Unit III: Ecosystem Ecology

Chapter 6-Population and Community Ecology

I. Levels of Organization• Organism→Species→Population→

Community→Ecosystem→Biosphere

I. Population Abundance and Distribution

• Populations are dynamic: constantly changing; inputs and outputs

• Population Characteristics:– Size (N): Total number of individuals within a

defined area at a given time. – Density: number of individuals per unit area. Used

to set hunting and fishing limits, food supply

Distribution: How they occupy space; how they are distributed with respect to one another

Clumping: most common; pack of wolves, flock of birds, school of fish; cluster where resources available, helps protect from predators, better hunting success.

Uniform distribution: common where resources are scarce; distance between neighboring individuals is maximized to avoid competition or avoid territoriality; penguins, creosote bush (terpenes)

Random dispersion: least common; position of each individual is independent of the other individuals; occurs in habitats where environmental conditions and resources are consistent; dandelions, oyster larvae carried by ocean currents.

– Sex ratio: proportion of males to females. – Age structure: percent of population in pre or post

reproductive age.

• Factors that influence population size: Environmental Resistance Factors– Density Dependent Factors: Influence an

individuals probability of survival and reproduction in a manner that depends on the size of the populationcompetition, predation, disease

– Density Independent Factors: have same effect on an individual's probability of survival fire, drought, hurricane, pest spraying

Limiting Factor/Limiting Resource: A resource that a population cannot live without and which occurs in quantities lower than the population would require to increase in size.

Carrying Capacity: (K) the population size of thespecies that the environment can sustain indefinitely, given the food, habitat, water and necessities available in the environment

Growth Models

• Growth rate: Number of offspring an individual can produce in a given period of time, minus deaths of the individual or offspring during the same period of time.

• Intrinsic Growth Rate (r) is the rate at which a population would grow if it had unlimited resources.

Exponential Growth Model

• Exponential Growth Model: Populations growing at a fixed rate. When population are not limited by resources their growth is very rapid, as more growths occur with each step in time. Represented by a J-shaped curve

Logistical Growth Model

• Populations can not grow at an exponential rate indefinitely.

Logistic growth: exponential at first but slows as population reaches carrying capacity. Sigmoid (sshaped) curve

• Population growth is affected by biotic or intrinsic factors that are built into the genetic basis of each species. This is known as biotic potential: is the maximum size a population would get it there were nothing holding it back.

• Together, biotic potential and environmental resistance determine carrying capacity (K)

Environmental Resistance + Biotic Potential =Carrying Capacity

Variations on the Logistic Growth Model

• Some populations will overshoot K, there will be a lack of resources and the population will experience a die-off/crash.

• Reindeer of St. Paul Island.

Variations to Logistical growth

Predator-Prey Cycles• Canada Lynx and

Snowshoe Hare-records estimated from Hudson Bay Co. show oscillations of abundance, with lynx pop. Peaking 1-2 years after the hare population

Reproductive Strategies

• Biotic or Intrinsic factors are specific to each species and include Age of reproductive maturity Number of offspring per reproductive event Number of reproductive events per lifetime These factors together are referred to as fecundity/fertility.

r strategists (r-selected species)High intrinsic growth rate because they reproduce often and producelarge number of off spring. Populations do not typically remain near K,but exhibit rapid growth followed by overshoots and die-offs

K strategists (K-selected species)Low intrinsic growth rate so pop increases slowly until reach K.Fluctuations are small

Survivorship Curves

• Late Loss

• Constant Loss

• Early Loss

Early Loss

Late Loss

Constant Loss

Species Interactions

• Competition: Struggle of individuals to obtain a limited resource

• Competitive Exclusion Principle: Two species competing for same limited resource cannot coexist

• Resource Partitioning: results from competition, two species divide a resources based on behavior or morphology.

Predation: Use or one species by another species

• True Predators: Kill prey and consume most of what they kill

• Parasitism: One species (the parasite) feeds on part of another organism (the host) usually by living on or in the host. Parasite benefits/host is harmed.

• Mutualism: two species or a network of species interact in a way that benefits both.

• Commensalism: an interaction that benefits one species but has little, if any effect on the other species

Mutualism

Commensalism

Indicator: Amphibians, Birds, Butterflies

Habitat Loss,Increases in UV,Parasites, Pollution(pesticides), Climatechange, OverhuntingButterflies as Indicator Species

Keystone: American Alligator

Dig holes that holdfreshwater during dryspells and serve asrefuges for aquatic lifeand provide freshwater

Foundation: create and enhance habitat that benefit other species

Beaver: build a dam,create a pond, whereother organisms live. Bat and bird speciesthat regeneratedeforested areas andspread fruit plants intheir droppings.

Composition of a Community Changes over Time

• Primary Succession

• Secondary Succession

• Aquatic Succession