Community Assembly

Like co-evolution, community formation is also complex and dynamic, resulting in a process known as ecological succession. Succession is the process whereby a site is colonized by plants and develops a successively more mature community over time.

Figure 7.31: In this example, it is also plain to see how the vertical structure of the habitat changes with community composition over time. Also, as succession progresses, the plant species that continue to colonize the site successfully will be more shade-tolerant (i.e. they would have to be able to compete better under low-light conditions). In other words, you can’t take a bunch of tree seedlings out into a field and expect to plant an old-growth forest. Image from URL: http://flowers4u.files.wordpress.com/2009/03/img137jpeg.jpg?w=300

Figure 7.31: In this example, it is also plain to see how the vertical structure of the habitat changes with community composition over time. Also, as succession progresses, the plant species that continue to colonize the site successfully will be more shade-tolerant (i.e. they would have to be able to compete better under low-light conditions). In other words, you can’t take a bunch of tree seedlings out into a field and expect to plant an old-growth forest.
Image from URL: http://flowers4u.files.wordpress.com/2009/03/img137jpeg.jpg?w=300

Primary succession begins with an area of bare rock being colonized, usually by mosses and/or lichens. As generations of lichens pile up on that patch of colonized area, wind-blown soil particles collect there, and as the rock becomes weathered, soil is formed. The site then becomes suitable for other types of plants to colonize. If conditions permit, the site will progress from a community of mosses and lichens to one of herbaceous plants, prairie. Again, conditions permitting, the site may gain some shrubs. Under the right circumstances it might become a savannah, or open forest. As generations pass, it might become a closed-canopy forest, which may, in this example, be the end of the progression. Without a change in conditions, the forest could persist. This is what is known as the climax community for the site.

Check Your Thinking: If the climax community of a site is reached, is that community static or is there a sort of equilibrium reached where the local species extinction rate approximates the rate of new arrivals?

 

Check Your Thinking: What happens if a species is removed from a climax community? Which has more influence on habitat structure, the plant species present on the site or the wildlife?

 

Site Disturbance & Secondary Succession

Succession proceeds along a trajectory, but that trajectory is often not linear. It might come in fits and starts. It might be set back considerably by disturbance to start from an earlier successional phase (also known as a seral stage), which might change the course entirely. It is dependent on the species of plants on the site, the environmental conditions that allow those plants to grow and compete, the resources available, etc.

Disturbance is an event that upsets or somehow changes the natural progression, or evolution of the site as compared to if that event had not occurred. Disturbances may be natural (e.g. lightning-caused fire, wind storm, disease outbreak, flood, etc.) or anthropogenic, or human-caused (e.g. prescribed fire, tillage, timber harvest, etc).

Different plant species have different requirements to thrive. An orchid that will grow in the understory of an old-growth forest will not compete well in the agricultural field abutting its borders. The light intensity, relative humidity, soil temperature, soil moisture, etc. would all be different and would likely be outside the range of conditions that the orchid needs to survive. By the same token, a plant that needs frequent, intense disturbance and lots of light will not compete well inside the old-growth forest.

If you were to, hypothetically, plow a prairie under for agricultural use and subsequently abandon it as bare soil, there would be an opportunity for plants from surrounding areas to colonize it. However, at the same time many species that were previously there would be lying dormant in the seed bank. Plants often produce a percentage of seeds with innate dormancy—they don’t germinate immediately. They will lie dormant for a time until conditions permit them to germinate. The seed bank is the accumulation of these dormant seeds over time. Even a site with nothing but bare soil has a potential plant community waiting for conditions to permit them to germinate and grow. That potential plant community may or may not resemble the previous community; some of the seeds may have arrived from another nearby community, and some species don’t persist as long as others in the seed bank. If it does resemble the original community type, there may be some differences in the exact species composition, but the original overall functions of that particular ecosystem would be retained. However, if the resulting community does not resemble the original, the trajectory of the succession on that site may be significantly altered.

When a site is set back in its successional path without being reduced to bare rock and then allowed to proceed again, it is known as secondary succession.

Especially in secondary succession, the site would have more than enough resources for plants during the early seral stages. However, as succession progresses, more and more of the site’s resources become tied up in the ecosystem. In other words, less space is available, and more and more water and nutrients are being used. At the same time, the structure of the community grows more complex, creating more niches (the range of conditions in that localized area available for habitation by a species) to be occupied by those species that have the ability to fill them. Generally, as succession proceeds, diversity increases, habitat structure gains complexity, and resources become more tightly controlled.

 

The Influence of Disturbance Regime on Plant Communities

Every site undergoes a disturbance event at some time or other. However, some disturbance events take place with fairly regular frequency. If a site has a history of a repeated periodic disturbance, that is called its disturbance regime.

Plant communities have two basic characteristics that determine their response to a disturbance. First is resistance, or the degree to which the community is able to withstand the disturbance without being significantly altered. The second characteristic is resilience, or the ability of the community to recover from disturbance. If a plant community has both high resistance and high resilience, disturbance events will not have much long-term effect at all. If its resistance and resilience are low, however, it would not take much of a disturbance to alter the community’s composition and structure.

Both frequency and intensity of disturbance are important when studying plant communities. High frequency/high intensity disturbance regimes (e.g. annual tillage of farm fields) tend to have plant communities dominated by annual plants with high seed production. Low frequency/low intensity disturbance regimes (e.g. understory of a temperate rainforest) would be characterized by perennial plants that invest more energy in vegetative structures such as rhizomes or woody stems.

Check Your Thinking: List some disturbance events and label them as either natural or anthropogenic.

 

Check Your Thinking: What kinds of predictions and inferences can you make about the site based on the plant community? How does ecological succession influence our ability to restore a site after disturbance or invasion?

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The MSP project is funded by an ESEA, Title II Part B Mathematics and Science Partnership Grant through the Montana Office of Public Instruction. MSP was developed by the Clark Fork Watershed Education Program and faculty from Montana Tech of The University of Montana and Montana State University, with support from other Montana University System Faculty.