Download Species Diversity in Continental and Marine Habitats Questions: 1

Species Diversity in Continental and Marine Habitats
Questions: 1) What determines the numbers and kinds of species that occur
together in one place? 2) Why do the numbers and kinds of species vary from one
place to another?
Species richness: the number of species in a local area or geographic region
Alpha diversity: the species richness of a local ecological community
Beta diversity: the change (turnover) in species composition over a relatively small
distance - often between different but adjacent habitats
Gamma diversity: total species richness of a large geographic region
Species richness increases in most groups in a latitudinal gradient from temperate to
tropical regions. Organisms restricted to temperate regions show an exception to
this pattern (e.g., Pinaceae has highest diversity at the mid latitudes).
Comparisons for #’s of tree species:
Northern Canada -1 - 5 spp per hectare
Eastern N. America -10 - 30 spp per hectare
South/Central America -- 40 - 100 spp per hectare
Animals that depend on plants will probably show comparable patterns of diversity
shown for plants in Table 15.1.
Similar latitudinal gradients demonstrated for continents are also found in the
oceans. Tropical coral reefs support the most diversity.
Ditto for freshwater systems.
What processes explain these patterns? Many hypotheses have been proposed
(Table 15.2). These hypotheses are not necessarily mutually-exclusive, and they can
be divided into two classes:
Equilibrial: adjustments of biota to current geological, climatic, and oceanographic
conditions
Nonequilibrial: reflect perturbations of the past (e.g., glaciation)
Global patterns to consider: the amount of sunlight changes with latitude
(hypotheses about productivity, harshness, climatic stability, habitat heterogeneity);
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so does the surface area (hypotheses about habitat heterogeneity)
Patterns of Diversity for particular geographic features
Peninsulas
can be thought of as an island connected to a mainland. Diversity decreases with
increasing distance to connection to continent.
latitudinal gradient may confuse the pattern, though. Historical perturbations may
also have an impact
Elevation
decreasing diversity with increasing elevation (could relate to temperature or water
availability)
Some exceptions are seen, of course: Orchids attain richest diversity on tropical
mountainsides
Fig. 15.8, 15.9
Aridity
species diversity decreases with decreasing water availability
Aquatic environments
patterns of diversity mirror terrestrial systems in regards to species-area curves and
latitudinal gradients.
In addition, highly productive regions can be species-rich or -poor and tend to occur
in small, patchy regions.
There may be an elevational gradient in terms of decreasing diversity with
increasing depth of the water column. This patterns seems to hold for marine
systems as well.
Rapoport’s rule: for areas of the geographic ranges of subspecies or geographic
races of a species North American mammals: taxa at higher latitudes have larger
ranges (Fig. 15.15)
Alpha and Beta diversity appear to be correlated. Along major geographic
gradients, as the diversity of species within a habitat or small area increases, the
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turnover of species between habitats and across the landscape also increases.
Summary of patterns
Alpha and gamma diversity increase with decreasing latitude, beta diversity
increases, geographic range decreases, and distribution of abundances among species
becomes more equitable.
Causes
Nonequilibrial mechanisms
Glaciation and climatic change
shifting poles
long-term climatic shifts
formation of mountains and other geological features
All these factors play into the diversification of life on the planet (evolution
happens)
The history of many taxonomic groups is characterized by rapid radiation
followed by stasis and eventual extinction.
Pleistocene history has shown that 1 to 2 million years is sufficient
evolutionary time for a lot of change in terms of adaptation and speciation.
Equilibrial mechanisms
Productivity
more productive environments support more species because, on average,
species can be more specialized and still maintain sufficiently large populations
to avoid extinction.
focus on producers and where they can release the most energy (surface of
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waters, where sunlight is most intense and conditions for photosynthesis are
most favorable)
Harshness and abiotic stress
stressful environments are not necessarily unproductive (e.g., salt marshes,
hotsprings, eutrophic lakes all have high levels of productivity)
life is constrained by some absolutes -- low temperature, high temperature,
desiccation, freezing, low oxygen levels, extreme pH or salinity all affect
biochemical reactions of life; few species can function at the extremes, but
those that do have adaptations to those environments (antifreeze in the blood
or tissues, ability to withstand drought, etc).
harsh environments may be areas where extinction rates are high, speciation
rates are low, and where few species are able to colonize
Habitat heterogeneity
close correlation between species richness and the complexity of vegetation
structure; reflects a tendency of coexisting species to use different niches
is structural complexity a cause or a consequence of diversity? (Both)
Area
tropics are spatially extensive as compared to temperate and polar regions;
large areas support more species than smaller ones
Biotic interactions
competition, predation, mutualism determine how physical resources of a
region are allocated among species to produce the observed species diversity
and community structure
Speciation and extinction rates
everything else being equal, one would expect the absolute rate of extinction
and speciation to be higher in the tropics because there are more species
there.
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All these factors contribute, but act on different temporal and spatial scales; some
are more direct and proximate than others.
Time:
longer periods of time increase opportunities for speciation and colonization
Space:
larger spaces provide larger targets for colonization, increased opportunities
for speciation, support larger populations (decreased extinction)
Continental Patterns and Processes
We’ve already reviewed Bergmann’s (larger animals and higher latitudes), Allen’s
(animals in hotter environments have longer appendages), and Gloger’s (animals in
humid environments have darker coloration) rules.
There are also geographic patterns regarding clutch and litter size -- larger clutches
and litter size at higher latitudes.
Areography: sizes, shapes and overlaps of ranges
Patterns
1. within most large taxonomic groups, the majority of species have restricted
ranges and only a few species occur widely over most of a continent
2. the majority of species have relatively small ranges (Fig. 16.4)
Boundaries of ranges are set by similar kinds of environmental limiting factors
dynamics of colonization, speciation, and extinction processes have similar effects on
the relative range sizes of unrelated taxa.
small ranges tend to be oriented N-S and large ranges tend to be oriented E-W in
North America; small and large ranges tend to be oriented E-W in Europe -- this
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reflects the physical geography (mountain ranges, river valleys, coastlines) of these
regions
Species with a small range relative to its body size are more prone to extinction.
Great American Interchange
following the formation of the Central American Landbridge, biotic interchange was
possible. Table 16.4 summarizes the major events.
Biogeographic isolation of South America allowed development of a distinct
endemic land mammal fauna; intermittent contact via stepping stone dispersal
routes allowed colonization by primates, armadillos, sloths, anteaters, porcupines,
pacas, guinea pigs, etc. Speciation and adaptive radiation followed.
Central American Landbridge ended the isolation. landbridge has served as a filter
with more exchange from North to South (Fig. 16.13); led to extinction of many
South American taxa
about 1/2 of South American spp are derived from North American ancestors,
whereas only 10% of North American spp are derived from South American taxa
Advantages of Northern Taxa as compared to Southern spp
1. better migrators
2. better survivors and speciators
3. better competitors
Convergence of Biotas
Convergence observed at the species level (adaptations to similar habitats) -- e.g.,
Fig. 16.19, 16.20
Can also be observed at level of biotas -- e.g., mammals of Australia and North
America (Fig. 16.21) -- physical resemblance may not directly relate to function of
taxa, though. For example, the marsupial cat is smaller, more insectivorous and less
arboreal than the ocelot.
There may be ecological convergence related to niche (fig. 16.23). Insectivorous
bird species inhabiting the chaparral and matorral have similar size and forage at
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similar heights in the vegetation.
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