Competition between Plants: What Biology Says

It's not as simple as two boxers in a ring

Russian Olive (Elaeagnus angustifolia) foliage and berries, as observed on a tree growing by an irrigation ditch in Fruita, Colorado

The following is a draft chapter from the book that Nikki Hill & I are writing, tentatively entitled, “Don’t Blame the Messengers: A critique of the ‘invasive plant’ narrative.” An earlier version of this chapter was shared with paid subscribers last year.

The most common accusation leveled by the “invasive plant” narrative is that introduced plants “outcompete” native species. As the story goes, they “aggressively” and “ruthlessly” “encroach on,” “choke out,” “push out,” “bully,” and “displace” native plants and that they “take over” ecosystems, thereby “degrading,” “disrupting” and “destroying” them, causing “ecological havoc.” The human response must be a declaration of “war” in which we “push back,” “stem the influx,” and “turn back the tide” by “fighting,” “battling,” and “combating” them, in order to “wipe out,” “purge” and “eradicate” them. We’re presented with a picture of horrible monsters we must slay in order to save helpless victims from being gobbled up. As an image to incite fear and enmity against supposed villains, it’s very effective, but how does it stand up as way of understanding ecological interactions?

Turning to science, let’s look at how biologists define “competition.”

James B. Grace, ecologist and senior research scientist with the US Geological Survey, says:

Any attempt to define competitive success must begin with a definition of competition. The variety of possible definitions of competition have been discussed numerous times and it is safe to say that there is no universally accepted definition.ⁱ

Okay, so we can see right away that this subject is not going to be cut-and-dry.

More helpfully, he then goes on:

Nonetheless, it can be argued that a conventional definition does exist, based on the methodologies used to study competition. Practically speaking, there exists a body of experimental data that constitutes our observational basis for discussing competition. In nearly all cases, these data were collected by allowing plants to grow either with or without neighbors of another species and, in many cases, by demonstrating that the plants were limited by some common set of resources. As a result, it can be argued that there exists a "conventional" definition of interspecific competition that is exemplified by the definition offered by Begon et al. (1986): "an interaction between individuals, brought about by a shared requirement for a resource in limited supply, and leading to a reduction in the survivorship, growth and/or reproduction of the competing individuals concerned.”ⁱⁱ

Paul Keddy, an ecologist who has studied plant population ecology and community ecology, and who literally wrote a science book called, Competition, explains:

Definitions of competition present a particular challenge because it is such a widespread phenomenon, and occurs in so many conditions. It may be difficult to find a definition that is sufficiently robust to encompass the riotous display of possibilities in nature, yet precise enough to clarify every particular circumstance where it is applied. Further, we may look for a definition that emphasized the mechanisms of competition, or its measurement by means of experiment, or its long term evolutionary consequences. Recent textbooks of ecology reveal a wide array of attempts to satisfy these conflicting objectives. Some authors even advocate that we no longer use the term.ⁱⁱⁱ [our emphasis]

That being said, Keddy then lets us know that the definition he’ll be using in Competition is: “the negative effects that one organism has upon another by consuming, or controlling, access to a resource that is limited in availability.”^iv

We could quote a dozen more definitions, as well as a dozen more reflections from biologists on the difficulty of coming up with definitions, but we hope you get the idea: understanding the ecological process of competition is not as simple as picturing two boxers in a ring.

To help see how biology can help us to understand competition, what follows are some basic concepts and terminology used by biologists to talk about it.

Keddy lists competition as one of three “fundamental forces” that “connect organisms in living systems,” the other two being predation and mutualism.

Predation is not a plant-plant interaction; think of a raptor eating a rodent, or a mammal grazing on grass (which is called “herbivory”). Plants can also be predators of animals, such as the Venus Fly Trap or Pitcher Plant, who trap and consume insects. Some plants (about 1% of all angiosperms^v) are parasitic to other plants, like Mistletoe or Dodder, but biologists don’t consider that to be a predator-prey relationship because the parasite doesn’t kill its host. Predator-prey interactions exist in all combinations of native and introduced species.

Mutualism occurs when one or both species benefit and neither is harmed. Mutualistic relationships have been observed between native plants, between native plants and introduced plants, and between introduced plants (including introduced species whose own native ranges don’t overlap). Mutualism also exists between plants and animals (pollination by insects) and between plants and fungi (mycorrhizal networks that capture or exchange nutrients for both parties).

Competition can be “intraspecific”—between members of the same species—or “interspecific,” between different species. As a rule, competition between introduced and native plants are interspecific (though there are exceptions, as we will discuss elsewhere).

The two major mechanisms of plant competition are direct interference and resource competition, which is also known as “exploitive competition.” An example of direct interference is allelopathy, when one plant produces a biochemical that negatively affects another. Direct interference is the less common of the two.

Resource competition mostly occurs for soil resources, water and sunlight.

Competition for soil resources happens below-ground, where plant roots uptake nutrients like nitrogen and phosphorus. Such nutrients are finite in supply, and replaced by processes that are often slower than the rate of uptake, and so in acquiring them, one plant denies them to another. The beneficiary of this competition might have roots that grow faster, longer or to a deeper depth than their neighbor.

Competition for water is also below-ground but is not exactly analogous to competition for soil resources, which is about “consuming, or controlling, access,” as Keddy put it. Craine and Dybzinksi clarify: “Competition for water is generally considered to occur by availability reduction, favouring plants that can withstand the lowest water potential”^vi [our emphasis]. That is, while the reduction in availability of nutrients to one plant might be mostly due to acquisition by another plant, the reduction of water available can happen for other reasons, including seasonal variation which, incidentally, “obscures the effects of competition.”^vii Here we might talk about the relative “drought tolerance” of different plants compared to each other.

Competition for sunlight is obviously an above-ground process. One of the most commonly observed forms of plant competition is when one plant limits the available light for another plant with its greater height, larger leaves, or a more spreading form. Keddy states: “the importance of size in determining competitive ability is well established.”^viii That being said, a plant that benefits by growing in the shade of another is a very common form of mutualism, and is often a key component of ecological succession.

Above-ground competition tends to be more “asymmetrical” than below ground, meaning there is a bigger difference between the respective competitive abilities of the neighboring plants. Competitive ability has two components: “The first is competitive effect: that is, the damage that each species can do to its neighbors. The second is competitive response: that is, the ability of each species to withstand the effects of competition from neighbors.”^ix Sports fans may think of these as offense and defense.

All of this is going to vary depending on location. Says Keddy:

While competition for resources may be ubiquitous, the actual nature of competition (e.g., exploitation versus interference, intraspecific versus interspecific, monopolistic versus diffuse, above-ground versus below-ground) probably changes dramatically from site to site. Too many ecologists have asked ‘‘is there competition?’’ between a pair of selected species rather than asking what the nature of the competition is and how it varies along gradients. Soil resource gradients are widespread, in which case gradients in competition intensity, and gradients in the relative importance of above- and below-ground competition, are likely common.^x

So, what you learn about the competition of species x and species y in one place may or may not fully apply in another. We are reminded of an Oregon farmer who told us, “When you’re learning how to farm on a particular piece of land, you’re learning how to farm on that particular piece of land.” On the subject of introduced species, differences from site to site—including but not limited to soil resources—might often be related to the human activities that introduced the species in the first place like urbanization, road construction, deforestation, dam-building, mining or—most significantly in terms of acreage—agriculture. Keddy addresses how field experiments can be thus affected:

[A] potential problem with most terrestrial experiments is that old fields are used as an experimental system. Old fields are a relatively new type of habitat created, in many cases, only a few hundred years ago and containing a mixture of native and exotic plants. Old fields might therefore not exhibit patterns found in native plant communities where species may have coevolved for thousands or even millions of years. Further, the patterns in such communities are largely ones of small-scale heterogeneity rather than of longer environmental gradients.^xi

We’ve given this subject a lot thought, being that we’ve spent a lot of time in old fields ourselves, as farmers who were paying close attention to plant life, both what we were planting and what volunteered. “Small-scale heterogeneity” is the truth. That would show up clear as day when we’d plant a cover crop for soil-building or weed-smothering over a large area. Buckwheat, a quick-growing, warm-weather crop with modest nutrient requirements, was especially illustrative at this. Although we’d broadcast the seed evenly across a space, germination was noticeably uneven, as was overall performance. In some spots, the plants were tall and exuberant, in others stunted and weak. The effects were patchy and unpredictable. What we were seeing, but could not easily explain, was the history of the field. Maybe that lush corner was a former animal pen, or where some manure had fallen off a truck, or there was an unseen seep. Maybe that bare spot was where a pile of gravel sat, or a sheet of plywood laid for a year, or some herbicide got sprayed. Who knew? Not us. We didn’t expect to learn anything about “natural” interactions between plants in places like this. We wonder how much you can, frankly.

This raises the subject of the experimental methodologies used for studying competition. You might, as we did, imagine researchers out in nature surveying plant communities, taking lots of measurement and samples, doing lab work for soil mineralogy and such, and returning year after year to track changes. (Which sounds like a great job!) While some of that certainly happens, much of the work is done under more controlled circumstances which aim to isolate and measure individual factors. This is ex situ (off-site) research.

Keddy describes a typical process:

It is not uncommon to find from 10 to 25 species in one small plot of grassland or wetland. How do all these species interact with one another? Why are some relatively common and others relatively rare? To explore competitive interactions in the community, one could simply choose two species and measure their relative competitive ability. But which two species should you choose?

...One alternative is to include all the species in one large experiment. In such an experiment, each species is grown with every other species. There are some problems with this approach. The first is the sheer size of the experiment – if there are n species you wish to study, there are (n2/2)-n possible pairs of plants to grow in mixture. (The term n2 is divided by two because when you grow species A with species B, you simultaneously grow species B with species A.) The second issue is a technical one. You must also grow plants of each species without interspecific competition, in monoculture, to use as reference plants for assessing the effects of interspecific competition. Do you grow the monoculture plants singly (without any competition), or do you grow the monoculture plants paired with a second individual of the same species (with intraspecific competition)? The first case is known as an additive design, whereas the second case is known as a substitutive design. Each has its strengths.^xii

So to investigate 10 plants species growing together, you would end up with forty interspecific pairs, and forty single-species references; 25 species would yield 238 pairs and as many references. The process of measuring, plotting and comparing the results is quite technical, and involves more formulae. If you dig into the details, you’ll understand why biology majors are required to take calculus and statistics courses.

Such methodology has both value and limitations. Regarding the former—and in a perfect world—the sincere goal is to tease out individual threads from a tangle and to quantify them in a meaningful way in order to better understand complex interrelationships. Many of these efforts have been sophisticated and rigorous.

Regarding the limitations—and in the real world—Oxford Bibliographies warns:

Experimental design carries its own assumptions, which are often not stated in published articles. One of the most difficult tasks in exploring published studies is the need to sift through large numbers of experiments in which investigators have haphazardly selected (a pair of) species and grown them in mixture, without adequately justifying their choice of species or the experimental design. Another difficult task is distinguishing between models that, at least in principle, have measurable inputs or make measurable predictions (or both) and those that do not and cannot be tested. Overall, the very ease of growing plants in mixture, as well as the ease of making new models, may have made some people careless, with the result that basic questions are remaining unaddressed.^xiii

Our own concern is that such experiments are usually conducted ex situ (off-site) from the subject plant community. Species pairs are grown out in plots (perhaps in an old field) or in containers (maybe in a greenhouse), which omits essential considerations like the impact and historical frequency of disturbance in the area of interest. One doesn’t need to be contrarian, but merely prudent, to consider experimental results as not unequivocal. When considering the value and limitations of a particular study, we recommend going straight to the “Methodology” section (if it’s not behind a paywall).

Crucially, such ex situ experiments, even when flawed, are cut from a different cloth than the horror stories regularly circulated by the “invasive plant” narrative, which are often based only on anecdote. Unfortunately, such evidence-free assertions are too often used to justify extermination campaigns that often inflict collateral damage on plants and animals alike.

In situ (on-site) fieldwork has its own challenges. John L. Harper, an ecologist who specialized in plant population biology, wrote:

Plants may modify the environment of their neighbors by changing conditions, reducing the level of available resources or by adding toxins to the environment. All of these effects can be shown quite clearly to operate in experiments with artificial populations, but there are probably no examples of plant interactions in the field in which the mechanism has been clearly and unambiguously demonstrated. Interactions dominate most situations that have been analysed in the field and the search for unique factors of competition may not be very sensible. Death or a reduced growth rate are often attributable to "competitive" effects, but interpretation of competition in the field may depend on the recognition of much more specific symptoms. There is a need for a more rigorous philosophy in the search for cause and effect.^xiv [our emphasis]

Ecologist Joseph H. Connell (1923-2020), whose work focused on community structure, competition, predation and the effects of disturbance, was famous for developing new methods for field experiments. He wrote:

The importance of competition in structuring natural communities can be evaluated in various ways. Obviously, the first task is to demonstrate unequivocally its occurrence in nature, yet this has often proved to be difficult. One difficulty is that the evidence often accepted as demonstrating competition can sometimes be produced by other types of interactions. In such cases, the competition supposedly demonstrated by an experiment or set of observations may be more apparent than real.^xv [our emphasis]

An example of such “apparent competition” that he named was “indirect interaction, via a shared enemy,” in which the population of one species appears to suffer competitive pressure from another, but the actual agent is a shared predator. He cited the example of two flowering plants in New Mexico: Hoary-Aster (then Machaeranthera canescens, now Dieteria canascens), an annual or short-lived perennial, and Broom Snakeweed (Gutierrezia sarothrae), a perennial sub-shrub. Seedlings of Hoary-Aster that emerge in the near vicinity of Broom Snakeweed don’t survive. But this wasn’t a case of direct interference (such as Broom Snakeweed exuding a biochemical that’s toxic to Hoary-Aster) or resource competition (like Broom Snakeweed denying soil nutrients to Hoary-Aster by uptaking them first); rather, Broom Snakeweed is a host plant to the Snakeweed Grasshopper (Hesperotettix viridis), who feeds on both Broom Snakeweed and Hoary-Aster, but does far more harm to Hoary-Aster. The farther away that a Hoary-Aster is located from Broom Snakeweed, the less often it is eaten by Snakeweed Grasshoppers. (Incidentally, the grasshopper is considered a “beneficial insect” by ranchers and farmers because the Gutierrezia genus is poisonous to domesticated livestock.^xvi) So what at first glance appeared to be plant-on-plant competition turned out not to be.

On a related but more general note, Keddy wrote:

It is important here to clarify the distinctions between competitive dominance and dominance. The word dominant is sometimes used to describe any organism that is abundant in a community. This use is misleading; abundance need not be the result of competitive dominance. Competitive dominance is abundance achieved as a consequence of exploitation of, and interference competition for, resources; that is, there is an active process of suppressing neighbors... [Instead,] a species may become dominant because of inherently better abilities to withstand environmental effects such as fire, [soil] infertility, or grazing... It seems useful to distinguish between situations where a species is dominant simply because of the inherent traits it has for tolerating the environment and situations where a species is dominant because it has traits for suppressing neighbors. The former type of dominance could occur in the absence of any competition.^xvii [our emphasis]

Examples of how grazing by introduced domestic livestock has suppressed some native plant species while encouraging the local or regional dominance of other native plant species has been observed in the western United States, where “winners” have included Joshua Trees (Yucca brevifolia) on Cima Dome in the Mojave National Preserve (where they are denser than anywhere else in their range), Mesquite trees (Prosopis spp.) throughout the Sonoran Desert, and Rabbitbrush (Chrysothamnus spp. or Ericameria spp.) in parts of the Great Basin. In all three of these examples, domesticated cattle preferred other vegetation, which opened up more space for the Joshua Trees, Mesquite and Rabbitbrush. With Mesquite, cattle additionally boost the dispersal of seed by distributing it in their manure.

So, simply observing the increased abundance of one plant species correlating with the decreased abundance of another is not by itself evidence that one was “choked out” by the other through competition. History and regional processes are highly important to understanding what has happened in particular locations. For example—as we discuss in detail in the chapter, “Scapegoating non-native plants for wasteful human water practices”—introduced Tamarisk (Tamarix sp.) and Russian Olive (Elaeagnus angustifolia) are widely accused of “displacing” native Cottonwoods and Willows in riparian areas throughout the western United States, but the decline of the native species was due to drastic changes to river systems and seasonal flood regimes from dams and irrigation diversion that made those habitats inhospitable to the natives and created conditions more suitable for the introductions. Both Cottonwoods and Willows depend on spring flooding for their seeds to disperse and germinate, but those floods no longer occur. This is truly a case of Tamarisk and Russian Olive being messengers, not competitors, who announce with their presence that we humans changed the playing field. Don’t blame the messenger.

Connell’s views of ecological interactions are informed by decades in the field, and he deserves to be taken seriously when he writes:

A common question in ecology is, What is the relative importance of competition, mutualism, predation, herbivory, parasitism, etc. in determining community structure? Given the possibility that, for example, herbivory can produce the effect of competition between plants, this question can scarcely be answered as stated. Perhaps the question could be more accurately answered if it were posed as, How much do direct interactions with resources, direct competitors, herbivores, mutualists, parasites, etc., plus the indirect effects these produce when acting together, affect community structure? ...The second question is a more complex one than the first, but, given what we now know, is probably a more realistic one.^xviii

Intriguingly, Keddy believes that, at present, the study of competition is still in its early days:

There exists a wealth of detailed observations on the natural history of selected living organisms, but the general principles (we no longer call them laws) remain elusive. There is at present no unified body of theories or laws for competition, although useful fragments are emerging. Instead, there is a rich mixture of observation, fact, experimentation, notion, concept, theory, belief, prejudice and models; the very diversity and volume of material presents a challenge to comprehension and synthesis. If these can be organized and sorted, such that evidence is separated from notion, and fact separated from belief, then we will have that least the beginnings of a body of true science.^xix

Contrast this humble, informed appraisal of the current state of competition research with the brashly militant tone of the popular “invasive plant” narrative. The first is cautious and measured while the second tends to be heedless and reductive. Science is not beyond criticism (indeed, we offer some in this book) but the gap between its claims and those of the “invasive plant” narrative is far wider than the common rhetoric acknowledges. We believe this should give people pause, especially if what’s delayed is a bulldozer or herbicide use.

Finally, we will quote biologist Mark Davis, author of the textbook, Invasion Biology, who writes:

More than 4000 plant species introduced into North America north of Mexico during the past 400 years are naturalized (established to various degrees), and these new species now represent nearly 20% of the continent’s vascular plant species. Yet there is no evidence that even a single long-term resident species has been driven to extinction, or even extirpated within a single US state, because of competition from an introduced plant species.

NOTES

i Grace, James, p. 55-56

ii Grace, James, p. 55-56

iii Keddy, P. A. Competition (Second edition), (Kluwer Academic Publishers, 2001), p. 4

iv Keddy (2001), p. 5.

v Biology Dictionary Editors. “Parasitism” in online Biology Dictionary. https://biologydictionary.net/parasitism/

vi Craine, J.M. and Dybzinski, R. (2013), Mechanisms of plant competition for nutrients, water and light. Funct Ecol, 27: 833-840. https://doi.org/10.1111/1365-2435.12081, p. 1.

vii Craine & Dybzinski, p. 5.

viii Keddy, Paul A. Plants and Vegetation: Origins, Processes, Consequences (Cambridge: Cambridge University Press, 2007), p. 212.

ix Keddy (2007), p. 193.

x Keddy (2007), p. 223.

xi Keddy (2007), p. 220-221.

xi iKeddy (2007), p. 204.

xiii Keddy, Paul A. & Cahill, James. “Competition in Plant Communities,” Oxford Bibliographies, last reviewed 31 Aug 2021. https://www.oxfordbibliographies.com/display/document/obo-9780199830060/obo-9780199830060-0009.xml

xiv Harper, John L. "Population Biology of Plants." (London: Academic Press, 1977), p. xvii-xviii.

xv Grace, James B. & Tilman, David, editors. Perspectives on Plant Competition (New York: Academic Press, 1990), pp. 9-10.

xvi Wyoming Agricultural Experiment Station. Bulletin 912 · Species Fact Sheet: Snakeweed Grasshopper (Hesperotettix viridis), September 1994.

xvii Keddy (2007), p. 196.

xviii Connell, Joseph, in Perspectives, p. 22.

xix Keddy (2001), p. 3.

xixi DAVIS, MARK A. “Biotic Globalization: Does Competition from Introduced Species Threaten Biodiversity?” BioScience 53, no. 5 (2003): 481. doi:10.1641/0006-3568(2003)053[0481:BGDCFI]2.0.CO;2.