de Wit Replacement Series
The de Wit replacement series is a widely used technique to assess the resource use and productivity in systems with competing species (especially plants). It is also very useful to evaluate the interspecific and intraspecific competition in different species.
Intraspecific competition is the competition for a resource (or more than one) between individuals of the same species, while interspecific competition is the competition between individuals of different species.
To use the de Wit series to test the competition between two different species we need to cultivate each of the species alone (monocultures) and paired with the competing species in increased percentages. For example, if we have species A and species B one reasonable way to set up the treatments of a de Wit series would be:
With these treatments, we can evaluate the productivity (yield) of both species when grown in monocultures (first and last row of the table). Having all the intermediate treatments with different proportions of both species allows us to evaluate the intraspecific and interspecific competition.
This graph below illustrates the outcome of an experiment designed to assess the productivity of two species when grown both alone and together, allowing for the analysis of intra- and interspecific competition.
In figure one, you can see the typical example of a de Wit series graph. At the bottom X-axis you can find the % of species A, while at the top X-axis you can find the % of species B, and at the Y-axis the yield of species A.
The blue line depicts a 1:1 relationship, indicating a balance point where the impact of intraspecific competition (among individuals of the same species) equals the impact of interspecific competition (between individuals of different species). This means that at any point along this line, adding one more individual of Species B has the same effect on Species A's yield as adding another individual of Species A. The red line is the data line, representing the actual data observed from the experiment. In this case it falls below the 1:1 line, suggesting that interspecific competition (between Species A and Species B) is stronger than intraspecific competition (among individuals of Species A). If we look at the 50%/50% data point we can see that the yield obtained is around 0.75 kg·m-2. However, the expected yield with no competition is 1 kg·m-2 (1:1 line at the same point), which means that the competition is reducing the yield in about 25%.
Figure 1: Example of a de Wit series graph.
To read this graph in a general context, you should understand that the farther the data line (red) deviates from the 1:1 line (blue), the greater the impact of interspecific competition compared to intraspecific competition. When interpreting such graphs, it is essential to consider not only the presence and proportion of each species but also how their interactions affect overall productivity.