Interspecific Competition II

Getting back to plants…

Although competition clearly appliesto plants, most of the theory iszoologically based

The Problem with Plants…

All plants have essentially same requirements:

LightWaterNutrients

These are not typically discrete packages

Shortage of one may affect ability tocompete for other needed resources

The Problem with Plants…

Cause and effect are very difficult to establish without experimentation

Coexistence explanations:

•Frequently focus on demonstratingthat spp are different in somecharacteristics

Not conclusive in terms of mechanisms~

SIX MECHANISMS OF COEXISTENCEIN PLANTS

1. Resource Partitioning

•Niche separation

•Intraspecific competition > Interspecific competition

2. Spatial Segregation

Example: Solidago

(Begon et al. 1996 p. 102)

Soil moisture andinterspecific comp.appear to determineeach realized niche

Six Mechanisms of Coexistence in Plants

3. Recruitment Limitation

•The dispersal/competitive abilitytrade-off

4. Pest Pressure

These four mechanisms all assume thatcoexistence is an equilibrium condition

Six Mechanisms of Coexistence in Plants

5. The Storage Effect

6. Density-Independent Mortality

•Overlapping generations

•Asynchronous recruitment

These two mechanisms are based onnonequilibrium as the condition

Mechanisms of Coexistence

Very difficult to demonstrate

Too much emphasis on adult plants?Maybe preemptive competition forregeneration niche the key?

Need to consider indirect abiotic effects?

Tilman et al. (1982) experimentalwork with planktonic algae

Two algae and abiotic Influences on competition

Asterionella formosa requires less SiO2

at lower temperatures

Synedra ulna requires less SiO2 at higher temperatures

Coexistence: temperature fluctuations, alternating competitive displacement

(Tilman et al. 1982)

Theory and Real Result

(Begon et al. 1996 p. 104)

Experimental Work Vital To Elucidate mechanisms of

coexistence

Particularly due to history of agriculturalresearch, standard experimentaldesigns developed

These include additive and substitutivedesigns

Experimental designs for plants

Simple Additive

Substitutive orReplacement Series

Full Additive

Some Experimental Results: Replacement series design

2 Species, 4densities

(Begon et al. 1996 p. 86)

Competitionuneven

A. fatua lessaffected thanA. barbata

Two Botanical Measures of Competition

Relative Yield Total (RYT):

Yield of X in mixtureYield of X in monoculture

Relative Resource Total (RRT):

Measure of performance in mixture

Both try to measure the extent to which 2 spp utilize different resources

Theoretical explorations of competitive exclusion and

coexistence

The logistic model of two-speciescompetition….

Outcomes of the model

(Begon et al. 1996 p. 108)

Competitiveexclusion

Unstable coexistence

Stablecoexistence