Worst case scenario: a Class 4 outbreak of Solanum leads to the destruction of civilization as we know it. But what happens to the world left behind? After the Zombie Apocalypse creates a planet overrun with undead, the balance of the natural world will be thrown into chaos.
The artificial structures humans made cannot last forever. Everything that people have built requires regular maintainance to remain running and intact. When zombies wipe out most of humanity, these things will fall into disarray.
The first attackers against human structures are the undead themselves, as they tear through houses, cars, and everything else to get to their prey. The next immediate threat is the military, which will be called on to fight the zombies. As with every other war, this will lead to the destruction of buildings and vehicles as they try desperately to defeat their enemy. In extreme cases, this can lead to wholesale bombings of entire cities, completely leveling them.
The buildings that remain intact are subject to the forces of nature. Rain, snow, wind, temperature, and other environmental factors will constantly attack them. With people, these effects were kept in check and fixed before damage was done. Without people, not so much.
Water has the power to destroy many things. Against wooden houses, untreated water exposure causes the buildup of mold and fungus. This rots the wood and makes it prone to breaking. Against metals, it destroys through the process of corrosion. This is when, exposed to water, metals oxidize and produce rust. Rust weakens the metal and makes it unable to support what it is holding. This can happen through rainfall or being nearby a water source, especially the ocean. This is a hazard to everything made of metal, though many large buildings will be able to stand for decades. Flooding can also destroy, as its sheer force running into obstructions can rip them away. One of the most powerful forms of water is ice. When water freezes into ice, it expands. If water is trapped inside cracks when it freezes, it has the power to drive it further apart. This can be can weaken support for buildings and damage roads.
Plant life has the power to bring buildings down. Everyone knows that things like vines are cut down so that they don't cover up a house. Without people, they will completely engulf it within a few short years. As with ice, plants can do damage through cracks. If seeds sprout in even tiny openings, it can open up the crack as it grows.
The most resistant to nature are hardened structures. Things made of concrete, stone, and granite are naturally strong against natural deterioration. Towns and cities with the best chances are those in very hot and dry or very cold climates. With water, plants, and moisture being a building's worst enemy, the dry desert will slow natural degredation. In cold and snowy places, the constant low temperatures nearly encase them against the outside world.
While the natural world will work to take back the former lands of humans, the damage might not be wholesale. A Solanum outbreak lasts for three to five years or longer, and then the dead completely rot away. While homes and buildings are damaged by the weather and environment, it would take longer (tens or even hundreds of years) for most things to completely collapse. This leaves many large and abandoned structures relatively livable for groups of survivors in an empty world.
While humans had inadvertantly taken their toll on their surrounding environment, zombies will have a more direct impact, especially on wildlife. After towns and cities are infected and survivors either lay low or flee, the undead will wander into the wilderness. Thousands or even millions of zombies will employ the same hunting techniques against animals that are used on people; small numbers locate prey and moan, which attracts others in a chain swarm. A horde of zombies can do great damage to a herds of animals.
All animals can sense Solanum and their first reaction is to flee from it. Only in certain instances will they fight back, such as being cornered or to defend their young, but mostly they will run. This will have devestating effects on animal populations, as constantly moving hordes displace them from traditional migration patterns and breeding grounds.
While virually all wildlife can run faster than humans, the ability of a zombie to never get tired remains. Animals will keep running from the undead and get exausted while the zombies will pursue until their target looses them or is caught. This is a similar hunting technique used by wolves and other long-endurance carnivores--just at a much slower pace.
Solanum turns people into ravenous killing machines, but not other creatures. If an animal is bitten or infected, it will simply die and nothing else. This makes their situation even more dangerous, as a zombie simply has to catch and bite and animal to kill it, instead of needing to overpower and devoure it. This also inhibits the ability of most predators to defend themselves if caught. Most predators rely on their jaws and teeth for attack and defense. If they bite a zombie and ingest infected blood, they will die even if they escape.
For obvious reasons, the natural winners in a zombie apocalypse are scavengers. Vultures, crows, and ever-present flies will have an abundance of food sources from human and animal corpses alike. Flies and decomposition bacteria are no exeption to Solanum's repelling effects. They can only indulge on the dead after a zombie's brain is shot or destroyed and the virus becomes inactive without a host. Once the virus dies the bodies become ordinary dead organisms, free for the living to scavenge flesh off of.
On land, the undead will sevearly reduce the numbers of wildlife, and some may be driven to extinction. Not just from being hunted and driven from their land, but the wider effect of the entire "food web" of an ecosystem being displaced. Most likely to survive are microbes, many small insect species, many flying creatures, many borrowing and sub-terrainean animals, and those who can live in trees and not need to travel on the ground.
One of the least affected areas from zombies would be the oceans. The undead do not require oxygen from air, so they can walk into water without drowning. On land masses, this gives them the ability to cross lakes of rivers. However, that is pretty much their only advantage in water.
Zombies can walk into the oceans like anywhere else and survive, but their actual impact is minimal. The repelling effect of Solanum is felt even here, driving away sea animals. They do not have the brain power to swim, so they are unable to catch most swimming creatures. Zombie coordination is bad on land, so it is even worse against the force of water. Their eyes cannot see clearly while submerged, depending in the visibility of the water. Their additional senses of smell and hearing are virtually rendered irrelevant. One danger of walking into the ocean is that infected blood from open wounds will seep out. Infected blood is normally thick and congeled, but can leak out from being wet. But with Solanum's repelling effects and the virus's inability to survive in the open environment long outside of its host, the damage is minimal.
Zombies are immune from saltwater corrosion, but not the sea's other powers. Most ocean water is cold, and long amounts of time submerged will severely restrict their already resricted movement. If they can get to deep water, they will be subjected to increased pressure, either pinning them to the ocean floor or crushing them entirely. This makes the possibility of a zombie moving from a major land mass to and island far off the mainland extremely difficult, if not impossible, unless brought there by some other means.
While coastal sealife would be damaged from zombies, most of the ocean would actually benefit from a zombie apocalypse. With the pressure of human overfishing removed and the undead's inability to effectively hunt sea animals, there would be an explosion of fish populations. This can provide an abundant food supply for the small numbers of remaining survivors.
The effect on the oceans depends heavily on the number of surviving human refugees. In World War Z, displaced peoples fled to water for protection. Large groups settling on islands and trying to survive from them would lead to stripping of sea life and resources around the area. A Class 4 outbreak would leave fewer people alive to need support, limiting the overall impact. As in civilized times, the greatest negative effects on the world's oceans will come from living people.