Figure 3.

Intraspecific competitiveness has the strongest effect on cancer risk in environments with low extrinsic mortality and strong trade-offs. When extrinsic mortality is low (μ = 0.01) and trade-offs between cancer defence and reproductive competition is high (β = 1), as intraspecific competition increases (α), cancer defences remain high until a critical threshold is reached (when intraspecific competition is greater than 1) and then dramatically declines (upper left). Cancer defence declines in all cases with α, but is sometimes low to begin with (upper right corner where extrinsic mortality is high). Parameters as in figure 1 and figure 2, and additionally β = 1, 2 or 5 (top, middle and bottom row, respectively), and μ = 0.01, 0.1 or 1 (left, middle and right column, respectively). Colours indicate the fitness consequences of each possible level of defence effort (0 ≤ d ≤ 1 as indicated on the y axis) for value of intraspecific competition (α) ranging from 0.01 to 10. More yellow indicates higher fitness, more blue low fitness; the black line additionally indicates highest fitness (optimum).