In many large ecosystems, conservation projects are selected by a diverse set of actors operating independently and allocating resources at different temporal and spatial scales. Though small-scale decision-making can leverage local expert knowledge, it also may be an inefficient means of achieving ecosystem-scale objectives. We investigated the value of coordinating restoration efforts in both space and time to maximize the restoration of aquatic ecosystem connectivity in the tributaries of the Laurentian Great Lakes. Tributaries to the Great Lakes are highly fragmented by tens of thousands of dams and road crossings that act as potential barriers to migratory fishes. We developed a mathematical optimization model that identifies portfolios of barrier removals which offer the greatest increase in upstream spawning habitat access for a given budget. We then parameterized this model using a recently developed database of the locations and estimated removal costs of hundreds of thousands of dams and road crossings. We used a return-on-investment framework to analyze potential efficiency gains from coordinating barrier removals at a range of spatial scales (county, tributary, state, lake, nation, and the entire basin) and temporal scales (a single “pulse” of investment vs. the same amount allocated as a series of two, five, or ten “trickle” investments).
We found that coordinating barrier removals across the entire basin is nine times more efficient at reconnecting fish to headwater breeding grounds than optimizing independently for each watershed. Similarly, a one-time pulse of restoration investment is up to ten times more efficient than annual allocations totaling the same amount. Both dams and road culverts are essential projects in optimal portfolios, highlighting the fact that improving road culvert passability is also essential for efficiently restoring connectivity to the Great Lakes. Regarding the efficiency of barrier removals overall, we find that the amount of accessible habitat in the basin could be doubled for less than $100 million, an amount that is relatively small compared to ongoing investments of $1.6 billion via the Great Lakes Restoration Initiative. Our results highlight the dramatic economic and ecological advantages of coordinating efforts in both space and time during restoration of large ecosystems.