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 CommunityBased on ecological roles, species

are described as:native

endemicnonnativeindicator

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

Fig. 6-3, p. 110

Remove dungAerate soilNutrient recycling

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 InteractionCompetition, 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)– Extinction

• Intense competition leads to resource partitioning

Animations/gause_v2.html

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.

Fig. 6-5cd, p. 113

Predator Avoidance: chemical warfare, warning coloration

Fig. 6-5ef, p. 113

Predator Avoidance: chemical warfare, warning coloration, mimicry

Fig. 6-5gh, p. 113

Predator Avoidance: deceptive looks, deceptive behavior

Species Interaction: Symbiosis

Symbiosis: Parasitism• 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.

Tomato horn worm and Braconid Wasp

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

ConditionsEcological 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

• Disturbances create new conditions– Eliminates species– Opens new niches, increases richness

• Intermediate disturbance hypothesis– frequent/moderate disturbances results

in increased species richness

Ecological Succession

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 timepopulation = (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 doublingr = 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

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?

Videos/World_AIDS_Day.mov

Number of deaths due to AIDS globally

2000—2004Millions

Number of deathsdue toAIDS

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2000 2001 2002 2003 2004

This bar indicates the range around the estimate.