Biodiversity and the number of populations within an ecosystem are deeply intertwined: more populations generally support greater biodiversity, and greater biodiversity helps stabilize the populations that exist. But the relationship runs in both directions and operates through several distinct mechanisms, from competition for resources to genetic exchange between isolated groups. Understanding how these two concepts connect explains why losing populations, not just entire species, can unravel ecosystems faster than most people realize.
Biodiversity vs. Population Size: Key Distinctions
Biodiversity typically refers to the variety of species in a given area (species richness), while the number of populations describes how many distinct groups of organisms exist, including multiple populations of the same species spread across a landscape. These are related but separate measurements. An ecosystem can have high species richness but low population counts if each species exists as a single, isolated group. Conversely, a single species can form dozens of populations scattered across patches of habitat.
Within most natural communities, a few species make up the majority of individuals while many species are relatively rare. This pattern holds across ecosystems worldwide. Widespread species tend to occur at higher densities than geographically restricted ones, meaning population size and distribution are closely linked. In structurally simple habitats like salt marshes, species abundance is highly unequal: a handful of species dominate. In complex habitats like fen wetlands, the same total number of individuals may be distributed more evenly across twice as many species. Habitat complexity, in other words, shapes both how many species can coexist and how their populations are distributed.
How Populations Enable Coexistence
The number of populations an environment can support depends largely on how species divide up available resources, a process called niche partitioning. When species compete for the same resources, the stronger competitor tends to eliminate the weaker one. But if species can carve out slightly different roles, stable coexistence becomes possible. The key condition is that competition within a species must be stronger than competition between species. When individuals of the same species compete with each other more intensely than they compete with neighbors of a different species, neither species can drive the other to extinction.
A clear example comes from burying beetles, where multiple species coexist by specializing in different-sized animal carcasses. Larger species dominate bigger carcasses through physical combat, while smaller species are pushed toward smaller carcasses. Each species occupies a distinct size niche, and this partitioning allows several populations to persist in the same habitat. The more ways resources can be divided, the more populations (and species) an area can sustain. This is why tropical forests with their enormous structural complexity support far more species than a grassland of the same size.
The Metapopulation Effect
Many species don’t exist as one large population but as a network of smaller, semi-isolated groups called a metapopulation. The number of these sub-populations has a direct effect on whether a species persists over time. When a local population goes extinct due to disease, drought, or random bad luck, it can be recolonized by individuals dispersing from neighboring populations. More populations in the network mean more sources of recolonization and a lower chance that the entire species blinks out at once.
The density and spacing of habitat patches strongly influence this dynamic. Populations that are close enough for occasional movement between them benefit from shared genetic material and demographic rescue. Populations that are too isolated lose both advantages and face higher extinction risk. This is why habitat fragmentation is so damaging: it doesn’t just shrink habitat, it severs the connections between populations that keep the whole network viable.
Genetic Health Depends on Connected Populations
The number of populations a species maintains across a landscape has a surprisingly strong effect on its genetic health. Small, isolated populations lose genetic diversity over time through random drift, and that loss increases vulnerability to disease, environmental change, and inbreeding. Research on declining kit fox populations found that even when a population’s effective size dropped to concerning levels, genetic diversity held steady, and the likely reason was low but consistent immigration from adjacent populations.
Even one migrant per generation can maintain local genetic diversity at levels comparable to the entire metapopulation. This finding underscores a practical point: a species with ten connected populations is in far better genetic shape than one with a single population of the same total size. Conservation efforts increasingly focus on maintaining landscape connectivity, ensuring that corridors exist for individuals to move between populations, rather than simply protecting the largest single group.
Why Population Loss Matters Before Species Loss
Monitored wildlife populations have decreased by an average of 73% between 1970 and 2020, according to the Zoological Society of London’s Living Planet Index. That figure represents population decline, not species extinction. Many of these species still exist, but with far fewer individuals in far fewer places. This distinction matters because ecosystems start losing function well before a species disappears entirely.
The relationship between biodiversity and ecosystem function follows a pattern: as you add species, ecosystem processes like pollination, decomposition, and nutrient cycling improve rapidly at first, then level off. This plateau creates what ecologists call functional redundancy. Multiple species perform similar roles, so losing one or two doesn’t immediately degrade the system. But this buffer only works above a critical threshold. Once enough populations and species are lost, the redundancy disappears, and losing even one more species can cause a sharp drop in ecosystem function. The danger is that ecosystems can appear stable right up until they cross that threshold.
Area, Fragmentation, and Species Counts
One of the most robust patterns in ecology is the species-area relationship: larger habitat areas support more species, following a predictable mathematical curve. But total area alone doesn’t tell the whole story. The same amount of habitat broken into scattered fragments supports fewer species than a single contiguous block, because fragmentation increases the ratio of extinction to colonization for individual populations.
This is where population number becomes the bridge between habitat and biodiversity. A large, connected habitat supports many populations of many species, each large enough to be resilient. Fragment that habitat, and you create smaller, more isolated populations with higher extinction rates. Species with traits that make them poor dispersers, those that can’t easily move between fragments, lose populations first. Over time, fragmented landscapes trend toward lower biodiversity even if the total habitat area remains unchanged.
Conservation Thresholds and Minimum Viable Populations
Conservationists use the concept of a minimum viable population (MVP) to estimate the smallest population size that gives a species a reasonable chance of surviving over a defined period, typically 100 years or more. A meta-analysis spanning 30 years of published MVP estimates found that this number is highly context-specific. Body mass, geographic range, reproductive rate, and the degree of environmental variability all influence the threshold. There is no universal magic number that applies across species.
What the research consistently shows, though, is that small and range-restricted populations face dramatically higher extinction risk. Species reduced to a single population, no matter how large, are more vulnerable than species spread across multiple populations in different locations. This principle drives modern conservation planning toward protecting networks of populations rather than pouring all resources into a single reserve. Maintaining multiple populations across varied habitats hedges against localized catastrophes and preserves the genetic and behavioral diversity a species needs to adapt over time.