Indirect impacts are impacts that are not a direct result of the strategic action, but occur away from the original source of impact or as a result of a complex pathway. Indirect impacts are often called secondary, tertiary etc. impacts, depending on how many steps there are between the original source and its impact. Examples of indirect impacts are a housing development resulting from a local land use plan that changes a water table and thus affects the ecology of a nearby wetland; and a European energy policy that promotes nuclear power and thus increases mining (and associated impacts) in Africa.
A subset of indirect impacts is generated impacts: where one type or phase of development attracts or facilitates another. Examples are:
- a new road which releases suppressed demand for travel, so that overall traffic levels post-road are higher than pre-road;
- new transport links to a remote area which trigger new housing and employment development. The whole purpose of improving access to remote areas is often to trigger these kinds of beneficial generated socio-economic impacts;
- one new technology (e.g. digital boxes) which triggers the purchase or development of another technology (e.g. "let's upgrade our television set while we're at it").
A straightforward technique for identifying indirect impacts is a causal chain diagram (or causal network diagram). The example below shows some of the indirect impacts on wildlife that could arise from a new housing development. They include secondary impacts (housing - land take - wildlife); tertiary impacts (housing - infrastructure - land take - wildlife); quaternary impacts (housing - infrastructure - construction traffic - disturbance - wildlife); and a few fifth and sixth order impacts.
This kind of diagram can suggest mitigation measures: mitigation of indirect impacts essentially involves breaking the link between one or more of the arrows, for instance:
- Can the demand for new (electricity, gas, water etc.) infrastructure be reduced, e.g. by producing/collecting more resources at the site itself, e.g. rainwater collectors, solar panels etc.?
- Can construction traffic be reduced, e.g. by recycling existing materials or minimising the waste that needs to be carried out?
- Can the impact of land take on wildlife be minimised, e.g. by building on agricultural/monoculture fields rather than more biodiverse areas?
A causal chain diagram could also suggest ways of facilitating positive impacts, by reducing the number of links between the original action and the final impact, and maximising the likelihood that the links will occur; and reducing the likelihood of negative impacts, by making the chain long and tenuous, with links that are unlikely to occur.