Ecological interactions (continued)

Lecture graphics

Any study of a pop'n or community should consider resources--they will virtually always play a role in organizing communities

other factors, such as weather, predation, or herbivory may be important

Resource (Tilman 1982): any substance or factor which is consumed by an organism and which can lead to increased growth rates as its availability in the environment is increased

There are only a few basic elements which are resources for living organisms [Keddy 1989 Table 1.1, p. 4]

Various classification systems have been developed for resources:

1. Trophic position

   Consumers of resources are commonly classified by trophic position--autotrophs vs. heterotrophs

   Problems w/ using trophic position to classify organisms:

   many animals change trophic level w/ changes in age, season, or climate

   The chemical composition of organisms is remarkably constant, which makes the distinction between autotrophs and heterotrophs seem particularly arbitrary

2. Temporal and spatial distribution

   Types of resources, based on how they vary w/ time: [Keddy 1989 Fig. 1.1, p. 6]

   Increasing: gradually increases over the active season of an organism, then suddenly declines

   Decreasing: produced suddenly at the beginning of a season, then gradually declines

   Pulsing (ephemeral): increase & decline rapidly

   Steadily renewed: continuously renewed over long periods

   More elaborate classification systems incorporate spatial variation (not just temporal variation) in resource availability [e.g., Keddy 1989 Fig. 1.2, p. 8]

3. Mode of consumption

   "Consumptive" competition: organisms harvest a fraction of a resource over a large area

   "Space" competition: organisms harvest all of a resource from a fraction of the area

4. Resource ratios [ref. Tilman 1985 Am. Nat. 125:827-852; economic parallels illustrated by Trepl 1994 Ecol. Modelling 75/76:71-82]

   Organisms usu. must forage for more than 1 resource --> Tilman's ideas about resource ratios

   Essential resources can not be substituted for each other, whereas

   Substitutable resources can be interchanged, to varying degrees

   Growth isoclines indicate resource type [Keddy 1989 Fig. 1.3, p. 10]:

   solid lines are zero growth isoclines, shaded areas indicate regions of negative growth

   Substitutable resources can be further classified, based on degree of substitution:

   perfectly substitutable resources

   complementary resources--have a synergistic effect, so that combinations produce more growth than equivalent amounts of either resource alone

   antagonistic resources--interfere w/ each other when consumed, so that a mixture of the 2 reduces growth relative to either resource by itself