Table 2. Basic classes of fundamental niche shapes, and corresponding stress interactions.

Niche shape	Type of interaction	Description	Functional form	Mechanism/example	M
Rectangular	Non-interacting	Growth rate is determined by the more severe of the two stresses and is completely unaffected by the other stress. As noted by Hutchinson, the corresponding niche shape is rectangular (GE Hutchinson, 1957)	g(S1,S2)=min{f1(S1),f2(S2)}. Note that f1(S1) and f2(S2) can be any two arbitrary functions	The cellular mechanisms related to the two stresses, and/or the limiting cellular resources used to alleviate their influence are distinct	M=1
Diagonal	Additive	Growth rate is determined by a linear combination of the stresses	g(S1,S2)=f(a1*S1+a2*S2), where f(a1*S1+a2*S2) is any arbitrary function of its argument, and a1, a2 are the amount of common resource required to cope with stresses S1 and S2, respectively	For each unit of either stress, a common cellular resource is dedicated to alleviate stress effects. The cellular resource may be ATP, a mineral, specific proteins (for example chaperons) and others	M=0.5
Convex Super-convex	Antagonistic	The combined stress incurred by a combination of two factors is less severe than the stress incurred by only one factor at the equivalent level	Multiple compatible functional forms	Concentrations of relevant cellular components under either stress alone are similar	0.5<M<1 M>1
Concave	Synergistic	The combined stress incurred by a combination of the two factors is more severe than the stress incurred by only one factor at the equivalent level	Multiple compatible functional forms	Concentrations of relevant cellular components under either stress alone are disparate	M<0.5