Ecology 2 Population Ecology

 A population is a group of individuals of a single species living in the same general area.  They rely on the same resources and are influenced by the same environmental factors. Soay Sheep from the Scottish island Hirta, an ancient and rare breed. Qu. Why do populations of some species fluctuate greatly while others do not?

 Density is the number of individuals per unit area.  In rare cases, biologists can physically count the number of individuals.  However, most often biologists use a variety of sampling techniques. Hector’s Dolphins If organisms move too quickly to count, biologists may use the Mark-Recapture sampling method to estimate population size.

 Additions to populations occur through births and immigration (the influx of new individuals from other areas.)  Removing individuals occurs by death and emigration (the movement of individuals out of the population)

 Demography is the study of the vital statistics of populations and how they change over time  Life Tables are age- specific summaries of the survival patterns of a population Life Table for Belding’s Ground Squirrels Age No. No. Death Avg. (yrs)alivedeathsratelife at startexp l

 A survivorship curve is a plot of the proportion or numbers in a population that are still at each age.  Assume 1,000 squirrels were present at the beginning, and plot the numbers vs. age. This survivorship curve indicates relatively constant rates of death; males have a lower survival rate overall than females. (Type II curve)

 Survivorship curves can be classified into three general types:  Type I—flat at the start, reflecting low death rates, then drops steeply as population ages  Type III—drops sharply at start, reflecting very high death rates for young, but flattens out as death rates decline

 The exponential model describes population growth in an idealized, unlimited environment.  Under these conditions, there are no restrictions on the abilities of individuals to harvest energy, grow and reproduce.

 Change in population size = (B + I) – (D + E)  (Births + Immigrants )- (Deaths + Emigrants)  N = (population size) ∆N = change in population size  t = time ∆t = time interval Simplified:  Thus: ∆ N ∆t = Births - Deaths

 Population ecologists are most interested in the difference between the per capita birth rate and the per capita death rate. This difference is the per capita rate of increase, or r:  r = b-d (Rate of increase = birth rate – death rate)  Zero Population Growth occurs when the per capita birth rate and death rate are equal. Births and deaths still occur, but they balance each other exactly.

 Birth rates exceed death rates. The population accumulates more new individuals per unit of time when it is large than when it is small, thus the curve gets increasingly steep. Can this be sustained? The population of elephants in Kruger National Park, South Africa grew exponentially for 60 years after they were first protected from hunting. The increasingly large number of elephants eventually caused enough damage to vegetation that a collapse in their food supply was likely, resulting in action by park managers.

 The logistic model describes how a population grows more slowly as it nears its carrying capacity.  Carrying capacity ( K ) is the maximum population size that a particular environment can sustain.

 As the population nears the carrying capacity, the per capita rate of increase approaches zero  The logistic model assumes that populations adjust instantly to growth. In reality, there is often a lag time before the negative effects are realized.

 K -selected organisms=  Populations of a roughly constant size whose members have low reproductive rates  The offspring produced require extensive postnatal care until they have sufficiently matured.  Humans are examples of a K -selected species.

 r -selected organisms=  They are populations that experience rapid growth of the J-curve variety.  The offspring produced are numerous, mature quite rapidly, and require very little postnatal care.  This population grows fast, reproduces quickly, and dies quickly. Bacteria and mice are examples of r- selected species

 Many factors that regulate population growth are density- dependent.  A death rate that rises as population density rises is density dependent.  In other words, there is a negative effect with a large population density. In a crowded population, increasing population density intensifies competition for declining nutrients and other resources, resulting in a lower birth rate.

 Without some type of negative feedback between population and the vital rates of birth and death, a population would never stop growing.  Density-dependent regulation provides that feedback, helping to reduce birth rates and increase death rates. Territoriality--Gannets nest virtually a peck apart. Up to a certain population density, birds can find a nesting site—beyond that, few additional birds can reproduce successfully.

 Competition for Resources  Territoriality  Disease—can be worse in crowded conditions  Predation—predators can encounter and kill more prey when crowded  Toxic Wastes—build up in crowded conditions  Aggressive interactions that increase with population size

 These factors occur when the birth rate or death rate does not change with population density.  A drought, for example will affect the death rate of grasses regardless of how crowded the plants are.

 Human population growth is no longer growing exponentially, but is still increasing rapidly  The rate of growth began to slow in the 1960s. Current models project an annual growth rate of.4%  By 2050, population could be 9 billion. This shows the movement of Sweden to Zero Population Growth where there is a low birth rate and a low death rate. This took about 150 years. Mexico should achieve this sometime after 2050.