Animal tracks

Grass deterioration, also known as animal tracks or desire lines, is an environmental feature first implemented in and preserved in. By keeping track of players' movements throughout town, the game is able wear down the grass in high-traffic areas. While originally intended to add to the town’s own unique character, the feature has led to desert-like conditions in extreme cases. Many players seek to prevent grass loss in their town, not only for aesthetic reasons, but because a lack of grass causes some insects (e.g., dung beetle) or furniture (e.g., Snowman Series) to be nearly impossible to collect.

In Animal Crossing: City Folk
Grass deterioration in City Folk is much more noticeable than in subsequent titles, so much so that patches of grass will shrink beneath a player's feet as they walk. The rate of regrowth is stable across seasons, however the rate of deterioration is more marked in the winter months. This became particularly evident shortly after the game's release in November of 2008, leading players to petition the feature's removal. Nintendo did not issue an official response, however customer service representatives assured players that the feature was intentional and suggested ways for players to encourage grass regrowth.

Game mechanics
Each grass tile has a health value ranging from 0-255, with zero being dirt and 255 being completely healthy. The first time (per game load) that a player walks across a tile, the value decreases between 4-11 points, depending on the season. Each day at 6AM the game restores as many as nine grass points to each tile, the exact number depending on the flora inhabiting the space as well as the tile's proximity to other healthy (or unhealthy) tiles.

Factors influencing grass wear
The most noticeable increase in wear occurs with the changing of the seasons. The following table illustrates the difference in deterioration rates per tile after one round of wear. Notice the sharp increase in wear during winter.

The first time a player steps on a grass tile (per game load) the game selects a value within the range appropriate for the current season and subtracts it from the tile's health value. A new round of wear can occur each time the town is loaded from memory. In addition to the initial game start, this occurs when partying over a Wi-Fi game, coming back from a friend’s town, and by the start of the day message at 6AM.
 * Factors that do not affect grass wear: Contrary to common belief, running over grass tiles instead of walking over them will not affect how many health points they lose, however it does increase the likelihood of the player stepping on a greater number of grass tiles, giving the illusion that the grass is decaying more quickly. Additionally, how many times a player walks over a space during the same game session is not considered by the game code, however this again increases the likelihood of walking over the neighboring tiles of damaged grass tiles in high traffic areas, again creating this illusion. Whether or not there is a pattern on a grass tile has no effect on its decay.

Factors influencing grass growth
Grass regrowth can be manipulated by the player to an extent. Certain flora will speed up regrowth, while others will have no effect. The effects noted in the table below are mitigated by a tile's proximity to other unhealthy tiles. If a tile is surrounded by healthy tiles, the effects will be maximized; otherwise the tile will grow more slowly. Grass health is adjusted for all tiles at 6 a.m. each day.

By providing five extra health points, flowers and trees can provide a total of up to nine health points per day. Note though that they only provide a growth bonus to the space they are occupying. Proximity to unhealthy spaces subtracts from the base growth rate, meaning that a grass tile with nothing on it would only gain three points of growth per day. As the minimum wear rate is -4, continually walking on a bare spot of grass makes it impossible for it to regrow.
 * Factors that do not affect grass growth: The presence of a weed, dropped item, buried item, or pattern. Watering flowers (or even empty spaces) and precipitation have no effect on growth. The type of flower or tree does not matter; all provide five bonus points.

In Animal Crossing: New Leaf
Grass wear is much more subtle in New Leaf and daily routes require extensive use before grass begins to fade. Even so, some players have reported extensive wear to the point of desertification. The addition of public works projects has also contributed to this process as a patch of dirt is left behind after a project is removed. While the exact wear and regrowth mechanics are unknown, the feature is thought to operate much as it did in.

Observed changes

 * The game no longer allows additional rounds of grass wear each time the game is loaded.
 * Grass regrowth is improved by three stages the day after a snow event and by one stage the day after a rain event.

Prevention
Short of spending the entire game inside the player's home, there is no way to avoid grass deterioration. Players can, however, take certain steps to limit it. In City Folk the player can reduce the number of times each day they load their game, as each play session allows for another round of wear. Players can also walk along pattern-designated paths. While patterns do not affect the wear rate, they provide a visual reminder for the player to limit their traffic to designated areas. Lining paths with flowers will keep the player on track and keep dirt patches from spreading past the path boundaries.

Repair
Complete regrowth from dirt to full health can take between 1-3 months to achieve depending on the extent of the damage and the factors influencing growth. There are a few ways in which a player can aid this process. Planting trees and flowers in the trampled area will speed up regrowth significantly. Using the growth rates from, assuming a grass health of zero, an isolated patch of dirt can achieve full health in 29 days (256/9=28.4...). Trees provide an additional advantage in that they serve as a physical barrier to prevent players from inadvertently stepping on worn areas and do not need to be watered. At the same time, their growth is limited by proximity and count restrictions which do not apply to flowers.

Once the trees and flowers are planted, the player must avoid walking in the affected area other than to keep the flowers watered. For players who are willing to time travel within the game, the regrowth process can be further accelerated by advancing the game clock forward one day at a time, being certain that all flowers are kept watered each day. Setting the game clock backward in time or moving forward more than one day does not result in additional grass regrowth. The player can take advantage of seasonal weather changes in and load their game in winter when it snows, or in the other seasons when it rains, allowing for a faster regeneration rate.