Community and Population
Ecology
Chapter 6
American Alligator
American Alligator: community structure
• Highly adaptable – around for nearly 200 million years
• Keystone species - A species whose loss from an ecosystem would cause
a greater than average change in diversity or abundance of other species,
community structure or ecosystem process.
– Gator holes – fresh water/food supplies, refuges
– Nest mounds – nesting/feeding sites for herons & egrets
– Balance predator populations
• Only natural predator is humans
• 1967 – endangered species list – 90% decimated
• Successful environmental comeback
Affect of Species Diversity on the
Sustainability of a Community
Species diversity increases the sustainability of
communities and ecosystems.
Species Diversity
•
•
•
•
Species richness combined with species evenness
Niche structure
Varies with geographic location
Species richness declines towards poles
Species Diversity
• Species richness combined with species evenness
determines diversity of an ecosystem
– Tropical Rainforest vs. Coniferous Forest
Species Diversity: communities
• Niche structure
determines diversity
– The number and types
of similar and different
niches along with how
the organisms within
them interact
• For example – stream
macroinvertebrates
Species Diversity: communities
• Niche structure determines diversity
– The number and types of similar and different niches along with how
the organisms within them interact
• For example – stream macroinvertebrates
Species Diversity: communities
• Diversity varies with geographic location (richness declines
towards poles)
Bird species richness in the Western Hemisphere (Hawkins et al. 2006).
Sustainability and Environmental Change
• Certain factors allow living systems to maintain
stability/sustainability (equilibrium)
• Inertia or persistence – ability to resist disturbance
– Tropical rainforests (high), grasslands (low)
• Constancy - ability to maintain population size within
limits
– Endemic species vs. invasive
• Resilience – ability to recover
– Tropical rainforests (low), grasslands (high)
Equilibrium: Area and Distance Effects
Animations/species_equilibrium.html
Richness and Sustainability
• Richness tends towards sustainability,
productivity and better recovery
– Not all of the ‘eggs are in one basket’
– NPP seems to peak with at least 10-40 producers
(difficult to distinguish ‘most important’)
Roles of Species in a Community
Based on ecological roles, species are described as:
native
endemic
nonnative
indicator
keystone or
foundation species
Ecological Niche
• Niche = unique role
• Native/endemic species = normal
• Nonnative species = invasives, aliens
– Spread in new, suitable niches
– Also – most crops and feedstocks
– Can reduce native populations (Africanized
honeybees)
– Lack predators, disease and competition
Indicator Species
• Early warning system for the health of an ecosystem
– Trout - absence means low DO
Vanishing Amphibians: indicator species
• 33% threatened,
43% declining
with no clear-cut
reason
– Habitat loss and
fragmentation
(drain/fill)
– Prolonged drought
(breeding sites)
– Pollution
(pesticides)
– Ultraviolet radiation
(ozone loss/egg
damage)
– Parasites (increased
susceptibility)
– Viral and fungal
diseases (skin)
– Climate change
– Overhunting
– Nonnative predators
and competition
Vanishing Amphibians
• Why we should care
– Indicates there is something wrong
– Skin – pharmaceuticals – painkillers, antibiotics
• Phantasmal poison frog – painkiller , Epibatidine, that
doesn’t have the side effects of morphine, but 200x stronger
Life Cycle of Typical Frog
Frogs Galore
Videos/Frogs_Galore.mov
Keystone Species
• Significant role in their food
web
• Elimination may alter
structure, function of
community
• Pollinators
• Top predators – Grey Wolf
in Yellowstone
Keystone Species: Dung Beetle
Remove dung
Aerate soil
Nutrient recycling
Fig. 6-3, p. 110
Sharks – garbage men of the sea
•
•
•
•
Keystone species
Remove injured, sick animals
Many are non-threatening
Provide potential insight into cures for
human diseases (immune system function)
Foundation Species
• Create habitats and ecosystems
– Beavers
• Create wetlands
– Elephants
• Push over trees, allow grasses & nutrient cycling
– Seed dispersers
• Bats’ & birds’ droppings
Species Interaction
Competition, predation, parasitism, mutualism, and
commensalism – affect resource use and
population sizes of the species in a community.
Adaptations allow some species to reduce or avoid
competition.
Interspecific Competition
• No two species can share vital limited resources
for long
• Resolved by:
–
–
–
–
Migration
Shift in feeding habits or behavior
Population drop (Gause’s Paramecium Experiment)
Animations/gause_v2.html
Extinction
• Intense competition leads to resource
partitioning
Resource Partitioning in Warblers
Species Interaction
• Predator-prey relationships – food webs
– Predators and prey both benefit – individual vs.
population (weak, sick, aged, least fit)
• Predator strategies
– Herbivores simply walk, fly or swim
– carnivores (pursuit/ambush – camouflage, poison)
Predator Avoidance: camouflage
• Prey strategies - Hard shell, speed, smell, spines, etc.
Predator Avoidance: chemical warfare, warning coloration
Fig. 6-5cd, p. 113
Predator Avoidance: chemical warfare, warning coloration, mimicry
Fig. 6-5ef, p. 113
Predator Avoidance: deceptive looks, deceptive behavior
Fig. 6-5gh, p. 113
Species Interaction: Symbiosis
Symbiosis: Parasitism
Tomato horn worm and Braconid Wasp
• Live in or on the host
– Parasite benefits, host harmed
• Parasites promote biodiversity – pick off weak,
aged, sick, keep population size low and allows other
species to move in.
Cowbird
Symbiosis: Mutualism
• Everybody benefits
• Nutrition and protection
• Gut inhabitant mutualism
– Humans and termites (enzymes for biofuels)
Symbiosis: Commensalism
• Benefits one with little impact on
other
• Epiphytes
• Bird nests
Review: How Species Interact
Animations/species_interactions.html
Communities Respond to Changing
Environmental Conditions
Ecological succession – The change in community
structure and composition due to changing environmental
conditions.
Precautionary principle – Measures taken to prevent or
reduce harm even if cause-and-effect relationships have not
been fully established scientifically.
• Primary succession
– gradual establishment of community in lifeless areas
– “Bare” – no soil
– 100s to 1,000s of years to establish fertile soil
• Secondary succession
– More common
– Not “bare”
– Has soil
Succession
Animations/succession.html
Ecological Succession
• Disturbances create new conditions
– Eliminates species
– Opens new niches, increases richness
• Intermediate disturbance hypothesis
– frequent/moderate disturbances results in increased species
richness
Succession’s Unpredictable Path – Intermediate Disturbance Hypothesis
• Doesn’t always arrive at a climax community
– Ever-changing mosaics of different stages
– Continual change, not permanent equilibrium
– Results in highest level of diversity
Precautionary Principle
• Lack of predictable
outcome should not
prevent conservation
– Consider health of ecosystem &
humans
Population Growth
• Populations differ
– Number
– Distribution
– Age structure
• No population can continue to grow indefinitely
– Limiting factors
– Competition
Population Dispersion/Distribution
Clumping
• Clumped Resources
• Protection
• Hunting success
• Mating or young-rearing
Populations Sizes Are Dynamic
• Vary over time
population = (births + immigration) - (deaths + emigration)
Populations Sizes Are Dynamic
• Changes in Age structure
– Pre-reproductive stage
– Reproductive stage
– Post-reproductive stage
Video: Bonus for a Baby
Videos/Bonus_for_a_Baby.mov
Limits to Population Growth
• Biotic potential is idealized capacity for growth
• Intrinsic rate of increase (r)
• Nature limits population growth with resource
limits and competition
• Environmental resistance = limiting factors
Limits to Population Growth
• Carrying capacity – biotic potential and
environmental resistance
– Exponential growth – unrestricted
– Logistic growth – restricted
Exponential and Logistic Growth
Rule of 70 = 70/r = years for doubling
r = growth rate
Caused by reproductive time lag
Animations/exponential_mice.html
White-tailed Deer
• Today: 25–30 million white-tailed deer in U.S.
• Conflicts with people living in suburbia
Overshoot and Population Crash of Reindeer introduced to
Bering Sea Island of St. Paul
Different Reproductive Patterns
• r-Selected species
– High rate of increase, little parental care
– Opportunists
– Do better when opportunities arise, exponential
• K-selected species
– Competitors
– Slowly reproducing, parental care
– Do better when near carrying capacity, logisitic
• Most species’ cycle between two extremes
Where each does well on the Sigmoid Growth
Curve
Videos/World_AIDS_Day.mov
Humans Not Exempt from Population Controls
•
•
•
•
•
Bubonic plague (14th century)
Famine in Ireland (1845)
AIDS
Technology, social, and cultural changes increase K
Expand indefinitely or reach carrying capacity?
Millions
Number of deaths due to AIDS globally
2000—2004
4.0
3.5
3.0
2.5
Number
of deaths
2.0
due to
AIDS
1.5
1.0
0.5
0.0
2000
2001
2002
2003
2004
This bar indicates the range around the estimate.