Lesson Objective Students will be able to describe the factors that affect the carrying capacity of environments.
Populations within Ecosystems Populations (group of organisms of a single species living in a given area) within ecosystems grow, shrink, or stay the same based on the how many are born (birth rate), die (death rate), join (immigration), and leave (emmigration) over time.
Exponential Population Growth Exponential growth occurs when resources are unlimited and environmental conditions are ideal. Birth Rate >>> Death Rate J-shaped curve results when number of organisms over time is graphed
However, in most ecosystems…. Resources are NOT unlimited….limiting factors slow population growth by increasing death rate and decreasing birth rate. Is demonstrated in the Logistic Model of Population Growth which incorporated limiting factors and recognizes a carrying capacity in each ecosystem. Results in an S-shaped curve (Sigmoid) when number of individuals over time is graphed.
Carrying Capacity is…. The total number of individuals the environment can support over an indefinite period of time.
What determines the carrying capacity? Limiting factors control the growth of populations. Some limiting factors are density-dependent and others density-independent. Ecosystem composition of plants and animals is determined by various abiotic factors, as well as biotic factors that act as limiting factors to growth. Disruptions of any of these factors shift the carrying capacity of ecosystems.
How do limiting factors work? Population Size Competition Predation Parasitism and disease Unusual weather Natural disaster can be limited by
Abiotic Factors….. The physical components of an ecosystem. Any nonliving part of the environment. Example: A bullfrog is affected by water availability, temperature, and humidity.
Abiotic Factors that influence carrying capacity… Temperature Precipitation Soil composition pH Humidity Salinity Amount of sunlight Availability of nitrogen
The importance of each factor varies from environment to environment. Abiotic factors vary from region to region and over time. For example, temperature varies from hour to hour, from day to day, from season to season, and from place to place. Subtle differences (i.e. from shade to full sun) makes a tremendous difference in terms of plant composition.
Abiotic Factors as Density- Independent Limiting Factors… Density-Independent limiting factors limit population growth in similar ways no matter how dense the population is at the time. Unusual weather (hurricanes, drought, floods) and natural disasters (wildfires) act in this way. These factors often result in a population “crash.”
Biotic factors….. The biological influences on organisms. Any living part of the environment with which an organism may interact. Example: Bullfrogs may be affected by the algae it ate as a tadpole, insects it eats as an adult, herons that eat bullfrogs, and other species it competes with for food and space.
Biotic Factors that influence carrying capacity…. All biological aspects of an ecosystem fall into this category. Vegetation composition often determines what species will be attracted to a given area due to food availability. Scientists note that competition, predator-prey relations, mutualism, and host-pathogen interactions are critical to consider when accessing carrying capacity.
Compare abiotic factors with biotic factors and give two examples of each.
Biotic Factors in greater detail.. Competition for resources Symbiotic Relationships Predator/Prey interactions
How does competition shape communities and affect carrying capacity of ecosystems?
Competition for Resources Interspecific Competition= an interaction in which two or more species use the same resource. Examples: when lions and hyenas compete for zebras,and when different plants in a forest compete for soil and sunlight. Often competition results in the reduction or complete elimination of one species from the area due to competitive exclusion.
Competition as a Density- Dependent Limiting Factor As population increases, individuals will compete for food, water, space, sunlight, etc. The more crowded an area is, the sooner resources will be used up. Those individuals who are better competitors survive and reproduce, those who aren’t, die out (survival of the fittest).
How do species interactions shape communities and how does that affect their carrying capacities?
Symbiotic Relationships A symbiotic relationship exists when there is a close, long-term relationship between two organisms. Parasitism, Mutualism, and Commensalism are all examples. Disruptions to these relationships can alter the flora and fauna of an area, thereby altering its carrying capacity.
a. Parasitism A symbiotic relationship in which one individual is harmed (the host) while the other benefits (the parasite) (+/-). Unlike predation, the host is not immediately killed. Parasites can be ectoparasites (outside the body) or endoparasites (inside the body). The host and parasite species are in an evolutionary “battle” to evolve better ways to resist infection/infect more.
Parasitism as a Density- Dependent Limiting Factor The denser the host population, the more rapidly parasites can spread from host to host. Example, pastures that are grazed upon by too many cattle often result the cattle becoming infested with worms passed from one cow to another. Example, the flu virus spreads rapidly through schools because there are a large number of students in one location.
b. Mutualism A mutual relationship exists when both individuals benefit (+/+). Mutual relationships, such as pollinators (ex. Bees) and flowering plants (ex. Lilies), are vital to the stability of some ecosystems.
c. Commensalism A relationship in which one organism benefits while the other is neither harmed nor benefits (+/0). For example, barnacles that are attached to a whale’s skin perform no known service to the whale; however, the barnacle benefits from consuming the food particles that are in the water that flows over the whales body as it swims.
How does predation shape communities and affect carrying capacity?
Predation as a Density- Dependent Limiting Factor…. Predator and prey populations fluctuate up and down over time. When prey populations are high (birth rate > death rate), predators have an easier time catching them and have plenty to eat. This often results in an increase in predator population size (birth rate > death rate).
Prey populations will likely drop as a result of more individuals being caught than are born (death rate > birth rate). As prey populations drop, predators begin to starve, increasing the death rate of the predator population (death rate > birth rate). Populations of herbivores and plants cycle in this same manner.
Role of Invasive Species in Carrying Capacity of ecosystems.
What is an Invasive Species? An invasive species is a species (plant or animal) that is introduced to an ecosystem, where it did not evolve in originally, and experiences exponential population growth. Invasive species often experience this growth because their new environment lacks predators and other environmental factors that limit their growth in their native environments.
Why are Invasive Species such a problem? Foreign species often compete with native species for food and habitat, OR they may prey on them. Generally, invasive competitors and predators have a greater negative affect on local species than native competitors or predators. This is often due to the fact that native species have not evolved strategies to “deal” with the invasive species because they have not lived “side by side” with that species over time.
How do Invasive Species infiltrate new ecosystems? Invasive (exotic) species are introduced into new environments in a variety of ways: Human introduction (intentionally or unintentionally) In the ballast of ships and boats In produce and other shipments from country to country Unintentional release from aquariums, private owners, etc.
So, what does this mean?? It means that invasive species generally end up affecting the carrying capacity of the ecosystem for the native species that are affected by the invasive species. A few examples of invasive species that have affected the carrying capacity of ecosystems include: the brown tree snake in Guam, the zebra mussel in North America, water hyacinth in Lake Victoria, and the Nile perch in Lake Victoria.
Nile Perch in Lake Victoria, East Africa Introduced intentionally to Lake Victoria in 1960 in order to increase sport fishing revenues. Nile perch, a voracious predator, found an ample supply of food in the abundant cichlid population native to the lake. By 1980, the Nile perch was the primary fish caught in the lake.
Further Affects….. Introduction of the perch resulted in increased deforestation for firewood for drying the large perch caught. Soil erosion therefore increased, resulting in decreased clarity (increased turbidity) of the water. As the algae population died off due to decreased light penetration, their decomposition (by aerobic bacteria) resulted in a decrease in dissolved oxygen in the ecosystem.
Brown Tree Snake in Guam Boiga irregularis (brown tree snake) probably arrived in Guam on a U.S. Navy ship after the end of WW II. It found no natural enemies and its populations exploded to over 3 million on the island. This nocturnal snake feeds on birds, reptiles, and small mammals.
Further affects…….. 9 of the 12 native rainforest birds are extinct in nature Example, the guam rail, a flightless bird, had a population of ~80,000 in 1968 but was extinct by 1986. The snakes have greatly affected the population size of many of the small reptile and mammals on the island. Snakes get into homes and their venom is mildly poisonous to infants. Snakes increase power outages by climbing on poles and electrical wires.
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