Chapter 8 Population Ecology

Types of Species in Communities Native species species that normally live in a particular community Nonnative species also referred to as “invasive” or “alien” species species that enter a new community either through immigration or introduction Introduction may be deliberate or accidental i.e. “killer bees,” Kudzu, zebra mussels, Asian oysters, domesticated animals

Indicator Species species whose decline or migration indicates a significant change (damage) to a particular community Serve as “early warning sentinels” of environmental degradation Examples: Amphibians Trout Birds Aquatic macroinvertebrates

Keystone Species species whose removal from its community may dramatically alter the structure and function of the community roles: pollinators top predators decomposers

Foundation Species species that alters its habitat in ways that benefit other species behaviors of such species may influence succession and increase species richness sometimes identical to the keystone species, while other times serves as a counterbalance Examples: elephants, kelp, eastern hemlock, mussels

Population Distribution Three general patterns: (see below) Most populations live in clumps although other patterns occur based on resource distribution. Figure 8-2

Why Clumping? Resource availability varies from place to place. Living in herds, flocks, or schools provides protection from predators and population declines. Fish, birds, caribou, antelope, zebra Predators that live in groups are afforded a better chance of catching prey and getting a meal. Wolves, hunting dogs Temporary animal groupings may occur for mating and caring for young. Dolphin, albatross

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

Population Age Structure: How fast a population grows or declines depends on its age structure. Prereproductive age: not mature enough to reproduce. Reproductive age: those capable of reproduction. Postreproductive age: those too old to reproduce. Populations with mostly reproductive individuals tend to increase. Populations with mostly post-reproductive individuals tend to decrease. Stable populations are equitability distributed among all three categories.

Limits on Population Growth: Biotic Potential vs Limits on Population Growth: Biotic Potential vs. Environmental Resistance No population can increase its size indefinitely. There are always limits to population growth in nature. Population change is a balance between: Biotic potential - the intrinsic rate of increase (r) or the rate at which a population would grow if it had unlimited resources and… Environmental resistance – all the factors that act to limit the growth of a population. Together these determine a populations 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 Exponential or geometric growth – starts slowly but accelerates rapidly as population increases J-shaped curve plotted on a graph of population vs. time Logistic growth – exponential growth followed by a steady population decrease until the population size levels off S-shaped curve Usually levels off at or near the carrying capacity Carrying capacity is not fixed

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

Population Growth Curves Exhibit four phases Lag – phase characterized by low birth rates, when the population is adjusting to a new environment Growth – phase which shows a dramatic increase in population size (B+I > D+E) Stationary – phase when then population is in dynamic equilibrium (B+I = D+E) Death – phase in which the population declines (B+I < D+E)

Can a Population Exceed Its Carrying Capacity? Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Some populations overshoot their carrying capacity. Reproductive time lag Causes a dieback or a crash Some populations may increase their carrying capacity by developing adaptive traits (i.e. natural selection) Some species maintain their carrying capacity by migrating to other areas.

Number of sheep (millions) Overshoot Carrying capacity Number of sheep (millions) Figure 8.4 Boom and bust: logistic growth of a sheep population on the island of Tasmania between 1800 and 1925. After sheep were introduced in 1800, their population grew exponentially thanks to an ample food supply. By 1855, they had overshot the land’s carrying capacity. Their numbers then stabilized and fluctuated around a carrying capacity of about 1.6 million sheep. Year Fig. 8-4, p. 164

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

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. Density dependent factors include biotic factors like disease, competition for resources, predation, and parasitism Some population control factors are not affected by population density. Density independent factors include abiotic factors like weather, fire, pollution, and habitat destruction

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. short-lived, rapidly reproducing species (i.e. algae, insects) Cyclic: populations fluctuate according to regular cyclic or boom- and-bust cycles. close predator-prey interactions Irregular: erratic changes possibly due to chaos or drastic change. populations that inhabit unstable or highly variable environments

Population size (thousands) Hare Lynx Population size (thousands) Figure 8.7 Population cycles for the snowshoe hare and Canadian lynx. At one time scientists believed these curves provided circumstantial evidence that these predator and prey populations regulated one another. More recent research suggests that the periodic swings in the hare population are caused by a combination of top-down population control—predation by lynx and other predators—and bottom-up population control. In the latter, changes in the availability of the food supply for hares help determine hare population size, which in turn helps determine the lynx population size. (Data from D. A. MacLulich) Year Fig. 8-7, p. 166

REPRODUCTIVE PATTERNS Some species reproduce without having sex (asexual). Offspring are exact genetic copies (clones). Others reproduce by having sex (sexual). Genetic material is mixture of two individuals. Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly (energetically). Major advantages: promotes genetic diversity, division of labor among the sexes may provide offspring greater protection through critical periods.

Reproductive Patterns: Opportunists and Competitors r-selected species: Large number of smaller offspring with little parental care K-selected species: Fewer, larger offspring with higher invested parental care Figure 8-9

Reproductive Patterns r-selected species tend to be opportunists while K-selected species tend to be competitors. Figure 8-10

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 populations, die at young ages. Number of individuals age

WORK CITED Population Ecology. (1998) Cyber Ed.