1. Population Ecology Freeman 2002 Jaguar in Brazilian rain forest
Solomon et. al. 1999
Freeman 2002
The focus of ecology can be local or global, specific or generalized, depending on what questions the scientiest is asking and trying to answer (solomon).
Ecology is the broadest field in biology, with links to evolution and every other biological discipline. It also may encomapss non-biolgogical sciences…earth sceince, geolgy, chemistyr, oceanography, climatology and meteorology. Humans are components of ecological relationships, and profoundly impact human
Jaguar in Brazilian rain forest
Humans on a crowded urban street
Ecology is the study of interactions among organisms (biotic factors) and their physical environment (abiotic factors).

2. Population Organism Ecosystem Community
Raven & Johnson 1999
Ecosystem
Community
Population refers to individuals within a species living in the same place at the same time

3. Populations as Units of Structure and Function
The population has unique, discernable properties and is an important unit of biological investigation.
Interests of population ecologists include
to understand how and why population sizes change.
to understand populations as functional subunits of communities.
The population is an important level of organization for the study of evolution
a population has common gene pool
evolutionary change happens to populations
Population ecology has important applications:
Conservation biology - predicting extinction risk, managing populations
Understanding, predicting human population growth
Solomon et al 2002)
Mexican poppies thrive on gravelly desert slopes and grow to a height of 40 cm.
Population of Mexican poppies (Eschscholzia mexiana) blooming in the desert after the winter rains

4. Density and Dispersion are important attributes of populations
Species Range Geographic extent of the occurrence of individuals in a species
Population Individuals within a species living in the same place at the same time;
Species consist of populations of varying size and number, with varying degrees of proximity and connectedness (think gene flow)
Population Density Number of conspecific individuals per unit area or volume at a given time
Dispersion Distribution, or spacing of individuals within a population, relative to each other
Aerial census for African Buffalo (Syncerus caffer) in the Serengeti of East Africa. (Campbell 2002)

5. Patterns of dispersion of individuals in a population
Clumped dispersion Many fishes, including these butterfly fish, often clump in schools. Adaptive function may relate to hydrodynamic efficiency of swimming, decreased predation risk, increased feeding efficiency. Within a school, individuals are fairly evenly spaced
Clumped
Uniform dispersion Not common in general, but is not uncommon among birds nesting on small islands. This is a breeding colony of King Penguins on South Georgia Island in the South Atlantic Ocean.
Within any given area individuals of a population can be distributed in a variety of ways, which form a continuum ranging from a perfectly uniform distribution thourhg randomness to a situation in which all individuals are found in groups or clumps.
Uniform distributions aren’t common. Ususally where environment is uniform and there is intense competition or antagonisms between individuals. Creosote bush toxins prevent germination. Animals defend similar sized exclusive territories
Random distributions are also rare. Only occure where environmental conditions are uniform, where there is no intense competition or antagonism, and where there is no tendency for organisms to aggregate. Very rare that all these conditions cooccur.
Clumping (aggregation) by far most common in nature.
1.      Environmental conditions seldom uniform throughout even relatively small area. Variations in soil, topography, distribution of other species, microclimate factors like moisture temperature light make for important habitat differences; organisms tend to move to those spots where conditions are most favorable
2.      Reproductive patterns, including sexual attraction, often favor clumping, Particularly true in plants that reproduce vegetatively and in animals whose young remain with the parent
3.      Behavior patterns often lead to active congregation in loose groups or in more organized colonies, schools, flocks, or herds
Random dispersion Populations of trees in tropical rain forests are often randomly dispersed, but in general, this dispersion pattern is rather rare in nature
Patterns of dispersion of individuals in a population

6. Uniform dispersion: Individuals are dispersed more evenly than expected from random occurrence in a habitat. Explanations include:
uniform territory sizes in relatively homogenous environments (e.g. penguins)
allelopathy -- production of toxins that inhibit growth of nearby plants (e.g. desert creosote bush and saltbush)
Campbell 1993
uniform or evenly spaced individuals may result from direct interactions between indivduals in the population. In plants, shading and competition for water or minerals may promote a tendency toward regular spacing. Some plants secrete chemicals that inhibit the germination and growth of nearby individuals that could compet for resources. Animals often show uniform dispersion as a rsult of territorial behavior and aggressive social interactions.
Human sunbathers in Sydney Australia. Humans often disperse uniformly.
(Solomon et. al. 1999)
Campbell 1993

7. Clumped dispersion (aggregation) by far most common in nature.
Environmental conditions seldom uniform throughout even relatively small area. Clumping of individuals often a response to clumping of resources
Clumped distribution of “Microhabitats” often explains clumping; local soil moisture, rotting log, limestone outrcrop…
Reproductive behavior patterns including sexual attraction, often favor clumping
Nonreproductive behavior patterns often lead to active congregation in loose groups or in more organized colonies, schools, flocks, or herds
Clumped; individuals are aggregated in patches. Eastern red cedar is often clumped on limestone outcrops where soil is less acidid than in nearby areas. Certain microhabitats of animals are favored and clupmped, like under logs where humidity is high

8. Population demographics affect increase (growth) or decrease (decline) in population size
Demography; study of changes in size and structure of populations
Important demographic variables include:
Sex Ratio
Age structure (cohort=individuals of same age, age class)
Age specific fecundity (birth) rate and mortality (death) rate
Ecologists assemble such demographic data in life tables, then use the life tables as the basis for further analysis of population dynamics

9. Population Dynamics; Changes in population size and demographics over time
Populations size changes in response to births and deaths, and to the movement of individuals into (immigration) our out of (emigration) the population
N1=N0 + B - D + I - E

10. Life Table of the 1978 Cohort of Ground Finches on Isla Daphne
Mortality high during first year of life; mortality then dropped for several years, followed by a general increase
Some of the fluctuation between years was probably related to annual rainfall; survival is related to seed production, which is closely correlated with rainfall
In drought years on the archipelago, seed production is low, nesting is low (most adults don’t breed), and adult survival is low
In years of heavy rainfall, seed production is high, most birds breed several times, and adult survival is high

11. Survivorship curves are graphical representations of age-specific survivorship
Survivorship = 1-mortality, = percent of cohort surviving to a given age
Top left figure shows generalized, hypothetical survivorship curves across a range of possibilities, and that are recurring in nature
Curve I characterizes humans, other large mammals, that have high parental care but produce relatively few young
Curve III characterizes some species with low parental care, large number of young (eg many fishes, marine invertebrates
Curve II Individuals roughly equally likely to die at any age. Characterizes some annual plants, some invertebrates (eg Hydra), some lizards, rodents, birds…

12. Life table for Red Deer on island of Rhum, Scotland.
Deer live up to 16, and females can breed at 4
Type I survival curve indicates a relatively consistent increase in the risk of mortality with age
The growth rate for most populations is strongly dependent on age structure
Raven and Johnson 1999
Raven and Johnson 1999
Life table for Red Deer on island of Rhum, Scotland.

13. Mathematical modeling is an important and widely-used approach to studying population dynamics
Using equations to simulate population dynamics over time
Illuminate complex processes and guide further research
Vary in predictive ability; rarely are perfect approximations

14. Biotic Potential and the Exponential Growth Model
All species have potential for explosive, exponential growth; absent resource limitations, growth would be exponential
Biotic potential (rmax) is the maximum rate at which a population could increase under ideal conditions
Exponential growth has been demonstrated experimentally in bacterial and protist cultures and in some insects.
At some point, environmental resisitance will curtail exponential growth; environmental limitations cause decreases in birth rates and increases in death rates
Time
Population Size
Raven & Johnson 1999
All populations have the potential for explosive growth becsue as the number of individuals in the populations increaseses, the number of new individuals added per unit of time accelerates, even if the rate per capita of populations increase remaines constant
Solomon et al 1999
Exponential growth in bacteria. Bacteria, dividing every 20 minutes, experience exponential population growth
Example of rapidly increasing population. European loosestrife is now naturalized over thousands of square miles of North American wetlands. Introduced in 1860, it has had a negative effect on may native plant and animal species

15. Biotic potential and the Exponential Growth Model
Rate of increase (slope of the curve) steadily accelerates, population increases exponentially.
b=per capita birth rate
d=per capita death rate
dN = (bN-dN) dt
dN/dt = rate of change in size of population
Population Size
Think about (bN-dN) ...
difference between births and deaths in absolute numbers
determines if population will grow, be stable, or decline dN dt
Time
Keeton & Gould 1993

16. (b-d) is the per capita growth rate
(b-d) is the per capita growth rate. It is the net rate of population change per individual
factor N out on right side of equation
dN = (b-d)N dt
divide both sides by N to “see” the per capita growth rate (b-d) dN
dt = (b-d)
----- N
back to...
Population Size
Time
(Keeton & Gould 1993)

17. Population Size Time dN = (b-d)N dt substitute r for b-d dN = (r)N
r = b-d = net rate of population change per individual at a given moment
Under these conditions of maximum birth rate and minimum death rate r is designated rmax
dN = rmaxN
rmax represents the intrinsic rate of increase, or biotic potential of the population
rmax varies widely among species
Population Size
(Keeton & Gould 1993)
Time

18. Population Size Time dN = rmaxN dt
Rate of population growth is a function of r and N; N is related to the number of breeders
N increases with each generation; therefore so does the rate of increase -- dN/ d t
Its due to this accelerating rate of increase that the slope of the curve becomes steeper and steeper
Population Size
Time
(Keeton & Gould 1993)

19. All other things being equal, a population with a higher intrinsic rate of increase will grow faster than one with a lower rate of increase
The value of rmax for a population is influenced by life history features, such as
age at onset of reproductive capability
number of young produced
Population growth predicted by the exponential model. The exponential growth model predicts unlimited populations increase under conditions of unlimited resources. This graph compares growth in populations with two different values of rmax:1.0 and 0.5

20. (Solomon et al 1999)
Human population growth. During the last 1000 years, the human population (globally) has been growing nearly exponentially

21. No population can continue to increase exponentially indefinitely
Environmental resistance:
Environment imposes limits on population growth
Food; water; disease; shelter from elements, predators…...
Carrying capacity (K):
Theoretical maximum population size that can be maintained indefinitely (assumes unchanging environment)
In reality, K changes with changes in environmental conditions
Logistic population growth:
Populations can be modeled taking carrying capacity of environment into account using the “logistic growth equation”
dN/dt = rN [(K-N)/K]

22. The term (K-N)/K causes growth in the simulated population to respond to environmental resistance
When N is small compared to K, [(K-N)/K] is close to 1 and growth is nearly exponential
When N is large compared to K, [(K-N)/K] approaches 0, as does population growth
Number of Individuals (N)
Time

23. Some assumptions and simplifications of the logistic model are either not true for most populations or do not apply equally to all populations
Each individual added to a population at a low level (N) has the same negative effect on population growth rate at low population at a high level (N)
Each individual exerts its negative effects immediately at birth
All individuals have equal effect on the population
Populations approach carrying capacity smoothly – don’t overshoot it
Carrying capacity is constant

24. How well does the logistic growth model fit the growth of real populations?
Experimental populations (bacteria, yeast, Paramecia…)
Some show sigmoidal growth fairly well, but conditions do not approximate nature (predators, competitors lacking).
Some, not all, experimental populations stabilize at some carrying capacity, and most experimental populations deviate unpredictably from a smooth sigmoidal curve
Natural populations
Introduced populations and decimated, recovering populations show growth patterns that generally support the concept of carrying capacity that underlies logistic population growth

25. Logistic Population Growth
Raven & Johnson 1999
Raven & Johnson 1999
A fur seal population on St. Paul Island, Alaska The numbers of male fur seals with harems were reduced to very low numbers due to hunting until After hunting was banned, the population increased dramatically and now oscillates around an equilibrium number, presumably the islands carrying capacity for this species (Campbell 2000)

26. Logistic Population Growth – Overshooting K
Lag time in many populations before negative effects of increasing population are realized
Hypothetical example: food becomes limiting, but birthrate not immediately affected because females use energy reserves to continue producing eggs for a period; population may then overshoot carrying capacity
Real life: In many of the populations that show sigmoidal-type growth, they oscillate around K, or at least overshoot it the first time
Number of sheep (in thousands)
(Keeton & Gould 1993)
Growth curve of the sheep population of Australia. Smooth curve is the hypothetical curve about which real curve fluctuates

27. Population Growth & Life Histories*
Conditions of high population density may favor life history traits different from those favored at low population density
High population size and life history
High population size; limited resources, slow or zero population growth
Traits favored may be those that enable organisms to survive and reproduce with few resources
Competitive ability and high efficiency at resource use may be favored in populations that tend to remain at or near their carrying capacity
Low population size and life history
Low population size; abundant resources, rapid population growth
Traits favored may be those that promote rapid reproduction; ie high fecundity, early maturity; efficiency of resource use not as important
* Life history
Life history of an organism includes birth, growth to reproductive maturity, reproduction, and death
“Life history traits” are characteristics that affect an organisms schedule of reproduction and death.

28. Life history of any individual will include these traits;
Life history of an organism includes birth, growth to reproductive maturity, reproduction, and death
“Life history traits” are characteristics that affect an organisms schedule of reproduction and death.
Life history of any individual will include these traits;
-size & energy supply at birth -rate and pattern of growth and development -number and timing of dispersal events -number and timing of reproductive events -number, size and sex ratio of offspring -age at death
Life History varies among organisms, lineages, based on variation in the allocation of time, effort, energy, etc, to activities and stages from birth, growth to maturity, reproduction, death
Consider the importance of life history traits in explaining demographic populations statistics…age-specific fecundity, mortality…
Flowering stalks of century plants (Agave shawii). Copyright G. J. James/BPS.
Avave: Some of these century plants have mobilized the energy stored during their long life to produce a large flowering stalk with hundres of flowers, literally reproducing itself to death.The low-growing bushy plants are the nonreproductive form of agave
Sockeye salmon. Ascend creeks, lay eggs in gravel beds in streams, then die.
Salmon spawning. Chugack National Forest, AK. Copyright J. Robert Stottlemyer/BPS.

29. “r-Selected populations” likely to be found in variable environments in which population densities fluctuate, or in “open” habitats where individuals likely to face little competition
“K-selected populations” likely to be living at a density near the limit imposed by their resources
Life history traits do often vary in ways shown in table
No demonstration of direct relationship between population growth rate and specific life history traits; concepts of r and K selection are mainly useful as hypothetical models

30. Population ecology and the evolution of life history traits
Because of the varying pressures of natural selection, life histories show high variability
among species and higher taxa
among populations within species
among individuals within a population
within individuals, depending on environmental conditions, availability of mates; consider the adaptiveness of plasticity in life history traits
Patterns exist in the way in which life history traits vary
Life histories often vary in parallel with environmental patterns
Life histories often vary with respect to each other (eg, delayed maturity & high parental investment tend to correlate with low fecundity and low mortality); such relationships between life history traits often reflect “trade-offs”…
Relationship between adult mortality and annual fecundity in 14 bird species Birds with high probability of dying during any given year usually raise more offspring each year than those with a low probability of dying. Wandering albatross; lowest fecundity (~.2 offspring/yr – single surviving offspring every 5 yrs) & lowest mortality. Tree sparrow; >50% chance of dying from one breeding season to another, produces average of 6 offspring per year

31. Organisms have finite resources to invest in components of their life history; Trade-offs between investments in reproduction and survival are a consequence
Selection favors (heritable) life history traits that allow individuals to maximize lifetime reproductive success; these traits will become more common in a population
Natural selection can not simultaneously “maximize” all the life history traits that can potentially contribute to the greatest lifetime reproductive output, because organisms have finite resources to invest; this mandates trade-offs.
Trade-offs occur between: -number & size of young; -number of young & parental care per young; -reproduction and growth; -reproduction and survival; -current reproduction and future reproduction --between investing in current reproduction and future reproduction
Experimental manipulation demonstrates trade-off between investing in current reproduction and survival Manipulating fecundity of female seed beetles by denying access to males or egg-laying sites causes a trade off in adult longevity and fecundity

32. Experimental manipulation demonstrates trade-off between investing in current reproduction and survival Manipulating clutch size in collared flycatchers (add or remove eggs) results in direct trade off between number of chicks raised that year and the next year’s fecundity (no effect of current fecundity on adult survival in this study)

33. Populations dynamics may be influenced by factors operating independent of population density, or in a manner that is dependent on population density.
Density-independent factors Factors that affect per capita birth rate (b) or per capita death rate (d), with the degree of the effect not influenced by (dependent on), population density
Typically abiotic, often weather-related; e.g. in insects, winters kill off all individuals except eggs and dormant larvae
Often random (unpredictable) e.g., blizzard, flood, fire
Effects may be indirectly related to density; e.g., social animals often able to endure weather by collective behavior -- sheep huddling in snow storm
Density-dependent factors Factors that affect the per capita birth rate (b) or per capita death rate (d), with the degree of the effect influenced by (dependent on), population density
Important density dependent factors; Competition, predation, disease
-increasing density may attract predators (more successful, efficient, hunting prey at high density) -increasing density may increase foster spread of contagious disease -increasing density may lead to depleted food supplies
Effects are proportional to population density; Density-dependent factors exert stronger effect as population increases
(Solomon et. al. 1999)
Fire may constitute a density-independent factor for some populations

34. 12 Bahamian islands; all islands have native spider populations, 4 have lizards, 8 do not have lizards
Lizards introduced in enclosures on 4 islands without native lizard populations.
After 7 years, spider densities were higher in lizard-free islands (enclosures).
Species diversity of spiders was also greater on lizard free islands.
Due to predation (lizards eat spiders), or competition (lizards and spiders compete for insect food)?
Effect of lizard presence on spider density. Tropical islands with lizards typically have few spiders. Spiller and Schoener (UC-Davis) tested the effect of presence of lizards on spider population density on Bahamian Islands
Few spiders occur on tropical islands inhabited by lizards
Solomon et. al. 1999

35. Hypothetical population dynamics in regulated population
Hypothetical population dynamics in regulated population. In these models, if birth or death rates or both are density dependent, population responds to increases or decreases in density by returning toward equilbrium density (zero positive or negative growth

36. Population “Regulation” and Reality
A regulated population is one whose dynamics are influenced primarily by density-dependent factors
Regulated populations experience interactions between density and carrying capacity
In reality, many populations are probably affected by both density-dependent and density-independent factors

37. Number of song sparrows on Mandarte Island (B. C
Number of song sparrows on Mandarte Island (B.C.) is a consequence of density-independent (winter weather) and density dependent factors

38. Age-specific demographics for 9-12 month cohort
Age interval (in months)
Number alive at beginning of age interval
Proportion alive at beginning of age interval
Number dying during age interval
Death rate for age interval
No. seeds produced per ind. during age interval
Age-specific demographics for 9-12 month cohort
death rate = 172/316=.544
per capita birth rate = 430

39. Cohort members began reproducing after 3 months of age;
Age interval (in months)
Number alive at beginning of age interval
Proportion alive at beginning of age interval
Number dying during age interval
Death rate for age interval
No. seeds produced per ind. during age interval
Cohort members began reproducing after 3 months of age;
Then reproduced throughout life; all dead by 2 years of age
Probability of age-specific survival decreases with age
Can calculate age-specific birth rates and death rates

40. factor N out on right side of equation
Population Size
N = (b-d)
t N
…and dividing both sides by N gives the per capita growth rate (b-d)...
Time
(Keeton & Gould 1993)

41. N = rN t Population Size Time
Under these conditions of maximum birth rate and minimum death rate:
r is designated rmax
rmax represents the intrinsic rate of increase, or biotic potential of the population
rmax varies widely among species
Population Size
Time
(Keeton & Gould 1993)

42. Under these conditions of maximum birth rate and minimum death rate:
N = rmaxN t
Under these conditions of maximum birth rate and minimum death rate:
r is designated rmax
rmax represents the intrinsic rate of increase, or biotic potential of the population
rmax varies widely among species
Population Size
Time
(Keeton & Gould 1993)