Lecture 18

Types of Interactions
	Benefit	Harm	No Effect
Benefit	Mutualism	Predation	Commensalism
Harm	Predation	Competition	Amensalism
No Effect	Commensalism	Amensalism	Neutralism

Individuals of one or both species suffer reduced fecundity, survivorship, or growth as a result of resource exploitation or resource interference by individuals of each species.

Terrestrial salamanders, Plethodon glutinosus and P. jordani, were studied in the southern Appalachian Mountains. Hairston (1980) conducted an experiment at two sites where both species naturally coexist.

	Treatment	Result after five years
Controls (3)	no manipulation	P. jordani more abundant than P. glutinosus
Experimental Removals (2)	removal of all P. jordani	P. glutinosus abundance increases
Experimental Removals (2)	removal of all P. glutinosus	P. jordani abundance does not change but year classes 1 and 2 increase

In the absence of P. glutinosus, there may be an increased fecundity or increased early survival in P. jordani. However, total abundance of P. jordani did not change.

Bedstraw plants, Galium, studied by Tansley (1917) are found in two soil environments in Britain. G. hercynicum is naturally found on acidic soils, and G. pumilum is naturally found on calcareous soils.

Grown alone, both species of Galium thrive on the acidic soils from G. hercynicum sites and on the calcareous soils from G. pumilum sites. Therefore, soil condition alone cannot explain the non-overlapping distributions of these two species.

This is an example of competitive exclusion, when these two species are together, only one species will persist. The outcome of competition depends on the soil type in which the plants are located.

Barnacles on the rocky shores of Scotland were studies by Connell (1961). Chthamalus stellatus adults are found only in the upper intertidal, and Balanus balanoides adults are only found in the lower intertidal.

If an area of rock surface in the lower intertidal is cleared of all barnacles, larvae of both Chthamalus and Balanus will settle and successfully grow. However, Balanus will smother, undercut, or crush Chthamalus in the lower intertidal over time. Chthamalus mortality is greatest at the time of maximum Balanus growth.

Balanus has a lower desiccation tolerance than does Chthamalus, so Balanus is limited to the lower intertidal and competition between these two species is limited to the lower intertidal. In this case, competition is direct, interference, which results in competitive exclusion of Chthamalus from the lower intertidal.

Three species of the ciliate Paramecium were studied by Gause (1934, 1935) in laboratory cultures. When grown alone, P. auralia and P. caudatum have very similar logistic growth curves. P. bursaria reaches a stable population size of approximately two-thirds of that seen in the other two species when grown alone under the same culture conditions. When grown together, P. auralia and P. caudatum both exhibit growth suppression, but P. caudatum, is eventually driven to extinction in these combined cultures. When P. bursaria is grown with P. caudatum, both species have lower stable population densities, but both persist over time and coexist.

Growth curves for P. auralia and P. caudatum when grown alone and together (Ricklefs, 1996, p 429, Fig. 19.4).

Growth curves for P. bursaria and P. caudatum when grown alone and together (after Begon, Harper and Townsend, 1996, p 268, Fig. 7.2).

Competition with coexistence characterize the interaction between P. bursaria and P. caudatum. Coexistence is possible because these two species exhibit spatial separation in mixed cultures. P. caudatum feeds on bacteria suspended in the medium and P. bursaria feeds on yeast cells at the bottom the culture vessel. Two species of freshwater diatom, photosynthetic protista (single celled algae), Asterionella formosa and Synedra ulna both use silica to make their test (Tilman, Mattson and Langer, 1981). When grown in culture alone, both species deplete available silicate from liquid medium, but Synedra reduces silicate to a lower level than does Asterionella. When grown together, Synedra competitively excludes Asterionella, even when Asterionella starts with a 10-fold greater population density than Synedra.

Growth curves for Asterionella and Synedra when grown alone and together (after Begon, Harper and Townsend, 1996, p 269, Fig. 7.3).

Competition in this case is indirect, it is an example of exploitation competition.

As in situations of intraspecific competition, interspecific competition can be either interference or exploitation. Results may be asymmetrical, two species do not necessarily have reciprocal inhibitory affects on each other. Competitive exclusion is, of course, an asymmetrical competitive outcome. In some cases, only one species is negatively affected by the competitive interaction.

The cattail species, Typha latifolia and T. angustifolia have separate water depth distributions in naturally occurring populations. Individual stems of T. latifolia are found in shallow waters and those of T. angustifolia are found in deeper waters (Grace and Wetzel, 1981). Transplant experiments (in the absence of competition) indicate that T. angustifolia can grown over a much wider range of water depths than it does in the presence of T. latifolia. The growth of T. latifolia at different water depths is the same in the absence of potential competition as it is in the presence of T. angustifolia in natural populations.

Standing crop biomass per unit area for T. latifolia and T. angustifolia in natural populations containing both species show the separate distributions of these species with water depth. Standing crop biomass per unit area for T. latifolia and T. angustifolia as a function of water depth in competition-free transplants shows the response of T. angustifolia (after Begon, Harper and Townsend, 1996, p 272, Fig. 7.4)

The outcomes of competition may range from symmetry to complete asymmetry, apparent amensalism.

The most extreme competitive interactions are those that are completely asymmetric and result in competitive exclusion.

These interactions are interference competition so extreme the interaction may seem to be amensalism.