Beyond BACI: Experimental designs for detecting human environmental impacts on temporal variations in natural populations

Abstract

Biological effects of environmental impacts are usually defined simplistically in terms of changes in the mean of some biological variable. Many types of impact do not necessarily change long-run mean abundances. Here, designs for detection of environmental impact are reviewed and some of their shortcomings noted. New sampling designs to detect impacts that cause changes in temporal variance in abundance of populations, rather than their means, are described. These designs are effective at distinguishing pulse and press episodes of disturbance and could be used for other variables of interest (size, reproductive state, rate of growth, number of species, etc.) for monitoring. The designs require sampling different time-scales before and after a proposed development that might cause impact. Cases are discussed in which there is a single control location. Inadequacies of this approach for detection of environmental impact are mentioned, with some discussion of the consequences for management of impacts that cause temporal change rather than alterations of the mean abundance of a population.

Effects of sewage pollution on the structure of rocky shore macroinvertebrate assemblages

“…The model consisted of two factors: Time (4 levels, random, orthogonal) and Location (1 impacted and 2 reference areas, fixed, orthogonal). In both cases, the design was asymmetrical (Underwood, 1991) due to the presence of a single impacted location. Therefore, the location term, and all terms involving location, was partitioned into two portions: the 1-degree-of-freedom contrast of Imp-v-Rs and the variability between reference locations (Rs).…”

Sewage pollution impact on Mediterranean rocky-reef fish assemblages

“…Unfortunately, "before data" are seldom available so that ACI (AfterControl/Impact) designs and alternative analytical procedures are often applied to carry out postimpact investigations (Underwood, 1991). Here, the presence of two sewage outfalls discharging untreated waters allowed to use a balanced symmetrical ACI design that was hierarchical and spatially replicated.…”

Influence of a breakwater on nearby rocky intertidal community structure

“…In order to examine the effects of the breakwater on the adjacent algal and animal assemblages, a beyond-BACI design (see Underwood, 1991Underwood, , 1992Underwood, , 1993Underwood, , 1994 for revision) was not possible due to the lack of a priori data. However, beyond-BACI designs can be modified and applied to situations where no before data exist and have been widely used (e.g.…”