What are the sound reasons (let's put aside the unsound reasons) why protecting or increasing biodiversity takes us further down the road to sustainability?
Are there real-world circumstances whereby higher biodiversity might make sustainability harder to achieve?
It can be observed from many past events that low levels of biodiversity such as mono cultures are susceptible to total destruction by a change in circumstances. The Irish potato famine is one familiar example.
High Biodiversity implies that there are more potential solutions to survival for new scenarios in the future.
High biodiversity also implies that more information about survival is stored in DNA that is active in the ecosystem.
Therefore the more biodiversity we have the more likely it will be for life to survive future unpredictable events.
I think the obvious counterargument is that if we all grow the most efficient crops (i.e. with the most outputs to the fewest inputs), we can achieve sustainability easier and therefore biodiversity gets in the way of sustainability the way localizing the economy does.
The problem though is that efficiency comes at the cost of safety margins and safety mechanisms. The Irish Potato Famine occurred because everyone was growing the same high yield strains of potatoes, because other crops were out of reach of the Irish peasants. While this was a very energy efficient solution, it wasn't sustainable. Therefore one of the concerns regarding sustainability needs to be robustness and this is where diversity comes in and mitigation of risks even if this comes with the loss of efficiency, at least if we define sustainability not to include massive famines or the like.
Diversity is the key mitigation strategy and it has two important components. First, highly diverse systems help control pests the way we currently use pesticides (this is a key strategy of permaculture for pest control btw). Secondly they limit damage because species-specific pests have greater trouble spreading.
A second important issue has to do with evolutionary ecology. One of the key things to note is that ecological niches are not static but in fact change as environmental factors change. Let's look at gypsy moths for example. During an outbreak of these pests one thing that is observed is that predators specialize in ways they wouldn't before. Animals that might eat gypsy moths in all stages may go for what they might find easier to catch, whether it is the actual moths (possibly bats), the caterpillers (some birds), or the like. As the outbreak subsides, many of these will actually re-generalize. Diversity of predators thus provides layers of protection in the case of pest outbreaks.
