Why biodiversity changes from place to place

Life on earth is so diverse, in large part because individual species are restricted to particular areas. If populations were to expand unfettered across continents, communities everywhere would contain the same species. But they don’t. Why communities vary is one of biology’s ‘big questions’, and UNM researchers have proposed new answers in a groundbreaking study published in the Proceedings of the National Academy of Sciences.

Work toward the new study began in 2011 when the big question about community variation was on the minds of Sabrina McNew, then a lab technician, and Christopher Witt, Curator of Birds at the Museum of Southwestern Biology and a professor in the Department of Biology. McNew and Witt were studying high-altitude birds in the Andes Mountains, trying to understand why the geographic ranges of bird species were so often small.

McNew, a graduate of Albuquerque’s Valley High School, was preparing for graduate school in parasitology. At the time, new evidence was emerging that avian malaria was an exceptionally diverse and common parasite-borne disease, even though most of its species were still unknown to science. Just as human malaria is a deadly disease that has shaped human history, avian malaria might profoundly affect birds.

“We started to think, ‘what if the ranges of bird species cannot expand because they might encounter dangerous, unfamiliar strains of avian malaria?’ And that started us down this path,” said Witt.

Peru, at the nexus of the Andes and the Amazon, is an ideal place to study why communities change from place to place because it is a premier hotspot of biodiversity, containing tremendously varied habitats and climates. McNew, Witt, and their assembled team surveyed birds and their malaria parasites at 18 locations in Peru — up, down, and around the mountains, and in rainforests and deserts. The team used DNA-based screening to uncover 1700 unique infections representing 400 species of avian malaria parasites.

“We started to think, ‘what if the ranges of bird species cannot expand because they might encounter dangerous, unfamiliar strains of avian malaria?’ And that started us down this path.” – Christopher Witt, professor, Department of Biology

The new study decisively answered the big question, but the answers were not exactly as McNew and Witt had predicted. They found that variation in rainfall strongly influenced how communities change from place to place. And although shifts in composition of the avian malaria communities barely affected the bird communities, the reverse was true — the bird community strongly affected the parasite community.

“Our analyses showed that the parasites are following their hosts, but they’re not driving the avian biogeographic patterns,” said McNew. “Rainfall was the main driver of bird community variation, with smaller contributions from temperature, elevation, and net primary productivity, [a measure of the energy production of plants]. Contrary to what we thought we would find, dispersal barriers and avian malaria had minor effects, at most.”

Several exciting results emerged from the McNew et al. analyses. For example, the number of bird species that occur in any given place (‘species richness’) was predicted by net primary productivity and elevation alone, with about 80% accuracy.

“This is a very useful model,” said Witt. “I believe it provides the best insight that we have about why the number of species varies among tropical bird communities, and it’s helpful that we did this work in Peru, where there is exceptional variation among bird communities, even among those that are in close proximity.”


Andean Cock-of-the-Rock, one of the Andean bird species screened for avian malaria in the new study by McNew et al. (photo by C. Jonathan Schmitt, UNM Class of 2012)

But different forces underlie species richness for the avian malaria parasites. McNew et al. found that the number of parasite species at any given site was predicted only by the number of bird species surveyed. “Again, the parasites seem to follow the birds,” said Witt, “and we can predict that, as more studies are conducted, the western Amazon basin and adjacent Andean forests will emerge as the global hotspot for avian parasite diversity, consistent with the dizzying variety of avian malaria that we have found so far.”

Between the start of the project and the new publication, McNew obtained her Ph.D. at the University of Utah, and she is currently a postdoctoral fellow at the Cornell Laboratory of Ornithology at Cornell University. During her training, she became skilled in statistical and spatial modeling, skills that allowed her to take the project beyond its original goals. For example, she used the models of bird and parasite diversity to create high-resolution maps of species richness and species turnover across Peru. These maps, made freely available along with the publication of the paper, provide rich information to guide the conservation of biodiversity. They show not only where many species occur together, but where strikingly different communities come into proximity and, accordingly, are expected to be of high conservation value.

UNM’s Museum of Southwestern Biology started its field research program in Peru in 2006, under Witt’s direction, and in collaboration with the Centro de Ornitología y Biodiversidad del Perú (CORBIDI), a Peruvian non-governmental organization dedicated to biodiversity science and conservation. The Field Museum of Natural History in Chicago also had conducted field research with CORBIDI in Peru, often cooperating with UNM on training students and planning expeditions. Researchers from the three institutions traveled to remote parts of Peru for more than a decade to compile the samples used in this study. They often used mules or horses to reach remote camps while carrying bulky liquid-nitrogen tanks that were needed to preserve high-quality frozen samples. “The physical challenges associated with this kind of fieldwork are daunting,” said Witt, “and the meticulous, plodding work of building natural history collections is often thankless, at least in the short term.”

Infrastructure for collections, such as the cutting-edge cryo-collections facility at the Museum of Southwestern Biology, funded by the National Science Foundation, was essential to conducting the new study. Its findings exemplify the tremendous potential of natural history museum collections for understanding biodiversity. The specimens used in this study are all stored in museum collections, available to extend the present research. Innovative software, such as UNM’s Arctos database and collections management system, make it possible for specimens and associated information to be searchable and accessible. In this way, museum collections infrastructure facilitates cross-institution collaboration and encourages novel, integrative research.

Understanding the rules that govern biodiversity is becoming urgent as the warming climate looms. “The ‘endless forms most beautiful’ that Darwin wrote of are still with us,” said Witt. “In the future, we’ll realize that now was the essential time to be documenting them and asking the big questions.”

CORBIDI co-authors on the paper included: Thomas Valqui, Director and long-time collaborator with UNM; Emil Bautista, UNM’s lead partner for fieldwork; and Paloma Ordoñez. Ms. Ordoñez passed away in 2020, during the early part of the pandemic, and the paper is dedicated to her memory.

UNM authors on the paper, in addition to McNew and Witt, included: Jessie Williamson, a current Ph.D. student; Lisa Barrow, Assistant Professor of Biology and Curator of Amphibians and Reptiles; Spencer Galen (M.S., 2015), Assistant Professor at University of Scranton; Ariel Gaffney (M.S., 2017), a forensic ornithologist with the United States Fish and Wildlife Service; Andrew Johnson, Senior Collection Manager at the Division of Birds of the Museum of Southwestern Biology; Jonathan Schmitt (B.S. 2012, M.S. 2015), Ph.D. student at Harvard University; and Ashley Smiley (B.S. 2014), Ph.D. student at University of California-Berkeley; and Shane DuBay (M.S., 2012), a postdoctoral fellow at University of Michigan. DuBay, who earned a Ph.D. from the University of Chicago after leaving UNM, contributed to the project from both UNM and the Field Museum. Additional Field Museum authors on the paper include a Ph.D. student, Heather Skeen, and bird curators John Bates and Shannon Hackett.

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