Chapter 10: Patterns in Species Richness
• Species richness = number of species in a community.
Diversity indices and rank-abundance diagrams
• Measure of the character of a community that is most commonly used to take into
account both species richness and relative abundance of those species is known as the
Shannon or Shannon- Weaver diversity index (denoted by H)
• Calculated by determining proportion of individual or biomass for each species (Pi)
that that species contributes total in sample. Then if S is the total number of species in
community (ie, richness) diversity (H) is
• 𝐻 = −∑𝑃𝑖 ln(𝑃𝑖)
• For a given richness, H increases with equitability and for a given equitability, H
increases with richness. H is diversity
• Rank-abundance diagrams plot Pi against rank, ie, the most abundance species takes
rank 1, the second most abundant species take rank 2.
o Steeper the slope of rank abundance diagram, the greater the
dominance of a common species over rare species in the community.
10.2 A simple model of species richness
• Assume for simplicity, that the resources available to community can be depicted as
one-dimensional continuum, R units long.
• Each species only uses a portion of this resource continuum, and these portions define
the niche breadth (n), and average niche breadth within a community (ñ)
• Overlap of niches can be measured by (o). Average niche overlap within community is
(ō).
1. For given values of ñ and ō, community will contain more species the larger value of R
• R (range of resources) increases species richness increases
2. For a given R, more species will be accommodated if ñ is smaller
• Species are more specialised.
3. If ō is higher, then more may coexist along the same resource continuum.
4. A community will contain more species the more fully saturated it is; conversely, it will
contain less species when more of the resource continuum is unexploited.
• If a community is dominated by interspecific competition, the resources are likely to
be fully exploited.
• Species richness will then depend on the range of available resources,
the extent to which species are specialist and the permitted extent of
niche overlap.
• We will examine a range of influences on each of these three.
• Predation is capable of exerting contrasting effects.
• Predators exclude certain prey species.
• In the absence for these species, the community may then be less than
fully saturated.
• Some available resources may be unexploited; therefore, species
richness may be reduced.
• Predation may tend to keep species below their carrying capacities for much of the
time.
• Reducing the intensity and importance of direct interspecific
competition for resources.
• May permit much more niche overlap (higher (ō)) and a greater richness
of species than in a community dominated by competition.
10.3 Spatially varying factors that influence species richness
10.3.1 Productivity and resource richness
• For plants, productivity of environment can depend on whichever nutrient or
condition is most limiting to growth.
• Productivity of animals follows the same trends as for plants, mainly as a result of
changes in resource levels at base of food chain.
• Higher productivity is correlated to wider range of available resources (higher R) =
likely to lead to increase in species richness.
However
• Higher supply of resources doesn't mean higher variety of resources.
• Therefore, abundance might increase instead of species richness.
• It is possible for rare resources in an unproductive environment to be abundant
enough in an productive environment for extra species, that are more specialised to
be added.
• In general, we might expect species richness to increase with productivity.
Plants
• Greater amount of resources (energy and water) leads to more evapotranspiration
and a greater requirement for water
• Species richness increased with water availability, but first increased, then decreased
with available energy (PET)
Animals
• Best correlation was consistently with PET
• Ectotherm - such as reptile - extra atmospheric warmth would enhance the intake and
utilization of food resources.
• Endotherm - such as birds- extra warmth means less expenditure of resources in
maintaining body temperature and more for growth and reproduction.
• Therefore, warmer environments may therefore support more species in total.
• Higher PET, higher species richness.
Paradox of enrichment
• High productivity leads to decline in species richness.
• High productivity leads to high rates of population growth, bringing about the
extinction of some of the species present because of a speedy conclusion to any
potential competitive exclusion.
• At lower productivity, environment is more likely to have changed before competitive
exclusion is achieved.
Humped relationship
• Species richness may be highest at intermediate levels of productivity.
• Declines at lowest productivities due to shortage of resource but also declines at
highest productivities where competitive exclusion speed rapidly to their conclusion.
• So, increase in productivity can and does lead to increased or decreased species
richness - or both.
10.3.2 Predation Intensity
• Predation increase species richness by allowing otherwise competitively inferior
species to coexist with their superiors (predator-mediated coexistence)
• Intense predation may reduce species richness by driving prey species to extinction.
• Therefore, there may also be a humped relationship.
• Greatest richness at intermediate predation intensity.
Ex. Predator-mediated coexistence by starfish on rocky shore
• Main influence of starfish Pisaster appears to be to make space available for
competitively subordinate species.
• (species fighting for space)
• It cuts a swathe free of barnacles and, most importantly, free of the dominant mussels
that would otherwise outcompete other invertebrates and algae for space.
• Removal of starfish led to reduction in number of species from 15 to 8.
• Predator-mediated coexistence finds a surprising application in field of restoration
ecology.
10.3.3 Spatial Heterogeneity
• Environment that are more spatially heterogenous can be expected to accommodate
extra species because they provide a greater variety of microhabitats, a greater range
of microclimates, more types of places to hide from predator and so on.
• Whether spatial heterogeneity arises from abiotic environment or is provided by
biological components of community, it is capable of promoting and increase in
species richness.
10.3.4 Environmental harshness
• Harsh environment - environment dominated by an extreme abiotic factor.
• Species richness is lower in extreme environments.
• Although it appears reasonable that intrinsically extreme environments should be a
consequence support few species, this has proved an extremely difficult proposition to
establish.
10.4 Temporally varying factors that influence species richness.
10.4.1 Climatic Variation
Temporal niche differentiation in seasonal environments
• Effects of climatic variation on species richness depends on whether the variation is
predictable or unpredictable.
• In predictable, seasonally changing environment, different species may be suited to
conditions at different time of the year.
• More species might therefore be expected to coexist in a seasonal environment than a
completely consistent one.
Specialisation in non-seasonal environments
• It would be difficult for specialist fruit-eater to persist in a seasonal environment when
fruits available for only a very limited portion of a year.
• Specialisation is found repeatedly in non-seasonal tropical environment
Unpredictable climatic variation (climate instability) could have a number of effects on species
richness
i. Stable environments may be able to support specialised species that would unlikely to
persist where conditions or resources fluctuated dramatically.
ii. Stable environments are more likely to be saturated with species
iii. Theory suggests that a higher degree of niche overlap will be found in more stable
environments.
• All these processes could increase species richness.
However, on the other hand.
• Populations in stable environment are more likely to reach their carrying capacities,
the community is more likely to be dominated by competition, and species are
therefore more likely to be excluded by competition.
10.4.2 Disturbance
• Previously mentioned in chapter 9, if a community is dominance controlled, there
tends in a community succession to be an initial increase in richness as a result of
colonisation, followed by a subsequent decline in richness due to competitive
exclusion.
Intermediate disturbance hypothesis
• Communities are expected to contain most species when the frequency of disturbance
is neither too high nor too low.
• Initially proposed to account for patterns of richness in tropical rainforest and coral
reefs.
• Supported by studies of algae on boulders on rocky shores and from studies of
invertebrates in small streams and plankton in lakes.
10.4.3 Environmental age: Evolutionary time
• It has often been suggested that communities that are 'disturbed' only on very
extended time scales may nonetheless lack species because they have yet to reach an
ecological or an evolutionary equilibrium.
• Some are closer to equilibrium, therefore, more saturated than others
• For example, many argued that tropics are richer in species than temperate regions
because tropics have existed over long and uninterrupted periods of evolutionary
time, whereas temperate regions are still recovering from the Pleistocene glaciations
where temperate biotic zones shifted in the direction of the tropics.
• However, the tropics were also disturbed during the ice ages by associated climatic
changes that saw tropical forest contracting to a limited number of small refugees
surrounded by grassland.
• An alternative explanation for lower species richness in temperate region invokes the
idea that species in tropics evolves faster because of higher rates of mutation in
warmer climates.
10.5 Gradient of species richness
10.5.1 Habitat area and remoteness: island biogeography
• Species-area relationship = number of species on islands decreases as island area
decreases.
• The relationship between species richness and habitat area is one of the most
consistent of all ecological patterns.
• One of the most obvious reasons why larger areas should contain more species is that
larger areas typically encompass more different types of habitat.
• However, MacArthur and Wilson (1967) believed that the number of species on an
island is determined by a balance between immigration and extinction; that this
balance is dynamic, with species continually going extinct and being replaced by the
same or different species.
• That immigration and extinction rates may vary with island size and isolation.
(Large, close islands should have highest species richness)
MacArthur and Wilson's theory makes several predictions
1. Number of species on an island should eventually become roughly constant through
time
2. This should be a result of a continual turnover of species with some becoming extinct
and other immigrating.
3. Large islands should support more species than small islands
4. Species number should decline with increasing remoteness of an island
• Besides, habitat diversity might also play an important role in species richness.
Finally it is important to reiterate that no aspect of ecology can be fully understood without
reference to evolutionary process, and this is particularly true for understanding island communities.
• On isolated islands, rate at which new species evolve may be comparable to or even
faster than rate at which they arrive as new colonist.
• The communities of which they are a part are clearly much more strongly affected by
local evolution and speciation than by the processes of invasion and extinction.
10.5.2 Latitudinal gradients
• Increase of species from poles to tropics.
1. Richness of tropical communities has been attributed to greater intensity of predation
and to more specialised predators.
• More intense predation can reduce the importance of competition,
permitting greater niche overlap and promoting higher richness.
• But predation cannot readily be forwarded as root cause of tropical
richness since this begs the question of what gives rise to the predators
themselves.
2. Increase species richness may be related to an increase in productivity as one moves
from pole to equator.
• More heat and light energy in increasingly tropical regions.
• Both these have tended to be associated with greater species richness,
though in some cases increase in productivity results in decrease of
species richness.
• Moreover, light and heat aren’t the only determinants of plant
productivity
• Tropical soils have lower concentration of plant nutrients compared to
temperate soils.
• Tropical soils are poor in nutrients because most nutrients are locked up
in the large tropical biomass.
• This though, leads to nutrient-poor soils and perhaps a wider range of
light regimes from forest floor to canopy far above.
• These in turn lead to high plant species richness and thus to high animal
species richness.
3. Some ecologist has invoked the climate of low attitude as a reason for high species
richness.
• Less seasonal than temperate regions, which allows species to be more
specialised. (narrower niches, higher richness)
4. The great evolutionary 'age' of tropics has also been proposed to be the reason for
great species richness.
• Repeated fragmentation and coalescence of tropical forest refugia
promoted genetic differentiation and speciation, accounting for much of
the high richness in tropical regions.
10.5.3 Gradients with altitude and depth
• Decrease in species richness with altitude has frequently been reported in terrestrial
environment.
• Some have reported increase of altitude leads to increase in species richness and
some said hump-shaped patterns.
• Declines in species richness have often been explained in terms of decreasing
productivity associated with lower temperatures and shorter growing seasons with
higher altitude, or physiological stress associated with climatic extremes near
mountaintops.
• Conversely, positive relationship between ant diversity and altitude is that
precipitation increased with altitude, resulting in higher productivity and less
physiologically extreme conditions at higher altitude.
• In addition, high-altitude communities almost invariably occupy smaller areas than
lowlands at equivalent latitudes and they will be more isolated from similar
communities than lowland sites.
• So, isolation and smaller area occupied results in decrease in species richness with
altitude.
In aquatic environments
• Change in species richness with depth shows some strong similarities to terrestrial
gradient with altitude.
• In open ocean, there is a rapid decrease in species richness with depth. (light can
hardly penetrate 30m below ocean for plants)
• In coastal regions, there is a hump-shaped effect where the peak is at 1000m depth.
And decreases as depth increases.
10.5.4 Gradients during community succession.
• Increase (colonisation) then decrease (competition) (in case of plants)
• There is something of a cascade effect with succession.
• One process that increases richness kick-starts a second, which feeds
into a third and so on.
• Earliest species (best colonizers and best competitors for open space)
• They immediately provide resources (and introduce heterogeneity) that
were not previously present.
• They provide new microhabitats, and for animals that might feed on
them they provide much greater range of food.
• Increase in herbivory and predation then feed back to promote further
increase in species richness (predator-mediator coexistence), which
provides further resources and more heterogeneity and so on.
• Besides, there is less temporal variation in forest than in an exposed
early succession stage.
(so, increase then decrease in richness might be for plants but there might be a cascading effect for
other organism (increases with succession)).
10.6 Patterns in taxon richness in the fossil record
• Cambrian explosion of taxa may have been an example of exploiter-mediated
coexistence.
• The Permian decline may reflect a species-area relationship when Earth's continents
coalesced into Pangaea.
• The changing pattern of plant taxa may reflect the competitive displacement of older,
less specialised taxa by newer, more specialised ones.
• The extinctions of many large animals in the Pleistocene may reflect the hand of
human predation and hold lessons for the present day.
