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How can seasonal changes cause population numbers to fluctuate?

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Line graph showing world population growth from 1700 to 2000 with a steep rise after 1950, followed by three projection scenarios for 2000 to 2100: high, medium, and low, reaching between 7 and 14 billion. — population Graph of the world's estimated human population from 1700 until 2000, with population projections extending until 2100.

Seasonal changes drive fluctuations in population numbers primarily by altering the availability of life-sustaining resources and regulating reproductive cycles.12

Resource Availability and Growth

Environmental shifts, such as temperature changes or the melting of snowpacks, directly influence the abundance of food. For example, in deep lakes, a spring thaw can trigger a surge in nutrient availability, leading to rapid growth in plankton populations. Similarly, in temperate mountain regions, the brief growing season causes a surge in plant productivity, which supports corresponding spikes in animal activity and reproduction. Conversely, when resources become scarce, populations may decrease due to increased mortality or emigration.23

Reproductive Cycles

Many species have evolved biological rhythms that synchronize reproduction with favorable seasonal conditions, such as the warmth and food abundance of spring. These cycles are often regulated by hormones influenced by external cues like photoperiodism (day length). By timing births to coincide with seasons where survival is most likely, species manage their population growth rates. Additionally, seasonal migration causes local population numbers to fluctuate as species move to lower altitudes or different regions to escape harsh conditions or follow food sources.143