Springs and wire plants: anachronistic defences against Madagascar's extinct elephant birds

Abstract

The extinction of large vertebrates in the last few millennia has left a legacy of evolutionary anachronisms. Among these are plant structural defences that persist long after the extinction of the browsers. A peculiar, and controversial, example is a suite of traits common in divaricate (wide-angled branching) plants from New Zealand. Divaricate architecture has been interpreted as an adaptive response to cold climates or an anachronistic defence against the extinct moas. Madagascar, a larger tropical island, also had a fauna of large flightless birds, the elephant birds. If these extinct ratites selected for similar plant defences, we expected to find convergent features between New Zealand and Malagasy plants, despite their very different climates. We searched the southern thickets of Madagascar for plants with putative anti-ratite defences and scored candidate species for a number of traits common to many New Zealand divaricates. We found many Malagasy species in 25 families and 36 genera shared the same suite of traits, the 'wire plant' syndrome, as divaricates in New Zealand that resist ratite browsing. Neither ecologically, nor phylogenetically, matched species from South Africa shared these traits. Malagasy wire plants differ from many New Zealand divaricates in lacking the distinctive concentration of leaves in the interior of shrubs. We suggest that New Zealand divaricates have a unique amalgam of traits that acted as defences and also confer tolerance to cold. We conclude that many woody species in the thickets of southern Madagascar share, with New Zealand, anachronistic structural defences against large extinct bird browsers.

Figure 1

Examples of Malagasy wire plants and spines. (a) Acacia delicatita with zigzag branching and very reduced spines, (b) Croton sp. resembling New Zealand divaricate with thin tangled branches, (c) heteroblastic Terminalia with, left, adult branch (>3 m), right, juvenile branch (<2 m) and (d) heteroblastic Alluaudia humbertii with, left, adult branch (>3 m), right, juvenile (<2 m).

Figure 2

Comparison of traits in juvenile (shaded) and adult branches of Poupartia minor, Terminalia seyringi, Operculicarya decaryi and A. humbertii. (a) leaf area, log mm², (b) ratio of thin (less than 3 mm in diameter) branches to total branch length in a 50 cm sample shoot, (c) branch angle and (d) breaking load–tensile force required to break off a sample shoot. A maximum force of 10 kg was applied. Error bars are standard deviations from the mean.