LRES 543, Agroecology

Plant Community Succession and Mechanisms Driving Temporal Dynamics

Agroecosystems continually undergo disturbances that limit the system to the earliest stage of succession. Agroecosystems can become more stable and at the same time remain productive by integrating intermediate disturbances that will varying successional stages.

• Primary succession: ecosystem developed on sites not previously occupied by other living

organisms.

• Secondary succession: ecosystem developed on sites previously occupied by other living

organisms that were disturbed.

Disturbances include fire, floods, wind, grazing pressures, man’s interventions, etc.

Three variations in disturbance:

1. Intensity of disturbance – determined by biomass amount lost and individuals eliminated
2. Frequency of disturbance – average amount of time between disturbances
3. Scale of disturbance – spatial size of disturbance

Three phases:

1. The entire biotic community changes the physical environment through complex interference.
2. Competition and coexistence among individuals and entire populations create changes in species diversity and abundance.
3. The system energy flow slows from production to respiration.

Table 17.1 describes ecosystem changes that occur after disturbance.

Intermediate Disturbance- Ecosystems are maintained by disturbances that occur at relatively moderate rates. Early successional traits, which enhance productivity, are present. At the same time, high species diversity is also present, adding to the overall stability of the ecosystem.

Patchy landscape – created by intermediate disturbances randomly spread across the landscape

in time and space. A mosaic of different successional areas is known as

patchiness. Patches enhance ecosystem diversity.

Table 17.3 displays advantages and disadvantages of successional stages to agroecosystems.

1. Plant annual crop as a pioneer species that grows rapidly and produces quick yield.
2. Addition of other annual crop species.
3. Plant a polyculture of annuals that play different roles. Soil development is enhanced.
4. 2^nd to 3^rd growing season, introduce short lived perennials.
5. Plant long-lived perennials, and trees and shrubs.
6. Agroforestry is adopted to maintain annuals, short lived perennials, and long lived perennials between tree and shrub species.
7. Endpoint is reached when trees are fully developed. At this point, the system is managed as is or disturbance is introduced to portions and/or the entire system to start the cycle again.

Figure 17.3 illustrates changes that take place through the 7-step agroecosystem development.

Through this process, removal of net primary productivity (NPP) and biomass accumulation must be balanced to maintain a healthy system. Figure 17.4 illustrates changes in NPP and biomass vs. time.

Managing Successionally Developed Agroecosystems – once these systems are developed, what can be done to effectively manage them?

1. A major disturbance can introduced to return the system to the first step.
2. Maintain the perennial/tree system as is.
3. Introduce disturbances in various locations throughout the agroecosystem to develop a mosaic of patches.

Agroforestry – managing a combination of trees and crops on the same land.

Disadvantages and advantages to this system.

Tropical Home Gardens – generally exhibit a highly diverse agroecosystem that involves agroforestry. These gardens are popular in the tropic and sub-tropic regions and are generally tied to social and economic factors.

Restoration efforts attempt to restore physical and chemical soil conditions in combination with seeding plant species that were encountered on a site prior to the disturbance. However, the end result is not always the desired outcome.

The combination of community assembly theory with restoration efforts strives to mimic the ecological principles that develop diverse communities. Community assembly theory involves controlling the addition and subtraction of species from a community at strategic growth periods to mimic successional trends.

Alternative endpoints - Community assembly experiments have displayed different end results of community development.

Basins of attraction – stable community composition endpoints

• species can increase or decrease in abundance
• certain species may go extinct and reappear later
• the basin of attraction does not change unless a disturbance acts upon it
• Once a disturbance does occur, the system can return to original basin of attraction, go extinct, or move to another possible basin of attraction

1. The natural community of the past was a function of former disturbances. The resultant community was likely created by the sequence of species establishment.
2. Once a natural community is disturbed, it may not return to its original composition.
3. History of species establishment and extinctions determines the outcome of community. Referred to as historical contingency.

1. The importance of individual species in the assembly process - Keystone species play an important role in creating and maintaining a community composition. These species can go extinct and later return.
2. The importance of the order species establish on a site – Competitive exclusion among species can determine which species will establish.
3. The importance of species attempts to restablish – Species tend to arrive in groups at different times throughout the community development. A failure to establish in one attempt might lead to success on the next attempt, changing the community structure.

Restoration case studies were reviewed to determine the extent to which the principles of community assembly are used. The majority of cases were end-oriented and little attempts were made to involve the community assemblage methodology.

Autogenic Succession – species act independently and if physical and chemical conditions are adequate, they will establish.

Allogenic Succession – early establishing species influence the subsequent establishment of later species.

Examples of restoration cases involving autogenic (vernal pools) and allogenic succession (oak-savannas in Chicago).

1. Gather species that were present or believed to be present in the past.
2. Introduce the species in a sequential order.
3. Control the order of introduction- even over long periods.

Few restoration efforts involve these concepts; however, in terms of large-scale reclamation, how cost feasible are these ideas? Also, regulatory framework guiding many reclamation projects on mine sites would consider introductions of new species at different times maintenance practices. Thus, each time a new species was introduced, the bond clock would be reset and the responsible party would never achieve final bond release.

Alternative community states are created by disturbances that eliminate essential species involved in positive feedback loops, and also by the establishment of other species that are capable of creating large changes. Disturbances that cause change in community assemblage through elimination and establishment of species are scale dependent.

• Alternative assemblages are created by the starting differences (origin) in plant community structure after a disturbance.

• History guides the development of the current plant community.

• the hazy distinction between the origin and persistence of assemblages
• tests of the origins of alternative communities require scale-dependent experiments

• Theory considers conditions at equilibrium in a constant environment that is closed to migration; however, field experiments are not in equilibrium and changes occur that create alternative communities. How do we effectively measure the changes?.

• Density data do not always describe the alternative state, instead biomass measurements should be performed.

• Experiments could demonstrate that a stable community might be returned to a community occupied by a very different state. This situation presents statistical problems.

• The experiment must include some form of disturbance to create the replacement alternative community.

Maintenance tests must prove that the established assemblages persist beyond the lifetime of individual species that have contributed greatly to the biomass of the assemblage.

Origin tests require a disturbance or pulse event that create alternative states that can be stable, self-sustaining, or self-replicating. Two scenarios can occur:

1. An influx of recruits causes a change in community structure creating the alternative state.
2. A disturbance eliminates original assemblage species, enabling the establishment of new species that create the alternative state. This scenario is more common than #1.

Both scenarios include:

Negative feedback – early colonizers alter the environment to favor the establishment of

competitively superior species.

Positive feedback – late-stage dominant species develop environmental changes that favor their

own existence.

Spatial and temporal scale of a pulse event must be large enough to create the alternative community state.

Scenario 2. Testing the Origin of Alternative Community States – Four Situations

1. Situations where both disturbance and recruitment can be manipulated.
2. Situations where only disturbance can be manipulated.
3. Situations where only recruitment can be manipulated.
4. Situations where neither disturbance nor recruitment can be manipulated.

Each situation poses problems with proper data collection, time of data collection, the scale of data collection, and the statistical analyses involved.

Rocky intertidal shores that contain Ascophyllum nodosum communities or communities with beds of mussels and barnacles.

Heathlands that are interspersed within full-statured North American forests.

Changes in community structure in both examples are dependent upon the scale and severity of the disturbance, recruitment of species, and the persistence of the species.