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Lectures 27 – Populations Regulation

March 27, 2000

(Chapter 21)

 

I.          Populations increase geometrically - but limited by carrying capacity

             resources

             disease

             predation

II.       Popns are dynamic

             examples from Europe

                         Great Tits varies fourfold

                         Pied Flycatchers varies annually by 50%

             long-term trends

                         Brown-headed Cowbird, Glossy Ibis, Cattle Egret very                             recently rare in e. US - now very abundant

                          Black Ducks & Red-headed Woodpeckers have declined

                          Finland (233 species) over last 100 yrs

                                     25% down

                                     22% fluctuating

                                     34% increased

                                     20% stable

 

III.     Popn growth patterns

              dN/dt = rN (continuous)

                        for discrete model:

                                    Nt+1=Nt*r

                                    Nt+2=Nt+1*r, but Nt+1=Nt*r so

                                    Nt+2=(Nt*r)*r, or

                                    Nt+2=Nt*r2, etc.

            so we can figure out population doubling time for a given r, or what r does it take to double a population in t years.  How?

                                    Nt=N0*rt : solve for Nt=2N0

                                    2N0=N0rt : divide both sides by N0

                                    2=rt

                                    log(2)=log(rt)

                                    log(2)=t*log(r), so if we know r,

                                    t=log(2)/log(r)

                                    or, if we know t

                                    log(r)=t/log(2)

                                    r=et/log(2)

                          r varies

                                      large birds = 10 -30%

                                      small birds = 50-100%

             Sigmoid curve

                        120 starlings in 1890 became 120,000,000 in 50 yrs

                        8 phaesants on Protection Island in Wash. turned into 1325 in 5      yrs.

                         Cattle Egrets NA in early 50s 2000 fold from 1956-71

                                     r=.21 for whole period

                                     r=.84 at peak growth period

                          House Finches released in 1940 = .23 from 62-71

IV.    But we’re not knee deep in cowbirds - why not?

              Regulation vs. limitation

                         Regulation is density dependent

                                     mortality and fecundity vary with popn size

                           too many Snow Geese, degrade nesting salt marshes – opening spring hunting season

                         Limitation density independent

                                     storms, etc.

              Limits -- 4 factors

                         habitat

                                     rails and bitterns decline with loss of marshes

                                     N. Spotted Owl declines as old-growth forest is cut

                           Red-cockaded WPs need old-growth pine, open understory, red heart fungus

                                     what does habitat provide?

                                                 nest sites

                                                 food

                                     habitat modification by humans

                                                 forest -> fields favors some species

                                                 managing forests cuts dead trees-lose snags,                                                   etc.

                                                 deer overbrowse understory

                                     wintering habitat

                                                 dense populations on wintering grounds

                                                 1 ha of winter = 5-8 ha breeding grounds

                         climate & food supply - often related

                                      Peruvian seabirds and anchovy popns/El Niņo

                                      Guano industry

                                      27-6 million

                                      starvation often over winter = leads to irruptions

                         disease (and parasites)

                                      Hawaiian honeycreepers decimated by avian malaria and pox

                                     why are island species more vulnerable? -

                                                 resistance is expensive

                                                 so it’s lost - just like flight

                                     Cycles in Red Grouse in Scotland linked to nematode

             Regulation

                         Social forces - territorial behavior especially

                                     when territories filled in primary habitat overflow forced to sub-standard

                                     sub standard fills up, floating population develops

                          Recruitment

                                     young produced locally

                                     immigrants

                          Example - Great Tits in Europe - especially Wytham Woods

                                      primary factor is juvenile survivorship over 1st winter

                                     Other factors also play a limited role

                                                 habitat

                                                 territorial behavior

                                                 dispersal

                                                 breeding season fecundity and survival density                                                             dependent (clutch size smaller when pop. high, but not as important as juvs over winter

                                     How is this demonstrated?

                                                 experimentally remove young and eggs on island

                                                 juvenile suvivorship 11->20%

                                                 adult survivorship 26->54%

                            population regulation by food limitation in non-breeding season seems to be the rule

                                     many populations  have been through bottlenecks:

                                                 Laysan Teal

                                                 Red-tails on Socorro Island

                            genetic evidence shows that others seem to have been through bottlenecks, even

                                                 Red-winged Blackbirds

                                                 Black-capped Chickadees

                                                 reduce genetic variability

                                                 important in speciation

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