Eat, Prey, Rain: New Model of Dynamics of Clouds and Rain Is Based On a Predator-Prey Population Model

Eat, Prey, Rain: New Model of Dynamics of Clouds and Rain Is Based On a Predator-Prey Population Model

  10:10:00 AM    Science Lover

---

What do a herd of gazelles and a fluffy mass of clouds have in common? A mathematical formula that describes the population dynamics of such prey animals as gazelles and their predators has been used to model the relationship between cloud systems, rain and tiny floating particles called aerosols. This model may help climate scientists understand, among other things, how human-produced aerosols affect rainfall patterns. The research recently appeared in the Proceedings of the National Academy of Sciences (PNAS).

A new mathematical model may help climate scientists 
understand, among other things, how human-produced 
aerosols affect rainfall patterns. (Credit: © Brian 
Jackson / Fotolia)

Clouds are major contributors to the climate system. In particular the shallow marine stratocumulus clouds that form huge cloud decks over the subtropical oceans cool the atmosphere by reflecting part of the incoming solar energy back to space. Drs. Ilan Koren of the Weizmann Institute's Environmental Sciences and Energy Research Department (Faculty of Chemistry) and Graham Feingold of the NOAA Earth System Research Laboratory, Colorado, found that equations for modeling prey-predator cycles in the animal world were a handy analogy for cloud-rain cycles: Just as respective predator and prey populations expand and contract at the expense of one another, so too rain depletes clouds, which grow again once the rain runs out. And just as the availability of grass affects herd size, the relative abundance of aerosols -- which "feed" the clouds as droplets condense around them -- affects the shapes of those clouds. A larger supply of airborne particles gives rise to more droplets, but these droplets are smaller and thus remain high up in the cloud rather than falling as rain.

In previous research, Feingold and Koren had "zoomed in" to discover oscillations in convective cells in marine stratocumulus. Now they returned to their data, but from a "top down" angle to see if a generalized formula could reveal something about these systems. Using just three simple equations, they developed a model showing that cloud-rain dynamics mimic three known predator-prey modes. Like gazelles and lions, the two can oscillate in tandem, the "predator" rain cycle following a step behind peak cloud formation. Or the two can reach a sort of steady state in which the clouds are replenished at the same rate as they are diminished (as in a light, steady drizzle). The third option is chaos -- the crash that occurs when predator populations get out of hand or a strong rain destroys the cloud system.



The model shows that as the amounts of aerosols change, the system can abruptly shift from one state to another. It also reveals a bifurcation -- two scenarios at different ends of the aerosol scale that lend themselves to stable patterns. In the first, relatively low aerosol levels lead to clouds in which development depends heavily on aerosol concentrations. In the second, high levels produce saturation; these clouds depend solely on the initial environmental conditions.

Using this so-called systems approach, says Koren, "can open new windows to view and understand the emergent behavior of the complex relationships between clouds, rain and aerosols, giving us a more useful view of the big picture and helping us to understand how shifting aerosol levels can lead to different climate patterns."

Read More

Scientists Find Universal Rules For Food-web Stability

Scientists Find Universal Rules For Food-web Stability

  8:11:00 PM    Science Lover

---

New findings, published in the journal Science, conclude that food-web stability is enhanced when many diverse predator-prey links connect high and intermediate trophic levels. The computations also reveal that small ecosystems follow other rules than large ecosystems: differences in the strength of predator-prey links increase the stability of small webs, but destabilize larger webs.

A Juvenile African Bush Viper (Atheris chlorechis) with a small frog, at night.
Researchers found that food-web stability is enhanced when many diverse predator-prey links connect high and intermediate trophic levels. (Credit: iStockphoto/Mark Kostich)

Natural ecosystems consist of interwoven food chains, in which individual animal or plant species function as predator or prey. Potential food webs not only differ by their species composition, but also vary in their stability. Observable food webs are stable food webs, with the relationships between their species remaining constant over relatively long periods of time.

Understanding complex systems such as food webs present major challenges to science. They can either be examined by observing natural environments, or by computer simulations. To enable computer simulations of such systems, scientists often have to make simplifying assumptions, keeping the number of model parameters as low as possible. Yet, the computational demands of such simulations are high and their relevance is often limited.

Innovative methodology

Scientists from the Max Planck Institute for the Physics of Complex Systems (MPIPKS) in Dresden, Germany, have developed a new method that allows them to efficiently analyze the impact of innumerable parameters on complex systems.

"By using a method called generalized modeling, we examine whether a given food web can, in principle, be stable, i.e., whether its species can coexist in the long term," says Thilo Gross from MPIPKS. Complex ecosystems can thus be simulated and analyzed under almost any conditions. "In this way we can estimate which parameters will keep ecosystems stable and which will upset their balance."

The method can also be used for examining other complex systems, such as human metabolism or gene regulation.

Generalists stabilize, specialists destabilize

Applying this innovative modeling approach together with colleagues at the International Institute for Applied Systems Analysis (IIASA) in Laxenburg, Austria, and Princeton University, USA, the scientists have succeeded in discovering not just one, but several universal rules in the dynamics of ecosystems.

"Food-web stability is enhanced when species at high trophic levels feed on multiple prey species and species at intermediate trophic levels are fed upon by multiple predator species," says Ulf Dieckmann of IIASA.

The scientists have also identified additional stabilizing and destabilizing factors within ecosystems. Ecosystems with high densities of predator-prey links are less likely to be stable, while a strong dependence of predation on predator density destabilizes the system. On the other hand, a strong dependence of predation on prey density has a stabilizing impact on food webs.

Differences between small and large systems

A further important finding is that food webs consisting of only a few species behave qualitatively different from webs consisting of many species.

"Small ecosystems apparently follow different rules than large ecosystems," says Ulf Dieckmann. "Systems with fewer species are more stable if there are strong interactions between some species, but only weak interactions between others. For food webs with many species, exactly the opposite is true. Extremely strong or weak predator-prey links in nature should therefore be the rarer the more species a food web contains," he concludes.

If you like this post, buy me a beer at $3!

Read More