Ecology: Intermediate Disturbance Hypothesis

Gap Dynamics

• Ecological succession is a process of replacement, where the species that fills the gap is determined by priority effect and environmental factors. This process is crucial in understanding the dynamics of ecosystems, as it allows for the adaptation and evolution of species in response to changing environmental conditions. The priority effect refers to the advantage gained by a species that is able to colonize a gap or newly available resource first, while environmental factors include factors such as light, temperature, and nutrient availability.
• Priority effect: whatever species has seeds at that time will fill the gap, but only if environmental factors are favourable. This is because the species that is able to colonize the gap first will have a head start on other species, allowing it to establish itself and potentially outcompete other species. However, if the environmental factors are not favourable, the species may not be able to survive or reproduce, allowing other species to fill the gap.

Gradually Changing Environments

• Environmental conditions change frequently, so that no species remains competitively superior for long enough to exclude other species. This is because environmental conditions can change rapidly, making it difficult for any one species to maintain a competitive advantage. For example, changes in temperature or precipitation patterns can affect the availability of resources, making it difficult for some species to survive.
• Each species encounters favourable and unfavourable conditions for reproduction, and conditions favourable for one species must be unfavourable to others. This means that the success of one species is often dependent on the failure of another. For example, a species that is well adapted to drought conditions may thrive, while a species that requires high levels of moisture may struggle to survive.
• Species should have low adult mortality, high fecundity, so that they can persist until opportunities for recruitment occur. This means that species that are able to survive to adulthood and produce large numbers of offspring are more likely to be successful in filling gaps or colonizing new areas. This is because they have a greater chance of producing offspring that can survive and reproduce, allowing the species to persist and potentially fill gaps.
• In addition, species that are able to adapt to changing environmental conditions are more likely to be successful in filling gaps. This can be achieved through a variety of mechanisms, such as changes in phenotype, behaviour, or physiology. For example, a species that is able to alter its growth rate or seed production in response to changes in environmental conditions is more likely to be successful in filling gaps.

Lottery Models

• The importance of being first: priority effect. This is a critical component of lottery models, as it emphasizes the importance of rapid colonization and establishment in determining the success of a species. The species that is able to colonize a gap first will have a significant advantage over other species, allowing it to establish itself and potentially outcompete other species.
• Species diversity is maintained through random allocation of space, where openings are filled at random by recruits from a large pool of potential colonists. This means that the composition of a community is determined by the random arrival of species, rather than by any inherent competitive advantages. This can result in a high degree of species diversity, as different species are able to fill gaps and establish themselves in the community.
• Species should have low adult mortality, high fecundity, so that they can persist until opportunities for recruitment occur. This is because species that are able to survive to adulthood and produce large numbers of offspring are more likely to be successful in filling gaps and establishing themselves in the community.
• Lottery models work best where different species have similar average reproductive rates and similar competitive abilities, so that no one species gains the upper hand. This is because the random allocation of space means that any species that is able to colonize a gap first will have an advantage, regardless of its competitive ability. This can result in a high degree of species diversity, as different species are able to fill gaps and establish themselves in the community.
• Lottery models are often used to explain the maintenance of species diversity in communities, particularly in ecosystems where there is a high degree of environmental heterogeneity. This is because the random allocation of space can result in a high degree of species diversity, even in the absence of inherent competitive advantages.