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Conservation Service
Cleaning up trash along waterways (trash in parks, etc, does not count unless it has a significant potential of ending up in the rivers and oceans)
planting trees
removing invasive plant species
installing erosion control devices
working at a native wildlife rehabilitation facility (e.g. Owl's Nest)
All service is great, but it is best to ask your Scoutmaster beforehand if you are not sure if a project satisfies a conservation requirement.
Conservation service does not have to be conducted with the troop, however you are free to coordinate an event with your fellow scouts if you would like. The following sites have regularly scheduled events that may satisfy the conservation requirement (always ask your Scoutmaster if you are unsure).