Genetics research at SEGA sites helping to unravel the mysteries of Darwin’s ‘entangled web’

The publication of Darwin’s ‘Origin of the Species’ over 150 years ago propelled evolution into the forefront of biological sciences – where it has been a cornerstone ever since. But there’s one area of Darwin’s theory that for a long time was considered so complex that it might never be figured out. His concept of the ‘entangled web’ in which complex interactions bind all species together to create complex communities and ecosystems. However the development of modern genetics techniques … AND research at a scale from individual genes to ecosystems being carried out at Southwest Experimental Garden Array (SEGA) sites is now revealing just how the entangled web works in practice.

In two key papers published in the last 12 months Professor Tom Whitham and colleagues demonstrate that fundamental aspects of how whole ecosystems are assembled and operate are controlled by genetics. “The idea that the genetic make-up of a single tree influences the whole community of organisms associated with the tree – from lichens on that tree’s trunk, to the bacteria in the soil its roots are growing in, to the pathogens on the tree’s leaves was never considered – until recently” explains Whitham. “Our research group is pioneering the field of community and ecosystem genetics - this is the genetics of everything”.

Trees are analogous to humans – in that each individual has its own distinct community of micro-organisms growing on and inside it. “But we have one big advantage working with trees instead of humans” says Whitham. “We can do experiments on trees, microbes, insects and lichens that you just can’t do on humans, including cloning the trees and putting them in stressful environments, in order to test fundamental principles.”
SEGA researchers have been studying cottonwoods - an important ‘foundation’ tree of riparian ecosystems in the US southwest. But the experiments on cottonwoods aren’t just relevant to riparian ecosystems. “This research is getting at fundamental principles that are potentially applicable to all biomes” Whitham asserts.
In recent papers published by Lamit et al. (in Journal of Ecology) and by Lau et al. (in Ecology) researchers used different cottonwood genotypes (individual trees with a unique genetic makeup), each of which was cloned and grown in large field experiments with the same environmental conditions. This common environment allowed them to investigate how the genetics of the trees influenced the communities of bugs and other organisms living on them. They found that some genotypes of trees supported lots of strongly interacting species and communities of insects, microbes, and lichens. Other tree genotypes supported fewer bugs and other species, with weak interactions in those communities living on them.
The trees which hosted more bugs in turn attracted more birds to feed on them. Genetics also affected the timing of things like leaf out in spring, leaf drop in fall, the shape (or ‘architecture’) of the tree and the amount of chemical defensive compounds the trees produced. Which also affected the timing of breeding for the insects and birds using the trees as habitat. It even influenced whether beavers found the trees tasty to eat or not. Literally thousands of species are affected by the underlying genetic make-up of the tree – with knock-on effects to birds and mammals, as well as decomposition and nutrient cycling rates. These findings show that species richness, species abundance and community interactions are ALL genetically based – adding a huge dimension to the understanding of Darwin’s entangled web.
Why is this so important? Well these complex communities of organisms living on the tree can have a big effect on that tree’s growth and survival. A tree with an overall positive feedback from the community will help the tree survive, while a tree with bugs and microbes that produce negative feedbacks is more likely to die or lose out in competition with other trees. “It really is life or death for the tree – and ultimately results in the evolution of whole communities!” explains Whitham. Including having a big influence on a tree’s ability to survive climatic and other environmental changes.

“The practical application of these findings is that it gives us a huge reason to maintain and encourage high genetic diversity”, says Whitham.  If each genotype of a particular tree species supports different communities of organisms, then higher tree genetic diversity means much higher overall diversity – and that higher diversity can confer much more resilience to things like climate change. “But if monoculture of plants are cloned from just one individual – and if something deadly turns up … it will likely kill the whole forest, instead of just one or two trees!” Whitham warns.

“But unlike so much of the ‘doom-n-gloom’ climate change research predictions – this genetics-based research can provide potential solutions for the future” says Whitham. “The power of SEGA is that it can identify genotypes and populations that can survive in future climatic conditions.”

Tree genotype mediates covariance among communities from microbes to lichens and arthropods. Louis J. Lamit, Posy E. Busby , Matthew K. Lau , Zacchaeus G. Compson, Todd Wojtowicz, Arthur R. Keith, Matthew S. Zinkgraf, Jennifer A. Schweitzer, Stephen M. Shuster, Catherine A. Gehring and Thomas G. Whitham. Journal of Ecology 2015, 103, 840–850.
Genotypic variation in foundation species generates network structure that may drive community dynamics and evolution. Matthew K. Lau, Arthur R. Keith, Stuart R. Borrett, Stephen M. Shuster and Thomas G. Whitham. Ecology 2016, 97, 733–742