The latitudinal gradient in species diversity - PowerPoint PPT Presentation

1 / 23
About This Presentation
Title:

The latitudinal gradient in species diversity

Description:

As soon a biologist sets foot in the tropics, terrestrial, freshwater, or marine, ... that show reverse latitudinal gradients (seals, penguins, sandpipers, conifers) ... – PowerPoint PPT presentation

Number of Views:2084
Avg rating:3.0/5.0
Slides: 24
Provided by: lfitzg
Category:

less

Transcript and Presenter's Notes

Title: The latitudinal gradient in species diversity


1
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.

2
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).

3
2 examples, mammals, just bats
  • Bat species richness (upper)
  • Bat feeding guild richness (lower)
  • Mammal richness (left)

4
  • 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.

5
  • 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.

6
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.

7
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?

8
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

9
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.

10
(No Transcript)
11
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.

12
Rohdes categories
  • Why are these circular?
  • Make a circular statement about the latitudinal
    gradient for some of these.

13
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).

14
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.

15
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.

16
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

17
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?

18
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.

19
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.

20
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.

21
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.

22
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.

23
(No Transcript)
Write a Comment
User Comments (0)
About PowerShow.com