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A magical calamity has wiped out an area about twice as big as Germany. The calamity also altered the climate in the region from one supporting forests and grasslands to a desert similar to the American southwest. Think badlands, not sand dunes. Aside from the climate changes, there are no lasting effects of the calamity; nothing comparable to nuclear fallout.

Assuming there was a small desert adjacent to the region from which desert life can spread, how long will it take that life to spread across the whole new desert?

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    $\begingroup$ small desert adjacent to the new one? $\endgroup$ Mar 10 '20 at 21:34
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    $\begingroup$ What exactly is "a desert similar to the American southwest"? As far as I understand, the "American southwest" consists of the states of Arizona and New Mexico; only part of their territory is desert -- most is only semi-arid. For example, almost one third of the territory of Arizona is covered by forests. For another example, they grow cattle and sheep in New Mexico. $\endgroup$
    – AlexP
    Mar 10 '20 at 21:48
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    $\begingroup$ @RodolfoPenteado Yes. That small desert is just a way to introduce desert-dwelling life to the region. I'm not asking for how long it takes desert-dwelling life to evolve, just how long it takes for desert-dwelling life to expand into a new desert. $\endgroup$
    – Ryan_L
    Mar 10 '20 at 22:03
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About 1 to 2 centuries.


You're talking about Ecological Succession.

https://en.wikipedia.org/wiki/Ecological_succession

and more specifically Secondary Succession...

https://education.seattlepi.com/ecological-succession-desert-5078.html

"... Desert ecosystems can be disturbed by fire, flash flood scouring or land clearing. After disturbance, succession is set back, but the soil has already been developed. Secondary succession in deserts is relatively quick compared to primary succession, but it takes much longer to establish a desert community than it does in less arid regions. For example, in the southwestern deserts of North America, it can take 76 years to establish perennial plant cover and 215 years for full ecosystem recovery. If the disturbance is great enough, secondary succession may produce an entirely different climax ecosystem."

also:

http://shawcloud.yolasite.com/soil-and-succession-and-symbiotic-relationships.php

and

https://desertbcraft.weebly.com/succession.html

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For wildlife to spread into the area, you need plants there first. Predators won't spread until prey starts living there. And prey uses plants for shelter from Predators and for food.

The diagram below shows the different mechanisms for seed dispersal -- wind, falling to the ground and germinating, falling to the ground and hitchhiking in animals' guts, or on fur or one clothing in the case of humans.

Wind dispersal is the farthest-reaching mechanism if there are winds present. Especially since your only viable plants verge on one side of a desolate zone.

The efficiency, and therefore the rate of dispersal is very complicated determined by the seed casing itself, wind speed, direction, gusting, probability of landing in a good spot, and not getting eaten before the plant reaches maturity and generates new seed.

You have a small advantage to accelerate the dispersal -- indirectly. The non-desert flora surrounding the desolate region will disperse seeds into the zone. These will attract birds and rodents, who will spread seeds in their scat and fur.

The topic of the mathematics of modeling seed dispersal is an ongoing topic for seedologists -- or whatever the discipline is called.

enter image description here

So unless you want to do a lot of very complicated probabilistic calculations, you can use a simple algebraic term and guess at an answer.
$$Rate = \chi \times d \frac{km | miles}{year|plant seeding cycle}$$ where $\chi$ is the probability a sufficient density of seeds will populate the adjacent area and where d is the average dispersal distance of seeds.

values for $\chi$ seem valid from 5% to 40% depending on species, weather, wind, the population of seed eaters, etc per seeding cycle.

values for d seem valid from 1 (1.6 km) mile to 10 miles (16 km)

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  • $\begingroup$ Nice detail and well compliments my answer (or my answer compliments yours). Thanks :) $\endgroup$
    – kleer001
    Mar 30 '20 at 17:27

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