Compe&&on pp in text

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1 Compe&&on pp in text Why is compe,,on important to consider? - Can species coexist? How large will popula&ons be? Defini,on: An interac(on among individuals for a common, limi&ng resource that results in reduced individual fitness How do we measure reduced fitness? correlates of fitness - what can be more easily measured): Growth, Survival, Fecundity What is compe,,ve exclusion? When one species eliminates another from a habitat by monopolizing resources and driving fitness of out- competed species to zero. Is compe,,ve exclusion common? Typical resources? - Food - Space (access to food) - Mates If you add more of a limi(ng resource to a habitat, what happens to growth, survival of individuals that rely on that resource? When are resources most likely to be limi,ng? à When popula(on density is high. K N &me 1

2 2 TYPES of compe,,on: 1) Intraspecific - - among individuals within a species ( conspecifics ) 2) Interspecific - - among different species ( heterospecifics ) 2 MECHANISMS of compe,,on: 1) Exploita,ve - - deple&on of resource to disadvantage of some other individuals 2) Interference - - behavioral interac&on that deprives another individual resource [e.g., territoriality] Niche concept The condi,ons and resources required for an individual or species to exist and reproduce. The full range of condi&ons and resources in which species can survive and reproduce = Fundamental Niche. The more narrow range of condi&ons and resources in which species actually survives and reproduces, in the presence of compe&tors and predators = Realized Niche. Are these fundamental or realized niches? 2

3 Evidence for compe,,on in streams: Evidence Type #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 compe&&on? How could you test for compe@@on? Niche shias if superior compe&tor removed. How does this figure show niche overlap? Rainbow trout consumes mostly terrestiral Dolly Varden intermediate Sculpin mostly benthic 3

4 Evidence Type #2: Niche ShiU Studies Does resource use change in presence of other species Examples A) 2 species of crayfish (Bovbjerg 1970) Where species do not co- occur, both species in riffles Where they do co- occur, one species displaced to pools B) 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 If sculpins experimentally excluded from cooler riffles, what should dace do? Move to occupy riffles How does this figure represent niche shiu? Greenhouse treatment (GH): - Native dolly varden switch to benthic prey (3A) - Benthic herbivore biomass decreased (3B) - algal biomass increased (3C) - insect emergence declined (3D) Are Rainbow Trout and - terrestrial spider biomass declined (3E) Dolly Varden compe&ng? Exotic rainbow trout treatment (RBT): - Native Dolly Varden switch to benthic prey (3A) - Benthic herbivore biomass decreased (3B) - algal biomass increased (3C) - insect emergence declined (3D) - terrestrial spider biomass declined (3E) What evidence needed? What about combined treatments? Conclusion: Exotic trout and habitat change similarly alter stream-riparian foodweb: - (1) trophic cascade (Fig. 3A,B,C) - (2) reciprocal subsidy (Fig. 3A,D,E) What type and mechanism of compe&&on??? 4

5 Strong evidence: Experimental studies Some studies experimentally demonstrate mechanism (or cause) of compe&&on on fitness. How do we measure fitness effect? Growth, survival, fecundity Recall, 2 types of compe&&on: Intraspecific and interspecific 2 causes or mechanisms: Exploita&ve and Interference Ques&on: Is compe&&on common in streams? That is, does Does it occur at ambient popula&on densi&es? 1) Exploita@ve compe@@on Intraspecific (one- species) compe&&on in caddifly grazer Helicopsyche (Fig. 3) Ho: An increase in grazer density does not lead to compe&&n How would we test this? Helicopsyche Normal food levels - Nega&ve slope (p < 0.01) and strong effect of increasing larval density on larval size What happens if you add food? - The density effect is less pronounced (smaller slope), but s&ll significant. Suggests food limita&on at ambient food levels ambient 1x 2x Fitness cost of increased density? (body mass à fecundity) Mechanism? (exploita&on) 5

6 Interspecific exploita,on (several examples) (#1) 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 exploita&on Elimia Streams with snails Neophylax (#2) Small- scale experiments in lab and field (Kohler 1992) Laboratory Glossosoma (G) and Bae(s (B) at field (= ambient) densi&es [Fig. 4] Glossosoma Bae(s à Algal biomass reduced by B and G à Add G and B growth declines à Add B and G growth declines Both species compete, i.e., each species affects growth rate of other species! Which species has larger effect on other species? Interes&ng note: Glossosoma survives beser at low resource levels, a result not shown here. [Why do you think?] 6

7 How could we test for effects of compe&&on at appropriate whole stream scale with maximum realism and no cage effects? (#3) Large scale natural experiment with Glossosoma - - Kohler and Wiley (1997) Large scale, i.e., whole stream manipulated Glossosoma popula&ons crashed and virtually eliminated from 6 streams in Michigan by parasite (Cougourdella sp.) over period of few years Findings: Func&onal Groups Algae? (compare before v. aoer) Grazers respond posi&vely (e.g., Fig b, c) (- - ), probably due to release from exploita&ve compe&&on since more algae present Some filter feeders also increased (Fig d, e, f) (- - ), probably due to lack of interference with Glossosoma (able to occupy space on rock surfaces)? Note that not all streams responded with similar magnitude or direc&on (e.g., Fig c, f) Outcomes of experiment at this whole stream scale may not be reproduced in small- scale lab experiments. Some responses consistent with small- scale results - - e.g., algae, Bae(s increase BUT, some responses not detected in small- scale experiments (e.g., rare grazers Goera, Neophylax increase) [Kohler & Wiley, Fig text] before aaer Glossosoma is an ecologically important grazer and a dominant species in these streams! 7

8 Interspecific interference compe,,on More oaen observed for space- limited, sedentary species 1) The territorial Leucotrichia - - microcaddisfly build silk cases asached to rocks, graze algae uniform spacing of defended territories Territory size increases with larval mass. Lecotrichia weed their territories to remove unpalatable blue- green algae and promote diatoms (Hart 1985) In Leucotrichia territories, edible algae (diatoms) >4x more abundant than outside territory If Leucotrichia removed from their territories, blue- green algae colonize and occupy the territory (Table 2). Leucotrichia defend territories and grazer Bae(s avoids! Before removal Aaer removal weeding effect 8

9 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 Leucotrichia s snipping mouthparts! Are these examples strong or weak evidence for compe&&on? 2) Territorial blackflies (filter- feeders) and grazing blepharicerids interference compe&&on (Dudley et al. 1990) Sta&onary blackflies snip at mobile blepharicerids In field, fewer blephs where more simuliids (Fig. 1) Manipula&ng simuliid abundance causes shias in bleph behavior (Fig. 2) fitness cost 9

10 end 3) Hydropsychids (caddisfly) Hemphill and Cooper (1983) aggressively displace blackflies More when we learn about disturbance 4) Trout species aggressively compete for preferred feeding sta&ons (Fausch work on cushroat and brook trout) 10

11 C) Na&ve Cushroat Trout and Brook Trout in Rocky Mtns. (Kurt Fausch lab) Brook trout introduced and Cushroats now restricted to coldest headwaters where brookies are excluded. Main mechanism for brook trout exclusion of cushroat trout is compe&&on ( compe&&ve exclusion ) Brook trout are fall spawners while cushroat trout spawn in spring, so in first summer, brook YOY are larger than cuts and appear to outcompete them for dria resources (interference for feeding sta&ons and exploita&ve depression of resource). In field experiments, cushroat young grow faster in first summer in absence of brook trout, offering evidence for compe&&on. Cushroats have a refuge in coldest headwater streams, but this a subop&mal thermal regime, so growth rate is low. What if species do not shia niches in absence of poten&al compe&tors? Niche par((oning: The ghost of compe((on past? 11