Tharina Bird

Colorado State University, Fort Collins, Colorado, United States

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Publications (2)3.48 Total impact

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    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.
    08/2014; 69(3):157-163. DOI:10.1080/0035919X.2014.941044
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    ABSTRACT: The evolution of cooperation requires benefits of group living to exceed costs. Hence, some components of fitness are expected to increase with increasing group size, whereas others may decrease because of competition among group members. The social spiders provide an excellent system to investigate the costs and benefits of group living: they occur in groups of various sizes and individuals are relatively short-lived, therefore life history traits and Lifetime Reproductive Success (LRS) can be estimated as a function of group size. Sociality in spiders has originated repeatedly in phylogenetically distant families and appears to be accompanied by a transition to a system of continuous intra-colony mating and extreme inbreeding. The benefits of group living in such systems should therefore be substantial. We investigated the effect of group size on fitness components of reproduction and survival in the social spider Stegodyphus dumicola in two populations in Namibia. In both populations, the major benefit of group living was improved survival of colonies and late-instar juveniles with increasing colony size. By contrast, female fecundity, female body size and early juvenile survival decreased with increasing group size. Mean individual fitness, estimated as LRS and calculated from five components of reproduction and survival, was maximized for intermediate- to large-sized colonies. Group living in these spiders thus entails a net reproductive cost, presumably because of an increase in intra-colony competition with group size. This cost is traded off against survival benefits at the colony level, which appear to be the major factor favouring group living. In the field, many colonies occur at smaller size than expected from the fitness curve, suggesting ecological or life history constraints on colony persistence which results in a transient population of relatively small colonies.
    Journal of Evolutionary Biology 12/2007; 20(6):2412-26. DOI:10.1111/j.1420-9101.2007.01407.x · 3.48 Impact Factor