Figure 1. The Lotka–Volterra model is one traditional way of exploring competition between two populations, in this case labelled N₁ and N₂. As the text describes, the two axes are the population size of the two species, and the lines show isoclines where population growth is zero. Depending upon the nature of interspecific competition, four outcomes are possible. Cases a and b top show competitive dominance, where one species can predictably eliminate the other. In case c, the two species reach a stable equilibrium, which allows long-term coexistence. In case d, competitive dominance again occurs, but the winning species is entirely dependent upon the starting size of the two populations, which is sometimes called contingent competition. The arrows show changes in population size with time. The solid dots represent the equilibrium points expected outcomes of these pairwise interactions. The open circle is an unstable equilibrium point.

Figure 2. Resource partitioning occurs when a group of species a through g harvests different sizes or kinds of resources. At one time, in a simplistic way, the amount of overlap in resource use was thought to measure the amount of competition between each pair of species. The amount of overlap is, however, shaped by other factors including evolutionary history sometimes called ‘the ghost of competition past’, intensity of present-day competition and patterns in the availability of resources.

Figure 3. Effect of competition upon two common species of desert shrubs was measured by comparing the water potential of plants having many neighbours line designated ‘control’ with plants where all neighbours of both species had been removed line designated ‘all removed’. For both species, the removal of neighbours significantly improved their water potential. ((a/b) after Fonteyn and Mahall, 1981)

Figure 4. Effects of above- and belowground competition from hardwood trees upon slash pine Pinus elliottii were assessed in four types of plots, from left to right, C=controls, S=reduced shading, T reduced root competition and ST=reduced shading and reduced root competition. See text for more details. Since reduced shading S did not increase pine growth, but the reduced root competition T allowed plants to nearly double in size, belowground competition appears far more important than aboveground competition. Corroborating evidence comes from the treatment ST, where reduced shading combined with reduced root competition was no different from merely reduced root competition. (after Putz, 1992)

Figure 5. The importance of different ecological factors probably changes along environmental gradients. Total competition likely increases from left to right, as biomass accumulates and small plants are increasingly shaded. The relative importance of root and shoot competition that is, below- and aboveground competition probably shifts as light becomes increasingly limited. Mutualism may be important at the far left where plants may ameliorate harsh conditions for their neighbours. Thus, the importance of mutualism, and competition, and above- and belowground competition, may depend upon the location of an organism. (after Keddy, 2001)