Ecology deals with the interactions of organisms and their physical and biological environment

Definitions

• Population : a group of individuals of one particular species which live in a particular geographic area

• Life history: Set of rules and choices which control an individual's schedule of reproduction, including: reproduction time, number of offsprings, life cyle and reproductive value

• Time to reproduction ( maturity, age of first reproduction). The reproductive maturation a few days in some insects or longer then ten years as in humans. A long developmental time requires an parental envolvment in the raising of the offspring. Some animal have a long maturity time but no extensive parental care ( sea turtles, large reptiles and fishes)

• Reproductive value: the number of offspring produced per reproductive episode and the probability of their survival.
  Reproductive value : = # of organisms that have offspring _* probability of offspring survival
  The reproductive value increases up to sexual maturity and declines with increasing age. At the end of its reproductive phase, the reproductive value of an organism is zero. Reproductive value is important for selection: selection is high if the reproductive value is high. Some animals produce millions of eggs in each reproductive event ( cod, sea urchins) . The more offspring the less investment in parental care. Large number of offspring is usually accompanied by high mortality rates. Larval mortality may be so high, resulting in a decrease of the population (year class variation in cod ). Fig. 54.4

• life cycle : some organisms assumes different morphological shapes and behavior during different stages in their life cycle ( haploid/ diploid life cycle , fern )
  Cnidarians can exist as either a polyp (an anemone like organism), or a medusa (a jellyfish like organism )   Jellyfish     life cycle    

• Reproductive effort: metabolic needs (growth, maintenance) compete with reproductive energy expenditures. Large energy consumption in reproductions leads to shorter life span Some organism reproduce once and then die (salmon)     Fig. 54.12 , tree rings

• Population structure: refers to the age and density of a population. Population density impacts competition for recourses, reproductive behavior and predetation. Large year classes dominate throughout their lifetime. Human population: France/India   Fig. 54.3-1     Whitefish   Fig. 54.3-2

• Tabulation of the above graphical information: Fig. 54.1. The example does not consider reproductive behavior, it shows only survival but no new additions of offspring. For each age interval shown in the table , survivorship, survival rate and mortality rate are listed. ( Cohorts are individuals who were born at the same time)
  A complete life table would consider reproductive behavior : # of offspring produced per age interval and expected # of offspring / age interval ( probabilty of surviving to end of age interval and average # of offspring produced)

• General demographic equation
  N_(t) = N_(o) + B_(t) - D_(t) + I_(t) - E_(t)
  N = number of individuals present at time =0 and time = t , B = number of individulas born, D = number of individuals who died, I = number of individuals immigrated, E = number of individuals emigrated
  

• Exponential Population Growth : Exponential population growth (unlimited growth) is only appropriate for describing the initial colonization of empty habitats.
  the differential equation describes unlimited population growth : dN/dt = rN
  N   is the population size (number of individuals),
  dN/dt   is the change of number of individuals over a period of time
  r   is the growth rate
  The rate of (net) growth : r = b (intrinsic birth rate ) - d (intrinsic death rate)
  integration of the differential equation   dN/dt = rN   yields :   N_t = N_o e^rt
  When there are no limits to growth r is at its highest : r_max
  exponential growth Fig. 54.5 a,b

• Exponential growth occurs only for a short time until the environment can no longer support the increasing number of individuals. Growth slows down and an S-shaped growth pattern (logistic growth) emerges as the number of individuals reaches carrying capacity.
  Logistic Growth : dN/dt = r [(K - N)/K] N
  K   is the maximum number of individual that an be supported in a particular environment (carrying capacity)
  If N is much smaller then K , the population growth is exponential. The maximum growth at K=N/2 ). When N = K the growth stops.  Fig. 54.5 c

• Factors determining carrying capacity ( number of individuals that can be supported) :
  Factors depending on the population density
  • competition for food, space
  • predation
  • disease (when crowded, more chance for disease)
  
  Population density independent factors
  • climatic factors: storms, extreme temperatures, drought
  Factors regulating population size  Fig. 54.7

• Disturbance and Equilibrium
  Equilibrium is established if the rate of birth = rate of death. Physical disturbances (severe weather), biological disturbance ( arrival of new predators, e.g. humans) prevent populations from reaching equibrium. Generally biological diversity is reduced, however, less severe disturbances can enhance diversity since some organism may be able to adapt to the disturbance
• Human impact on natural populations
  Exploitation of natural population has dramatically reduced their size and led to the extinctions of many species. North American Buffalo, moa. Recently some attemps have been made to manage rather than exploit a natural populations Fisheries: if a small extent of the population is killed, then competition decreases, relative amount of food supply increased and growth rate should increase.
   Fig. 54.14-1  Fig. 54.14-2  graph

• Human Population Growth
  The 20 th century saw an explosives increase in human population increased pressures on ecology, food production, and socio-economic factors. Resources are populations are unevebly distributed
  US population    Fig. 51.16-1  Fig. 51.16-2