Is optimal foraging a realistic expectation in orb-web spiders?

Edwards, Will, Whytlaw, Poppy, Congdon, Bradley C., and Gaskett, Chris (2009) Is optimal foraging a realistic expectation in orb-web spiders? Ecological Entomology, 34 (4). pp. 527-534.

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1. Explanations for web relocation invoking optimal foraging require reliable differentiation between individual sites and overall habitat quality. We characterised natural conditions of resource variability over 20 days in artificial webs of the orb-web spider Gasteracantha fornicata to examine this requirement.

2. Variability in catch success was high. Day-to-day catch success in 90% (18/20) catch sites fitted negative binomial distributions, whereas 10% fitted Poisson distributions. Considered across trap sites (overall habitat), variance in catch success increased proportionally faster than the mean (i.e. Taylor's Power Law, variance = 0.54mean1.764).

3. We compared the confidence intervals for the expected cumulative catch in randomly drawn sequential samples from a frequency distribution representing the overall habitat (based on the parameters for Taylor's power law) and the frequency distribution of expected cumulative catch within each individual catch site [via randomisation based on the mean and negative binomial exponent (k)].

4. In all cases and across all sample sizes, median values for the power to differentiate habitat and catch sites never exceeded 0.2, suggesting that principles involved in optimal foraging, if operating, must be accompanied by a very high degree of uncertainty.

5. Under conditions of high resource variability, many days must be spent in a single catch site if movement decisions are based on an ability to differentiate current catch site from overall habitat. Empirical evidence suggests this is never met. This may explain why proximal mechanisms that illicit quickly resolved behavioural responses have been more successful in describing web relocation patterns than those associated with optimal foraging.

Item ID: 5494
Item Type: Article (Research - C1)
ISSN: 1365-2311
Keywords: Gasteracantha, negative binomial, optimal foraging, Taylor’s Power Law, web residency, tropical biology
Date Deposited: 04 Nov 2009 00:02
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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