Why are some species rare? Mathematical models of plant-soil feedbacks in diverse communities

Background/Question/Methods

Among species, rarity is common.  Species can be rare for many reasons,
such as habitat or resource limitation, and strong top-down control.
Presumably, few species have chosen rarity as a strategy, and thus must
be constrained to be rare by forces from which evolution provides no
escape.  Abundance is ultimately controlled by the strength of
intraspecific density dependence, itself determined by the organism's
ecology.  We develop a suite of mathematical models of diverse plant
communities where species compete for resources with contrasting levels
of complexity and structure, and parallel models for interactions
between competing plants and their natural enemies in the soil.  We
find equilibrium population sizes in relation to the strength of
intraspecific density dependence in both cases, and then extend the
models to incorporate interactions between resource and natural enemy
limitation, and to allow evolution of plant strategies that trade off
resource collection efficiency and defense against pathogens.

Results/Conclusions

Our models show that different forms of resource limitation, whether
having a higher R*, or access to only those resources that other species
have not used, can but need not necessarily lead to lower abundance of
species that might be judged as competitively inferior.  Similarly,
species attacked by the most efficient natural enemies can but need
not have lower abundance.  However, when strategies that enhance
resource collection ability trade off with defenses against natural
enemies, the patterns of abundance more closely match those of
resource limitation.  When those strategies can evolve, communities
develop a more predictable and consistent structure.