Spatial patterns of social spider colonies in Namibia reflect habitat features and dispersal modes

Abstract

Colonies of web building social spiders may persist at a site for several generations; therefore, their placement in the habitat is critical for survival. This study focuses on the development of spatial distribution patterns by means of different dispersal modes in a social spider species (Stegodyphus dumicola Pocock, 1898, Eresidae) in central Namibia. Social spiders disperse varying distances from the parent colony by ballooning or short distances by budding. Using spatial point pattern analyses we aimed to identify the dispersal pattern from source colonies and what drivers affect the resulting distribution patterns. The distribution of suitable vegetation constrained pattern development in the three study plots with mapped colonies. In a plot with isolated large trees, colonies were only aggregated over short distances and the average cluster size was small. First generation colonies were established within the same clusters by solitary ballooning females. In the two study plots with more evenly distributed shrubs, clusters were generally larger and new colonies were formed mainly by budding. In these plots, newly established colonies and older source colonies were either associated in suitable habitat patches or segregated from each other. Possible explanations include high mortality of either older colonies and replacement by newly established colonies or of dispersers in the vicinity of established colonies, or selection of “empty” sites by dispersing individuals based on cues from conspecific colonies. In conclusion, both the distribution of the vegetation and the mode of dispersal may explain the development of spatial patterns in S. dumicola. Our results highlight the importance of spatial pattern analyses for inferring underlying causes of distribution of sedentary organisms.

Keywords:

• Araneae
• ballooning
• dispersal
• habitat heterogeneity
• pair correlation function
• point pattern analyses
• spatial association

ACKNOWLEDGEMENTS

We thank two anonymous referees and the associate editor for helpful comments on a previous version of the manuscript. The Ministry of Environment and Tourism granted permissions for this project. We thank the following students and colleagues for assistance in the fields: Chris Bird, Kerstin Birkhofer, Karin S. Coates, Nicholas Krone and Nadja Schilling. We further thank the families Schmidt (Rooisand) and Hammond (Oupembamewa) for permission to work at their farms. Dr Joh Henschel recommended potential study regions and his assistance greatly contributed to the success of this project. This is publication No. 845 of the Mitrani Department of Desert Ecology.