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Competition. Peter B. McEvoy Insect Ecology Ent 420/520. Prevalence of current competition in natural communities as judged by recent reviews. Schoener (1983) Connell (1983) Strong, Lawton, Southwood (1984) Lawton and Hassell (1984) Denno, McClure, Ott (1995). Review by Denno et al. .

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Peter B. McEvoy

Insect Ecology

Ent 420/520

prevalence of current competition in natural communities as judged by recent reviews
Prevalence of current competition in natural communities as judged by recent reviews
  • Schoener (1983)
  • Connell (1983)
  • Strong, Lawton, Southwood (1984)
  • Lawton and Hassell (1984)
  • Denno, McClure, Ott (1995)
review by denno et al
Review by Denno et al.
  • History of Competition studies
  • Failure to find evidence of competition probably lies with the methods
    • Observational studies of pattern (e.g. niche and habitat overlap) were inappropriately used to infer process. Is high overlap indicative of presence or absence of competition?
    • Experimental studies of competition in phytophagous insects were few and far between (e.g. in reviews by Connell and Schoener)
    • Recent application of more powerful methods
Denno, R. F., M. S. McClure, and J. R. Ott. 1995. Interspecific interactions in phytophagous insects: Competition reexamined and resurrected. Ann. Rev. Entomol. 40:297-331.
  • Scope. 193 pair-wise species interactions (all major feeding guilds except pollinators and detritivores): occurrence, frequency, symmetry, consequences, and mechanisms of competition.
  • Result. Interspecific competition in 76% of interactions, often asymmetric, and frequent in most guilds (sap feeders, wood and stem borers, seed and fruit feeders) except free-living mandibulate folivores.
review continued
Review Continued
  • Phytophages more likely to compete if they were closely related, introduced, sessile, aggregative, fed on discrete resources, and fed on forbs or grasses
  • Interference competition was most frequent between mandibulate herbivores living in concealed niches
  • Host plants mediated competitive interactions more frequently than natural enemies, physical factors, and intraspecific competition
narrative and vote counting reviews can be misleading
Narrative and vote-counting reviews can be misleading…
  • Move from
    • Narrative or qualitative reviews
    • Vote-counting reviews that assess the frequency of reported outcomes
    • Such comparisons can be misleading because they fail to consider the magnitude of treatment effects and ignore the importance of sample size in determining the statistical power of individuals studies
  • Meta-analysis offers a remedy
paradigms lost
Paradigms Lost
  • Top Down Effects
    • Herbivores rarely compete
    • Herbivores rarely influence plant population and community dynamics
  • Bottom Up Effects
    • Much of the green is inedible or unsuitable
    • Some downright toxic




effects of carnivore removal meta analysis by schmitz et al 2000
Effects of Carnivore RemovalMeta-analysis by Schmitz et al. 2000
  • Direct positive effect of carnivore removal on herbivores
  • Indirect negative effect of carnivore removal on plants
    • Plant biomass
    • Feeding Damage
    • Reproductive output (seeds)




effects of herbivore removal meta analysis by bigger and marvier 1998
Effects of Herbivore RemovalMeta-analysis by Bigger and Marvier 1998

A meta-analysis of 246 experimental contrasts between plants protected from herbivores versus plants exposed to herbivores

Herbivores often impose important effects on plant biomass -- equal in magnitude to those imposed by plant competition.

Effects of invertebrate herbivores are significantly stronger than those of vertebrates, in contrast to widely held views.

Thus future studies should investigate how herbivory and plant competition interact to determine the distribution, abundance, and diversity of plants.



persistent questions
Persistent Questions
  • Are herbivores limited by natural enemies? …to such a degree that they rarely compete or reduce plant biomass?
  • Are herbivores limited by food resources? Quantity or Quality? What are the elements of Host Quality?
  • Are plants limited by herbivores?
rules of the game are changing humans have transformed constraints on plant growth
Rules of the Game are ChangingHumans have Transformed Constraints on Plant Growth


  • Resource limitation
  • Recruitment limitation
  • Predators and pathogens
  • Disturbance
  • Conditions (temperature/climate)
  • Temporal variation




paradigms gained experiments on competition review by denno mcclure ott 1995
Paradigms GainedExperiments on CompetitionReview by Denno, McClure, Ott 1995
  • Is competition mediated by
    • Carnivores
    • Diffuse interactions among many herbivores
    • Changes in plant quality




concept alert
Concept Alert!
  • Competition and Coexistence
  • Character displacement
  • Resource partitioning
  • Exploitation vs Interference
  • Coevolution of competitors – interaction a prerequisite
  • Null hypothesis: unsaturated, non-interactive communities
concept alert mechanisms of competition
Concept Alert! Mechanisms of Competition
  • Exploitative competition occurs when individuals, by using resources, deprive others of the benefits to be gained from those resources
  • Interference Competition results when individual harm one another directly through fighting or killing, or indirectly by aggressively maintaining a territory or producing chemicals that deter other individuals
asymmetric competition
Asymmetric Competition
  • For equal competitors, strong intraspecific competition may mitigate interspecific competition
  • For asymmetric competitors, strong intraspecific effects in the superior competitor may not preclude a strong interspecific effects on the inferior competitor
key questions
Key Questions
  • Conditions for coexistence. What conditions lead to coexistence vs. competitive exclusion?
  • Frequency and intensity. How common is interspecific competition in nature, what kinds of species are likely to compete?
  • Niche differentiation. What is extent of niche differentiation among coexisting competitors? Is competition a cause of morphological and ecological differentiation among species?
predictions that follow from conventional theory
Predictions That Follow From Conventional Theory
  • Potential competitors should exhibit niche differentiation
  • Niche differentiation often manifest as morphological differentiation
  • With any community, species with little or no niche differentiation are unlikely to coexist (limiting similarity), likely to differ in habitat (habitat distribution)
niche separation along a resource axis
Niche Separation Along a Resource Axis

Narrow niches with little overlap (d > w)

Broad niches with greateroverlap (d < w)

conditions for coexistence related to carrying capacities k1 and k2 and nich overlap d w
Conditions for Coexistencerelated to carrying capacities K1 and K2 and nich overlap (d/w)

Decreasing d/w (increasing overlap) reduces likelihood of stable coexistence except for competitors with similar carrying capacities

morphological differentiation among harvester ants
Morphological Differentiation Among Harvester Ants

As species diversity of ants increases, the variability in mandible length for Veromessor pergandei decreases

character displacement in harvester ants
Character Displacement in Harvester Ants

Observational studies

Mandible sizes of this species vary from site to site such that they always differ from those of local competitors (shown by arrows)

history of competitive displacement in aphytis parasitoids
History of Competitive Displacement in Aphytis Parasitoids
  • Aphytis chrysomphali
  • A. lingnanensis
  • A. melinus

Displaced by

Displaced by

mechanism of competitive displacement in aphytis
Mechanism of Competitive Displacement in Aphytis

Minimum host size required for female progeny is larger for inferior competitorA. lingnanesis (right arrow) than for superior competitor A. melinus (left arrow) for red scale distributions on wood, leaves, fruits

females of 4 species of parasitoids attacking cocoon stage of swaine jack pine sawfly
Females of 4 Species of Parasitoids Attacking Cocoon Stage of Swaine Jack Pine Sawfly

Note differences in wing area and ovipositor length

ratios in ovipositor lengths in parasitoids attacking sawfly pupae too close for coexistence
Ratios in Ovipositor Lengths in Parasitoids Attacking Sawfly Pupae: Too Close for Coexistence?
methodological difficulties in proving or disproving competitive exclusion principle
Methodological difficulties in proving or disproving Competitive Exclusion principle
  • When two species compete and coexist, we can suppose that they do so as a result of some niche differentiation.
  • But this supposition remains unconfirmed until such differentiation is observed and shown to ameliorate the effects of interspecific competition.
  • It is often difficult to establish that there is such differentiation, and it is impossible to establish the absence of it. We may simply have looked in the wrong place or in the wrong way.
modern theory moving away from assumption of community at equilibrium
Modern theory moving away from assumption of community at equilibrium
  • Disturbance, colonization, successional development
    • Unpredictable gaps: the poorer competitor is a better colonizer (Zwölfer 1973)
    • Unpredictable gaps: the preemption of space
    • Fluctuating environments: independence in species fluctuations
    • Ephemeral patches with variable life-spans: the poorer competitor reproduces more rapidly
    • Aggregated distributions - e.g. carrion flies (Ives 1988, Ives 1991)
  • Apparent competition: coexistence mediated by a shared natural enemy
    • No effect on coexistence. Predation modifies the interaction between competitors without necessarily making coexistence more likely
    • Promotes coexistence. Predation more likely to enhance diversity if predator prefers to feed on the superior competitor
Effects of Species Interactions on Biological InvasionsApparent Competition in Leafhoppers(Settle and Wilson 1990)



Leaf Hopper

Variegated leaf hopper







Host Plant


relative strength of intraspecific and interspecific competition
Relative Strength of Intraspecific and Interspecific Competition
  • Density-dependent per capita reproduction
  • Equivalence of intraspecific and interspecific competitors indicated by equal slopes
apparent competition or negative effect of one species on another acting via a shared parasitoid
“Apparent Competition” or Negative Effect of One Species on Another Acting Via a Shared Parasitoid
  • Percent GLH parasitism
  • Ratio of parasitoids to GLH

Increase with VLH relative density

non linear frequency dependent parasitism
Non-linear, frequency-dependent parasitism

Because risk of parasitism is higher in GLH than VLH, VLH contributes a small fraction of parasitoids even when VLH is an intermediate fraction (i.e. up to 50%) of the total hopper population but contribution accelerates thereafter

populus angustifolia narrowleaf cottonwood
Populus angustifolia(Narrowleaf Cottonwood)
intraspecific competition in pemphigus aphids studied by whitham
Intraspecific Competition in Pemphigus Aphids studied by Whitham
  • Narrow “window” for colonization in spring. As much as 83% of over-wintering Pemphigus betae population migrates in 3-day span to immature leaves of Populus angustifolia were they gall and become encased in plant tissue
habitat selection by pemphigus aphids in response to resource limitation and competition
Habitat selection by Pemphigus aphids in response to resource limitation and competition

Successfully developed gall of Pemphigus betae on leaves of Populus angustifolia

Aborted gall

size of mature leaf is critical to ultimate fitness of individual colonizing mother
Size of mature leaf is critical to ultimate fitness of individual colonizing mother

(A) Percent aborted declines…

(B) Aphids per gall increases…

(C ) Stem mother weight increases..

…with increasing leaf size

stem mothers seek out and colonize large leaves
PercentStem mothers seek out and colonize large leaves

Distribution of Leaves Available on Tree

Distribution of Leaves used by stem mothers

Territorial AphidsTypical fighting posture of Pemphigus betae females. The larger female usually wins the better territory, which is about 3 mm long at the base of the leaf

Whitham 1979

Intraspecific competition in females of the aphid Pemphigus betaeWhen dominant female removed, displaced female moved down to occupy the better position

Whitham 1980

when more than one aphid colonizes a leaf the advantage goes to the aphid lower on the leaf
When more than one aphid colonizes a leaf, the advantage goes to the aphid lower on the leaf
  • Effect of gall position and numbers of aphids on females fitness
  • Leaves drawn to scale and effects of position given in boxes
  • Trends in leaf size, aphids per gall, abortion rate, stem mother weight
model of habitat selection
Model of Habitat Selection
  • The Fretwell and Lucas model of habitat choice assumes that fitness is negatively correlated with density of competitors.
  • As best habitat is colonized and densities become high, poorer but sparsely inhabited habitats are likely to be used
  • Fitness similar across habitats. Prime habitat with more occupants becomes similar to poorer habitat with fewer occupants

At d2

At d3

fretwell lucas model applied to pemphigus betae aphids
Fretwell Lucas model applied to Pemphigus betae aphids
  • Shows habitats (leaves) of varying quality (leaf size) and differences in population density per habitat (galls per leaf)
  • Lines show expected fitness (aphids per gall) among leaf sizes and gall densities.
  • Solid line indicates how average fitness for 1,2,3 stem mothers per leaf is even across densities, consistent with model predictions
conclusions from pemphigus study
Points made

Not all leaves on tree are available or suitable

When available resources accurately defined, found to be limiting at least within tree (we don’t yet know tree to tree picture)

Points requiring further study

Host variability as defense against herbivores

Intertwining of suitable factors: nutrition, density of conspecifics, leaf abscission, enemies

Conclusions from Pemphigus study
insects on seaside daisy
Insects on Seaside Daisy
  • Erigeron glaucus Seaside daisy

Platyptilia emissalisPTEROPHORIDAE is a congener of species studied by Karban

herbivore example spittlebugs plume moths and thrips on seaside daisy karban
Herbivore Example: Spittlebugs, Plume Moths, and thrips on Seaside Daisy (Karban)
  • Combines experimental and observational study of interspecific competition between folivorous insects on seaside daisy Erigeron glaucus
  • The study illustrates asymmetric competition between unrelated insects belonging to different feeding guilds
natural history
Natural History
  • The Calendula plume moth Platyptilia williamsii (Lepidoptera: Pterophoridae) is a multivoltine, shoot-feeder.
  • The meadow spittlebug Philaenus spumarius (Homoptera: Cercopidae) is a cosmopolitan, univoltine, sap-tapper with a very broad host plant range.
experimental design
Experimental Design
  • Treatments manipulated the density of each insect: 2 levels of moths (presence, absence) x 2 levels of spittlebugs. Presence was represented by a single fixed density for each insect (1 moth, 5 spittlebugs). Would have been better to have 3 levels of each factor. Why?
  • Responses were measured as persistence of each insect and approximate population growth rates.
resource renewal rates and competition mechanisms
Resource Renewal Rates and Competition Mechanisms
  • Leaf-axils are a limiting resource for spittlebugs (McEvoy 1986) and mechanical barriers to feeding confer resistance (Hoffman and McEvoy 1985, Hoffman and McEvoy 1986).
  • Karban measured resource renewal rates (production of new leaves), simulated effects of plume moth by binding leaves together, and found similar results with simulated and observed damage.
multiple scales of observation
Multiple Scales of Observation
  • Important to look across scales, see how description of competition changes with scales, and to link processess occuring on different scales
  • Karban investigated multiple scales
    • temporal scales (Within  Between Years)
    • spatial scales (Plant PatchRegional Patterns of Association)
    • organizational scales (Two focal species in this paper Other species including a thrips, a leafhopper, and an aphid are treated in other papers).
results insects on seaside daisy
Results Insects on seaside daisy
  • Asymmetric interactions. Persistence times and net reproductive rates of spittlebugs are reduced by addition of plume moth; but there is no detectable effect of spittlebugs on plume moths
  • Individualistic behavior. Each species affected strongly by a different biotic factor
    • Spittlebugs by interspecific competition
    • Plume moths by vertebrate predation
    • Thrips by clonal variation
regional patterns of coexistence
Regional Patterns of Coexistence

Observational study

Karban surveyed patterns of association among plume moths (PM), spittlebugs (S), and thrips (T) along the length of the CA coast

Percentage of rosettes with each insect

unsettled points
Unsettled Points
  • Side effects in handling of plants and insects: Plants grow naturally in dense clumps, whereas plants in gardens were isolated, widely-space individual ramets. How might species interactions be modified by these differences in host dispersion?
  • Insect Density: What is the probability of finding a given density of each insect in nature? Why would it be better to examine a range of densities and frequencies in these experiments rather than a single, fixed density?
  • Insect Fate: What is the fate (e.g. emigration, survival, reproduction) of individuals that “disappear”? Why is it important to know their fates? How might the fate of spittlebugs differ on single ramets isolated in a garden and multiple ramets clustered in clones in nature?
  • Evolution: What effect might competition with plume moths have on the evolution of resource exploitation by spittlebugs?