Community Ecology and Species Interactions Study on Species Interactions and Diversity Regulation, Interactions, and Keystone Dynamics, and Structural Foundations and Diversity, and Niche Partitioning
Course Logistics and Research Participation
- Metacognition Research Project: The semester began with a survey on metacognition. This is part of a research project investigating the effect of exam records on how students think about their own learning.
- Survey Schedule: The second round (follow-up) of the survey will be released on Sunday and will remain open through Wednesday.
- Participation Eligibility: Students do not need to have completed the first survey to participate in the second. Anyone who missed the first survey is still welcome to participate.
- Extra Credit System: Participation contributes to a pool of extra credit.
* Each exam wrapper completed earns 0.1 extra credit on the course average.
* Each survey filled out (two in total) earns 0.1 extra credit.
* The maximum potential extra credit from these activities is 0.6 points (two wrappers and two surveys).
Ecological Systems and Community Scale
- Scaling Up in Biological Systems: The course has progressed through different organizational levels:
* Subcellular Level: How signals pass through a cell.
* Organismal Level: How signals (e.g., from the pancreas) travel to different cells (e.g., muscle cells) to regulate blood glucose.
* Population Level: How the size and health of single-species populations are regulated.
* Community Level: The focus shifts to how the composition of a community is regulated, moving beyond single species to all organisms in a particular area. - Defining Ecological Community: A community consists of all the living organisms (species) present in a specific location. It encompasses all populations of all species in that area.
- Central Questions in Ecology: Research typically focuses on "who lives where and why."
- Community Systems Model:
* Components: The different species present in the area.
* Processes: Species interactions, represented by arrows connecting components. These interactions include predation, competition, mutualism, commensalism, and herbivory. - Interaction Webs vs. Food Webs: While traditional food webs focus on trophic (feeding) relationships, interaction webs provide a more complex view by accounting for non-trophic interactions. These include:
* Competition between species at the same trophic level.
* Indirect effects like commensalism.
* Mutualistic relationships.
* These interactions have the potential to drive which species survive and coexist, thereby regulating community structure.
- Study Overview: Dobson and colleagues surveyed 24 mountain ponds and focused on the interactions between four specific aquatic species.
- Community Components:
* Midge larvae (Diptera larvae): Aquatic larval form of a flying insect.
* Daphnia rosea: A small aquatic planktonic organism.
* Larval salamander: Found at the bottom of ponds.
* Daphnia pulutus (also referred to as Daphnia pulitus, Daphnia puliche, Daphnia coliche, and Daphnia pull ups): A larger species of Daphnia. - Interaction Labeling Logic:
* Plus Sign (+): Indicates the interaction increases the population size of that species.
* Minus Sign (-): Indicates the interaction decreases the population size of that species. - Analysis of Interactions:
* Midge larvae and Daphnia rosea: Predation (+/−). Feeding on Daphnia provides energy/nutrients to midge larvae while decreasing the Daphnia population.
* Larval salamander and Daphnia pulutus: Predation (+/−).
* Between the two Daphnia species: Competition (−/−). They compete for the same resources (filter feeding on organic matter). Both populations would be higher if the other were absent.
* Larval salamander and Midge larvae: Commensalism (+/0). An indirect interaction where the salamander's presence positively affects the midge larvae. - System Disruption Scenario (Removing Larval Salamanders):
1. Removing the larval salamander causes a "release from predation," leading to an increase in the Daphnia pulutus population.
2. The increase in Daphnia pulutus leads to a decrease in Daphnia rosea because Daphnia pulutus is the superior competitor.
3. The decrease in Daphnia rosea leads to a decrease in midge larvae because their primary prey item is less available. - Conclusion on Commensalism: Larval salamanders indirectly increase the population of midge larvae by feeding on their competitor (Daphnia pulutus) and thus protecting the population of the midge larvae's prey.
Competition and Niche Partitioning
- Connell’s Barnacles (Scotland): An experiment investigating why two barnacle species coexist in a consistent pattern in the intertidal zone.
* Thamelus: Found in the upper intertidal zone.
* Balanus (also referred to as Valinus): Found in the middle and lower intertidal zone. - The Experiment: Connell removed Balanus from rocks and observed the response.
- Result: Thamelus expanded its range to occupy the whole rock in the absence of Balanus.
- Key Concepts:
* Fundamental Niche: The full range of environmental conditions where a species is able to survive.
* Realized Niche: The actual restricted range a species occupies when competitors are present. - Niche Overlap and Competitive Exclusion:
* Balanus and Thamelus have a large overlap in their fundamental niches in the middle/lower zones.
* Balanus is the superior competitor and excludes Thamelus from the lower zones.
* Thamelus survives in the upper zone because it is better at surviving desiccation (drying out), while Balanus cannot survive there.
* The upper zone serves as a "refuge from competition." - Niche Partitioning: Two species can coexist as long as their fundamental niches do not totally overlap. If they did overlap completely and one was superior, the inferior competitor would be lost from the community.
Predation and the Green World Hypothesis
- The Green World Hypothesis: Proposed by Nelson Hairston, Lawrence Slabotkin, and Fred Smith. It posits that the world is green because predators keep herbivore populations in check, preventing them from defoliating and destroying plant communities. Control comes from the "top down."
- Bob Paine’s Starfish Experiments (Pacific Northwest):
* Top Predator: Pisaster ocratius (purple and orange starfish).
* Primary Prey: Blue mussels.
* The Experiment: Paine removed Pisaster from rocky outcrops by physically throwing them into deeper water.
* Result: Species richness dropped significantly, from 15 species down to 8, then to 7, eventually becoming a monoculture of blue mussels. - Keystone Species Definition: A species that has a greater effect on community composition than its own abundance or biomass would suggest.
* Pisaster is a keystone species because it reduces competition for space by feeding on blue mussels, which are superior competitors. This allows other species to coexist.
* Metaphor: Similar to an architectural keystone in an arch; if removed, the structure collapses.
Trophic Cascades in Salt Marshes
- Interaction Diagram: Blue Crabs $\rightarrow$ Snails $\rightarrow$ Salt Marsh Grass.
* Crabs eat snails (predation).
* Snails eat grass (herbivory).
* Crabs have an indirect positive effect on grass populations. - Doctor Silliman’s Research:
* Exclusion cages showed that without snails, grass flourished (the "chia pet" effect).
* In areas where snails were unchecked, they created "battlefronts" that killed large fields of grass.
* Tethering experiments showed that snails in healthy grass were killed by blue crabs at a rate of 90%. - Trophic Cascade: The indirect effects from predators (top) to herbivores (middle) to plants (bottom).
- Economic/Ecological Risk: Overfishing blue crabs can lead to a snail population explosion, causing a collapse of the foundational marsh species and the services the marsh provides.
- Top-Down vs. Bottom-Up Control:
* Top-Down: Predators control the structure (e.g., Silliman’s Maryland marshes).
* Bottom-Up: Nutrient availability (e.g., Nitrogen) and resource supply control the community (e.g., Northeastern marshes in Massachusetts).
* General Rule: Most communities are controlled by a combination of both mechanisms.
Disturbance and Diversity
- Defining Disturbance: Density-independent abiotic factors such as fires, floods, or physical disruptions.
- New Zealand Stream Data: Researchers estimated disturbance by counting overturned rocks and measuring species taxa.
- Intermediate Disturbance Hypothesis: Species diversity (number of taxa) is maximized at intermediate levels of disturbance.
* Low Disturbance: Superior competitors have enough time to exclude all other species, leading to lower diversity.
* High Disturbance: Only a few resilient species can tolerate the extreme conditions, leading to lower diversity.
* Intermediate Disturbance: Opens up habitat by occasionally removing superior competitors, allowing more species to coexist. - Human Impact: Human-caused disturbances tend to be skewed toward the high-intensity end of the scale.
Questions & Discussion
- Question regarding Daphnia poll ups (Daphnia pulutus) and Midge larvae: How does the negative-negative interaction between Daphnia species relate to midge larvae?
* Response: The two Daphnia species are in competition. If one increases (like Daphnia pulutus when salamanders are removed), it negatively impacts the other (Daphnia rosea). Since the midge larvae depend on Daphnia rosea, the midges ultimately decrease. - Question on Commensalism: Why is the relationship between salamanders and midges described as commensalism?
* Response: One species (salamander) has a positive effect on the other (midge) through indirect interactions (eating the midge's competitor's competitor), but the midge doesn't necessarily have a reciprocal effect on the salamander. - Question on Global Consistency: Would you expect the same result in all salt marshes?
* Response: No. Variations exist. Some regions show strong top-down control (crabs), while others are bottom-up (nitrogen). It is often a combination of both mechanisms.