Question Amount of harvest from different states and probabilities

[andia]

Hi Leonardo,

I have couple of questions regardless to harvest in the models and probabilities of transitions:

1) In your first tutorial video about non spatial models, you established one model with no harvest and a second model with harvest of 20 acres. Are these 20 acres from the total amount of acres (in your example 1,000 acres)? or this 20 acres correspond to 20% of the probability of harvest? If you set the harvest probability as 1 (100%), and you harvest 20 acres (is this 20%?), do this means the probability of harvest will be 0.2? I am confuse about where these 20 acres are taking from, the total acres in the simulation or just the specific state that is being harvest, or is just an adjustment of the probability?

2) What if you leave the mean probability of that harvest that occurred naturally (predation for example) and then on top of that you add harvest (external-human). Is this valid, or you always have to adjust to 1?

3) What if you have more than one kind of harvest at different states, how you can establish the harvest for each state? For example, if you take 20 acres of harvest for state A and 20 acres for state B, this means from the total of each state? or for all the area you consider in the model? how you establish that in the model if there are no age differences between states? and if you already have some area that is harvested?

4) This last question is about probabilities. I understand in any state transition model probabilities per state should add to 1 (as in the table below). However, in your example, they do not necessarily add to 1. Is this just because is an example? I understand they have to add to 1 in this cases, and this is how I am adding them into the model in SyncroSim. Is this correct, or I am missing something in your software?

Fate (t+1) HC IC DC CS S
HC 0.8086 0.1152 0.0147 0.0000 0.0000
IC 0.1272 0.7307 0.0000 0.0000 0.0000
DC 0.0330 0.1061 0.9847 0.0000 0.0156
CS 0.0312 0.0480 0.0000 0.9717 0.0000
S 0.0000 0.0000 0.0006 0.0283 0.9844
1.00 1.00 1.00 1.00 1.00

Thank you,

Andia

[leonardo-frid]

Hi Andia,

Good questions, here are my responses:
1. When you set a transition target the model targets that amount of area for each timestep independent of how much area is actually eligible. At the start of the simulation 20% of the 1000 acres are conifer ranging in age from 20 to 100 years old. The minimum age for harvest is 40 so initially there will be about 0.75 * 200 = 150 acres eligible for harvest. With a target of 20 acre, the initial probability of harvest will be 20/150 = 0.133. However, the amount of area eligible for harvest will always be changing so when you set a target – the total amount is constant but the probability is not. In the tutorial example, there is not enough old conifer to sustain a harvest target of 20 acres per year so at about year 8, the amount of area harvested drops dramatically.

2. I don’t fully understand your question, are you asking why not use a probability in the pathways for harvest rather than a value of 1? If you set a probability and harvest, the harvest target will override the probability you set. If you would like to see some “harvest” happen with a probability and an additional amount with a target, you need to define two transition types and pathways: for example a type called “probabilistic harvest” that has the probability you intend in the pathway and another called “targeted harvest” where you set a transition target for the annual area undergoing the transition.

3. If harvest is possible in multiple states, you would still see only 20 acres harvested when you set a target of 20 acres. The amount harvested in each state would depend on two things: the amount of area in each state and the relative probability of the pathway in each state. For example if set a harvest target of 20 ac and I had harvest possible in state A with an area of 100 ac eligible and a probability of 0.1 and in state B with and area of 100 ac eligible and a probability of 0.3 then I would expect to see 3 times as much harvest of state B than state A so 15 ac harvested in state A vs. 5 acres in state B. The amount of harvest that you expect to see in state 1 is going to be equal to P1 x A1 x (Target area)/[sumproduct across all cells (area and probability)] where P1 is the probability of your transition in state 1 and A1 is the area in state 1.

4. You do not need to include probabilities for the diagonals (i.e., the “no change” transitions) of the transition matrix. The software will automatically calculate the “no change” probabilities. Also, because STSMs allow for more than one transition per timestep of different types (for example harvest and fire might occur in the same year), it is OK if the sum of your probabilities exceeds 1.

• This reply was modified 5 years, 2 months ago by Tom RoeAdmin.

[andia]

Dear Leonardo,

Thank you so much for your answers to my questions. This makes much more sense.

I know my questions sounds weird in some way. I am trying to use your software to model changes in marine plants and invertebrates on coral reefs and not forest or plants on land.

I have other questions regardless to the harvest. When harvest occurs at any age and there are no age classes specified, how do you will add the harvest?
Or do you have to always specify age (even if it is the same theoretical age) and assign an area to each state?
When I do not assign an area (i.e. area of the state 1), I do not get zeros as result, so I am wondering, how is the calculation in this case:
P1x(A1?)(Target Area))/[sumproduct across all cells (area and probability)] change to P1x(Target Area))/[sumproduct across all cells (area and probability)] ? In other words is A1 excluded?
By Target Area you refer to the Total Area being use in the model, correct?

Thank you,

Andia

[leonardo-frid]

Hi Andia,

1. If there are no age constraints on harvest or any other transition for that matter, you can leave the age fields empty. State classes will be eligible for these transitions regardless of their age.

2. If you do not set a target, the actual probability for the transition will be used to determine how many cells undergo the transition. That’s why you still see harvest occurring if you remove the target. If you don’t want a transition to occur at all you can set a target area of zero, or you can set a Transition Multiplier (under the Advanced tab) of zero.

3. In the formula, the Target Area represents the area target that you set under the Transition Targets tab.

Hope that helps,
Leonardo

[andia]

Hi Leonardo,

Thank you for your answers. They are very helpful.
Now, it makes more sense, and I have been able to adjust the harvest properly. 🙂
Thank you.
Have a great weekend.

Andia

[Author]

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