Species Diversity
Species Diversity
• In this section we will explore what is
species diversity and what are the factors
that can influence it
Species Diversity
• In actuality, most studies estimate
diversity, they do not get a complete count
• Estimates are based upon the number of
species and their relative abundance
Species Diversity
Species Diversity
• In describing a general theory of diversity,
Brown has termed ‘capacity rules’ and
‘allocation rules’
• Capacity rules define those characteristics
of any given environment which affect its
capacity to support life, broadly influencing
the total number of organisms
• E.g. BBS routes, S, body weight and AB
Species Diversity
• Allocation rules determine the inherent
‘divisibility’ of those resources and the
ways in which the available resources are
subsequently partitioned amongst species
(think about it as biotic interactions)
Species Diversity
• Studies in community frequently
incorporate aspects of species diversity
• Patterns of species diversity in time and
space form the basis of many important
ecological models, including mechanisms
of succession, explanations of latitudinal
gradients, hypotheses of mass extinctions,
and relationships between diversity and
stability
Species Diversity
• Can be used to assess the larger
conservation value of an area (e.g.
indicator group)
• Also used to evaluate the success of
nature reserves (or restoration projects) or
assay the effects of environmental
perturbations
Primary Factors
• There are several factors are thought to
influence species diversity; the two most
studies are productivity and structural
complexity or heterogeneity
Primary Factors
• Primary productivity: provides direct and
indirect energy for plants and
subsequently, animals
• Has been shown to be both positively and
negatively associated with diversity
(maybe dependent upon which part (all or
a narrow part) of the resource spectrum
has been added)
Positive Correlation
Positive Correlation
Potential
evapotranspiration is
a combination of
temperature and solar
radiation and
represents the
maximum amount of
water that would be
lost when water is not
a limiting factor
Not surprisingly, PET
is strongly correlated
with latitude
Negative Correlation
the ‘paradox of enrichment’
Sometimes diversity
decreases when
nutrients or other
resources that increased
productivity are added to
a system
Particularly evident in
aquatic systems
(monopolization of
resources by algae,
simplifying the
ecosystem)
Negative Correlation
the ‘paradox of enrichment’
In many systems diversity
peaks at intermediate levels
Furthermore, there are many
examples of one community
increasing in diversity along a
productivity gradient while the
diversity of another group is
decreasing along the same
gradient
Correlated: Spatial Heterogeneity
• The number of factors that contribute to
spatial hetero are numerous and can
include the organisms themselves (i.e.
‘structural organisms’) which can result in
a positive feedback loop
Spatial Heterogeneity
• For plants, geologic processes influence
spatial distribution of resources/minerals
• For animals, plants generate vertical
structure and complexity by the roots,
stems, branches, and leaves of woody
plants
Spatial Heterogeneity or
Complexity
• Structural complexity
may increase species
diversity by creating new
‘additional’ niches that
were not there
Spatial Heterogeneity or
Complexity
• Fig 8.3
Spatial Heterogeneity
• Relationship between
habitat structural
complexity, measured
as foliage height
diversity and bird
species richness. This
relationship is
typically positive (A),
but has been shown
to be negative in
some cases (B)
Correlated: Latitude
Latitude Gradients
Correlated: Sample Area
• With few exceptions, large areas have
more species than small areas
• This is not a trivial observation as the
underlying mechanisms include most of
those that are potentially important in
regulating diversity
• The pattern, species-area curve, can give
insight into the total number of species
and how rapidly species are accumulated
Species-area Curve
• Species-area curves also yield information
as to how quickly species are accumulated
where S = kAz
S is the number of species
k is a constant
A is the area of the sample
z rate at which the number of species with
area (ranging from 0.2 – 0.4)
Species-area Curve
Species-area Curve
Different taxonomic
groups will
generate different
species- area
curves (in
intercept, slope, or
both)
Species-area Curve
• Differences in the ecological processes
regulating the diversity of different
landscapes, but those differences can only
be deduced by comparative studies of the
ecosystems themselves, rather than
differences in the shapes of the curves
(although not all agree on this)
Comparisons
Comparison of the
avifauna of four
habitat types in
the neotropics
Temporal Comparisons
Comparison of
avian community
in three years.
During the
‘drought’ breeding
species were
accumulated more
rapidly than other
years, despite
there being
significantly fewer
species
Species-area Curve
• Differences in the ecological processes
regulating the diversity of different
landscapes, but those differences can only
be deduced by comparative studies of the
ecosystems themselves, rather than
differences in the shapes of the curves
Correlated: Age of Sample Area
•
Three primary processes that influence
the diversity/age correlation (on time
scales of hours to millennia)
(1) Dispersal and migration
(2) Biologically produced changes in
heterogeneity
(3) The balance between speciation rates
and extinction rates
Dispersal and Migration
• Different species
and different
taxonomic groups
have different
dispersal
capabilities
• e.g. plants and
oceanic islands
Biologically Produced Changes in
Heterogeneity
• Much of the increase in diversity is
correlated with succession changes
associated with plant community and in
the long-term, the addition of nitrogen and
organic matter to the soil
Evolutionary Increase
• Over long periods with appropriate
conditions, evolution can contribute to the
species diversity (assuming speciation
rate is higher than extinction rate)
• e.g. Pleistocene refugia model
Diversity and Disturbance
• Disturbance, and the time to recover, can
be both positively and negatively
associated with species diversity and that
can vary across spatial scales
Infrequent,
massive
disturbances
Frequent,
less severe
disturbances
Diversity and Disturbance
Highest diversity is achieved
at a medium level of
disturbance. However,
when small boulders were
attached to the substrate
(hence negating the
disturbance effect), the
structure complexity
resulted in higher species
diversity
Temporal Aspects: Ecological Time
• Using a chrono-series, we can better
understand how a process such as
succession works
Temporal Aspects: Ecological Time
• Age of
succession is
related to both
increased
species diversity
(2nd Y-axis) as
well as increased
productivity.
Temporal Aspects: Ecological Time
• Species can also be
accumulated over time if a
new habitat has been
‘created’
Temporal Aspects: Evolutionary
Time
• We frequently see
an increase in
diversity over time
within a taxonomic
group. Why?
Temporal Aspects: Evolutionary
Time
• However, it is
probably just as
frequent that diversity
bounces around
some mean with
speciation and
extinction being
somewhat in
equilibrium