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May 27th, 2015

Explaining biodiversity patterns: An Antarctic case study

An aerial view of the Dry Valleys of Antarctica, where researchers conducted a study on biodiversity in soil-dwelling bacteria. Photo by Mike Gooseff.

Scientists have developed a new theory to explain patterns in biodiversity, and tested it on bacteria in the soils of the Dry Valleys of Antarctica. Their study was published today in the Proceedings of the Royal Society B.

The study, whose coauthors include three investigators from the McMurdo Dry Valleys Long-Term Ecological Research (MDV LTER) site, integrates niche and metabolic factors affecting species distribution to test a general diversity hypotheses, which can be used to separate simple from more complex variable effects and aims to explain the cause of biodiversity patterns.

When unraveling the forces driving a species’ distribution, both its environmental niche and metabolism play crucial roles. While an organism’s niche sets the environmental conditions under which it can survive and reproduce, its metabolism allows the organism to grow, survive and reproduce. Together, the niche and metabolic requirements limit, “local species richness (alpha-diversity) and the turnover in species composition between sites (beta-diversity).” However, the degree to which these two factors affect biodiversity patterns along environmental gradients has been unclear.

To better understand this relationship, the researchers developed three baseline hypotheses that predict how niche and metabolic principles shape alpha and beta-diversity: (1) a taxa’s niche extent will vary positively with temperature (in other words negatively with elevation, since elevation inversely relates to temperature), (2) increasing niche extent will increase alpha-diversity, but decrease beta-diversity, and (3) alpha-diversity along the gradient of an environmental variable will follow from the frequency distribution of niches along that gradient.

Predictions were tested on bacteria distributions along pH, salinity and elevation (temperature) gradients in the Dry Valleys. The low temperatures and primary productivity of the area, plus limited water and soil nutrients, limited confounding environmental variables.

Upon testing and analyzing sample results, the study found agreement with their hypotheses. Other literature suggests that the previously observed positive alpha-diversity-temperature relationship is due to the combination of more organisms in warmer temperatures and productive environments. However, prediction (1) shows that temperature is a stronger variable in alpha-diversity relative to a productive environment. Broader niches found at lower elevations, as follows in prediction (2), suggest increased growth rates associated with higher temperatures leading to increased alpha-diversity and reduced beta-diversity. When applied to pH gradients, prediction (3) illustrates a hump-shaped trend that is significantly higher than global soil values, but comparable to regional soil pH values. This suggests the combination of both niche evolution and regional conditions mediate pH’s effect on alpha-diversity, which expands beyond just physiochemical effects. When applied to a salinity gradient, prediction (3) demonstrates decreased alpha-diversity, which may be explained by the soil’s low water availability imposing biological stress on the bacteria.

The researchers conclude that niche-based distributions mediated by metabolic effects of temperature (and pH and salinity) impose stronger constraints on alpha and beta-diversity than other variables. These general lessons can be employed to predict major biodiversity patterns along different environmental gradients and varying scales. Such tools will help elucidate the ecological understanding of Earth’s broad range of biomes.

Study authors include lead author Jordan G. Okie of Arizona State University; new MDV LTER lead investigator and CU-Boulder faculty Mike Gooseff; and MDV LTER investigators John Barett of Virginia Tech and Cristina Takacs-Vesbach of the University of New Mexico.