1. Natural environmental variability (disturbance) is important in structuring systems

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1 What we ow thik we kow about disturbace 1. Natural evirometal variability (disturbace) is importat i structurig systems The more frequet the disturbace, the more r-selected species we expect to fid (compare the plat commuities of prairie ecosystems that experiece fire o a decadal scale with those for forests that bur o a scale of ceturies) r-selected species are ot iheretly bad there are umerous atural systems with disturbace regimes appropriate to r- selected life history strategies (e.g. the riparia habitat of Furbish s lousewort) The issue is resposes to o-atural disturbace (upredictable ad uremittig) such that formerly K-selected species are replaced by r-selected oes Particularly if these r-selected commuities are oly the sum of the resposes of the idividual species to the eviromet (e.g. urba impacted systems) 2. However there are more types of biological iteractios tha we have yet discussed what do we kow about whe/how they operate? 1

2 Importat biological iteractios 1. Niche: physiological ad biotic limits that restrict species distributios 2. Competitio: Rarely is just oe of these operatig: 3. Disturbace: a challege to our uderstadig! 4. Successio: 5. Symbiosis: mutualism, commesalism, parasitism 6. Herbivory/predatio: food webs, eergy flow 7. Facilitatio: the presece of some species ehaces others 8. Recruitmet limitatio: the limitig stage for may species is ofte early i life 9. Ecosystem fuctio: utriet ad eergy fluxes 10.Islad biogeography: how big/how coected 11.Ecotypes/Geetic diversity: the ability of populatios to adapt to local ad/or chagig coditios 2

3 2 ad 6: competitio vs trophy Two primary possibilities exist for desity depedet populatio regulatio Startig with plats: They could be regulated by light/utriets (bottom-up) i which case we re dealig primarily with plat-plat competitio for resources or plats could be regulated by predatio (grazig) from herbivores (top-dow). Liebig tells us that oly oe of these will be operatig at a particular time The data suggest that whe oly two trophic levels are preset, herbivores cosistetly drive their resources (plat commuities) to extictio hece we ever see oly 2 trophic levels i aturally occurrig systems 3

4 The case study of St. Matthew Islad reideer August 1944: 29 reideer released 1957: 1,350 reideer, most i excellet coditio Some cocers wrt tramplig ad overgrazig of liche 1963: 6,000 reideer 1966: the islad is covered with skeletos. 42 live reideer: 41 of which are females. the oly male had severely deformed atlers (perhaps idicative of a iability to reproduce). o faws. The reideer of St. Mathew Islad go extict i mid

5 Aticosti Islad, Québec 1895: Heri Meier attempts to make Aticosti a hutig paradise releasig 220 deer (he tries ad fails to itroduce biso ad elk) Chages i forest species compositio is evidet by the 1920s Most deciduous uderstory vegetatio is destroyed ad the oly sigificat deer predator the black bear is extirpated by the 30s or 40s (likely i respose to loss of fruit-bearig plats) By 2004 the deer umber 120,000 or ~15 deer.km -2 compared to the 2.km -2 across the rest of Québec The commercial harvest of balsam fir severely affected Potvi, et al Ecosciece 10:

6 Moose i Newfoudlad May , 4 moose are itroduced to the islad of Newfoudlad I the absece of predators, the populatio reached desities of ~4.km -2 compared to other areas of NA where desity is ~0.1.km -2 (e.g. Otario moose desities are ~ km -2 with populatios geerally regulated by wolf or bear predatio o juveile moose) Severe grazig impacts o balsam fir (ad reduced carryig capacity) have reduced the NFL populatio to ~ km -2 Similarly to Aticosti Islad, moose are impactig forest regeeratio settig the stage for their ow demise 6

7 Deer i souther Otario OMNR is proposig a expaded hutig seaso to deal with a estimated 25% icrease i deer populatios over the past 10 years, icreases i crop losses ad deer vehicle collisios. Will that actually work? What would? 7

8 The precedig case studies help us uderstad why (uder atural coditios) we ever see herbivores regulatig plat resources: they would drivig themselves to extictio by exhaustig their resources Oly two trophic levels seems to be so iheretly ustable, that it is ever selected for Whe circumstaces create such a situatio it ever persists 8

9 What we cosistetly see i atural systems (at least terrestrial oes) is bottom-up regulatio o plats (i.e. competitio amog plats for water, light ad utriets) ad top-dow regulatio o herbivores by carivores Herbivores are feedig o plats, they may reduce plat biomass or chage species compositio but ot extirpate plats as we ve see i our case studies of (athropogeically caused), uregulated grazig 9

10 Plats (producer commuities) 1. If plat populatios are primarily regulated by abiotic factors (light, utriets, temperature, water) 2. Competitive iteractios amog plats for access to those factors is the importat 10

11 Some really cool work by James MacMaho ad Doald Phillips at Utah State Phillips ad MacMaho. Ecology. 69: Creosote bushes do t start out uiformly spaced. They germiate i clumps (where there is sufficiet moisture). Mappig creosote roots Over time competitio amog plats for water ad utriets leads to higher mortality amog ear eighbours, evetually creatig the regular distributio Roots showed oly 4% overlap! 11

12 Plat size vs plat desity MacMaho ad Phillips work also hits at the role of plat size (small seedligs/plats ca exist at higher populatio desities tha larger idividuals) 10 6 Lema #.m -2 auals pereials Sequoia biomass 12

13 1. Ecologists sometimes frame the apparet ecessity of carivore regulatio of herbivores as a questio: why is the world gree? Meaig why is there this apparet surfeit of plat material that o herbivore has evolved to exploit? 2. the explaatio appears to be that uregulated herbivore populatios cosistetly drive their plat resources (ad the themselves) to extictio (e.g. St. Mathews Islad) plats ca t escape. a. Such iheret istability will ot persist i atural systems b. What are the implicatios for athropogeically impacted systems where humas seem itet o predator removals? 3. However herbivores ca sigificatly ifluece plat species distributios! Plats & Herbivores There will be caveats to these geeralities i aquatic systems 13

14 While herbivores are ot directly regulatig the total umber of plats, (plats are primarily regulated by utriets ad light ad are competig amog themselves for access to those resources) Nevertheless: Whe herbivores selectively eat some species of plats, other species of plats that are ot grazed gai a competitive advatage. Ecosystems are ot static! Remember what happeed whe too may sheep shifted the Australia grasslad systems to woody shrub systems? (differetial grazig) 14

15 Some data I: Differet sized grazers Exclosures with two differet mesh sizes placed i both forest ad tudra sites i four locatios i Swede ad Norway Reideer were the most abudat large grazer Lemmigs ad voles were the most commo small grazers The relative abudace (recall Tutorial 6!) of five of the fiftee most commo plats was sigificatly iflueced by voles ad lemmigs Oly a sigle plat species was sigificatly iflueced by reideer. Our results suggest that itese ad localized selective foragig by small mammals may have a more marked effect o vegetatio tha trasiet feedig by reideer. Scales of space ad time are importat! The etire grazer commuity is importat Elisabeth S. Bakker, Ritchie, M.E.; Olff H.; Milchuas, D.G. ad Kops, J.M.H. (2006) Herbivore impact o grasslad plat diversity, habitat productivity ad herbivore size. Ecology Letters 9:7,

16 The data II: Ugulate grazer pressure Plat Biomass (g.m -2 ) above ad below groud } % 10 species Total species # Hart, R.H Cattle grazig itesity ad plat biodiversity o shortgrass steppe after 55 years. Plat Ecology 155:

17 Early i the 1900s, a group of moose maaged to cross the ice to Isle Royale Recostructig their populatio dyamics i the early years suggests the populatio grew eruptively, experiecig high desities followed by starvatio ad severe populatio crashes Somewhere aroud 1950 (aecdotally 1949), either a pack of wolves or a sigle pregat female also crossed o the ice, settig up the curret dyamic 17

18 Two of the more iterestig aspects of the story occurred i the 80s ad the late 90s 18

19 Parasites ca be as importat as covetioal predators i populatio regulatio (remember exploitatio) The abrupt declie i wolves i the 80s is believed to be due to accidetal itroductio of parvo virus from domestic dogs The declie of moose i the 90s was apparetly due to a fatal tick bore disease More recetly, moose populatios have rebouded ad while it is still too early to be sure, there is some evidece that both populatios are comig ito equilibrium with each other ad the eviromet (although fluctuatios are always to be expected). 19

20 While herbivores ca exhaust their food resources, there is o evidece that aturally occurrig predator populatios ever drive their prey to extictio (eve though humas do) Why? No-huma predators prey most successfully o the slowest, weakest, least fit members of the prey populatio. As prey populatios are culled, oly healthy idividuals who are capable of escape survive. Predators themselves the declie as system carryig capacities fall. Cotrast this with sport hutig or wholesale resource harvestig (clearcuttig or dragger fishig) 20

21 Take home messages? Fig 4.4 KR8C Predators do t kill healthy, mature prey, relyig istead o feedig o the old, youg ad ill. Parasitic ifectios break out i dese populatios, leadig to the death of idividuals weakeed by ifectio. As predators/parasites cut the prey populatio dow to oly those idividuals capable of escape, the predator/parasite populatio will declie (uless they ca switch prey). Healthy prey populatios the rebuild their umbers, settig up the ext roud of dyamics. Havig removed most atural predators from may ecosystems leaves us facig ecoomic ad ethical elemets of herbivore regulatio 21

22 Predator restoratio i Yellowstoe: re-itroductio of wolves Whe wolves were extirpated i the early s from much of the US ad souther Caada, the populatios ad desity of moose, deer ad elk soared (depedig o locatio) I Yellowstoe, high umbers of ugulates led to overgrazig. may species of trees like aspe ad cottowood declied (Robbis ). the majority of aspe ad willow trees i the Lamar regio of Yellowstoe were 70 to 100 years old with scarcely ay ew sapligs (Ripple & Beschta, ) 1 Lessos from the wolf. Scietific America 290: Wolves ad the ecology of fear: Ca predatio risk structure ecosystems? Biosciece. 54:1-22. Kristie Mahoey 22

23 While ot without cotroversy, the data suggest that 10 years later... Elk umbers are dow, elk have also shifted grazig areas/habits Icreased recruitmet of willow, aspe ad cottowood trees With icreases i tree desity, sogbirds ad beaver umbers are up Beaver dams ad pools of slowmovig water attract otters, muskrats, moose, birds, ad isects Stream bak erosio has bee substatially reduced Coyote umbers are dow Voles ad mice (coyote s mai prey) up Fox, eagle, hawk ad owl umbers up Visitors to Yellowstoe sped about $35 millio i Motaa, Idaho ad Wyomig up 22% Park visitors rak the wolf as oe of the primary aimals they come to see 71% of busiess owers thik wolf recovery is resposible for icreased tourist travel 23

24 Ayway, what are predators doig? Terrestrial herbivore desities geerally appear to be limited by topdow exploitative factors (predators, parasites, parasitoids, pathoges ) predators/parasites cut prey populatios dow to oly those idividuals capable of escape, but do ot extirpate all their prey Our topic for Tuesday 24