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Teachers & SchoolsStudy Guide

Before moving onto other population characteristics that are of interest to ecologists, let’s briefly look at how the principles of population growth we just discussed (see previous topic for a refresher) are actually useful in *real life*. Recall from the Monterey Bay example that, after the collapse of the sardine population, the fishing industry became more careful about regulating its practices. The golden question for industries that harvest natural living resources like lumber, seafood, and so forth is

"How many trees, or lobsters, or sardines, or saffron orchids, should we 'harvest' each year for a maximum yield?"

At first you might think, "That’s a silly question. If you want maximum yield, harvest them all." But, most industries want to survive beyond a single or a few harvests. The real golden question becomes

*"What’s the maximum sustainable yield for this population?" *

In other words, these industries want to know the maximum number of individuals that can be harvested right now that will allow the population to return to its **carrying capacity** as quickly as possible, making it ready for another productive harvest. If you look at the logistic growth figure below, you will quickly see that when a population is exactly halfway to its carrying capacity, it is growing at its fastest rate. This means that if an industry stops harvesting at half its carrying capacity, the population can quickly recover and allow for a successful harvest time after time. The golden question, then, has a golden answer. The **maximum sustainable yield** (**MSY**) for a population is exactly equal to half of its carrying capacity. So, there you have it. Problem solved.

Yeah, right. It is never that simple, is it? Though the math makes sense, it turns out that calculating the MSY for a population is nearly impossible because calculating the carrying capacity for a population is difficult. First of all, how do you count the number of tuna in the sea, or even in the small area you want to harvest? Second, even if you could count all of the tuna, how do you know if it has reached its carrying capacity?

What’s more, the carrying capacity for a population can change from day to day, month to month, or even year to year for lots and lots of various reasons. This means that even if we could calculate the carrying capacity for a population in 2010, we couldn’t be sure that our number would be accurate for 2011.

Lastly, individuals in populations do not contribute to birth and death rates equally. If we accidentally harvested too many of the breeding individuals, or not enough of the dying individuals, we would miss the MSY by a long shot, no matter what our math told us. Math lies! In theory, MSY is a nice concept, but the complexities and messiness of real life make putting it into practice quite a challenge. Despite this difficulty, an accurate understanding of the principles of population growth can lead industries that harvest natural living resources to be more careful and precise in how they sustain their target populations. Before the 1950s, few people thought about these things. Such ignorance led to disastrous consequences.

In addition to population growth rates, ecologists are also interested in the **survivorship traits**, **age structures**, and **distributions of populations**. These population characteristics are tied to the **life histories** of the individuals in the population.

Life histories describe

- The age of first reproduction

- Probabilities of survival and reproduction at each age

- Litter size

- Litter frequency

- Longevity for individuals in a population

Life histories can be summarized in **life tables** that display all of the information listed above. You might be interested to know that biologists didn’t invent life tables. Life insurance companies did. Even today, life tables are used by insurance companies to determine life insurance rates for people at different ages based on the probability of survival—actually, the probability of death—for each age. The lower the probability of dying, the lower the insurance rate. Our suggestion: Buy life insurance as early in life as you can.

Taken together with growth rate statistics, life histories allow ecologists to more accurately describe and understand the populations they study.

**Brain Snack**

Currently, many governments are trying to determine the MSY for fisheries. You can read about the recent changes to the European fishing policy here.