Evidence in the scientific literature supports the hypothesis that the Pleistocene, the geological epoch which lasted from about 2.5m to 11,700 years ago, spanning the most recent global glaciations, was a time of radical climate change, when very cold and dry periods alternated with very warm and wet periods.
In a study published in the Biological Journal of the Linnean Society, researchers linked to FAPESP’s BIOTA Program show how these climate changes affected the distribution and evolution of 15 bird species endemic to the Atlantic Rainforest biome in Brazil.
Led by Cristina Yumi Miyaki, a professor at the University of São Paulo’s Bioscience Institute (IB-USP), the study was conducted under the aegis of the Thematic Project “Dimensions US-BIOTA São Paulo: a multidisciplinary framework for biodiversity prediction in the Brazilian Atlantic forest hotspot”, supported by FAPESP in Brazil, and by the National Science Foundation (NSF) and National Aeronautics & Space Administration (NASA) in the United States.
“We used modeling techniques to compare the distribution of these 15 species 20,000 years ago, when the Last Glacial Maximum occurred, with their distribution today. The results show that all these birds currently inhabit a smaller area than in the past,” Miyaki toldAgência FAPESP.
The starting-point for the analysis, she explained, was the forest refugia hypothesis put forward by German scientist Jürgen Haffer in 1969 and independently by Brazilian scientist Paulo Vanzolini a few months later. According to this theory, climate change in the Pleistocene altered the distribution of tropical forests such as the Amazon and Atlantic Rainforests. These biomes expanded to the utmost during periods of warm wet weather and dwindled to relatively small fragments in cold periods.
In these isolated fragments, the theory runs, organisms that depended on this type of vegetation were separated from others belonging to the same species and differentiated over time, giving rise to new lineages, populations and even species.
“This is a hypothesis raised to explain why the biodiversity found in tropical rainforests is so much greater than in other biomes,” Miyaki said.
To determine when warm moist periods and cold dry periods occurred, scientists typically use sediment records found in caves or fossilized pollen evidencing the kind of vegetation that existed there in the remote past.
“From all this geological and biological data, including the humidity and temperature levels considered ideal for the occurrence of a given species, we make inferences about its past geographic distribution,” Miyaki said.
Two models were used to simulate species distribution 20,000 years ago: CCSM3 (Community Climate System Model) and MIROC (Model of Interdisciplinary Research on Climate). Current distribution was estimated on the basis of records compiled in previous studies and museum collections.
“The models showed that generally speaking the ranges considered ideal for the survival of these species were larger in the past. This is what we call potential habitat distribution,” Miyaki said.
The next step was an analysis of population genetic structure and diversity based on mitochondrial DNA sequencing for the 15 species included in the study: Sclerurus scansor cearensis (Rufous-breasted leaftosser (Ceara)); Thamnophilus ambiguus (Sooretama slaty antshrike); Sclerurus scansor scansor (Rufous-breasted leaftosser); Synallaxis ruficapilla (Rufous-capped spinetail); Automolus leucophthalmus (White-eyed foliage-gleaner); Xiphorhynchus fuscus (Lesser woodcreeper); Xiphorhynchus atlanticus (Northern lesser woodcreeper); Conopophaga lineata (Rufous gnateater); Conopophaga melanops (Black-cheeked gnateater); Myrmoderus loricata(White-bibbed antbird); Myrmoderus squamosa (Squamate antbird); Pyriglena leucoptera (White-shouldered fire-eye); Schiffornis virescens (Greenish mourner); Tachyphonus coronatus (Ruby-crowned tanager); and Myiothlypis leucoblephara (White-browed warbler).
Miyaki and her team analyzed DNA sequences specifically collected for this and previous studies as well as public gene sequences available from Genbank, a public database maintained by the National Center for Biotechnology Information (NCBI), which is part of the United States National Library of Medicine (NLM).
“Genetic diversity indicates the degree of variability of a gene that exists in the individuals of a given species,” Miyaki said. “It enables us to evaluate whether there are differences between groups found in the southern Atlantic Rainforest and those found further north, for example. Depending on the level of differentiation, we can conclude that significantly differentiated groups are differentiated populations of the same species.”
Next the researchers correlated the changes in potential habitat distribution detected by the models with the results of the genetic analysis in order to understand how changes in range and distribution affect evolutionary processes.
“Generally speaking, the reduction in geographic distribution seems not to have affected the genetic diversity of these species,” Miyaki said. “However, we observed that each species appears to respond slightly differently from the genetic standpoint to climate changes and habitat reduction, so it isn’t possible to build up a single evolutionary history for this entire universe of diverse organisms, even if we focus only on birds.”
For Miyaki, although large-scale compilations like this study are interesting to detect trends, they are no substitute for detailed studies of each species in itself.