PSLE Science Reality Lab Vol No.243 | “Species Occupancy = 70%” — Was the Species Actually Seen at 70% of the Sites?

Stable internal ID: PSLE-SCI-REALITY-0243

Wait, what? A wildlife report says, “Estimated species occupancy = 70%.” A student translates this into, “Scientists saw the animal at 70% of the sites.”

That translation can be wrong even when the 70% figure is perfectly valid.

Wild animals are often difficult to detect. A frog can be present in a pond without calling during a survey. A bird can be present in a forest plot but hidden. A fish can be present in a lake but missed by the sampling method. Ecologists therefore distinguish presence from detection. Modern occupancy studies can use repeated surveys to estimate the probability that sites are occupied while also accounting for the fact that a present species may not be detected on every visit.

That makes occupancy a powerful Reality Lab object. It looks like an ordinary percentage, but it carries a scientific model underneath it. The 2026 PSLE Science assessment objectives require learners to interpret and analyse information, evaluate observations, information and methods, and communicate explanations and reasoning. The 2023 Primary Science syllabus also develops healthy scepticism and the ability to assess the reasonableness, accuracy and quality of scientific information.

Quick Answer

No. “Occupancy = 70%” does not automatically mean the species was visibly detected at exactly 70% of the sites.

In an occupancy study, the scientific question is usually about whether a defined site is occupied by at least one individual of the species. Because animals can be present but missed, repeated surveys can be used to estimate both:

the probability or proportion of sites that are occupied; and
the probability of detecting the species during a survey when it is actually present.

Occupancy also does not tell you how many animals live at each occupied site.

The Owned Learner Job

This article owns one narrow evidence-transfer job: how to evaluate a wildlife monitoring statement, chart or map that reports species occupancy without turning the occupancy estimate into a simple observed detection percentage or an animal count.

It does not teach ecological population modelling as a standalone topic. It does not own habitats, adaptations, biodiversity, sampling, probability or statistics. It applies existing Primary Science reasoning to one real-world scientific communication object: a reported occupancy value.

The Field Notebook: Seen Is Not the Same as Present

Imagine a class monitoring a fictional night frog at five ponds. Each pond is visited on three evenings under the same survey method. A plus sign means the frog was detected. A minus sign means it was not detected.

Pond	Visit 1	Visit 2	Visit 3
A	+	−	+
B	−	+	−
C	−	−	−
D	+	+	−
E	−	−	+

The frog was detected at least once at four of the five ponds. At Pond C, it was never detected.

Can we conclude the frog was definitely absent from Pond C?

No. A nondetection can mean two different things:

the species was genuinely absent; or
the species was present but the survey failed to detect it.

That is the key evidence problem occupancy models were developed to address. USGS explains that failing to allow for imperfect detection can bias estimates of site occupancy.

Three Different Numbers That Learners Must Not Merge

Number	What it describes	What it does not automatically describe
Detection fraction	How many surveyed sites produced at least one detection	True occupancy when detection is imperfect
Estimated occupancy	Occurrence at defined sites, often adjusted for imperfect detection	Number of animals at each site
Abundance or count	Number of individuals, or an estimate/index related to number	Percentage of sites occupied

A report can contain all three. They answer different scientific questions.

Observed, Claimed and Inferred

Suppose the report says, “Estimated occupancy = 0.70.”

Observed: a pattern of detections and nondetections across repeated surveys.
Modelled or estimated: occupancy and detection probability under the stated design and model.
Supported claim: the study estimates occurrence across the defined sites at the stated level.
Unsupported rewrite: “The species was physically seen at exactly 70% of sites.”
Another unsupported rewrite: “70% of all animals were found.”
Another unsupported rewrite: “Every occupied site contains the same number of animals.”

The scientific habit is to keep the observation layer and the inference layer separate.

Why Repeated Visits Matter

One visit gives only one detection opportunity. Repeated visits create information about detectability.

Imagine the species is detected at Pond B on Visit 2 but missed on Visits 1 and 3. Because the species was detected at least once, we know it can be present even when a survey returns a minus sign. That is direct evidence that nondetection cannot always be treated as absence.

Repeated visits do not magically solve every problem. The visits must still be comparable enough for the model and the ecological question. Weather, season, observer skill, survey duration and animal behaviour can affect detectability. But repeated detection histories give scientists more information than one visit per site.

Worked Case 1: The Quiet Frog

A monitoring team surveys ten ponds three times. The frog is detected at six ponds at least once. A model that accounts for imperfect detection estimates occupancy at about 0.75 for the defined pond sites.

Which statement is strongest?

“The frog was seen at 75% of ponds.” — too strong, because 75% is the occupancy estimate, not the raw detection fraction.
“The frog was seen at six of ten ponds, and the occupancy analysis estimates that about three-quarters of the defined pond sites are occupied.” — better.
“Exactly 75 frogs live in the area.” — unsupported; occupancy is not abundance.

Worked Case 2: Same Occupancy, Different Abundance

Region A has ten survey sites. Seven are occupied, and each occupied site contains roughly one or two animals. Region B also has seven occupied sites, but several sites contain large groups.

The two regions can have similar occupancy while having very different numbers of animals.

Therefore a headline such as “occupancy unchanged” does not automatically mean population size is unchanged. The communication object names site occurrence, not total abundance.

Worked Case 3: Same Detections, Different Detectability

Two teams each detect a bird at five of ten sites.

Team X surveyed at dawn when the bird sings loudly. Team Y surveyed at midday when the bird is much harder to hear.

The same raw detection fraction does not necessarily imply the same true occupancy. If detectability differs, the evidence needs to be interpreted with the method in view.

This is a classic PSLE Science habit in a real-world setting: if a method changes how easily an outcome can be observed, the observed results cannot be compared casually.

Worked Case 4: The Site Itself Changes

Study P defines one site as a 100 m × 100 m forest plot. Study Q defines one site as a whole 10 km grid cell.

Can you compare “70% occupancy” from the two studies as if they measured the same thing?

Not automatically. USGS research notes that occupancy depends on how a site is defined, including plot size. A larger site gives an animal more space in which to be present. The meaning of “occupied site” therefore depends on the sampling unit.

The Representation Check: Map Colour, Dot or Number?

Occupancy can be communicated in several forms:

a single overall percentage;
a map of estimated occupancy probabilities;
dots showing actual detections;
a table of detection histories;
a graph comparing occupancy across years.

Do not treat these as interchangeable.

A detection dot means the species was detected at that place and survey. A coloured occupancy surface may represent an estimate. A site with no dot can still have non-zero estimated occupancy if the model and other evidence allow for missed detection.

The Baseline Check: Occupancy of What Sites?

Before interpreting 70%, identify the denominator.

70% of which survey sites?
How large is each site?
Were all sites inside the same habitat type?
Were sites selected across the whole region or only convenient places?
Does the percentage refer to this season, this year or a longer period?

A percentage without its site definition is an incomplete scientific object.

The Method Check: Could a Present Animal Be Missed?

Ask what the survey actually does.

Does an observer listen for calls?
Does a camera need the animal to pass in front of it?
Does a trap sample only part of the habitat?
Does the survey happen at the time when the species is active?
Could weather reduce visibility or sound?
Are repeated visits independent enough to be informative?

If the answer to “Could it be present but missed?” is yes, then nondetection needs cautious interpretation.

Do Not Overcorrect: A Detection Still Matters

Healthy scepticism is not permission to ignore observations. If a reliable survey detects the target species, that is evidence of presence at that time and location under the survey definition.

The asymmetry is important:

a valid positive detection can establish presence for that observation;
a nondetection does not always establish absence when detection is imperfect.

Science becomes stronger when we understand exactly what a positive and a negative observation can support.

Alternative Explanations for Fewer Detections

Suppose detections fall from 60 sites last year to 40 sites this year. That might mean occupancy declined, but consider other explanations before deciding:

the species may be less detectable this year;
survey timing may differ;
weather may differ;
survey effort may be lower;
observers or equipment may change;
site boundaries may change;
the species may genuinely occupy fewer sites.

The purpose of alternatives is not to avoid conclusions forever. It is to identify which evidence would distinguish the explanations.

What Evidence Strengthens an Occupancy Claim?

Sites are clearly defined.
Site selection is appropriate for the intended region or population.
Repeated surveys allow detectability to be evaluated.
The survey method is consistent or method changes are accounted for.
The season and observation period are stated.
Detection histories are preserved rather than collapsed too early.
The report distinguishes detections from occupancy estimates.
The uncertainty of the estimate is communicated.

What Weakens the Claim?

One visit per site is treated as perfect proof of absence when nothing is seen.
The site size or denominator is not stated.
Survey effort differs greatly between groups.
Occupancy is described as if it were animal abundance.
A modelled occupancy value is presented as the raw percentage of sites with detections.
Maps of habitat suitability are confused with occupancy observations.
A change in detection method is ignored.

How Far Can the Conclusion Travel?

A careful conclusion might be:

Using repeated survey data and a method that accounts for imperfect detection, the study estimates that about 70% of the defined sites were occupied during the study period.

That does not automatically tell you:

that the species was physically seen at 70% of sites;
how many animals lived at each site;
how many animals lived in the whole region;
whether every apparently suitable habitat patch was occupied;
whether occupancy will remain 70% next year.

Each of those is a new question requiring more evidence.

Tempting but Invalid Reasoning

“No animal was seen, so none was there.” Not if the method can miss present animals.
“Occupancy 70% means 70% of the animals were found.” Occupancy concerns sites, not the fraction of animals detected.
“Two regions have the same occupancy, so they have the same population size.” Occupied sites can contain very different numbers of animals.
“A larger site and a smaller site are equivalent sampling units.” Site definition changes the meaning of occupancy.
“A habitat suitability map proves occupancy.” Suitable conditions and actual occurrence are different evidence objects.
“Accounting for imperfect detection makes the estimate exact.” A model can reduce a known bias while still carrying assumptions and uncertainty.

PSLE-Style Transfer Case: The Hidden Beetle

A class checks eight logs for a nocturnal beetle on three evenings. The beetle is detected at five logs at least once. A report later gives an estimated occupancy of 0.70 after accounting for imperfect detection.

A pupil writes, “The beetle was seen at 70% of the logs.”

Improve the answer:

The beetle was directly detected at five of the eight logs during the surveys. The reported 0.70 occupancy is an estimate of site occurrence that also considers the possibility of missing the beetle when it is present, so it should not be rewritten as the raw percentage of logs where it was seen.

Practice: Detection or Occupancy?

Practice 1

A bird is detected at a site on one of four visits. Was the site occupied during at least part of the survey period?

Explained answer: A valid detection provides evidence of presence. The three nondetections show that detection was not perfect on every visit.

Practice 2

No frog is heard on one night. Can absence be concluded?

Explained answer: Not automatically. We need to know how likely the survey is to detect a frog that is present and whether repeated surveys provide more evidence.

Practice 3

Two forests both have 80% occupancy. Must they contain the same number of birds?

Explained answer: No. Occupancy reports occurrence across sites, not the number of individuals at each occupied site.

Practice 4

A map is coloured green where habitat is suitable. Can every green cell be counted as occupied?

Explained answer: No. Suitability and observed or estimated occupancy are different. Survey evidence is needed to support occurrence claims.

Delayed Independent Return: The Doorbell Test

Later, imagine you ring a friend’s doorbell and no one answers. Does that prove nobody is inside? Perhaps not. The person may be wearing headphones or away from the door.

Now translate carefully. Wildlife surveys are not doorbells, and animals are not people in houses. The analogy carries only one evidence structure: failure to detect something does not always prove absence when the detection process is imperfect.

Route to Existing Canonical PSLE Science Owners

Parent and Tutor Teaching Guide

Use a simple hidden-object game rather than a wildlife capture activity. Place a few paper animal cards under opaque cups. Let a learner inspect each cup using a rule that sometimes misses the card—for example, viewing through one small opening without moving the cup. Repeat the inspection several times.

Ask two questions separately: “Where did you detect the card?” and “Where might a card still be present despite nondetection?” The goal is to make detection error visible without pretending the classroom game reproduces a professional occupancy model.

Then introduce the word occupancy. Keep the language modest: scientists can use repeated survey patterns to estimate site occurrence while accounting for imperfect detection. Do not ask the learner to calculate advanced model parameters. The PSLE Science job is to understand what the reported number does and does not mean.

Authoritative Sources

Quiet Return

The next time a wildlife graphic says “occupancy = 70%”, do not turn the number into a sighting count.

Ask: What was detected, what could have been missed, how was a site defined, and is this number about occurrence or abundance?

That is the Reality Lab habit: respect the observation, respect the model, and never make either one pretend to be the other.

Small Group Tutorials