Transcript Interspecific Competition

Competition
• Population growth is almost always controlled by density.
• Density regulation implies:
1. Resources are limited
2. Individuals in the population are competing with one another
Carrying capacity (K). The resource threshold for population
increase vs. decrease is at K. After that point, the death rate
is greater than the birth rate. Resource limitation.
Intraspecific, density-dependant, competition and population regulation
As density of individuals (in the same living space) increases, tadpole
growth rate decreases. At high density, they have lower final weight.
WHY?
Intraspecific, density-dependant, competition and population regulation
As density of white clover
increases, mean weight per plant
decreases sharply.
Resources are finite- increasing
the number of individuals
decreases the amount available to
each individual.
As density of horseweed (Erigeron canadensis) decreases,
mean weight of each individual plant increases. Low
density, each plant has access to more resources.
As density of harp seals (Phoca groenlandica) increases,
resources are depleted- it takes longer to reach the needed
weight for reproduction. (reproduction is density-dependant)
As corn plant density increases- grain yield decreases.
(reproduction is density-dependant)
Population responses to increasing density vary...
Some species have mechanisms that ameliorate issues related to densitydependent regulation. The area an animal visits during a year is the home
range. Usually not defended. Provides access to resources. Larger
animals need more space. Carnivores need more space than omnivores or
herbivores.
Many animals also have definable
territories. This is the area that a given
individual (or small group of individuals,
e.g., a lion pride) defends against intruders.
Provides stable access to resources,
shelter, and sometimes secures mating
opportunities.
•Plants do not have territories, but
they do aggressively acquire and
maintain growing space.
•One of the most prominent
examples is root competition.
•Plants compete for resources via
root networks that gather water and
nutrients
•Belowground competition
influences aboveground growth
Quercus alba, Dysart Woods
•In many plant communities (esp. forests), light
is an important limiting resource.
•Individuals that can acquire canopy space
often have long & productive lives...in the
understory, carbon is limited, and individuals are
less successful.
•Getting tall is a good idea
Bottom line- resources are scarce, populations cannot grow
indefinitely. Individuals respond to this scarcity in a variety of ways.
Most ecological systems are complex & messy: landscapes are heterogeneous,
resources are patchy, and climate conditions dynamic.
Population growth is regulated by this messiness, but also depends on resource
depletion due to intraspecies competition.
But, monocultures are rare in nature, so we have to think more broadly about
population regulation- and consider the role of other species (ie, interspecies
competition)
Often in nature, many species compete for the same resources
(light, nutrients- food, shelter).
Individual species have differing traits, capabilities, resource requirements.
Competition between species is often decided by these trait differences and
resource requirements. *Key point* populations competing for resources will tend
to deplete that resource- so the species (or individual) that can survive at the
lowest value of that resource wins.
Ecologists have used simplified model systems to understand interspecies
competition, and then tested that theory on more natural systems.
Alfred Lotka and Vittora Volterra independently, and
simultaneously, derived a model to explain the outcome of
competition between two species
“Lotka-Volterra Model” predicts four outcomes of competition
between two species (X & Y):
1) X always wins
2) Y always wins
3) Outcome depends on the initial density of each
4) X & Y coexist
For any combination of population densities- Lotka-Volterra
predicts one of four outcomes.
Classic experiment by G.F. Gause used Paramecium
Generally supported the
Lotka-Volterra Model.
Separately each species grew
more than when combined
(suggests interspecific
competition greater than
intraspecific competition)
P. aurelia drove P. caudatum to
extinction.
Further work by Tilman with diatoms also
supported Lotka-Volterra
Separately each species does
well, and lowers the silicate
level in the habitat.
When grown together, one
species drives the other to
extinction by lowering the
silicate level below tolerable
levels for the other species.
Over time, access to
resources can changeshifting the competitive
balance between two
species.
The competitive balance
among species can be
shifted along resource
gradients.
The competitive balance among species can be
shifted along resource gradients.
Gradients often represent a spectrum of stress tolerance and competitive ability
Niche, Potential Niche & Realized Niche
• Competition is an important force in natural settings,
and lab environments.
• Experimentation and theory have combined to support
the Competitive Exclusion Theory: Two species
cannot coexist (in the same space) if they require
exactly the same resources.
G.E. Hutchinson was puzzled by the tension between
competitive exclusion and the Diversity of Life....
Niche differentiation takes
many, sometimes subtle
forms...
Niche differentiation takes many, sometimes subtle
forms... In this case the teeth of these critters makes
up a nice gradient…suggests differentiation
Niche is a multidimensional
hyperspace. Obscure words that
simply mean that many features of
the environment dictate whether an
organism is successful. It means
that if two organisms share exactly
the same requirements for some
resource or environmental featurethey can still coexist as long as
some other feature differs.
Niche is a multidimensional hyperspace. If you could
sum across the variation in many of the dimensions- you
can think about a “space” and separation of spaces in
which the organism resides