Chapter 4: Ecosystems: How They Change

12/10/2010 12:19


1)      What are the two fundamental kinds of population growth curves? What are the causes and consequences of each?


fundamental growth curves:

                               I.      J – curve (exponential)

J-curve appears only temporary and its slope is as an explosion and fall

·      --   in extra conditions

·       --  unusual disturbance – new specie, sudden change of habitat, elimination of predator

 Two causes of population (J-curve) explosion:

1.      natural mechanisms may cause population to level off and continue in a dynamic equilibrium – as a S-curve

2.      in the absence of natural enemies, the population keeps growing until exhausts essential resources (usually food) and die off precipitously due to starvation, diseases related till malnutrition, producing a reverse of the J-curve

3 consequences of J-curve crush:

1.      ecosystem was not seriously damaged – recovery of plant food – recovery of herbivore’s population and than J-curve may repeated (insects)

2.      after initial J, natural mechanism may come into play as the ecosystem recovers, thus bringing the population into S-balance

3.      damage to the ecosystem is so severe that recovery is limited, and small surviving populations eke out an existence in a badly degraded environment.


                            II.      S – curve (held in balance)

It represents dynamic equilibrium

·    --     the typical feature of natural ecosystems

·      -- S-curve rises up to the carrying capacity and then oscillates between slightly above and slightly below depends on environmental resistance and biotic potential


2)      Define biotic potential and environmental resistance, and give factors of each. Which generally remains constant, and which controls a population’s size?


Environmental resistance = the totality of factors such as adverse weather, predators and diseases that tent to cut back populations and keep them from growing or spreading. (contrast biotic potential); it controls a population`s size

·         --Lack of food or nutrients

·         --Lack of water

·         --Lack of suitable habitat

·        -- Adverse weather conditions

·         --Predators

·         --Disease

·         --Parasites

·         --Competitors


Biotic potential = Reproductive capacity. It is the potential of a species for increasing its population and/or distribution. The biotic potential of every species is such that, given optimum conditions, its population will increase; it remains constant

·         --Reproductive rate

·        --Ability to migrate (animals) or disperse (seeds)

·        -- Ability to invade new habitats

·         --Defense mechanisms

·         --Ability to cope with adverse conditions

3)      Differentiate between the terms critical number and carrying capacity. What is density dependence?


Critical number = the minimum number of individuals of a given species that is required to maintain a healthy, viable population of the species. If a population falls below its critical number, it will almost certainly become extinct.

Carrying capacity = the maximum population of given species that an ecosystem can support without being degraded or destroyed in the long run. The carrying capacity may be exceeded, but not without lowering the system`s ability to support life in the long term

Density dependence = attribute of population – balancing factors, such as predation, that increase and decrease in intensity in proportion to population density.

·         Density depending factors depend on population density (number of individuals per unit area). Factors of environmental resistance are density dependent. As population density decrease, environmental resistance lowers and this allow the population to recover.


4)      Distinguish between mortality and regulation in populations.


Mortality is caused by biotic and abiotic factors, unusual disturbances …

Regulation in populations is caused only by density dependent factors (respecting dynamic balance of an ecosystem)


5)      Describe the predatory-prey relationship between the moose and wolves of Isle Royale. What other factors influence these two populations?


At the beginning, only moose lived at the island – extremely good conditions – J-curve. Later, small pack of wolves reached the island. Fewer wolves represented low environmental resistance for the moose, so the moose population increases. Than, the abundance of moose represents optimal conditions for wolves, so the wolves increase. More wolves mean higher predation on the moose, so the moose population falls. The decline in the moose population is followed by a decline in the wolf population. There was dramatic fall in moose population at the island. Mortality was caused by other factors: deep snow (adverse weather), infestation of ticks (parasites).



6)      Distinguish between intraspecific and interspecific competition. How do they affect species as a form of environmental resistance?


Intraspecific competition = between individuals belong to one species; individual as a part of a population has identical requirements for success (completely overlapping niches). If any requirement – any resource – is in short supply, the most intense competition for the resource will come from members of the same species.

                               I.      Territoriality (see q.7)

                            II.      The Spoils (individuals unable to claim territory – usually young – can seize their opportunity as they mature and older members with territories weaken or perish)

                         III.      Self-thinning (in crowded conditions – plants will have thinner stalks, poorer fruit or flowers; animals growing on the rocks in coastal ecosystem)

Interspecific competition = between species; species may compete for some scarce resource and when they do, their ecological niches are said to overlap

                         I.      Plants

·        -- affected by topography, type of soil, moisture, light, … in location

·         --single species cannot utilize all of the resources in a given area, therefore, any resources that remain may be claimed by other species having different adaptations

·         i.e. different type of root system

                      II.      Animals

·         strong factor influencing the distribution and abundance of species

·         in short run, interspecific competition can reduce the success of species and even eliminate them from the environment – it can help regulate population

·         in long run, interspecific competition helps drive natural selection, bringing about greater specialization of ecological niches and allowing resources to be divided up among species



7)      What is meant by territoriality, and how does it control certain population in nature?


Territoriality = the behavioral characteristic exhibited by many animal species, especially birds and mammalian carnivores, to mark and defend a given territory against other members of the same species. It is a kind of intraspecific competition.

·         It causes the dispersal of populations

·         Let healthy population to survive (fighting)

·         Breeding is restricted to only those individuals capable of claming and defending territory; thus, population growth is defined in a density-dependent manner


8)      What problems arise when a species is introduced from a foreign ecosystem? Why do these problems occur?


Invasive behavior

·       --  Devastation of current nature species

·       --  Devastation of environment (overgrazing, overgrowing of forest, overgrowing waterways)


Problems occur because in new environment there is different environmental resistance (problems are if there is low env.res.) for foreign species and new species cannot fit into local web of nature regulation relationships. (they will die, they will need extra artificial conditions or they became invasive)

·         i.e.: rabbits from England into Australia, Chinese chestnut into USA, water hyacinth from South and Central America into Florida



9)      What is selective pressure, and how does it relate to natural selection?


Selective pressure = an environmental factor (i.e.: different climatic condition) which causes individuals with certain traits (alleles – variations of genes) that are not norm for population to survive and reproduce more than the rest of the population. The result is a shift in the genetic makeup of the population. Selective pressure is fundamental mechanism of evolution (relate to natural selection)


Natural selection = the evolutionary process whereby the natural factors of environmental resistance tend to eliminate those members of a population which are least well adapted to cope with their environment and thus, in effect, tend to select those best adapted for survival and reproduction.


10) Describe several basic trains that adapt an organism for survival and reproduction.


Adaptation for:

·         --coping with climatic and other abiotic factors

·         --obtaining food and water (animals) or nutrients, energy and water (plants)

·         ---escaping from or protecting against predation and for resistance to disease-causing or parasitic organisms

·        -- finding or attracting mater (animals) or pollinating and setting seed (plants)

·         --migrating (animals) or dispersing seeds (plants)


11)  What factors determine whether a species will adapt to a change or whether the change will render it extinct?


 Adaptation occurs by selective pressures eliminating those individuals that cannot tolerate new condition. For adaptation to occur there must be some individuals with traits (alleles – variations of genes) that enable them to survive and reproduce under new conditions.

4 keys to survival:

                               I.      Geographical distribution (eagle X house fly)

                            II.      Specialization to a given habitat or food supply

                         III.      Genetic variation within a gene pool of the species

                         IV.      The reproductive rate relative to the rate of environmental change


12) How many evolution lead to the development of new species (specialization)?


Key features of specialization

·         New species are not formed by scratch; they are formed only by gradual modification of existing species

·         The gene pool of a species may be formed in many different directions when different populations are isolated and face different selective pressures


2 essential premises:

                                                                                    I.      Reproductive isolation

                                                                                 II.      Subpopulation face different conditions

·        Evolution of new species can appear when a population spread over a broad and will face various selective pressures. If the population splits so that interbreeding among subpopulations does not occur, the different pressures may result in the subpopulations evolving into new species (unable to interbreed) i.e.: Artic fox and Gray fox (thousand years ago – 1 species); Finches (Darwin collected at Galápagos)



13) What are plate tectonics, and how does it help to explain the past movement of continents? How does tectonic movement affect present-day conditions?


Plate tectonics = the theory that postulates the movement of sections of the Earth’s crust, creating earthquakes, volcanic activity, and the buildup if continental masses over long periods of time

Nowadays effects:

·        -- Tsunami

·         --Earthquakes and volcanic eruptions

·        -- It can shifted climate in the long term


14) Define the terms ecological succession and climax ecosystem. How do disturbances allow ecological succession?


Ecological (or natural) succession = process of gradual and orderly progression from one ecological community to another. i.e.: over the course of year, a grassy field may gradually be replaced by woodland, and in time the woodland may develop into mature forest


Climax ecosystem = the last stage in ecological succession. Ecological succession process doesn’t go on indefinitely. Climax ecosystem is the stage of development of an ecosystem. In such ecosystem, populations of all organisms are in dynamic balance between themselves and existing abiotic factors. i.e. 6 major biomes


Disturbances allow ecological successions are:


                   I.      Primary succession - Facilitation

·         Pioneer species start the process, but as they grow, they create conditions that are favorable to longer-lived colonizers and the process in general is driven by the changing conditions that pave the way for other species

·         i.e.: moss invades bare rock and acts as a collector, accumulating a layer of soil sufficient for additional plants to become established

                II.      Secondary succession

·         DISTURBANCES: Area has been cleaned by fire or by humans and left alone, or overgrazed

·         Reinvasion to the area – through a series of district stages called secondary succession

·         Starts with preexisting soil but in different conditions, thus with different species than before disturbance


             III.      Aquatic succession

·         Soil particles erode from the land and settle out in ponds or lakes, filling process is supported by aquatic vegetation`s detritus

·         Terrestrial species from the surrounding ecosystem can advance in the ecotone and aquatic species must move farther out in the lake

·         lake disappears altogether – climax community may be a bog or forest


             IV.      Climate Change   (I think, everybody knows by heart J )


15) What role may fire play in ecological succession, and how may fire be used in the management of certain ecosystems?


Fire and Succession

Fire - natural (if started by lightning) and important abiotic factor, some species even depend on fire (release of seeds)

Slow and periodic fire helps to

·         release nutrients from dead organic matter

·         remove excessive fuel and kill competing species in pine forests – fire as a management tool in such ecosystem

·         remove deadwood which becomes the breeding ground for wood-boring insects and those can attack live trees


-          crown fires do not occur naturally (they can be caused by fire-prevention because there is huge amount of deadwood)



16) What is non-equilibrium theory of ecosystem stability? What evidence is there for the theory?


Non-equilibrium theory of ecosystem stability

Ecosystems are not in steady balance but they face disturbances. This controversy between “equilibrium” and “patchiness of ecosystem” is known as ecological succession.



·        -- In a forest, the species of trees commonly vary independently in space. It is difficult to predict what species will tend to be found associated together in given stand at a given time.

·        -- The distribution of species is similarly patchy (unsteady), reflecting the patchiness in conditions.

·         --Neither conditions, nor distribution of species are uniform!



17) What is meant by ecosystem resilience? How is this accomplished, and what can cause it to fail? How does this relate to environmental tipping points?


Ecosystem resilience = the tendency of ecosystems to recover from disturbances through a number of processes known as resilience mechanism (i.e. succession after a fire). Resilience ecosystem is the one which maintains its normal functioning – its integrity – even through a disturbance.


Accomplished by resilience mechanism:

·        -- Nutrient release to soil

·        -- Regrowth by remnant roots and seeds

·         --Invasions from neighboring ecosystems

·        -- Rapid restoration of energy flow and nutrient cycling

·         --Successional dynamics


Limits of resilience:

·        - removal of forest by humans, no reforestation (instead of it overgrazing) will lead to dramatic soil erosion and degraded state will persist


Environmental tipping points = a situation in a human-impacted ecosystem where a small action or reaction catalyzes a major change in the state of the system. i.e. fire or logging as a kind of decision point for an ecosystem and that situation can move in a better or worse direction – resilience in ecosystems is that better direction


18) Succinctly (briefly) describe ecosystem management. What is meant by adaptive management?


Ecosystem management = decision making style with respect to ecological sustainability, integral terrestrial and aquatic ecosystems, it integrates ecological concepts at a variety of spatial scales (i.e. leaving dead logs and soil litter in place to enhance biodiversity of small animals), it incorporates the perspectives of landscape ecology (preserve the range of landscapes), management must be adaptive to new researches, it includes the human element – all stakeholders are included as important elements in stewardship of the resources.

Adaptive management – managers are given the opportunity to learn from experiments and to employ new knowledge from forestry and landscape science in their place, thus, management must be adaptive because it is still evolving model, paradigm.


19) Examine the key message from MA Board and evaluate how the points affect you now and will impact you and your children in future.


“Living Beyond Our Means. Natural Assets and Human Well-being”

At the heart of this assessment is a stark warning. “Human activity is putting such strain on the natural functions of Earth that the ability of the planet’s ecosystems to sustain future generations can no Langer be taken for granted“


Key message from MA Board is quite clear for ST students, for more you can see page 110.

[Download Pdf file]