LECTURE 28 - Assemblages & Ecosystems Flashcards
communities def
Communities: Two or (usually) more species that occur together in
space and time
assemblages def
Assemblages? Less well defined, but can be taken simply as a
group of species that live together with no assumptions made about
how or whether they interact with each other
communities over time
- Stable communities maintain consistent species richness & composition but they are an exception as change in species composition is the norm in nature
- Change is driven by local colonisations (new arriving in the are) and extinctions of species (old disappearing or dying out)
- Ecologists often use models to understand how communities change. One common model is ecological succession, which describes how different species replace each other over time in a predictable pattern following a disturbance such as the ecological succession.
- When a disturbance, like a wildfire or a volcanic eruption, affects an ecosystem, there are predictable patterns of how species recolonize and establish themselves. This process of recovery follows a sequence that ecologists study.
- New communities are being assembled by human activity / Human activities, such as urbanization, deforestation, and the introduction of non-native species, can disrupt natural ecosystems. This can lead to the assembly of new, often human-altered, communities.
- New communities are often homogeneous in many parts of the world –
‘biotic homogenisation’ / Due to human activities like globalization and the spread of invasive species, some communities around the world are becoming more similar or “homogeneous.” This can lead to a loss of biodiversity as unique local species are replaced by a smaller set of widespread species.
ecological succession
Ecologists often use models to understand how communities change. One common model is ecological succession, which describes how different species replace each other over time in a predictable pattern following a disturbance
succession in general
Succession - general
* Early ideas related to forests: tree falls down, creating gap for light [One common starting point is when a large tree falls, creating an opening in the forest canopy. This opening allows more sunlight to reach the forest floor.]
- Light unsuitable for certain species (esp. shade-tolerant), creates high
quality environment for other species
[The increased sunlight in the gap can be unsuitable for some tree species that are adapted to shade. However, it creates a favorable environment for other plant species that require lots of light. So, the change in light conditions creates opportunities for different types of plants to grow.] - Changes in species composition and abundance, growth rates in lower canopy and ground level strata
[With more light, the plant species composition changes. Shade-tolerant species might decline, while light-loving species become more abundant. This affects the overall structure of the forest, including plants at different heights, like the lower canopy and the forest floor.] - Dominant species in system change over time.
[Over time, the plant species that dominate the area can change. Initially, the gap might be filled with fast-growing pioneer species. As the forest matures, other, more shade-tolerant species might become dominant. This shift in dominant species characterizes different stages of succession.] - Various biogeochemical
processes associated with the presence of certain species also
change
[Different plant species have different effects on the soil and nutrient cycling. As the dominant plant species change, so do the biogeochemical processes in the ecosystem. For example, some plants might fix nitrogen in the soil, while others do not. These processes can influence nutrient availability for other plants and organisms.] - New dominant species move in [As succession progresses, new species take over and become dominant. This can lead to further changes in the environment, creating a dynamic process of ecological change.]
- Equivalent changes often seen with animals, fungi etc
pioneer species
Pioneer Species: the first plants to colonize a previously uninhabited or disturbed area, such as a newly formed volcanic island, a cleared field, or a recently abandoned construction site.
- grow in sun : prefer / tolerate full sunlight
- fix nitrogen : Some pioneer species have the ability to fix atmospheric nitrogen into a form that plants can use. This is an essential nutrient for plant growth, and these species help enrich the soil with nitrogen.
- good dispersal : typically have effective mechanisms for dispersing their seeds over long distances. This helps them reach new, open areas and establish themselves quickly.
- small seeds : can be easily transported by wind, water, or animals.
- rapid growth
- short generation time/ short life cycle
- poor competitors : ay not compete well with other, more established plants because they are specialized for colonizing open environments.
climax species
Climax species are plants that dominate and thrive in the later stages of ecological succession when the environment has become more stable and mature.
- shade tolerant
- slow growth
- long-lived
- good competitors
types of sucession
primary
secondary
primary succession
the process of ecological development in an area that initially lacks soil and is devoid of living organisms. It typically occurs in areas that have been completely sterilized or where no life has existed before.
* Bare area without soil
e.g. sand-dune, bare rock
secondary succession
the process of ecological development in an area that occurs in habitats that have already been influenced by living organisms and have some existing soil. This type of succession typically happens in areas that have been disturbed or altered by events like fires, logging, or abandoned farmland. Secondary succession is a process where an ecosystem regenerates and gradually returns to its previous state.
- In a habitat modified by other species
e.g. forest gaps, abandoned agricultural fields
models of succession
Models of succession
Facilitation
* early arriving species make environment more favourable for later species [ pioneer species establish themselves and die off, they contribute organic matter to the soil.]
Tolerance
* neither negative nor positive interactions between early
and late species
Inhibition
* early species inhibit later species
ME : early-arriving species inhibit the establishment of later-arriving species. This inhibition occurs because the early species dominate resources or alter conditions in a way that makes it challenging for other species to take hold.
As time progresses, conditions might not become more favorable for late-successional species. Instead, the early species maintain their dominance, and the community remains relatively stable with limited species turnover.
resilience
the ability of a natural community or ecosystem to recover and return to a stable state after it has been disturbed. [measure of how long it takes for nature to bounce back to a balanced condition after something disrupts it.]
When studying resilience and succession, it’s essential to have clear criteria for defining the conditions before and after a disturbance. These criteria help scientists measure and understand the changes that occur.
For example, in Australia, a common criterion used to define pre-disturbance conditions is “before 1788.” This means that researchers often use the state of ecosystems and communities as they existed before European colonization as a reference point for understanding natural conditions.
PROBLEMS with the “before 1788” criteria
Problems with the ‘Before 1788’ Criterion:
Using the “before 1788” criterion in Australia has its challenges. It assumes that the state of ecosystems before European settlement is the ideal or natural condition. However, this assumption can be problematic for several reasons:
a. Indigenous land management practices were in place before 1788, which means that ecosystems were already influenced by human activity.
b. It doesn’t account for the fact that ecosystems change naturally over time.
c. It doesn’t consider the dynamic and shifting nature of ecosystems or the impacts of other disturbances like climate change.
Ecosystems def
The community of living organisms considered in
conjunction with the abiotic components of their
environment, interacting as a system
Biogeochemical cycles
how elements and compounds essential for life, like C, N, P, S move through living organisms, the atmosphere, the Earth’s crust (geology), and back into living organisms. These cycles involve both (bio), (geo), and (chemical) processes.
- Energy flows through the biosphere
- Materials are recycled [Unlike energy, which flows in one direction, materials (elements and compounds) are recycled in ecosystems. For example, carbon, nitrogen, and other elements are used by living organisms, returned to the environment through processes like decomposition, and then reused by other organisms.]
- Ecosystem productivity is controlled by efficiency of recycling as well as by energy available
- Materials transported in the atmosphere (H2O, C, N, S)–> global cycles [These elements move through the Earth’s atmosphere, and their distribution and movement have a global impact on climate, ecosystems, and the environment.]
- Phosphorus, potassium, calcium and magnesium move through
soil–> local ecosystem cycles
