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Population Dynamics
Chapter 8
Sea Otter – the other, other white meat
Why are sea otters considered keystone species?They control urchin populations which feed on kelp, hence they keep the kelp forests healthyWhy did their populations decline?Originally due to hunting, now chemical pollution is suspected
Characteristics of a populationSize – number of organismsDensity – number /spaceDispersion – spatial distributionAge distribution – pre-breeding, breeding, or post breeding age
Population dynamics – how these factors change due to environmental stresses
Population growthPopulation change (growth) = (births + immigration) – (deaths + emigration)
ZPG – zero population growth is when incoming equals outgoing
Biotic potential – max growth for that particular populationIntrinsic rate of increase – rate of growth with unlimited resources
High intrinsic growth ratesReproduce early in lifeHave short time between generationsReproduce many timesHave many offspring each time
Roaches, mice, fish, flies
Environmental ResistanceThese are the vast assortment of environmental factors which help keep populations from growing out of control
This is a way a population finds an equilibrium point
Fig. 9.3, p. 200
POPULATION SIZE
Growth factors(biotic potential)
Favorable lightFavorable temperatureFavorable chemical environment(optimal level of critical nutrients)
Abiotic
BioticHigh reproductive rate
Generalized niche
Adequate food supply
Suitable habitat
Ability to compete for resources
Ability to hide from or defendagainst predatorsAbility to resist diseases and parasitesAbility to migrate and live in other habitatsAbility to adapt to environmentalchange
Decrease factors(environmental resistance)
Too much or too little lightTemperature too high or too lowUnfavorable chemical environment(too much or too little of critical nutrients)
Abiotic
BioticLow reproductive rate
Specialized niche
Inadequate food supply
Unsuitable or destroyed habitat
Too many competitorsInsufficient ability to hide from or defendagainst predatorsInability to resist diseases and parasitesInability to migrate and live in other habitatsInability to adapt to environmentalchange
Carrying capacityBiotic potential and environmental resistance will determine the population a given area can hold and sustain indefinitely
A population must not drop below the minimum viable population or lowest number needed to keep population from disappearing due to environmental resistance
Fig. 9.5, p. 201
2.0
1.5
1.0
.5
Nu
mb
er o
f sh
eep
(m
illio
ns)
1800 1825 1850 1875 1900 1925
Year
Logistic growthExponential growth (J curve) is not possible forever because resources and space eventually run out. When a population reaches a certain point, environmental resistance increases causing the population size to stabilize. This is known as logistic growth (s curve) and this generally happens to all populations
Fig. 9.4, p. 201
Time (t) Time (t)
Po
pu
lati
on
siz
e (N
)
Po
pu
lati
on
siz
e (N
)
K
Exponential Growth Logistic Growth
Can you overshoot your carrying capacity?
Absolutely, it happens all the time
When you have too many individuals for the area to support you will have a population crash
If the overshoot was not too drastic, and the crash was small the population re-stabilizes
Types of population curvesStable – nearly flat lineIrregular – widely fluctuating pattern with no periodicity Cyclic – regular growth and crash at set intervals, usually seasonalIrruptive – normally stable, but with a random spike or crash
Fig. 9.7, p. 202
Nu
mb
er o
f in
div
idu
als
Time
Irruptive
Stable
Cyclic
Irregular
Top-down or bottom-up?Evidence seem to show both happening
Top-down – predators hunt and kill prey keeping their population stable
Bottom-up – prey are the food source that allow predators to keep the populations up
Types of reproductionAsexual – cloning, single parent donates both parts of DNA (bacteria)
Sexual – two parents donate DNAFemales have to give birth more (males do not as in asexual)More genetic errors from combiningMating is more damaging, and energy intensiveDoes provide more genetic diversity, hence a stronger species
R-selected speciesAlso known as r-strategists and fill generalist nicheHave many offspringReach reproductive age earlyShort time between generationsLittle to no parental care and adapted to unstable climate (low survivorship)Short life span (usually under a year)Algae, rodents, bacteria, annual plants and insects
Fig. 9.10a, p. 205
r-Selected Species
cockroach dandelion
Many small offspringLittle or no parental care and protection of offspringEarly reproductive ageMost offspring die before reaching reproductive ageSmall adultsAdapted to unstable climate and environmental conditionsHigh population growth rate (r)Population size fluctuates wildly above and below carrying capacity (K)Generalist nicheLow ability to competeEarly successional species
K-selected speciesK-strategists or competitors, specialist nicheFewer, larger offspring (usually develop inside)Mature slowly (often protected while vulnerable)Lower population growth rateLong lived with stable population near carrying capacityDepend heavily upon suitable habitatLarge mammals, birds of prey, long lived plants such as oaks, redwoods, some cacti
Fig. 9.10b, p. 205
Fewer, larger offspringHigh parental care and protection of offspringLater reproductive ageMost offspring survive to reproductive ageLarger adultsAdapted to stable climate and environmental conditionsLower population growth rate (r)Population size fairly stable and usually close to carrying capacity (K)Specialist nicheHigh ability to competeLate successional species
elephant saguaro
K-Selected Species
Survivorship curveLate loss - typical for k-strategistsEarly loss – typical for r-strategistsConstant loss – for species in the gray area in-between k and r strategists with intermediate reproductive patterns
Song birds, lizards, and small mammals
Fig. 9.11, p. 206
Per
cen
tag
e su
rviv
ing
(lo
g s
cale
)
100
10
1
0
Age
Conservation biologySensible use of natural resourcesOriginated in 1970’s – uses current scienceInvestigate human impact on the biodiversityDevelop practical approaches to maintain biodiversityMaintain – endangered species, wildlife reserves, ecological restoration, ecological economics, environmental ethics
Assumptions of conservation bioBiodiversity is necessaryHumans should not affect extinction or vital environmental processesProtecting ecosystems is the best way to protect
Based on Aldo Leopold’s ethical principle, that if we maintain the earth’s life-support system it is appropriate
Human impact on ecosystemsFragmentation – breaking up large tracts with roads, fences, towns, etc.Habitat loss/degradation – pollution, lumber, mining, etc.Simplifying ecosystems – lower biodiversity through habitat change (monocultures)Strengthening species – pesticide use, antibiotics
Human impact continuedPredator elimination – wolves, coyotes, bear, etc.Introduce alien speciesOverharvest potentially renewable resources – trees, soil, other biomass (grasses, nuts, etc)Interfere with natural chemical cycling – clear cutting, monocultures, pesticides (we kill and simplify a system)
Way to go humans!! You’re the best!Goals for the future (if we want to be a part of it)
Maintain balance between human impacted simple ecosystems and natural rich ecosystemsSlow down rates at which we alter nature for our own purposeRealize that we never do merely one thing, everything is interdependent and unpredictable
How can you helpUse consumer power – buy products that are friendly to the environmentUse voting power – elect officials that will strive to protect the environmentEducate – most people have no idea about the consequences of their actionsIdentify “mother culture” that says spend, buy, consume and learn to tune it outExploit nature for its aesthetics and renewable resources
That’s all folks
Have a nice day