Course: BIS 105 Environmental Biology

Unit 4 Learning Module

Movements of Populations

Migration and Other Movements Affect Population Densities

Populations change not only through births and deaths, but also via immigration, movement of individuals into a population from other areas, and emigration, movement of individuals out of a population. If we add per capita rates of immigration and emigration into our equation for population growth rate, it becomes:

r = (b + i) – (d + e)

growth rate = (birth rate + immigration rate) – (death rate + emigration rate)

Most species have some means of dispersal – movement of offspring away from the parents. This “behavior” reduces competition within the population, promotes colonization of suitable habitat, and improves reproductive success. Some dispersal mechanisms take advantage of natural energy in the environment. For example, dandelion seeds grow “parachutes” which allow wind to carry them far from their parents – and sometimes entirely out of a population. For the same reason, immobile animals such as corals often produce motile larva. Mobile animals often evolve behaviors which ensure dispersal. A lone gray wolf which leaves its birth pack must find a mate and an unoccupied territory in order to reproduce; within the pack, usually only the alpha male and female have offspring. Dispersal behaviors are common in the living world; have you - as a teenage high school student, begun to feel stirrings of the wish to leave home?

Wind carries dandelion seeds away from their parent plants. The parachute adaptation allows for dispersal, reducing competition within the population and promoting colonization of suitable habitat.

Migration, the direct, often seasonal movement of a species, is a predictable change for some animal populations. Many northern hemisphere birds, such as Swainson's Hawks, migrate thousands of miles southward in the fall and return north to nest in the spring in order to follow summer’s long days which provide extra hunting time and a greater abundance of food.

Entire populations of Swainson’s Hawks migrate annually from North America to South America and back. Migration can affect all four factors of the growth rate equation: rates of birth, death, immigration, and emigration.

Apparently, energy benefits outweigh costs for this annual long-distance commute. Elk migrate vertically – up the mountains in spring as snow recedes and down the mountains in fall as winter advances. Monarch butterflies migrate in “shifts”; somewhat like a relay team, successive generations divide the task of moving from Mexican wintering grounds to northern summer habitats. Such migrations do not add to or subtract from populations as much as they move entire populations from one set of boundaries and environmental conditions to another. Some species, such as Peregrine Falcons, have both migratory and non-migratory forms, so their populations may grow or decline with migration. Gray Whales migrate 12,500 miles from Alaska to Mexico for calving, but at least one population limits its northward journey to the Oregon coast. Seasonal densities of migratory species vary considerably, but resources and environmental benefits vary as well. Migration can affect all four factors of the growth rate equation.

Other types of movement are less predictable, but still may affect population growth.

- Nomadism, regular, wide-ranging wandering behavior, allows some species to compensate for fluctuating food sources. Normally arctic species, Snowy Owls occasionally venture as far south as Texas, southern Russia, and northern China. Bohemian waxwings are notoriously nomadic, feeding on highly variable berry supplies.

- Irruptions or invasions are irregular movements, often caused by food source failures. Owls such as Great Grays and Boreals occasionally invade northern US states from their Canadian homes when rodent populations decline. Some may remain to nest following such an irruption.

- Range expansion involves the gradual extension of a population beyond its original boundaries. Recent examples in the US include Cardinals, now common in northern areas where they were originally absent. The Swainson’s Thrush follows an indirect and unnecessarily long migration path - retracing, scientists believe, a range expansion from 10,000 years ago. Intentional introductions of non-native species such as the House Sparrow and reintroductions of extirpated species such as Peregrine Falcons throughout the Eastern US are human-initiated colonizations, which are often followed by range expansions.

- Closely related to range expansion is colonization, but the latter often involves newly created, or at least newly found, habitats. Illustrating both range expansion and colonization, the small red-eyed dragonfly spread throughout Europe in the late 20^th century and colonized Britain in 1999.