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Rats are what can be a part of multiple trophic levels.  Being an omivore, they prey on both the primary consumers and the producers.  Producers are organisms that feed off the energy from the sun, such as plants or plankton.  Rats will eat seeds produced by the plant and receive nutrients and the energy originally from the sun, acting as a primary consumer.  However, it is common that rats consume organisms considered primary consumers, such as insects, frogs, and small birds, who eat the producers, making the rat act as if it were a secondary consumer.  The rat will gain more energy as a primary consumer compared to being a secondary consumer because less and less energy is transferred as organisms are traced up the food chain.  The rat can be eaten by tertiary and quatrenary consumers, as depicted.  Specifically the web describes its predators to be the snake and the hawk, but other large birds such as the falcon should be noted as well as other medium sized carnivorous mammals such as the coyote, wolf, or fox.

It should also be noted that rats are also considered scavengers, because they can also prey on dead rats and other dying mammals as well as garbage, holding no caloric value.  However, often  being near human civilization means digging through our garbage that often can be food and dairy products such as cheese are snatched.

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A rat's burrow acts as a home base to a group of rats.  These burrows are made of gathered materials, such as leaves and twigs and even garbage.  When possible, these burrows are dug under ground.  Much like the sewage systems where these can commonly be found, these burrows have a  distinct organization to them, often including designated chambers for things such as food storage and multiple escape ways.  The nest is always the most protected part of the burrow as to protect the young offspring of the Norway rat.

This picture depiction describes a Norway rat's burrow underground and underneath pieces of slate rock.  Labeled are the escape hole that a rat will give itself and its nest underground, the hardest place to get to for predators.

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Here is a graph of the survivorship curve of the brown rat. It is an example of Type 2 survivorship. The offspring has an equal chance of survival at all stages of life.

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Why is the rat population not growing exponentially?
In areas with humans, rats mainly survive off of food that humans throw in the garbage. So if the human population began to dwindle in an area, a rat's food source is compromised, therefore causing rats to die due to lack of food.
Additionally in the wild rats have many predators including: wolves, foxes, cats snakes and lizards.
In human populated areas, humans are another preditor of the rat. Humans view the rat as a problem as they have killed more people through disease than all the major wars have combined!
Therefore due to humans trying to eradicate them, animals hunting on them in the wild and a food source that is not always sustainable, the rat population does not continue to grow exponentially.

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Versus other small mammals, such as squirrels, the rat produces more offspring, and cares for them less than the squirrels care for their offspring. Therefore, it can be concluded that rats have many offspring so the chance of some of them surviving is higher. But when compared to other organisms such as the human or the mussel, the survivorship curve is much closer to the squirrel than these other organisms. Therefore the survivorship of a brown rat follows the Type 2 curve.

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Rats mate in large groups. Females can mate up to 500 times with competing males during her estrus period (the time in which she can reproduce).
Female rats are capable of producing 60 offspring a year.
Their young leave the nest at 3 to 4 weeks, only a mere number of days after they have opened their eyes.

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The brown rat's dispersion pattern is clumped. They live in territories, which are based on a hierarchy system determined by size and age. These territories can be as large as 50 meters wide. The male rats are normally at the top.

These following factors could have a negative effect on the rat population, and therefore cause the population to not grow indefinitely.
Density Dependent Factors:
-- Predators Include: wolves, foxes, owls, cats, snakes, lizards + humans
-- Not enough food due to the rats not being able to get as much garbage

Density independent Factors:
-- Human predation: humans set up rat traps
-- Natural disasters: floods. Because rats live in burrows underground, floods have a very negative effect on their living situation.

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Anonymous(2016). Brown Rat (Rattus norvegicus). Wildscreen Archive. Accessed on May 11, 2016. http://www.arkive.org/

Foto, John (Photographer). Young Brown Rat [Photograph]. Available from https://www.flickr.com/

KellyHyle (blogger) (circa 2015) Wetland Food Chains [Photograph] Tackker Accessed on May 11, 2016
Available from https://tackk.com/1p4pe1

Kez245 (vlogger) (2014)Wild Rat Family  [Video] YouTube.  Accessed on 12 May 2016.  https://youtu.be/M5kdl2xKAoE

Schwartz(illustrator).(1981). [Untitled drawing of a rat burrow]. Available from http://icwdm.org

Shefferly, N., Myers, P. and D. Armitage(2014). Rattus norvegicus brown rat. Animal Diversity. Accessed on May 11, 2016. http://animaldiversity.org

[Untitled photograph of a rat]. Available from http://www.conserveireland.com

[Untitled photograph of a rat trap]. Available from http://www.amazon.com/

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Rats can be found in all biomes except the tundra,
These include:
1. taiga
2. desert
3. grassland
4. chaparral
5. forest
6. rainforest
7. mountains

Today, rats are in abundance in areas of human civilization, especially port cities in and out of which ships travel.  For this reason, these rats have been able to travel all around the world on ships.  Due to their port city dwelling, rats have adapted to become strong swimmers and are even called the "water rat".  The population originated in northern China, but after some had scuttled onto ships, the species' geographic boundaries quickly broadened.  This first reported sightings in the western side of the world occurred in 1770; what's more is that it was first reported in Europe in 1720 and by the turn of the nineteenth century, they could be found in every single european country.  They can inhabit anywhere that provides food and shelter, from woodlands to sewages.  Sewage systems, specifically, require a certain intelligence to live in its complex network of piping, luckily the rats have a strong learning capacity and can easily memorize these complex patterns.  Also, places dark and isolated such as the sewer would not serve many animals well as they rely so much on their sight, but the Norway uses its extremely attentive sense of smell to scope out food and touch from its whiskers and paws to feel the vibrations of its surroundings.   Similar to a bird, the Norway rats will build burrows.