Interactions between Populations and Ecosystems

Population ecologists deal with the many dynamics of population. Ecologists consider how many species differ in how they respond to environmental changes such as food shortages, disease, parasites, predation, droughts, floods and many other factors.

Factors that Affect Population Size

Some factors affect population's density (the number of individuals in a specific area):

Birth rate
Death rate
Immigration of new members of the species
Emigration of existing members

Environmental Changesthat affect Population Size:

The availability of food
The availability of water
The availability of specific nutrients
Space
The balance of predators and prey
Competition with other species
Diseasessuch as West Nile Virus
Parasites

Human Actions that Affect Population Size

Habitat loss or protection
Pesticide
Herbicides
Poisoning
Hunting
Erosion
Introduction of invasive species

Climate Conditions that affect Population Size

Global Warming
Sunlight Temperature
Humidity
Natural disasters, such as:
- Tsunamis
- Volcanoes
- Hurricanes
- Tornados
- Avalanches

Carrying Capacity: How many Individuals can an Ecosystem Support?

Carrying Capacity is the maximum number of individuals from a species that an ecosystem can support indefinitely, without permanently damaging the ecosystem upon which it depends. Carrying capacity may also specific the ecosystem's features such as food, water, habitat and other necessities.

Environmental changes can affect a habitat's carrying capacity for different species. For example, the destruction of rainforest above, would affect the habitat's carrying capacity for the many species that depend on it.

Graphing Population Change

Many species follow a similar growth pattern when they are initially introduced to an environment.
The Log Phase represents, how the population size increases as the species moves into the environment and starts to reproduce.
The Lag Phase reflects how the population size starts fluctuating when environmental factors start affecting the population size.

What Happens when Predators and Prey Live Together

Decades of Observing Moose and Wolf Interactions

Predators can actually benefit prey population. Try to imagine what would happen to a population of moose on an island without predators. Without wolves or other large predators, a moose population could overgraze the vegetation and damage the ecosystem. Ultimately, many ungulates could starve do death. Conversely, without moose, the wolves would starve.

In a famous study of predator-prey relations, scientists spent decades studying wolf and moose populations on an island (Isle Royale National Park in Lake Superior). Before the study, some scientists speculated that the wolves could eventually overpopulate and kill of the moose population. Then, without enough large prey, the wolves would eventually die off.

However, this has not happened. Scientists are puzzling over how the wolves to limit their own population size. There are some great websites to follow this research. (I will add a good website, once it becomes available next month.)

Graphing Predator prey relations

This graph by Lotka-Volterra represents how predator populations tend to increase or decline in response to changes in prey population size. Try to interpret the graph. Do both populations rise and fall at the same time, or does one population lag behind the other?