It is easy to agree that 'diversity' is valuable, but what kind of value is it exactly?
Take linguistic diversity, passionately defended by anthropologists and linguists who want to preserve all 6,500 thousand spoken languages from extinction. If you look carefully at their arguments, they are all about the usefulness of languages and not about the value of diversity per se. For example, it is claimed that minority languages contain useful local knowledge about plants, animals, ecosystems and so on which would be lost if those languages died out. Or that minority languages are important for individual self-esteem and community functioning among ethnic minorities, which in turn supports their flourishing and well-being.
However, if the value of diversity lies in what it does for us rather than what it is in itself, then whether and how far diversity is valuable (or even disvaluable) turns out to depend on the context. Diversity has no value in its own right; it is merely one tool among others that we may use to achieve things that do matter. For example, if we are worried that important botanical knowledge achieved over many generations may be lost as the last people to speak the language that encodes it die off, then it makes sense to send academic botanists to collect that knowledge and publish it in a universally accessible scientific database. But it makes little sense to try to preserve that knowledge by somehow getting more children to learn the language and then continue living in the same precarious relationship to the natural ecosystem in which that specific knowledge would be needed and communicated. That confuses the substrate with the information it contains. It would be like trying to preserve knowledge of Euclidean geometry by teaching everyone ancient Greek.
Likewise, indigenous peoples around the world suffer from high levels of social and psychological dysfunction, and arguably this is related to a loss of cultural identity. Supporting language preservation might be one way of helping such peoples. However, it is likely that multiple other factors are also responsible for their problems, including a traumatic history of abuse by governments and enduring political and economic exclusion and neglect. According to the standard set by the values actually at stake, language preservation is not the only way to help. Indeed, some forms of language preservation may well be disvaluable in so far as they perpetuate the political and economic isolation of indigenous people.
The same goes for diversity in other domains, such as the number of species or the different shapes and colours of humans employed by a company. Biodiversity is said to be important because it allows us to take advantage of the outcomes of numerous evolutionary pathways that we did not follow (for example, more efficient photosynthesis pathways or antibiotics) that would be imaginatively or materially expensive to reproduce. Workplace diversity is said to be valuable because it promotes higher productivity in certain creative problem solving contexts (although see elsewhere where I have argued that its real value is demonstrating fairness).
Some things, like happiness, are intrinsically valuable, meaning that they are valuable in themselves and not for the sake of anything else. Other things, like money, are only instrumentally valuable, meaning that they are only valuable because they are useful to achieving things that do matter in themselves. Note that value is not in the eye of the beholder. Only a fool would value money for itself rather than for its functions as a means of exchange and store of purchasing power. This simple picture is complicated by the fact that some things combine instrumental and intrinsic value, meaning that we have two sources of value for them. Friendship may fall into this category, and knowledge, since these are both wonderful in themselves and also help us to achieve other intrinsically valuable things like happiness.
It is clear that diversity is instrumentally valuable. Is it possible that it is also intrinsically valuable, like friendship or knowledge? In that case the value of diversity would be made up of an intrinsic invariable component plus an instrumental component that is contingent, i.e. varies with the circumstances.
Value of Diversity = Intrinsic Value (invariable) + Instrumental Value (contingent)
If that were the case then more diversity would be more valuable and worth pursuing even if it didn't have a positive impact on other intrinsically valuable things (so long as the contingent effects of more diversity didn't turn out to have a net negative impact).
However, I don't think it is the case that diversity in itself has any intrinsic value, i.e. regardless of what it is diversity of and how that relates to other real valuable things. I don't think a world with more variety is automatically better than a world with less.
Imagine a scenario in which you have 10,000 different kinds of something (species, say). It is common for diversity enthusiasts to focus on the supposedly terrible loss we would experience in moving from 10,000 to only 9,999 different species. But is it actually the loss of diversity that we are moved by here, or something else?
First, if diversity were valuable in itself then you should not only believe that a world with 9,999 species would be worse than one with 10,000. You should also believe that, ceteris paribus (other things being equal), a world with 10,001 different species would be better, 10,002 would be even better, and so on. In other words, if diversity is intrinsically valuable then you should should want to not merely prevent the loss of existing kinds, but deliberately produce more species (which shouldn't be particularly complicated or expensive, especially for simpler organisms). Failing to produce more of a good thing is bad in just the same way as failing to prevent the loss of a good thing.
Second, diversity is a meta-property, i.e. a property of sets of things rather than of the things themselves. Therefore you ought to be indifferent to the particular items which make up the set and whether they survive. You should prefer a world in which one existing species was destroyed but two new ones came into existence to a world in which the original 10,000 species persisted.
Those who disagree with these conclusions may have to acknowledge that the loss of a single species is not important because it represents a loss of the meta-property called diversity but for other quintessentially conservative reasons connected with its particularity not generality (i.e. its contingency). The loss of a species or a language then resembles the loss of a life: a particular thing with a particular history and connections to other things has ended and it leaves a hole where it used to be that will not be made whole by the creation of new lives. But that of course has nothing to do with diversity.
Note: This essay was inspired by Peter Wells' Saying The Unsayable About Saving The Unsavable on 3 Quarks Daily