spiders has been intensively studied, and the effects on the aggregation of individuals in several spider species on plants include variation in prey abundance, availability of predator-free refuges and smoother microclimate conditions. The emergence of inflorescences at the reproductive time of the plants changes branch architecture, and could provide higher prey abundance for the spiders. The distribution of spiders between inflorescences and vegetative branches was compared on four widespread plant species in a Brazilian savannah-like system. Inflorescences attracted more spiders than vegetative branches for all plant species sampled. The influence of branch type (inflorescence and vegetative) on spider distribution was also evaluated by monitoring branches of Baccharis dracunculifolia DC. in vegetative and flowering periods for 1 year, and through a field experiment carried out during the same period where artificial inflorescences were available for spider colonization. Artificial inflorescences attached to B. dracunculifolia branches attracted more spiders than non-manipulated vegetative branches for most of the year. However, this pattern differed among spider guilds. Foliage-runners and stalkers occurred preferentially on artificial inflorescences relative to control branches. The frequencies of ambushers and web-builders were not significantly different between treatment and control branches. However, most ambush spiders (65%) occurred only during the flowering period of B. dracunculifolia, suggesting that this guild was influenced only by natural inflorescences. The experimental treatment also influenced the size distribution of spiders: larger spiders were more abundant on artificial inflorescences than on vegetative branches. The hypothesis that habitat architecture can influence the spider assemblage was supported. In addition, our observational and experimental data strongly suggest that inflorescences can be a higher quality microhabitat than non-reproductive branches for most plant-dwelling spiders.

Distribution of plant-dwelling spiders: Inflorescences versus vegetative branches

To assess the relative impact of a range of habitat variables on spider abundance, field and laboratory experiments were conducted on populations of the urban wall spider Oecobius navus in suburban Perth, Western Australia. Habitat characteristics investigated were: substrate type, wind speed, rainfall, sunlight exposure, relative humidity, air temperature, substrate temperature, artificial lighting and prey type/abundance. In the field, O. navus was found to be associated with high humidity, low air tempera- ture and shelter from sunlight and rainfall. Oecobius navus was more abundant at sites with greater prey abundance. The most common prey item was the red meat ant Iridomyrmex chasei. Juvenile spiders were more abundant than adult spiders; however, patterns between spider abundance and habitat variables were similar for both adults and juveniles. Laboratory experiments showed that O. navus preferred to build webs on wooden substrates, and pitted limestone walls. These findings indicate that O. navus may be vulnerable to desiccation and/or thermal stress, and thus survives better on sheltered walls.

Habitat preferences of the urban wall spider Oecobius navus (Araneae, Oecobiidae)

1. Distribution and web characteristics were examined for the dune-burrowing spider Seothyra henscheli in the Namib Desert to determine how these spiders relate to spatially heterogeneous habitat features that influence foraging costs and benefits, growth rate and survival. In particular, we set out to determine the relative importance of (i) habitat selection, (ii) site-dependent growth and survival, and (iii) restricted dispersal, to the occurrence of S. henscheli in different macro- and microhabitats. 2. Spider webs were mapped along transects that crossed dune regions with dense vegetation hummocks, steep dune plinths and bare, gravelly interdune plains. Spiders were more abundant in the hummocks than in the interdunes and plinths. Translocation of spiders from the hummocks to the interdunes and plinths confirmed that spider density was correlated with habitat quality. 3. In a comparison of web sites with dummy sites in terms of factors related to sand stability and food availability, it was found that, except for the avoidance of extreme conditions, web-site characteristics did not differ from dummy sites. Web geometry indicated that good sites were characterized by stable sand and abundant ants. However, many spiders occurred at poor sites when good sites were vacant nearby. 4. It is concluded that S. henscheli do not actively choose web sites, but have restricted dispersal. Site-dependent growth and survival may explain the pattern of local abundance. Site tenacity may be a result of the spiders' inability to predict site quality coupled with high costs of relocation. It is inferred that spiderlings tend to remain near their mother's site, which has a previous record of success. 5. Tolerance of variable conditions of dunes and the ability to sustain populations even at poor sites enable these spiders to occur in extreme desert conditions.

J. R. Henschel

Papers

A test of habitat selection at two spatial scales in a sit-and-wait predator : a web spider in the Namib Desert dunes