Transcript Exploitative Interactions - Cal State LA

Community Diversity
dynamics of community species
composition
Community Diversity
• At its simplest, community diversity can be
thought of as the raw number of species
occupying a small area (sometimes called
alpha diversity)
Community Diversity
• Generally, the species abundance in a
community will follow a lognormal
distribution
Community Diversity
• Metrics that aim to capture community
diversity should include two aspects of
diversity:
• absolute number of species (species
richness)
• relative abundance of each species (species
evenness)
Community diversity – species
richness and evenness
Community Diversity Metrics
• Two measures used to quantify community
diversity that incorporate both species richness
and evenness:
• Shannon-Wiener Index
s
H’ = -Σpilogepi
i=1
(H’ increases with increasing species richness and evenness)
• Simpson’s Index
γ = Σpi2
(γ= the probability that any two individuals drawn from the
sample will be the same species)
Community Diversity – Rank
Abundance Curves
• Can also visualize relative abundance and
species diversity in a community by plotting
relative diversity of species against their rank
in abundance
• In these plots, greater evenness in species
abundances is indicated by lower slope
Community Diversity – Rank
Abundance Curves
Community Diversity –
Phylogenetic Diversity
• evolutionary trees are increasingly being used
as a tool to assess community diversity
• A phylogenetic tree representing the
relationships of all the species in a community
can be constructed and used to determine the
evolutionary scope of the community
• based on summing ‘branch lengths’ for trees
Flora of a biodiversity hotspot, the
cape of South Africa
Forest et al. 2007
Community Diversity
• Factors that influence community diversity
include:
• environmental complexity
• species interactions
• disturbance
• history
Community Diversity –
Environmental Complexity
• Increased environmental heterogeneity /
complexity translates into increased niche
“space” for species to occupy, hence,
increased species diversity
• complexity can be influenced by different
factors for different types of organisms…
Community Diversity –
Environmental Complexity
• Animals may rely primarily on spatial habitat
complexity, e.g., volume of tree space available
for occupation by birds (MacArthur)
• Plants may rely more on chemical / nutrient
composition of substrate to divide up niche space
among different co-occurring species, reduce
effects of competition
• microscopic organisms (e.g., plankton) also avoid
competitive exclusion, increase community
diversity through chemical heterogeneity of their
environment
Community Diversity – Species
Interactions
• predation can act to increase community
diversity if the predator acts to suppress a
dominant competitor or consumer species,
therefore allowing resources (niche space) to
be divided among a larger number of species
Community Diversity – Species
Interactions
• competition can act to increase community
diversity if the competitive interaction causes
the species to partition resources (niche
space) more finely
Example using Darwin’s finches: Competition
drives character displacement in beak size
Studies of niche breadth in Anolis spp. fit the competition hypothesis.
In this case, prey length was used as an indicator of “niche breadth.”
Narrow niche breadth
in island with 5 species
Broader niche breadth
in island with one species.
Community Diversity – Disturbance
• Disturbance: a discrete event in time that
disrupts population, community or ecosystem
structure and changes resources or the
physical environment
• Joseph Connell: Intermediate Disturbance
Hypothesis
• species diversity will be highest in
communities that experience intermediate
levels of disturbance
Community Diversity – Disturbance
• High disturbance: favors dominance of a few
ruderal/weedy species with fast life cycles
• Low disturbance: favors dominance of a few
competitively dominant species
• Intermediate disturbance: enough time for a
wide variety of organisms to colonize; not
enough time for competitive exclusion
Sousa 1979
Measuring Diversity in Two
Redwood Forest Communities
Field Guide
# individuals
pi
lnpi
pi(lnpi)
2
0.05556
-2.890291761
-0.16058461
4
0.11111
-2.197234577
-0.244134734
5
0.13889
-1.974073026
-0.274179003
6
0.16667
-1.791739469
-0.298629217
7
0.19444
-1.637631647
-0.318421097
8
0.22222
-1.504087397
-0.334238301
9
0.25
-1.386294361
-0.34657359
Community Dynamics - Succession
Communities are not static, but constantly
change in response to disturbance,
environmental change, and their own
internal dynamics
Succession
• The process of succession occurs as the
composition and diversity of organisms in a
community change following exposure of a
new area for colonization
Succession
• Primary succession
is a process that
occurs on newly
exposed geological
substrate (little to
no organic activity
in substrate)
Succession
• Secondary succession is a process that occurs
when disturbance removes a community
without destroying the soil
Succession
• Pioneer community – the first species to
colonize an open area after disturbance
Succession
• Climax community –
the final assemblage of
species achieved in
succession: the
populations of the
climax community
remain stable until
perturbed by
disturbance
Succession
• Time required to get from pioneer to climax
community is highly context dependent:
• Intertidal boulder – 1.5 years
• Taiga – 1500 years
• Temperate forest – 150 years
Succession
• Generalities regarding the process of
succession:
• species composition changes
• species diversity increases
Succession
• Several mechanisms have been proposed to
explain how succession occurs:
• Facilitation (Clements 1900s)
• Tolerance (MacArthur and Horn 1970s)
• Inhibition (Connell and Slatyr 1980s)
Succession
• Facilitation - pioneer species are the only ones
capable of colonizing during early stages of
succession, pioneer species modify their
environment in such a way that it becomes less
suitable for themselves and more conducive to
the growth of other (later successional) species.
Succession
• Tolerance – all species tolerant of the conditions
in a particular environment (even climax species)
can be present in pioneer community. Less
emphasis on earlier species modifying
environment for later species; final species
composition of climax community simply reflects
the environment’s selection of all species that can
tolerate environmental conditions.
Succession
• Inhibition – proposes that earlier occupants of an
area modify the environment in a way that makes it
less suitable for both early and late successional
species. Late successional species can only invade an
area if space is opened up by the death of earlier
successional species. The climax community in this
model comprises species that are long-lived and
resistant to physical damage and biological factors
(competition, etc.). Earlier successional species more
vulnerable to physical and biological stresses.
Inhibition Model: algal succession on
intertidal boulders (Sousa 19979)
control
treatment
(Ulva sporelings
removed by hand)
Observed process of succession is often a
combination of different models
Community dynamics
• Climax community stability – some form of
disturbance likely to occur fairly frequently in at
least some parts of a community / ecosystem
• Community stability defined as the ability of a
community to tolerate disturbance or the
ability of a community to reassemble itself after
a disturbance (resilience)