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Ecological Assembly Rules

By: Alejandro Baladrón Julian

Invasive giant knotweed spreads in a coastal community in Oregon. Image courtesy by R. Emanuel, OSU.

Invasive giant knotweed – a late successional species outgrowing the earlier species.
Image courtesy by R. Emanuel, OSU.

As an ecologist I am often in the field and am often struck by the same question, if all these plants were suddenly wiped out, what would I find after six months, after five or ten years? According to the idea of ecological succession, we should expect a transition of species over time. Early successional plants usually thrive in sites affected by disturbances, such as fires, flooding, insect outbreaks, and tree logging, are eventually eliminated by other species. Late successional plants often use the environmental resources more effectively or adversely impact the colonists by overgrowth or chemical toxins.

But is this transition of plant species predictable? Or, is this transition a random process? Are there multiple but limited successional pathways that can result in similar climax stages of plant communities? If successional pathways are multiple but limited, then we may need to re-open a controversial debate in ecology: the existence of natural rules which assemble species in a certain way driving the temporal change of biological communities.

The term assembly rule has been applied to a variety of patterns in developing and established biological communities. Assembly rules are processes in nature that may help explain the existence of vegetation in various ecosystems. For example, certain grasses are found in a New Jersey wetland marsh, why a forest in New England is dominated by hemlock instead of sweet birch, or why a single invasive plant species can completely dominate a plant community.

Assembly rules depend on various factors including historical patterns of speciation, migration of species, dispersal, abiotic environmental factors, and biotic interactions, none of which are mutually exclusive. What constitutes evidence for assembly rules continues to be debated by ecologists. However, most agree that some assembly rules are trivial.

The complexity of assembly rules makes it difficult to not only prove their existence but also as the explanation for natural changes in biological communities. The rules are generally regarded as hypotheses that need to be tested case by case.

To test assembly rules, scientists collect field data and run computerized simulations. Then, using random data they run another simulation and compare the results to find a pattern, the assembly rule. However, many studies are inconclusive. As a result, many scientists hypothesize the rules are changing biological communities over time but cannot prove their existence.

The Enemy of My Enemy, Is My Friend

Some assembly rules are based on biological interactions. The observation that predators cannot invade a new community without prey is a straight forward example of an assembly rule. But, there are also less obvious assembly rules. Predators can shape the species composition of a community in more complex ways, especially by feeding on a strong competitor species. If a predator prefers to feed on a superior prey, a strong competitor, rather than weak prey, it will affect species diversity, as the competitively superior species will no longer be able to push the weaker competitors out of the habitat.

Early goldenrod, Solidago juncea,  Image courtesy by Prairie Moon Nursery.

Early goldenrod, Solidago juncea. Image courtesy by Prairie Moon Nursery.

An experiment conducted by Carson and Root (2000) illustrates how predation can define plant communities in time. Their study area near Ithaca, NY, demonstrated the impact of excluding arthropods from plant communities. Some plots were regularly sprayed with pesticides, excluding arthropods, while other plots were not sprayed. They found that the plots without arthropods had increased amounts of the dominant species, Solidago, which caused a decrease in the species richness of understory herbs. It could be said that as an enemy of Solidago, the arthropods, is a friend of the understory herbs.

Timing is the Key to Success

The term assembly rule has also been applied to the observation that different sequences of species invasion in communities can produce different species composition. For example, a species often persists in a community if it arrives first. However, if it arrives later in a sequence nutrient and space resources may be limited.

Access Denied

Can species loss be irreversible? Studies of different ecological communities have demonstrated that when a species is deleted from a community, it can cause a cascade of subsequent extinctions. Most striking, is that those communities are sometimes closed to reinvasion by the extinct species. This assembly rule can explain why restoration of natural communities with former extinct species sometimes fails.

Understanding the rules

Based on the various assembly rules ecologists must answer a number of questions. What is the best way to restore a degraded plant community and obtain a specific species composition? How can we do to create or reassemble plant communities resistant to invasion by exotic species? But before we can answer these questions it is critical to understand how species arrive in an area, survive, and interact with other species.

Great Ecology Ecologist, Tyler Nicol, monitors the Woodbridge test plots.

Monitoring test plots at Woodbridge Waterfront Park. Image by Great Ecology.

For example, Great Ecology is conducting a Test Plots Monitoring study at the Woodbridge Waterfront Park, a 185-acre habitat restoration project. By using different plant treatments (control plots, seeded plots, mixed plots with seeds and plugs, or plots with seeds and woody species) we will obtain valuable information about the general ecosystem framework –  where planted and spontaneous species co-occur and assemble. As a result, we can determine which plants are best suited to survive, which spontaneous plants can outcompete target species, or what specific plant treatments or assemblies of target species are more resistant to invasion. With this information, we will be closer to providing effective management and restoration guidelines for creating a natural and fully functional wetland.

Although we may not be able to change the rules governing species assembly, we can try to make them more “flexible”, and in turn make the most of our landscapes.

References
Peter J. Morin. 2011. Community Ecology. Wiley-Blackwell. Second Edition. New Brunswick, NJ.

Götzenberger, L., de Bello, F., Brathen, K.A., Davison, J., Dubuis, A., Guisan, A., Lepš, J., Lindborg, R., Moora, M., Pärtel, M., Pellissier, L., Pottier, J., Vittoz, P., Zobel, K. and Zobel, M. 2011. Ecological assembly rules in plant communities—approaches, patterns and prospects. Biological reviews 87: 111-127.

Carson, W.P. and Root, R.B. 2000. Herbivory and plant species coexistence: community regulation by an outbreaking phytophagus insect. Ecological monographs 70: 73-99.

 

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