Lecture #K5 – Population Ecology, continued – Dr. Kopeny

4/22 lecture

No population can continue to increase exponentially indefinitelyEnvironmental 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]

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(N)

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

Time

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

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

Logistic Population Growth

http://www.pinnipeds.fsnet.co.uk/species/species.htm

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 1911. 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)

Raven & Johnson 1999

Raven & Johnson 1999

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

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Growth curve of the sheep population of Australia.Smooth curve is the hypothetical curve about which real curve fluctuates

(Keeton & Gould 1993)

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.

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.

•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…

Salmon spawning. Chugack National Forest, AK. Copyright J. Robert Stottlemyer/BPS.

Flowering stalks of century plants (Agave shawii). Copyright G. J. James/BPS.

•“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

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

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

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)

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

Fire may constitute a density-independent factor for some populations

(Solomon et. al. 1999)

Solomon et. al. 1999

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

•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)?

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

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

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