Known numbers of seeds from two species (
and 
) of annual plants are mixed together in different proportions and planted in five small plots of soil in the spring. The plants grow, flower, and produce seeds. It is found that the percentage of seeds of species and species in the harvest is usually different from the proportion that was planted, although the total number of seeds produced is the same as the number of seeds planted. The data are plotted on the graph below.
What do the data indicate about the ecological relationship between species and species ?
Select an Answer
and 
are mutualistic for low percentages of seeds.
and are mutualistic for high percentages of seeds.
and compete when both and seeds are present.
competes successfully against at all percentages of and seeds.
is a parasite of when is rare.
View Correct Answer
The graph shows that the survival of either of the two species depends on the relative abundance of each species at the time of seed planting. In a mutualistic relationship both species benefit, but the data do not show that this is true at either low or high percentages of species X, ruling out choices (A) and (B). Species Y does not compete successfully against species X in all mixtures, ruling out choice (D). There is no evidence for parasitism, ruling out choice (E). Competition occurs in all seed mixtures and species X is a stronger competitor in most of the seed mixtures.