Abundance vs. occupancy
Abundance asks how many individuals occur in a defined area or population; occupancy asks what proportion of defined sites are occupied or used. Neither is the raw count alone when individuals or occupied sites can be missed during sampling.
Scope: Wildlife and plant surveys across defined sites and sampling periods; abundance and occupancy estimates depend on design, scale, closure assumptions, and detection · Last updated
Ask two different questions
A colony may contain many birds at one site but occupy few sites across a region; another species may occur sparsely at most sites. Abundance describes individual numbers or density, whereas occupancy describes distribution among sampling units. The size and placement of those units can change the occupancy value even when the organisms have not moved. [1][2][5]
Separate state from observation
A species can be present yet undetected because it is quiet, hidden, inactive, temporarily away, or poorly sampled by the method. Occupancy models use repeated detection and nondetection histories to estimate detection probability alongside occurrence. Count models likewise need to consider whether every individual has the same chance of being observed. [1][3][4]
Match effort to the estimate
Occupancy surveys usually require many defined sites and repeat visits within a period when occupancy is assumed not to change. Abundance can require counts, distance sampling, marked individuals, or other designs with additional assumptions. Standardizing duration, area, observers, weather, and methods makes comparisons more defensible but does not automatically eliminate bias. [1][4][5]
Interpret trends at the same scale
Falling local counts with stable occupancy can signal thinning across a still-broad range, while increasing occupancy can reflect colonization without high local numbers. Compare like sites, seasons, and methods, report uncertainty, and preserve nondetections and effort. Opportunistic sightings are valuable records but do not by themselves estimate either population abundance or site occupancy. [2][3][4][5]
Related guides
Identify it and save the field note.
Where this guide comes from
Source-checked editorial guide. Last updated . This guide teaches identification and field skills; it is not a substitute for expert verification when it matters.
- U.S. Geological Survey — Occupancy models to study wildlife ↗
- U.S. Geological Survey — Estimating abundance from presence-absence data ↗
- U.S. Geological Survey — Multispecies occupancy modeling ↗
- U.S. Geological Survey — Integrating count and detection-nondetection data ↗
- U.S. Geological Survey — Dynamic occupancy models and survey design ↗
