The standard expectation is that ecological development produces both taxonomic and functional diversity. More species means more functions, because each species brings its own metabolic capabilities. This assumption connects species counts directly to ecosystem capability: lose species, lose functions.
Sveen and colleagues tracked soil microbial communities across chronosequences — sites of different ages developing from the same starting conditions. As ecosystems matured, functional diversity increased. Taxonomic diversity decreased. The two measures, assumed to track together, diverged.
The mechanism is specialization through redundancy loss. In young soils, many different species perform the same functions. The community is taxonomically rich but functionally redundant — multiple species decompose the same substrates, fix nitrogen through the same pathways, cycle the same nutrients. As the ecosystem develops, competitive dynamics eliminate redundant species. The survivors are those occupying unique functional niches. The community becomes more efficient — more distinct functions per species — but each function is now performed by fewer species.
The system gains capability while losing the buffer that protects capability. A young soil with ten species decomposing cellulose can lose three of them and still decompose cellulose. A mature soil with one cellulose specialist loses that function entirely when the specialist is lost. The vocabulary of the system expands. The insurance against any word disappearing contracts.
This is a general property of optimization under competition. The competitive process that produces specialization — the engine of functional diversification — simultaneously strips redundancy. Development does not proceed from fragile to robust. It proceeds from robust and limited to capable and fragile. What looks like ecosystem maturation is also ecosystem exposure. The system becomes better at more things and less able to survive the loss of any one of them.