Ecosystems: How they change Chapter 4

Chapter Introduction Prior to 1988 fires were allowed to burn their course Unless close to human habitations. Park service policy. Yellowstone Park policy changes and fires were put out at all cost.

Yellowstone Fires burned 36% of the park Succession began quickly. The events have shown that the total landscape recovers, populations of herbivores and predators, and biodiversity are even enhanced.

Dynamic Balance Nature is in balance when left alone. An ecosystem is a dynamic system in which changes are constantly occuring. Additions (births) and subtractions (deaths) are constantly occuring and the population fluctuates around a median (s-curve).

Definitions Equilibrium: births and deaths are more or less equal over time. Exponential increase: 2;20;200;2000 Population explosion: exponential growth with respect to a poulation. Population plotted over time results in a j-curve.

J-curve Come about when there are unusual disturbances Introduction of a foreign species Elimination of a predator Sudden alteration of habitat. Can happen for a period of time but then 1 of 2 things happen Natural mechanisms take over and the dynamic equilibrium (s-curve) is returned. With no enemies the population continues to grow until it exhausts essential resources (usually food) and then dies off quickly.

The herbivore For the herbivore any one of three possibilities may happen. Food resource may recover, allowing the herbivore to recover S-curve may result if the ecosystem recovers Damage is too severe, recovery is limited and a small population exists in a degraded system.

Animal populations J-curves are temporary because animals will die off as resources are depleted. Except for human populations because of advances in medicine, and ability to think/reason.

Biotic potential Ability of populations to increase. The number of offspring that a species may produce under ideal conditions. To have effect on size of future generations the young must survive and reproduce (known as recruitment).

Reproductive strategies Produce massive numbers of offspring with no maternal care. May lead to low recruitment. Lower reproductive rate with special care for young until they make it to adulthood and compete with members of the population for resources.

Environmental Resistance The biotic and abiotic factors that may limit a population’s increase. Reproductive ability (biotic potential) remains relatively constant, recruitment varies substantially. If recruitment is at replacement level (just enough to replace adults), the population remains at equilibrium.

Carrying capacity Maximum population of a species that a given habitat can support sustainably.

Population Density Whether a population grows, remains stable or decreases is a result of an interplay between biotic potential and environmental resistance.

Density dependence As population density increases, environmental resistance becomes more intense and causes increases in mortality. As population grows food becomes more scarce, habitats get smaller… Density independence: causes of mortality are independent of population size. Frequently true for abiotic factors like fire, deep freezes…

Critical number Minimum population base that is needed for a population to recover/survive. Threatened: population declining rapidly. Endangered: population is near its critical number.

Lesson 4.2

Top-down Control of a population by predation. Controlled because higher trophic levels are eating lower trophic levels.

Bottom-up Most important control Result of scarcity of resources.

St. Matthew Island Demonstrated that no population can escape ultimate limitation by environmental resistance, although the form and consequences may differ. 1944 29 reindeer cross ice to island. By 1963 the animals reached a population of around 6,000. vegetation was scarce and herd was malnourished.

Predator removal Same consequence as introducing an animal with no natural enemies (St. Matthew Island).

Competition Interspecific: competition between species. Lions vs. tigers for prey, habitat, etc. Intraspecific: Competition within a species. Bees vs. bees for nectar, habitat, etc. Territoriality: competition for territory by members of the same species. Competition for scarce resources led to Natural selection (survival and reproduction of individuals who possess trait or lack traits that make them more successful).

Resource Partitioning Splitting a resource such as habitat into parts so that each can survive (example: warblers from chapter 2, figure 2-16)

Lessons to be learned Lessons from introduction of (undesirable) species are two: Regulation of populations is a matter of complex interactions among the members of the biotic community. The relationships are specific to the organisms in each particular ecosystem.

Lesson 4.3

Mechanisms for population control Predation Predators and prey become well adapted to each other’s presence Intraspecific (within a species) Competition May lead to improved adaptations of a species Interspecific (between species) competition Promotes adaptations that allow them to specialize in exploiting a resource.