While multiple reasons make long-term demographic studies
disproportionally valuable, much of their importance comes from the
ability to detect and quantify rare events. We argue here that rare
events can be critical for understanding important ecological and
evolutionary processes. We highlight the additive or interactive nature
of anthropogenic rare events with environmental rare events that may
cause outsized changes in vital rates and therefore in population
dynamics. Rare events may also generate complex responses in populations
due to interactions between demographic processes and evolutionary
responses. Complex, non-linear dynamics of populations may include
threshold, extreme responses such as long transients, tipping points,
regime shifts and collapse. When occurring locally, rare events may also
exacerbate spatial heterogeneity with consequences for demographic
processes. In sum, these effects represent substantial challenges for
prediction, especially when considering the increase in the frequency of
rare extreme events, and emphasise the need for long-term studies. Our
perspective attempts to integrate the occurrence of rare events in
variable environments and the consequences for the overall fitness,
growth rates, and the spatial-temporal dynamics of populations.