A general scenario to evaluate evolution of grassland birds in the Neotropics. Norambuena, H.V. & Van Els, P. IBIS. DOI: 10.1111/ibi.12905 VIEW
When researchers think of the Neotropics, their imagination automatically takes them to dense tropical forests or to the Andean altitudes, while the more experienced ones will also think of the Patagonian glaciers or the cold waters of the Humboldt current. But very few will focus their attention on the grassland systems of this biogeographic region. Although the Neotropical grasslands have not attracted the attention of many ornithologists or evolutionary biologists (except for botanists), they occupy at least 15% of the surface of this region. In this ecosystem there are around 300 species of birds; of these, about 50 species are specialists of this habitat. These specialists include the large Rheas (R. americana and R. pennata), endemic paleognate birds that can measure up to 1.8 m, a large radiation of grass-seed-eating tanagers (Sporophila), several morphologically extraordinary Tyrannidae, as well as the small and subtly-coloured Pipits (Anthus; Fig. 1). The latter colonized the Neotropics shortly after the expansion of grasslands and had their definitive systematics and evolutionary history resolved only recently (Van Els & Norambuena 2018).
Figure 1 Correndera Pipit (Anthus correndera), a specialist grassland bird of the southern Neotropics.
Origin of the Neotropical grasslands
Unlike in the Holarctic region, Neotropical grasslands are not distributed continuously and constitute a mosaic of different types of grasslands in the temperate (pampa and Patagonian steppe), montane (puna and paramo), and (sub)tropical (cerrado, campo, Llanos) zones (Fig. 2). Neotropical grasslands are distributed in a fragmented way around the Amazon rainforest and reach their maximum extension in the pampas system that extends to Patagonia. Neotropical grasslands originated during the Paleogene, C3 cold-adapted grasslands were not widespread until the mid-Cenozoic, and C4 warm climate grasslands expanded much later, during the late Neogene (Strömberg 2011). Grasslands and forests often competed with each other during climatic oscillations, particularly in the Amazon Basin. In spite of their relatively young and dynamic origins, the development of Neotropical grasslands promoted the diversification of several groups of birds.
Figure 2 Neotropical grasslands: A) Sipaliwini grasslands in Surinam, where several morphologically well-defined near-endemic subspecies of widespread Neotropical grassland birds occur (Mittermeier et al. 2010; photo: John C. Mittermeier). B) Extensive wheat fields replacing pampas grasslands in Argentina, modern habitat of the poorly known Pampas Pipit (A. chacoensis; photo: Paul van Els). C) High-elevation Puna grasslands in Chile, habitat for White-winged Diuca-Finch or White-winged Cinclodes (photo: Heraldo V. Norambuena). D) Patagonian grasslands in Chile, home to Lesser Rhea (photo: Heraldo V. Norambuena).
Grassland bird evolution
We present a synthesis of the hypotheses that explain the tempo and mode in which Neotropical grassland birds originated. These hypotheses centre around several ideas. For example, by far the greatest diversity of grassland birds occurs in southern South America, with species occurring north of the Amazon generally being a subset of these. The dynamic nature of the extent of grasslands during climatic cycles means that populations north and south of the Amazon must have been connected at least periodically, but where did they connect? And did dispersal of grassland bird species occurring in both lowland and highland habitats occur primarily from the lowlands to puna and paramo, or vice versa? Although phylogeographic and evolutionary studies are still in an initial state in this group, some patterns can already be distinguished: vicariance/dispersal dynamics seem to be different for grassland than for forest birds. The phylogeographic patterns of Neotropical open-habitat birds are uniform over large areas of continuous habitat, presumably because of their greater capacity for dispersal. However, dispersal may also lead to diversification, such as recently found by investigation of intra-tropical migratory patterns in Tyrannus savana (Gómez-Bahamón et al. 2020). Due to the seasonality of many Neotropical savannas, multiple species may have (thus far, poorly known) intratropical movements, which can lead to diversification between groups with different times of breeding. We hope that this summary of our knowledge on evolution of grassland birds, including gaps to be filled, will motivate the development of new investigations and the discovery of new processes that explain the diversification of this fascinating group of birds.
Gómez-Bahamón, V., Márquez, R., Jahn, A.E., Miyaki, C.Y., Tuero, D.T., Laverde-R, O., Restrepo, S. & Cadena, C.D. 2020. Speciation associated with shifts in migratory behavior in an avian radiation. Current Biology 30: 1312–1321. VIEW
Mittermeier, J.C., Zyskowski, K., Stowe, E.S. & Lai, J.E. 2010. Avifauna of the Sipaliwini savanna (Suriname) with insights into its biogeographic affinities. Bulletin of the Peabody Museum of Natural History 51(1): 97-122. VIEW
Norambuena, H.V. & Van Els, P. 2020. A general scenario to evaluate evolution of grassland birds in the Neotropics. Ibis 29: 317–386. VIEW
Strömberg, C.A. 2011. Evolution of grasses and grassland ecosystems. Annual Review of Earth and Planetary Sciences 39: 517–544. VIEW
Van Els, P. & Norambuena, H.V. 2018. A revision of species limits in Neotropical pipits Anthus based on multilocus genetic and vocal data. Ibis 160: 158–172. VIEW
Top right: Correndera Pipit Anthus correndera © Heraldo V. Norambuena