Direct gradient analysis

Lecture graphics

Multivariate analyses are required for community data because we're interested in the response of many species, simultaneously

Multivariate analyses are used to summarize redundancy, reduce noise, elucidate relationships, and identify outliers

Multivariate analyses can relate communities to other kinds of data (e.g., environmental, historical data)

Results from multivariate analyses are designed to improve our understanding of communities, esp. community structure

Direct gradient analysis

Used to display distribution of organisms along gradients of important environmental factors

Devised by Ramensky (1930) and Gause (1930), but used extensively in ecological research after about 1950 (Whittaker)

An example:

Dix and Smeins (1967) took 100 community samples to represent the range of vegetation present in Nelson County, North Dakota

Homogeneous stands of 0.1 ha were sampled by recording frequency in 30, 0.5 × 0.5 m quadrats

Numerous environmental variables were recorded for each stand

Defined indicator species of a drainage class as a species w/ frequency at least 10% greater in that class than in any other class

Defined indicator value as drainage class of the indicator species {drainage classes vary from 1 (good) to 6 (poor)}

Goal: summarize frequency of all species --> single number for each stand

Stand Index Number = {(rel. freq. × indicator value)/{(rel. freq. of indiv. sp)} × 100

e.g., Stand 17 (sample data) {RF=rel. freq., IV=indicator value}:

Spp.	RF	IV	RF x IV
Stco	20	1	20
Stvi	10	-	--	(not an indicator for any drainage class)
Acmi	15	2	30
Lica	5	3	15
Other	50	-	--
	40*		65

^*sum of RF for spp. w/ IV (20+15+5)

Stand Index ₁₇ = (65/40) × 100 = 162

For all stands, stand index varied from 100 to 600

Divided this 500-unit gradient into 10, 50-unit classes:

Species frequency

Class	Stand w/in 50-uinit class	A	B	C
100-149	4
	9
	12
		XA	XB	XC
150-199

and so on ...

=========> Fig. 2 [Dix and Smeins 1967, p. 33]

They could have plotted frequency over the entire 500-unit gradient, but the graph would have been messy--10 drainage classes "smooths" the graph, making interpretation easier

The purpose of direct gradient analysis is to organize community and environmental data to answer questions such as:

1. Precisely which environmental factor in a complex of factors principally affects distribution of organisms and communities?

   While direct gradient analysis can be used to identify ecologically important environmental factors, experimental manipulations are needed to more precisely determine the importance of various environmental factors

2. How can environmental factors best be measured or estimated?

   Dix and Smeins derived an index for drainage based on the plants themselves: this may be easier, more accurate, and less expensive than other measures of drainage or soil moisture

3. What additional environmental gradients affect community composition?

   Often difficult to evaluate because secondary gradients are overshadowed by primary gradients

4. What general principles emerge from direct gradient analysis to characterize the combining of individual species into communities?