population dynamics A look at the factors that tend to increase or decrease the size of a population.

POPULATION DYNAMICS AND CARRYING CAPACITY Most populations live in clumps although other patterns occur based on resource distribution. Figure 8-2

(a) Clumped (elephants) Figure 8.2 Natural capital: generalized dispersion patterns for individuals in a population throughout their habitat. The most common pattern is clumps of members of a population throughout their habitat, mostly because resources are usually found in patches. (a) Clumped (elephants) Fig. 8-2a, p. 162

(b) Uniform (creosote bush) Figure 8.2 Natural capital: generalized dispersion patterns for individuals in a population throughout their habitat. The most common pattern is clumps of members of a population throughout their habitat, mostly because resources are usually found in patches. (b) Uniform (creosote bush) Fig. 8-2b, p. 162

(c) Random (dandelions) Figure 8.2 Natural capital: generalized dispersion patterns for individuals in a population throughout their habitat. The most common pattern is clumps of members of a population throughout their habitat, mostly because resources are usually found in patches. (c) Random (dandelions) Fig. 8-2c, p. 162

The population size of a species in a given space at a given time is determined by the interplay between BIOTIC POTENTIAL and ENVIRONMENTAL RESISTANCE. Biotic potential = growth rate with unlimited resources. Environmental resistance = all the factors acting jointly to limit population growth.

(environmental resistance) POPULATION SIZE Growth factors (biotic potential) Favorable light Favorable temperature Favorable chemical environment (optimal level of critical nutrients) Abiotic Biotic High reproductive rate Generalized niche Adequate food supply Suitable habitat Ability to compete for resources Ability to hide from or defend against predators Ability to resist diseases and parasites Ability to migrate and live in other habitats Ability to adapt to environmental change Decrease factors (environmental resistance) Too much or too little light Temperature too high or too low Unfavorable chemical environment (too much or too little of critical nutrients) Low reproductive rate Specialized niche Inadequate food supply Unsuitable or destroyed habitat Too many competitors Insufficient ability to hide from or defend Inability to resist diseases and parasites Inability to migrate and live in other Inability to adapt to environmental

Population characteristics Density~ # of individuals per unit of area •counts •sample size estimate •indirect indicators •mark-recapture Dispersion~ pattern of spacing •random~ unpredictable, patternless spacing (a) •clumped~ patchy aggregation (b) •uniform~ even spacing (c)

Four variables change population size: NATALITY = birth rate MORTALITY = death rate IMMIGRATION = rate of organisms moving in EMIGRATION = rate of organisms moving out

Changes in Population Size: Entrances and Exits Populations increase through births and immigration Populations decrease through deaths and emigration

Demography: factors that affect growth & decline of populations Birthrate (natality, fecundity)~ # of offspring produced Death rate (mortality) Age structure~ relative number of individuals of each age Survivorship curve~ plot of numbers still alive at each age

Limits on Population Growth: Biotic Potential vs Limits on Population Growth: Biotic Potential vs. Environmental Resistance No population can increase its size indefinitely. The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

Exponential and Logistic Population Growth: J-Curves and S-Curves Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. Figure 8-4

Environmental Resistance Carrying capacity (K) Population size (N) Biotic Potential Exponential Growth Figure 8.3 Natural capital: no population can continue to increase in size indefinitely. Exponential growth (lower part of the curve) occurs when resources are not limited and a population can grow at its intrinsic rate of increase (r) or biotic potential. Such exponential growth is converted to logistic growth, in which the growth rate decreases as the population becomes larger and faces environmental resistance. Over time, the population size stabilizes at or near the carrying capacity (K) of its environment, which results in a sigmoid (S-shaped) population growth curve. Depending on resource availability, the size of a population often fluctuates around its carrying capacity, although a population may temporarily exceed its carrying capacity and suffer a sharp decline or crash in its numbers. Time (t) Fig. 8-3, p. 163

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Figure 8-6

Population overshoots carrying capacity Number of reindeer Population Crashes Number of reindeer Figure 8.6 Exponential growth, overshoot, and population crash of reindeer introduced to the small Bering Sea island of St. Paul. When 26 reindeer (24 of them female) were introduced in 1910, lichens, mosses, and other food sources were plentiful. By 1935, the herd size had soared to 2,000, overshooting the island’s carrying capacity. This led to a population crash, with the herd size plummeting to only 8 reindeer by 1950. Carrying capacity Year Fig. 8-6, p. 165

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Over time species may increase their carrying capacity by developing adaptations. Some species maintain their carrying capacity by migrating to other areas. So far, technological, social, and other cultural changes have extended the earth’s carrying capacity for humans.

Population life history “strategies” r-selected (opportunistic) Short maturation & lifespan Many (small) offspring; usually 1 (early) reproduction; no parental care High death rate K-selected (equilibrial) Long maturation & lifespan Few (large) offspring; usually several (late) reproductions; extensive parental care Low death rate

Opportunistic or r-Selected Species cockroach dandelion Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species

Competitor or K-Selected Species elephant saguaro Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete Late successional species

REPRODUCTIVE STRATEGIES Number of individuals Time Carrying capacity K species; experience K selection r species; r selection K

Survivorship Curves: Short to Long Lives The way to represent the age structure of a population is with a survivorship curve. Late loss population live to an old age. Constant loss population die at all ages. Most members of early loss population, die at young ages.

Percentage surviving (log scale) SURVIVORSHIP CURVES Percentage surviving (log scale) 100 10 1 Age

Type I: high survivorship until late in life Survivorship Curves Type I: high survivorship until late in life Type II: constant death rate at all ages Type III: death rate is highest early in life

Population Density and Population Change: Effects of Crowding Population density: the number of individuals in a population found in a particular area or volume. A population’s density can affect how rapidly it can grow or decline. e.g. biotic factors like disease Some population control factors are not affected by population density. e.g. abiotic factors like weather

Population density affects population growth.

Types of Population Change Curves in Nature Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. Stable: fluctuates slightly above and below carrying capacity. Irruptive: populations explode and then crash to a more stable level. Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles. Irregular: erratic changes possibly due to chaos or drastic change.

Population limiting factors Density-dependent factors •competition •predation •stress/crowding •waste accumulation Density-independent factors •weather/climate •periodic disturbances

R-strategists populations are most affected by these. DENSITY INDEPENDENT FACTORS = affect a populations’ size regardless of its population density. Weather Earthquakes Floods Fires . . . Natural disasters R-strategists populations are most affected by these.

DENSITY DEPENDENT FACTORS = affect a populations’ size depending on its population density. Predation Disease Availability of food and water Space Negative Feedback!!

INTERNAL FACTORS = might include density-dependent fertility or size of breeding territory. EXTERNAL FACTORS = might include predation and disease.

Limiting Factors on Growth Essential resources Food Minerals Water Refuge Carrying capacity Logistic growth

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