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OUR Ecological Footprint

Chapter 16 To what extent does competition control species’ presence and abundance?

Objectives I Define facilitation Define competition Types of competition Types of resources Liebig’s Law of the minimum D-D regulation via intraspecific competition Plant responses Animal responses Competition added to logistic growth models

Facilitation Sp 1 aids Sp 2 An alternative to competition Not altruism; no cost to facilitator Understudied area

Which species helps which species? Does it cost the helper species? Does the commensal relationship change through time?

Competition: use or defense of a limiting resource that reduces availability to others

Types of competition : Exploitation vs. interference Intraspecific vs. interspecific

Competition may occur through exploitation (indirect) of shared resources or (direct) interference (individuals defend resources actively). exploit interfere

Superior competitors can persist at lower resource levels.

Allelopathy: chemical (interference) competition. This plant distribution is caused by: H 1 : Allelopathy H 2 : Seed predation by small mammals No resolution: need experiment considering both simultaneously.

Types of Resources Plants abiotic biotic Animals abiotic biotic

For sessile animals, space is an important resource. For most plants, space is not considered a resource.

Limiting Resource: If resource is scarce relative to demand. Renewable resource: constantly regenerated e.g. prey, nutrients Non-renewable resource: occur in fixed amounts and can be fully re-used e.g. space, hiding places

Liebig’s Law of the Minimum: Populations are limited by the single resource that is most scarce. A population increases until the supply of the limiting resource is insufficient; then growth stops. Applies to resources that do NOT interact to determine population growth rate. How realistic is this ‘Law’?

***Do these results support the Law of the Minimum? Explain.

N and P act synergistically to promote growth. More than 1 limiting resource…

Law of constant yield: *** What are 2 conclusions? N Figure C

Intraspecific competition + density- dependent population regulation Negative responses: Growth is…. Sexual maturity is…. Birth rate is … Death rate is…

Density-dependent regulation (via intraspecific competition) of growth. *** Summarize 2 conclusions. Figure 1

Density-dependent regulation of time to reach sexual maturity. ***Does age or weight determine sexual maturity? Explain. Figure 2

Density-dependent regulation of birth rate Figure 3

Higher density (> competition) leads to lower birth rate and (probably) lower population growth. Figure 4

Intraspecific competition contributes to density-dependent birth and death rates; hence to regulation of population size. Figure 5

Logistic growth model dN/dt = r N(K - N)/K) As N approaches K, intraspecific competition increases. (K - N)/K) gets smaller. Population growth slows.

Logistic equation is based on intraspecific competition depressing r, the per capita population growth rate; population size continues to increase until N = K.

Interspecific competition reduces equilibrium level of population below carrying capacity. At equilibrium, each competing species reduces K for the other species.

*** How much is K of sp 1 reduced by one individual of competing sp 2? By all individuals? K1K1 Figure 6

Lotka-Volterra addition of interspecific competition to logistic equation: (dN/dt) = rN[(K-N)/K] becomes: (dN 1 /dt) = r 1 N 1 [(K 1 -N 1 -  1,2 N 2 )/K 1 ]  1,2 = competition coefficient effect of each individual of sp 2 on sp 1 effect of sp 2 on sp 1’s growth rate Similarly, for species 2: (dN 2 /dt) = r 2 N 2 [(K 2 -N 2 -  2,1 N 1 )/K 2 ]

***What is  (competition coefficient) for the effect of 1 ind. of sp. 2 (N 2 ) on sp. 1 (N 1 ) or equivalence of sp. 2 for sp. 1? K1K1 Figure 7

Objectives II Niche concepts Interspecific competition ‘Ghost of competition past’: how deduce today whether competition in past? Classic experiments Competitive exclusion hypothesis Field experimental studies: how document that competition is occurring?

Niche : Ecological role of species in community Ranges of conditions and resource qualities within which the species persists Often conceived as a multi- dimensional space (hypervolume)

From 1 to n niche axes --> hypervolume of niche space

Niche metrics

Intraspecific competition: creates selective pressure for broader resource use. Interspecific competition: may select for narrower resource use. Food size utilized Frequency within pop. w/o with w/o with Food size utilized Frequency within pop.

Invasion by new species brings niche overlap may --> reduce niche breadth may--> allow more species packed into community.

Niche breadth decreases with interspecific competition.. Without competing sp. With competing sp..

Niches defined by two dimensions. ***Under what conditions do sp ) have overlapping niches? 2) compete? Figure 8 AB

Niche differences among congeneric trees. Note differences on different rock types.

‘Ghost of competition past’: What evidence is used to deduce that interspecific competition has occurred in the past?

1) Niche separation via resource partitioning among related species.

1) Niche separation via resource partitioning

1) Reduced niche overlap: closely related species use different parts of resource gradients when sympatric. Figure 9

2) Habitat shift: Use of habitat often changes depending on presence or absence of closely related species. Expand habitat when other species absent. Figure 10

3) Character displacement when in sympatry.

3) Character displacement: Morphological traits and food selection of species shift depending on presence or absence of closely related species.

4) Competitive exclusion may occur when new competitors are introduced.

Failure of species to coexist in lab cultures led to the Competitive Exclusion Hypothesis: 2 species can’t coexist on the same limiting resource. Figure 12

Two species can coexist if they are limited by different resources. Figure 12A

Outcome of interspecific competition is sometimes dependent on abiotic conditions °C 32.3 °C Figure 13

5) Competitive release: Densities of organisms often increase when densities of competing species are reduced. Figure 14

The winner in competition: Can not always be predicted by solo performance. Can change through time owing to genetic responses to selection (rapid evolution). Can change depending on the presence of other species (e.g. predators) = apparent competition

Observation: One predator feeds on many competing prey species. Does it control diversity? Hypothesis: If predation controls competition among prey and maintains prey species diversity. Prediction: When predator is removed, dominant competitors exclude other species and reduce species number. Conclusion: The outcome of competition can be altered by predation. Figure 15

3 species of toad/ frog tadpoles ***What is the main conclusion?

Removing nutrient limitation of stress-tolerant plants alters the outcome of competition.

Field Experiments: How can we demonstrate competition is occurring? The following slides are sample test questions for you to work on at home as ICA 5. See website to print worksheet to use. Do ONLY questions Worksheet is due on Thursday, October 15 at lecture.

Question 1: Buttercups The figure on the next slide illustrates the distribution of two species of buttercups along a transect across ridge (high land) and furrow (low valley) grassland. 1.In one sentence summarize the results. 2.Provide two alternative hypotheses (If…then) for the observed pattern. 3.Draw or describe one complete experiment that would test both hypotheses. 4.What specific results from the experiment would provide support for your hypothesis 1 above?

Figure for preceding ? Draw the contrasting curves for species 1 and 2. F R F R F Distance along transect (m) No. of plants Sp 1 peaks on furrow (F) Sp 2 peaks on ridge (R )

Question 2. Observation: zones of sp. B + C do not overlap. What are two alternative hypotheses / predictions? Experimental design? C B

What explains zonation of barnacles? Results: Together: Only B survives with B + C in B zone. Only C survives with B + C in C zone. Alone: C grows in B but B can’t grow in C zone. What are two conclusions?

Question 3: Galium sp. + Common garden experiment to study interspecific competition: Observation: 2 closely related sp. grow in different habitats or soils. What are 2 altenative hypotheses H 1 : H 2 : Describe the experimental design. What are 2 key results? What is main conclusion? Is competition symmetric between the two species?

Question 4. Desmodium sp. X 2 What are two questions being addressed by this experiment? Summarize two key results. What is the main conclusion? Which species is the better competitor? Is competition symmetric between the two species?

Question 5: 2 grasses Observation: An area of serpentine rock is adjacent to an area of granite rock. Grass 1 on granite soil is replaced abruptly by Grass 2 on serpentine soil. What are two hypotheses that explain their distribution? Question 1: Are the two grasses restricted to their respective substrates by soil factors? ***Design an experiment to answer the ?

Results: Grass 1 grows well on granitic soil but fails to grow on serpentine soils. Grass 2 grows equally well on serpentine and granitic soils. ***What is the main conclusion for grass 1? For grass 2?

***Design two experiments to determine whether competition limits the distribution of these two grasses.

Results: Experiment 1 1:1 mix on serpentine, Grass 1 dies, Grass 2 thrives. 1:1 mix on granite, Grass 2 dies, Grass 1 thrives. Experiment 2 When Grass 2 is cleared from serpentine, Grass 1 does not spread into area. When Grass 1 is cleared from granite, Grass 2 spreads into area. Is competition symmetric or asymmetric? What is the main conclusion about grass 1’s competitive ability? Its tolerance of serpentine soil stress? What may be a general principle relating to competitive ability vs. tolerance of stressful conditions?