The latitudinal gradient in species diversity

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The latitudinal gradient in species diversity

• One of the oldest and most fundamental patterns
  concerning life on earth (Willig et al. 2003)
• Source Encyclopedia of biodiversity and Willig
  et al. 2003. Annual Review of Ecology and
  Systematics.

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The latitudinal gradient

• The pattern is abundantly clear. As soon a
  biologist sets foot in the tropics, terrestrial,
  freshwater, or marine, extant or fossil, almost
  any taxonomic group, the increase in species
  richness is apparent.
• In general there are more species at lower
  latitudes.
• There are lots of taxonomic groups that show
  latitudinal gradients, and some that show reverse
  latitudinal gradients (seals, penguins,
  sandpipers, conifers)
• Willig et al. plotted results from a literature
  survey and showed most studies (150) found a
  negative relationship between latitude and
  richness, although some found no relation, a
  modal relation, or positive relation (20 studies
  each).

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2 examples, mammals, just bats

• Bat species richness (upper)
• Bat feeding guild richness (lower)
• Mammal richness (left)

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• Communities at work? consider the graphs for bats
• This example appears to demonstrate that when
  samples are chosen carefully to include real
  communities (species are presumably interacting),
  a clear pattern of latitudinal gradient emerges.
• Another example exists for nonvolant mammals.

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• These figures show results of studies testing for
  the latitudinal gradient in species diversity in
  a variety of taxa.
• The extent of latitude influences the outcome.

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Why does the gradient exist, and is it relevant
to conservation biology?

• The questions are
• What causes the latitudinal gradient?
• How can I use this information to benefit
  conservation goals?
• Understanding the patterns and processes of
  biodiversity is useful to conservation because
• Lets fill in the blanks
• 1.
• 2.
• 3.
• 4.
• 5.

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Why does the gradient exist, and is it relevant
to conservation biology?

• Understanding the patterns and processes of
  biodiversity is useful to conservation because
• Comments?

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Why does the gradient exist, and is it relevant
to conservation biology?

• Understanding the patterns and processes of
  biodiversity is useful to conservation because
• Conservation biology is about biodiversity
  conservation, and understanding the patterns and
  processes that result in biodiversity that we are
  concerned about seems like a good idea.
• Important for identifying priority areas for
  conservation
• Important for understanding what processes may
  result in loss of biodiversity
• Important for understanding why some places are
  speciose and others are depauperate
• Helps understand how communities are organized
• Aids in understanding the concept of ecological
  function

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A grand proliferation of hypotheses

• Land areaThe tropics are big and bigger areas
  hold more species.
• Evolutionary time The tropics are old and there
  has been more time for speciation to occur.
• Cradle High rates of speciation in the tropics
  (Dhobzansky)
• Refugia Refugia set up allopatric speciation
• Ecological time Disturbances in high latitudes
  havent allowed enough time for higher latitudes
  to catch up in species richness. (like glaciers)
• Climatic stability Stable climate releases
  environmental pressures and allows
  specialization.
• Climatic predictability Predictable seasons
  allow adaptations to environments and organisms
  become specialized.
• Productivity High productivity in tropics allows
  more resources that can be partitioned and
  specialized upon.
• Competition Trends in specialization is the
  result of avoiding competition.
• Rarefaction/predation Rarefaction by natural
  events or predation keeps population size of each
  species low, and this allows more species to
  persist.
• Museum Species hang on in the tropics after
  theyve dispersed and arrived there.
• Spatial heterogeneity More heterogeneous
  environments have more niches, and organisms
  radiate to fill niches.

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Consider the hypotheses

• Each hypothesis has only qualitative predictions
• Cant do experiments to disentangle hypotheses
• Similar mechanisms underlie several of the
  hypotheses.
• Some hypotheses are too specific (aridity, host
  diversity, solar angle) to explain the ubiquity
  of the gradient.

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Rohdes categories

• Why are these circular?
• Make a circular statement about the latitudinal
  gradient for some of these.

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Rohdes categories Circular and insufficiently
supported

• Circular hypotheses
• These cant explain the gradient alone- they
  require latitudinal variation in some factor
  extrinsic to the hypothesis.
• e.g., Competition is intense where there are lots
  of species living together, and there are lots of
  species living together in the tropics, and
  competition has caused species packing in the
  tropics, which means lots of species can live
  together.
• Insufficiently supported
• The proposed factor hasnt been shown to vary
  monotonically with latitude (like area, or
  habitat heterogeneity, or geographic range size).

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Lets look at effects of scale

• 3 kinds of scale effects are relevant to the
  gradient
• Scale-invariant
• Rank-invariant
• Scale-dependent
• If s-a differs at different latitudes, then its
  hard to know if diversity is due to area sampled
  or just latitude, or both.
• Comment about autocorrelation When sampling
  points are spatially autocorrelated, it is
  difficult to know the true significance of the
  pattern.

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Workable themes or categories of latitudinal
gradient hypotheses

• Time
• Older communities are more diverse
• Ecological disturbance, local dispersal
• Geological perturbation, speciation/extinction
• The above cant be tested, but glaciation story
  is used as evidence
• Spatial heterogeneity
• Tropics are more complex never demonstrated
• If rely on biotic heterogeneity, then another
  mechanism has to account for the gradient in
  biotic heterogeneity
• Interspecific interaction
• Intensity of species interactions results in more
  species in the tropics (sensu species-packing)
• Must have underlying pattern in diversity of
  interactions before any species interaction
  itself can generate a pattern of latitudinal
  diversity.
• Population
• Requires a latitudinal pattern in demographic
  traits, and this hasnt been shown
• It would also have to be produced by some other
  factor such as species packing
• Evolutionary rate
• High rates of speciation in the tropics.

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Geographic area

• The tropics are bigger. The pattern is thus
  driven by the s-a relationship. If landmasses are
  distributed at random, naturally less area of
  polar-temperate areas initiates the latitudinal
  gradient.
• Increased productivity in the tropics.
• The interaction between the two, the gradient is
  produced.
• Arguments dont ask whether area is important,
  rather authors argue about how important area may
  be.
• The area hypothesis is still argued.
• earth is spherical, so a band around the equator
  is bigger than at the poles.
• arrangement of polar, temperate, tropical is
  symmetrical, with tropical zones adjacent, giving
  the tropics a huge contiguous area

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Productivity

• Basic idea is more productivity is correlated
  with species richness
• This is hotly debated
• One flaw is there is no known mechanism why or
  how species diversity should increase to a
  maximum depending on energy availability. Why
  shouldnt population densities simply increase?

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Ambient energy

• Environments at high latitudes have average
  conditions that are not optimal for lots of
  species.
• Solar energy inputs create environments that
  affect organisms physiological response to
  temperature.
• This hypothesis accounts for others, such as the
  climatic stability, environmental stability,
  environmental predictability, seasonality, and
  harshness.
• Circular? few species at high latitudes because
  high latitudes are harsh, so low latitudes are
  best for species diversity.
• But physiological characteristics can help assess
  the favorableness of environments, and it is true
  that harsh environments are species-poor.
• Theres a natural reason why there are few
  reptiles in high latitudes. Too cold!
• Similarly, sunshine and temperature underlie
  butterfly richness in Great Britain, due to
  basking and physiological requirements.

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Rapoport-rescue (Rapoports Rule)

• Species geographical ranges are smaller in the
  tropics. Because of this, there are more species
  in the tropics.
• Rescue accidentals are species that normally
  would not occur, but are more likely to disperse
  or spillover into unfavorable areas. That is, not
  long-term members of the assemblage of species
  without re-colonization.
• There seemed to be concordance between taxa that
  fit latitude gradient and those that exhibit RR.
• One proposed mechanism for RR
• Organisms with broad tolerances are favored in
  high latitudes (climatic variability hypothesis).
  Broad tolerance leads to large ranges.
• Narrow tolerances in the tropics leads to smaller
  ranges.

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Rapoport-rescue (Rapoports Rule)

• Some recent work points out flaws with the logic
  of this proposed mechanism. RR is not holding up
  to scrutiny.
• RR doesnt hold up well with real data.
• Some strong examples of the latitudinal gradient
  do not exhibit Rapoports Rule.

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Evolutionary speed

• Depends on temperature-induced rates of
  speciation
• Latitudinal patterns of temperature are
  associated with shorter generation times, higher
  mutation rates, and faster selection pressures in
  the tropics.
• This differs from the ambient energy hypothesis,
  because evolutionary speed does not depend on
  community saturation and equilibrium, as does the
  ambient energy hypothesis.
• Data are only beginning to come in, using
  phylogenetic independent contrasts. If they tend
  to show that speciation rates are not different,
  then the popularity and use of this hypothesis
  will slow down.

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Geometric constraints hypothesis for the
latitudinal gradient in biodiveristy A Novel
Idea

• Random placements of species ranges within a
  bounded domain. Bounded domain is an area
  circumscribed by a physical or physiological
  barrier.
• If model world is unbounded, no gradient is
  produced.
• If the model world has edges of continents, edges
  of freshwater etc. then various types of
  gradients are produced.
• Novel idea because
• Does not require environmental gradients to be
  correlated with latitude
• Does not require biota to respond to
  environmental gradients
• Nonbiological gradients can be produced by
  placing species at random in an area closed in by
  a physical barrier or a physiological barrier.
• This restricts species distributions.
• Because the world really does have these
  boundaries, gradients are produced. The models
  fit some empirical datasets, too.
• The type of boundaries produces different forms
  of latitudinal gradients.

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