Ecological Interactions; Chapters 13, 14, 15; Competition(13), Predation Mutualism

Interspecific competition Mutually negative interactions between members of different species Intraspecific competition in Ch. 11, important density-dependent factor slowing popln. Growth Schoener classified types of competition –consumption of shared, exhaustible nutrients –preemption or occupation, e.g., barnacles –overgrowth (e.g., taller tree shades out shorter tree) –chemical interaction, allelopathy in plants –territory, defense of an exclusive space –encounter, finding and using a carcass first

Lab studies of Paramecium cultures support Schoener models Gause’s exptecs. P. aurelia higher popln growth tolerates high density P. caudatum lower popln. growth can’t tolerate high density Each sp. grown in isolation – expectation? Growth together, outcome expected?

Paramecium caudatum and Paramecium bursaria P. caudatum lost in interactions with P. aurelia P. caudatum and different species, P. bursaria grown together in lab in tubes coexisted Mechanism? – P. caudatum fed on suspended bacteria – P. bursaria fed on bacteria on bottom of tubes Interpretation in terms of interactions in nature?

Dave Tilman’s work (Univ. Minnesota) Chemostats- nutrient replenishment Asterionella formosa grown alone achieves high popln size by using up silicate levels

Dave Tilman’s work (Univ. Minnesota) Synedra ulna diatoms achieves somewhat lower population sizes when grown alone, it reduces available silicate levels also Synedra seems to survive at lower silicate levels, but lower density

These two diatom species grown together, outcome? Why? Both species used up silicate as populations grew, Synedra can live at lower conc., it won.

Competitive exclusion principle Naturalists noted early that 2 interacting species with very similar needs often resulted in extinction of one. Comp. excl. principle – “complete competitors” (species with exact similarity in ecological requirements) cannot coexist Why? Any slight growth advantage of one species would lead to extinction of other Assumptions of comp. excl. principle: – “complete predators”, environment is stable

Competition outcomes may be influenced by non-resource factors Pioneering species of plants varied in seed germination as a function of early season soil temperature (Bazzaz)

Why are germination rates impt.? determined seedling establishment influenced competition of various seedlings ultimately influenced what species were present for first year or two, why important?

Competitive abilities change along environmental gradients Can be caused by: –changing carrying capacities for a species under various conditions –changes in the physical environment that determine resource availability Pickett and Bazzaz, summer annual plant competition in a wet/dry gradient –A) monoculture grown across wet/dry gradient –B) mixture of species grown across gradient –Biomass measured of the plants

Several environmental factors may affect competitive ability in nature Saltmashes common on Atlantic coast Substrate is mud, intertidal region constantly being drowned by tides, environmental stresses on plants high…

Coexistence of species Role of evolution Partitioning of resources observed Partitioning reduces competition Did partitioning evolve from competitive interactions? Competition can influence natural selection