spatially autocorrelated wildlife counts

Dear list,

I have a few beginners' questions and I would greatly appreciate your
comments and suggestions.

I am trying to simulate the distribution of a ungulate species across
a landscape, where animals of this species are spatially aggregated
(exact aggregation level is unknown so I'll have a range of values). I
know the "true" size of this population because it results from my
previously built biological model. After spreading these, let's say 10
000, animals across the landscape, I'll simulate counts using fixed
transects.

My response variable is number of animals per cell. I am assuming a
zero-inflated negative binomial distribution and, to simulate spatial
autocorrelation, I'm planning to use spatial lag models (number of
animals in each cell should be affected by number of animals in
neighbour cells). Because my landscape matrix has 250 000 cells, I'm
using the function powerWeights and not invIrW (spdep package).

My questions are:
- approximately 70% of cells should have no animals (hence the
zero-inflated distribution) but after applying the spatial weights,
these cells are populated. How could I control this (i.e., "forbid"
70% of cells to have animals allocated)?

- the total population is known so, after incorporating spatial
autocorrelation, I need to have exactly 10 000 animals across the
landscape. Any advice on how to do this? Also, is there a way to have
sum(uncorrelated variable)=sum(correlated variable)?

Finally, I'd also appreciate any comments on my approach to this
simulation. Please, do let me know about alternative approaches!

Thanks in advance!


--
Ana Nuno