Plant community concepts and attributes.


Review accuracy and precision: how do you think you fared with respect to accuracy and precision along the gradient?

          Accuracy – the closeness of a measured value to a true value, and it is dependent on having a good measuring device or system.

          Precision – the closeness of repeated measurements to the same item. (A ruler that is marked off in the wrong place and is too short may give a very inaccurate measure of a fish’s length, but if used carefully and repeatedly in the same manner, the ruler will give nearly the same numerical value; it would give a precise or consistent measure).



1.     Population -  a group of individuals of a single species living in close proximity and capable of interbreeding.

a.     Populations within a relatively small geographic area (e.g. whose pollinators have ready access to all individual members) are called local populations

2.     Communities consist of populations of different (and interdependent) species living in the same location.

3.     Ecosystem consists of living organisms that interact with one another and with the nonliving (abiotic) environment.

4.     Cover is the percentage of plot area beneath the canopy of a given species.

a. The canopy of an overstory species creates a microenvironment that smaller, associated species must contend with. The overstory canopy, therefore, exerts a biotic control over the microclimate of the site. It is assumed that a comparison of cover for each species or life form in a given canopy layer will reveal the relative control or dominance that each species exerts on the community as a whole, such as relative amounts of nutrients or other resources each species commands.

b. Canopy coverage, then, can be expressed as the percentage of ground covered by canopy, when the edges of the canopy are mentally projected down to the surface.

15. Relative cover is the cover of a particular species or life form as a percentage of total plant cover. Relative cover will always tally up to 100%, even when absolute cover is quite low.

16. Density is the number of plants rooted inside the plot. Population density – the number of individuals in a given area (e.g. 5 blackberry bushes per square meter).

17. Relative density is the density of one species or life form as a percent of total plant density.

18. Frequency is the percentage of total plots that contains at least one rooted individual of a given species or life form. (Sociability). [Frequency is an artifact of plot size, and so is a more artificial statistic than either cover or density].

19. Relative frequency is the frequency of one species or life form as a percentage of total plant frequency.

20. Dominance -  the dominant species is that species that contributes the most cover or basal area to the community, compared with other overstory species. This definition is based on physiognomy. If more than one species contribute relatively equal cover, the species are said to be codominant.

21. Species richness – the number of species in some area within a community.

Each species is not likely to have the same number of individuals. One species may be represented by 1,000 plants, another by 200, and a third by a single plant.

          a. Generally, the more favorable the environment the greater the species richness within that environment.

          b. The more extreme the environment, the fewer the number of species within that environment, and the greater the chances of dominance by a single species.

                   i. plants may be restricted to extreme environments because they are poor competitors in less extreme (more favorable) environments.

                   ii. they have adapted to the extreme environment (temperature, precip, substrate), in the absence of other biotic competition.

                   iii. plants that occupy extreme environments that other plants cannot, can tolerate more favorable environments elsewhere, but only in the absence of competition.

22. The distribution of individuals among the species is referred to as species evenness, or species equitability.

          a. where S = the # of species in a sample

          b. Habitat 1 = aaa bbb ccc ddd  (high evenness) – similar # of indivs of each species.

          c. Habitat 2 = a bbbbbbbb cc d  (low evenness) – community has relatively common and relatively rare species.

          d. In both habitats, S = 4; i.e. the species richness is the same.

          e. An important aspect of the numerical structure of communities is completely ignored when the composition of the community is described simply in terms of the # of species present. It misses the information that some species are rare and others common.


23. Species diversity is a combination of richness and evenness; it is species richness weighted by species evenness, and there are formulae that permit the diversity of a community to be expressed in a single index number.


Richness and diversity are quite different from one another. Altho, the two are often positively correlated, environmental gradients do exist along which a decrease in richness is accompanied by an increase in diversity.

          Community A has 5 species but an uneven # of individuals in each species, has a lower species diversity than Community B, with 4 species that have a very similar # of individuals in each.


          a. Generally, there is a gradient of increasing species diversity (and richness) from the poles to the equator, and from high elevations to low elevations.

          b. These gradients follow complex environmental gradients of increasing warmth, among other factors.

It is also generally understood, albeit with exceptions, that diversity increases as any particular stress lessens.

i. an exception might be that a semiarid grassland (water stress) may be   more diverse than a woodland or forest; but it is also more diverse than an arid shrublands (even greater water stress).

           c. The pattern we found in species richness, with respect to extreme environments, may likewise be found with annuals over time, where the influencing parameter is moisture.

                   i. in very dry years, diversity usually falls, while dominance of one plant becomes apparent.

                   ii. in more favorable years, wetter years, diversity usually increases: there are fewer dominants,  and more rare species present.


What can we say about the first and last transect we sampled along the elevation gradient with respect to species (or life form) richness – the playa at 3,000’ and the transect at 8,000 and 8700’?




Spatial dispersion

The spatial distribution of plants and animals on the landscape can yield information about population biology, physiology, and interactions with other organisms. The investigation of spatial patterns has become a specialized field of study in plant ecology, and there are many methods available for estimating patterns. Animal spacing is difficult for animal and behavioral ecologists to determine because animals are mobile, and methods for determining the patterns of spatial use differ from those used for plants and other sessile organisms.


With respect to plants, we recognize three types of spatial dispersion (spatial spread):

a.      Clumped (sometimes referred to as “under dispersed” or “contagious”).

b.     Random

c.     Uniform (sometimes referred to as “over dispersed” or “regular”).


The type of spatial pattern you see depends in part on the scale of your view. We would find that pines, for example, were distributed in an extremely clumped fashion if we considered the whole of North America, but within a single forest, pines may be uniformly or randomly distributed.

Consider for a moment the range of factors that may act throughout a plant’s life to create a final pattern we observe in the field.

a.      Seed dispersion is affected by the method of seed dispersal

i.                    wind

ii.                  gravity

iii.                water

iv.               animals

b.     Seed dispersion is also affected by seed predation

i.                    Granivores may avoid certain areas

ii.                  Drop of discard seeds after collection

iii.                Forget where their “stashes” are located.

c.     Seedlings are strongly affected by nutrients, moisture, and light in their immediate environment, and the spatial distribution of these abiotic factors is often heterogeneous, particularly in deserts.

d.     Physiological constraints alone may influence patterns of occurrence:

i.                    Plants species found only near washes or permanent sources of water because they cannot survive in the surrounding drier environment.

e.      In some cases, biotic and abiotic factors interact:

i.                    Under certain desert shrubs, seedlings can find refuge from harsh sunlight and obtain access to higher concentrations of water and nutrients.

ii.                  These nurse plants facilitate seedling survival and subsequent reproduction.

f.       Other shrubs may act as competitors to seedlings or other plants, usurping limited resources such as water and nutrients (exploitative competition).

g.     Allelopathy is a special case of interference competition, whereby allelopathic plants give off chemicals (thru leaching leaves or loiter or by root exudation) which inhibit the growth and survival of plants growing nearby.


Dispersal patterns among plants may clue us into the kind of interaction taking place between plants.

a.                         Sampling is based on the premise that positive interactions will produce positive spatial relationships (clumping) between partners; where one parent is found the probability is high that the other will be found nearby.

                                                                          i.      The two populations attract one another and exist in a nonrandom, clumped pattern.

b.                        Similarly, negative interactions will produce negative spatial relationships;

                                                                          i.      the two populations appear to repel one another and exist in a nonrandom, regular pattern.

c.                         If there is no interaction between populations, then the location of one individual has no influence on the location of others;

                                                                          i.      the two populations are said to be randomly distributed with respect to each other.



Discussion of competition from the reading.







Elevational gradients 

If we were to fly from the equator to the North Pole, stopping every few hundred miles along the way, we would see dramatic changes in the vegetation, from tropical rainforest to tropical dry forest, to grasslands, temperate deciduous forest, the coniferous forests of the taiga, and finally to the lichens and small prostrate plants of the tundra.

Similar changes in vegetation occur throughout the world as one climbs in altitude.  Climbing from valley bottom to mountain top in southern Nevada, we see changes in both the species present and in the overall structure (or “physiognomy”) the vegetation - from desert scrub to a mixture of shrubs and tall yuccas to areas dominated by trees.

Such changes in vegetation may be related to elevational gradients in moisture and temperature. 

a.                         Air temperature drops an average of 0.6° C for every 100 m gain in elevation. 

b.                        Species, plant and animal alike, have different physiological tolerances for extremes of moisture and temperature. 

c.                         In southern Nevada, the high temperatures and aridity of lower elevations pose serious challenges for many plant species. 

d.                        Low temperatures can also pose distributional barriers; for example, Saguaro cacti do not grow naturally in Las Vegas because they cannot withstand our low winter temperatures.


But biotic factors may also affect where a species lives.  Within the set of all species that can reach a site and are physiologically capable of dwelling there (a list that could be long indeed), interactions among species can affect which species thrive and which fail. 

Competition among species (or interspecific competition) may play an important role in determining plant species distributions. 

          a. A poor competitor might be prevented from establishing in portions of habitat with perfectly suitable abiotic conditions. 

          b. For example, a grass species that is very efficient at drawing water from the soil may exclude a shrub species from the area; in other words, the two species cannot coexist because of competition for limiting moisture. 

          c. In contrast, another pair of species may coexist without adverse effects. 

          d. One species might even depend on the presence of another to persist, such as succulents that require “nurse” plants to provide shade for seedlings.


A gradient analysis is a way of analyzing the change in community structure and gaining some insight into why species are distributed as they are. 

Along an elevational gradient, we assume that important physical variables, such as temperature and moisture, change gradually. 

a.                         Do the distributions of species change gradually along the gradient as well? 

b.                        Or do communities change abruptly in composition across sharp zones of transition, which might indicate an important role for biotic factors? 

c.                         Can you think of other gradients in which plants might be distributed differentially?


 In this exercise, we will ask the following questions:

1.     How are plant life forms (growth forms) distributed with respect to one another along an elevational gradient in the Spring Mountains?

2.     Do the distributions of life forms reveal evidence or information about changing environmental parameters?