Population ecology

I.  Introduction to ecology

A.  Ecology:

• studies interactions among organisms -- biotic factors
• studies interactions between organisms and their non-living, physical environment -- abiotic factors
  • abiotic factors:  water, temperature, pH, wind
• extremely broad field with links to evolution, many biological and nonbiological disciplines

B.  Areas of ecology

-  concentrates on levels of organization above that of individual organism

• population ecology
• community ecology
• ecosystem ecology
• biosphere ecology

1.  Population ecology

-  focuses on studying populations

• population:  a group consisting of members of the same species that live together in the same area at the same time
• ex:  study population of moose, how individuals within it live, interact with each other and physical environment
  • how members compete for limited resources
  • what is population density
  • how is population dispersed
  • patterns of population growth

2.  Community ecology

• populations organized into communities
• community:  all populations of all the different species that live and interact together within an area
  • study of how species compete for limited resources
  • study of how species develop symbiotic relationships with one another

3.  Ecosystem ecology

• ecosystem encompasses a community in a specific area and links it to abiotic environment
• interactions among living organisms of a community
• interactions between the organisms and their abiotic environment
  • how temperature, humidity, light influence organisms living in Blue Ridge mountains

4.  Biosphere ecology

• biosphere is a global ecological system comprising all the Earth's communities
• organisms on biosphere depend on one another and on Earth's physical environment
  • atmosphere
  • hydrosphere
  • lithosphere
• effects of deforestation or increased carbon dioxide emissions on global warming

II.  Populations and population ecology

A.  Introduction

- population is a group of individuals of the same species in a given area

-  population ecology:

• how population interacts with environment
• intraspecific competition -- food, mates, other resources
• role of environment on population dynamics

- populations exhibit specific characteristics:

• population density
• population dispersion
• population birth/death rates
  • population growth rates
• population age structure

- example:  Norway rats in pens -- overcrowding study (John Calhoun)

- pens large enough to support 48 rats comfortably

- initial population of 32 and 56 grew to 80

• females abandon normal nesting behavior
• females abandon care of young earlier
• mortality among females and young increases (young, 96%)
• reproductive failure -- even healthiest rats produced fewer litters, no offspring survive to maturity when put in uncrowded situations
• cannibalism among males
• male "sleepwakers", no interest in other animals, sex
• male probers
  • hyperactive, obsessed with sex with both genders
  • no normal courtship patterns, unusually aggressive in mating with females
• our world, our times....

B.  Features of populations

-  structure and changes in a population measured primarily by looking at population density

1.  Population density:  number of individuals of a species per unit area or volume at a given time

• number of mushrooms/square meter
• number Daphnia/liter water

a.  measuring population density:

• photograph herd of antelope -- count
• mark-recapture for small animals
  • mark/tag (50 mice)
  • recapture -- fraction of those marked among total recaptured = fraction of all marked individuals in whole population are fraction
    • recapture 50, 5 tagged --> 10% population marked
    • 50 marked mice are 10% of population (500)
• often population too large to study in it's entirety -- studied by sampling it as above

b.  different environments or habitats vary in population density of any species they can support

• red grouse in treeless moors of northwest Scotland at two locations 2.5 km apart
  • one location population density stationary during 3 year period
  • in other location population density doubles in first two years and declines to original density in third
    • change in habitat?

2.  Population dispersion:  a population is distributed, or dispersed, over its geographic range in accordance with the distribution of its resources

-  individuals of a population exhibit characteristic patterns of spacing, or dispersion, relative to each other

• clumped dispersion
• uniform dispersion
• random dispersion

a.  clumped dispersion

• occurs when individuals are concentrated in specific parts of the habitat
• results from patchy distribution of resources in the environment
  • because of differences in soil, temperature, topography, other physical factors, plants grow in patches
  • interaction among plants/organisms also determines this -- plant species A inhibits B, enhanced by C, insect species D lives only on A
• caused in animals by the presence of family groups and pairs
• caused in plants by limited seed dispersal and asexual reproduction
  • aspen grove originate asexually from single plant
• occurs when association advantageous to social animals
  • school of fish -- reduces risk of predation on single individual
    • more eyes
    • clumping prevents predator to single out an individual

b.  uniform dispersion

• occurs when individuals more evenly spaced than would be expected from a random occupation
• results from antagonism between individuals
  • territoriality and breeding behavior
    • aggressive interactions among nesting birds -- since they strike at one another to defend territory (young) nests place just beyond striking reach of neighbors
  • plant species produce chemical substances that inhibit growth of their own seedlings or other plants
    • in deserts of southwestern US creosote bush releases various pungent substances into soil around it
      • materials inhibit growth of other plants of same species -- population spaces out uniformly, as adaptation for using limited water resources of desert (other spacies also affected -- whole community)

c.  random dispersion

• occurs when individuals in a population are spaced in an unpredictable manner that is unrelated to the presence of others
  • trees of some species randomly distributed in a tropical rainforest
• occurs infrequently if at all as environmental factors affecting dispersion usually do not occur at random

III.  Mathematical models describing population growth

-  population ecologists try to understand general processes shared by different populations -- develop mathematical models describing dynamics of populations

A.  Growth rates

-  all natural populations tend to grow, as organisms have enormous potential for reproduction

- thus size of populations (N) change over time (t); changes due to:

• natality - rate at which organisms produce offspring (b, birth rate)
  • b is number of births in a certain period divided by N
• mortality - rate at which organisms die (d, death rate)
  • d is number of deaths in a certain period divide by N

- the intrinsic growth rate (r) of a population is b - d (r = b - d)

-  thus growth rate of a population of size N over time period t is:  DN/Dt = rN

• growth rate proportional to the size of the population
  • large populations grow faster than small populations of the same organism (interest on investment)
  • r positive, r negative, r = 0

-  example: what is monthly growth rate of specific mouse population?

• N= 1,000
• t = 30 days
• 300 births per month -- b = 300/1,000 = 0.30
• 280 deaths per month -- d = 280/1,000 = 0.28
• r = 0.30 -0.28 = 0.02
• thus  DN/Dt = 0.02 (1,000) = 20

- note that in addition to b and d, migration will affect population size

• migration:  movement from one region to other
  • immigration -- when individuals enter a population
  • emigration -- when individuals leave a population
• thus r = (b - d) + (i - e)

B.  Types of population growth 

-  the maximum rate at which a population can increase under ideal conditions is its biotic potential (r_max)

-  (r_max) depends on many factors

• age at which reproduction begins
• fraction of lifespan during which organism is capable of reproducing
• number of reproductive periods per lifetime
• number of offspring produced during each period of reproduction

-  large organisms -- elephants, whales -- have smallest biotic potentials

-  small organisms -- bacteria have greatest biotic potentials

1.  Exponential population growth

-  some organisms grow by repeatedly dividing in two -- population doubles in size periodically

• population of N bacterial cells grows from N to 2N, 4N, to 8N, and so on

-  can also occur in species with lower biotic potentials

• example:  8 pheasants introduced in 1937 on Protection island on coast of Washington State
• no predators to inhibit growth, population grew to nearly 1,900 birds in 6 years
• episode ends in 1942 -- military occupies island and shoots all pheasants

- thus exponential growth implies that the size difference between any two generations, DN, is proportional to N

• the larger the population gets, the faster it grows

2.  Logistic population growth

-  certain populations exhibit exponential growth for short periods of time

-  however, organisms cannot reproduce indefinitely at their biotic potential since environment sets limits -- environmental resistance

• with crowding, limited food and living space
• with crowding organisms more susceptible to predators
• d increases as b decreases

- thus number of individuals in a population (N) controlled by ability of environment to support them

• as N increases so does environmental resistance
• over periods of time DN/Dt may become 0
  • all natural populations stop growing at a certain maximum size, carrying capacity (K)
• K is the largest population that can be maintained for an indefinite period of time by a particular environment assuming there are no changes in the environment

-  logistic model describes population growth when size of population is regulated by environmental resistance

• S-shaped curve
• initial period of exponential increase
• leveling out as carrying capacity for environment approached

- model described by logistic equation:  DN/Dt = rN(1 - N/K)

• as N increases, N/K gets closer to 1, growth rate keeps getting smaller
• at carrying capacity, N/K is 1, growth rate = 0

-  population can remain very stable in size for years as long as no major environmental changes occur -- example

-  in many cases population grows logarithmically, temporarily rises higher than K 

• then experiences a population crash -- an abrupt decline to low population density
• examples:
  • bacterial growth
  • herd of 26 reindeer introduced in Alaskan island in 1910
    • carrying capacity determined largely by availability of winter forage
  • population increased exponentially for 25 years -- 2000 reindeer
    • vegetation overgrazed -- plant life wiped out
    • over next decade reindeer die from starvation  -- population of 8
    • recovery of vegetation (artic/subartic) takes 15 - 20 years
    • carrying capacity for reindeer greatly reduced

IV.  Factors affecting population size over time

A.  Density-dependent factors that limit population size:

• density-dependent factors (DDF)
  • factors that affect a crowded population more than a sparse one
  • effects of DDF on population growth become stronger as a population density increases
    • slow population growth by increase in d and/or decrease in b
  • conversely decrease in population density results in enhancement of population growth by DDF
    • DDF decrease d and/or increase b
  • tend to regulate population size at relatively constant size near K

- example of density-dependent factors:  competition for resources such as living space, food, cover, minerals, sunlight

• two types of intraspecific competition
  • contest competition
    • dominant individuals obtain adequate supply of limited resource at expense of other individuals in population
    • intraspecific competition among red grouse
      • low red grouse populations-- birds less aggressive, most birds establish feeding territory
      • larger populations -- hard to establish territory, more birds than territories, birds more aggressive -- birds without territories die
    • relatively small fluctuations in population size by death of individuals unable to compete successfully
  • scramble competition
    • all individuals in population share limited resource equally -- at high population densities none of them obtains adequate amount
    • Isle Royale, lake Superior moose
      • some moose walk to island, 1900
      • moose population up to 3000, 1934 -- all edible vegetation consumed, massive starvation
      • cycle repeated itself
    • populations often oscillate over time

- density dependent population limitations:  nutrient availability

- density dependent population limitations:  waste accumulation

- density dependent population limitations: stress factors associated with high density

B.  Density-independent factors that limit population size

• density-independent factors
  • any environmental factor that affects the size of a population but is not influenced by changes in population density
  • affect large and small concentrations of organisms equally
    • harsh weather kills 90% of individuals in a particular population whether there are 30 or 3,000
    • a severe fire kills organisms without regard for their density
    • arctic mosquito populations

V.  Reproductive strategies of populations  -- different patterns of reproduction

- name of the survival game is reproduction:

• for a species, number of new offspring not as important as how many offspring make it to reproductive age and number of successful offspring they produce

- organism has a lifestyle adapted to its reproductive pattern

• reproductive pattern set by genes

- assume an idealized organism: 

• perfect life history assuring reproduction at biotic potential
  • produces maximum number of offspring
  • cares efficiently for all offspring
  • majority of offspring survive to reproductive maturity
  • organism reaches reproductive maturity early
  • organism reproduces frequently
• no such a case -- can't put all energy into reproduction
  • needs to ensure its own survival
  • organisms must do what is required to survive as individuals and as populations
    • allocate all energy to individual
    • allocate all energy to reproduction

- thus a population has a life history strategy that evolved through natural selection of traits (energetic compromise):

• reproductive characteristics
• body size
• habitat requirements
• migration patterns

-  two main extremes of populations with regards to reproductive strategies:

• r-selected species
• K-selected species

A. r-selected species:  evolved traits that lead to high population growth

• small body size
• early maturity, early reproduction
• short-lifespan -- often reproduce only once
• large broods
• little or no parental care

- r-selected species:

• opportunistic species
  • found in variable, temporary, unpredictable environments
  • found in environments where probability of long term survival low
    • example:  insects find suitable environment
      • reproduce rapidly
      • expand throughout area -- exploit resources
      • only live a few years in such environment
      • disperse to find other areas
    • example:  sub-arctic mosquito population

B.  K-selected species:  evolved traits that maximize the chance of surviving in an environment where the number of individuals is near K

• because populations of species maintained at K, no need for increased reproductive rates
  • organisms in sharp competition with other members of population for limited resources
• individuals have long life-span
• slow development
• late reproduction
• large body size
• low reproductive rate -- repeated reproduction
• invest in parental care of young

-  K-selected species, example:

• Tawny owls
  • pair bond for life
  • regulate their reproduction in accordance with resources, especially food
    • in average year 30% of birds do not breed
    • if food supply limited, many of those fail to incubate eggs
  • thus behaviorally regulate population size so number stays at or near the carrying capacity of environment

- note that there is a wide continuum between r-section and K-selection

VI.  Survivorship and reproductive strategies

-  reproductive strategies that organisms possess associated with different patterns of survivorship

• survivorship:  proportion of individuals in a population that survive to a particular age

A.  Type I survivorship

• example:  humans, K-selected species
• young have high probability of living
• probability of survival decreases more rapidly with increasing age

B.  Type III survivorship

• probability of mortality is greatest early in life
• those individuals that survive early mortality have high probability of survval
• probability of survival increases with age
• example:  oysters (free swimming larval stages, before shell secretion); r-selected species

C.  Type II survivorship

• probability of survival does not change with age
• probability of mortality equal across all age groups
  • results from random events that cause death with little bias

VII.  Population ecology and the human population