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 .