Regional Species pool: List
Regional Species pool>Dispersal and immigration>Environmental conditions> Species interactions
Species interactions:
Coexistence with other species is also required for community membership.
Species may be excluded by competition, predation, parasitism,
or disease
Biotic resistance
occurs when interactions with the native species exclude the invader.
Resource partitioning:
Competing species are more likely to coexist if they use resources in different ways
- theorized to reduce competition and increase species diversity
Resource ratio hypothesis:
species of diatoms can coexist by using resources in different
proportions
(Tilman 1977).
if disturbance, stress, or predation keeps the
dominant
competitor from becoming too
abundant, competitive exclusion
cannot occur, and coexistence can be maintained.
G. E. Hutchinson (1961) - the paradox of the plankton
lake phytoplankton communities have very high species richness (30–40 species), all using the same limited resources, in a homogeneous environment.
Hutchinson’s model has two components
tc – time required for one species to exclude another (depends on
species growth rates)
• te– time it takes environmental variation
(disturbances) to impact population growth
If tc << te, coexistence cannot be achieved.
In a fluctuating environment, tc >> te, competitive exclusion can still occur.
Coexistence can occur only when tc = te.
Intermediate disturbance hypothesis
species diversity will be greatest at intermediate levels of disturbance (Connell 1978).
At low levels of disturbance, competition regulates diversity.
At high disturbance levels, many species cannot survive.
Huston (1979) competitive displacement
The best competitor uses the limiting resources, reducing the weaker competitor’s population growth to the point of extinction.
Huston’s dynamic equilibrium model
disturbance frequency and rate of competitive displacement to determine species diversity.
Hacker and Gaines tested this in a New England salt marsh:
The middle intertidal zone had greatest species richness.
Figure 19.18 Positive Interactions: Key to Diversity in Salt Marsh Communities
High disturbance: Stress controls richness
Low disturbance: Competitive exclusion by Iva
Moderate disturbance: Facilitation by Juncus
The intermediate disturbance hypothesis
Menge and Sutherland (1987) considered predation separately
considers disturbance and predation to be similar—a dominant competitor is killed or damaged, creating opportunities for other species
Lottery models and neutral models emphasize
all species have equal chances of obtaining resources that were made
available by disturbances, and this allows coexistence.
• Species
must have similar interaction strengths and growth rates and be able
to respond quickly to disturbances
Diversity in marine organisms is greatest at coral reefs, yet reefs
are in nutrient poor water.
Darwin viewed this as
a paradox, and noted that reefs were like an oasis in an ocean desert.
Complementarity hypothesis
As species richness increases, there will be a linear increase
in
community function.
Each species added has an equal effect.
Redundancy hypothesis:
Additional species reaches a threshold.
As more species are
added, there is an increasing overlap in their functions, or
redundancy among species.
Driver and passenger hypothesis
Driver” species have a large effect; “passenger” species have
a
minimal effect.
• Addition of driver and passenger species
will
have unequal effects on community function.
A variation on the driver and passenger hypothesis:
It assumes there could be overlap between the two
The Diversity–Stability Theory
species richness is positively related to community stability—
The tendency of a community to remain the same in structure and
function, or to
return after a disturbance.
The portfolio effect
diversified communities are more stable. This term gets its name from investing – the more diverse your investment portfolio, the more stable your net worth will be against market fluctuations
Doak et al. (1998) showed that it is a statistical inevitability that community biomass will vary less with greater species diversity
a is a measure of species evenness
Large a = low
evenness
As evenness is reduced, variability in community biomass
is increased
Monoculture
Greater variability in crop yields in response to disease
Zoonotic diseases are hosted by wildlife and are infectious to humans
Diseases: hantavirus
(SNV), Zika virus, Ebola, avian influenza,
and
SARS-CoV2 (COVID-19).
Factors that affect emergence are
complex but may include climate change, species invasions, pollution,
and land use conversion.