Changes occur in an ecosystem resulting from changes in climate human activity, introduction of non-native or indigenous species, and changes in populations.

We will talk about each of these individually and more in depth:

Climate
http://en.wikipedia.org/wiki/Climate_change

Climate changes affect each biome and each species individually. Different species and populations migrate, begin, and become extinct at different rates, climate changes, could cause the extinction of existing ecosystems and the formation of new ecosystems.

Ecological changes in climate (temperatures, flooding, warming, and freezing) will probably eliminate some species, and these species losses may cause the elimination of entire ecosystems.

For example we could lose cold-adapted systems such as arctic and alpine communities to global warming, and low laying areas or islands to be flooded by the sea level rising. All of these changes could have a disastrous effect on the ecology of earth and its ecosystems, as we know it today. All climate changes affect each species way of life, the way they eat, sleep, and survive and interact with each other.

Human activity
http://en.wikipedia.org/wiki/Biodiversity
http://conserveonline.org/workspaces/human.activity.index/globio_report.pdf

The invention of industry and agriculture and spread of these practices throughout the world have allowed humans to prefer a large portion of the useable parts of the Earth. Estimation shows that humans currently use approximately 40 percent of the energy of the Sun captured by organisms on land. Use of such an inordinately large proportion of the Earth's productivity by a single animal species is unique in the history of the planet.

The vast use of the natural resources on earth has caused each species to fight for the natural resource’s it needs to survive: land, water, food sources and available mate to reproduce. By humans using or destroying so much of the resources that are available today, we are currently causing species to become threatened, or extinct.

The human population continues to expand at the rate of approximately 80 million persons per year and may reach 10 billion sometime in the 21st century. Changes to the atmosphere caused by complex technology and the increasing population threaten to cause major disruptions to ecosystems. Among the most important changes are global warming and the release of greenhouse gases—including carbon dioxide, methane, and chlorofluorocarbons—into the atmosphere.

Human Impacts on the environment

• Global Warming
• Use of CFC's and impacts on the Ozone Layer
• Overuse of Pesticides
• Use of PCB's
• Deforestation Destruction of Wetlands
• Urban Encroachment
• Endangered Species
• Acid Rain
• Water Contamination
• Use of Fossil Fuels
• Dams for Hydro Electricity
• Disposal of Radioactive Wastes
• Oil Spills
• Smog Air Pollution
• Soil Erosion
• Introduction of Invasive Species
• Heavy Metals
• Salting Roads
• Crop Irrigation
• Irrigation, land erosion and pesticide run off

Below are a few examples of the impact of humans on an ecosystem and its populations:

Roads are often built through forest for industrial purposes, such as mining/mineral exploration, oil and gas interests. Secondary, more uncontrolled development, result in deforestation, with subsequent erosion and loss in biodiversity. Sustainable development planning should therefore take into consideration the effects of the entire road network, not just the individual new segments that are continuously added on.
 

This chart shows how simply putting a road in somewhere can affect all species in that area, including humans.



Despite the enormous advances made in understanding the biosphere over the past few decades, there is clearly much more to learn. Many would agree that we are just beginning to perceive the complex process that keeps the biosphere hospitable to life.
 

How Animal Populations Interact

Populations of animals interact with each other and their environment in a variety of ways. One of the primary interactions a population has with its environment and other populations is due to feeding behavior.
The consumption of plants as a food source is referred to as herbivory and the animals that do this consuming are called herbivores. There are different types of herbivores. Those that feed on grasses are referred to as grazers. Animals that eat leaves and other portions of woody plants are called browsers, while those that consume fruits, seeds, sap, and pollen are called frugivores.
Populations of animals that feed on other organisms are called predators.

The populations on which predators feed are called prey. Often, predator and prey populations cycle in a complex interaction. When prey resources are abundant, predator numbers increase until the prey resources wane. When prey numbers drop, predator numbers dwindle as well. If the environment provides adequate refuge and resources for prey, their numbers may again increase and the cycle begins again.

The concept of competitive exclusion suggests that two species that require identical resources cannot coexist in the same location. The reasoning behind this concept is that one of those two species will be better adapted to that environment and be more successful, to the point of excluding the lesser species from the environment. Yet we find that many species with similar requirements do coexist. Because the environment is varied, competing species can use resources in different ways when competition is intense, thus allowing space for one another.

When you have two species that influence (either positively or negatively) from each other, in a relationship referred to as symbiosis. The various types of symbiosis include:

• parasitism - one species (parasite) benefits more than the other species (host)
   
• commensalism - one species benefits while a second species is neither helped nor injured
   
• mutualism - both species benefit from the interaction

Here are a few more vocabulary words that you should know:

• Demography — The study of population numbers, distribution, trends and issues.
   
• Migration — The movement of people and all living species from one place to another.
   
• Population density — The number of people and all living species in a measured land area.
   
• Population distribution — The patterns of population in an area.
   
• Population projection — Models that estimate scenarios of population growth or decline

Introduction of non-native species
http://en.wikipedia.org/wiki/Invasive_species

The word non-native species has many similar and interchangeable terms that are used for the same situation: alien, exotic, escaped, feral, foreign, invasive, non-native, immigrant, non-indigenous, and some that I pry missed.

The definition from the United States Environmental Protection Agency is: introduced species are “species that have become able to survive and reproduce outside the habitats where they evolved or spread naturally" (EPA, 2003).

Humans move many species from their native ranges to places all over Earth, both deliberately and accidentally. Where those species establish new, self-maintaining populations, they constitute a biological invasion.

An invasive species can be in the form of a: plant, animal, disease, or insect.

The majority of species introductions fail, plant and animal specie success rates of over 50% are rare, usually only
10-40%. of game fish, birds, mammals can survive as an invader in a new community, due to the inability to adapt to the area.

The difference between invasion and immigration probability depends on geography and human activity.

Even for species that eventually become pests, it often takes many attempts to become established.

These species are most likely to be successful invaders:

• High reproductive rates, pioneer species, short generation time
   
• Long-lived
   
• High dispersal rates
   
• Vegetative or cloned reproduction
   
• High genetic variability
   
• Broad native range
   
• Habitat generalist
   
• Broad diet
   
• Human introduction

The sort of ecological communities, which are more likely to be invaded:

• Climatically matched with original habitat of invader
   
• Early successional
   
• Low diversity of native species
   
• Absence of predators on invading species
   
• Absence of native species morphologically or ecologically similar to invaders
   
• Absence of predators or grazers in evolutionary history (“naïve” prey)
   
• Absence of fire in evolutionary history
   
• Low-connection food web
   
• Simple communities

Here are a few examples of exotic species:
http://en.wikipedia.org/wiki/Invasive_species
http://en.wikipedia.org/wiki/Exotic_species

Examples of invasive weeds:
Invasive plant species are among the greatest threats to the integrity of natural areas.
http://www.invasive.org/weeds.cfm

Changes in population size

A population is define as a group of one species of organisms occupying the same general area, using the same resources, and acted upon by the same environmental factors. Populations cannot grow indefinitely, many populations will become stable over a period of time while others will show sharp increases followed by similar decreases.

Logistic Growth
Populations seldom undergo unrestricted growth under constant conditions for long periods. Given the capacity for increase of bacteria, this is just as well! More commonly, as populations grow individuals affect one another's access to vital resources such as food and space. This is an example of intraspecific competition (intra = within). Consequently, birth rates tend to decrease and death rates increase, as populations become more crowded -- so-called density-dependent effects on birth and death rates. At some point, the carrying capacity (K) of the environment is reached by the population and there is no more growth (i.e., births balance deaths). The carrying capacity is the number of individuals, which can be supported by an environment.

Interspecific Competition
Populations do not live alone, just as individuals do not. When individuals of the same species, or of two different species, depend on a common important resource then competition occurs. Competition can be defined as "interactions between individuals brought about by a shared requirement for a resource in limited supply leading to a reduction in survivorship, growth, and reproduction of individuals." Interspecific competition (inter = between) is the competition between two or more different species for a resource.

Predation
Predation is an important interaction between species that affects population levels of species. Predation can be defined as occurring when individuals of one population eat living individuals of another. There are several models for predation, but all models have two parts: the birth rate of the predator increases as the number of prey increases, and the death rate of the prey increases as the number of predators increases.

Density-Dependent and Density-Independent Factors:

Density Dependent Factors - Increasing population size reduces available resources and this eventually limits population growth. In restricting population growth, a density-dependent factor strengthens as the population size increases, affecting each individual more strongly. On the other hand, the population growth declines because the death rate increases, birth rate decreases or both. If there is a reduction in the food supply or other major resources, could also restrict the reproduction cycle resulting in less offspring.

The animal species’ battle to establish territories is a behavioral trait that may restrict population growth. Predators concentrate in areas where there is a high concentration of organisms. As long as the natural resources are available in sufficient quantity, the population will remain constant. As the population decreases so, do the predators.

Density-Independent Factors - Examples are weather, climate, and natural disasters such as freezes, seasonal changes such as unusually hot or cold temperatures, hurricanes, and fires. All of these factors are unrelated to population size but affect everyone in the population regardless of population size or location.

Experiments for Home and Classroom

You can try these desert experiments at home. You can create a mini-pond at home and grow your own algae, see how cyanobacteria grow mostly in the warm, upper layers of a lake or see how warm temperatures speed up bacterial growth. Desert plants grow better in the heat than plants imported from colder places. See how plants require the right temperature in order to grow. A little bit of air or water, pollution goes a long way. Find out how.
http://chainreaction.asu.edu/ecology/trythis/

Here are 12 ecology-related experiments for you to try:
Before starting any of the experiments listed be sure to read the instructions carefully.
http://www.galeschools.com/environment/experiment/index.htm

Build a small ecosystem and make a mold terrarium, watch tiny blue, green and white plants grow on leftover food.
http://www.exploratorium.edu/science_explorer/mold.html

Next Page:  Fluctuation in Population Size (top)