Heat waves are transient environmental events but can have lasting impacts on populations through lethal and sub-lethal effects on demographic vital rates. Sub-lethal temperature stress affects individual energy balance, potentially affecting individual fitness and population growth. Environmental temperature can, however, have distinct effects on different life-history traits, and the net effect of short-term temperature stress on population growth may lead to different population responses over different time frames. Furthermore, sublethal temperature responses may be density dependent, leading to potentially complicated feedbacks between heat stress and demographic responses over time. Here, we test the hypotheses that: (i) populations subjected to higher heat wave temperatures and longer heat wave durations are more negatively affected than those subjected to less intense and shorter heat waves, (ii) heat wave effects are more pronounced during density-dependent population growth phases, and (iii) population density patterns over time mirror the short-term population growth rate responses. We subjected experimental populations of the marine copepod Tigriopus californicus to short-term heat stress perturbations (“heat waves”) at two different time points during a 100-day period. Overall, we found that population growth rates and density responded similarly (and moderately) to heat wave intensity and duration, and that the heat wave effects on populations were largely density-dependent. We detected heat wave effects on population growth and density at low densities, but not at high densities. At low densities, we found that population growth declined with heat wave duration for the more intense heat wave intensity group, but did not detect an effect of heat wave duration within the less intense heat wave intensity group. Our study demonstrates that while ephemeral density-independent factors can influence population vital rates, understanding the longer-term consequences of transient perturbations on populations requires understanding these effects in the context of density dependence and its relationship to temperature. Higher densities may buffer the negative effects of intense heat waves and confer some degree of resilience.

Please refer to the methods section of the manuscript.

Please refer to the README_tig_pop_data.text file.