Abundance
Theory suggests that abundance is largely driven by processes relating to life history and interactions. Life history theory captures the effects of several key demographic processes that directly affect abundance, and the related fast–slow life history continuum distinguishes between species with “slow” life histories (K- strategists), which are regulated by density-dependence, and those with “fast” life histories (r- strategists), which are more strongly influenced by growth and reproductive rates. Whereas in theory, r- strategists should have higher abundance thanK- strategists owing to their higher population growth rates , there are significant indirect effects from the environment that make it difficult to generalise the relationship between life history strategies and abundance. For example, K- strategists can be quite abundant in resource-poor environments, where resource limitations constrain the feasibility of fast life history strategies . Conversely,r -strategists could hypothetically remain rare if disturbance occurs so frequently that there is insufficient time to rebuild populations between disturbances.
Interactions also play a significant role in regulating local abundance through resource–consumer dynamics and density-dependent effects. Consumers are limited by the availability of the resources they require and the pressure they exert on them; this may in part explain the widely observed negative correlation between body size and abundance , which may be attributable to the higher energetic requirements of larger organisms . Density-dependence, whether interspecific or intraspecific, can play a strong role in regulating population size. Predators and pathogens may constrain the abundance of their prey via cyclical (e.g., or non-cyclical (e.g., ) effects. Finally, negative intraspecific density dependence, which is stronger in rare species than in common species , may be an adaptive mechanism that promotes persistence in some naturally rare species .