Incorporating native plants into landscapes has exploded in popularity, as more people look to support pollinators, conserve water, and restore a little bit of natural habitat in their own backyards. But it’s not only the species you plant that matter, the plant’s genetic diversity matters too. We spoke with Matt Olson, a botanist at Texas Tech University, about why genetic diversity matters, and how paying attention to it can make native planting efforts significantly more effective.
Genetic diversity and species diversity can be thought of as a continuum. There are a number of ways to measure species diversity. One can count the numbers of species in an ecosystem, called alpha diversity. You can assess whether those species have similar numbers as one another or if some are very rare; this measure is called evenness. Or you can think about how distantly related different species are to one another, a measure called phylogenetic species diversity. Ecosystems with greater species diversity are often more resilient to disturbances. Thus, species diversity is important for ecosystem health related to factors like soil health, supporting a diversity of pollinators, and providing a variety of foods and other resources for insects and animals.
Genetic diversity is a measure of the variability in the genetics of individuals within a population of a species. Populations with greater genetic diversity are able to evolve more quickly in response to changes in the environment, such as human disturbances or climate change. Genetic diversity is also important for ecosystem health. For instance, in poplar trees it has been shown that increasing the genetic diversity increases insect and microbial (think soil fungi that help absorb nutrients) diversity. These effects can propagate up through other organisms in the food web. Greater genetic diversity also provides a mechanism for a population to respond to stress because a variety of different individuals increases the chance that some will survive. Finally, higher genetic diversity often produces more diverse and stronger seeds for the next generation because inbreeding is reduced. Inbreeding often leads to genetically inferior offspring, so inbreeding over many generations can have long-lasting effects on the persistence of populations.
Genetic diversity is critical for restoration projects. Restoration projects are often stressful situations for plants; the environment may not be ideal for growing in the area at first because of past misuse of the area. We can think of it as a sort of bet-hedging. If one genotype cannot persist, another genotype may have what it takes. Adding more and more genotypes increases the chances of adding a genotype that will be successful. This goes beyond environmental factors like heat, nutrients, and water availability. Some genotypes may grow better than others with the specific microbiota that are in the soil, increasing the chances of successful restoration. Once the plants become established, more genetic diversity will aid in population persistence through avoidance of inbreeding depression (decreased vigor from mating among relatives) and will draw in a larger number of insects and microorganisms with a knock-on effect of increasing overall species diversity that in turn improves overall ecosystem function and resiliency.
That’s an interesting question and it depends on the situation and the species. In some instances, I would say, “no, you can’t have too much diversity.” Evolution across generations will weed out the poor performing from the better performing genotypes. However, evolution requires generations and a restoration biologist may not have much time and likely wants to see immediate results. Adding the “wrong kind” of diversity may slow or not allow the establishment of the population during restoration. There is something the evolutionary biologists call “local adaptation.” It is when populations of a species are adapted to their local environment. It can sometimes happen over very small distances. For instance, seed transfer zones for replanting Ponderosa pines after harvest or fires can be as little as a 200-foot change in elevation, meaning that if you want to replant, you should choose seeds within 200 feet of the original elevation or the trees are not likely to do well. Thus, if you mix too much genetic diversity from across a variety of elevations, the restoration may end up failing at first or take longer than expected as the poorer genotypes are selected against over generations.
This is a big question in evolutionary and conservation biology that has real world implications. It is especially important for long-lived species like trees which may live over a century and will experience future climate regimes that are different from the current climate or the climate in the recent past. The concept of local adaptation comes into play here as well because plants are locally adapted to the climate where they and their ancestors have grown for centuries. Long lived species are likely better adapted to the pre-industrial climate than the current climate because there have not been sufficient generations for meaningful adaptation to the new climate regime. Matthew Fitzpatrick and Stephen Keller have been at the forefront of this work. They think in terms of the mismatch between a population currently growing in an area and a population that would be best adapted to a future climate in that same area. To make their predictions they use spatial and statistical models that take into account genetics, the climate where they are growing now, and the predicted future climate. Using this information, we can predict which seed sources will do best in future climates. The problem is that local adaptation is likely highly complex and accounting only for climate change may not account for other parts of the environment that may not change, such as soil type. Also, it is difficult to verify these mismatch predictions because we need to do real-world experiments using seeds grown in different environments. Nonetheless, a lot of progress is being made, and we will soon know much more about which seed sources will do best in future climates for many regions.
Assisted gene flow is exactly what I was discussing in the previous question. It is when humans move seed sources to new locations to try to match the genotypes for the present or future climate. The answer to your question of whether it is necessary or controversial partially depends on the species and the goal. For native species with less economic importance, it would likely be quite expensive to conduct assisted gene flow. However, this must be weighed against an increased chance that these species may go extinct. As an evolutionary biologist working on trees, I often think in timescales of thousands of years, which I have found is uncommon among my friends and relatives! In many cases, I don’t think assisted gene flow is necessary because over generations genotypes of plants will move and become naturally adapted to new locations. There is an argument that our current climate change is happening at an unprecedented rate and that adaptation is too slow to keep up, but I think that, given enough time (maybe thousands of years!), many species will migrate to new locations that are consistent with the climate under which they have adapted historically. Of course, not all species can migrate. Some have small populations which makes it perilous to move, or they have specific environmental needs that do not allow them to move to new locations. These species will likely go extinct. I’m not happy about that outcome, but it seems like where we are headed.
This is where the controversy really comes in: some folks are concerned that if humans move genetic diversity from one location to another, that individuals carrying this diversity will mate with local populations and change their genetic makeup, potentially resulting in locally mal-adapted progeny. In a sense, humans may be influencing the evolution of the species in ways whose consequences we cannot fully predict, and this could potentially be detrimental or even catastrophic. This is a real concern, but my sense is that over the long term this genetic diversity will naturally be available in the new populations through the process of long-distance gene flow, which is the natural movement of genetic diversity through seed and pollen dispersal. Thus, assisted gene flow is just a case of speeding up a natural process that will be going on anyway.
The decisions we make regarding assisted gene flow, however, may be different for species with economic importance: think crops that are necessary for feeding us, forests that are necessary for growing wood, and trees that we are planting in parks. For these species there is already a lot of assisted migration going on. For instance, parks departments in large cities are choosing trees from seed sources to the south, with the expectation that they will be better adapted to future climates because they and their ancestors have grown for many generations in hotter climates in the south. The same thing is going on for selecting seed sources for replanting forests. The mismatch models that I described previously are meant to more precisely identify the seeds that will grow best in the future.
For crop species that are planted each year, seed companies are designing seeds to tolerate rising temperatures and increased drought, but this is not technically assisted migration.
I would say, plant seed collected from the local area. They are more likely to be locally adapted to the climate and will have a higher probability of establishing. If you can collect from multiple populations in the local area to maximize genetic variation, that is even better.