Contemporizing island biogeography theory with anthropogenic drivers of species richness

Abstract

Aim: Island biogeography theory states that species richness increases with habitat diversity and decreases with isolation from source pools. However, ecological theory must incorporate effects of human activity to explain contemporary patterns of biodiversity. We contemporized island biogeography theory by conceptualizing island trajectories of how species richness changes over time with accelerating land development and economic trade, which increase extinction and immigration rates, respectively. With this contemporized theory, we then articulate and empirically assess expected relationships of native, introduced and total species richness with natural and anthropogenic metrics of habitat diversity and isolation from source pools. Location: Greater Caribbean region.Time period: Database finalized in 2020. Methods:We built a database of 1,042 native and introduced reptiles and amphibians (herps) for 840 Caribbean islands. For each island, we calculated natural and anthropogenic metrics of island habitat diversity and isolation from source pools and used linear model averaging to assess the expected relationships under the contemporized theory for 15 major herp clades.

The great acceleration of island saturation by species introductions in the Anthropocene has altered species-area relationships

“…Thus, the more linear SARs of the total species assemblages (i.e. the assemblages that contain 240 introduced and native species) is likely from a reduction in the influence of isolation and 241 increased immigration from the human-mediated introduction of species 14,19 . Theory suggests 242 that isolation should reduce the rate of immigration to banks 1 causing the slow process of in situ 243 speciation to be the main driver of species accumulation on isolated islands.…”

“…This is because native species will have little time to adapt to the increasing pressures of novel competitors and predators as well as having less habitat to sustain large populations due to the conversion of natural land cover for anthropogenic purposes. While the land development can initially increase habitat diversity by opening new habitat into which introduced species can immigrate 27 , as development continues, habitat diversity is likely to decrease leading to reductions in species richness 19 . Therefore, the relationship between area and habitat diversity may become decoupled as human activity increases on islands, which can further alter the area-defined species richness equilibrium of islands and influence the shape and strength of SARs.…”

“…The island groups included in the greater Caribbean region are the Greater and Lesser Antilles, Bahamas, Turks and Caicos, Southern Antilles, Trinidad and Tobago, Cayman Islands, and the Mexican, Belizean, Honduran, Columbian, and Nicaraguan Islands as defined by Hedges et al 31 . In addition, we included Bermuda as in Helmus et al 14 due to its high level of trade with the Caribbean and 75% of its reptile and amphibian species also occur in the Caribbean 19 (see also 32 ). These islands were then grouped into island banks (referred to as islands in the main text) based on Powell and Henderson 33 and the underwater topography (see Supplementary Information Fig.…”

“…The two most specious genera of Caribbean terrestrial vertebrates are Anolis lizards and Eleutherodactylus frogs 44 . We used a dataset of island-level herp records compiled from an extensive literature search to determine the native, introduced, and total (native + introduced) species richness for each island bank 19 that was updated with new information.…”

“…We primarily used Powell and Henderson 33 to obtain lists of the species for most of the islands in the greater Caribbean region but needed to add records of species from islands not included in this publication. For these other islands, we obtained species records from field guides and books as well as and extensive literature search (see 19 ). For introduced species we used Hailey, Wilson and Horrocks 45,46 and added to that introduced species records found through literature searches on Google Scholar, Web of Science, and natural history and occurrence notes of species checklists and Caribbean, herp, and invasion ecology journals.…”