Succulent plants of Africa and America: Do these lookalikes live alike?

There are numerous examples in nature of distantly-related organisms converging on similar shapes that have proved useful to each. This convergent evolution can generate strikingly similar but independently evolved forms such as the streamlined bodies of dolphins and ichthyosaurs (a group of extinct marine reptiles); the wing shapes of birds and bats and the similar body shapes of the placental wolf and the thylacine (a recently extinct wolf-like marsupial). Such similarities have long been assumed to result from exposure to similar environmental conditions and selection pressures.

Old World and New World Succulents.
They look alike, but are they alike? Photos: Cathy Shields.

A recent AoB paper considered the similarities between the habitats of American and African succulent plants. The spurges, milkweeds and iceplants of Africa and the ancestrally distant cacti of America are outwardly very similar in appearance, and these resemblances have been explained by similarities in local climate. This study aimed to quantify the environmental spaces in which the two groups of plants exist, and therefore show whether similarity in form is indeed a corollary of similarity in habitat.

The selected study sites were hotspots of succulent abundance and diversity on each continent. In these dry, warm areas, storage of water and prevention of water loss are priorities for plants and so spherical or globular growth forms have evolved in each group. The authors analysed local climate data and employed GIS and niche equivalence modelling to compare the American and African succulent hotspot sites. What they found were surprisingly many differences in variables such as rainfall and temperature between the sites, and these differences outnumbered any similarities.

The authors concluded that the resemblance between the succulents on each continent may be explained by factors that were not included in their climate analyses, such as soil type, distance from the sea, and possibly important contributions of water to the plants by fog and dew. They also point out that the “similarities” (to the human eye) between these groups may be quite subjective, and that more robust measurements of similarity may be required when claiming convergence of growth forms.

Original Paper:
Alvarado-Cárdenas, L. O., Martínez-Meyer, E., Feria, T. P., Eguiarte, L. E., Hernández, H. M., Midgley, G., & Olson, M. E. (2013). “To converge or not to converge in environmental space: testing for similar environments between analogous succulent plants of North America and Africa”. Annals of Botany, 111(6), 1125-1138. DOI:10.1093/aob/mct078