Synergy

When scientists study a situation there is a tendency to regard each factor in isolation, on the very reasonable grounds that it is both easier and clearer. The real world however, rarely works like that and factors are often synergistic, that is the effect of their combination is greater than either alone. For example a combination of two anti-cancer drugs may be required to knock out a tumour´s resistance and to kill it, neither drug is effective alone. A recent paper from the Smithsonian Tropical Research Institute draws attention to how this might apply to tropical forests.
http://www3.interscience.wiley.com/journal/123190996/abstract?CRETRY=1&SRETRY=0

Consider logging, hunting and fire. Logging of tropical forests of course destroys habitat, but in those rare examples where it is controlled the loss of species is less than you might think, they often still persist in any remaining fragments. Hunting in pristine forest tends to hampered by difficulty of access, and large scale fire is relatively rare as the canopy keeps lower levels damp and hard to burn. However logging needs roads and this opens up previously inaccessible areas to hunters, as well the as the effect of loggers themselves. It is estimated that a single logging camp in Indonesia consumed 33,000 kg of bushmeat per year. Ease of access and increased demand together can devastate animal populations, particularly of large animals.
Logging also opens the forest canopy, drying out the undergrowth beneath, and greatly increasing the chance of fire. Most tropical forest plants lack the fire defences such thick bark or fireproof seeds of pampus species, and so are wiped out by fire.

Another example is the way that native species can be exposed to foreign pathogens imported by humans from elsewhere, a form of "pathogen pollution". Native species of have no, or very little, natural immunity to these diseases. As with the great human plagues of the middle ages, this can devastate whole populations. Animals in small habitats fragmented by logging or agriculture can be wiped out. To make matters worse, many diseases are exacerbated by "environmental stressors" such as pesticides, UV, and pollutants damaging the immune system

The world is a very very complicated place, events combine in unexpected ways, and the Law of Unexpected Consequences is as draconian as it ever was. We need to be aware of this in our thinking.