Botanical gardens as networks for “chaperoned” assisted migration of the world’s flora
Assisted migration (AM), the purposeful introduction of lagging species to areas that are newly favorable as a result of climate change, is controversial because it a) has the potential to cause invasions by diseases and pests or by the introduced species itself; b) is currently practiced on an ad hoc basis; c) ignores the difficulties and failures inherent in reintroduction projects; and d) would have to be done repeatedly as species’ favored climates pass over areas into which they are introduced. Here we propose an institutional framework for “chaperoned” AM of plants, in which species are introduced from natural sources to semi-natural settings at botanical gardens. Botanical gardens offer the potential for monitoring of diseases and invasibility and can artificially ameliorate climate to ensure reintroduction success through, for example, irrigation and shading. A network of gardens, if located appropriately, could serve as “stepping stones” with each sequentially a “receiver” and “donor” from/to other gardens as climate changes. We analyze the current locations of the world’s 3106 botanical gardens to determine where, climatically and geographically, there would be gaps in the network of gardens capable of exchanging species under a chaperoned AM scheme.
The coverage of botanical gardens varies by geographic location and climatic zone. Across continents, Africa has the lowest density of botanical gardens (131 total, or ~0.004 / 1000 km2) and Europe the greatest (842 total, or 0.08 / 1000 km2). Across climatic zones, the coldest “hardiness” and “heat” zones have the least representation in every biogeographic province, meaning that chaperoned AM could lead to an accumulation of species in the most poleward gardens unless further investments are made in establishing gardens as high latitudes become warmer. However, the potential intensity of translocations at each garden varies as a function of the diversity of the region from which it could translocate, making gardens located in warmer areas (but not the hottest areas) generally most important for maintaining diversity. Islands have few gardens relative to their richness, geographic spread, and area, suggesting that they must be paired with like climates on mainland areas to conserve their flora. Currently only 20% of the world’s botanical gardens belong to Botanical Gardens Conservation International, the world’s largest framework for conservation at botanical gardens. Hence, effort must concentrate not only on establishment of new gardens but also of infrastructure to support coordination of conservation activities.