Ecology Exam 3

Facilitation Interactions

Commensalism (+/0)

• Biological interaction in which members of one species is benefitted while the other is neither benefited or harmed

 Mutualism (+/+)

• Each species has a net benefit 

• Allows organisms to use resources that would otherwise be “off the table”

• Ex: Cockroaches digesting wood via bacteria in gut 

Symbiosis (+/0)(+/+)(-/+)

• Close long-term biological interaction between two organisms 

• Can be mutualistic, commensalistic, or a parasitic relationship 

The role of ectomycorrhizae and how does it shape ecosystems?

• The role of it is nutrient and water uptake as well as protecting roots from pathogens and environmental extremes

• Non penetrative fungus grows between root cells and forms a mantle around the root 

The role of arbuscular mycorrhizae and how does it shape ecosystems?

• The role of it is enhancing nutrient uptake (phosphorous) and promote plant growth and resilience especially stress conditions

• Penetrative fungus that penetrates the cell wall of root cells forming a branched network called arbuscule  

What’s the difference between facultative vs obligate mutualists and generalist vs specialist mutualists?

• Facultative vs obligate mutualists 

  1. A facultative mutualism can live without its mutualist partner, although performs better with partner 

  2. Obligate mutualism depends on its partner, in that the survival and/or reproduction are not possible in the absence of the partner (ants feeding leaves to fungus then feeding fungus to their offspring)

Generalist vs specialist mutualist 

1. Generalist are when multiple species can perform the function

   • Strategy safe (lots of options), not efficient (poor fidelity), and costly

2. Specialists are reliant on only one other species  

   • Strategy risky (few options) but efficient (great fidelity)

Trophic mutualism 

• Receives energy or nutrients from it’s partner

Habitat mutualism 

• Provides a place to live for the other partner

Cheaters

• Increase offspring production by overexploiting their mutualistic partner  

How positive interactions affect the abundances and distributions of populations as well as the structure of ecological communities  

• Increase availability of resources, creating new habitats, and reduce stress ultimately leading to larger populations, wider distributions, and more diverse communities 

• Ants bore holes into trees for protection and in return will kill any other plant or insect that comes near the plant (reduces competition for both)

Community Structure and Diversity 

Community

• A group of co-occurring, interacting species 

Community Structure

1. Taxonomic 

   • Measure of how similar 2 organisms or populations are (share common ancestor)

2. Resources

   • Guild: What is being eaten

3. Function

   • Functional Group: how it's being done (eaten)

Richness

• The number of species in a community

Evenness

• Relative abundances compared with one another

Species dominance/abundance

• The abundance rank tells us if there is a dominating species within a community

Shannon Index

• Way to measure how diverse a group of species is

• P(i): proportion of individuals in the (i)th species to total community

• s = number of species in a community

• Values Range: 0-4 () being low diversity, 4 being high)

Rank abundance curves

• Graph that shows relative abundance of a species in a community

Direct vs indirect interactions

• Direct: immediate impact one organism has on another 

• Indirect: one organism influences another through a third organism or environmental factor

Interactions vs trophic webs

• Trophic webs: energy flow through feeding relationships 

• Interactions: all types of relationships between organisms

Why is it a challenge to determine the number of species in a community?

• The complexity of the ecosystem, the difficulty of sampling inaccessible areas, and the constant flux of species interactions and environmental changes

How can species accumulation curves help with this problem?

• Can help us determine when most or all of the species in a community have been observed, until you don’t see/get any new species (levels off) in a community have been observed

What information is revealed by rank abundance curves?

• Species richness and evenness within a community

What are keystone species?

• An organism that has a disproportionately large impact on its ecosystem relative to its abundance 

How do you differentiate between a keystone species and a dominant species?

• An organism that has a disproportionately large impact on its ecosystem relative to its abundance while a dominant species is simply the most abundant or has the highest biomass, exerting influence through sheer number

Are top predators always keystone species?

• NO because trophic cascade don’t always flow from top to bottom

• Starfish eat barnacles allowing for other species growth. Without starfish, the barnacles would overtake everything. Starfish are keystone but NOT apex predator

What are ecosystem engineers?

• Any organism that creates, significantly modifies, maintains or destroys a habitat. These organisms can have large impact on species richness and landscape level heterogeneity of an area

What is ecosystem redundancy? 

• The presence of multiple species within a community that perform similar roles or functions

Disturbance and Succession

What is disturbance?

• Any abiotic physical force that causes mortality or loss of biomass in a community

  • ex: waves, currents, wind, flooding, chemical composition, fire, volcanism

What factors influence its impact on a community?

• The intensity and frequency of the disturbance event

• The resilience and resistance of the community to recover from the disturbance

• The existing biodiversity and species composition within the community

• The spatial and temporal scale of the disturbance

• Environmental conditions such as soil type, moisture levels, and climate

• The presence of invasive species that may alter recovery dynamics.

What do community resilience and community resistance to disturbance mean?

• Resistance: the ability of a community or ecosystem to maintain its structure and/or function in the face of disturbance

• Resilience: ability and rate of an ecosystem to recover from a disturbance and return to its pre-disturbed state

What is Succession?

• The non-seasonal, directional and continuous pattern of change in the species found at a site, following a disturbance or biotic agent or the formation of a new habitat

What defines whether succession is primary or secondary?

• Primary succession occurs in lifeless areas where soil has not yet formed, such as after a volcanic eruption

• secondary succession takes place in areas where a disturbance has destroyed an existing community but left the soil intact, such as after a forest fire.

What types of disturbance are associated with each?

• Primary Succession: Typically associated with disturbances like volcanic eruptions, glacier retreats, or landslides that create new landforms devoid of soil.

• Secondary Succession: Commonly linked to disturbances like forest fires, floods, hurricanes, or human activities such as logging and farming that disrupt existing ecosystems but do not eliminate the soil.

What is a climax community?

• late successional community that remains stable until disrupted by disturbance

How long does succession take?

• Ecological succession can take anywhere from a few years to several thousand years, depending on the specific conditions of the environment and the severity of the disturbance that initiated the succession.

Are there predictable patterns in succession?

• Communities are typically most diverse and complex at intermediate stages of succession

• Other properties of communities change also:

  • the ratio of biomass to productivity increases

  • the net accumulation of biomass slows and stops

  • the proportion of nutrients available to the community that is in organic materials increases

  • detrital food chains increasingly predominate over grazing chains

What are alternative stable states in succession?

• Sometimes different communities develop in the same area under similar environmental conditions. These communities can persist over time, as they are maintained by feedback mechanisms that stabilize their structure and function.

What are the typical characteristics of pioneer and climax species?

• Pioneer species: These are typically the first organisms to colonize disturbed areas. They are often hardy species that can tolerate harsh conditions, such as high levels of sunlight, poor soil quality, and extreme temperatures. Examples include lichens and mosses, which help to create soil by breaking down rock and organic matter.

• Climax species: These species are usually larger, longer-lived, and more complex than pioneer species. They thrive in stable environments and contribute to a more diverse ecosystem. Examples include mature trees in a forest, which provide habitat and resources for various organisms.

How do these relate to r- and K- selected life history strategies?

• Pioneer species, such as lichens and mosses, are typically r-selected, meaning they reproduce quickly and in large numbers to exploit available resources in unstable environments.

• Climax species, which include mature trees, are often K-selected, characterized by slower growth rates, longer lifespans, and a focus on nurturing fewer offspring, allowing them to thrive in stable ecosystems.

What are the general patterns of how species richness changes through succession?

• During ecological succession, species richness generally increases over time as pioneer species are replaced by more competitive species. Initially, the species diversity is low, but as the environment stabilizes, more species can establish themselves, leading to a peak in species richness during the mid-succession phase. Eventually, as the ecosystem reaches its climax community, species diversity may stabilize or slightly decline due to increased competition among K-selected species.

What are facilitation, tolerance, and inhibition in succession?

• Facilitation: This process occurs when early successional species modify the environment in ways that benefit later species, such as improving soil quality or creating shade, which allows more sensitive species to thrive.

• Tolerance: In this model, species can establish themselves in a community without the need for early species to modify the environment; instead, they succeed based on their own adaptations and resilience to the conditions present.

• Inhibition: This concept suggests that early colonizers may hinder the establishment of later species through competition or resource depletion, thus delaying succession until the initial species die off or are removed.

How do ecosystem variables ( eg. light availability, soil carbon and nitrogen pools), change during succession?

• During succession, ecosystem variables such as light availability, soil carbon, and nitrogen pools undergo significant changes:

  • Light Availability: Initially, light levels may be high, but as pioneer species grow and form a canopy, light availability for understory species decreases, influencing the types of vegetation that can thrive.

  • Soil Carbon Pools: As organic matter accumulates from decaying plant material, soil carbon pools generally increase, enhancing soil fertility and supporting more complex plant communities over time.

  • Nitrogen Pools: Early successional species, particularly nitrogen-fixing plants, can elevate nitrogen levels in the soil, promoting nutrient availability for later successional species and facilitating the transition to a mature ecosystem.

What brings about these changes?

• These changes are primarily driven by ecological succession, where the gradual replacement of one community by another alters the physical and biological environment, leading to shifts in light, nutrient availability, and soil composition.

Factors Influencing Diversity

What is the typical pattern regarding species richness and latitude?

• Species richness is higher in the tropics (0 degrees latitude)

• Medium disturbance

What hypothesis explains this pattern?

• The Intermediate Disturbance Hypothesis: species diversity is maximized in ecosystems at intermediate levels of disturbance, neither too frequent nor too rare (a medium rock in the water)

1. Species diversification rate is higher in the tropics 

2. Diversification rates are similar, but evolutionary time is greater in the tropics

3. Higher productivity results in more abundant resources in the tropics 

How does productivity influence species richness? Why?

• Processes such as disturbance, stress, predation, and positive interactions can mediate resource availability, thus promoting species diversity by providing more resources to support a greater number of species

What different interactions among species can affect species richness in communities?

• Competition 

• Predation

• Mutualism

• Parasitism 

What kinds of ‘filters’ determine which species are found in a community?

1. Regional species pools and dispersal ability

   • The regional species pool provides an upper limit on the number and types of species that can be present in a community 

2. Environmental conditions

3. Species interactions 

• Competition

• Predation

• Mutualism

• Parasitism

Primary Production, secondary production 

What is the difference between plant production and plant biomass?

• Plant biomass is the total amount of living organic matter (dry weight) present in plants within a given area. Plant production is the rate at which that biomass is being created (rate of biomass accumulation)

Definition for and relationship of gross primary production (GPP)

• Total carbon produced by primary producers (total energy by autotrophs)

Definition for and relationship of respiration

• The process by which organisms exchange gases (O2 and CO2) with their environment, enabling them to produce energy through cellular processes like breathing and cellular respiration

Definition for and relationship of net primary production (NPP)

• Carbon accumulated (in stored form) by primary producers (also can be expressed as GPP - R(p)). Usually ~25-50%

Definition for and relationship of net ecosystem production (NEP)

• NEP = GPP - ER (ecosystem respiration)

Definition for and relationship of net ecosystem production (NEP)

• Exchange of CO2 with the atmosphere

• NEE = GPP - (AR + HR)

• AR:  autotrophic respiration

• HR:  heterotrophic respiration

• Helps understand the health of the ecosystem, how carbon is being budgeted

What are the controls for GPP? 

• Climate through its influence on photosynthetic rate, nutrients, light, mixing (if in water). Example is gyers are nutrient poor because of the climate

• Leaf area index (LAI) - Leaf area per unit of ground area

What are the controls for NPP?

• Climate influence on primary production can also be mediated by factors such as  nutrient availability

What is Allochthonous?

• Introduced from a different environment, habitat, or geographical area

What is Autochthonous?

• Indigenous to the place rather than descended from migrants or colonists

What is the critical depth? 

• Place in water column where integrated (over time) P = integrated R

• Depends on: angle of incidence, extinction coefficient, Light compensation intensity (through this, respiration rate)

Compensation depth? 

• Photosynthesis = Respiration

• P > R: Biomass increases

• P< R: Biomass decreases

How do these change with the season? 

• Compensation Depth and Critical depth both deepen in spring/summer with increased sunlight and become shallow in winter/fall

How does water column stability (thermocline, mixed layer depth, pycnocline) change with the season, and what is the effect of this on aquatic primary production? 

• As temperatures rise in spring and summer, the thermocline becomes more pronounced, leading to a stable stratification of the water column. This stability reduces mixing, which can limit nutrient availability in the upper layers, thereby affecting aquatic primary production. Conversely, in winter and fall, the mixed layer deepens due to cooling surface temperatures, increasing mixing and enhancing nutrient distribution, which can boost primary productivity.

Know the basic trophic levels - primary producers, herbivores, carnivores (predators), and detritivores/decomposers. 

• Primary producers: autotrophs

• Primary consumers: herbivores

• Secondary consumers: carnivores (predators)

• Tertiary consumers: apex predators

• Detritivores: break down dead organic material (bugs&fungi)

• Decomposers: break down dead organic material (bacteria&fungi)

How do you calculate gross, assimilation, and net growth efficiencies?

• GPE: (produced/consumed) 

• AE: (assimilated/consumed) 

• NGE: (produced/assimilated) 

• Assimilation efficiency depends on food quality

• Assimilation vs Consumption: consumption is the total amount of carbon that an organism intakes while assimilation is the portion of carbon absorbed in its system after digestion and metabolism

 How do these efficiencies differ for different types of consumers, and how do these differences relate to those consumers’ physiology (e.g., heat metabolism) and diet (herbivores vs. carnivores)?

• Herbivores on land consume a much lower proportion of autotroph biomass than herbivores in most aquatic ecosystems 

• Herbivores are constrained by predators and never reach carrying capacity 

• Autotrophs have defenses against herbivory (chemicals, spines, etc)

  • Plants of resource-poor environments have stronger defenses than in resource-rich environments 

  • Unicellular algae lacks chemical and structural defenses

• Phytoplankton are more nutritious for herbivores than terrestrial plants

  • Terrestrial plants have structural components with little nutrients (wood and cellulose)

Nutrient cycling

What is a nutrient source and sink?

• Source: a part of the biosphere where a nutrient is released or liberated from a source faster than it is absorbed (carbon in the atmosphere)

• Sink: a part of the biosphere where a nutrient is taken up or absorbed faster than it is released (deposits)

How do phosphorus, nitrogen, and carbon originate into ecosystems?

• Phosphorus: rock weathering (mechanical and chemical)

• Nitrogen: atmospheric fixation by bacteria

• Carbon: Photosynthesis and decomposition (atmosphere)

How is litter ultimately recycled through a soil ecosystem?

• Decomposition → mineralization → soluble nutrients 

What is meant by nutrient spiraling in a river ecosystem?

• Nutrients are recycled repeatedly as water flows downstream and they are transferred between dissolved inorganic forms, organisms, and detritus 

• Depends on amount of biological activity, water velocity, and form of the nutrient