Competition for Two Resources

What happens if two species compete for two resources.? There are two possible scenarios: (1) One of the species has lower R* values for both resources than the other species. In this case, the ZNGIs of the two species will not intersect (Figure 4a). (2) One of the species has a lower R* value for one of the resources, but a higher R* value for the other resource. Hence, the ZNGIs will intersect (Figure 4b).

An example of the first scenario, without intersection of the ZNGIs, is illustrated in Figure 4a. Here, two unicellular green algae, Chlorella and Monoraphidium, compete for phosphorus and light. Chlorella has lower R* values for both phosphorus and light than Monoraphidium. In this case, the outcome of competition is intuitively easy to predict. Since

Both species go extinct

Chlorella wins

Chlorella

wins

Monoraphidium Chlorella

Monoraphidium Chlorella

0 2 4 6 8 10 Light availability (|mol photons m-2 s-1)

Cyclotella

/ A

wins

a/

AJ

/ A

Asterionella wins

-*-

Silicate availability (|M)

Silicate availability (|M)

Figure 4 Competition between two species. The thick solid lines indicate the zero net growth isoclines of the species. The predicted outcome of competition is indicated in the graphs.

(a) Competition for phosphorus and light between the green algae Chlorella and Monoraphidium. Chlorella always wins.

(b) Competition for phosphorus and silicate between the diatoms Cyclotella and Asterionella. The lines indicated by Cyc and Ast correspond with the consumption vectors of Cyclotella and Asterionella, respectively. The region between these two lines allows stable coexistence of Cyclotella and Asterionella. The symbols indicate the outcome of laboratory competition experiments: Asterionella wins (asterisks), Cyclotella wins (filled circles), stable coexistence of Asterionella and Cyclotella (triangles). (a) Redrawn from Passarge J, Hol S, Escher M, and Huisman J (2006) Competition for nutrients and light: Stable coexistence, alternative stable states, or competitive exclusion? Ecological Monographs 76: 57-72, with permission from Ecological Society of America. (b) Reproduced from Tilman D (1980) Resources: A graphical-mechanistic approach to competition and predation. American Naturalist 116: 362-393, with permission from The University of Chicago Press.

Chlorella has lower R* values for both resources, Chlorella is obviously the superior competitor for both resources and will therefore always outcompete Monoraphidium. We tested these predictions in laboratory experiments. The outcome of the experiments matched the predictions based on the ZNGIs of these species: Chlorella always won.

For the second scenario, with intersecting ZNGIs, take a closer look at the example of two other microscopic algal species in Figure 4b. Here, the common freshwater

silicate

maltose diatoms Cyclotella and Asterionella compete for phosphorus and silicate. Cyclotella has a lower R* for silicate, while Asterionella has a lower R* for phosphorus. In other words, Cyclotella is a better competitor for silicate, while Asterionella is a better competitor for phosphorus. Cyclotella will therefore dominate if both species compete for silicate. This is predicted to happen when the silicate availability is low. Conversely, Astrionella will win if both species complete for phosphorus. This will happen when phosphorus availability is low. So far, the theory is similar as competition for a single resource. However, it gets interesting in the intermediate region, the transition from phosphorus to silicate limitation, where the species compete for phosphorus as well as silicate. Here, both species may coexist. But do they?

To answer this question, we need more information. In addition to the R* values of both species, we need to know their so-called consumption vectors. The consumption vector of a species reflects in which relative amounts a species consumes its two resources. Cyclotella has a steeper consumption vector than Asterionella; that is, Cyclotella consumes relatively more phosphorus than silicate, while Asterionella consumes relatively more silicate than phosphorus. In other words, both species consume more of the resource for which the other species is the better competitor. Whenever this is the case, competing species may coexist. In our example, Cyclotella and Asterionella will coexist for all resource availabilities in the intermediate region labeled 'stable coexistence'. David Tilman, who is well-known for his theoretical and experimental contributions to ecology, tested these predictions in the 1970s. The outcome of his laboratory experiments largely confirmed the predictions, except for two experiments close to the boundary between the regions of coexistence and extinction of Asterionella (Figure 4b).

So what happens in the opposite case, when two species both consume more of the resource for which they are themselves the better competitor? (Graphically, the consumption vectors of the two species in Figure 4b would be reversed.) In this case, competition does not allow coexistence. Instead, in the intermediate region, the species that becomes dominant first will monopolize both resources and will thereby prevent the growth of the other species. The winner is the species that comes first.

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