Seeds on the Run:A Model of Seed Dispersal

Sara Garnett, Michael Kuczynski, Anne RoyerGK-12 workshop

12/5/12

On the run?

• Most organisms don’t spend their whole lives in the place they were born

• Dispersal: movement of organisms away from a given population or parent– Natal, adult

• Many reasons dispersal may be beneficial

Reasons for DispersalNatal dispersal Adult dispersal

Reduce competition with relatives

Avoid inbreeding

Find better habitat, resources

Plants disperse too!

Effects of Dispersal

• Clear benefits to dispersal– Avoid inbreeding– Reduce competition, lower population densities– Make use of better habitats

• Why is there variation in dispersal ability?

• How does this variation affect communities?

Why does dispersal matter? In the early 1970s, two ecologists were trying

to figure out why the many species of trees found in tropical forests were so evenly distributed. They started with dispersal.

Winnie Hallwachs Westsocnat.com

Dan Janzen Joseph Connell

Null hypothesis Two mature trees growing in a forest are

setting and dispersing lots of seed. Where would you expect most of the resulting seedlings to grow?

aha-soft.com archigraphs.com

Null hypothesis Seeds, and seedlings, end up mostly clustered

under the parent tree.

aha-soft.com archigraphs.com

Predict It! (Graph #1)

Distance from mother tree

Num

ber o

f see

dlin

gs

near far

Null hypothesis

If we assume dispersal alone dictates where adult trees will be, what distribution of adult trees would this result in?

aha-soft.com archigraphs.com

Null hypothesis

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archigraphs.com

Null hypothesis – fail!

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What we actually see looks more like this.

archigraphs.comarchigraphs.com

What could turn this into this?

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aha-soft.com archigraphs.com

HINT #1

the tropics are full of diversity, but it’s not all spider monkeys and morpho butterflies

HINT #2• Specialist organisms are especially common in

the tropics – many herbivores and disease organisms tend to attack a single victim species.

• Do you think they would prefer to feed in high-density or low-density patches?

aha-soft.com archigraphs.com

Predict It! (Graph #2)

Seedling density

Like

lihoo

d of

seed

ling

surv

ival

high low

The Janzen-Connell Hypothesis

• Most seeds fall near the tree• Specialist diseases and herbivores will be

more abundant in those high-density areas• Seedlings near the parent tree will experience

higher mortality rates• Rare seeds that disperse far are most likely to

survive to adulthood

I = # seeds per unit areaP = probability that seed will maturePRC = “Population Recruitment Curve,” I*P. The likelihood of an adult tree ending up there.

Janzen 1970

The Janzen-Connell Hypothesis

Can we use these assumptions to build a model (game) that works (produces predicted results)?

• Your seeds are more likely to land close to the mother tree

• Seeds that land close to the tree are more likely to have bad things happen to them

The Environment

• The game board consists of three zones representing different distances of dispersal from the parent plant

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3

Game pieces (aka: fun with tiddlywinks!)

• Seeds/plants are represented by tiddlywinks• When you begin your turn you take control of

a new seed• Role a die to see how far the seed disperses– 1-3 = Zone 1– 4,6 = Zone 2– 6 = Zone 3 =

Decide your fate!

• After your seed has dispersed draw a fate card to see what will happen to your seed

• If you have any other plants on the board they must also draw a fate card

End of the game

• After each player has gone through 10 turns the game ends

Time to graph!

• Add up the number of seeds/plants in each zone and graph this data

• Calculate the average height (number of tiddlywinks) for the plants in each zone and graph this data

• Report your group’s data to the entire class so we can create graphs of the pooled data

Extensions

• Do you think our game-model worked – produced results that reflect the hypothesis? If not, what would you change to make it work? (This is the process theoretical biologists use!)

• Can you think of other mechanisms that could create this pattern? How would you model them in game form?