Transcript Global Patterns in Species Richness

Global Patterns in Species
Richness
BY
MuzvondiwaJ.V.
Guiding Principles
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From the tropics through temperate areas to the poles, there is a
gradient in species richness, with the highest richness in the
tropics and the lowest in the polar areas. Many biotic and abiotic
factors can explain the changes in species richness over these
large scales.
Some communities in similar habitats from different parts of the
world, such as plants in deserts, converge in numbers of species.
Others, such as lizards in deserts, do not.
It is estimated that there are 12.5 million species on Earth.
To preserve biodiversity, we can focus on saving countries with the
highest numbers of species or saving areas with the highest
numbers of endemics.
The preservation of biodiversity is important because recent
experiments show that communities perform best when they
have a full complement of species.
Introduction
• Canada has a system of 34 parks located in nearly all of
the principal biomes
• The number of species they contain is commonly called
species richness
• Review of species of parks (Rivard, 1999)
– Compared species list when the parks were established to
a recent survey
– Some species were lost from certain parks
• Missing species were mainly those hunted by humans
– Species richness increased in some parks
• Influx of species also associated with humans (rats, starlings,
sparrows, and pigeons)
• Other species have been deliberately introduced, including game
species
Introduction
– Rivard and colleagues found that changes in
species richness were related to climate,
particularly mean annual potential evaporation
– Change in species richness in Canadian parks was
highest in the warmer, southern areas because
these contain the most humans
• Understanding species richness
improve management practices
would
Explanation of Species Richness
Gradients
a. Background
– Generally, the number of species in any habitat increases from
polar areas through temperate areas, and reaches a maximum
in the tropics
– Species richness is also increased by topographical variation.
Hence, the increase in birds and mammals in the West
• Mountains provide a wide range of habitats, thus increasing species
richness
– Richness of trees in North America is not well linked to
latitudinal gradients
• Trees do not grow well in deserts, even though there is a decrease in
latitude and an increase in topographical variation
• Tree richness is linked to rainfall level
– Understanding theories that explain species richness is
important in order to conserve biodiversity on Earth
Explanation of Species Richness
Gradients
b. Biotic explanations
i.
Spatial heterogeneity theory
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There are more plant species in the tropics than any other
climatic zone
These plant species support higher numbers of
herbivorous animal species, and hence more carnivores
Increased plant richness increases richness in herbivores
by
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Increasing the numbers of monophagous herbivores
Creating a more diverse architectural complexity
Theory does not explain why there are more plant species
to begin with
Explanation of Species Richness
Gradients
ii. Competition theory
– Advanced by Theodosius Dobzhansky (1950)
– In temperate climates, natural selection operates
through harsh environmental extremes and species
are r-selected
– In the more constant tropical temperatures, species
are thought to be more K-selected, to compete more
keenly and interact more
• Increased competition narrows the breadth of niches
available, allowing more species to pack along the resource
axes
– Little data gathered to test this theory
Explanation of Species Richness
Gradients
iii.
Predation theory
– Proposed by Robert Paine (1966)
– Contrary to the competition hypothesis
• Argues that predators and parasites in the tropics hold
prey populations down to low levels such that more
resources remain and competition is reduced
• Allows for more species to coexist
• Increased richness promotes more predator species
Explanation of Species Richness
Gradients
– Evidence from intertidal communities (
• Pisaster (starfish), top predator
• Food web was fairly constant
• Pisaster preyed on another predator Thais (whelk), and
on chitons, limpets, acorn barnacles, and bivalves
• If Pisaster was removed, diversity decreased from 15 to
8 species
– Mytilus increased in numbers, crowding out other species
– Pisaster prevented any species from monopolizing the space
– Pisaster was termed a keystone species
Explanation of Species Richness
Gradients
– Darwin (1859) also noted where predation
allowed the coexistence of many prey, when he
observed more grass species in areas that were
grazed
– In order for this theory to explain tropical
richness, predation would have to be intense on
the majority of species at all trophic levels
• No data to test such a theory
Explanation of Species Richness
Gradients
iv.
Animal pollinators theory
– Most tropical plants are pollinated by animals
– Close associations develop between plants and
specific pollinators, increasing the reproductive
isolation between plant populations—increasing the
rate of speciation
– Coevolution of plants and pollinators ensures high
rate of speciation
– Theory may apply to terrestrial systems, but cannot
easily explain the similar diversity gradients in aquatic
ecosystems
Explanation of Species Richness
Gradients
b.Abiotic theories
i. Time theory
• Species richness increases over time. Ancient,
unglaciated communities in the tropics contain more
species. More temperate areas were glaciated and
species richness has not yet returned to pre-glacial
levels
• Sanders (1968) provided evidence in support of the
evolutionary time theory
– Compared bottom-dwelling invertebrate diversity in glaciated
and unglaciated Northern Hemisphere lakes that occur at
similar latitudes
Explanation of Species Richness
Gradients
– Lake Baikal (an ancient unglaciated lake) in the former Soviet
Union had 580 species of benthic invertebrates
– Great Slave Lake (a glaciated lake) in north Canada had only 4
species in the same zone
• Southwood (1961)
– Investigated insect herbivore diversity on British trees
– Found good correlation of insect richness with the length of
time the species of tree inhabited Britain
– However, American ecologist D. Strong (1974) found insect
species diversity on each tree species was better correlated
with the area over which a tree species could be found
Explanation of Species Richness
Gradients
ii. Area theory
– Based on the notion that larger areas contain more species
because larger areas can support larger populations
– Bigger populations result in fewer extinctions
– Large areas with climatic similarity will have greater
species richness
– The tropical regions of both hemispheres are adjacent,
creating one large area with a similar climate. The result is
increased species richness
– Area theory does not seem to account for the relatively
few species in the vast contiguous landmass in central Asia
nor in the deep sea (in comparison to shallow tropical
surface waters).
Explanation of Species Richness
Gradients
iii.
Productivity theory
– Proposes that greater primary production by plants
results in greater overall richness (Wright 1983)
– Currie and Paquin (1987) showed that the diversity of
tree species in North America is best predicted by the
evapotranspiration rate. Annual evapotranspiration is
correlated with primary production
– Gratehouse and Carey (1987) demonstrated a
correlation between richness of British butterflies, and
sunshine and temperature during the months they
were on the wing (suggesting a relationship between
energy and species richness)
Explanation of Species Richness
Gradients
– Theory fails to explain
• Why tropical seas have low productivity but high
richness
• Why eutrophic lakes and lakes polluted with fertilizers
have high productivity but low richness
• While in North America, there is evidence for the
evapotranspiration theory, the pattern does not hold
for broad comparisons between continents
– The temperate areas of eastern Asia support 729 tree species,
while climatically similar North America supports 253 species,
and Europe supports 124 species
Explanation of Species Richness
Gradients
iv.Evolutionary speed theory
– Klaus Rohde (1995) proposed that effective
evolutionary time promotes high species richness.
– High effective evolutionary time is the result of
evolutionary speed and geological time during which
an ecosystem has existed under more or less the same
conditions
– Evolutionary speed is promoted by high temperatures,
which foster
• Shorter generation times
• Higher mutation rates
• Increased selection
Explanation of Species Richness
Gradients
– Generation time is accelerated by temperature, but no
evidence for higher mutation rates or increased selection
– Evolutionary speed theory predicts
• High richness in the tropics
• High richness in the deep sea
• Accounts for high richness in areas next to these areas
Explanation of Species Richness
Gradients
v. Conclusion
– Eight theories are neither exhaustive nor mutually exclusive,
and can be combined in many permutations
– Some biotic explanations are insufficient: explanations that
invoke increased competition, predation, or disease are
secondary explanations. A primary explanation is still needed
– There may be good correlations between abiotic variables and
species richness; however, it is not clear why increased
productivity promotes diversity and not simply higher
population densities of just a few species
– The most realistic way to examine causes of richness gradients
was proposed by Ricklefs and Schluter (1993)
• Different processes may act on different scales
– Biotic factors: local scale
– Evolutionary processes: provincial or global scale (Figure 15.4)
Community Similarity
a. Communities from different parts of the globe
are similar
– Similarity in vegetation in climatically similar areas
around the globe
• Ex. Cacti-like plants occur in deserts around the globe
– If species converge in morphology, can
communities converge in species richness?
• Similar species diversity in similar habitats
• Dissimilar species diversity in similar habitats
Community Similarity
b.Eric Pianka (1986): Most comprehensive field
studies
– Examined desert lizard species around the world
– 61 species in Australia, 22 in southern Africa, but
only 14 species in North America
– Australia had more lizard species than southern
Africa, no matter what the habitat was
– Suggests that there are indeed strong
evolutionary constraints in southern Africa
Community Similarity
c. Robert Whittaker (1972)
– Named the differences in richness
• Within habitats, alpha (a) diversity
• Between habitats, beta (b) diversity
• Overall difference in diversity between two
geographical regions, gamma (g) diversity (which is the
product of alpha and beta diversity)
– Comparison of Australia and southern Africa
Community Similarity
d.John Lawton and colleagues (1993)
– Examined convergence in diversity in guilds: the
actual way species utilize a common resource
– Ex. Bracken fern, Pteridium aquilinum
• Widespread
• Over last 20 years, surveys of insects were conducted
• Species assemblage varies remarkably, giving no
evidence of taxonomic similarity in the fauna
• Variation in total number of insects exploiting bracken
fern is partly a function of how common and
widespread the plant is in each geographical region
Community Similarity
– Effects of biotic interactions
• Distribution of species across resources on the plant is
idiosyncratic from locality to locality, with numerous vacant
niches
• Parts of plants go unutilized in certain areas of the world. It
does not look like there is convergence of feeding types
across regions nor does it look like competition is an
important factor
• Only pattern: leaf seems to be more exploited than the rest
of the plant
– Summary of main rules governing species richness of
insects on bracken fern. Different factors are
important at different scales
Global Species Richness
• 2 million species have been classified, and best estimates
suggest that about 12.25 million exist, with a maximum
estimate of 118 million
• Insects represent 2/3 of the total number of species. Why?
• In any given taxon, it is not the smallest or the largest
organisms that are the most abundant, but the species that
are intermediate between smallest and medium-sized
• Plot of number of species on Earth against body size,
measured in length. Insects fall right where the number of
species is highest
• In order to get a more accurate measure of the number of
species on Earth, we need to obtain a better estimate of
the number of insect species
Global Species Richness
• Based on observations by Terry Erwin (1982), an
estimated 30 million species of arthropods must be in
the tropics alone
– Estimate is based on one canopy of one tree species in
Panama:
• He found 1100 species of beetles
• 160 species specific to that species of tree
• Beetles represent 40% of all arthropods, so he estimated that
there must be 400 arthropods specific to that tree canopy, and a
total of 600 insect species for the whole tree
• 50,000 different species of tropical trees would result in 30 million
arthropods on tropical trees all.
• Add insects in the soil and insects in temperate forests and
grasslands, results in an estimate of 100 million
Global Species Richness
– Problems with estimate
• Many tropical insects probably utilize more than one
tree species
• Many insect species that were taken to be newly
discovered had actually been discovered earlier
• Most probable estimate—between 5 and 10 million
(Gaston, 1991)
Preserving Species Richness
• Conservation of biodiversity differs from
single-species focus because the aim is to
conserve the habitats that contain the most
species
• Which habitats contain the most species?
– Generally, tropical habitats have the most species
– Ex. La Selva Forest Reserve in Costa Rica (13.7
km2) has almost 1500 plant species, which is more
than is found in Britain (243,500 km2)
Preserving Species Richness
• Methods for identifying areas for conservation
– Target countries with the greatest number of species
(megadiversity)
• Mexico, Columbia, Ecuador, Peru, Brazil, Zaire, Madagascar,
China, India, Malaysia, Indonesia, and Australia
• Together, these countries hold up to 70% of the diversity in
those groups of organisms
• Megadiversity approach works well because large countries
with large areas garner most of the available international
funds
• Drawback: Although they contain the most species, they do
not necessarily contain the most unique species. What is
needed is a measure of uniqueness (number of endemics)
Preserving Species Richness
– Norman Myers and his colleagues (2000) identified 25
"hot spots" for endemic forest plants (representing
1.4% of the world’s total land area, that together
contain
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133,149 endemic plant species (44% of the world’s total)
9645 endemic vertebrate species (35% of the world’s total)
Tropical Andes has the most endemics
Protecting these hot spots would prevent the extinction of a
larger number of endemics than would protecting areas of
similar size elsewhere
Preserving Species Richness
• Areas that are enriched in endemics of one taxon are
often rich in endemics of another
• Drawback to preserving species richness through
endemism is that most areas rich in species and
endemics are in the tropics. Thus, other major habitats
would be ignored
– Best strategy may be to take into account
richness, endemism, and type of habitat
Species Richness and Community
Function
1.Concerns for loss of biodiversity (Ehrlich and
Wilson, 1991)
– Moral responsibility to protect what are our only
known living companions in the universe
– Humanity has already obtained enormous
benefits from biodiversity in the forms of foods,
medicines, and industrial products
– The wide array of essential services provided by
natural communities
Species Richness and Community
Function
2.Natural communities provide essential
services
– Maintenance of the correct gaseous composition
of the atmosphere
• Prevents global warming and loss of soil biodiversity
– Maintenance of a reservoir of natural enemies to
prevent pest outbreaks
– Maintenance of a reservoir of pollinators to
pollinate crops and other plants
Species Richness and Community
Function
3. More diverse communities perform better than
less diverse ones
– Shahid Naeem et al. (1994) experiment
• A series of 14 environmental chambers, each 1 m2
• Replicated terrestrial communities that differed only in their
biodiversity
• The communities consisted of 9, 15, and 31 species, spread
across four trophic levels, with species-poor communities
being subsets of the more diverse communities
• Experiment ran 6 months
• Results: as richness increased two- to threefold, so did
community productivity
Species Richness and Community
Function
– David Tilman and his colleagues (Tilman 1996; Tilman
et al., 1996) hypothesized that diversity increases both
resistance and resilience
• Studies directly manipulated plant diversity and measured
plant production and nitrogen extraction rates from the soil
• 140 plots, each 3 m x 3 m, with comparable soils
• Sown with seeds of 1, 2, 4, 6, 8, 12, or 24 species of prairie
plants, and replicated 20 times
• Results: More diverse plots used nutrients more efficiently
than less diverse ones and exhibited increased productivity
• Same results were found in a sample of 30 unmanipulated
native prairies
• Reason: In more diverse communities, interspecific
differences in soil use among plant species allow fuller use of
nitrogen, the main limiting nutrient. So as species richness
increased, more nitrogen from the soil was used up
Species Richness and Community
Function
– Relationship between productivity and diversity is
not linear
• Strongest at lower levels of diversity and weaker at
higher levels of diversity
• Productivity would probably increase dramatically as
tree richness increased from 0 to 40 species, but might
increase little after that (diversity matters, but only up
to a point)
Applied Ecology
a. Loss in species richness weakens community resistance
to invasion by exotic species
– Elton (1958) suggested that species-rich communities
would be more resistant to invasion from exotic species
• Richer systems would more fully utilize limiting resources, and few
would be left over for invaders to use
– Elton also suggested that increased richness would reduce
the severity of plant disease
• Transmission rates are proportional to the host species population;
therefore, in rich communities there are fewer individuals of any
one host
– Increased plant richness should lead to increased diversity
of insect herbivores and hence increased predator and
parasite richness
Applied Ecology
b. Field experiments to test Elton’s ideas (Knops et al.,
1999)
– Small biodiversity experiment with 140 plots that were 3 m
x3m
• Each plot was weeded to control the species that inhabited the
plots
• Small plots has either 1, 2, 4, 6, 8, 12, or 24 native grassland
species
– Large biodiversity experiment had 342 plots that were 13
m x 13 m
• These plots were not weeded as carefully, but were large enough
to examine the effects of richness on disease and insect herbivore
attack rates
• Large plots has either 1, 2, 4, 6, 8, or 16 native grassland species
Applied Ecology
– Results
• The frequency of invaders decreased with increased
species richness and the biomass of the invaders also
declined
• The level of fungal disease also decreased with richness
• Arthropod species richness increased with plant species
richness
• Overall, as species richness increases, fewer species
invade the area, plants are less susceptible to disease,
and insects are more diverse. It appears that Elton was
right!
Summary
• Species richness increases from poles to the equator. A
variety of biotic and abiotic factors have been proposed to
explain this phenomenon. Most explanations are
incomplete or seem secondary. Over different scales, the
predictive power of the explanations varies considerably.
For instance, predation and competition may be important
on the local scale, evolutionary time may be important on a
large scale
• Convergence in species richness. Species richness between
areas has been examined with reference to the terms a , b ,
and g diversity. a diversity exists within habitats, and b
diversity obtains between habitats. The total diversity g is
the product of a and b.
Summary
• Estimating the total richness of species on Earth
is fraught with difficulty, but a figure of about
12.5 million species has been proposed
• A variety of methods have been proposed for
managing richness and biodiversity on Earth,
including consideration of megadiversity and hot
spots
• Evidence is emerging that species richness can
affect ecosystem function: In general, species-rich
communities perform better than species-poor
communities
Discussion Questions
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Summarize the evidence for and against abiotic and biotic factors
as drivers of the increase in species richness from the poles to the
tropics
Although most groups of organisms become more species-rich
toward the tropics, sometimes the reverse pattern is found, as
with aphids and parasitic insects. Why do you think this might be?
To preserve biodiversity on Earth, should we focus primarily on
species-rich areas, on "hot spots" of rare or endemic species, on
underrepresented habitats, or on some combination thereof?
List emergent properties that communities might exhibit, such as
stability, resistance to invasion, and other such properties.