The X axis is number of individuals of that sex in that bracket (as well as, in this case, the percentage of the population). The Y axis lists age brackets in increments of 5 years: the bottom bar is 0 to 4 years of age the next is 5-9 years, etc. The bars on on the left are for the males. In this particular age structure graph, the bars on the right are for this country’s females (note the label in the upper right corner). You can think of an age structure graph as being two bar graphs. Take a moment and study the graph below to see what you can learn from it. A population’s age-structure can be represented by a population pyramid (though these pyramids might not be very pyramidal in shape, as you’ll see below). Populations can be characterized by the number of individuals of various age brackets. Each red circle represents an individual organism. Uniform (a), random (b), and clumped dispersion patterns (c). Since each plant’s location is a function of where a wind-blown seed randomly lands, the resulting plants will often be randomly distributed (“B” below). Another dispersion pattern is seen in the way dandelions spread out over a lawn.

This is seen in bird colonies, where territorial interactions force nesting birds to spread themselves out at a safe distance from their neighbors (who would be happy to eat an unguarded chick in a neighboring nest). However, individuals within a population can also, in specific circumstances, space themselves out in a uniform pattern (see “A” below). In an African savannah, there will be dense arrays of wildlife around watering holes.Ĭlumping (“C,” below) is the most common dispersion pattern. For example, in an intertidal zone, you’ll find sea anemones clumped around tide pools. Within a population’s range, a population will generally clump around a particular resource or habitat. For example, a liter of seawater may contain as many as 100 billion viruses.Īggregating anemones: an example of clumped dispersion. However, when an organism inhabits a three-dimensional space, like air or water, density is usually expressed as individuals/unit volume. For example a 1990 study of lions in Serengeti National Park determined, in one area of the park, a density of one lion/4 km 2. What’s the size of this population? How would you determine its density?ĭensity is usually measured as the number of individuals/unit area.

Let’s continue by looking at some others. Size is perhaps the most obvious way to characterize a population. Now there are about 350,000 (source: Africa Geographic). These numbers are less precise, but it’s thought that there were about 12 million African elephants in the early 1900s. And the high scenario sees the current rate continuing.Īs human populations have grown, populations of wildlife have fallen. The medium projection sees population growth continuing, but at a slower rate. Why? because in many parts of the world, birth rates are falling. The low projection sees human population reaching a maximum size in about 2040, then starting to decline. What about the future? It depends on a variety of factors, especially birth rate (the number of births/year).

The Earth’s human population was about one billion in 1800. The diagram to your left, which uses data from the United Nations, shows how the size of humanity’s population has changed over the past two hundred years. In biology, a population is defined as a group of organisms of the same species inhabiting a particular location. Introduction: Key Attributes of Populations

Dispersio patters of of orgaisms