What is disturbance?
- Any relatively discrete event in time that disrupts the ecosystem and
- changes population / community structure (generally killing or damaging dominant species)
- resource availability
- substrate availability
- the physical environment.
- (fire, hurricane, flood, volcano).
- Disturbances result from forces originating outside of the community and are outside of the normal range of the system’s perturbations.
What are the two key features of disturbances? What is the relationship between the two?
- Disturbances occur on a continuum of frequency and intensity. There’s a negative correlation between frequency and intensity.
What are the possible sources of disturbance?
- Abiotic or biotic
- Natural or anthropogenic
What is succession?
The repeatable change in community structure / composition through time following a disturbance. In plant communities, this is caused by internal dynamics. Abiotic and biotic factors determine which species appear when during different successional stages. Both short term and long term environmental fluctuations affect the development of a community. Contemporary thinking suggests that succession is complex and rarely leads to a single predictable climax community.
What is secondary succession?
Succession after a disturbance that kills most species but not all, leaving behind some species, propagules (seedbank), or organic matter (soil or sediment) from the previous ecosystem.
What is primary succession?
Succession on newly exposed mineral substrate or disturbances that remove virtually all traces (species and organic matter) of the prior ecosystem. No soil is present. Biomass is nonexistent.
What are the three conceptual models of succession?
- Facilitation (the presence of some species facilitate the colonization of the area by other species/ secondary and primary)
- Inhibition (the initial community composition is simply a function of who gets there first. The growth of one species inhibits the growth of another / secondary succession/ longer living late successional species are only established after the death of early successional species)
- Tolerance (secondary and primary / initial community composition is a function of who gets there first, late successional species tolerate early successional species as they’re better competitors, but eventually exclude them through competition)
What are the assumptions made in the three conceptual models of succession?
- Organisms are sessile
- Succession is triggered by disturbance and proceeds without further changes in the biotic environment. (autogenic succession with changes in species composition are driven solely by internal forces)
- Initial colonists change the environment (shade, deeper litter etc.)
Explain climax community
Taken to their logical extreme, some models suggest that in the absence of additional disturbance, succession leads to a more or less stable point called a climax community. The assumption being made here is that communities are driven purely by autogenic processes. In reality, allogenic processes ensure that communities are never really at equilibrium.
What is the intermediate disturbance hypothesis?
Species diversity will be highest when disturbances are at an intermediate frequency.
How do we observe succession?
- Direct observation throughout time.
- Chronosequence: A group of related communities that differ in development due to differences in age. (“space for time distribution”). The comparison of chronosequences allows for the study of the changes in a community throughout succession.
- What is the relationship between the concept of pioneer vs climax species and the concept of r vs K species?
- Pioneer species are similar to r species and climax species are similar to K species.
Explain pioneer species.
- First species on bare site.
- Must be stress tolerant
- Can handle low soil conditions
- Often favors N fixing species. (lots of light and space available, little N available)
- Favors species with wind dispersed spores and seeds.
- Early successional species make the environment more suitable for later species.
What is the influence of the frequency and intensity of disturbances on the species diversity of the ecosystem?
When disturbances aren’t frequent, K species dominate and competitive exclusion occurs, lowering diversity. When disturbances are too frequent, only a few taxa are able to withstand the high levels of disturbance and r species dominate. The intermediate disturbance hypothesis states that diversity will be highest at an intermediate level of disturbance where a mix of both r and K species will occur.
What are the reasons why prairie grass has left Iowa? What are the implications of this for disturbance management?
- Bison are gone
- Fire suppression and fragmentation.
- To bring back the prairies, controlled fires in fragments, mowing, and grazing can be facilitated to maintain the ecosystem at arrested development. Maintaining areas at various successional stages by controlled fragmented fire will largely increase diversity. Additionally, as fires become less frequent, their intensity increases due to increased fuel. Thus, fires at controlled frequencies are important for the avoidance of more intense disturbances. Fires require proper management - not just suppression.