| 1. |
- Birkhofer, Klaus, et al.
(författare)
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Effects of extreme climatic events on small-scale spatial patterns: a 20-year study of the distribution of a desert spider.
- 2012
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Ingår i: Oecologia. - : Springer Science and Business Media LLC. - 1432-1939 .- 0029-8549. ; 170:3, s. 651-657
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Tidskriftsartikel (refereegranskat)abstract
- Individuals of most animal species are non-randomly distributed in space. Extreme climatic events are often ignored as potential drivers of distribution patterns, and the role of such events is difficult to assess. Seothyra henscheli (Araneae, Eresidae) is a sedentary spider found in the Namib dunes in Namibia. The spider constructs a sticky-edged silk web on the sand surface, connected to a vertical, silk-lined burrow. Above-ground web structures can be damaged by strong winds or heavy rainfall, and during dispersal spiders are susceptible to environmental extremes. Locations of burrows were mapped in three field sites in 16 out of 20 years from 1987 to 2007, and these grid-based data were used to identify the relationship between spatial patterns, climatic extremes and sampling year. According to Morisita's index, individuals had an aggregated distribution in most years and field sites, and Geary's C suggests clustering up to scales of 2 m. Individuals were more aggregated in years with high maximum wind speed and low annual precipitation. Our results suggest that clustering is a temporally stable property of populations that holds even under fluctuating burrow densities. Climatic extremes, however, affect the intensity of clustering behaviour: individuals seem to be better protected in field sites with many conspecific neighbours. We suggest that burrow-site selection is driven at least partly by conspecific cuing, and this behaviour may protect populations from collapse during extreme climatic events.
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| 2. |
- Birkhofer, Klaus, et al.
(författare)
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Spatial-pattern analysis in a territorial spider: evidence for multi-scale effects
- 2006
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Ingår i: Ecography. - : Wiley. - 1600-0587 .- 0906-7590. ; 29:5, s. 641-648
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Tidskriftsartikel (refereegranskat)abstract
- Territorial animals maintain a certain distance to neighbouring conspecifics, presumably leading to a regular spatial pattern through social spacing. Nevertheless, most animal populations are assumed to show aggregation at certain distance ranges, reflecting the scale dependency of spatial patterns. Leucorchestris arenicola (Araneae: Sparassidae) is a burrow-living spider species that shows territorial behaviour against conspecifics. A multi-scale approach in spatial analysis revealed that territory owners had fewer neighbours than expected under spatial randomness at distances up to 6 m. Behavioural field experiments showed that territory owners were able to perceive and react to burrow constructing neighbours up to at least 4 m distance from their own burrow. At larger distances individuals were often more aggregated than expected under spatial randomness. Analysing adult and immature relationships showed attraction between different development stages at small distances and avoidance at larger distances. The analysis reveals diverse spatial patterns in a territorial and cannibalistic species, showing that both behaviour and environment affect pattern development at different distances. The study outlines the importance of multi-scale approaches for spatial analysis and the need for accompanying experiments to facilitate the interpretation of results.
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| 3. |
- Norgaard, Thomas, et al.
(författare)
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Vision in the nocturnal wandering spider Leucorchestris arenicola (Araneae : Sparassidae)
- 2008
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Ingår i: Journal of Experimental Biology. - : The Company of Biologists. - 1477-9145 .- 0022-0949. ; 211:5, s. 816-823
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Tidskriftsartikel (refereegranskat)abstract
- At night the Namib Desert spider Leucorchestris arenicola performs long-distance homing across its sand dune habitat. By disabling all or pairs of the spiders' eight eyes we found that homing ability was severely reduced when vision was fully abolished. Vision, therefore, seems to play a key role in the nocturnal navigational performances of L. arenicola. After excluding two or three pairs of eyes, the spiders were found to be able to navigate successfully using only their lateral eyes or only their anterior median eyes. Measurement of the eyes' visual fields showed that the secondary eyes combined have a near full (panoramic) view of the surroundings. The visual fields of the principal eyes overlap almost completely with those of the anterior lateral eyes. Electroretinogram recordings indicate that each eye type contains a single photopigment with sensitivity peaking at similar to 525 nm in the posterior and anteriomedian eyes, and at similar to 540 nm in the anteriolateral eyes. Theoretical calculations of photon catches showed that the eyes are likely to employ a combination of spatial and temporal pooling in order to function at night. Under starlit conditions, the raw spatial and temporal resolution of the eyes is insufficient for detecting any visual information on structures in the landscape, and bright stars would be the only objects visible to the spiders. However, by summation in space and time, the spiders can rescue enough vision to detect coarse landscape structures. We show that L. arenicola spiders are likely to be using temporal summation to navigate at night.
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