Biotic Interactions: Not All They’re Cracked Up to Be?

Image Credit: Danyell Odhiambo/ICRAF, CC BY-NC-SA 2.0

Local Adaptation to Biotic Interactions: A Meta-analysis across Latitudes (2020) Hargreaves et al., The American Naturalist, https://doi.org/10.1086/707323

The Crux

Local adaptation is a process whereby individuals native to a given area are better-suited to live in that environment than foreign individuals, and those local individuals will out-compete foreign individuals. This adaptation to local conditions can range from a predator that is better at finding and catching prey, to a plant that is more efficient than another at taking nutrients from the soil, or to a host that has evolved defenses against a local parasite. Despite a wealth of literature and science that has been dedicated to the study of local adaptation, it is not clear what it is about the environment that commonly drives it.

Early studies of local adaptation measured abiotic (non-living) factors like temperature and the amount of light, but this ignores the fact that all environments include biotic factors like other species and any interactions with them. A small amount of studies have shown that biotic interactions (i.e. interactions with other species) can drive local adaptation, but there isn’t a consensus on how common of a pattern that is. Today’s authors used a meta-analysis of previous studies to test how these biotic interactions affect local adaptation.

What They Did

Studies of local adaptation often use a reciprocal transplant or common garden design (see Did You Know below) to test how adapted (or maladapted) a population is to its habitat. The studies the authors collected for this meta-analysis used one of two of these different designs, ones that left biotic interactions intact and those that altered the environment. An example of the latter would be a competition experiment where two plant species were grown in a plot that had been previously cleared of weeds and/or other plants. In contrast, a study that left the biotic interactions intact would not weed the plots, but instead grow the plants of interest along with whatever vegetation was already there. 

The authors used these studies to ask a series of questions. First, do biotic interactions affect the strength and frequency of local adaptation? Second, do biotic interactions affect fitness? And last but not least, does altering the environment by removing biotic interactions result in false maladaptation? This could happen in cases where local adaptation is detected when biotic interactions are left intact, but local individuals appear to be maladapted to local conditions when biotic interactions are removed.

Did You Know: Reciprocal Transplant vs. Common Garden

Ecologists use many different designs to test for many different effects and patterns, and two common methods are the common garden and reciprocal transplant. In the reciprocal transplant, individuals from Habitat A are moved to Habitat B, while individuals from Habitat B are moved to Habitat A. This allows the experimenter to test if some advantage (or disadvantage) is due to some inherent difference among the environments, or if individuals in one habitat are simply better in some metric than the individuals in the other.

Common garden experiments, on the other hand, involve taking individuals from two or more populations and raising them in the exact same environment. This is meant to remove any differences due to the habitats themselves, so that an experimenter can simply test the individuals against one another.

 

long term experimental plots Experimental plots like these are commonly set up using competing plant species or varieties, and often weeds and other pests are removed using pesticides, ridding the experiment of natural biotic interactions. (Image Credit: IRRI Photos, CC BY-NC-SA 2.0)

What They Found

Detecting local adaptation was not dependent on biotic interactions, as there was no difference among studies that removed or left those interactions intact. Likewise, the strength of local adaptation was either not dependent on biotic interactions (for studies that tested local adaptation using both intact and altered environments) or stronger in studies that only tested for local adaptation using altered environments than it was in studies using only natural, unaltered environments. 

Biotic interactions did affect fitness, as transplant fitness (i.e. the fitness of foreign individuals) was almost twice as high when biotic interactions were removed. Altering the environment to remove biotic interactions led to false detections of maladaptation in 13 out of 19 studies, but it was not a significant pattern. 

Problems

One the paper’s questions not included in this breakdown was concerned with the importance of biotic interactions in the tropics vs. temperate areas, and they found that (in the tropics) biotic interactions are important for detecting local adaptation but do not affect the strength. Although over 100 studies were collected and analyzed for this meta-analysis, most of the studies in this meta-analysis came from temperate areas, while a minority were conducted in tropical areas, which often have much higher species richness. This makes it hard to draw conclusions about the generality of these results. While I don’t think this is a flaw of the researchers by any means, it is simply an artifact of the data that is available. It’s hard to include studies in a meta-analysis that don’t exist!

So What?

Studies of local adaptation are nothing new, and although the science behind it has progressed over the years, we still lack information on what selective forces are driving it. This study has shown that, while biotic interactions seem to be important drivers of local adaptation in the tropics, the lack of a relationship between local adaptation and biotic interactions in temperate areas points towards a stronger relationship with the abiotic environment. That is to say, biotic interactions are less predictable in time and space for temperate organisms and thus are not likely to be drivers of local adaptation.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, or follow him on Twitter here.

 

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