Compe&&on. Why is compe,,on important to consider?

Transcription

1 Compe&&on Why is compe,,on important to consider? 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.

2 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? When popula(on density is high. K N &me

3 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]

4 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?

5 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 shi`s if superior compe&tor removed.

6 Evidence Type #2: Niche ShiS 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

7 C) Na&ve Cughroat Trout and Brook Trout in Rocky Mtns. (Kurt Fausch lab) Brook trout introduced and Cughroats now restricted to coldest headwaters where brookies are excluded. Main mechanism for brook trout exclusion of cughroat trout is compe&&on ( compe&&ve exclusion ) Brook trout are fall spawners while cughroat trout spawn in spring, so in first summer, brook YOY are larger than cuts and appear to outcompete them for dri` resources (interference for feeding sta&ons and exploita&ve depression of resource). In field experiments, cughroat young grow faster in first summer in absence of brook trout, offering evidence for compe&&on. Cughroats have a refuge in coldest headwater streams, but this a subop&mal thermal regime, so growth rate is low.

8 What if species do not shi` niches in absence of poten&al compe&tors? Niche par((oning: The ghost of compe((on past?

9 Evidence Type #3: 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 it occur at ambient popula&on densi&es?

10 1) Intraspecific (one species) compe&&on in caddifly grazer Helicopsyche (Fig. 3) Experimental density increases from 0.25x to 2x ambient Helicopsyche Normal food levels ( ) Nega&ve slope (p < 0.01) and strong effect of increasing larval density on larval size Addi&onal food provided ( ) S&ll nega&ve slope (p < 0.05) but effect much smaller. Implies compe&&on IS occurrying at normal streambed densi&es of grazer Note increased growth rate at at ambient (1x) density ( ) ambient 1x 2x Fitness cost of increased density? (body mass) Mechanism? (exploita&on)

11 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

12 (#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 Algal biomass reduced by B and G ( ) Add G and reduce B growth ( ) Add B reduces G growth ( ) Both compete, i.e., each species affects growth rate of other species! (Not shown): Glossosoma survives beger at low resource levels. [Why do you think?]

13 How could we test for effects of compe&&on at appropriate whole stream scale with maximum realism and minimal 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

14 Findings: Func&onal Groups Algae? (compare before v. aqer) 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 smallscale experiments (e.g., rare grazers Goera, Neophylax increase) [Kohler & Wiley, Fig text] before a`er Glossosoma is an ecologically important grazer and a dominant species in these streams!

15 Interspecific interference compe,,on More o`en observed for space limited, sedentary species 1) The territorial Leucotrichia microcaddisfly build silk cases agached to rocks, graze algae uniform spacing of defended territories Territory size increases with larval mass.

16 Lecotrichia weed unpalatable bluegreen algae with scissors like mandibles (Hart 1985) If Leucotrichia removed, patches free of bluegreen algae decrease ( ) (Table 2). Edible algae (diatoms) >4x more abundant in Leucotrichia territory than outside Leucotrichia defend territory and Bae(s avoids! weeding effect

17 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!

18 2) Territorial blackflies (simuliids) and interference compe&&on a) 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 shi`s in bleph behavior (Fig. 2) What is fitness cost of modified behavior?? smaller body size at maturity

19 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 cughroat and brook trout)