Table 1.

A glossary of model terms and their definitions.

model term	definition
module occupancy (N)	the number of colonists per module
preferential allocation (b)	the difference between the amount of resources allocated to modules colonized entirely by mutualists and modules colonized entirely by exploiters, expressed in units of per capita fitness
symbiotic service cost (z)	the cost that mutualists pay to generate host benefits; equal to the difference in mutualist and exploiter fitness within a mixed module
initial allocation (v)	the initial resources allocated to all symbionts within a module (mutualist and exploiter) in order to establish symbiosis
half-saturation constant (α)	the proportion of preferential allocation that goes into modules with a 50 : 50 mix of mutualists and exploiters, determining whether the function is linear (α = 0.5), saturating (α > 0.5) or accelerating (α < 0.5)
mutualists (m versus M)	symbionts that provide a costly service to their hosts; the proportion of mutualists in the population is m, while the number of mutualists within a module is M
exploiters (1 − m versus N − M)	symbionts that consume host resources without generating any services; the proportion of exploiters in the population is 1 − m, while the number of exploiters within a module is N − M
proportion of module occupancy N (PN)	an empirically determined frequency of modules with N colonists