Although interaction effects are frequently found in experimental studies, field researchers report considerable difficulty in finding theorized moderator effects. Previous discussions of this discrepancy have considered responsible factors including differences in measurement error and use of nonlinear scales. In this article we demonstrate that the differential efficiency of experimental and field tests of interactions is also attributable to the differential residual variances of such interactions once the component main effects have been partialed out. We derive an expression for this residual variance in terms of the joint distribution of the component variables and explore how properties of the distribution affect the efficiency of tests of moderator effects. We show that tests of interactions in field studies will often have less than 20% of the efficiency of optimal experimental tests, and we discuss implications for the design of field studies.

Statistical difficulties of detecting interactions and moderator effects

I examined the effects of habitat fragmentation on the distribution and abundance of mamma- lian carnivores in coastal southern California and tested the prediction that responses to fragmentation var- ied with the body size of carnivore species. I conducted track surveys for nine native and two exotic carnivore species in 29 urban habitat fragments and 10 control sites. Fragment area and isolation were the two stron- gest landscape descriptors of predator distribution and abundance. Six species were sensitive to fragmenta- tion, generally disappearing as habitat patches became smaller and more isolated; three species were en- hanced by fragmentation, with increased abundance in highly fragmented sites; and two species were tolerant of fragmentation, with little to no effect of landscape variables on their distribution and abundance. Within urban habitat fragments, the carnivore visitation rate increased at sites with more exotic cover and closer to the urban edge, a pattern driven largely by the increased abundance of fragmentation-enhanced carnivores at edge sites. Finally, body size, in conjunction with other ecological characteristics, partially ac- counted for the heterogeneity in responses to fragmentation among carnivore species. These differential sensi- tivities are useful criteria for choosing appropriate focal species for ecological research and conservation planning, a choice that depends on the scale of fragmentation in a region and the commensurate responses of carnivore populations at that scale.

http://www.edc.uri.edu/nrs/classes/nrs534/NRS_534_readings/Crooks_2002.pdf

Relative Sensitivities of Mammalian Carnivores to Habitat Fragmentation

Observations in ecology: you can't make progress on processes without understanding the patterns.

Two recent letters to the editor of The Plant Cell ([Gutierrez et al, 2008][1]; [Udvardi et al, 2008][2]) highlighted the importance of following correct experimental protocol in quantitative RT-PCR (qRT-PCR) In these letters, the authors outlined measures to allow precise estimation of gene

Real-time quantitative RT-PCR: design, calculations, and statistics.

A food web graphically represents the paths of nutrients and energy through the living components of an ecosystem and the context in which individuals exploit their prey and avoid their enemies. Temporary ponds are excellent arenas for the study of food webs because they are discrete communities that can be mimicked in containers that approach the realism of natural habitats. Artificial ponds permit repeatable initial conditions and sufficient replication of independent experimental units in complex experiments to test hypotheses about the control of structure and function in natural communities. I used a combination of observations of natural ponds, “experimental natural history” of artificial ponds in my study area, and controlled experiments in an array of 144 replicate ponds to develop, then test, hypotheses about how the structures of food webs are regulated. Understanding food webs begins with population biology. Amphibians use the aquatic larval stage of their biphasic life cycle to exploit ephemer...

https://www.esa.org/history/Awards/papers/Wilbur_HM_MA.pdf

Experimental ecology of food webs: complex systems in temporary ponds

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D. R. Causton

Papers

The Design of Experiments: Statistical Principles for Practical Applications.