What is the difference between direct and indirect relationships




















Inflation occurs when overall prices in an economy rise. All this means is that one variable affects the another but through a third variable. They do not affect each other directly. However, Group A does not affect Group C directly. So, what if one variable affects another variable through a third variable, and they both move in the same direction? In that case, there is an indirect but not inverse relationship. Therefore, they communicated through an interpreter.

We can say that the two presidents had an indirect relationship. One could not make the other happy, angry, nervous, or anxious directly. Menge identified seven classic models of these sequences, and identified two new models during his research Figure 4. Keystone Predation: A series of classic ecological experiments by Paine in the s helped to elucidate this indirect effect model.

In keystone predation, the removal of a prey species by the predator indirectly increases the abundance of a competitor of the prey species. Exploitation Competition: When two predators consume the same prey, the loss of abundance in the prey species can negatively affect the second predator. Menge found that exploitive competition was the least common interaction, appearing only 2. Apparent Competition: Apparent competition occurs when an increase in the abundance of one species leads to a decrease in the abundance of a second species due to enhanced predation by a shared predator.

Indirect Mutualism: An indirect mutualisum takes place when positive effects on two consumer species when each negatively impacts a competitor species of the other predator's main prey species. Indirect Commensalism: Indirect commensalism is similar to indirect mutualism except that one of the predator species will also consume the main prey of the other predator species.

Habit Facilitation: In habitat facilitation, one species indirectly improves the habitat of a third species by its interactions with a second species. The plant is positively affected by the decrease in herbivory caused by a predator reducing herbivore numbers. Indirect Defense: Indirect defense occurs when a non-prey species leads to the indirect decrease of a consumer species either by reducing the prey species via competition, which leads to a reduction in the consumer species, or when one prey species leads to an increase in its predator abundance, which then preys more heavily on the third species.

Apparent Predation: Apparent predation can occur by two avenues: a non-prey individual that has an indirect positive effect on a consumer or the negative effect of a predator on a non-prey species. P, H, and B refer to predator, herbivore, and basal species, respectively. The lower case letters indicate types of interactions involved, where p indicates predation, c indicates interference competition, f indicates provision of food, and if indicates inhibition of feeding.

The detection of indirect effects can be complicated in ecological experiments due to the fact that responses can contain a mix of direct and indirect effects.

By using the appropriate experimental design and data collection, it is possible to isolate and test pathways. One way to approach the detection of indirect effects is a press versus pulse design Bender et al. In a press experiment, the experimental manipulation is sustained whereas in a pulse experiment the perturbation is transient.

Theoretically, press experiments should evidence both direct and indirect effects while pulse experiments should only present direct effects.

However, some indirect effects can occur quickly Menge, , so alternative methods for detection of indirect effects, such as path analysis, are needed Wootton, Do direct or indirect effects have a greater impact in structuring communities? In a review of experimental studies, Schoener reported that only about one quarter of all community change reported resulted from indirect effects while the remainder resulted from direct effects. This review seemed to support the view that indirect effects were not very important relative to direct effects.

Thus, it seems that the relative roles of these factors vary among studies and that under certain circumstances indirect effects could be exerting as strong an influence on community structure as direct effects. Bender, E. Perturbation experiments in community ecology: theory and practice.

Ecology 65 , 1—13 Crumrine, P. Partitioning components of risk reduction in adragonfly-fish intraguild predation system. Ecology 84 , — Heil, M. Protective ant-plant interactions as model systems in ecological and evolutionary research.

Annual Review of Ecology, Evolution, and Systematics 34 , — Holland, J. Consumer-resource theory predicts dynamic transitions between outcomes of interspecific interactions. Ecology Letters 12 , — Izzo, T. Cheating the cheater: domatia loss minimizes the effect of ant castration in an Amazonian ant-plant. Oecologia , — Menge, B. Indirect effects in marine rocky intertidal interaction webs: patterns and importance.

Ecological Monographs 65 , 21—74 Detection of direct versus indirect effects: were experiments long enough? When there is a direct or inverse relationship, x and y are proportional to each other in some way. A direct relationship is proportional in the sense that when one variable increases, so does the other. Using the example from the last section, the higher from which you drop a ball, the higher it bounces back up.

A circle with a bigger diameter will have a bigger circumference. If you increase the independent variable x , such as the diameter of the circle or the height of the ball drop , the dependent variable increases too and vice-versa. Pi is always the same, so if you double the value of D , the value of C doubles too. The gradient of the graph tells you the value of the constant.

Inverse relationships work differently. If you increase x , the value of y decreases. For example, if you move more quickly to your destination, your journey time will decrease. In this example, x is your speed and y is the journey time. Doubling your speed halves the journey time, and increasing the speed by ten times makes the journey time ten times shorter.

You will understand a relationship more fully when you study the tabular data that often accompanies the graph as shown in Figures 5 and 7 , or the material in which the author elaborates on the variables and relationships being studied. Steep vs. Gentle Slopes. When you have been out running or jogging, have you ever tried, at your starting pace, to run up a steep hill? If so, you will have a good intuitive grasp of the meaning of a slope of a line.

You probably noticed your lungs starting to work much harder to provide you with extra oxygen for the blood. If you stopped to take your pulse, you would have found that your heart is pumping blood far faster through the body, probably at least twice as fast as your regular, resting rate. The greater the steepness of the slope, the greater the sensitivity and reaction of your body's heart and lungs to the extra work.

Slope has a lot to do with the sensitivity of variables to each other, since slope measures the response of one variable when there is a change in the other. The slope of a line is measured by units of rise on the vertical y-axis over units of run on the horizontal x-axis.

A typical slope calculation is needed if you want to measure the reaction of consumers or producers to a change in the price of a product. For example, let's look at what happens in Figure 7 when we move from points E to D, and then from points B to A.

The run or horizontal movement is 80, calculated from the difference between and 80, which is Let's look at the change between B and A. The vertical difference is again 20 - 80 , while the horizontal difference is 80 - We can generalize to say that where the curve is a straight line, the slope will be a constant at all points on the curve.

Figure 8 shows that where right-angled triangles are drawn to the curve, the slopes are all constant, and positive. Now, let's take a look at Figure 9, which shows the curve of a negative relationship. All slopes in a negative relationship have a negative value. We can generalize to say that for negative relationships, increases in one variable are associated with decreases in the other, and slope calculations will, therefore, be of a negative value.

A final word on non-linear slopes. Not all positive nor negative curves are straight lines, and some curves are parabolic, that is, they take the shape of a U or an inverted U, as is demonstrated in Figure 10, shown below. To the left of point C, called the maxima, slopes are positive, and, to the right of point C, they are negative.

You can determine the slope of a parabola by drawing a tangent touches at a single point line to any point on the curve. You can see below that a point such as R is then selected on the line, and a right angled triangle can be constructed which joins points R and B. We can then calculate the rise over the run between points B and R from the distance of the height and the base of the triangle. So, we can generalize to say that the slopes of a non-linear line are not constant like a straight line and will vary in sign and in value.

You will find that a knowledge of slope calculations enhances your understanding of the dynamics of graphs. It will likely improve your marks in economics, since many test questions require you to illustrate your thinking with graphs. A person from an Eastern culture once observed, "A picture is worth a thousand words.

Without them, we would be forced to examine thousands, or tens of thousands, of bits of statistical information to determine economic relationships. Many economic researchers over the years have done that work for you, and it gets expressed in nice little packages called graphs. They convey information easily, efficiently, and effectively, and can stimulate good thought and discussion. Notes 1. Miller, B. Abbott, S.



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