Niche filling dynamics in the Australian Wet Tropics

How to tease apart the mechanisms behind patterns of diversity in the wild.

A major challenge in ecology and evolution is to unravel the processes that generate and maintain the uneven distribution of life on Earth. A common approach is to regress species richness, the number of species counted at a place, on variables representing contemporary and historical environmental conditions. However, numerous hypotheses for richness gradients implicitly or explicitly make predictions about mechanisms acting on how species occupy niche space, the degree of ecological differentiation among species, and therefore to trait dispersion within assemblages, which cannot be addressed using data on species richness alone. For instance, contemporary hypotheses predict that areas of higher primary productivity have a larger available niche space that might be filled with more species. Moreover, historical hypotheses predict that areas that remained stable through geological time (i.e., over millennia) suffered less from extinctions and allowed the accumulation of more species, lineages and trait diversity over longer periods.

Image: A view of the Australian Wet Tropics landscape taken from the Thornton Peak. Photo by: Stephen Zozaya..

FROM THE COVER: read the article on which this post is based …
Oliveira, BF, Flenniken, JM, Guralnick, RP, Williams, SE, Scheffers, BR. (2020) Historical environmental stability drives discordant niche filling dynamics across phylogenetic scales. J Biogeogr. 47: 807–816.

To test these predictions, we focused our study in the Australian Wet Tropics (AWT), a region that has undergone significant historical habitat shifts, with rain forests expanding and contracting over the Quaternary period. Here, we investigated how spatial patterns of niche space occupancy conform to current environmental conditions (Net Primary Productivity, NPP) and historical stability of environmental conditions since the Last Glacial Maximum (LGM, ~21 kyr BP). Following Robert Helmer MacArthur in his seminal paper (MacArthur, 1965), we estimated two contrasting ways for species to occupy niche space: packing and expansion. Niche packing occurs when increasing richness leads to a denser packing of species into a niche space, whereas niche expansion occurs when species diversify into novel regions of niche space resulting in increased richness. We used datasets of unusually high phylogenetic scope, including all known vertebrates (mammals, birds, amphibians, and reptiles) occurring in the AWT, species distribution data derived from 20 years of standardized surveys, and trait data of measurements taken preferentially from specimens occurring at the region.

A series of previous studies conducted at the AWT showed that historical environmental fluctuations played an important role in shaping current biodiversity patterns in the region, including species distribution ranges, richness patterns and genetic structure of populations. Our study indicates that historical processes also determined dynamics of niche space filling and occupancy. Across multiple taxonomic groups, stable areas tend to be “expansive”, with niche dynamics driving assemblages of higher functional diversity than expected by chance. Although current environmental productivity and historical stability are linked, productive regions cannot support high biological diversity if they are located in unstable areas. As a result, the net effect of NPP on richness and niche occupancy patterns was offset by a stronger effect of historical environmental instability, reiterating the importance of historical processes for the generation of current diversity gradients.

Our broad phylogenetic sampling is of particular utility for examining inter-clade interactions in the filling of niche space. We identified discordant niche occupancy patterns across analyses of differing phylogenetic scale (e.g., mammals versus all vertebrates). At lower phylogenetic scales (i.e., within a given constituent vertebrate group), increases in species richness lead to niche differentiation, a characteristic of niche expansion. In contrast, at higher phylogenetic scales (i.e., the scale of vertebrates), increases in richness increases the sharing of trait space by species from different constituent classes, a characteristic of niche packing. These seemingly contradictory results suggest that dynamics of niche space occupancy depend on phylogenetic scale. Further cross-phylogenetic scale analyses may allow researchers to have a more complete picture of the processes that generate and maintain the diversity of life.

Written by:
Brunno F. Oliveira – Postdoctoral Researcher, Department of Wildlife Ecology & Conservation, University of Florida/IFAS, Gainesville, Florida, USA
Robert P. Guralnick – Curator of Biodiversity Informatics, Florida Museum of Natural History, University of Florida, Gainesville, Florida, USA
Stephen E. Williams – Professor, Centre for Tropical Environmental & Sustainability Science, James Cook University, Townsville, Australia
Brett R. Scheffers – Assistant Professor, Department of Wildlife Ecology & Conservation, University of Florida/IFAS, Gainesville, Florida, USA

Additional information:
Brunno F. Oliveira: @666brunno999,
Robert P. Guralnick: @robgural,
Brett R. Scheffers: @BrettScheffers,

Cited literature: MacArthur, R.H. (1965) Patterns of Species Diversity. Biological Reviews 40:510–533.

White-lipped tree frog (Litoria infrafrenata) occurring at the is one of the world’s biggest tree frogs. Photo by: Stephen E. Williams.

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