Changes in Ecosystems
Ecological Success
•	What is Ecological Succession?
Natural, gradual changes in the types of species that live in an area
•	Can be primary or secondary
•	The gradual replacement of one plant community by another through natural processes over time

Primary Succession
•	Begins in a place without any soil: Sides of volcanoes, landslides, flooding
•	First, lichens that do not need soil to survive grow on rocks
•	Next, mosses grow to hold newly made soil
•	Known as PIONEER SPECIES
•	Lichens break down rock to form soil. Low, growing moss plants trap moisture and prevent soil erosion
•	Soil starts to form as liches and the forces of weather and erosion help break down rocks into smaller pieces
•	When lichens die, they decompose, adding small amounts of organic matter to the rock to make soil
•	Simple plants like mosses and ferns can grow in the new soil
•	The simple plants die, adding more organic material (nutrients to the soil)
The soil layer thickens, and grasses, wildflowers, and other plants begin to take over
•	These plants die, and they add more nutrients to the soil
•	Shrubs and trees can survive now
•	Insects, small birds, and mammals have begun to move into the area
•	What was once a bare rock, now supports life

Secondary Succession
•	Begins in a place that already has soil and was once a home of living organisms
•	Occurs faster and has different pioneer species than primary succession
•	Example: after forest fires

Climax Community
•	A stable group of plants and animals that is the end result of the succession process
•	Does not always mean big trees
-Grasses in prairies
-Cacti in deserts




Two types of limiting factors:
Density-dependent and Density-independent
Population density describes the number of individuals in a given area
Density – Dependent
•	Density-dependent factors include disease, competition, predators, parasites, and food
•	Disease, for example, can spread more quickly in a population with members that live close together
Density – Independent
•	Density-independent factors can affect all populations, regardless of their density
•	Most density-independent factors are abiotic factors, such as temperature, storms, floods, drought, and major habitat disruption
Organism Interactions
•	Population sizes are controlled by various interactions among organisms that share a community
•	Predation and competition are two interactions that control populations
Predation – Populations of predators and their prey experiences cycles or changes in their numbers over periods of time
Competitions
•	Is Density – Dependent
•	When only a few individuals compete for resources, no problem arises
•	When a population increases to the point at which demand for resources exceeds the supply, the population sizes decreases

Crowding
•	As populations increase in size in environments that cannot support increased numbers,
•	Individual animals can exhibit a variety of stress symptoms
•	These include aggression, decrease fertility, and decreased resistance to disease
•	They become limiting factors for growth and keep populations below carrying capacity