The study of the influence of the environment on fluctuations in population size and composition
A group of individuals of the same species that occupy the same area, use the same resources, and have a high probability of interacting and breeding with each other.
the number of individuals per unit area or volume
the pattern of spacing within the individuals of a population
Common techniques used to gauge population size:
- Mark-recapture method
- Indirect indicators (burrows, nests, etc)
Changes in population density caused by:
May indicate a heterogeneous environment, with organisms congregating in suitable microclimates
May be related to competition and result from interactions between individuals. Territoriality can cause uniform dispersion.
Uncommon. Indicates no strong interaction between individuals or a fairly consistent habitat
the study of vital statistics of a population, such as birth and death rates
presents age-specific survival data for a population. Constructed using a cohort of organisms from birth to death, and finding the proportion of the cohort surviving at each age group
shows the number or proportion of members of a cohort still alive at each age. Often based on a starting pop. of 1000, so other species with different population sized can be compared
Types of survivorship curves:
- Type 1: Low young/middle age mortality; rapidly increases at old age (Organisms with few offspring and high parental care)
- Type 2: Mortality relatively constant
- Type 3: High early mortality rate; the few that survive are likely to reach adulthood. (Organisms with many offspring and little/no parental care)
Gives the age specific reproductive rates in a population. Usually only follows female reproductive output. (Avg. # of daughters per female in each age class; proportion of females breeding times the number of female iffspring they produce)
Life history of an organism
Reflects evolutionary trade-offs between survival and reproduction. Traits include:
- -Age at first reproduction
- -How often an organism breeds
- -Number of offspring produced during each reproductive cycle
Big-bang reproduction. Putting all reproductive resources into a single reproductive effort. Common in organisms with unpredictable environments and low chances of survival.
Making repeated reproductive efforts over a period of time. More common in organisms with predictable environments with high competition, where a smaller population gives a higher chance of survival
Tradeoffs and life histories
Increased reproduction and higher care in reproduction increases chances of survival for the child generation, but decreases chance of survival for parent generation. (Finite energy budget; cannot maximize all life history traits simultaneously)
Rate of increase
Ignoring im/emmigration, change in N (pop size) = births - deaths. Birth (b)/death (m) rate can be expressed as per capita rates: instances per individual per unit of time. So change in N per time unit = bN - mN
Change in population at any one instant: dN/dt = rN (differential calc)
Per capita rate of increase (r)
r = b - m
Zero population growth
r = 0 (No change in population)
- Happens under ideal conditions
- intrinsic rate of increase (rmax): fastest per capita rate of increase possible for a species
- dN/dt = rmaxN yields exponential growth
Carrying capacity (K)
the maximum sustainable population size that a particular environment can support at a particular time
Logistic Growth model
- per capita rate of increase decreases as population nears K.
- dN/dt = rmaxN((K-N)/K)
seen in some populations when individuals benefit from increased population. (physical support, social interaction important to learning, etc.)
Few, large offspring produced regularly. Seen when population density is an important factor.
Many small offspring produced early on in life. Seen when population density is less important.
Density dependent/density independent
- Density dependent: when death rate increases and birth rate falls with increasing density. Negative feedback may limit population in density dependent populations (limited territorial space, increased competition, transmission of disease, accumulation of toxic wastes, predation, intrinsic (chemical) factors)
- Density independent: when death/birth rate does not change with increasing density
Study of variations and fluctuations in population size and the factors that cause them. Examples:
- -Herbivores: severity of winter
- -Dungeness crab: density dependent cannibalism, changes in temp, ocean currents
A group of populations in which im/emigration may significantly influence individual population sizes
Predator cycles follow the cycles of their prey. In snowshoe hares, 90% of deaths are due to predation by lynx, although winter food supply played a small role.
The movement from the first configuration to the second
influences present and future growth. Larger proportions of individuals at reproductive age or lower results in more rapid growth now or in the near future.
Takes multiple human needs in estimating carrying capacity. Ecologically productive areas used to determine a country's ecological footprint: Arable land, pasture, forest, ocean, built-up land, and fossil energy land (vegetative area required to absorb CO2 from burning fossil fuels) Estimated 2 hectares per person (1.7 considering land conservation and parks)
Global human population
- Annual population rate is low (~0/1%) in developed nations, and high (~1.4%) in developing nations. 80% of world's pop in developing countries.
- Human population is unique; can be consciously controlled
- Earth's carrying capacity for humans is ~10-15 billion