Genetic diversity: a poor cousin of species diversity?

In July, Functional Ecology published the Special Feature: A Mechanistic Understanding of Global Change Ecology. We have invited the authors of the papers to write about their paper. In this post, Julia Koricheva (@korichevalab) author of  The relative importance of plant intraspecific diversity in structuring arthropod communities: A meta‐analysis writes about her paper why she wrote it, what she found and where the big gaps are.


Beyond species diversity

A lot of research over the last few decades has focussed on the extent and consequences of biodiversity loss due to human activities. The majority of these studies explored the effects of species extinctions and showed that reduced species diversity has negative effects on ecosystem functions and services such as productivity, community stability, and trophic interactions. However, genetic variation within species is also an important, albeit less well studied, element of the multidimensional biodiversity concept, and a lot of diversity is harboured within species (Fig. 1).

Cauliflower, Brussel sprout, green cabbage, broccoli and kohlrabi are all cultivars of the same plant species, Brassica oleracea and a result of artificial selection for either leaves, stems, buds or flowers. Attribution: Coyau / Wikimedia Commons / CC BY-SA 3.0
Fig. 1. Cauliflower, Brussel sprout, green cabbage, broccoli and kohlrabi are all cultivars of the same plant species, Brassica oleracea and a result of artificial selection for either leaves, stems, buds or flowers. Attribution: Coyau / Wikimedia Commons / CC BY-SA 3.0

Within-species genetic diversity might also be directly affected by human activities such as agriculture. For instance, crop fields are often planted with a single cultivar or variety. Does this loss of genetic diversity within species matter for biodiversity and ecosystem functioning as much as losses of species diversity? This was one of the questions my co-author Dexter Hayes, who collected literature for this project as a part of his Masters’ project, and I wanted to address. To answer this question, we collected data from 60 experiments where different plant genotypes were grown either individually or in mixtures. Sixteen of these experiments manipulated both species and genetic diversity within the same study. For example, in the UADY tree diversity experiment in Yucatan, Mexico (Fig. 2), the number of genotypes of one of the component tree species, big-leaf mahogany (Swietenia macrophylla) was manipulated in addition to number of tree species. Experiments like UADY are important because they provide the opportunity to directly compare the effects of species and genetic diversity within the same ecosystem.

UADY experiment - Photo by Luis Abdala-Roberts
Fig 2. Species polyculture in the UADY tree diversity experiment, Yucatan, Mexico. Photo by Luis Abdala-Roberts.

We also wanted to know whether some of the negative effects of biodiversity loss could be reduced by increasing genotypic diversity within species. For instance, could planting mixtures of crop cultivars provide benefits similar to intercropping (planting several different crop species together)? Tooker and Frank recently reviewed the potential of using crop cultivar mixtures for pest control. They concluded that “increasing the diversity of genotypes within crop fields holds great promise to reduce disease, decrease pest abundances and increase crop yield”. But the above review was narrative and did not assess the magnitude of the effect of crop genetic diversity (e.g. by how much pest and pathogen damage on crops is reduced in mixed cultivar planting).

We decided to conduct a quantitative review of the literature on plant genetic diversity effects in both natural and agricultural ecosystems. Our focus was on plant genetic diversity effects on arthropods, as they are very important for ecosystem functioning and include many different trophic groups such as herbivores, predators, parasitoids, pollinators, detritivores, etc. To review the literature, we used a method called meta-analysis. It involves extracting  summary data from each study on the subject (in our case, differences between mean arthropod species richness or abundance in genetic monocultures and mixtures) and combining them across studies to calculate the overall effect size.


Surprising results

Overall, genotypically diverse plant stands harboured larger number of arthropod species and individuals than genetic monocultures. Interestingly, natural enemies of herbivores such as predators were more positively affected by increase in plant genetic diversity than herbivores themselves. This indicates that genetic impoverishment within plant populations might have community-level consequences and change the tropic structure of associated arthropod communities.

Remarkably, when we examined studies which manipulated both plant genetic and species diversity within the same experiment, we found that magnitudes of plant genetic and species diversity effects on arthropods were nearly identical. This result provides good news for arthropod conservation, as it suggests that protecting plant genetic diversity within species might have similar positive effects on arthropod diversity as protecting plant species richness.

On the other hand, we found limited support for Tooker and Frank’s suggestion that genotypically diverse cultivar mixtures can be used as an effective pest management tool, as overall herbivore damage was not reduced in genotypic mixtures and only damage by generalist herbivores was significantly lower in genetically diverse mixtures. Therefore, the potential for using plant genetic mixtures as a way to control pests appears to be limited. However, mixtures of crop cultivars can provide many other benefits even if they do not reduce herbivore abundance and damage. For instance, cultivar mixtures have been shown to have higher yield than monocultures and are also effective in reducing the spread of fungal pathogens.

Overall, our results highlight the importance of incorporating both species and within-species genetic diversity into predictions of the effects of biodiversity loss


Want to know more?

You can read our meta-analysis on plant genetic diversity effects on arthropod communities here (Open Access). See also this blog which discusses the results of another recent meta-analysis on ecological importance of intraspecific variation.

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