Sampling in ecology is quantifying or measuring organisms’ populations in a given area. Sampling is often used to measure the biological diversity of plant and animal species and can help us estimate their density, distribution, or migratory patterns. It is important to note that not all species are equally detectable. Generally, accounting for a species is easier if the species is larger in size or has a greater abundance of individual members.
All ecological sampling surveys begin with an objective. Objectives are specific questions that the sampling survey is designed to answer. In other words, objectives provide the reason for quantifying a population in the first place.
Objectives also inform the structure of the sampling survey. The objectives dictate what will be measured, what strategies may be optimal in taking those measurements, or how much ground should be covered in the study. Without objectives, the survey data could become unfocused or useless altogether.
Biodiversity Sampling
The sampling frame is the list of the organisms (plants, animals, bacteria, or insects) of a population from which a survey is taken. Sampling frames vary between studies because the studies themselves are usually designed to answer different questions about a target population.
Target populations are the ecological resource, the species or organism, of interest. When studying a target population, ecologists have to consider the bias involved in their study. A sample is biased if all individuals from the sampling frame were not equally likely to have been detected.
A survey team commits a measurement bias if they only account for a fraction of the sampling frame. This sort of measuring error would give ecologists values that do not completely cover the scope of the sampling surveys objective, and therefore could leave out crucial data points.
Biases are the result of taking a sample that fails to include all groups of the target population. Every sampling method and every sampling survey contains some sort of bias. Meaning that not all target populations and organisms will ever be measured with equal accuracy. Sampling biases might occur because some species are easier to detect than others, or because there is a particular set of limitations to consider when sampling a given species. Understanding biases is crucial for assessing the trustworthiness of a sample.
To rectify sampling biases, ecologists may double sample. Double sampling involves randomly choosing certain species from the total survey and measuring them a second time. Double sampling tests inconsistencies and potential defects in survey analysis.
Species can only be accounted for if they are detectable, which means that surveys may detect megafauna or large vertebrates much more than insect species. Not all species are available at the same times of day, during the same seasons, or in the same distributions, researchers therefore have to survey ecological communities at specific time windows.
To maximize species richness for all species, sampling teams have to take measurements when the most members of the target population are detectable. Variables include season, weather conditions, time of day, and the presence of other species and predators. Some species (especially plants) are more abundant in specific seasons, which gives ecologists a limited time window to measure those populations. Therefore, sampling during all four seasons will offer more accurate data than sampling during one or two. Sampling populations at differential time windows widens the possibility of encountering novel organisms or distributions of previously observed organisms.
The most trustworthy samples are taken over extended periods. Samples taken at varying times offer more data about a community of organisms. Too few samples can give researchers an inaccurate representation of which organisms are present in a region, how they are distributed, or how dense their populations are. Temporal variance in data is significant because it allows ecologists to draw inferences about how ecological communities change through time. Over time, representation, representation in a community’s species richness, distribution, and general behavior can change.
When surveying the populations of an ecosystem, researchers may be motivated to answer questions such as: what is the average population size? How does the average population size change over time?
To understand how population sizes vary through time, four relevant factors should be kept in mind: 1) the birth rate of the individual organisms, 2) their death rates 3) the introduction of nonnative individuals that have migrated from a separate population, 4) the removal of individuals that migrate out of the population being observed. Using these four measures provides insight into the rate of change within a population.
Populations may have a steady rate of growth, grow exponentially or even grow exponentially up until a certain point. A population’s growth may slow, or be halted completely by environmental factors, such as the presence of predator species or the lack of resources necessary for survival.
