EE 4- Interspecific Competition and Tilman’s Resource-Based Models

Levels of Organization: Biological systems are organized from the Chemical level (atoms, molecules, macromolecules) to the Cellular level (organelles, cells), then through tissues and organs to the Biosphere (Earth and all its communities).
Population: A group consisting of organisms of the same species.
Community: Populations of different species inhabiting the same area.
Interspecific Competition: A conceptual model where two populations (e.g., Cat population and Dog population) exert a negative impact on one another through resource competition.

$R^*$ Definition: The equilibrium resource availability at which per capita birth rate and mortality rate ( $mA$ ) are balanced. This is the minimum level to which a species can reduce a resource ( $R$ ) in the environment.
The $R^<em>$ Rule: When two species compete for one limiting resource, the species with the lower $R^</em>$ deterministically outcompetes the other.
Example: In competition for Phosphorus ( $P$ ), Asterionella outcompetes Cyclotella because it can reduce the resource to a lower level.
Resource Supply Point: A characteristic of the environment representing the total amount of resource available; at equilibrium, consumption must balance supply.

Zero Net Growth Isocline (ZNGI): A line on a graph of two resources ( $R1$ and $R2$ ) representing the resource levels where population growth is zero ( $rac{dN}{dt} = 0$ ). * The region above and to the right of the ZNGI favors population growth ( rac{dN}{dt} > 0). * The region below/outer zone favors decline ( rac{dN}{dt} < 0).
Impact Vectors: These show the direction and ratio in which a population influences and consumes resources.
Conditions for Coexistence: * ZNGIs must intersect, representing a trade-off in resource usage. * Each species must consume more of the resource that most limits its own growth (intraspecific competition must be greater than interspecific competition). * If Species A consumes more of the resource that limits Species B, coexistence is not stable, and the outcome may depend on initial densities.

Physiological Trade-offs: Species traits are constrained by allocation; for example, a plant's allocation to leaves for light capture comes at the expense of root development for nutrient sequestration from the soil.
Multispecies Models: Under equilibrium conditions, the number of coexisting species cannot exceed the number of limiting resources.
Diversity Factors: In nature, spatial or temporal variation in resource supply allows for the coexistence of many species even with limited resources.