Chapter 26: Population Ecology Flashcards
1
Q
why does population ecology matter
A
conservation of biodiversity
- over-hunting contributed to the decline of megafauna such as mammoths, sabre-toothed cats, etc.
2
Q
one of the ways we, humans, interact with and impact populations of other species
A
harvesting from wild ecosystems
3
Q
population ecology
A
study of how and why populations of organisms grow and decline
- patterns of population change can be influenced by biotic or abiotic factors such as competition, disease, or resource availability
4
Q
the 4 rates of change
A
these determine whether populations grow, shrink, or stay the same
1) birth rate
2) rate of immigration
= population increase
3) death rate
4) rate of emigration
= population decrease
5
Q
population
A
all individuals of a single species that live together in the same place at the same time
- we can describe a number of characteristics of any population and, in combination, these characteristics help us predict what will happen to populations over time
6
Q
geographic range
A
overall space in which each population lives
i.e. a population of snails might inhabit a small tide pool
7
Q
habitat
A
- specific environment in which a population lives, as characterized by its biotic and abiotic features
8
Q
population size
A
(defined as N)
- number of individuals making up the population at a specified time
9
Q
population density
A
number of individuals per unit area/per volume of habitat
usually declines with increasing body size among species
- provides info about its relationship to the resources it uses
- i.e. if a population of 200 oak trees occupies 10 000m^2, the population density is 1 tree per 50m^2, whereas if it was 200 oak trees over 5 hectares, then the density is 1 tree per 250m^2 (second population is less dense than the first and its members will have greater access to resources)
WHEN DENSITY INCREASES INDIVUDLAS WILL FLOCK ELSEWHERE
10
Q
species with a large body size vs a small body size: population densities
A
the larger ones tend to have lower population densities than the small ones
11
Q
ecologists use pop. size and density for what
A
- monitoring and managing populations of endangered species, economically important species, and agricultural pests
- for large-bodied species, a simple head count may provide accurate information about population size
12
Q
african elephant and ecologist example
A
- ecologists survey the size ad density of populations of African elephants by flowing over herds and counting individuals (you can genetically mark them instead)
- researchers use variation here to estimate pop. size in tiny organisms that live at high pop. densities
- for example, you might collect water samples of known areas such as a lake and examine them through a microscope, developing data
13
Q
dispersion
A
populations vary in the spatial distribution of individuals or populations within the range of the population or species
14
Q
most common dispersion
A
clumped
- may be the result of number of ecosystem drivers: I,e, distribution of essential resources
i.e. certain pasture plants are clumped in small scattered areas where cowpats have fallen and locally enriched the soil
- can also occur in species where seeds, eggs, or larvae lack dispersal mechanisms and offspring grow and settle near their parents
15
Q
social animals and clumping
A
occur together in groups of various size, social individuals cooperate in rearing offspring, feeding, or defence against predators
things such as resources and social (breeding) could cause clumping
16
Q
uniform dispersion
A
can occur when individuals repel or avoid one another, often because resources are in short supply
[resources are limited]
- for example: creosote bushes are uniformly distributed in the dry scrub deserts of the American Southwest, mature bushes deplete the surrounding soil oof water and secrete toxic chemicals, making it impossible for seedlings to grow
= this chemical warfare is called ALLEOLOPATHY
- another example: seed-eating ants and rodents living at the bases of mature bushes eat any seeds that fall nearby
- the distributions of species of plants and animals can be uniform and interrelated
17
Q
territorial behaviour
A
the defence of an area and its resources
- can also produce uniform dispersion in some species
18
Q
random dispersion
A
occurs when environmental conditions don’t vary much within a habitat and individuals are neither attracted to not repelled by others of their species (conspecifics)
[resources are abundant]
19
Q
ecologists and random dispersion
A
- use formal statistical definitions of random to establish a theoretical baseline for assessing the pattern of distribution
- individuals are distributed unpredictably
i.e. some spiders, burrowing clams, and rainforest trees examine random dispersion
20
Q
whether the spatial distribution of a population appears to be clumped, uniform, or randoms depends on
A
(In part) on the size of the organisms ad of the study area
- oak seedlines for example, may be randomly dispersed on a spatial scale of a few square meters but over an entire mixed hardwood forest, they are clumped under the parent trees
- therefore the dispersion of a population may depend on the researchers scale of observation
21
Q
the dispersion of animal populations often
A
- varies through time in response to natural environmental cycles
- few habitats provide a constant supply of resources throughout the year, and many animals move from one habitat to another on a seasonal cycle, reflecting the distribution of resources such as food
- tropical birds and mammals are often widely dispersed in forests during the wet season when food is widely avlaible, during the dry season though, the spices gcrowd either into narrow gallery forests
22
Q
age structure
A
a statistical description of the relative numbers of individuals in each age class
- individuals can be categorized generally as pre-reproductive (younger than the age of sexual maturity), reproductive, or post-reporuductive (older than maximum age of reproduction)
23
Q
populations composed of many indivudlals
A
- many pre-productive individuals must have grown rapidly in the recent past
- these populations will keep throwing
24
Q
what do population ecologists attempt to determine
A
proportion of unreproducing individuals
- relevant to conservation of any species in which individual are rare or widely described
25
in a specie where only females produce young/...\
the number of females has a larger impact on population growth than the number of males
- in many species, one male can mate with several females and the number of males may have little effect on the populations reproductive output
26
In northern elephant seals Example
in northern elephant seals, mature bulls fight for dominance on the beaches where the seals mate
- while a few males each may inseminate many females and sire many young, most males do not
- thus the presence of other males in the group may have little effect on the size of future generations. In animals that form life-long pair bonds, such as geese and swans, the number of pairs influences reproduction in the population
27
generation time
influences a populations potential for growth or decline
- its the average time between the birth of an organism and the birth of its offspring
- generation time is usually short in species that reach sexual maturity at a small body size
- their populations often grow rapidly because of the speedy accumulation of reproducing individuals
28
immigration
movement into populations
29
emigration
movement out of the population
30
demography
statistical study of these processes that change a populations size and density through time
- ecologists use demographic analysis to predict a populations growth
I.E. demographic data on northern spotted owls helped convince the courts to restrict logging in the owl's primary habitat, the old-growth forests of the Pacific Northwest
31
although every species has a characteristic life span
few individuals survive to the maximum age possible
- mortality results from starvation, disease, accidents, predation, or inability to find a suitable habitat
32
life table
summarizes the demographic characteristics of a population
- to collect life table data for short lived organisms, demographers mark a cohort at birth and monitor their survival until all members of the cohort die
- researcher might sample the pop. for 1-2 years, recording the ages at which individuals die and extrapolate these results over the species life span
33
cohort
group of individuals of similar age
34
in any life table
the life spans of organisms are divided into age intervals of appropriate length
- for short-lived species, days, weeks, or months are useful
- where longer-lived species years, or groups of years will be better
35
age specific morality
proportion of individuals alive at the start of an age interval that died during the age interval
36
age-specific survivorship
probability of surviving to a particular age or stage within a population
37
for any age interval, the sum of age-specific mortality and whatnot is
must always equal 1
38
life tables also summarize the proportion of ....
the proportion of that cohort that survived to a particular age, to identify that any random selected newborn will be alive at that age
| age-specific survivorship
39
age specific fecundity
life tables also include data on age-specific fecundity
- the average number of offspring produced by surviving females during each age interval
- in some species, including humans, fecundity is highest in individuals of intermediate age
- younger individuals have not yet reached sexual maturity, and older individuals are past their reproductive prime
40
T/F: fecundity can increase steadily with age
T; FOR SOME PLANTS AND ANIMALS
41
Survivorship curve
displays the rate of survival for individuals over a species or populations average life span
- ecologists have identified three generalized survivorship curves
42
type 1 survivorship curve
1) type 1 curves:
- reflect high survivorship until late in life that produce few young and reduce juvenile mortality with extended parental care
- large mammals, such as tall mountain sheep, produce only one or two offspring at a time and nurture them through their first year
- at that time, the young are better able to fend for themselves and are at a lower risk for mortality (Compared to younger animals)
| **as you get older, curve moves down** (humans)
43
what could cause the picture of survivorship in mammals to change
| hint: pregnancy
if one starts with the time of conception, as opposed to birth, the change would then also reflect problems of pregnancy and health of mothers
44
type 2 survivorship curve
- reflect a reflectively constant rate of mortality
- In all age classes, a pattern that produces steadily declining survivorship
- many lizards, and song birds, face a constant probability of mortality from predation, disease, and starvation = type 2 pattern
45
type 3 survivorship curve
- curves reflect high juvenile mortality, followed by a period of low morality once offspring reach a critical age and size
46
type 3 survivorship example (desert shrub)
- a desert shrub from the Middle East experiences extraordinarily high mortality rate in its seed and seedling stages
- researchers estimate that, for every 1 million seeds produced, fewer than 1000 germinate and only about 40 individuals survive their first year
*once a plant becomes established, however, its likelihood of future survival is higher and the survivorship curve flattens out. Many plants, insects, marine invertebrates, and fishes exhibit type 3 survivorship*
47
the analysis of life tables reveal
how NS has produced different life histories--- the lifetime patterns of growth, maturation, and reproduction
- maximize the number of surviving offspring an individual produces
48
energy budget
- every organism is constrained by a finite energy budget
= total amount of energy it can accumulate and use to fuel its activities
*when it accumulates more energy than it needs, it makes deposits to this account, storing energy as starch, glycogen, or fat*
49
what happens when the individual expends more energy than its harvests, it makes withdrawals from its energy stories
- but unlike a bank account, an organisms energy budget cannot be overdrawn and no loans against future "earnings" are possible
- organisms use the energy they harvest for 3 broadly defined functions: maintenance (preservation of good anatomical and physiological condition), growth, and reproduction
WHEN AN ORGANISM DEVOTES ENERGY TO ANY OF THESE:
= balance of energy budget is reduced, leaving less energy for the other functions
50
Organisms use different ways to manage their energy *3 ways*
1) organisms that enter states of inactivity or dormancy can maximize the time over which they use stories energy
2) a extreme instance is animals and plants that ca survive freezing, a strategy for conserving energy
3) Hibernation and estivation in animals are other examples
- hibernating animals use periods of reduced body temperature to weather, pan intended, prolonged periods of cold
51
estivation
inactivity during prolonged periods of high temperature
- however, some plants and fungal spores have adaptations making them resistant to heat and desiccation
52
migrating birds on long flights get energy...
by metabolizing fat as well as other energy reserved when necessary, such as muscle or digestive tissue
53
Travel Sockeye Salmon example
- hatch in headwaters of stream where they feed and grow for about a year before assuming their adult body form and swimming to the ocean
- they remain at sea for a year or two, feeding voraciously and growing rapidly
- eventually, salmon use a sun compass and geomagnetic and chemical cues to return to the rivers and streams where they hatched
- fish swim upstream: males make nests and try to attract females, females lay thousands of eggs
= they spend all their energy reserved on this upstream journey and reproduction, their condition deteriorates and they die
*reproduce in one single episode*
*grow until old age*
54
Deciduous trees example
- deciduous trees in the temperate zone, such as oaks begin their lives as seeds in late summer
- their acorns remain dormant until the following spring or a later year
- after germinating, trees collect nutrients and energy and continue to grow throughout their lives until they achieve a critical size where they can produce thousands of acorns annually
- growth and reproduction occur simultaneously
*reproduce repeatedly*
*grow until old age*
*produce thousands of seeds*
55
European red deer example
- born in spring and the young remain with their mothers for an extended period, nursing and growing rapidly
- the young then forage alone
- female red deer begin to breed after reaching adult size in their third year, producing 1 or 2 offspring annually until they die at about 16 years of age
*reach adult size fairly early*
*reproduce repeatedly*
*produce only 1-2 young/spring*
56
what factors produce the traits between the deer, salmon, and tree?
life history traits
- modified by NS, they exhibit adaptations that increase their fitness
therefore, a life history=a highly integrated strategy of suite of selection-driven adaptations
57
ecologists analyzing life-histories
compare the number of offspring with the amount normally produced per reproductive event
- also consider the number of reproductive episodes in the organisms lifetime and the timing of first reproduction
- these characteristics often evolve together, a change in 1 trait is likely to influence the other
58
fecundity vs parental care
female duck: has 1000 units of energy
- 4 reproduction it lays 10 eggs with 100 units each
salmon: has a higher fecundity
- will lay 1000 eggs each with 1 unit of energy
59
passive parental care
amount of energy invested in each offspring before birth
- usually provided by the female: yolk in egg, endosperm in seed, or in the placenta for mammals
60
active parental care
- the care once an organism is born: a red deer doe nurses its single fawn for up to 8 months before weaning it
species producing many offspring in a reproductive episode provide little active parental care to each offspring
species with little offspring at a time provide much more care to each one
61
semelparity
devoting all stored energy to a single reproductive event (salmon for example)
- uses all of its energy budget on reproduction
62
iteroparity
reproduce more than once
( trees and deer)
- only devote some of energy budget to reproduction with the balance allocated for maintenance and growth
advantages:
- continued growth may result in increased fecundity at a later age
- but organisms that don't survive until the next breeding season lose the potential advantage of putting energy into maintenance and growth
63
larger individuals vs smaller individuals (reproduction)
FOR PLANTS, INVERTEBRATE, FISHES, AND REPTILES ONLY
larger individuals produce more offspring than smaller ones
64
early reproducers tend to be
smaller and less healthy than individuals that delay reproduction in favour of other functions [diff in energy investment]
BUT
- delayed reproducers increase chance of survival and future fecundity by becoming larger or more experienced but there is a chance that it will die before the next breeding season, leaving behind no offspring
* a finite energy budget and the risk of mortality establish a trade-off in the timing of first reproduction *
65
which form of reproduction will be favoured by NS
delayed
- IFF: a sexually mature individual is likely to survive to an older age, if organisms grow larger as they age, and if larger organisms have higher fecundity
early
- IFF: adult survival rates are low, if animals don't grow larger as they age, or if larger size doesn't increase fecundity
66
guppies
(Caribbean Island of Trinidad)
- guppies bear young in shallow mountain streams
- John Endler and David Reznick were studying the environmental variables influencing the evolution of their life history patterns
(males stop growing at sexual maturity=smaller, bright scales as a courtship display, females=dull and keep growing larger)
- guppies live in diff streams
- 2 species of fish eat guppies: killfish which usually kills baby guppies and large pike cichlid which prefers mature guppies are rarely hunts small, immature guppies
- Reznick and Endler found that the life history patterns of guppies vary per streams with diff predators
Streams with Pike Cichlids: male and female guppies mature faster and begin to reproduce at a smaller size and younger age than their counterparts in streams where killfish live
- female guppies here reproduce more often (smaller and numerous young) + they reproduce at an earlier age
- this helps avoid predation since pikes don't go after the young
Streams with killfish:
- grow quickly to a size that is too large to be consumed by killfish
- helps avoid predation since killfish want the young babies
67
Guppies: further investigation by Endler and Reznick
- the life history differences were correlated with the distributions of the 2 predatory fish
- but the 2 researches wanted to know if these differences may result from eachother
-conducted controlled lab experiments where they bred guppies from each kind of stream for 2 generations
- raised in identical conditions w/o predators
= maintain their life history conditions, providing evidence of a genetic basis for the observed life history differences
THEY ALSO EXAMINED THE ROLE OF PREDATORS IN THE EVOLUTION OF THE SIZE DIFFERENCE
- they raised guppies for many generations in the lab under 3 experimental conditions: alone, with killifish, or with pikes
- the guppy lineage w/ killfish became larger and maturity because the smaller ones were eaten with reduced reproduction
- the guppy lineage w/ pikes showed a trend towards earlier maturity, individuals that matured at a larger size faced a greater likelihood of being eaten before they had reproduced
68
models of population growth
1) exponential growth
- when populations experience unlimited growth
2) logistics growth
- when population growth is limited
- often because available resources are finite
= simple models are tools that help ecologists refine their hypotheses, but neither provides entirely accurate predictions of population growth in nature
69
- tracking changes in the number of individuals in a population generally involved monitoring four types of demographic change
1) the number of births
2) the number of deaths
3) the number of immigrants to a population
4) the number of emigrants from a population
in the simplest versions: ecologists define births as the production of offspring by any form of reproduction and ignore the effects of immigration and emigration
70
exponential models
- population size increases steadily by a constant ratio (bacteria + prokaryotes + SOME multicellular organisms)
Bacteria:
- reproduce via binary fission, population doubles in size in each generation
- bacterial populations grow quickly under ideal temperatures and with unlimited space and food
- consider a population of the human intestinal bacterium E.COLI
71
time vs e.coli populations
- generation time=20 miutes
1 bacterium
after 20 mins: goes to 2 bacterium
after 40 mins: 4 cells
after (2 gens)
after 3 gens: 8 cells
after 8 hours (24 generations)=17 million
after 1 day (72 generations), the population will number nearly 5x10^21 cells
72
over a given time period, a change in population can be described with what equation
a change in population size= number of births-number of deaths
*N=population
t=time
B= no. of births
D= no. of deaths*
N/t= change in pop. size over time
N/t=B-D
= equation applies to any population where we KNOW THE EXACT NUMBERS OF BIRTHS AND DEATHS or we know the per capita birth and death rates making the equation:
N/t=(b-d)N
or
dN/dt=(b-d)N
= exponential model of population growth
73
ecologists usually express births and deaths as
per capita (per individual rates), allowing them to apply the model to a population of any size
74
per capita birth rate (b)
- number of births in the population during the specified time period divided by the population size: b=(B/N)
if in a pop. of 2000 field mice, 1000 mice are born then
b=1000/2000=0.5
75
per capita death rate (d)
- no. of deaths divided by the population size
d= (D/N)
if in a pop of 2000 field mice, 200 mice die per month then
d= 200/2000=0.1
76
b-d
per capita growth rate of the population
- symbolized by r, which is always expressed per individual per unit time
- using the per capita growth rate, r, in place of b-d, the growth equation is
dN/dy=rN
77
if the birth rate exceeds the death rate
r is + and population is growing
0.5-0.1=0.4 (field mouse ex)
*populations will grow as long as the per capita growth rate is positive*
78
if the death rate exceeds the birth rate
r is - and the population is shrinking
79
birth rate=death rate
r=0
- population isn't changing
= ZERO POP. GROWTH
80
graph of exponential population
characteristics J shape
- getting steeper through time
- population grows at an ever-increasing pace because the change in a populations size depends on the number of individuals in the populations and its per capita growth rate
- predicts unlimited population growth
81
intrinsic rate of increase
maximum per capita growth rate under ideal conditions
- symbolized rmax
exponential growth equation:
dN/dt=rmax(N)
- populations increase very quickly
- rmax caries inversely with generation time: species with a short generation time have a highest intrinsic rates of increase than those with a long generation time
82
what factors limit the growth of populations
- as a population gets larger it uses more vital resources, perhaps leading to a shortage of resources
- individuals may have less energy available for maintenance and reproduction, causing decreases in per capita birth rates and increases in per capita death rates
- energy in food isn't equally available, and if an animal is hunting the cost to benefit ratio diminishes, affecting return on investment
= affects per capita growth rate, causing population growth to slow or stop
83
carrying capacity
the maximum number of individuals that an environment can support indefinitely
- symbolized as K (defined for each population)
- the spring and summer flush of insects in temperate habitats support large populations of birds, but fewer insects are available in autumn and winter, representing a seasonal decline in K for birds
- explains why birds migrate South in autumn
- other cycles are annual, i.e. variation in water levels in wetlands
84
logistic model of population growth
- assumes that a populations per capita growth rate, r, decreases as the population gets larger
- S shape: populations slowly approaching K and remaining at the level
- pop. grows slowly when pop is small because few individuals are producing, it also grows slow with a large pop. because per capita pop. growth rate is low
- pop grow faster at intermediate sizes when a sizeable no. of individuals are breeding and the r value is fairly high
- population growth slows as the population size approaches K
mathematically: K-N
- tells us how many individuals can be added to a population before it reaches K
- expression: (K-N)/K=indicates the proportion of the carrying capacity is still available so we multiply rmax and yield.....
dN/dt=rmaxN((K-N)/K)
- as N increases
85
how does r vary with population size
in a very small pop. (when
NK), few additional resources are available, now (K-N)/K is small and the per capita growth rate is very low
when the size of the population exactly equals K, (K-N)/K=0 as does the population growth rate, the situation defined as 0 pop. growth
86
logistic models assumes
vital resources become increasingly limited as a pop. grows
- therefore, the model Is a model of intraspecific competition
87
intraspecific competition
the dependence of 2_ more individuals in a population on the same non-infinite resources
88
limiting resources
- when these resources limit growth and reproduction and thus, pop. growth
i.e. food, water, nesting sites, space (for sessile species), refuge from predators, [sunlight, water, inorganic nutrients and growing space=plants]
89
in some dense populations, the accumulation of poisonous waste products
- reduce population growth rates
- most natural populations live in open system where waste are consumed by other organisms or flushed away
- but the buildup of toxic wastes is common in some settings
i.e. yeast cells ferment sugar and produce ethanol as waste giving us alcoholic bevs.
- alcohol content of wine usually doesn't exceed 13% by volume, this is because 13% is the point where the ethanol concentration poisons the yeast and they are unable to survive
90
how well do species conform to the predictions of the logistic model?
- in simple lab cultures, relatively small organisms such as paramecium, flour beetles, and crustaceans, show a S-shaped pattern of population growth
- large animals introduced into new environments sometimes exhibit a pattern of population growth that matches the predictions
91
logistic model flaws
*SOME ASSUMPTIONS OF THE MODEL ARE UNREALISTIC*
- for example, the model predicts that survivorship and fecundity respond immediately to changes in a populations density
- many organisms exhibit a delayed response [time lag] because fecundity has been determined by resources availability
= may reflect conditions when individuals were adding yolk to eggs or endosperm to seeds
- when food becomes scarce, individuals may survive and reproduce using reserves of stored energy delaying the impact of crowding and intraspecific competition until stored reserves are depleted
- this means N could overshoot K
- when deaths outnumber births, N drops below K temporarily, and time lags often cause a population to oscillate around K
92
in general (logistic): the assumption that the addition of new individuals to a population always decreases survivorship and fecundity is unrealistic
- in small populations, modest population growth may not have much impact on survivorship and fecundity
- in fact, most organisms require a minimum pop. density to survive and reproduce
- some plants flourish in small clumps that buffer them from physical stresses, whereas a single individual living in the open would suffer adverse effects
- in some animal populations, a minimum population density is necessary for individuals to find mates or hunt
93
density dependent
influence of density dependent factors can increase or decrease with population density; either way can be an example of a density-dependent environmental factor
- the logistic model includes the effects of density dependence in its assumption that per capita birth and death rates change with population density
94
numerous labs and field studies show what about crowding
- highest pop density (crowding) decreases individual growth rate, adult rate, and survival of plants and animals
- organisms living in a very dense pops are unable to harvest enough resources; they grow slow and tend to be small, weak, and less likely to survive
95
gardeners understand crowding
hence why they thin out their populations of plants to achieve a density that maximizes the no. of vigorous individuals available for harvest
96
crowding and its negative effect on reproduction
- when resources are in short supply, each individual has less energy for reproduction after meeting its basic needs for maintenance
- hence, females in crowded populations produce either fewer offspring or smaller offspring that are less likely to survive
- in some species, crowding stimulates developmental and behavioural changes that can influence pop. density
- migratory locusts for example can develop into either solitary or migratory forms in the same pop.
- migratory individuals have longer wings + more body fat, characteristics that allow long-distance dispersal
97
high population density and the frequency of migration
high population density increases the frequency of the migratory forms
- so as locusts move away from the area of high density, decreasing the size and thus the density of the OG populations
= this data confirms the assumptions of the logistic equation, they do not show that natural populations are regulated by density dependent factors
- experimental evidence is necessary to provide evidence that an increase in pop. density causes it to increase
98
Robert Eisenberg
- experimentally increased the numbers of aquatic snails in some ponds and decreased them in others
- he also maintained natural densities in control ponds
- adult survivorship didn't differ between experimental and control treatments
- but there was a gradient in egg production, from few eggs (snails in high density ponds), to more (medium density, considered the control) to low (low density)
- survival rates of young snails declined as density increased
- after 4 months, densities in the 2 experimental groups converged on those in the control, providing strong evidence of density-dependent population regulation
AT THIS STAGE: intraspecific competition appears to be the primary density-dependent factor regulating pop. size
- competition btwn populations of diff species can also exert density-depoendent effects on pop. growth
## Footnote
- after 4 months, 2 groups=control
- density-dependent regulation—as the high-density group experienced more competition and resource limitations, their growth slowed, while the low-density group had more resources and grew faster.
99
predation can also cause density-dependent population regulation
as a particular prey species becomes more numerous, predators may consume more of it because it is easier to find and catch
- once a prey species exceeds some threshold density, predators may consume a larger percentage of its population, amounting to a density-dependent effect
I.e. on Rockey shores in california, sea stars feed mainly on the most abundant avlaiable invertebrates, when one prey species becomes common, predators feed on it disproportionately reducing its numbers, then they switch to now more abundant alternative prey
100
several density-dependent factors influence a population at the same time
spider example
- on small islands in the West Indies, spiders are rare wherever lizards are abundant but common where the lizards are rare or absent
- to test whether the presence of lizards limits the abundance of spiders, David Spiller ad Tom Schooner built fences around plots on islands where these species occur
- they eliminated lizards from experimental plots but left them in control plots
- after two years, spider populations in some experimental plots were 5 times denser than those in control plots, suggesting a strong impact of lizard populations on spider populations
- In this situation, lizards had two density-dependent effects on spider populations: first, lizards ate spiders; second, lizards competed with spiders for food.
101
Predation, parasitism, and disease can cause...
density-dependent regulation of plant and animal populations
- infectious microorganisms (I.e. those causing malaria) can spread quickly in a crowded population
- if crowded individuals are weak or malnourished, they are more susceptible to infection and may die from diseases that healthy organisms would survive
- effects on survival can be direct or indirect
102
density-independent factors
-some populations are affected by density-independent factors that reduce the size of a population regardless of its density
- if insect population Is not physiologically adapted to high-temperature, for example, a sudden hot spell can kill 80%, whether they number 100 or 100 000.
- fires, earthquakes, storms, and other natural disturbances can contribute directly or indirectly to density-independent mortality
- such factors do not cause a population to fluctuate around its K, these density-independent factors can reduce but do not regulate population size.
103
density-independent factors have a particularly strong effect on populations of ...
small-bodied species that cannot buffer themselves against environmental change
- their populations may grow exponentially for a time, then shifts in climate or random events may cause high mortality before populations may grow exponentially for a time, then shifts in climate or random events may cause high mortality before populations reach a size at which density-dependent factors would regulate their numbers
104
when conditions improve, populations grow exponentially....until..
another density-independent factor causes them to crash again
105
small Australian insect example of populations growing exponentially until density-independent factor causes them to crash
THIRP EX.
- a small Australian insect, a thrip eats the pollen and flowers of plants in the rose family
- these thrips can be abundant to damage blooms
- populations grow exponentially in spring when many flowers are avaliable and the weather is warm and moist
CRASHING:
- populations crash predictably during summer because thrips don't tolerate hot and dry conditions
- after the crash, a few individuals survive in remaining flowers, and they are the stock from which the population grows exponentially the following spring
THUS....
- although the density-independent effects of weather limit populations of thirds, the availability of flowers in summer (a density-dependent factor) regulates the size of the starting population of thirds the following spring
106
density-dependent and density-independent factors
- density-dependent factors can interact with density-independent factors and limit population growth
107
food shortage and high population density
- food shortage caused by high population density (density-dependent factor) may lead to malnourishment
- malnourished individuals may be more likely to succumb to the stress of extreme weather (a density-independent factor)
108
how can populations be affected by density-independent factors in a density-dependent manner
- some animals retreat into shelters to escape environmental stresses such as floods or severe heat
- if a population is small, most individuals can be accommodated in available refuges
- but if a pop. is large (exceeds the capacity of shelters), only some individuals will find suitable shelter
- the larger the population, the greater the % of individuals exposed to the stresses
= THUS,both density-dependent and density-independent factors influence the size of populations of thirps
109
extrinsic control
= external factors that affect the variety of life on earth
- such as the relationship between a cycling species and its food or predators
- a dense population may exhaust its food supply, increasing mortality and decreasing reproduction
110
example of impact of overpopulation
- die-off of large numbers of African elephants in Tsavo National Park in Kenya is an example of the impact of overpopulation
- there, elephants overgrazed vegetation is most of the Park habitat
- in 1970, the combination of overgrazing and a drought caused high mortality of elephants
- the picture is not always clear because experimental food supplementation does not always prevent decline in mammal populations, suggesting a level of intrinsic control
111
casual observation reveals
considerable variation in the rapidity of changes in sizes of population in different species
- weeds often appear in a garden overnight whereas oak trees in a forest remain stable for years
112
why do some species have the potential for explosive population growth
- how NS has moulded life history strategies adapted to different ecological conditions
113
- some ecologists recognize two quite different life history pattern
r-selected species
K-selected species
114
r-selected species
- adapted to rapidly changing environments
- success depends on the flooding the environment with a large quantity of young because only some may be successful
- small body size means that r-selected species lack physiological mechanisms to buffer themselves from environmental variation
= populations of r-selected species can be reduced by chances in abiotic environmental factors (e.g. temperature or moisture) so that they may never grow large enough to reach K and face a shortage of limiting resources
115
T/F: K selected species are better with logistic pops.
T
- because they thrive in stable environments, have limited reproduction, and their populations are regulated by carrying capacity (K), leading to an S-shaped growth curve.
116
although r-selected species appear to have poor tolerance of environmental change
- they appear adaptable to rapidly changing environments
117
K-selected species
- expected to thrive in more stable environments
- generally large, with long generation times, and produce offspring repeatedly over their lifetimes w/ offspring receiving substantial parental care (energy reserves in an egg or active care ensuring most survive in the early stages of life)
- have a low rmax= populations grow slowly
- success of K-selected life history therefore depends on production of a relatively small number of high-quality offspring that join an already well-established population
118
ensuring most offspring survive in the early stages of life applies to what forms of survivorship
type 1 and type 2
119
generalizations about r-selected and K-selected species can be misleading
e.x. deer mouse and little brown bat
DEER MOUSE:
- occurs widely in N.A.
= in southern Ontario, adults weigh 12 to 31 g, females produce average litters of 4 and each bear 4-5 litters per year
- females become sexually mature at age 2 months and breed in their first year
- occasionally, deep mice live to age 3 in the wild
LITTLE BROWN BAT:
- also occur extensively across N.A.
- the bats weigh 7-12g, females may breed a year after they are born, but many do not reproduce until they are two years old
- in the wild, they can live over 30 years
= therefore....
deer mouse=r-strategiest
brown bat=K-strategist
120
what makes the comparison between brown bats and deer mice difficult
- deer mice living in Kananaskis near Calgary mature at 1 year and may have 2 litters per year
w/ 5 young per litter
- therefore, compared to little-brown bat the Kananskis deer mice are r-strategists BUT compared to Ontario deer mice, they are most likely K-strategists
121
issues with r-strategists and K-strategists division
oversimplified:
- elephants are big and meet all K-strategist criteria
- insects are small but still meet K-strategists ideals because of reproductive techniques
- codfish, are big but meet most of their criteria with r-strategists such as their patterns of reproduction
122
population cycles
- population densities fluctuate between species-specific lows and highs in a multiyear cycle
= arctic populations of small rodents vary in size over a 4 year cycle, whereas snowshoe hares, ruffed grouse, and lynx have 10 year cycles
- food supply and predators for a cycling population are themselves influenced by a populations size
123
intrinsic controls
- as animal population grows, individuals undergo hormonal changes that increase aggressiveness, reduce reproduction, and foster dispersal
- dispersal phase of the cycle may be dramatic
- when populations of Norway lemming, a rodent that lives in the Scandinavin Arctic reach their peak density, aggressive interactions drive younger and weaker individuals to disperse
124
dispersal of many thousands of lemmings during periods of population growth
has sometimes been incorrectly portrayed in nature films as a suicidal mass migration
125
cycles in populations of predators could be induced by time lags
- between populations of predators and prey, and vice versa
- the 10 year cycles of snowshoe hares and their feline predators, Lynx:
- hare pops exhibit a 10 year fluctuation even on islands w/o lynx
- therefore, lynx are not solely responsible for pop. cycles in hares
- the database of the fluctuations was often the numbers of pelts purchased by the Hudsons Bay Company
= fur price influenced the trapping effort and the numbers of animals harvested
= economical reality brought into question the relationship between the numbers of pelts and actual population densities of lynx and hares
126
Charles Krebs
- studied hare and lynx interactions in Yukon (Kluane)
- used fenced experimental areas to add food for hares and/or excluding predators
w/o predators: densities of hares doubled
w/ food: densities of hares tripled
w/ food, w/o predators: densities increased x11
= therefore, they concluded neither food availability nor predation is solely responsible for population cycles in snowshoe hares (its the interaction)
127
at least a billion ppl are malnourished or starving, have access to non-clean water, and live w/o shelter and proper hc
- represents a lack of fair and equitable distribution and access to resources not a global lack of resources or calories
128
how did demographers identify 3 ways in which we have avoided the effects of density-dependent regulating factors
1) humans have expanded their geographic range into every habitat on Earth
- our ancestors lived in tropical and subtropical grasslands but then dispersed
- we solved ecological problems: made fires, made shelters and tools, etc.
- spread survival and language skills
2) increased K, the carrying capacity in habitats we occupy
- the change began to occur about 11 000 years ago, when populations in diff parts of the world began to shift from hunting and gathering to agriculture
- irrigating crops, domesticated animals, and used innovations to increase food availability
3) advances in public health reduces the effects of critical population-limiting factors such as malnutrition, poor hygiene, and disease
- we've been able to reduce death rates sharply especially infant mortality
= births can greatly exceed deaths
129
annual growth rate for human population
averaged roughly 2.2% (r=0.022 new individuals per year)
- in 2011, the worldwide annual growth rate for the human population averaged about 1.15% (r=0.0115 new individuals per individual per year)
- population experts expect that rate to continue to decline, but even so, the human pop. will continue to grow at least until 2050
130
Western Europe and Japan vs Africa/Central and South America/Asia
WE and Japan: zero pop. growth or negative growth
Africa/Central and South America/Asia: increases in population over the next 15-40 years
- movement of people can impact populations, (i..e refugees leaving the Middle East and going to Canada)
131
differences in age structure are a major determinant of differences in....
population growth
- uniform age structure of countries with 0 growth, w/ approximately equal numbers of ppl of reproductive and pre-reproductive ages, suggests that individuals have just been replacing themselves and that these population will NOT experience a growth spurt when children mature
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- Narrow-based age structure of countries with negative growth illustrates a continuing decrease in population size
- reproductive-aged people have been having few babies and the small number of pre-productive people may not have enough children to replace themselves
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-Countries with rapid growth have a broad-based age structure, with many children born during the 15 years
- each of these population age structures can be accompanied by political, social, and environmental challenges
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- Increasing populations of young people may overwhelm resources and infrastructure locally, and lack of equitable distribution of resource may lead to poverty and further social inequity in these regions
- overall increasing global population increases global resource consumption and neg. impacts on the environment such as increased fossil fuel emissions and loss of habitat for agricultural production
132
demographic transition model
- the relationship between a country's population growth and its economic development can be depicted by this model
- it describes historical changes in demographic patterns in the industrialized countries of Western Europe; we don't know whether it accurately predicts the future for developing nations today
- therefore, during a country's pre-industrial stage, birth, and death rates are high, and the population grows slowly
133
ecological footprint
sum total of resources we extract and the cost to the Earth of absorbing our waste, we need to meet the needs for food, fuel, and fibre for people across the globe more equitably, and reduce the waste and environmental costs associated with meeting these needs
134
Industrialization
begins a transitional stage
- when food production rises, healthcare improves, and death rates decline, resulting in increased rates of population growth
- later, as living conditions improve, birth rates decline, causing a drop in rates of pop. growth
- when the industrial stage is in the full swing, population growth slows dramatically
- people move from countryside to cities, and urban families often have fewer children later in life....0 population growth is reached in the post-industrial stage
= eventually, the birth rate falls below the death rate, r, falls below zero, and population size begins to decrease
135
population age structure
relative numbers of individuals of different ages
136
life history theory
-trade-offs in energy allocation are inescapable (energy must be allocated in certain ways)
-natural selection acts on life histories to adjust energy allocation in a way that maximizes total lifetime reproduction of offspring
137
life history traits
- age at maturity
- size at maturity
- number of offspring
- size of offspring
- frequency of reproduction
