Table 2. Scaling exponents (, , ) for the different species.

Species				Scale-free	Exponential	Fractal
eco	1.9(1)	3.6(3)	1.3(4)	Yes	No	Yes
sce		3.0(2)	1.5(1)	No	Yes	Yes
ath		1.5(1)	2.1(2)	No	Yes	Yes
cel	2.6(1)	1.6(1)	1.8(2)	Yes	No	Yes
dme	3.0(1)	1.6(1)	1.3(2)	Yes	No	Yes
mmu	2.9(1)	1.7(1)	2.0(1)	Yes	No	Yes
hsa		2.9(2)	2.0(1)	No	Yes	Yes

According to the values of the scaling exponents, the seven species listed are grouped into two categories: scale-free fractal networks and exponential (non-scale-free) fractal networks. The scale-free networks have a power-law degree distribution with exponent , and the non-scale-free fractal networks have an exponential degree distribution with . Notice that none of the networks are small-world. Instead, they are characterized by fractal/modular structures.