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.

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. Intraspecific, density-dependant, competition and population regulation

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 density- dependent 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 change- shifting the competitive balance between two species.

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 feature- they 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