Population dynamic are influenced strongly by life history traits and population density Chapter 53, Sections 4 and 5

Overview  In every species, there are trade-offs between survival and reproductive traits such as frequency of reproduction, number of offspring, and investment of parental care.  Life history : the traits the affect an organism’s schedule of reproduction and survival.  A life history entail three main variables:  When reproduction begins.  How often the organisms reproduces.  How many offspring are produced per reproductive episode.

“Trade-offs” and Life Histories  No organisms could produce unlimited numbers of offspring and provision them well.  There is a trade-off between reproduction and survival.  Example: Researchers in Scotland found that female red deer that reproduced in a given summer were more likely to die the next winter.  Plants and animals whose young are likely to die often produce many small offspring.  Example: plants that colonize disturbed environment usually produce many small seeds, only a few of which may reach a suitable habitat.  Example: Animals that suffer high predation rates, such as quail, tend to produce large numbers of offspring.

 Extra investment on the part of the parent greatly increase the offspring’s chance of survival.  Example: Walnut trees provision large seeds with nutrients that help seedling become established.  Example: Primate generally only bear one or two offspring at a time.  Such provisioning and extra care can be important in habitats with high population densities.  Ecologists have attempted to connect differences in favored traits at different population densities with the logistic growth model.  Selection of traits that are sensitive to population density and are favored at high densities is known as K-selection, or density-dependent selection.  Selection for traits that maximize reproductive success in uncrowded environment is called r-selection, or density-independent selection

Population Change and Population Density  A birth rate or death rate that does note change with population density is said to be density-independent.  A death rate that rises as population density rises is said to be density-dependent, as is a birth rate that falls with rising density.  Dune Fescue Grass Example  At the University of Wales, scientists found that mortality of dune fescue grass is mainly due to physical factor that kill similar proportions of local population regardless of its density (density-independent).  Reproduction by dune fescue declines as population density increase, in part because water or nutrients become scarce.

Mechanisms of Density-Dependent Population Regulation  Without some type of negative feedback between population density and the rates of birth and death, a population would never stop growing.  Density-dependent regulation provides feedback, halting population growth by reducing birth rates or increasing death rates.  Competition for Resources  Increasing population density intensifies competition for nutrients and other resources, reducing reproductive rates.  Example: Farmers minimize this effect on the growth of wheat by applying fertilizer.  Toxic Waste: the accumulation of waste contributes to regulation of population size.  Predation  Occurs if a predator captures more feed as the population density of the prey increases.  Example: As the population of the collared lemming increases, predation by the snowy owl increases.

 Territoriality  Limits population density when space becomes the resource for which individuals compete.  Example: cheetahs use a chemical marker in urine to warn other cheetahs of their territorial boundaries.  Disease  Occurs when the transmission rate of a disease increases as a population becomes more crowded.  Example: tuberculosis and the flu strike a greater percentage of people in densely populated cities than in rural areas.  Intrinsic Factors  Intrinsic physiological factors sometimes regulate population size.  Example: White-footed mice express aggressive behaviors and hormonal changes in field enclosures. This delays sexual maturation and depresses the immune system.

Population Dynamics  All populations for which we have long-term data show some fluctuation in size.  Such population fluctuations from year to year or place to place, called population dynamics, are influenced by many factors and in turn affect other species.  Example: fluctuations in fish populations influence seasonal harvests of commercially important species.

Population Dynamics: Stability and Function  Populations of large mammals were once thought to remain relatively stable over time, but long-term studies have challenged that idea.  Example: Moose population on Isle Royale in Lake Superior.  Harsh weather, particularly cold winters, can weaken the moose and reduce food availability, decreasing the population size (1995 collapse).  When moose numbers are high, an increase in the density of parasites can cause the population to shrink.  A collapse of the moose population (1975) correlates to the introduction of wolves into their habitat (predation).

Population Dynamics: Immigration, Emigration, and Metapopulations  When a population becomes crowded and resource competition increases, emigration often increases.  Immigration and emigration are particularly important when a number of local populations are linked, forming a metapopulation.  Local populations in a metapopulation can be thought as occupying discrete habitat patches.  Habitat patches vary in size, quality, and isolation.  If one population becomes extinct, the patch is occupied can be recolonized by immigrants of another population.