Photoperiod (day length) plays an important role in regulating the timing of migration as changes in photoperiod influence temperature, rainfall, and ultimately primary productivity. Most plants respond to changes in photoperiod by producing seeds, new growth, and/or fruit at specific times of the year. A common animal response to shifts in vegetative production is migration. A classic example of a migrating species is the Serengeti wildebeest which migrates from areas of low resource production to areas of high resource production on an annual basis. Wildebeests occur throughout the Serengeti ecosystem which extends along the border region of Tanzania and Kenya. Forty percent of the Serengeti is comprised of grassland habitat which is the primary food source for wildebeests. Seasonality in rainfall throughout this region results in a dichotomous climate in which most of the rainfall occurs in the wet season from January-May, with very little rainfall occurring during the dry season (JuneAugust). Grass production is high during the wet periods and virtually nonexistent during much of the dry season. Throughout the year, wildebeest will move from areas of low grassland productivity to areas of high grassland productivity.
Migratory species often modify the habitat in the area they migrate to, themselves having a seasonal impact on the environment. For example, many waterfowl congregate at breeding grounds where they reproduce and feed prior to migrating back to their winter habitat. It is not uncommon for breeding grounds to have tens of thousands of birds concentrated in relatively small areas for 23 months at a time. During these events, foraging birds have direct impacts on the plants they feed upon. Grubbing (digging up plant roots) by large numbers of waterfowl can have detrimental consequences on the environment as these areas are stripped of vegetation and plant community composition is significantly altered. However, for some plant species, heavy grazing (foraging on aboveground plant material) actually increases the overall net aboveground primary production, and plays a critical role in maintaining aboveground biomass and species composition of the vegetation. This occurs because although birds are eating plants, they are not killing them; and while foraging they are defecating on the ground, essentially adding fertilizer for plant growth. In wintering grounds, migratory waterfowl primarily roost in wetlands and spend most of the day foraging in upland habitats. Daily foraging forays to uplands followed by returns to roosting sites creates an agent for nutrient transport between habitats, which in turn can influence local ecosystem processes. The transfer of nutrients into wetlands have positive impacts on many aquatic plant and animal species; however, large numbers of migratory waterfowl roosting in small wetlands can result in high levels of nutrient loading which can be detrimental to water quality and ultimately the aquatic ecosystem.
Seasonal shifts in primary productivity driven by changes in photoperiod and/or rainfall have consequently played an important role in selecting for behavior of many species such as wildebeests and waterfowl, to move between hospitable habitats on an annual basis. This in turn influences the habitat in the areas they migrate to. Furthermore, top predators, such as lions and foxes, gain a substantial amount of their annual energy budget from migratory animals as they migrate through, or into, their ranges. Thus, these migratory behaviors in turn have direct and indirect impacts on the environment, which can result in large ecological effects that ripple throughout the food web.
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