Measuring Frequency

What is Frequency?

Frequency is the number of times a plant species occurs in a given number of quadrats. Frequency is usually expressed as a percentage and is sometimes called a Frequency Index. The concept of frequency indicates the probability of finding a species in a series of quadrats examined in an area of interest. No counting or measuring is involved -- just a record of species present in each quadrat.

Use of Frequency Data

Frequency is most often used to compare plant communities and to detect changes in vegetation composition over time.  In this way frequency can be used to assess vegetation trend.

Frequency is also used to quantify and describe the distribution of a species in a community.  If a scientist or land owner examines quadrats across a landscape, the proportion of plots that contain a plant of interest give an indication of how widely distributed that plant species is across that landscape. 

For example, if I were to examine 450 quadrats across a shrublands site in the Chihuahuan desert and I found desert marigold (Baileya multiradiata) in 54 of the quadrats, then my frequency would be 12%.  
54 quadrats ÷ 450 quadrats examined = .12 or 12%. 

I would furthermore conclude that desert marigold occurs on the site but it is not common or widely spread across the site.

Frequency is used to describe the abundance of a species of interest but, it should not not be used to compare abundance of difference species.  For example, if you are using a 50x50 cm quadrat and you find lupine has a frequency of 45% and the frequency of tarbush is 20% you cannot conclude there is more lupine than tarbush. These are very different kinds of plants and the size of quadrat you use will affect how often the plant is recorded "in" the quadrat.  If you want to compare different species consider using cover or biomass (to be discussed in future modules).

Frequency depends on Plot Size

Remember, frequency is the proportion of times a plant occurred in the quadrats you examine.  Suppose  you examined a plant community the determine the abundance of Blackeyed Susan (Rudbeckia hirsuta) using 100 quadrats that are 25 cm² in size. If you found that Blackeyed Susan occurred in 15 of the 100 plots you examined than the frequency of this plant is 15%. Now, lets say you decide to repeat your examination with a large quadrat, for example 1 m². You can imagine that Blackeyed Susan may occur more frequently in these larger plots thus the frequency would appear to be greater even if the plants did not change on the environment. The bigger the plot, the more likely a plant is to occur in the plot.

Advantages of Assessing Frequency

Frequency is widely used among land management agencies such as the US Forest Service, Bureau of Land Management, and US Park Service to monitor change in vegetation communities.  From a monitoring perspective, frequency is often the measurement of choice, because frequency is:

• Highly repeatable.  Individual observers may disagree on how many plants occur, or how much they weigh, or how much area they cover. But, it is easy to determine and agree if a species occurs in a plot or not.

• Fast and easy to measure. Little equipment is required and no fancy protocols are needed.

• Frequency of perennial plants is less sensitive to seasonal  changes or year to year variation.  Plus, ecological forces such as fire or grazing will probably affect amount of a plant that remains, but, they will likely not affect the presence or absence of a perennial plant.

• Can describe distribution of species in a community or across a landscape.

• Highly sensitive to changes resulting from seedling establishment, so it can be an early indication of community change or invasion by annual plants.

Limitations of Assessing Frequency

Despite its wide use in grasslands, shrublands, woodlands, deserts, and forests there are several shortcomings to using frequency that must be considered when designing monitoring protocols.

• Frequency is highly influenced by the size and shape of the quadrat used.  It may be difficult to determine the "right" quadrat size for assessment and the "right" quadrat size may change as the abundance of the plant changes over time.

• If frequency changes, it is not clear which vegetation parameter has changed:  Cover? Density? Distribution across the landscape? Or, changing abundance in a small area?

• The key to good frequency data is excellent plant identification skills. This is easier said than done. Either a well-experienced observer is needed or many hours must be initially spent to ensure good identification of plants.