Download Chapter 36: Population Growth Population Concepts

Chapter 36:
Population Growth
Population Concepts
Population:
• interbreeding group of same species
Carrying Capacity:
• maximum population size an ecosystem
can sustainably support
Critical Number:
• minimum population size required for its
survival
Growth Rate:
• change in population size per unit time
e.g. 10% increase per year
• accounts for births and deaths
Growth rate = Birth rate - Death rate
e.g.
10% = 15% - 5%
A population is stable, at equilibrium if:
Birth rate = Death rate
(immigration, emigration also affect population size)
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Population Growth Patterns
Unchecked population growth will be
exponential
• population increase by the same factor
over time (e.g., doubling per generation)
• can result in a population explosion
Exponential growth will eventually resolve
into one of 2 basic growth patterns…
“J-curve”
Carrying
Capacity
“S-curve”
(@ equilibrium)
“J-curve” Growth
Occurs when populations undergo exponential
growth beyond carrying capacity followed by a crash
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“S-curve” Growth
S-curve growth (aka “logistic”)
• population level hovers around carrying capacity
due to environmental resistance
• typical of established species in stable
ecosystems
J-curve growth
• exponential growth beyond carrying capacity
followed by crash
• associated w/ introduced species, loss of
predator, habitats that fluctuate
• may be temporary & resolve to S-curve over time
Environmental Resistance
All factors (biotic & abiotic) that limit or resist
population increase
Density-dependent factors:
• environmental resistance factors that change in
response to population density
• usually biotic (predators, disease, food supply)
• provide more resistance as population expands,
less as it shrinks (keeps pop. at carrying capacity)
Density-independent factors:
• resistance that is unrelated to population density
• usually abiotic (changes in weather, fire,…)
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Predator/Prey Population Cycles
Predators provide a form of density-dependent
environmental resistance:
• predator numbers
increase/decrease
in response to prey
populations
Hare population size
(thousands)
120
Lynx
9
80
6
3
40
Lynx population size
(thousands)
Snowshoe hare
160
• keeps prey
populations in
check
0
0
1850
1875
1900
1925
Year
Key Terms for Chapter 36
• population, growth rate
• carrying capacity, critical number
• J-curve, S-curve growth
• environmental resistance
Chapter 37:
Communities & Ecosystems
1. Biological Communities
2. Energy Flow in Ecosystems
3. The Cycling of Matter
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1. Biological Communities
Communities and Niches
Community:
• all interacting, living organisms in a given region
• consists of many different & diverse species,
each with a unique niche or “role”
Niche:
• sum of a species use of resources
• food, living space, environmental requirements…
• unique for each species, but can overlap
between species
How do Species Interact?
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Competition
Intraspecific: within same species
• most common type of competition
• members of same species share same niche
• important part of natural selection
Interspecific: between different species
• less common since different species tend to
have different niches
• occurs between species with overlapping niches
• e.g., plants competing for space, sunlight & water
Competitive Exclusion
If 2 different
species occupy
same niche, one
will outcompete
and eliminate the
other
• evolution
selects for
species with
unique niches
Predator & Prey
All species that don’t produce their own food
(e.g., photosynthesis) or feed on dead, waste
material (detritus feeders) prey on other
organisms:
3 basic Predator – Prey relationships:
*Carnivore – Herbivore
Herbivore – Producer (plant)
Parasite – Host
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Predators have an Important Role
Keep other species in balance, their removal can
lead to overgrowth of prey, severely
disturbing ecosystem balance.
Species with essential roles for ecosystem
balance are called keystone species:
• usually predators that prevent domination
of the ecosystem by one species
e.g.
starfish control mussel populations
otters & lobsters control urchin populations
wolves control deer populations
Symbiotic Relationships
Symbiosis: a close, prolonged association
between two species
Mutualism (both species benefit)
• e.g., plants & insects; coral & algae
• many species depend on such relationships for survival
Commensalism (one benefits, other unharmed)
• e.g., barnacles on a whale, epiphytes
Parasitism (one benefits, one is harmed)
• e.g., mistletoe, tapeworms
• host usually survives
2. Energy Flow in Ecosystems
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What is an Ecosystem?
“a distinct biological community and its
abiotic environment”
biotic = “living” or “derived from living things”
•
living organisms and their products, remains
abiotic = unrelated to “living things”
•
non-living matter (water, air, minerals…)
“ecosystems are functional units of
sustainable life”
Examples of Ecosystems
• ecosystems are defined by their plant life
• biomass varies greatly among ecosystems
• biomass = dry weight of living material
Energy and Matter in Ecosystems
Energy constantly flows through ecosystems
• solar energy is the ultimate source (usually)
• converted to chemical PE via photosynthesis
• passes through the “food web”, gradually being
dissipated as heat
Matter is recycled within ecosystems
• organic material is continually built and broken
down using the same elements (requires energy!)
**Earth continually gains and loses energy, while
matter is essentially confined to the planet**
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Energy flows in/out, Matter is recycled
Chemical
cycling
Energy
flow
Chemical
energy
Light
energy
Heat
energy
Chemical elements
Energy is Transferred by Feeding
There are 3 major trophic (feeding) categories:
1) Producers (autotrophs)
• convert energy of sunlight to food energy
by photosynthesis
2) Consumers
• require food produced by other living organisms
• feed on producers, or each other
• herbivores, carnivores, omnivores
3) Detritus Feeders
• feed on nonliving organic matter
• dead organisms, organic waste
• include the decomposers (mostly bacteria & fungi)
• break down organic material completely
• very important for recycling nutrients
**Without detritus feeders, ecosystems would collapse!**
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Trophic Relationships
Trophic levels of the “Food Chain”:
1st trophic level = producers
• “photosynthesizers” (plants, phytoplankton)
2nd trophic level = herbivores
• primary consumers – eat producers
3rd trophic level = carnivores
• secondary consumers – eat herbivores
4th trophic level = “bigger” carnivores
• tertiary consumers – eat carnivores
**There is also a “detritus food chain”**
Trophic level
Quaternary
conusumers
Killer whale
Hawk
Tertiary
consumers
Terrestrial
food chain
Snake
Tuna
Secondary
consumers
Herring
Mouse
Aquatic
food chain
Primary
consumers
Grasshopper
Zooplankton
Producers
Plant
Phytoplankton
Energy Transfer is Inefficient
Tertiary
consumers
10 kcal
100 kcal
Secondary
consumers
Primary
consumers
Producers
~10% of food
energy is
transferred to
each
successive
trophic level!
1,000 kcal
10,000 kcal
1,000,000 kcal of sunlight
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Where does this “Food Energy” go?
Most is lost as heat due to respiration
• used to meet the energy needs of the organism
Some is not consumed
• not all potential food is eaten
Not everything is digested
• digestion is usually incomplete, and some material
is indigestible (fiber, etc)
Only ~10% is incorporated into organic molecules
• i.e., potential food for other organisms
• thus biomass decreases by ~90% at each trophic level
Energy & Human Food
Production
Trophic level
Human
meat-eaters
Secondary
consumers
Primary
consumers
Human
vegetarians
Corn
Cattle
Corn
Producers
Due to inefficient food energy transfer, animal-based
food production is much less efficient and more
expensive than plant-based food production.
3. The Cycling of Matter
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All matter in Ecosystems is Recycled
We will focus on the cycling of 3 key elemental
nutrients between abiotic reservoirs and organic
material:
Carbon
Nitrogen
Phosphorus
The
Carbon
Cycle
CO2 in atmosphere
Photosynthesis
Cellular
respiration
• the cycling
of carbon
between
CO2 &
organic
molecules
Burning of
fossil fuels
and wood
Higher-level
Primary consumers
consumers
Carbon compounds
in water
Detritus
Decomposition
Summary of the Carbon Cycle
Carbon from CO2 in the atmosphere or water is
fixed into organic molecules by photosynthesis.
• this is how carbon enters the food web
Carbon in organic molecules eventually returns to
atmosphere (or water) in molecules of CO2 due to:
• respiration
• fires
• volcanic activity
• burning of fossil fuels*
***Increased CO2 due to fossil fuel burning results in
global warming and lowering of ocean pH***
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The
Nitrogen
Cycle
Nitrogen in atmosphere (N2)
Nitrogen
fixation
Assimilation
by plants
Nitrogen-fixing
bacteria in root
nodules of legumes
Denitrifying
Nitrates bacteria
(NO3–)
Detritivores
Decomposition
Nitrifying
bacteria
Ammonium (NH4+)
Nitrogen-fixing
soil bacteria
• the cycling
of nitrogen
between
N2, inorganic
ammonium
& nitrate ions
& organic
molecules
Summary of the Nitrogen Cycle
Nitrogen fixation is one of the most important
processes on earth!
• converts nitrogen compounds in the atmosphere
(mainly N2) to ammonium ions (NH4+)
• nitrifying bacteria convert NH4+ to nitrate (NO3-), the
most easily assimilated form of nitrogen for plants
• plants cannot use atmospheric nitrogen (N2) directly
• nitrogen thus enters the food web through plants
Atmospheric nitrogen is fixed by:
• cyanobacteria (blue-green algae) in water
• various soil bacteria and fungi on land
…more on the Nitrogen Cycle
Denitrification is a microbial process that
produces N2 from nitrates
• returns nitrogen to the atmosphere, completes
the cycle
Most nitrogen actually cycles between plants,
consumers, & detritus feeders which metabolize
organic nitrogen compounds back to NH4+
**Human activity (synthetic fertilizers, fuel burning) results in
“excess” fixed nitrogen entering ecosystems**
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The Phosphorus Cycle
Rain
• the cycling of
phosphorus
between
inorganic
phosphate
& organic
molecules
Weathering
of rocks
Geologic
uplift
of rocks
Plants
Runoff
Consumption
Sedimentation
Soil
Plant uptake
–
of PO43
Leaching
Decomposition
Summary of the Phosphorus Cycle
Phosphorus originates as inorganic phosphate
(PO43-) in rocks, leaches into soil & water, and is
incorporated into organic compounds by plants…
Decomposition of dead tissue & animal wastes
release inorganic phosphate back into soil to
re-enter the food web via plants.
**Agricultural runoff (synthetic fertilizer, animal waste),
untreated sewage release an excess of phosphorus
(& nitrogen) into ecosystems, causing imbalance**
• due to “overgrowth” of algae, bacteria & certain plants
Key Terms for Chapter 37
• ecosystem, community, niche
• competition, predation, keystone species
• symbiosis: mutualism, commensalism, parasitism
• producer, consumer, autotroph, heterotroph
• herbivore, carnivore, omnivore, detritus feeder
• trophic levels, food web, biomass
Relevant Review Questions:
1-3, 5, 7, 8, 10, 11, 13
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