The Scope Effect in Multiple Species Valuation

Scope insensitivity in stated preference studies has been widely debated in empirical literature with mixed evidence. We investigate this in the context of threatened species conservation across Australia by conducting three discrete choice experiments. The choice experiments followed the same structure where species, used as attributes, were shown in the same order, but the designs varied in the number of species presented as options to be conserved: three, five or eight. For three and five species cases, an explicit partial-profile design was used. Using data collected through online surveys, we use a mixed-logit model in willingness-to-pay (WTP) space to estimate WTP for reducing each species’ extinction risk. WTP to conserve a species differs with the number of species presented. We observed a clear pattern: the higher the number of species in a design, the lower the people’s WTP for a species. This observation is reinforced when comparing the results to a related study using the same species but a single-species design. Results suggest that we failed to find evidence of the ‘scope’ effect. Therefore, policies to conserve threatened species need to be holistic and consider all threatened species collectively as WTP estimates reported from single-species-based studies may not be additive