1. Competition
Definition: An interaction among individuals for a common, limiting resource that results in reduced fitness of some of the individuals
Correlates of fitness: growth, survival
How does ‘competitive exclusion’ occur?
Extensive loss of individual fitness causing population to become locally extirpated.
Necessary condition:
Limiting resource: One whose supply limits growth, survival, fitness
Typical resources:
 food
space (access to food)
mates

2. When are resources limiting?
 When population density is high. [e.g., logistic growth to K] K
time N
2 TYPES of competition:
1) Intraspecific -- among individuals within a species (“conspecifics”)
2) Interspecific -- among different species (“heterospecifics”)
2 MECHANISMS of competition:
1) Exploitative -- depletion of resource to disadvantage of some other individuals
2) Interference -- behavioral interaction that deprives another individual resource [e.g., territoriality]

3. Evidence for competition in streams:
Evidence 1: Niche Overlap Studies
Do species overlap in resource use?
Example: Fish use different microhabitat niches in a Mississippi stream [Baker & Ross 1981]
Is the “strong” or “weak” evidence for competition?
How could you test?
HOW COULD YOU TEST??? Remove one species and see if others “fill niche”

4. Evidence 2: Niche Shift Studies
Does resource use change in presence of other species
Examples
1) 2 spp. Of Crayfish (Bovbjerg 1970)
Where not co-occurring, both species in riffles
Where co-occur, one species displaced to pools
2) Fish: Sculpins and speckled dace (Baltz et al. 1982)
Prefer similar water velocity and substrate size
In cooler, upstream reaches … sculpins dominate riffles
In warmer, downstream reaches few sculpins, dace occupy riffles

5. 3) Native Cutthroat Trout and Brook Trout in Rocky Mtns
3) Native Cutthroat Trout and Brook Trout in Rocky Mtns. (Kurt Fausch Lab)
Cutthroats now restricted to coldest headwaters where brookies introduced.
Main mechanism for brook trout exclusion of cutthroat trout is competition
Brook trout are fall spawners while cutthroat trout spawn in spring, so in first summer, brook YOY are larger than cuts and appear to outcompete them for drift resources (interference for feeding stations and exploitative depression of resource).
In field experiments, cutthroat young grow faster in first summer in absence of brook trout, offering evidence for competition.
Brook trout thermally excluded only in coldest headwaters, giving cutthroats a refuge, but this a suboptimal thermal regime for cutthroat trout as well, so growth rate is low.

6. The “ghost of competition past”?
What if species do not shift niches in absence of potential competitors?
The “ghost of competition past”?

7. Evidence 3: Experimental studies
Experimental demonstration of fitness effect and mechanism
What is “fitness effect”?
Growth, survival, fecundity
What is “mechanism”?
Exploitative competition (resource depletion by one species)
Interference competition (behavioral interactions that reduce foraging time)
Question: Is competition common in streams? That is, does it occur at ambient pop’n densities?

8. Exploitative competition
Intraspecific (one-species) competition in grazer Helicopsyche (Fig. 3)
density increases from 0.25x to 2x ambient…
Helicopsyche
Unsupplemented food levels (----)
- Negative slope (p < 0.01) and strong effect of increasing larval density on larval size
What if additional food is provided? (----)
- Still negative slope (p < 0.05) but effect much smaller
Competition appears weak at ambient (1x) density (----).
[Compare mean streambed mass to experimental masses at 1x density]
0.25 ambient 1x 2x
Fitness cost of increased density? (body mass)
Mechanism? (exploitation)

9. Interspecific exploitation (several examples)
a) Snail and caddisfly in southeast US -- Hill (1992)
In Streams – natural experiment
6 streams with snail + caddisfly, 6 with caddisfly only
Fitness correlate: fat reserves and body mass of caddisfly [Fig. 9.5a]
Effect on stream food resources [Fig. 9.5b]
In Lab -- Dietary overlap, both species food limited
CONCLUSION: Elimia reduces fitness of Neophylax via exploitation
Elimia
Neophylax
Streams with snails

10. b) “Small-scale” exp’ts in lab and field (Kohler 1992)
Laboratory—Glossosoma, Baetis at field (ambient) densities [Fig. 4]
Baetis
Glossosoma
 Algae reduced by B and G (--)
 G reduces B growth (--)
 B reduces G growth (--)
Both compete, i.e., each species affects growth rate of other species.
(Not shown): Glossosoma survives better at low resource levels. [Why do you think?]

11. Large scale experiment with Glossosoma -- Kohler and Wiley (1997)
How could we test for effects of competition at appropriate whole stream scale with maximum realism and minimal “cage” effects?
Large scale experiment with Glossosoma -- Kohler and Wiley (1997)
Large scale, natural field experiment
Glossosoma virtually eliminated from 6 streams in Michigan by parasite (Cougourdella sp.) over period of few years

12. Findings:
Functional Groups
Algae? (compare before v. after)
Grazers respond positively (e.g., Fig. 9.15b, c) (--), probably due to release from exploitative competition
Some filter feeders also increased (Fig. 9.15d, e, f) (--), maybe due to lack of interference with Glossosoma?
Note that not all streams responded with similar magnitude or direction (e.g., Fig. 9.15c, f)
Small-scale cage experiments don’t always “scale up.”
Some responses consistent with small-scale results -- e.g., algae, Baetis increase
BUT, some responses not detected in small-scale experiments (e.g., rare grazers Goera, Neophylax increase)
[Kohler & Wiley, Fig text]
after
before
Glossosoma is an ecologically important grazer and a dominant species in these streams!

13. Interspecific interference
More often observed for space-limited, sedentary species
1) The terretorial Leucotrichia -- “microcaddisfly”
build silk cases attached to rocks, graze algae
uniform spacing of defended territories
Territory size increases with larval mass.

14. Lecotrichia “weed” unpalatable bluegreen algae with scissors-like mandibles (Hart 1985)
If Leucotrichia removed, bluegreen algae grow (Table 2).
Edible algae (diatoms) >4x more abundant in Leucotrichia territory than outside
Leucotrichia defend territory and Baetis avoids!

15. Removing Leucotrichia … opens space for other consumers
Increase midges [Fig. 9.6]
Increases Glossosoma and blackflies [McAuliffe Fig. 6]
Figures show how other insects avoid snipping Leucotrichia

16. 2) Territorial blackflies (simuliids) and interference competition
a) Dudley et al. (1990)
nip at mobile blepharicerids
In field, fewer blephs where more simuliids (Fig. 1)
Manipulating simuliid abundance causes shifts in bleph behavior (Fig. 2)
ultimately smaller body size at maturity

17. 3) Hydropsychids (caddisfly)
Hemphill and Cooper (1983)
aggressively displace blackflies
More when we learn about disturbance
4) Trout species
aggressively compete for preferred feeding stations
(Fausch work on cutthroat and brook trout)

18. - response of periphyton? - response of other grazers?
• Field experiment -- Exclusion of Glossosoma from artificial substrates [Fig. 7, Kohler 1992]
(Measure insect density each month)
- How would you do this??
- response of periphyton?
- response of other grazers?
 Baetis, other grazers increase
 many non-grazer taxa also respond
Some evidence that Glossosoma depresses resources for other stream grazers.
Glossosoma negatively influences growth (fitness) of other species?
How to test?