09 sample survey strategies

sample survey advantages

• cost effective and efficient

• can gain insights into the broader population trends and patterns

non probability sampling

• convenience based

• judgement based

• targeted sampling

simple random sampling

each unit has an equal chance of being selected

systematic sampling

samples taken at regular intervals after a random start

stratified sampling

the population is divided into strata, and random samples are taken from each

cluster sampling

the population is divided into clusters, and random samples of clusters are taken

simple random sampling process

• identify all possible sampling units and randomly select a pre-determined subset #

• units size can vary

• most appropriate when habitat is homogenous

• simple and easy to implement

• provides unbiased parameters

• could produce bias in situations where animals or habitats features of interest are heterogenous

stratified random sampling process

• population of sampling units is divided into strata and samples are randomly taken from each strata

• most appropriate for when the habitat and/or organisms of interest are occurring in clear groups or environmentally derived pattern

• improves statistical precision by accounting for known variability and ensures representation from all strata

• does require a prior understanding of strata

systematic sampling process

• samples taken at regular intervals after a random starting point

• common in habitat studies

• easy to implement and ensures even coverage

• can introduce bias if measured variables can a cyclic or other pattern to is distribution

cluster sampling process

• population is divided into clusters, and random samples of clusters are taken

• used when animals are naturally grouped

• cluster size can affect precision

• reduces effort and cost when population is large and easier to implement when it is widely spread

• increases variability between clusters, reducing precision

fixed radius plot

• cruising timber method

• record DBH, species, and other variables of all trees within a fixed area

• allows the direct calculation of

  • stem density

  • basal area

• typically extrapolated to represent per-acre or per-hectare estimates

• time and labor intensive

variable radius plot

• cruising timber method

• size of your vegetation plot varies based on the tree size

• larger diameter plots for large trees, smaller diameter plots for small trees

• prism or angle gause is used to determine if a tree is inside or outside plot boundaries

plot based disadvantages

• labor intensive and time-consuming

• limited spatial coverage

• potential for bias

• habitat disturbance

• inflexibility

plotless sampling

• estimate population parameters based on distance measurements

• used to assess the density of natural resources without the need for traditional plots

line intercept sampling

• used to estimate the coverage and other parameters of 2-d objects within a study area

line transect assumptions

• objects on the line are detected 100%

• animals are counted only once

• observers do not influence animal movements

• objects are fixed at point of initial observation

• distances are measured accurately

• transect lines are located probabilistically

point count survey

• standard method used to estimate the abundance and diversity of a bird species within a specific area

• observers stand at predetermined points and count all birds seen or heard within a fixed time period and distance

point and count advantages

• relatively simple to conduct

• provide quantitative data on bird populations

• can be repeated over time to monitor changes

• non-invasive

camera trapping

• non-invasive technique used to study and monitor wildlife populations

• involves the placement of automated camera systems in the field to capture images or videos of animals as they pass by

camera trapping advantages

• cost effective, non-invasive

• allows for long-term monitoring of wildlife populations without disturbing the animals

• provides visual record of species and can capture rare or elusive species

camera trapping limitations

• camera placement

• battery life

• image quality

• may not be suitable for all species/habitats

• data analysis can be time-consuming

capture-recapture studies

• involve sampling the population of interest two or more times

• each captured animal uniquely marked to estimate abundance and other population parameters while accounting for imperfect detection

time to event models

• use data from radio-tagged individuals to estimate abundance, survival, recruitment, and account for detectability

• making assumptions about the random sampling and independence of survival times

occupancy modeling

• focus on estimating the proportion of suitable habitat occupied by the species of interest

• requiring assumptions about site closure and independent detections

• provides a less labor-intensive alternative to abundance estimation

double sampling

• easy to measure or inexpensive indicators are measured across a large subset of experimental units then more rigorous or expensive indicators are measured on a smaller subset

• usually look for a strong correlation between the two, allowing us to extrapolate and increase confidence and inferential scop

• can be adapted to plot, point, or line-based surveys