The SIMILARITY INDEX (SI) of the ecological site is dictated by many factors. Historically, herbivory by mammals and invertebrates above and below the soil surface, extensive fires, and periods of drought were major disturbances to the land. The kinds of plants that are present on an ecologcial site may be desirable or undesirable for a particular use. For example, if cattle have been grazed at a heavy stocking rate on a site for a long period of time, some of the plants that have increased over that period of time are not preferred by cattle. Plants preferred by cattle have decreased over this period of time. Any disturbance of the ecological site will affect the SI. Disturbances are a natural occurance on all sites and are necessary to maintain ecological structure and function. For contest purposes, the SI will be determined by comparing the present vegetation (species composition by weight at the end of the growing season in an ungrazed condition) to the presumed original dominant plants on that site historically and before European settlement. Specific ecological site descriptions can be obtained from the Natural Resource Conservation Service (NRCS). For example, if we were judging the SI for a Shallow Savanna Site, we would determine the compostion of plant species. By convention, however, we can count no more than the percent allowable on the Ecological Site Guide. The SI is expressed as a percentage from 0 to 100%. Plants native to the site count in percent composition toward the SIMILARITY INDEX (SI). Plants native to the site but not specifically listed in categories are counted as "other."

Judging the SI of a Shallow Savanna Site:

Dominant Plants

Site Composition Maximum

Observed Composition

Percent Counted Toward SI

Site 1

Site 2

Site 3

Site 1

Site 2

Site 3

Grasses

70%

big bluestem  switchgrass  Indiangrass	25	__10__	______	______	__10__	______	______
little bluestem	15	__10__	______	______	__10__	______	______
Canada wildrye  sideoats grama  purpletop  dropseed  Scribner panicum  blue grama  other natives	30	__15__	______	______	__15__	______	______
invasives	0	__10__	______	______	___0__	___0__	___0__