1. Ecosystems: How They Change
Chapter 4
Ecosystems: How They Change

2. Introduction

3. Dynamics of Natural Populations
Section 4.1
Dynamics of Natural Populations

4. 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

5. 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

6. 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

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Carrying Capacity
Maximum population of a species that a given habitat can support without the habitat being degraded over the long term

17. 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

18. 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

19. 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

20. 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

21. Mechanisms of Population Equilibrium
Section 4.2
Mechanisms of Population Equilibrium

22. 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

23. Predator-Prey Dynamics

24. 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

25. 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.

26. 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

27. 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.

28. 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

29. 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

31. 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

32. 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

33. 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

34. Interspecific Competition
Competition different species compete

35. 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

37. 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

38. 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

39. 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

41. 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

42. 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

43. Evolution as a Force for Change
Section 4.3
Evolution as a Force for Change

44. 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

45. 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

46. 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

47. Adaptations to the Environment
All traits of any organism can be seen as features that adapt to the organism for survival and reproduction
“Fitness”

48. 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)

49. 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

50. Limits of Change
When facing a new, powerful selective pressure, species have only three choices
Adaptation
Migration
Extinction

51. 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

52. Migration
Surviving populations may migrate and find an area where conditions are suitable to them

53. Extinction
Failing the first two possibilities, extinction is inevitable

55. 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

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

57. 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

59. Darwin’s Finches

60. Ecosystem Responses to Disturbance
Section 4.4
Ecosystem Responses to Disturbance

61. 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.

62. 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

63. 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

64. 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

65. Typical Succession Pattern
Lichens  mosses  grasses  shrubs  pine trees  hardwoods 

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

67. Primary vs. Secondary
Main difference between primary and secondary succession is that secondary succession starts with preexisting soil

68. 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

69. 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.