Biodiversity

1
Biodiversity Ricklefs definition Biodiversity
is a measure of the variety of organisms within
a local area or region, often including genetic
variation, taxonomic uniqueness, and
endemism. Ricklefs is slightly more precise when
he defines diversity as an ecological
measure Diversity is a) the number of taxa in a
local area or region or b) A measure of the
variety of taxa in a community that takes
into account the relative abundance of each one.
2
Definition a is usually called species richness.
As a count of the number of species, it does not
take into account relative abundance. It would
claim the diversity of a community with 100 of
species A and 1 each of species B,C,D, and E was
5, the same as one with 20 individuals of each
species. Depending on the details of the measure
used, definition b may be called a dominance
index or, if information theory is used to
generate the index, it is called Shannons or the
information theory diversity index. The dominance
index for a community is calculated as
3

• The Shannon information theory diversity index is
  calculated
• as
• Since H is an index related to logs, two
  corrections have
• been suggested for it
• Instead of using H, compare it as a ratio to the
  maximum
• possible diversity for the community (if all
  species were
• equally abundant, called H). This ratio is
  called evenness.
• b) Make the value more proportional to the number
  of species
• by using the H as the exponent of an
  expression that doesnt
• have an accepted name. The reported diversity
  is then 2H. Note that your text (pg. 417)
  suggests using logs base e. The result is
  comparatively identical, but isnt an information
  theory measure.

4
Heres a sample calculation of the two diversity
indices for a community containing 5
species Species richness 5 Relative
abundances Species individuals pi pi2
A 15 .123 .015 B 20 .164 .027
C 75 .615 .378 D 4 .033 .001 E
__8___ .066 .004 Total 122
0.425 D 1/0.425 2.352 H 1.142 (using
ln rather than log base 2)
5
Obviously, a more diverse community has more
species and no single (or a few) species
predominant in the community. But what determines
how many species we are likely to find, and what
determines how many species there are in the
world? The answer to the second question is
fairly straightforward There are about 2 million
species that have been officially named. Some
groups are well known, others have only a small
fraction named. You can probably guess which are
well known
6
Birds there are about 9000 species. Half had
been named by the middle of the 19th century.
Today a few species are named each
year. Mammals there are about 4000 species,
and less than 10 are added each year (almost all
small rodents). Are they the most diverse animal
fauna? NO!
7
Globally, the most accepted estimates of total
taxonomic diversity expect that if we could find
and name them all, wed have somewhere between 10
and 30 million species. The majority of the total
are insects, and among insects the largest number
are beetles. J.B.S. Haldane wrote that The
creator has an inordinate fondness for beetles.
About 20 of all species are beetles, and we
find new species frequently and in large
numbers Terry Erwin has collected beetles from
trees in tropical forests for many years. In one
series of studies he used insecticidal fogging to
collect insects in the Tambopata Reserve in
Panama. The results are staggering.
8
The species overlap between two plots only 50m
apart in his study area was only 8.7 (i.e. 91.3
of the beetles were different over this short
distance). There were 163 species of beetles
unique to one species of tree, Luehea seemannii,
in its canopy. Since beetles make up about 40 of
all insects, Erwin estimated that this tree
canopy harbored 400 unique species of insects
(160 beetles and 240 other insects). What about
the other parts of the tree trunk and roots? We
can only guess how many others may live there.
Estimates (E.O. Wilson 1992) suggest about twice
as many species in the canopy as on and in the
ground. So, there are maybe 600 species of insect
unique to this tree species. There are an
estimated 50,000 tropical tree species, so
9
There may be 30,000,000 tropical insect
species. What about the rest of the world?
Tropical rainforest is the most diverse biome on
earth. Many fewer species would be added by
expanding the range of habitats included. There
are also errors of unknown size in each step of
the estimation process. So, with no better
information to make an estimate of total global
diversity more precise or more accurate, the
loose estimate of from 10 30 million is a
useful start.
10
While it is generally accepted that tropical
rainforest is the most diverse biome on earth,
there are also indications of a large scale
pattern in diversity globally. In general, the
diversity of species declines with latitude, i.e.
as you move from the tropics toward the poles.
Your text uses the number of ant species to show
the pattern
Amazon rain forest
11
Many taxa have been surveyed in North America.
Mammals also fit the latitudinal pattern, so do
lizards. In each case equal areas are compared,
so it isnt just that the range is larger at
lower latitude Mammals lizards
12

• Ecologists have generated a number of different
  (but in many cases not completely independent)
  hypotheses to explain the latitudinal gradient in
  diversity. Well consider the hypothesis in turn.
  They are
• Evolutionary time 8. Competition
• Ecological time 9. Disturbance
• Climatic stability 10. Predation
• Climatic predictability
• Spatial heterogeneity
• Productivity
• Stability of primary production

13
These mechanisms are likely working at different
spatial scales. At the geographical or regional
scales evolutionary time ecological
time climatic stability or predictability hypoth
eses invoking increased resource
partitioning spatial heterogeneity competition
productivity and stability of productivity At
the local scale disturbance predation
14
Evolutionary time based on the assumption that
diversity increases with the age of a
community. In the northern hemisphere, relatively
recent glaciations mean that temperate and polar
communities are very recent compared to tropical
communities. This is argued to explain their
relative paucity in species. However, we now know
that, though tropical rainforest persisted
through the 4 cycles of Pleistocene glaciation,
the community was not untouched. Rainfall
patterns changed during glacial epochs, and
continuous tropical rainforest was fragmented
during glaciation (in the Amazon Basin into at
least 7 separate continuing forest areas the
areas between were probably savannah.). So, there
are old and relatively recent communities in
tropical rainforest.
15
Heres the simple view of what the evolutionary
time hypothesis suggests
16
Ecological time this hypothesis suggests that
the time since glaciation has been too short for
dispersal of species to occupy their full range
of habitats (and latitudes). When glaciers
receded, there was newly open habitat. How
rapidly can species migrate/disperse to occupy
suitable places? Dispersal capacity for most
species suggest that this is at most a minor
factor.
17
Climatic stability Here reference is made to
the amplitude of the seasonal cycle of climate.
If there is little seasonality, species can live
there with little need for broad tolerance
capability (a broad niche with respect to
physical climatic variables). However, if there
is high amplitude seasonal variation, either
broad tolerances or other adaptations are
necessary. If niches are broad, how many species
can be crowded in? Fewer than if niches are
narrower. Other adaptations? These can include
hibernation, diapause (loosely, insect
hibernation), dormancy (plants), and, possibly,
the evolution of migration.
18
Along the same gradient (physical conditions or
some resource) you can pack more species with
narrow niches
3 spp.
Species Abundance
breadth
Resource or climatic gradient
6 spp.
Species Abundance
Resource or climatic gradient
19
Climatic predictability If a climate has high
amplitude cycles, but those cycles occur
predictably each year, then adaptations can be
selected to manage the problem. Most (if not all)
of the other adaptations to climatic stability
may well fit better here, as adaptations to
predictable cycles. Species may time important
phases of the annual life cycle to fit
environmental conditions. For example, in the
desert of the American southwest, there are two
periods when rainfall is more likely
20
Heres a comparison between rainfall at Omaha,
Nebraska and Phoenix, Arizona.
Note the difference in the scales along the
y- axes.
In the southwest desert there are two groups of
plants. Ones seeds germinate in the cool-wet
(winter annuals), and the others germinate in
warm-wet, and flower after summer flash floods.
21
Spatial heterogeneity there is a strong
correlation between the structural complexity of
a habitat and the number of species it holds.
Text figure 23.5 shows this for birds from
deciduous forest in eastern North America
Just to show you this wasnt a carefully chosen
habitat, the same relationship is found for birds
over a wide range of habitats in both North
America and Australia
22
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23
If there are more possible specializations in
habitat conditions (indicated by such measures as
foliage height diversity), then more species may
be able to co-occur by specialization on a
particular part of the gradient.
sp. 1 sp. 2 sp. 3
Fewer resources or possible habitat
specializations, so fewer species
Species Abundance
Resource
More resources or, more species
sp. 1 sp. 2 sp. 3 sp. 4 sp.
5 sp. 6
Species Abundance
Resource
24
Productivity More productivity means more food
available, and the potential for greater
specialization. Thus, everything shown on those
last graphs applies here, as well. One way of
seeing this is to look at how a graph of
resources available can be subdivided into equal
areas, and how many such areas pack in when there
are more resources available.
25
Your text (fig.23.3) shows that productivity and
habitat complexity interact to some degree. High
productivity without a means to specialize
(complexity) will simply lead to larger
population sizes rather than increased diversity.
26
Sometimes productivity is estimated by using
potential evapotranspiration. This is a measure
that combines evaporation from the soil surface
and transpiration. It indicates energy input into
a system. There is a pattern relating PET to
species richness in a number of major taxonomic
groups. Ricklefs shows the relationships for
North America.
27
Stability of primary production Species can
only successfully partition resources finely if
the resources do not vary too substantially in
abundance over time. Some call this hypothesis
one of temporal heterogeneity to parallel the
spatial heterogeneity hypothesis, but clearly
with an opposite result on diversity. In the
attempt to avoid predation on fruits and/or
seeds, some plants attempt to become
unpredictable in providing resources by
adopting a strategy of masting, producing a heavy
crop one year, but very little the next. That
unpredictability should reduce the diversity (and
number) of granivores or frugivores feeding from
the plant.
28
Competition this hypothesis has a lot of
overlap with others that invoke small niche
breadth as being key to high diversity. For
example, it is believed that tropical forest
species are highly K-selected. Competition is
fierce, the populations are near their equilibria
in the face of both intra- and interspecific
competition. There is strong selective pressure
to reduce niche breath to a region of niche space
where a species is superior. The resulting narrow
niches allow more species to be packed into the
same resource space. Just the opposite broader
niches, lower diversity would be expected where
r-selection predominates, i.e. harsh, high
latitude habitats.
29
Disturbance (and the intermediate disturbance
hypothesis) Disturbances reduce the density of
individuals, and thus the intensity of
competition. You can consider this to be the
inverse of the competition hypothesis in many
senses. The intermediate disturbance hypothesis
says that when disturbances are rare, the
community becomes filled with the K-strategists.
When disturbances are frequent, those capable of
rapid growth to recover following the disturbance
should predominate, i.e. r-strategists. At an
intermediate frequency and intensity of
disturbance, members of both groups may be
present, and thus diversity may be maximized.
Heres a figure abstracting this idea
30
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31
When disturbances occur frequently, populations
can never grow to an abundance where resources
become limiting. Thus, competition among the
species cannot be important. However, species
that are slow to mature and reproduce cannot
tolerate frequent disturbance, i.e.
K-strategists. The species that can tolerate
frequent disturbance are those that mature and
reproduce rapidly. Those species, r-strategists,
also tend to have high rates of dispersal, and
are therefore likely to find disturbances soon
after they occur.
32
If disturbances occur rarely, then populations do
reach a size where resource limitation is
important. Thus, competition among the species
occurs, and dominant competitors can exclude
poorer competitors from the community. Species
diversity is lower due to the loss of these
inferior competitors. But, at an intermediate
frequency of disturbance (and each place Ive
talked about frequency you could substitute
intensity) both r- and K-types may persist
together, and diversity should be higher. There
is evidence both supporting and not supporting
the intermediate disturbance hypothesis
33
First, evidence that doesnt work the effect of
burning on plant species diversity on Konza
Prairie in Kansas. Here, species diversity
declines with increasing frequency of burning,
but there is no intermediate peak
34
But here is evidence that supports the
hypothesis. It was initially constructed by Joe
Connell for the intertidal. Here it was applied
by Wayne Sousa to colonization of shoreline
boulders by green algae. Each line is a species.
Ulva, the line that starts first, is a pioneer.
Note that it has disappeared by the end. Both the
species that arrive late and Ulva are present at
the middle of the sequence
35
Sousa also knew that wave action (the main
disturbance force), would be most likely to roll
small boulders (frequent disturbance), least
likely to roll large ones, and roll medium sizes
and intermediate number of times. What was the
diversity on the different boulder classes?
36
Predation here the hypothesis parallels what
was described for keystone predators. The
intensity of competition among prey species is
reduced, thus permitting a greater number of
species to persist. This is especially true when
the predators follow a type III functional
response. If predation is prey frequency
dependent, then prey diversity can increase.
37
When we talk about diversity, we have to careful
to identify the scale we are talking
about. Within a single community, there is a
diversity of species present. At the local level,
we call this a diversity. There is a second,
larger level in which we identify the variation
in the species lists of communities in different
habitats within a region. This is ß
diversity. Finally, there is a third level that
considers the total species diversity over the
whole region. This level is called ? diversity.
? diversity average a diversity x ß
diversity
38
This relationship can be viewed in a slightly
different way. ? diversity indicates the entire
species pool available in a region. Not all
species from this pool are present in each
community. Instead, there is a process called
species sorting that separates the total pool
into groups of species that can successfully
coexist in single communities. One of the best
(and few) studies to investigate sorting was work
done by Paul Keddy of the University of Ottawa.
He planted seeds of 20 different wetland species
(his regional pool) into 120 different wetland
areas, among which physical and chemical
conditions differed. He then followed what
happened for 5 years
39
One species failed to germinate in any of the 120
sites. 5 others didnt persist in any community
for the full 5 years. That left 14 species to be
sorted by the differences in physical and
chemical conditions of the individual plots.
Ricklefs plotted the results in terms of the
average number of species in individual plots,
separated into those from plots with fertile
soil and those from infertile soil.
fertile soil