Biodiversity: the variety in life both within and between species and higher taxa (Any group or rank in a
biological classification into which related organisms are classified)
Most ecological studies focus on the taxonomic component of biodiversity, species richness
(Species richness is simply the number of species present in a sample, community, or
taxonomic group). But this is only one component concept biodiversity dependign on
questions asked other considerations could be important.
species richness only one component of species diversity alone it misses out vital info on the
evenness of relative abundance of individuals of different taxa ( Evenness measures the variation
in the abundance of individuals per species within a community). All else being equal,
communities with greater evenness are considered to have greater species diversity. Diversity
Indicies= Shannon–Wiener and Simpson’s
even these measures contain no information about the relative uniqueness and importance of
the taxa. Are the species contained within the taxa endemic to that area or widespread
at largest scales, species richness is almost always
the only diversity measure available
The generality of LDG
Most taxanomic groups for which we have good data show a decrease
in diversity from the equator to the poles: past 400 million years but
more pronounced today
emphasis on higher order plants and vertibrate studied and disproportionate amount in the new
world. Few studies have addressed macro-scale diversity patterns of invertebrates, parasites, bacteria
and soil organisms, even though these groups comprise the majority of global biodiversity
In some instances, taxa with good data are found
to deviate from the classical pattern
Exceptions to the latitudinal gradient generally do not occur deep in the taxonomic
hierarchy (e.g. they are much more likely to occur at the family level than at the phylum
level). generality of latitudinal gradient more questionable when higher taxonomic
groups deviate from the global trend
Investigating and Analysing Geographical
Patterns of Biodiversity
measuring biodiversity: scale
Biodiversity is scale-dependent; that is, diversity depends strongly on the size of
the units used in data collection.
3 attributes of scale particularly important in species richness studies.Grain- the
area represented by each sampling point (Quadrat size). Focus- the area of the
analytical unit (single or attribution of data points). Extent- total geographic area
studied
species richness depends on them because of species area effect: As area increases more individuals,
habitats and biomes, are incorporated, resulting in higher species richness
control for the effect of area through sampling techniques: using sample units of equal area and
shape
Different factors are likely to explain diversity patterning best at
different scales. Potential explanatory variables make sense at some
scales but not others.
The diversity of any region depends upon the richness of the smaller
areas that it contains, and on the compositional turnover between these
areas (turnover: the compositional change in species assemblages
through either time or space)
Whittakers Scheme. Alpha diversity refers to finer scale species
richness. Gamma diversity to coarser scale. Beta Diversity to species
turnover at all scales
Explaining the LDG
Latitude is not ecologically meaningful (interactions between organism and their environments), but correlates with variation in ecologically meaningful variables such
as climate and area. Both product of contemporary processes and historical factors influencing speciation,
extinction and dispersal only just starting to notice both linked
regardless of complexity of proposed causes LDG fundamentally a result of
spatial variation in speciation/immigration (increasing species richness) and
extinction/emigration (decreasing species richness)
hypotheses
geometric hypotheses
Area has two important, related ecological effects.
First, diversity tends to increase with area.
However, this would imply that greater diversity
should exist on all other great circles, but it does
not, so something about latitude must explain.
Tropical habitat more extensive than polar also
because tropical zones contiguous where as
temperate or polar are seperate
second, species of larger biomes are able to maintain larger ranges increasing the chance of allopatric
speciation (speciation by geographic isolation,something extrinsic to the organisms prevents two or more
groups from mating, eventually causing that lineage to speciate) and reducing extinction probability via
larger potential populations and the provision of niche refugia (location of an isolated or relict population of
a once more widespread species)
little testing, criticism some places dont fit the trend eg hawaii vs antarctica
the mid domain effect
diversity gradients can arise in the absence of environmental gradients from the random placement of
species’ ranges in constraints of a bounded domain. species' ranges would tend to overlap more toward
the center of the domain than towards its limits, forcing a mid-domain peak in species richness