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Bluegill are a type of sunfish that can be found in freshwater lakes of North America. Bluegill have an intricate predator-prey relationship with bass, an apex predator in many lakes. Bass eat young bluegill until bluegill become large enough and fast enough to evade the bass. The environmental conditions for the best bluegill populations is a delicate balance of numerous factors. Conservation groups and fishing enthusiasts would want to know the effect on the size of both populations of fish cohabiting the same lake.

Ecology is the scientific study of the relationship between organisms and their living and nonliving environments. Unlike studies done in the lab, ecological studies require the researcher to sample the organism being studied in its original environment (a lake in the case of the bluegill). This comes with a whole set of challenges. One of them is to make sure that the organisms being sampled provide a good picture of the overall population. This can be important in situations where you cannot be sure that all organisms in a population are sampled (imagine trying to capture every mosquito around a lake or finding every camouflaged moth in a forest). Sampling uses specific techniques that should provide a more unbiased estimate of a population.

Sampling Methods

Different sampling methods are used depending on the kind of organism being sampled, for example, whether it is nonmoving, slow moving, or fast moving. The method to choose depends on how the organism you are studying is spaced throughout its habitat. A researcher should consider what they want to learn about the organism they are working with. These include abundance (individuals per total sample), population size, density (individuals per specific area, and species distribution (how the organism is distributed across the area being studied).

There are four commonly used sampling methods. Point sampling is used for species that are easy to spot or widely spaced.

While it can be used for stationary species, it can also be used for mobile species (e.g., by counting individuals that move past a camera). Line or belt transect methods are used for species that are not evenly dispersed throughout the environment. This method is best for stationary or slow-moving species. Quadrat sampling is good for determining the density of a population by sampling small areas and counting every organism inside that area. It can also be used to determine abundance. Finally, the mark-and-recapture technique is best for situations when it is difficult to directly count every individual, as with fast-moving organisms.

The next challenge with sampling is how to distribute your sampling locations in that habitat. Systematic sampling creates a grid and takes samples at regular intervals. Random sampling creates a random pattern of samples and works well for species where individuals move around a lot. Stratified sampling uses a random sampling method, but breaks up the population to be sampled into sections or strata, for example, based on habitat.

Population Size and Environmental Factors

Populations constantly change in response to changes in their environment. Estimating population size before and after those environmental changes allows scientists to understand how populations respond under specific conditions.

Numerous factors can affect a population's size. These factors can affect the carrying capacity of a habitat - how many organisms of a given species it can support. These factors include:

• Temperature

• Water availability and quality

• Food availability and quality

• Predator-prey relationships

• Habitat availability and quality

• Light availability

• Pollution

In this lab, we will look at a predator-prey relationship. It is generally hypothesized that the prey population size will decrease after the introduction of a predator. To test this hypothesis, the prey population size can be estimated before and after introducing predators.

To rule out the effect of any other environmental factors described above, the environmental conditions for both sampling events should be as similar as possible to make the comparison as valid as possible. For example, sampling in the same season, in the same sample locations, and with the same or similar equipment will eliminate the effect of these parameters on the estimated population size. That way, it is easier to attribute any changes seen in population size to the predator introduction.

Estimating Population Size

Estimating how many individuals are in a population is an essential tool in any ecologist's tool kit. When selecting samples to make population size estimates, it is crucial that the samples truly represent the population being studied. Randomness and

sample size have a large impact on the validity of a population estimation.

Randomness is a strategy used to address the uneven distribution of populations. Populations tend to cluster around areas where food is readily available or where they are safe from predators. When samples are collected randomly throughout an area, different levels of population density have a better chance of being represented in the sample. To ensure that sampling areas are truly chosen at random, computer programs are now used to remove human bias.

A sample size is the number of individuals measured compared to the total number of individuals. A large enough sample needs to be collected so that less commonly occurring members of the population are represented. Many factors affect the validity of a sample size, such as the complexity of the environment or the complexity of the sampled population's distribution. In general, the larger the sample size the more accurate the estimation is thought to be.

Mark and Recapture

One technique for estimating population size is mark and recapture, which works exactly the way it sounds: members from the population under study are captured, marked so that they can be tracked (e.g, tagging bluegill with a piece of plastic), and then released back into the population. Once they have had enough time to mix evenly back into the population, but not so much time that births, deaths, or migrations occur, the population is resampled to find out what percentage of the sample has the mark (a tag) from the first capture.

The total population size is then estimated using a simple proportion:

N = M * Z / R

• Where N is the total estimated population

• M is the number of organisms marked in the first sampling

• S is the number of organisms captured in the second sampling

• R is the number of marked organisms recaptured in the second sampling

The result is the estimated population size at the time of marking, not at the time of recapture.

The mark-and-recapture estimate makes several assumptions:

• Marks are not lost or overlooked.

• The population is not changed by reproduction, immigration or emigration, or death between the marking session and the recapture session.

• Marked and unmarked bluegill have the same chance of being caught in both the first and second sampling sessions.

All methods for population estimation are just that estimates based on these assumptions.

At the same time as you calculate the population, you can also weigh and measure the length of the animal to see if there are other effects: do predators preferentially chase larger prey and lead to smaller or thinner prey? The effects of predators can be more complex than just leading to lower prey population numbers.

In part of this lab, you will investigate the effect of introducing bass predators on a lake's bluegill population. Bluegill and bass have an intricate predator-prey relationship where bass eat young bluegill until bluegill become large enough and fast enough to hide and evade the bass. Once bluegill reach adulthood, they are no longer easy prey and spread out to the whole lake.

The environmental conditions for the best bluegill populations is a delicate balance of numerous factors. Young bluegills compete for shallow lake habitat to avoid bass, and adult bluegills compete strongly with each other for food. If there aren't enough young bluegill available, bass population numbers decrease. The first step in determining if the predator-prey relationship is balanced is to measure the bluegill population over time.

Population Cycles for a Predator-Prey System

While an introduction of a new predator may have an immediate effect on the prey population size, in some ecosystems predator and prey species continuously affect each other's population numbers. This can lead to cyclical swings or oscillations in population size of both species that, on a graph, look like overlapping hills and valleys. What drives these changes? For example, a predator population may increase if more prey are available to hunt. This can cause the prey population size to decrease, in turn driving down the predator population numbers. These lower predator numbers can then lead to a higher prey population size. The population cycles can vary in amplitude (the difference between peak and trough of each oscillation) and period width (how long one cycle lasts) based on factors specific to each predator-prey pair. These cycles can also be impacted by external factors, such as seasonal availability of food supply for the prey.