Chapter 4

Ecosystems: How They Change

Introduction

Section 4.1

Dynamics of Natural Populations

Dynamics of Natural Populations

• In any population, births and deaths will cause the population to grow or shrink

• If births and deaths are ore or less equal over time, the population is said to be in equilibrium

Population Growth Curves

• Exponential Increase– Every species has the capacity to increase its

population when conditions are favorable – Growth under absolutely ideal conditions will be

exponential• Ex – a pair of rabbits producing 20 offspring, 10 male,

10 female, may grow by a factor of 10 each generation– When graphed, produces a J-Shaped Curve

• Leads to population explosions

Population Growth

• Populations may increase exponentially for a time, but then one of two things may occur– 1. Natural mechanisms may cause the population

to level off and continue in a dynamic equilibrium• When this happens it is graphed as an S-Shaped Curve

– 2. In the absence of natural enemies, the population keeps growing until it exhausts essential resources and then dies off• When this happens it is graphed as the opposite of a J-

Shaped Curve

Two Types of Growth

The J-Curve (blue) demonstrates population growth under optimal conditions, with no restraints. The S-curve (green) shows a population at equilibrium. The horizontal line (red) shows the carrying capacity of the environment for that population. Notice how the J-curve spikes well above and then crashes below the carrying capacity, whereas the S-curve rises up to the carrying capacity and then oscillates between slightly above and slightly below it

Equilibrium Populations

• Natural ecosystems are made up of populations that are usually in the dynamic equilibrium– Represented by an S-shaped curve

• J-curve come about when there are unusual disturbances, such as the introduction of a foreign species, elimination of a predator, or the sudden alteration of a habitat

Gypsy Moth CaterpillarAn introduced species that has often caused massive defoliation of oak trees, now seems to been brought under natural control in forests

Biotic Potential

• The ability of a population to increase is known as biotic potential– Number of offspring that a species may produce

under ideal conditions– Biotic potential of different species varies greatly• To have an effect on future generations, offspring must

survive and reproduce– Survival through the early growth stages to become part of

the breeding population is called recruitment

Recruitment

• Replacement level recruitment – When just enough offspring are born to replace

the adults• Population will remain in equilibrium

– If there is less offspring, populations will decrease, and if there are more offspring, populations will increase

Reproductive Strategies

• Two common reproductive strategies in the natural world– r – selected species, K-selected species• Variable r = population growth rate• Variable K = carrying capacity

r – Selected Species

• 1 – produce massive numbers of young, but then leave survival to the whims of nature– Often results in very low recruitment– Species has a high biotic potential, but the

population will not increase because of high mortality of the young

– Organisms with this strategy are usually small, with rapid reproductive rates and short life spans

K-Selected Species

• Much lower reproductive rate (lower biotic potential)– Care for and protect young until they can compete

for resources with adult members – Larger, longer lived, and well adapted to the

normal environmental fluctuations

Environmental Resistance

• Biotic and abiotic factors tend to cause mortality in populations, and limit a population’s increase– Biotic factors that cause resistance• Predators, parasites, competitors, lack of food

– Abiotic factors that cause resistance• Unusual temperatures, moisture, light, salinity, pH, lack

of nutrients, fire

Carrying Capacity

• Maximum population of a species that a given habitat can support without the habitat being degraded over the long term

Density Dependence

• The size of a population generally remains within a certain range hewn environmental resistance factors are density dependent– As the number of individuals per unit area

increases (population density) , environmental resistance becomes more intense and causes an increase in mortality that ceases population growth, and vice, versa

Density Independence

• Factors in the environment that cause mortality no matter what the population density is– Frequently true of abiotic factors• Ex – deep freeze during spring germination, fires

Critical Number

• The survival and recovery of a population depends on a certain minimum population base, which is referred to as the population’s critical number

Endangered Species Act

• Calls for the recovery of two categories of species– Species whose populations are declining rapidly

are classified as threatened– Population that is near what scientists believe to

be its critical number, is classified as endangered• These definitions, when officially assigned by the U.S.

Fish and Wildlife Service, set into motion a number of actions aimed at the recovery of the species in question

Section 4.2

Mechanisms of Population Equilibrium

Mechanisms of Population Control

• Top-Down Regulation– Control of a population by predation

• Bottom-Up Regulation– Most important control of a population occurs as a

result of the scarcity of some resource

Predator-Prey Dynamics

Parasites

• Affect the populations of their hosts organisms in a density-dependent manner– As population density of the host increases,

parasites and their vectors (agents that carry the parasites from one host to another), will have little trouble finding new hosts

Plant-Herbivore Dynamics

• Overgrazing– If herbivores eat plants faster than the plants can

grow, the plants will eventually be depleted and the animals will suffer.

In 1944, a population of 29 reindeer (5 males, and 24 females) was introduced onto St. Matthew Island, where they increased exponentially to

about 6,000 and then died due to overgrazing

Predator Removal

• Eliminating predators or other natural enemies upsets basic plant-herbivore relationships in the same way as introducing an animal without natural enemies. – Ex – Sea urchins harm coastal marine ecosystems

of eastern Canada• This is due to the over harvest of lobsters which are a

predator of sea urchins.

Keystone Species

• In the West Coast rocky intertidal zone, a sea star species feeds on mussels (herbivores that feed on plankton), thus keeping mussels from blanketing the rocks– As a result, barnacles, limpets, anemones, whelks,

and other invertebrates are able to colonize the habitat

Keystone Species

• Ecologist Robert Paine experimentally removed the sea star from the shoreline, and the mussels crowded everything, decreasing biodiversity

• Paine referred to the star as a keystone species– Species that has a crucial role in maintaining the

integrity of an ecosystem

Competition

• Species may compete for scare resources• When they do, their ecological niches overlap• Competition is a form of bottom-up regulation

because it occurs only when a resource is in limited supply

Intraspecific Competition

• Competition from members of the same species– Territory

• Territoriality refers to individuals or groups defending a territory against the encroachment of others of the same species

• In territoriality, what is being protected is an area suitable for nesting, for establishing a harem, or for adequate food resources

– Self – Thinning• When crowded conditions lead to competition for

resources

Effects on Species

• Competition for scare resources led Charles Darwin to identify the survival of the fittest as one of the forces in nature leading to evolutionary changes in species– “Fit” meaning the ability to have offspring– Certain organisms have adaptations to their

environment that makes them more “fit” than others, thus passing on that desired adaptation

Interspecific Competition

• Competition different species compete

Introduced Species

• Rabbits– 1859 rabbits were introduced into Australia from

England to be used for sport shooting• Rabbits had no natural enemies capable of controlling the

population, therefore, it exploded. – Devastated native marsupials and sheep– Was temporarily brought under control by introducing a virus to

the rabbits» Over time rabbit became resistant to virus and population

continued Exploding» Today rabbits are still Australia’s most destructive pest

animal, costing farmers $100 million in agriculture each year

In the first photo, the island is largely devoid of vegetation and heavily eroded. Following the eradication of the rabbits in 1988,the island vegetation

recovered spectacturaly

Introduced Species

• American Chestnut– Prior to 1900, dominant tree in eastern deciduous

forest of the U.S. was the American chestnut– In 1904 a fungal disease called chestnut blight was

accidentally introduced when some Chinese chestnut trees carrying the disease were planted in New York

– Fungus spread and killed nearly every American chestnut tree by 1950

Introduced Species

• Zebra Mussel– Introduced into the Great Lakes with the discharge

of ballast water from European Ships– Mussels are now spread throughout the

Mississippi River basin and cause ecological and commercial damage• Displace native mussel species and clog water0intake

pipes

Lessons from Introduced Species

• Regulation of populations is a matter of complex interactions among the members of the biotic community

• Relationships are specific to the organisms in each particular ecosystem

• Therefore, when a species is transported over a physical barrier from one ecosystem to another, it is unlikely to fit into the framework of the relationships in the new biotic community

Introduced Species

• In some cases, the introduced species simply joins the native flora or fauna, or will be put under too many environmental strains and die out

• In other cases, the species becomes INVASIVE– When the conditions are favorable, and there are

no natural predators, the species will thrive and outcompete native organisms

Section 4.3

Evolution as a Force for Change

Selective Pressures

• Most young organisms in nature do not survive; instead they fall victim to various environmental resistance factors– Parasites, predators, drought– These factors are known as Selective Pressures• Each factor can affect which individuals survive and

reproduce and which are eliminated

Natural Selection

• In nature, there is a constant selection, and consequently, a modification of a species’ gene pool toward features that enhance survival and reproduction within the existing biotic community and environment – Usually in response to selective pressures– Because the process occurs naturally, it is referred

to as NATURAL SELECTION

Natural Selection

• Discovered independently by Charles Darwin and Alfred Wallace

• Concept was first published by Darwin in his book On The Origin of Species by Means of Natural Selection (1859)

• The change in gene pool of a species by natural selection over the course of many generations is the main idea behind biological evolution– This is significant because Darwin and Wallace conducted their

research, mainly on observations.• Modern understanding of DNA, mutations, and genetics wasn’t

published until after Darwin’s publication

Adaptations to the Environment

• All traits of any organism can be seen as features that adapt to the organism for survival and reproduction– “Fitness”

Adaptations to the Environment

• Essentially all characteristics of organisms can be grouped as follows: – Coping with abiotic factors (climate)– Obtaining food and water (animals), or nutrients,

energy (plants)– Finding or attracting mates (animals) or pollinating

and setting seed (plants)– Migrating (animals) dispersing seed (plants)

Modifications of body shape and color that allow species to blend into the background and thus protect their populations from predation are among the most amazing

adaptations. First picture = spanworm, second picture = leaf katydid

Limits of Change

• When facing a new, powerful selective pressure, species have only three choices– Adaptation– Migration– Extinction

Adaptation

• The population of survivors may gradually adapt to the new condition through natural selection– For adaptation to occur, there must be individuals

with traits (alleles – variations of genes) that enable them to survive and reproduce under the new conditions

– Also must be enough survivors to maintain a viable breeding population

Migration

• Surviving populations may migrate and find an area where conditions are suitable to them

Extinction

• Failing the first two possibilities, extinction is inevitable

Keys to Survival

• Four key variables among specie that will affect whether or not a viable population of individuals is likely to survive new conditions:– Geographical distribution– Specialization to a give habitat or food supply– Genetic variations within the gene pool of the

species, – Reproductive rate relative to the rate of

environmental change

Vulnerability of different organisms to environmental changes. A summary of factors supporting the survival and adaptation of species, as opposed to their extinction.

The Evolution of Species

• Prerequisites– Original population must separate into smaller

populations that do not interbreed with one another. • Reproductive isolation is important so that genes stay

separate

– Separated subpopulations must be exposed to different selective pressures

Darwin’s Finches

Section 4.4

Ecosystem Responses to Disturbance

Ecological Succession

• Over the course of years, a grassy field may gradually be replaced by a woodland, and in time the woodland may develop into a mature forest– This phenomenon of transition from one biotic

community to another is called ECOLOGICAL SUCCESSION• Occurs because the physical environment may be

gradually modified by the growth of the biotic community itself.

Ecological Succession

• Pioneer species start the process• As pioneer species grow, they create conditions

that are favorable to more longer-lived colonizers– This process is called facilitation

• Succession does not occur indefinitely – A stage of development is eventually reached in

which there is a dynamic balance between all of the species and the physical environment. The final state is called a climax community

Primary Succession

• If the area has not been occupied previously, the process of initial invasion and then progression from one biotic community to another is called primary succession. – Gradual invasion of bare rock or gravel surface

Secondary Succession

• When an area has been cleared by fire or by humans and then left alone, plants and animals from the surrounding ecosystem may gradually reinvade the area– This is known as secondary succession– Example – abandoned agricultural field turning

back into a forest

Typical Succession Pattern

• Lichens mosses grasses shrubs pine trees hardwoods

Reinvasion of an agricultural field by a forest ecosystem occurs in the stages shown.

Primary vs. Secondary

• Main difference between primary and secondary succession is that secondary succession starts with preexisting soil

Aquatic Succession

• Succession occurs because soil particles inevitably erode from the land and settle out in poinds or lakes, gradually filling them in

• Aquatic vegetation also produces detritus which contributes to filling in pond/lakes– As the buildup occurs, terrestrial species can cross

over and live there• Lake/pond bog forest

In this photograph, taken in Banff National Park in the Canadian Rockies, you can visualize the lake that used to exist in the low-level area. It is now filled it with

sediment and covered by scrub willow. Spruce and fir forest is gradually encroaching.