1.4. Secondary and cumulative effects

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Last update: June 2023

Understanding the primary effects of infrastructure and transportation on biodiversity is essential for the implementation of adequate mitigation. However, it is equally important to maintain a wide perspective and see these impacts in their broader context. Small, incremental effects may merge to a more significant but unintended impact on nature or trigger a cascade of effects that lead to a much broader change than what may be expected initially.

Secondary effects refer to the unintended consequences that result from the interaction among primary effects, and the interplay with environmental conditions and other driving factors at a landscape or regional level. They typically relate to larger areas and longer time frames than primary effects and may refer to a cascading chain reaction of events that occur in response to a primary event. For example, if a road is fenced to prevent wildlife-vehicle accidents, the primary effect and objective is increased traffic safety. The consequent barrier effect on wildlife may be an unintended secondary effect in this respect. Increased barrier effects may lead to lowered fitness in individuals that are cut off from important resources, a consequent reduced reproductivity in the populations and eventually a decline in local populations.

When the impact gradually builds up over time, from, e.g., incremental infrastructure development and/or urban sprawl, or from repeated or sustained exposure to a particular activity or event, the effects are described as cumulative. For example, if a bird community is repeatedly exposed to high traffic noise over a long time that reduces mating success by masking vocalisations of birds, the cumulative effect could be a decrease in the most sensitive birds and a loss of biodiversity. Cumulative effects also appear when different infrastructures augment each other’s impact. For example, when roads and railroads are bundled, their combined barrier effect is multiplied and not only added.

To avoid or at least minimise the risk of unintended large-scale impacts, planners must consider secondary, cascading effects, and the accumulation of, and interaction between impacts. These broad- scale effects can be rather obscured and may need a long time to manifest, but when they become visible, their mitigation is significantly more demanding and will require extended collaboration among multiple stakeholders. In addition, as these effects unfold, their impact may not have a direct correlation with the pressure. Instead, there will be thresholds to how much impact a species or community can accommodate before conditions reach a tipping point and change suddenly, comprehensively, and, in some cases, irreversibly. Mitigating such complex effects requires broad-scaled approaches and frameworks that can trigger local actions. ‘Thinking globally and acting locally’ must be the guiding principle for environmental and social movements and it very much applies to mainstreaming biodiversity and transportation.

For example, the European Green Deal recognises the importance of addressing complex effects and finding mitigation approaches that jointly promote sustainable development. The EU’s first ever Nature Restoration Law aims to restore degraded ecosystems and increase biodiversity by 2030. The law includes a target to restore at least 25,000 km of rivers to a free-flowing state and to plant 3 billion trees across the EU by 2030. Similarly, the EU’s biodiversity strategy for 2030 aims at increasing the coverage and quality of protected areas, improving the management of Natura 2000 sites, restoring degraded ecosystems, and integrating biodiversity into other policies. These policy tools provide strong incentives for the transport sector to integrate biodiversity into plans, strategies, and infrastructure design.


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