Ecosystems: Populations and Succession

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Chapter 4

• Ecosystems Populations and Succession

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Population Dynamics

• Each species in an ecosystem exists as a
  population
• Population reproducing group
• Population equilibrium balance between births
  and deaths

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Population Growth Curves

• Exponential Increase
• Population explosion
• When the size of a population is plotted over
  time, two basic kinds of curves can be seen
  S-curves and J-curves

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Two types of growth curves
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Biotic Potential versus Environmental Resistance

• Biotic potential the ability of populations to
  increase
• Recruitment survival through the early growth
  stages

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Two strategies

• 1. Produce massive numbers of young, with little
  or no care result in low recruitment despite
  high biotic potential
• Low recruitmenthigh mortality
• 2. Lower reproductive rate, but then care and
  protect young to enhance recruitment

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Carry capacity

• Usually limited by environmental resistance
  biotic and abiotic factors that limit population
  increase
• The upper limit of any particular animal that an
  ecosystem can support is called the carrying
  capacity

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Population balance

• Population balance is a dynamic balance
• Additions (births) and subtractions (deaths) are
  occurring continually and the population may
  fluctuate around a median

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Density Dependence and Critical Number

• Population density the number of individuals
  per unit area
• Density dependent environmental resistance
  becomes more intense as population density
  increases
• Critical number the minimum population base
  that survival and recovery of a population
  depends on

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Endangered Species Act

• Threatened species whose populations are
  declining rapidly
• Endangered if the population is near what
  scientists believe to be the critical number

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Mechanisms of Population Equilibrium

• Many factors may cause mortality in a population,
  but those that are density dependent are capable
  of actually regulating the population, keeping it
  around an equilibrium
• Top-down regulation control by predation
• Bottom-up regulation control occurs by the
  scarcity of some resource

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Population control

• Predator Prey dynamics

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• Parasites also tend to be density dependant

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Plant Herbivore Dynamics

• Overgrazing eating of plants faster than they
  can grow
• Ex An unchecked population can explode, over
  graze then crash as a result of starvation
• Predator Removal (elimination of predators)

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Predator Removal

• Keystone Species plays a critical role in
  maintaining ecosystem biotic structure
• Example starfish on the rocky pacific northwest
  intertidal zone

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Competition

• Niches overlap when two species compete for the
  same scare resource
• Competitive exclusion elimination of one of the
  species due to competition
• Interspecific competition competition between
  members of different species

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Territoriality

• Protecting and defending an area of land from
  which adequate food resources can be obtained in
  order to rear a brood successfully
• Usually against members of same species
  intraspecific competition

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Introduced species
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Introduced species

• American Chestnut chestnut blight brought in on
  Chinese chestnut
• Pests inscts, cats, goats, zebra mussel
• Plants
• Water hyancinth an escaped ornamental plant in
  Florida is now under maintenance control

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Introduced plant species
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Lessons

• 1. The regulation of populations is a matter of
  complex inactions among the members of the biotic
  community
• 2. The relationships are specific to the
  organisms in each particular ecosystem

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Remedies?

• Introduction of a natural enemy
• Do single natural enemies exist?
• We they fit in a survive the new ecosystem?

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Definitions

• Genetic variation genetic differences among
  individuals
• Gene pool All the DNA of a species
• Biological evolution change in the gene pool of
  a species over the course of generations
• Selective pressures factors play a role in
  determining if an individual survives and
  reproduces or is eliminated

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Natural Selection

• Every factor of environmental resistance is a
  selective pressure resulting in the survival and
  reproduction of those individuals with a genetic
  endowment that enables them to cope with their
  surroundings.
• Constant selection in nature
• A modification of the gene pool towards features
  that enhance survival and reproduction within the
  existing biotic community and environment.

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Darwin

• Survival of the fittest
• Survival of those individuals having traits that
  best enable them to cope with the biotic and
  abiotic factors of their environment.
• Biological Evolution the modification of the
  gene pool that occurs through natural selection
  over the course of many generations

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Fitness

• Features that adapt an organism for survival and
  reproduction

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Adaptations

• Coping with climate
• Obtaining food and water / nutrients
• Escaping or protecting against predators
• Resistance to disease or parasites
• Finding or attracting mates reproduction
• Migrating
• Dispersing seeds

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Selective Pressures
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Limits to change species only have three
alternatives

• Adaptation adapt to new conditions
• Migration migrate to a new area
• Extinction

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Will a population survive a new condition key
variables

• Geographic distribution
• Specialization to a given habitat or food supply
• Genetic variation within the gene pool of the
  species
• Reproductive rate relative to the rate of
  environmental change

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Factors affecting survival of a species
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Speciation

• The final product may be so different from the
  population that started the process that it is
  considered a different species

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New species

• Two or more species can develop from one if you
  have reproductive isolation

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Ecological Succession

• The phenomenon of transition from one biotic
  community to another
• Primary succession initial invasion then
  progression form one biotic community to the next
• Secondary succession - reinvading an area after
  clearing by fire or humans

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Secondary succession
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Aquatic succession

• Lakes and ponds are gradually filled in
• The climax community may be a bog or a forest

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Fire and Succession

• Fire is a natural and important abiotic factor
• Periodic fires are instrumental in maintaining a
  balance in favor of pines, grasses, or redwood
  trees
• Ecosystems that depend on recurrence of fire to
  maintain existing balance are now referred to as
  fire climax ecosystems

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Nonequilibrium Systems

• Disturbances such as fires, floods, windstorms
  and droughts are important in structuring
  ecosystems
• They remove organisms, reduce populations, and
  create opportunities for other species to
  colonize the ecosystem

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Carry Capacity

• The maximum population that a given habitat can
  support without the habitat being degraded over
  the long term

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One question

• What is the earths carrying capacity for humans?