BIOB50H3 Chapter Notes - Chapter L#18: Intermediate Disturbance Hypothesis, Keystone Species, Starfish
LECTURE #18, pages 424-434, 438-443
CHAPTER 19: SPECIES DIVERSITY IN COMMUNITIES
- found the SNV infection prevalence increased as
species diversity declined species diversity buffers
the transmission of zoonotic pathogens to wildlife and
humans
COMMUNITY MEMBERSHIP (19.1)
- the abundance and distribution of organisms are
dependent on 3 interacting factors: (1) regional
species pools and dispersal ability (species supply),
(2) env. Conditions, (3) species interactions
Species supply is the “first cut” to community membership
- reigona species provides the absolute upper limit on
the # and types of species that can be present w/I
comm. the regional pool supplies species to
communities
- humans have greatly expanded the regional species
pools of comm. By serving as vectors for dispersal (eg.
Ships ballast-mediated invasion allow for some
species to move from one area to another)
oi.e zebra mussel invading the Great Lakes
Environmental conditions play a strong role in limiting community membership
- a species may be able to get to a community, but fail to become a member of the
comm. b/c it can’t tolerate the env./abiotic conditions there
eg. Abiotic conditions of a lakes make it a good place for fish, but not terrestrial
plants
it is not smart to rely on physiological constraints to exclude all species while a lot
of species die (that are released by the ballast water b/c are in unfamiliar
conditions), some can still survive
Who interacts w/ whom makes all the difference in community membership
- the final reason for why a species can disperse or not is if it can co-exist w/ other species
some might depend on others for growth, others might be excluded from a
community by competition/predation/parasitism/disease
-biotic resistance: the failure of non-native species to become incorporated into comm.
b/c native species can exclude or slow down the population growth of non-native
species
eg. Native seed web moth breeds and feeds on the seedpods of the invasive gorse
shrub
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How are species invasions enhanced by climate change?
-5 potential consequences
1. CC alters the pathways (transport and introduction) of non-native species eg. CC
would better link areas that are geographically separate (eg. Non-native green crabs
were introduced to the SF bay form the East Coast of North America and colonized
the Pacific during warm years crab larvae were transported in stronger, warmer
northward flowing currents to estuary locations near Oregon and Washington and
survived there
othus: changes in coastal currents caused by global wamring create new
pathways of dispersal for non-native species
2. Alteration of env. Constraints on non-native species that allows some species to
overcome physiological changes/or biotic constraints on their persistence outside of
their native range eg. Green crabs can’t persist in cold water, but they were able to
survive in semi-warmer winters
3. Alteration of the distributions of existing invasive species climate change can
expand their geographic range in dramatic ways
4. The impacts of non-native species are altered eg. Green crabs can outcompete
the red rock crab (that is more dominant in the colder parts of the estuaries)
5. Effects on the management of non-native species
RESOURCE PARTITIONING (19.2)
- each resource available in a
community varies along a resource
spectrum; this spectrum represents
the variability of an available
resource (not the amount)
resource use of each species falls
along this spectrum and overlaps
w/ the resource use of other
species to varying degrees more
overlap = more comp (w/ the
extreme being competitive
exclusion)
- possible ways in which resource partitioning might result in higher species richness in
some comm. Than others
1. Species richness could be high in some comm. b/c high degree of partitioning (i.e
if all species have less overlap = more indiv. Can be packed into it)
olow overlap could be b/c evolution of specialization or character
displacement
2. Species richness could be high in some comm. b/c the resource spectrum is broad
Early studies suggested the resource partitioning was the main mechanism of coexistence
-Robert MacAruthur equilibrium theory of island biogeography
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Studied warmbers in the forests of North America these forests are home to the
Setaphaga warbler species that migrate form the tropics each spring to breed and
feed on insects
oMacarthur looked at the feeding habits, nesting
locations and breeding territories of 5 species to
see how they might coexist
Found warblers use diff. parts of the habitat in diff. ways
oEg. Yellow rumped warbler fed on the middle
parts of trees to the forest floor, white breasted
fed on the middle of the of a tree (and both
inside/outside of the tree canopy), the
blackburnain and cape may, both feed on the tree
top
Suggests: were able to coexist b/c were partitioning
resources
- Found a positive relationship between bird species diversity
and height diversity
-Tilman study on diatoms fighting over silica (mentioned
before) proposed the resource ratio hypothesis: species
coexist by using resources in diff. ratios or proportions
oProposed that diatoms despite using the same
nutrients, would be able to coexist by acquiring
same set of limiting nutrients, in diff. ratios
oWhen Tilman grew the 2 diatoms in differing ratios of
silica and phosphorus, found that Cyclotella was able to dominate only when
ratio of Si:P was low, while Asterionella outcompeted it wen Si:P was high only
when the ratio of Si:P were limiting to both species, could they coexist
- This variability in resources occurs in the env. i.e variation in soil nitrogen and
moisture
RESOURCE MEDIATION AND SPECIES DIVERSITY (19.3)
Processes that mediate resources can allow species to coexist
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