What’s so Good about Biodiversity in Crop Production?

In a 2012 book, Donald Maier asked, “What’s so good about biodiversity?” He describes how difficult it is to critique principles of biodiversity because all the value of the natural world has been bestowed on the term. It doesn’t help that biodiversity also gets tangled up with human diversity issues. Nevertheless, critique must be done. The Cause of biodiversity, as in “protect endangered species,” has distorted our view of biodiversity as a cause-and-effect cause. It’s often a misleading correlation, one that is not useful for crop management.

Although I’ve written a lot on this topic, I’ll do a last (?) roundup of the topic here. Various details follow, but the thing to remember about biodiversity in crop production is that it is not biodiversity itself that gives the benefits. When we observe benefits, they result from specific interactions between specific species.

The Diversity of Biodiversity Meanings

“Biodiversity is everything.” In popular use, the term is often used as a catch-all for all the organisms in a place. This use has nothing to do with the actual diversity of the organisms or species other than noting that life is diverse almost everywhere. This “everything” biodiversity is used to move people to action; “Save the earth’s biodiversity.” This is the biodiversity movement, the Cause, where the term originated, and conservation of biodiversity is the goal here. Whether or not this Cause motivates you, this meaning is not useful in for crop production.

“Biodiversity” functions as a useful umbrella term that sounds more scientific than a word such as “nature.” – Burlingame, 1999

The meaning that I want to address is that of the diversity of species. This is most often measured by the number of species, termed richness, but can also be the relative abundance of species in a population (evenness). Here, I will focus on the number of species in a field and the soil.

The Assumption: More is Better

The idea that biodiversity drives ecosystem function – more biodiversity produces better function – is deeply engrained in ecology, but not without dissent (Picasso, 2018; Devictor and Meinard 2020). Agroecology takes the more [biodiversity] is better assumption and applies it to agriculture (Duru et al. 2015). A related assumption is that agriculture is degraded nature, so nature naturally has higher biodiversity. Therefore, agriculture should aim for higher biodiversity so it can be better, like nature.

Better can mean many things. If you are trying to conserve biodiversity, i.e. nature, then more is better. More elephants, wilderbeasts, and rhinos are better than fewer. However, if you are after function, or as in agriculture, function with the overriding goal of producing food, then more biodiversity is not necessarily better. There are reasons why ecological research does not apply to agriculture, but several recent papers in ecology also challenge the relationship between biodiversity and function. Their results suggest:

1. Biodiversity is not an actual cause of function, or
2. Biodiversity is a cause, but it may not work in nature like it does in controlled experiments.

Biodiversity; Cause or Correlation?

In the first paper (Schoolmaster Jr. et al., 2022), a rigorous analysis of causation vs. correlation finds, “there is no causal relationship found between biodiversity and ecosystem functioning.” The authors find the links are correlations; biodiversity is not the cause. What is the cause? The analysis suggests that species composition drives ecosystem functioning; specific species with specific interactions.

This overturns the conclusions of 100s of papers. Unsurprisingly, some of the first ecologists to declare the causal biodiversity-ecosystem relationship challenged the new findings (Grace et al., 2022). The authors met the challenge with a seemingly robust defense. I am no expert here on causal analysis, but this seems a major blow to the biodiversity-ecosystem-function idea (Arif and MacNeil, 2022; Maier, 2023).

An even more recent paper (Dee et al., 2023) analyzes data from 43 grasslands in 11 countries using some of the robust causal analysis tools recommended by Schoolmaster et al. The authors find that species diversity is an actual cause, but that increasing species number in unmanaged environments caused a decrease in productivity, a common ecological measurement of ecosystem function. This contradicts results in planted experiments (see also Lisner et al. 2022) which provide most of the evidence of higher biodiversity leading to higher function.

I suspect there will be more papers, continuing the back-and-forth on this topic of the past two decades. Nevertheless, these results do not strengthen the arguments for biodiversity in crop production, where “more is better” has been applied to cover crop mixtures, crop rotation, and soil biodiversity.

Crop Diversity in Space; Cover Crop Mixtures and Intercropping

I have written a lot about cover crop mixtures. Bottom line: other than the bet-hedging strategy, the best mixture is often similar to the best monoculture, and the latter is much easier to find. More here.

For intercropping, a recent review (Jones et al., 2023) found consistent yield benefits in just 43.8% of trials. A different recent review (MacLaren et al., 2023) found the most consistent intercropping strategies to be:

1. Legumes + Non-legumes (especially in low-N soils), and
2. Relay crops

Again, it is not the diversity of these successful intercrops that provides the benefits.

Diversity in Time, Crop Rotation

Crop rotation is a proven strategy for controlling some insect and disease pests. However, it is not the diversity itself that provides this benefit. It is often the abrupt change in vegetation interacting with specific pest problems that is the source of this benefit. The actual cause of yield-related benefits from crop rotation may be active crop time, and not crop diversity. Are you seeing the trend here? The place where a diverse mixture may be the cause of benefits is in longer term perennials, where mixtures avoid the gradual buildup of soilborne pathogens (van Ruijven et al., 2020).

Soil Biodiversity; Not As We Thought

The idea continues with regard to soil biodiversity; more is better. From this, other assumptions flow:

1. Nature has higher soil biodiversity, and therefore higher function.
2. Plant diversity promotes soil diversity.
3. Farming decreases soil biodiversity because of its lower plant (crop) diversity and disturbance with tillage, fertilizers, and pesticides.
4. Agriculture is a degraded nature and therefore we should work to restore biodiversity levels and function.

Back in 2018, there were signs the reality was not so simple. I wrote of the research that found higher bacterial diversity in cornfields than under tallgrass prairie. More recently, in surveys of European soils, two studies come to the same conclusion: cropland soils have higher biodiversity than in forests or unmanaged grasslands (Köninger et al., 2023; Labouyrie et al., 2023). This holds for fungal and bacterial diversity, and also protists, nematodes, arthropods, and annelids.

These results call into question all the ideas above, and all the principles of agroecology and regenerative agriculture based on them. Again, it is not biodiversity of the soil that causes it to function well or poorly; it is the specific composition of soil organisms. Counting species to compare ecosystems is the ecological equivalent of a beauty contest. And indeed, we are prone to view diverse landscapes as more beautiful than corn fields, but beauty is not why we grow crops.

Biodiversity and Poker; It’s not the Diversity that Wins

Comparing biodiversity to specific composition of species is like a hand of poker. It’s possible to win with a diverse mixture of cards in your hand:

But a diverse hand does not guarantee a winning hand because while one card can substitute equally for another in terms of diversity, one card is not equal to another in terms of the rules of the game (nor with composition-driving mechanisms). This hand beats the one above even though they have the same diversity.

Why? Because of the specific cards and their interactions—they are in order—with other cards.

The specific mixture of specific cards determines whether you should hold ’em or fold ’em, just as the specific species and their interactions (“just-so biodiversity”, according to Maier, 2012) determine whether a mixture will provide benefits.

And, as with annual crops, a monoculture can win over a more diverse hand 🙂.

Self-correcting Science

People speak of Big Ag. There is also Big Biodiversity; the values of a movement that skew science in favor of always higher biodiversity. Big Biodiversity remains a Cause; conservation of endangered species is just as much a movement as when the word biodiversity was first used. But is it a cause-and-effect cause? The evidence is trending towards “no”. Science seems to be correcting itself on this view. To his credit, Maier (2012) tried to point this out to us over a decade ago. What’s so good about biodiversity? For crop production, the answer is “not much.” We’d be better off focusing on actual mechanisms in specific species and their interactions.

References

Arif, S., and M.A. MacNeil. 2022. Applying the structural causal model (SCM) framework for observational causal inference in ecology. Ecological Monographs: e1554. doi: 10.1002/ecm.1554.

Burlingame, L.J. 1999. Review of The Idea of Biodiversity: Philosophies of Paradise. Isis 90(3): 621–622.

Dee, L.E., P.J. Ferraro, C.N. Severen, K.A. Kimmel, E.T. Borer, et al. 2023. Clarifying the effect of biodiversity on productivity in natural ecosystems with longitudinal data and methods for causal inference. Nat Commun 14(1): 2607. doi: 10.1038/s41467-023-37194-5.

Devictor, V., and Y. Meinard. 2020. Empowering biodiversity knowledge. Conservation Biology 34(2): 527–529. doi: https://doi.org/10.1111/cobi.13367.

Duru, M., O. Therond, G. Martin, R. Martin-Clouaire, M.-A. Magne, et al. 2015. How to implement biodiversity-based agriculture to enhance ecosystem services: a review. Agron. Sustain. Dev.: 1–23. doi: 10.1007/s13593-015-0306-1.

Grace, J.B., M. Loreau, and B. Schmid. 2022. A graphical causal model for resolving species identity effects and biodiversity–ecosystem function correlations: comment. Ecology 103(2): e03378. doi: 10.1002/ecy.3378.

Jones, S.K., A.C. Sánchez, D. Beillouin, S.D. Juventia, A. Mosnier, et al. 2023. Achieving win-win outcomes for biodiversity and yield through diversified farming. Basic and Applied Ecology 67: 14–31. doi: 10.1016/j.baae.2022.12.005.

Köninger, J., C. Ballabio, P. Panagos, A. Jones, M.W. Schmid, et al. 2023. Ecosystem type drives soil eukaryotic diversity and composition in Europe. Global Change Biology.

Labouyrie, M., C. Ballabio, F. Romero, P. Panagos, A. Jones, et al. 2023. Patterns in soil microbial diversity across Europe. Nat Commun 14(1): 3311. doi: 10.1038/s41467-023-37937-4.

Lisner, A., M. Konečná, P. Blažek, and J. Lepš. 2023. Community biomass is driven by dominants and their characteristics – The insight from a field biodiversity experiment with realistic species loss scenario. Journal of Ecology 111(1): 240–250. doi: 10.1111/1365-2745.14029.

MacLaren, C., W. Waswa, K.T. Aliyu, L. Claessens, A. Mead, et al. 2023. Predicting intercrop competition, facilitation, and productivity from simple functional traits. Field Crops Research 297: 108926. doi: 10.1016/j.fcr.2023.108926.

Maier, D.S. 2012. What’s So Good About Biodiversity?: A Call for Better Reasoning About Nature’s Value. Springer Netherlands.

Maier, D.S. 2023. Chasing Biodiversity Off the Scientific and Conservation Tracks. Philosophy, Theory, and Practice in Biology 15(0). doi: 10.3998/ptpbio.4337.

Picasso, D.V.D. 2018. The “Biodiversity–Ecosystem function debate”: An interdisciplinary dialogue between Ecology, Agricultural Science, and Agroecology. Agroecology and Sustainable Food Systems 42(3): 264–273. doi: 10.1080/21683565.2017.1359806.

van Ruijven, J., E. Ampt, D. Francioli, and L. Mommer. 2020. Do soil-borne fungal pathogens mediate plant diversity–productivity relationships? Evidence and future opportunities. Journal of Ecology.

Schoolmaster Jr., D.R., C.R. Zirbel, and J.P. Cronin. 2022. A graphical causal model for resolving species identity effects and biodiversity–ecosystem function correlations: Reply. Ecology 103(2): e03593. doi: 10.1002/ecy.3593.

Shade, A. 2017. Diversity is the question, not the answer. ISME J 11(1): 1–6. doi: 10.1038/ismej.2016.118.