Lecture 15 Understanding the Structure and Function of Communities

Transcription

1 Lecture 15 Understanding the Structure and Functin f Cmmunities Fd chains Primary Prductin and Energy Flux HSS Green Wrld Hypthesis Fretwell Hypthesis Tp Dwn vs. Bttm Up Examples Fd Webs Ecsystem-based Management Cmmunity Structure: Fd Chains Fd Chains A diagrammatic representatin f the flw f energy thrugh trphic levels in an eclgical cmmunity. Trphic Levels functinal classificatin f rganisms in an ecsystem accrding t feeding relatinships Essentially grupings f species based n what they eat and what eats them. Terrestrial Marine/ Aquatic General Patterns f Fd Chains Eltn s Pyramid (1940 s) One f the funders f eclgy. 3 cnsumers 3 cnsumers 2 cnsumers Grazers & Detritivres Net Primary Prductin 2 cnsumers Grazers & Detritivres 1) Differences acrss trphic levels 1) MORE animals LOWER n the fd chain 2) MORE bimass sequestered in LOWER fd chain 3) HIGHER turnver and Smaller animals LOWER in fd chain Help Litter Detritus 1

2 General Patterns f Fd Chains Lindeman (1942)- pineer f thinking abut ecsystems in terms f energy flux Therm-dynamics Explains: 1) Energy transfer between trphic levels (1-25%) 2) Fd Chain lengths 3-5 Cmmunity structure: the number f species and their relative abundance in a cmmunity 75-99% f energy is lst between trphic levels Cmmunity Structure: Net Primary Prductin NPP is imprtant t fd chain and cmmunity structure NPP is extremely variable acrss ecsystems ( gc/m 2 per year) S fd chains are driven by prductivity right?! Cmmunity Structure: Green Wrld Hypthesis HSS (Hairstn, Smith, Slbdkin 1960) Why is the wrld green?? Nt prductivity. Tp dwn Cntrl (Trphic Cascade) 1) Nutrients are seldm limiting fr plants. 2) Herbivres are ptentially strng regulatrs f plant ppulatins. BUT 3) Predatrs keep herbivre densities lw and allw plants t prliferate Prblems with HSS 1) Often nt supprted experimentally 2) Many plants are defended chemically and structurally (cactus spines, lignins, kelp secndary cmpunds. Wrld is green because it green isn t always edible. Cmmunity Structure: Fretwell-Oksanen (Fretwell 1977) (Oksanen 1981) Explains deviatins frm HSS by cnsidering number f trphic levels. 1) Odd numbered trphic levels= Green 2) Even number = Brwn Trphic Level HSS A B Pssible Fd Chains E 2

3 Tp Dwn (Trphic Cascades): Tp Dwn vs. Bttm Up Predatrs cntrls herbivres Cnsequently, herbivres have little effect Plants are abundant Primary Prductin is nt limiting (nt regulating) Predictins: Remval f predatrs shuld release herbivres, which will then decrease plants Bttm-up mdel (Prductivity) : Nutrients limit plant abundance Primary Prductin is limiting Plants limit herbivres, herbivres limit carnivres, etc. Predictins: Remval f tp levels=little effect n lwer levels But if yu remve lwer trphic levels, big effect n upper trphic levels Tp Dwn Cntrl Keystne Species : A species that has a disprprtinate effect n the cmmunity (Paine 1969) Keystne species ften: 1) Cntrl dminant cmpetitr (Mussels) 2) Cntrl dminant herbivre (Urchin) 2) Mdify habitat (Bears, Elephants in Serengeti) Remval f few results in massive change t the cmmunity structure Imprtant t ecsystem integrity Examples 1) Starfish and Mussel 2) Alaskan Sea Otter/ Urchin/ Kelp 3) Aleutian s Invasive Rat study Tp Dwn: Starfish and Mussel Fd Web Cmplexity and Species Diversity (Paine 1966 American Naturalist) Tp Dwn: Otter/ Urchin/ Kelp Killer Whale Predatin n Sea Otters Linking Oceanic and Nearshre Ecsystems (Estes, Tinker, Williams, Dak 1998) Hypthesis: Diversity will decrease in areas where Pisaster is remved cmpared t cntrls Pisaster- Vracius predatr Mytilus- Space cmpetitr Experiment: Remve Pisaster Results: Lss f diversity: 25 t 1 species. Pisaster Ochraceus (Estes and Duggins 1995) Otters almst hunted t extinctin During recvery kelp frests recvered Due t relaxatin f urchin herbivry Orcas switch diets and eat tters Suddenly tter ppulatins crashed Urchin grazing increased Kelp frests were denuded and were lst Criticisms: 1) Only cunted primary space hlders 2) Was nly dne at ne lcatin Only takes a few pds t switch diet 3

4 Tp Dwn: Invasive Rats n Aleutian s Intrduced rats indirectly change marine rcky intertidal cmmunities frm algae- t invertebrate-dminated (Kurle, Crll, Tershy 2008) Tp Dwn: Invasive Rats n Aleutian s Rat Infested Rat Free + + Birds Limpets/ Snails - Algae Sessile Inverts (nt eaten by birds) Bttm Up: Big Blue Big Sur Why is the cean blue in Big Sur? Fd webs Fd webs: Mre cmplex than fd chains Shw interactins between all species r functinal grups in a cmmunity (Bruland et al. 1991) Upwelling prvides nutrients: Phsphate, Nitrate, Silicic Acid Irn (Fe) is frm cntinental shelf Rivers have lw sediment lads Big Sur is Fe limitated 4

5 (Gary Plis 1991) Fd Webs: Cachella Valley Characterize and Quantify All interactins Get very cmplex- very fast!! Useful fr predictin: Identify Keystne Species Identify Strng Interactins/ Pathways Identify Ptential Indirect Interactins Understand Tp-Dwn/ Bttm-Up effects Understand hw cmmunities interact Great Sharks Smaller sharks and rays Fd Webs: Wh Cares? Marine Subsidies f s Marine Terrestrial (Myers et.al. 2007) Fd Webs: Ecsystem-Based Based Management 3 cnsumers 2 cnsumers Grazers & Detritivres Detritus 5

6 Lecture 16 Bidiversity Species-Area relatinships Bigegraphy SLOSS debate Latitudinal gradients in bidiversity Bidiversity Bidiversity # f species in an area Tw different ways t measure bidiversity: 1. Species richness- number f species 2. Species richness and evenness Richness = 3 Evenness Lw Diversity Lw Richness = 3 Evenness High Diversity High Bidiversity Three different ways t measure species richness: α (Alpha) Diversity- within an ecsystem Shannn Index- accunts fr bth richness and evenness. p i= prprtin f individuals fr each species n= number f species K= cnstant Imprtance f Bidiversity Imprtance f Bidiversity 1. Gd Measure f Ecsystem Health -metric fr change 2. Ecsystem Stability (species redundancy) 3. Cnservatin Tl -htspts (bang fr the buck) β (Beta) Diversity- between ecsystems r alng a gradient (prprtin f diversity cmpared t the average) γ (Gamma) Diversity- ver an entire regin 1

7 Equilibrium Mdel f Bigegraphy Why d farther islands have less species? Distance will affect dispersal rate f species: -difficult t survive -island becmes a small target -mre frces need t act tgether t make a successful clnizatin DISTANCE EFFECT Distance lwers lcal clnizatin rates Clnizatin Rate Extinctin Rate DISTANCE EFFECT DISTANCE EFFECT Clnizatin Rate Far Clse Extinctin Rate Rate Far Clse S far S clse 4

8 Species Area Relatinship (One f the few laws in eclgy) A= Area S= c and z= cefficients # Species Patterns f Bidiversity S=cA z Lg Species Lg S= Lg c + z lg A Big islands have mre species than small islands Bigegraphy Tw bservatins abut islands: 1) LARGER islands have MORE species 2) CLOSER islands t mainland have MORE species than distant islands Why d we see these patterns n islands? 1) Increased habitat hetergeneity 1) Many examples where this is nt the case 2) Equilibrium Mdel f Bigegraphy Area Lg Area -Include mre habitats -Equilibrium prcesses ( Bigegraphy) Equilibrium Mdel f Bigegraphy Hyptheses: (MacArthur and Wilsn 1967): 1) diversity represents balance between the lcal rates f clnizatin and extinctin 2) Therefre: diversity is at equilibrium births deaths Equilibrium Mdel f Bigegraphy Why d larger islands have mre species? Nn-equilibrium Hyptheses: Mre habitats explains sme Can supprt higher trphic levels maybe Equilibrium Hypthesis: Sustain larger ppulatins therefre: lwering lcal extinctin rates Rate Stable Equilibrium Ppulatin 2

9 ISLAND SIZE EFFECT Assumptins: Permanent mainland surce pl f species (P) All P species have same dispersal capacity All P species have same chance f ging extinct n the island Clnizatin Rate Extinctin Rate ISLAND SIZE EFFECT ISLAND SIZE EFFECT Clnizatin Rate Extinctin Rate Small Large Rate Small Large S small S large 3

10 Equilibrium Mdel f Bigegraphy Why d farther islands have less species? Distance will affect dispersal rate f species: -difficult t survive -island becmes a small target -mre frces need t act tgether t make a successful clnizatin DISTANCE EFFECT Distance lwers lcal clnizatin rates Clnizatin Rate Extinctin Rate DISTANCE EFFECT DISTANCE EFFECT Clnizatin Rate Far Clse Extinctin Rate Rate Far Clse S far S clse 4

11 DISTANCE and AREA EFFECT Gain r Lss f Species Far Clse Small Large Patterns f Bidiversity Effects vs. Area Effects shws species/ area relatinship, but weaker than islands Suggests that prcesses specific t islands are imprtant clnizatin extinctin dispersal is s high that area desn t affect clnizatin much HOWEVER- Habitat Fragmentatin makes mainland sites behave like s!! Clearcut picture Strng Relatinship S far/small S clse/large Prtecting Diversity: SLOSS? Prtecting Diversity: SLOSS? SLOSS single large r several small reserves? Why might large reserves be better? 1. Less edge effects 2. Larger cntiguus ppulatins 3. Prtects species with large hme-ranges 4. Supprts higher trphic levels 5. Increased habitat diversity within ne reserve SLOSS single large r several small reserves? Why might small reserves be better? 1. Mre habitat diversity acrss reserves 2. Spreads risk f extinctin 3. Easier plitically 4. Easier enfrcement? VS. VS. 5

12 Latitudinal gradients f bidiversity Observatin: Overall species richness declines at higher latitudes e.g. Trpics mre diverse than tundra Latitudinal gradients f bidiversity Predatry Birds: Falcns (Rangel et.al. 2004) Plants Latitudinal gradients f bidiversity Latitudinal gradients f bidiversity: WHY? Predatry Birds: Falcns Deep Sea Invertebrates Case study: Bird Diversity (Jhn Terbrgh 80 s) (Rangel et.al. 2004) Plants Cmparative study f Bird Diversity: Manu, Peru vs. Nrth Carlina Gal: Identify the mechanisms that allw greater diversity in the trpics (Rex et.al. 2000) 6

13 Cmparing species richness: Peru vs Carlina (50 ha plts) Ri Llullapichis, Peru Nrth Carlina Results frm Manu 1. Extremely high alpha diversity in trpics: 319 bird species n 1 km 2 plt 2. Big territries, lw densities: median densities: 2 pairs/km2 many rare species: 84 spp denisities < 1 pair/km2 (cmpare: lark buntings Clrad 200 pairs/km2) 3. Cmparisn t Nrth Carlina (50 ha plts): 207 species versus 40 species (167 mre) 5X USED A GUILD APPROACH- Species using same resurces Carlina Peru same guild, mre finely divided (mre specializatin) Results f Cmparisn using Guild Apprach Guild Apprach: Diversity is higher in the trpics because: 1. New guilds 33% same guild, brader resurce base 2. Mre species per guild 50% Increased diet specializatin Increased habitat partitining new guild 7

14 Trpical flycatchers shw extreme mrphlgical & diet variatin Mre species/guild due t diet specializatin extreme diet specializatin nly stingless bees Laughing Falcn snake specialist Mre species/guild due t vertical habitat partitining Hyptheses fr Bidiversity Gradients Difficult questin t answer: Speciatin (eclgical time) Maintenance (eclgical time) Examples f Gradients: Latitudinal Gradient Tpgraphic Relief East-West Gradients Penninsular Lws Wave expsure Gradients Depth Gradients 9

15 Hyptheses fr Bidiversity Gradient 1) Climate stability ver evlutinary/gelgic time Glaciers had less impact in trpics Greater speciatin rate Lwer extinctin rate 2) Climate stability ver eclgical time Lw variatin in resurces allws fr greater speciatin Intermediate levels f disturbance prmte early and late successin spp. 3)Mre habitat hetergeneity Explains in β diversity but des nt explain in α diversity Hyptheses fr Bidiversity Gradient 4) Cmpetitin higher in trpics Physical factrs nt as imprtant/limiting as in temperate areas Cmpetitin leads t species diversificatin 5) Predatin higher in the trpics Predatrs keep # s dwn Cmpetitin less imprtant N cmpetitive exclusin ccurs 6) Patterns f Prductivity Stable prductivity allws fr greater specialzatin Pulsed prductivity results in lwer species richness. Deep-Sea: lcal cnditins dn t vary much Surface prductin is much mre variable in nrth Summer Vlunteers Needed fr Lab Wrk figurski@bilgy.ucsc.edu 3 cnsumers 2 cnsumers Grazers & Detritivres Detritus 10