Competition

5.3.4.1 Background, Principle and Aim

It is known that the geographical and ecological distribution of most organisms is more re- stricted than their physiological tolerance would allow (Lampert and Sommer 1999). One explanation can be the interactions between different species. Organisms can influence the availability of resources (or light, see Huisman et al. 1999; Litchman et al. 2004) or can themselves be a food resource for other organisms. Here, a resource is defined as a con- sumable factor, for which an increase in availability leads to increase per capita reproductive rates through at least some range of its availability (Tilman 1977, 1982, 1986, 1987 and 2004). Interactions between organisms

sharing the same resource are defined as competition. These interactions can be indi- rect when based on common resources, or direct when they consist of immediate damage to or interference with the competi- tor. Competition can result in reduction of survival, growth and/or reproduction of the individuals concerned.

G. F. Gause demonstrated experimentally that only one species survives if two spe- cies are competing for the same resource (Gause 1934a and 1934b). In his experi- ments, three protozoan species of Parame-

cium were investigated competing for the

same resource. All three species were grown successfully when cultivated in sepa- rate cultures. P. aurelia excluded P. cauda-

tum to extinction when grown in mixed cul-

ture, although they reached nearly identical densities when grown in separate cultures (Figure 5-16; original data from Gause, 1934a). P. caudatum and P. bursaria coex- isted at much lower densities than when grown alone. Although both existed togeth- er in the same batch, they were separated

Figure 5-16: Growth of P aurelia and P. caudatum in separate and mixed cultures.

0 5 10 15 20 Time (d) separate cultures mixed cultures P. aurelia P. caudatum

Static Algae Tests

Material and Methods 51

in space. P. caudatum lived and fed on bacteria suspended in the medium, while P. bursaria used yeast cells at the bottom of the tube (Gause 1934a and 1935). These results showed clearly that only one species will survive when two or more species compete for the same resource in the same habitat. The concept of niche may explain the result of coexistence by spatial separation. This concept was introduced to explain such events. Niche is defined as combination of resources and environmental conditions (Lampert and Sommer 1999) that allows reproduction, growth and survival of individuals of a species. If a competitor is pre- sent, the niche of another species is reduced. If the niche of the inferior species is completely occupied by the superior competitor, the inferior species becomes extinct. On the other hand, a coexistence of competing species is possible by niche differentiation. P. caudatum and P.

bursaria coexisted in separate niches where space and resources were differentiated. The

concept of the niche and the above mentioned experiments lead to the competitive exclusion principle (Gause’s principle). Summarizing the principle it can be said that two competing species can coexist in a stable environment by differentiation of niches, for example different resources, habitats, or use of different sources for the same resource. In case there is no niche differentiation possible, one species will displace the other one.

Sommer concluded that Tilman’s concept of coexistence (Tilman 1977 and 1982) is not only confined to artificial two-species systems, but also valid for natural assemblages (Sommer 1983 and 1985). Competition of two or more species for resources was frequently investigated and models were developed and applied to describe the mechanisms of competition in more or less detail and complexity. One of the first and most cited models to describe competition was the Lotka-Volterra model (Lotka 1925; Volterra 1928). In general, the Lotka-Volterra models predict four possible outcomes of competition: (1) one species is eliminated, (2) the other species is eliminated, (3) both species coexist and (4) either species is eliminated depending on starting conditions. The classic Lotka-Volterra models are the simplest representation of nonlinear predator-prey interactions. The classic models give no information about the underlying mechanisms of competition but can be developed into more detailed models that reveal/describe dependencies in a more accurate way. An abstract concept of the classic Lotka-Volterra equations like the maximum population size can be replaced by physiologically relevant properties (Goudriaan and DeWitt 1973).

Ahn (2002) used two different models to describe his competition experiments, a Lotka- Volterra and a cell quota based model. They concluded that the simulation model that was based on a cell quota model was superior to the Lotka-Volterra model, because it provided detailed explanations for underlying mechanisms of competition and is more flexibly applica- ble to various environmental conditions. The above mentioned results support the implemen- tation of a cell quota submodel into the developed algae model. Therefore, a use as core

model for complex ecosystem models, that can take multi-species interactions at higher trophic levels into account, is possible. The competition experiments that were performed during this work were not designed to investigate the basic mechanisms of competition. The aim was to test and verify the capability of the developed model to describe competition, and thus to indicate the potential for an extension of the model to a more complex structure for simulations of e.g. aquatic ecosystems. Two combinations of algae competition were tested in batch cultures: D. subspicatus vs. P. subcapitata and C. terricola vs. C. pyrenoidifera. The experimental data was used to verify model simulations of algae competition and to confirm literature data of model parameter values.

Static Algae Tests

Material and Methods 53

5.3.4.2 Preparation and Setup

Competition tests for D. subspicatus vs. P. subcapitata were performed using OCED 201 nutrient medium. Test duration was 41-71 days. C. terricola vs. C. pyrenoidifera were tested using WARIS-H nutrient medium with test duration of 24-37 days. All experiments were car- ried out at constant illumination of 76±10 µE∙m-2_∙s-1 _{at 24±1°C in a growth incubator or a} growth chamber. In order to ensure exponential growth of the algae to be used as inoculum, an exponentially grown pre-culture was prepared 3-4 days prior to test start and cultivated at main test conditions. Nutrient medium was freshly prepared prior to test start. For each repli- cate, 100 mL of nutrient medium was filled into a 250 ml Erlenmeyer flask and sealed with a cellulose plug. Algae cell density was determined on specified sampling days by cell counting using a bright-lined Thoma© counting chamber. Each sample was counted four times per algae species in order to reduce standard deviations. The flagellated species C. terricola and

C. pyrenoidifera were immobilized before counting by adding 2 µL of Lugol’s solution to each

2 mL sample. As a side effect, the cells partly formed lumps and clusters and the distribution within the counting chamber was not randomized, although the sample was shaken. This resulted in higher standard deviations when counting the cells. The three competition tests with D. subspicatus vs. P. subcapitata started with three different ratios of initial cell densi- ties, namely 1:1, 1:3 and 3:1 with three replicates per ratio each. Three tests with C. terricola and C. pyrenoidifera were performed with ratios of 1:1 and 1:3; two tests with a ratio of 3:1; each with three replicates per ratio. An overview of performed competition experiments is given in Table 5-10.

Table 5-10: Overview of performed competition experiments

Test Species Duration _(d) Inoculated cell density _(cells∙mL-1₎ Light intensity _(µE∙m-2_∙s-1₎ Temperature _(°C)

1-3 D. subspicatus vs. P. subcapitata 47 1·104 _{: 1·10}4 _{76±10 24±1} 1-3 D. subspicatus vs. P. subcapitata 47 1·104 _{: 3·10}4 _{76±10 24±1} 1-3 D. subspicatus vs. P. subcapitata 72 3·104 _{: 1·10}4 _{76±10 24±1} 1-3 C. terricola vs. C. pyrenoidifera 37 1·104 _{: 1·10}4 _{76±10 24±1} 1-3 C. terricola vs. C. pyrenoidifera 33 1·104 _{: 3·10}4 _{76±10 24±1} 1-2 C. terricola vs. C. pyrenoidifera 20 3·104 _{: 1·10}4 _{76±10 24±1}

5.3.5 Growth Inhibition