Biodiversity: beyond counting species
According to a definition from the United Nations 1992 Convention on Biological Diversity, biodiversity is “variability among living organisms from all sources [and] includes diversity within species, between species and of ecosystems.” And yet conservation organisations often quantify the ongoing biodiversity crisis by loss of species. So why doesn’t the number of species reflect variety in nature?
What influences diversity?
Biodiversity results from a dynamic balance between extinction, migration and speciation (the creation of new species) – an equilibrium that’s disrupted by human activities such as overexploitation of natural resources. But those artificial causes aside, geography has a big influence on why some places are more diverse than others.
How is biodiversity distributed?
One notable pattern is an increasing gradient in diversity with latitude (from poles to tropics), which suggests that life has an ideal temperature range. There’s also an uneven distribution among freshwater, marine and terrestrial habitats.
About 15 per cent of macroscopic species (visible to the naked eye) are aquatic, while 85 per cent live on land. This discrepancy is explained by three factors. First, productivity: land plants produce more energy to power ecosystems if they absorb sunlight in the open (unfiltered by water). Second, habitat complexity: rainforests and reefs are biodiversity hotspots because plants and corals form 3D structures that become homes to other organisms. Third, physical differences between media: water is denser than air, for instance, so animals expend more energy moving through it.
What’s wrong with counting species?
The number of species in a particular place, ‘species richness’, has several shortcomings. One problem is that it only reflects diversity at a low level of biological classification (taxonomy). So if I said one area contains two species and another has three, you might conclude that the second is more diverse. But if all three species in the second area are in the same family (let’s say lion, cheetah, domestic cat) and the first contains two different families (cat, dog) then you could argue that the place with fewer species has greater diversity.
Another issue with simply counting species is it doesn’t consider individuals – the diversity within each species. A population with few members has a small gene pool, which gives natural selection less raw material – distinct individuals – if an environmental change (such as a new disease) forces the population to adapt or go extinct. So a habitat is more diverse when the relative abundance of individuals living there is similar among species, known as ‘species evenness’.
So how is diversity measured?
Clearly ecologists don’t collect every individual of each species from an area. Instead, they extrapolate from samples, which might involve identifying species in video recordings, reading DNA from the environment or even acoustic surveys. But sampling alone is vulnerable to bias because numbers of recorded species are higher when an area is large or more time is spent there, and so scientists use statistical models to estimate species and individuals. That gives an ‘index’ that combines richness and evenness. An index is an abstract figure, however, which explains why conservationists still publicise numbers of species.
Why does biodiversity matter?
Just as diversity within species (related to evenness) offers a population the potential to adapt to change, diversity between species (greater with richness) gives an ecosystem better resilience. If one prey species becomes unavailable, for example, a predator can find an alternative source of food.
Unfortunately, we humans don’t really preserve biodiversity, we protect specific species – charismatic creatures with aesthetic appeal, such as pandas, even though vertebrates are only one of the 32 phyla (a high taxonomic level) that make up the animal kingdom. The rest are invertebrates and include insect pollinators vital to plants, the producers that energise almost all of Earth’s ecosystems. Because we’re selfcentred, the best way to protect biodiversity itself may be to promote its benefits to ourselves – ‘ecosystem services’ that supply things like food, fuel and pharmaceuticals.