community ecology key question how does species diversity affect the sustainability of a community
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Community Ecology
Key QuestionHow does Species Diversity Affect the
Sustainability of a Community
Organisms have a Habitat and a Niche in an Ecosystem
Example: Zebra MusselsHabitat: freshwater lakes, rivers and streams
Niche: feeds on green algae
Habitat vs. niche
• Habitat– Space an organisms
inhabits• Where it lives• Its address
– Several species can share the same habitat
– Based on biological requirements
• Niche– Functional role of an
organism• Its job
– unique to each organism– How it affects other
organisms and the environment
Key QuestionWhat role do species play and how do they
interact in a Community?
Niche Structure = how many occur, how unique are they, and how do species occupying different niches interact
Role of Species in an Ecosystem• Native: Normally live and thrive in a particular ecosystem
• Alien: Deliberately or accidentally introduced to an ecosystem - An invader
• Indicator: Marks a specific condition in the ecosystem or serves as an early warning to degradation
• Keystone: maintains diversity in the ecosystem, positively affects other organisms, top predator keystone species regulate populations
• Foundation species: Help create habitats and ecosystems
Organisms living in direct contact can benefit or harm each other as they interact
Relationships Between Organisms
• Symbiosis – Long term relationship between organisms of two different species living in direct contact– Mutualism – benefits both organisms
– Parasitism – benefits one and harms the other organism
– Commensalism – benefits one organisms while the other is not affected
• Trials of Life Pt1• Trials of Life Pt2• Trials of Life Pt3• Trials of Life Pt4
mutualism
Mutualism between acacia trees and ants
Ants feed on nectarand protein rich sacks
Attack and sting invadingInsects and eat fungal spores
Prune back nearing vegetation
Tree is protected from invasionProvides home and food for
the ant
Parasitism negatively effects population growthParasite = organism that takes nourishment from a host organism
sea lamprey
Parasitism – Misletoe
Commensalism
What type of relationship exists between these organisms?
Predator/Prey RelationshipsAre not symbiotic – One species (the predator) feeds
directly on another species (the prey) - they are not in long association with each other
Predation• Plays an ecological role in evolution by
natural selection– Predators kill less fit members of a
population– Survivors reproduce offspring with
adaptations that help them avoid predation
• Controls population levels– Adaptations to decrease the impact of
predators (protection mechanisms)• clumping• run, swim, fly• highly developed sight and smell• shells, bark, thorns, chemicals (stinging
nettles, skunk, octopus)• camouflage (insects, birds)• mimicry (viceroy butterfly)• Deceptive markings• Warning coloration
Predator – prey relationships are usually cyclical
Platable or harmless speciesLarval stage of hawkmouth moth Unpalatable or poisonous snake
Coral snake Milk Snake
Cross-mimicry: benefiting bothPredators learn to avoid coloration pattern
Peppered Moth
Interspecific Interactions – effects on population
Key Question
How do Communities Respond to Changing Environmental
Conditions?
Ecological Succession• Communities gradually change their structure and
species composition in response to environmental conditions(depend on biotic and abiotic factors)
- 2 types1. Primary succession – establishment of a community from barren ground – can take thousands of years to form fertile soils2. Secondary succession – in response to disturbance - can be positive as nutrients are cycled and new niches open up
Intermediate Disturbance Hypothesis - Fairly frequent but moderate disturbance leads to increased species biodiversity (richness and evenness
Primary succession – gradual establishment of community in a lifeless area
Secondary succession
Succession on Mt. St. Helens
Population
• A group of the same kind of organisms (of a single species) living in a given space at a given point in time
• Defined by:– type – time– Space
Key Question
What limits the growth of populations?
Populations establish various distribution patterns (dispersion)
• 3 types– Clumping
• Most common – example: schools of fish• Size / location varies with availability of resources
– Species clump toward resources• Provides protection from predators and therefore population
decline• Increases ability to catch prey• Groups form for mating and caring for young
– Uniform dispersion• Provides access to scarce resources
– Random dispersion (fairly rare)
Clumped dispersion: buffalo, swans, fish, lupine
Four rates determine change in population size
– Natality = birthrate: number of births per unit time
– Mortality = death rate: number of deaths per unit time
– Immigration: number of individuals moving into a population over time
– Emigration: number of individuals moving out of a population over time
Additions = removals
Effect onPopulation
size
positive
negative
positive
negative
Zerogrowth
Dispersal
• The ability of populations to spread from a central place– Active – animals– Passive – plants: move by other organisms or mechanisms– Barriers block dispersal
• Physical: mountains, water, land forms, food source• Behavioral: pack mentality, breeding preferences, territorialism,
tradition
Limiting Factor Principle– Any abiotic or biotic factor may be critical to the
success of organisms and affect the growth of a population (even if other factors are optimal)
The best soils for agriculture have no or few limiting factors.
Population Growth Limiting Factors• Density Dependent - Effect is independent of how big
population is• Weather: wind, rain, drought, fire, hurricane• Human activities: pesticides, clear cutting
• Density Dependent – Effect depends on the density of the population– 1. Competition for shared or limited resources
• Resources / food• Space - crowding stress
– Dense populations result in higher infant mortality
– 2. Predation– 3. Parasitism or diseases that spread
Stress due to crowding limits population
Predators – Density dependent limiting factor
Population Growth Curves
No population can grow indefinitely
Species have a “biotic potential” for population growth
Intrinsic rate of increase (r) = inherent reproductive capacity– unlimited resources– no limiting factors
• Examples: bacteria, geese, humans
– Results in exponential growth
• J-shaped growth curve
Exponential GrowthThe larger a population gets the faster it grows
Lag phase:Positive growth
starts slowly
Boom phase:Exponential growth
Example of exponential population growth in nature
Carrying Capacity (K)• The maximum population a habitat can support
without degrading the environment
– Abiotic and biotic factors limit population size• Competition• Scarce resources
– Environmental resistance – combination of all factors that limit population growth
– Carrying capacity is determined by the combination of biotic potential and environmental resistance
Homeostasis – A Stable equilibrium
• Growth rate decreases as a population reaches carrying capacity
• Growth rate averages zero• Population reaches equilibrium = Maintenance of stability in
numbers of individuals
• Viewed over a long period of time• Takes into account fluctuations in population density
• Populations are dynamic and changing
Homeostasis – maintaining stability
Populations usually carrying capacity and growth stabilizes
Logistic growth = growth with limits curve (s-shaped)
Homeostasis
Fig. 6-11, p. 119
Exponentialgrowth
Environmentalresistance
Population stabilizes
Carrying capacity (K)
Bioticpotential
Time (t)
Popu
latio
n si
ze (N
)
Fig. 6-12, p. 119
Population recoversand stabilizes
Carrying capacity
Populationruns out ofresources and crashes
Exponentialgrowth
Populationovershootscarryingcapacity
Year
Num
ber o
f she
ep (m
illio
ns)
Reproductive strategies and population fluctuations(most organisms have reproductive patterns between (r) and (K))
• 2 categories– K-strategists (Deer, lion, whale, human) = competitors
• Large organisms, long lives, produce few offspring, provide care for offspring
• Populations stabilize at carrying capacity• Controlled by density-dependent limiting factors
– r-strategists (insects, mice, fish) = opportunists• Small, short life, produce many offspring (high capacity for
population increase)• Produce many offspring but don’t reach carrying capacity• High mortality among young (little parental care)• Populations fluctuate wildly (boom and bust model)• Controlled by density-independent limiting factors
Growth Rate CurvesK-strategist vs. r-strategist
Predator – prey relationships cause population fluctuations
Class dataTreatment Ave K value Ave r value
control 5.3 .57
Saline - low 8 .44
medium 9 .3
high 8 .36
PO4 - low 12.7 .68
medium 9 .50
high 14.5 .52
NO3 - low 14 .26
medium 6.5 .30
high 7 .39
Shade - low 9.5 .287
medium 6 .219
high 5 .107
Ph 5 9 .288
Ph 6 22 .231
Ph 7 7 .357
Ph 8Ph 9
7.3
.92
.057