Foliage Density of Branches and Distribution of Plant Dwelling Spiders
Foliage Density of Branches and Distribution of Plant Dwelling Spiders
Foliage Density of Branches and Distribution of Plant Dwelling Spiders
and
ABSTRACT
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 espécies de plantas, que formam um gradiente de nı́veis de complexidade estrutural, descrito através do número de
folhas por unidade de volume do ramo, numa região de cerrado no sudeste do Brasil. A abundância de aranhas entre as espécies 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 três 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 atraı́ram mais aranhas do que os com baixa densidade de folhas. A
distribuição de guildas de aranhas errantes foi comparada entre ramos de três espécies de plantas com diferentes nı́veis 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
possuı́am uma maior densidade de folhas. Aranhas caçadoras 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, área superficial de folhas e
textura dos ramos.
PLANT STRUCTURAL COMPLEXITY, THE ARCHITECTURAL ARRANGE- abies (Gunnarsson 1990), leaf surface area in different species of
MENT OF BIOMASS IN SPACE, has been recognized as one of the main Eucalyptus (Evans 1997), and needle density and branching com-
factors that determine the diversity and abundance of plant-dwelling plexity in Douglas-fir branches Pseudotsuga menziesii (Halaj et al.
spiders (Gunnarsson 1990, Scheidler 1990, Hurd & Fagan 1992, 2000). However, these studies were restricted to comparisons within
Halaj et al. 1998). Although causal mechanisms remain obscure, a particular plant species or a few plant species of the same genus in
some variables have been proposed to explain this pattern, such temperate systems. In addition, the effect of foliage density on spider
as prey abundance (Allen et al. 1975, Nentwig 1993), availabil- abundance is usually confounded with the effect of biomass of the
ity of refuges against predators (Askenmo et al. 1977, Gunnarsson branches.
1996), and milder microclimate conditions (Henschell & Lubin Spiders form a very diverse group, and several species may
1997). A positive influence between plant architecture and spi- actively choose favorable microhabitats (Turnbull 1973, Morse &
der abundance was demonstrated through indices of structural Fritz 1982, Morse 1990, Nentwig 1993). Their requirements for
complexity such as foliage density in big sage Artemisia tridentata structural characteristics of microhabitats can change among guilds,
(Hatley & MacMahon 1980), density of needles in spruce Picea since they differ in relation to hunting strategies, refuge and repro-
ductive behavior, and morphological and physiological features such
1Received 7 June 2002; revision accepted 26 January 2005.
2
as visual acuity, tolerance to sunlight and shade, moisture, and ther-
Corresponding author. Current address: Depto. de Biologia, Setor Ecologia –
CCBS, Universidade Federal de Mato Grosso do Sul – CP 549, CEP 79070-900, mal conditions (Uetz et al. 1978, Uetz 1991, Foelix 1996). Thus,
Campo Grande, MS, Brazil; e-mail: altsouza@nin.ufms.br we expected that spider guild distribution could be associated with
416
Distribution of Plant-Dwelling Spiders 417
METHODS
and 1900 h, as made for natural branches. The branches were col-
lected with the same procedure used to sample natural branches, to
allow data comparisons.
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