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.

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 the landscape, the proportion of plots that contain a plant of interest give an indication of how widely distributed a species is across the 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 be used to compare abundance of difference species.  For example, if you are using a quadrat (50x50 cm) 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 the proportion of time a plant occurred in the quadrats you examine.  If you examined 100 quadrats and found that PLANT B occurred 15 times than the frequency of PLANT B is 15%.  Now, lets say you did your examination with a large quadrat, say 1 m^2.  If you decided to try to repeat the same examination with a small quadrat, say 25 cm²  you can imagine that PLANT B may not occur in as many plots. 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 US Forest Service, Bureau of Land Management, and US Park Service to monitor change in vegetation communities.  From a monitoring perspective, why do you think frequency is often the measurement of choice?

• 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 rooted perennial plants is less sensitive to seasonal  changes or year to year variation.  Plus, ecological forces 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 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.