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Water, Ice, Wind and Gravity


The four agents of erosion, or the transport and removal the products of weathering, are water, wind, gravity, and glaciers. *Note: water and ice are sometimes thought as one agent, making three agents of erosion total (water, wind, and gravity).

Water, whether in the form of a stream, an ocean, or even heavy precipitation, transports sediments. In the image below, the water has worn away the rock and these rock sediments have been transported elsewhere.


During wind erosion, moving sediments are picked up by the wind and are transported depending on the strength of the wind and the mass of the sediment.


Gravity also causes erosion. This process can happen gradually over time or more quickly, such as during a landslide or avalanche.


Finally, glaciers transport sediment when they move. This process occurs very slowly, but as a glacier moves across land, it picks up and distributes sediments. In the image below, you can see how the contact between the ice and the earth disrupts sediments. These sediments become trapped in the ice and moved.


As you can see in the image below, glaciers aren't just ice.


Environmental conditions do change in the oceans, but the terms climate and weather are not usually applied.


In the atmosphere, climate is longer-term changes in rainfall, temperatures, humidity, pressure, winds, etc., while weather is short-term changes (days to weeks) changes in the same factors.

in the ocean environments, temperature and pressure play important roles, but pH and salinity are also important factors. Needless to say that humidly in the oceans is not a factor as it's 100% water!

Every marine or lake organism evolves over million of years to fit into a given environmental niche. If the environment is fairly stable, the environment will continue to thrive. You can think of long-term marine environments as roughly the same as "climate" on the land. If environmental changes happen rapidly, like say El Nino, this might be analogous to weather.

During El Nino or La Nina events, vast schools of fish migrate to areas of the oceans that have the right temperature for them to thrive in. When things settle down again, they migrate back. A hurricane on land/surface of the ocean can also have big short-term impacts to the shelf regions of the oceans too - you could argue this is "oceanic weather."

Organism evolve in certain temperature/pressure/salinity conditions but when those conditions changes (like they are now with global warming) species are challenged to adapt or die out. Right now corals are struggling to adapt to longer-term changes in the ocean temperature and ph conditions, brought on by atmospheric above land climate change.


It will decrease the frequency of some genes and increase the frequency of others.


Natural selection decreases the frequency in a population of genes that decrease fitness and increases the frequency of genes that increase fitness. **Note that fitness in ecology refers to an individual's ability to survive and produce viable offspring.

For example, say we have a population of ants that live on the jungle floor. Half of the ants are dark brown and half are a lighter brown (50% dark brown, 50% light brown).

A pollutant enters the soil and the color of the soil changes to a very dark brown. Before, both dark brown and light brown ants survived on the forest floor, but now the light brown ants are more visible. They're consumed by insects and birds and other predators at a higher rate. Thus, their numbers decline.

Before the pollutant, four out of eight ants in this population are dark brown, or 50%. After the soil has been polluted, four out of five ants are dark brown, or 80%.
Created by Kate M.

Genes that are beneficial will be selected for and genes that are harmful will be selected against.

Note: natural selection operates on phenotypes, the expression of the gene, not the genotype. The example above assumes that the light brown ants are all of one genotype and the dark brown ants are all of another genotype. Learn about genotypes and phenotypes here.


Removing wolves affected much of Yellowstone because wolves are top predators and arguably keystone species.


Removing wolves from the park affected much of Yellowstone because wolves are top predators and arguably keystone species.

Predators are often very important to an ecosystem because they control population numbers of other species, mainly their prey. Think of a very simple food web where birds eat insects which feed on plants. If there are no more birds, no insects will be consumed, leaving more insects alive in the food web. With more insects alive, they will eat more of the plants.

This same concept applies to wolves and Yellowstone, except the food web and effects of wolves are far more complex. Wolves feed on elk, and without the wolves, the elk population exploded. The elk fed on young aspen trees, so the park had very few young aspen trees.

Without the predation of wolves, the elk remained in one place and fed on vegetation by the rivers, which had tremendous effects. With significantly less vegetation, the riverbanks began to erode and the rivers widened. The temperature of the river warmed because there was no shade cooling the river, so the abundance and distribution of fish species changed. Birds that nested by the river no longer had a riverbank to build their nests on. Beavers used willow trees on the banks of the river for their dams, but there were no more willow trees by the river because of the elk, so the beavers disappeared.

Before removal(simplified):
Image: KM
Once wolves had been removed(simplified):
Image: KM

To read more about the effects of removing the wolves, see this link from Yellowstone National Park on reintroduction, this article on the controversy surrounding the reintroduction of wolves and if this has saved Yellowstone, or this link on the beaver-willow lack of recovery.


More diversity means better chances of survival.


Increased genetic diversity leads to increased chance of species survival. Species with a limited variety of phenotypes and where all members of the species are similar to one another have a smaller chance of coping with environmental variability compared to a species with greater diversity.

In the image below, we have a species with high genetic diversity and one with lower genetic diversity. If a disease is introduced to the area and both species can contact the disease, species 1 is better situated to survive. Even if some phenotypes are not well-adapted to the disease and are wiped out, there are other phenotypes remaining in species 1. This is not true for species 2, a loss of one phenotype dramatically reduces diversity. All it would take is for a single event that the remaining phenotype is poorly adapted to, say a drought, for this species to go extinct.

Image created by Kate M

Its very important to note that natural selection operates ony on phenotype, and selected phenotypes which are controlled by genes can be transferred from one generation to the next. After considerable time, selection pressure may change populations in one of possible three ways: these could be either stabilising/directional/disruptive selection.


It could be seen from the illustration that maximum range of phenotypic features, i.e. variation are saved after disruptive selection, while after stabilising selection only a narrow range of phenotypic variation would remain in population . Loss of variability due to stabilising selection could increase the threat of extinction.


It is evident from the above illustration that disruptive selection may actually divide a population in subpopulations with distinct characters. For example, zebu cattle of Gangetic plains and other parts of south asia, bisons of european grass lands and yak of himalayas are all descendents of a distant common ancestor:- disruptive evolution allowed them to survive in different climatic conditions in different continents.Then there are other related groups like wild gaurs (indian bison), and semi-domesticated mithuns.

Stabilising selection and decrease in genetic diversity may allow the organism to live within a small, distinct habitat in a certain way. Any change in its environment may threaten its existence.


Species diversity is measured by determining the number of species present in a given area or community and calculating how evenly distributed each species is.


Species diversity is measured by determining the number of species present in a given area or community and calculating how evenly distributed a species is within that community. Indices of species diversity are used which may give more or less weight species that are dominantly found in the landscape. The Simpson's Index and the Shannon's Index are two examples of diversity indices.

There are multiple ways the number and evens can be measured in the field depending on the ecosystem in question, the resources (money, technology available, time, the amount of people available to collect data, etc) of the scientist, and the context in which species diversity is being measured.

Surveys done by scientists, camera traps, environmental DNA, and light traps are all examples of methods used to measure species diversity.

Biologists can traverse a landscape repeatedly, at different times of the day, at different times of the year, in different weather conditions, and so forth and record species presence over time to assemble a rough idea of species diversity. This is often done when one is interested only in a specific type of organism, such as mammals.

Camera traps may be used to determine the number of carnivores present at a site. Camera are placed throughout the study site and take a photo whenever triggered by movement. These images are then sorted and categorized to determine the number of carnivore species present at a site. Data is typically collected over an extended period of time and the camera traps may be moved to new locations to obtain more data.

Scientists setting up a camera trap:

A newer technique is to use environmental DNA, or DNA that can be collected indirectly from the animal through skin the animal has shed, DNA left in the soil, feces, or the water even. DNA collected is then compared to a database of known species to determine what species is present in the landscape. Environmental DNA can be collected to determine the number of fish species in a river. Read about a recent example here.

Another example of a method that may be appropriate depending on the study question is to use light traps to sample insect diversity. This is done by holding up a light to attract insects and then containing the insects in a net or some other sort of trap. Again, this type of sampling may be done repeatedly at different times of the year to determine an accurate estimate.

Scientist using a light trap:

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