The life of every animal and plant depends upon other biota.¹ When one species is removed for human use, other biota, not directly touched and perhaps even unnoticed by humans, can suffer because they depend on that species. Even worse, such damage can cause a second species' irreversible decline into extinction.

When an ecosystem service (e.g. soil retaining ground moisture) is lost or weakened it can harm many species. 
 

Lets take a look at a simple system involving Sea Otters, Kelp and Sea Urchins. They all depend upon each other. Sea urchins eat kelp, sea otters eat sea urchins, and kelp provides predator protection for sea otters. 

KELP LOSS 

If we let industry harvest too much kelp, Sea Otters leave or decline in population because they have nowhere to sleep and hide from predators such as sharks and Orcas. They must move to find other protected foraging areas. When Sea Otters leave or decline sea urchins thrive and eat too much kelp and the remaining kelp forest dies. 

SEA URCHIN LOSS 

If we let industry harvest too many sea urchins, Sea Otters have to find other food and may leave. When Sea Otters leave or decline sea urchins can rebound and destroy a kelp forest preventing sea otters' return. 

SEA OTTER LOSS 

If we let sea otters decline sea urchins thrive and can destroy a kelp forest. The California kelp forest was decimated by sea urchins after Southern Sea Otters were hunted to near extinction by 1900.

For another example, lets look at how woodpeckers, Monterey pine trees bark beetles and pine pitch canker interact. 

ANIMALS, PINES & INSECTS 

Bark beetles carry the dreaded Pine Pitch Canker. Woodpeckers eat insects which damage trees, including Bark beetles. Woodpeckers depend on dead standing trees for homes mainly because dead trees have softer wood.  Woodpeckers can carve a nest cavity much easier in soft, dead wood than in hard live wood.

This means that to minimize Pine Pitch Canker we must not disturb Woodpeckers or their homes - dead standing Monterey pine trees. When all dead standing trees are cut down, woodpeckers leave. When woodpeckers leave, bark beetles abound and kill many trees and spread the Pine Pitch Canker which kills many more trees. 

Humans cannot dine on rocks or dirt, but plants must. Since essentially everything we eat is either a plant, or an animal which ate plants, it is vital to our survival that we "Stop treating soil like dirt." -David Brower 

LICHENS 

Lichens create the original soils from which plants can grasp nutrients. It is reasonable to say that all terrestrial life owes its existence to lichens. 

TREES 

Trees, flowers and plants depend on Soil for food (nutrients). Trees also depend on mycorrhize fungus to help their roots absorb water. 

Fruiting Trees depend on Bees for pollination. 

FLOWERS 

Forest flowers and plants depend on trees for shade and wind protection as well as soils for nutrients. 

BEES 

Bees depend on flowering plants and trees for food. 

Humans depend upon Bees for not only honey but, for fruit from trees they pollinate. 

INSECTS

Birds eat insects. Widespread eradication of insects would harm bird populations as well.
 

Fish, such as Steelhead trout (Anadramous Salmon) depend on trees to shade and cool their streams so they do not die of overheating. 

Freshwater fish, including steelhead, depend on insects for food. 

FUNGI & MICRO-ORGANISMS 

Decomposers depend on dead trees for food. When a Park is "cleaned up" by removing its dead trees, decomposers decline. 

Redwoods and other giant trees depend on a tiny Mycorrhize fungi which greatly help the trees absorb water and nutrients from the soil. Pesticides can kill the fungi and thus indirectly kill a forest. 

All interdependent biosystems have some resilience or elasticity that allows each species to expand and shrink by small amounts without irreversibly destroying the system. However when humans take biota from the system, that elasticity is often stretched beyond its limit just like a rubber band pulled too tight or a balloon blown up too much. 

A big problem is how we have only the faintest hint of an idea how close the limit of extraction is for any species in an ecosystem web. The only way humans have ever found out how close we are is when we have extracted too much. But of course by then it is too late. 

A less recognized problem is how the risk of irreversibility is much closer when we remove amounts of two or more interdependent species, rather than just one. Thresholds of irreversibility get even closer, extracting smaller amounts of biota can cause species extirpation, such as when we remove multiple interdependent species such as sea otters and sea urchins, or Monterey pines and woodpeckers. 

When a key species is killed it can start an ecological chain reaction that devastates an entire ecosystem.