Chapter 40 Part 2 - Population Ecology

Population ecology

study of populations in relation to environment

• includes environmental influences on density and distribution, age structure, and population size

population

group of individuals of a single species living in the same general area

density

number of individuals per unit area or volume

• is the result of an interplay between processes that add individuals to a population and those that remove individuals

dispersion

pattern of spacing among individuals within the boundaries of the population

how to determine population size

extrapolation from small samples, index of population size, or the mark-recapture method

mark-recapture method

population size estimated by

N = (M x C) / R

• M = number marked first time

• C = total sampled second time

• R = number of recaptures

• N = population size

immigration

influx of new individuals from other areas

emigration

movement of individuals out of a population

Patterns of dispersion

• Clumped dispersion - individuals aggregate in patches

  • may be influenced by resource availability and behaviour

• uniform dispersion - individuals are evenly distributed

  • may be influenced by social interactions like territoriality

• random dispersion - position of each individual is independent of each other

  • occurs due to absence of strong attractions or repulsions

demography

study of the vital statistics of a population and how they change over time

• death rates and birth rates are of particular interest to demographers

life table

age-specific summary of the survival pattern of a population

• best made by following the fate of a cohort

  • cohort - individuals of the same age

survivorship curves

1. Type 1 - low death rates during early and middle life, then an increase among older age groups

2. Type 2 - constant death rates over life span of organism

3. Type 3 - high death rates for the young, slower death rate for survivors

How do demographers take into account of gender

• often ignore males and focus on females in a population since only females produce offspring

  • view populations in terms of females giving rise to new females

Idealized situations

• useful to study populations growth since they help us understand capacity of species to increase and the conditions that facilitate this growth

Zero population growth

occurs when birth rates equals death rate

Calculating change in population size

ignoring immigration and emigration

Change in Number (delta N) / Change in Time (delta T) = R = B-D

• N - population size

• B - births

• D - deaths

• R - change in population size during a given time interval

Exponential Growth Model

dN/dt = rN

dN/dt - change in numbers over change in time

r - population growth rate

N - population size

Intrinsic rate of increase

r - intrinsic rate of increase

• per capita rate where an exponentially growing population increases in size at each instant in time

• rate of reproduction (r) is at its maximum when under ideal conditions

takeaway on exponential growth

• it is population growth under ideal conditions

• when population size is small and many resources, growth is often close to exponential

• cannot continue indefinitely

  • will be limited by resources eventually

Carrying capacity (K)

maximum population size that a particular environment can sustain

logistic growth model

instantaneous rate of population growth = r x N [ (K-N) / K ]

• r - intrinsic (maximum) rate of growth

• N - current population size

• K - carrying capacity

in logistic model, when population is small, per capita growth rate is….

near maximum ( = r )

takeaway messages about the logistic model

per capita growth rate is

• near maximum ( = intrinsic rate of growth, r) when population size, N, is small

  • growth is near exponential

• slows as population size, N, increases toward carrying capacity, K

• Zero when population size, N, reaches carrying capacity, K

• negative when population size, N, exceeds carrying capacity, K

what shape does the logistic model of population growth have

Sigmoid (s-shaped) curve

The Logistic Model and Real Populations

• growth of laboratory populations of paramecia fits an S-shaped curve

  • they grow in a constant environment lacking predators and other competitors

• some populations overshoot K before reaching an approximately stable size

Density-dependent selection (K-selection)

selects life history traits that are sensitive to population density

• birth rates fall and death rates rise with population density

• this is an example of negative feedback that regulates population growth

factors that affect density-dependent birth and death rates

• competition for resources

• terriroriality

• disease

• intrinsic factors

density-independent selection (r-selection)

selects life history traits that maximize reproduction

• birth rates fall and death rate do not change with population density

competition of resources and density-dependent birth and death rates

in crowded populations, increasing population density intensifies competition for resources and results in a lower birth rate

territoriality and density-dependent birth and death rates

competition for territory may limit density in many vertebrates and some invertebrates

• ex. cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries

disease and density-dependent birth and death rates

in dense populations, pathogens can spread more rapidly

• in human populations, respiratory diseases influenza, COVID, measles and tuberculosis are spread through the air when an infected person sneezes or coughs

• ex. sea star wasting, white-nose syndrome in bats

intrinsic factors and density-dependent birth and death rates

intrinsic physiological factors operating within an individual organism sometimes regulate population size

• ex. in white-footed mice at high population densities, hormonal changes within individuals delay sexual maturation and depression the immune system

population dynamics

population fluctuations from year to year or place to place

• focuses on the complex interactions between biotic and abiotic factors that affect populations

metapopulations

local populations within populations

• can be thought of as occupying discrete patches of suitable habitat in a sea of unsuitable habitat

  • can vary in size, quality, and isolation from other patches that affect how many individuals move among populations

immigration, emigration, and metapopulations

• when population increases and resource competition increases, emigration often increases

• immigration and emigration may become important when local populations are forming a metapopulation