Richness, abundance, density…and other ways to argue about biodiversity
An opinion piece, written by Ben Youngman, WAI’s Science and Research Lead.
International Day for Biological Diversity tends to arrive with a familiar parade of charismatic megafauna. Whales breach. Pandas chew bamboo with suspiciously good media training. Somewhere, a tiger stares moodily into the middle distance. Meanwhile, beneath the surface of Lake Wānaka, a freshwater invertebrate the size of a fingernail quietly goes about holding an ecosystem together with absolutely no expectation of applause. Typical.
Biodiversity in the Upper Clutha is often framed through what we can easily see: braided rivers, alpine wetlands, kānuka shrublands, or if you’re lucky – the occasional kererū behaving like it has discovered fermented fruit again. Yet some of the region’s most important biodiversity exists out of sight, in freshwater ecosystems teeming with microscopic algae, aquatic insects, native fish, fungi, and microbial communities that collectively keep lakes and rivers functioning.
And “functioning” is doing a lot of heavy lifting here. Freshwater biodiversity is not just an ecological accessory. It regulates nutrient cycling, supports water clarity, stabilises food webs, and underpins the resilience of ecosystems facing warming temperatures, invasive species, and land-use change. In plain terms: these organisms are the unpaid operations staff of the catchment. Remove enough of them, and the whole system starts behaving like a board meeting with no agenda.
The trouble is that biodiversity itself has become a strangely slippery concept. The term is used widely across policy, business, and consulting, but often, people mean entirely different things when they say “improved biodiversity.”
Take species richness, abundance, and density. They sound interchangeable. They are not.
Species richness is simply the number of different species present in an area. Think of it as the length of the guest list. A stream with ten aquatic insect species is considered richer than one with five. Useful, yes, but incomplete. A lake can appear biodiverse because many species are technically present, even if most exist only in tiny numbers while a handful of tolerant species dominate the system.
That is where abundance comes in. Abundance measures how many individuals of each species are present. You may have high richness, but if one pollution-tolerant species dominates while everything else hangs on by a thread, the ecosystem is sending a fairly clear distress signal.
Density adds another wrinkle. It measures the number of organisms within a defined area or volume. In freshwater ecology, density matters because organisms are not spread evenly across landscapes. One square metre of healthy streambed can contain hundreds of macroinvertebrates. Another nearby patch may resemble an ecological ghost town.
Here is the awkward part: depending on which metric you choose, you can tell entirely different stories about environmental outcomes. A restoration project might increase abundance but reduce richness. A lake may retain similar richness while losing sensitive species and becoming dominated by generalists. Technically, biodiversity remains. Practically, ecological integrity is quietly eroding in the background.
This is why biodiversity debates are increasingly shifting toward the need for common language, standardised metrics, and ecosystem function-based assessments. Counting species alone is no longer enough. We need to understand interactions, resilience, and whether ecosystems are still capable of doing the jobs society depends on them to do.
For the Upper Clutha, this matters enormously. These lakes are globally rare deep-water ecosystems, but they are not immune to pressure. Stormwater runoff, warming temperatures, invasive species, and catchment development all chip away incrementally. Rarely does biodiversity collapse all at once. More often, ecosystems are slowly negotiated into mediocrity.
International Day of Biological Diversity is therefore less about celebrating nature as a postcard and more about recognising biodiversity as infrastructure. Living infrastructure. Complex, dynamic, occasionally inconvenient infrastructure. And unlike roads or pipes, this infrastructure evolved over millennia for free. Which, in economic terms, is usually the point at which humans become dangerously complacent.
This International Day of Biological Diversity, perhaps the challenge is not simply to protect more biodiversity, but to understand it better. Because if we cannot agree on what healthy ecosystems actually look like, measuring progress becomes an unhelpful and fragmented process.
The Upper Clutha and the Queenstown Lakes District have an opportunity to lead differently: by combining community action and local knowledge, rigorous science, and clearer ways of measuring the living systems that sustain us.
If you want to find out more about what biodiversity there is in the Upper Clutha – check out Wilderlab and their interactive Environmental DNA (eDNA) map!