Species abundance and distribution

1
Species abundance and distribution

• How are species organized in communities?
• How do we describe attributes of species
  abundance and distribution?
• What factors influence relative abundance?

2
Communities
Are communities Discrete ecological units that
have evolved to enhance their interdependent
functioning? or Loose collections of
species that tolerate the same environmental
conditions?
3
Clementsian view of communities - emphasizes
ecological and evolutionary relationships among
species - holistic concept of communities.
Only understand contribution of individual
species in context of ecosystem function (energy
flow, trophic relations etc) - species
interactions contribution to community stability
Gleasonian view of communities - plant
association is not an organism, scarcely even a
vegetational unit, but merely a coincidence -
individualistic concept of communities. Natural
selection acts only on individuals. No
organization beyond species level.
4
Clementsian view - Communities can be defined
by the presence of essential species e.g.??
Juniper-Ash association in 30 yr old fields in
southern Illinois - Species distributions
should reflect community organization. Ecological
limits of species should co-occur as one
community transitions to another (ecotone)
5
Clear associations separated by ecotones
Indistinct associations no clear ecotones
Where have we seen ecotones?
6
Little evidence for ecotones at geographic
scales Tree species distributions for species
that co-occur in Kentucky do not overlap.
7
Similarly, at smaller scales species occur
outside sites where they are dominant
8
Neo-gleasonian view reflected in null models of
biodiversity and biogeography
Can we explain the number and abundances of
species in a community without including any
biology?
9
Species richness
Alpha diversity number of species present within
a fixed locality or habitat, S (also called
species richness e.g., number of spp present in
the Paxton prairie) Beta diversity turnover
(difference) in species between habitats -
additional diversity encountered in a new site
(e.g. how many new species encountered in Loda
cemetary)
10
Total spp richness is often difficult to measure
Species accumulation curve Paxton prairie
Estimated (our data) 2007 60 spp 2008 67
spp Observed (John Taft) 145 spp

11
Rarefaction provides alternative means of
comparing richness among communities
Compare species richness at a fixed number of
individuals sampled
12
Species diversity
Weighs the contribution of each species according
to its abundance (more diverse communities are
those in which relative abundances of all species
are similar) Many different indices of species
diversity are used Fishers Alpha,
Shannon-Weiner, Simpson (all give similar results)
13
Simpson Index of species diversity D 1
Where pi is the proportion of all individuals
belonging to the ith species
?pi2
D will be greatest if all species have the same
relative abundance (pi)
14
Species similarity
Within a locality, how similar is the species
composition comparing one sample to another?
(essentially another measure of Beta
diversity) Jaccard similarity Cj
j/(ab-j) Where j spp in both sites a
species only in site a b species only in site b
15
How similar are quadrats at Paxton prairie?
Within edge
Within interior
Edge vs Interior
Are plots from the edge more similar than
expected assuming spp are distributed randomly?
Paired t-test within edge vs. across plt0.01
16
How do species vary in abundance in communities?
Species are never equally abundant in a
community A few species (dominants) attain high
relative abundance Relative dominance declines
with increasing diversity
17
Rank abundance plot - Smoky Mt taxa
Species are ranked from left to right on x axis
from most to least abundant. Y axis shows
relative abundance ( of all individuals) on a
log scale
Forest birds
Deciduous forest
plants fir forest
18
Species relative abundances often follow
log-normal distribution
Can normalize by grouping species into abundance
classes on log scale
19
Log-normal distribution of herbs and snakes
Number of species
Doubling size classes of abundance
20
Why should species abundances follow a
log-normal distribution? So ubiquitous that
reflect an underlying general mechanism?? No
clear answer. May (1975) and others argued that
results as consequence of the Central Limit
Theorem (interacting effects of many random
processes) Non-biological systems also have
properties of the log-normal (e.g. wealth
distribution in the USA world distribution of
human populations).
21
Summary
Most communities are open with species
distributed independently of other species, and
ecotones are not often apparent (except at steep
soil or climate gradient) Species differ in
their commoness in communities, with a few
dominants accounting for a large proportion of
individuals. Our understanding of rank abundance
relationships is incomplete Species richness
refers to number of species present species
diversity takes abundance into account
22
Species-Area relationship
At large spatial scales species richness
increases with the area sampled as a log-log
relationship (power law) Log S log c z log
A Where A Area, c is intercept and z is
slope Values of z are often reported to be
around 0.2 - 0.3 (mostly data collected on
islands - where species richness data are best
described)
23
Values of z tend to be lower for mainland areas
than islands Why?
24
Why does richness increase with area?
Species-area relationship is an example of a
power-law relationship Slope of the relationship
does not vary over orders of magnitude variation
in area Also call this a scale-invariant
relationship Suggests single underlying
mechanism???
25
Scale-invariant relationships of general interest
to physicists (looking for elegant explanations
of everything)
Size distribution of earthquakes Size
distribution of forest fires Size distribution of
extinction events in fossil record Size
distribution of cities Size distribution of stock
market fluctuations Distribution of citations of
scientific papers!
Scale invariance in these examples thought to
reflect unpredictability of effect of single
events of similar magnitude
26
Does the species-area relationship reveal a
single mechanism?