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
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
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
d.
Low temperatures
can also pose distributional barriers; for example, Saguaro cacti do not grow naturally in
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
2.
Do the
distributions of life forms reveal evidence or information about changing
environmental parameters?