l33-Ecosystem Function and Functional Diversity
Review of Ecosystem Control Factors
Yesterday's lesson focused on the factors controlling food chain lengths within ecosystems.
Most food chain lengths are observed to be quite short.
The major limiting factors for food chain length include:
Energy available in the ecosystem.
Ecosystem size.
Dynamic stability: This is closely tied to disturbances. In a longer food chain, a disturbance requires waiting on many more species to recover, which leads to considerable instability.
Angus provided a study on this, comparing stream size and the food chains within them against the types of disturbances experienced. This study analyzed:
The effect on the length of the chain.
The amount of energy passed between each trophic level.
The interaction strengths between different trophic levels.
Definition and Activity of Ecosystem Function
Everything discussed regarding direct interactions and diversity contributes to understanding the function of our ecosystem.
Verbatim Definition: Ecosystem function is "the collective life activities of plants, animals, microbes, and the effects these activities, feeding, growing, excreting waste, etcetera, have on the physical and chemical composition compositions of their environment."
Essentially, ecosystem function boils down to what is happening in the ecosystem and how it affects the environment.
It is important to note that function implies showing activity; it does not necessarily mean or imply a purposeful role for a species or group, though it can be framed that way for clarity.
Functional Groups and Traits
Within ecosystems, we can group species that perform similar roles in a specific ecosystem process into functional groups. Examples include:
Pollinators.
Seed dispersers.
Decomposers.
These species often share similar traits relating to those processes, which are referred to as functional traits.
Traits can include:
Behavioral traits: Migration or predatory behaviors.
Physical traits (examples provided in discussion).
Analysis of these traits often incorporates species abundance for statistical purposes. We apply measures such as richness and evenness to understand the contributions of these traits to separate processes.
Redundancy in Ecosystem Processes
Redundancy is a critical concept in ecosystem function. It occurs when multiple species in an ecosystem share the same functional traits for a particular process.
Example of Process and Process :
If two species (e.g., a green species and a purple species) both possess the traits (visualized as triangles and squares) required for Process , the loss of the green species would not affect the process because the red species provides redundancy.
However, if only the green species possesses a specific trait (visualized as a "plus" sign) needed for Process , the loss of that species could lead to the loss of that process.
This leads to three possible relationships (lines/curves) on a graph of Taxonomic Diversity against Functional Diversity:
Linear Relationship: A straight for line where each new species sampled adds unique functional diversity.
Asymptotic Curve (Redundancy): As more species are sampled, the curve flattens out because we begin seeing overlaps and repeated traits (redundancy).
Rare Species Contribution: A curve that shoots up at higher levels of sampling; this suggests that more rare species, which are not sampled initially, are functionally unique and contribute significantly to functional diversity.
The Cedar Creek Experiment and Functional Groups
Cedar Creek was previously discussed in terms of the portfolio effect and stability (where increased species richness leads to greater community-wide stability).
In a study on plant biomass, researchers tested the effect of planting different functional groups and varying the number of these groups in plots.
Results on Species Diversity: An increase in species richness leads to a relatively quick increase in plant biomass.
Results on Functional Diversity: It takes longer to reach the carrying capacity or asymptote for biomass. The study showed that at least different functional groups are needed to cover the necessary processes for maximum biomass production.
Functional groups used in the study included:
Legume species.
grass species.
A combination of both.
The researchers observed interaction facilitation, meaning species within and between groups contribute to the success of others.
Mechanisms of the Richness-Function Relationship
There are four primary mechanisms proposed for why richness affects ecosystem function:
Complementarity Effect: Species occupy different niches and use resources in different ways; as more species are added, a higher proportion of resources are utilized, leading to higher function. This is represented by a linear increase in function.
Sampling Effect: Communities with higher species richness have a higher probability of containing a species with very high functioning by random chance. (Note: This is difficult to distinguish from complementarity).
Redundancy: Ecosystem function reaches a threshold as species are added because functional roles begin to overlap.
Facilitation: Certain species have a positive, non-redundant influence on the functioning of other species.
Composition and Multitrophic Diversity
Research shows that community composition (the diversity of functional traits) often explains variability in ecosystem function better than pure species richness.
Using sums of squares to explain variability in models (taking the log to visualize the spread of small numbers), the general trend across trophic groups is that composition is more important for driving function than richness.
When considering multiple trophic levels (multitrophic diversity), there is a net positive effect on ecosystem services:
Regulating Services: Soil carbon, pollinator abundance, pest control, and resistance to pathogens.
Cultural Services: Benefits for human enjoyment, such as flower cover or bird diversity.
Case Study: Procolotis mariae in Venezuela
A study at the Amazon Headwaters in Venezuela examined a system with upwards of different fish species.
While there is much redundancy among omnivores and insectivores, the detritivore species Procolotis mariae provides a unique, significant function.
Functional Role: Procolotis mariae acts like a vacuum cleaner, filtering the water column and reducing algae and sedimentation.
Experimental Results: Researchers split a stream; in the half where Procolotis mariae was removed, there was a visible increase in sedimentation.
Carbon Cycling: The study showed the fish is critical for the carbon cycle. Pre-manipulation showed no difference between sites. Post-removal results included:
Increased organic standing crop (likely algae).
Significantly decreased organic carbon flux (the movement of carbon between different habitats).
The fish also migrate seasonally, which is important for cycling carbon throughout the ecosystem.
Human Impact: Local populations harvest these fish for subsistence. Over-harvesting has led to a shrinking trend in the body size of the fish and the mesh size of nets used, which threatens to diminish the fish's functional contribution to the carbon cycle.
Nutrient Runoff and Ecosystem Remediation
The Mississippi/Missouri River Example: These rivers run through the agricultural "breadbasket" of the US. High concentrations of nitrogen from crops and pasture enter the system, leading to eutrophication.
The Eutrophication Process:
Excess nutrients cause algae blooms.
When the algae die (due to lacking consumers), microbes decompose the algae.
This decomposition uses up oxygen, causing hypoxis and leading to fish die-offs and damaged coastal habitats.
Proposed Solutions:
Natural Floodplains: Recreating floodplains allows water to flatten out and interact with macrophytes. This retains more nitrogen in the soil and plants rather than it moving quickly through a ditch.
Bioreactors: Used in New Zealand, these are biological components (holes filled with wood chips) that treat agricultural runoff. Diverse microbial communities on the wood chips use the carbon source to perform denitrification, consuming the excess nitrogen.
Constructed Wetlands: These address several issues associated with runoff. Planting species like Carex helps shade the stream, which deters algal growth by reducing light penetration.
Lake Ellesmere (Tehuahara) and Hysteresis
Lake Ellesmere (Tehuahara) shifted from a clear state with macrophytes to a turbid, sedimented state with high nutrients following a major storm and compounded land-use changes.
Changes in Function:
A shift in who conducts photosynthesis (from macrophytes to algae).
Loss of wave attenuation and sediment stabilization formerly provided by macrophytes.
Increased turbidity prevents light from penetrating deeper areas, which may prevent macrophyte recolonization.
Restoration Challenges: Changing it back requires addressing hysteresis. Hysteresis is the phenomenon where the amount of energy (or nutrient reduction, such as phosphorus) required to return a system to its initial stable state is significantly greater than the energy that caused the initial shift.
Future Research Directions
Understanding the importance of functional trait diversity as a driving factor for function.
Studying how biodiversity affects stability and function at different scales (from genetics to landscapes).
Applying these concepts across different ecosystems and trophic levels.
Addressing the "chicken and egg" argument: Does diversity cause function, or does function cause diversity?
Assessing the impact of global climate change and human economic decisions on ecosystem functioning.
Advancing restoration techniques to recover original ecosystem functions.
Questions & Discussion
Question Regarding Fungi: A student asked if fungi were excluded from the definition of ecosystem function, as they were not mentioned in the list of plants, animals, and microbes.
Response: The professor clarified that fungi are indeed a critical part of the ecosystem and are implied in the definition (often categorized with microbes or as their own group), noting that many people in the department specifically work on fungi.
Discussion on Traits: When asked what a trait of a species is, students suggested behavioral traits like migration or predator-prey behaviors.
Discussion on Lake Ellesmere: A student proposed replanting the lake in small chunks to help it recover. The professor noted that while this is a common idea, its success depends on factors that might keep the lake in its alternative stable state and the risk of future storms.
Post-Lecture Exchange: A student mentioned collecting insects for a course () and discussed finding a fly. The professor mentioned the possibility of using a flyswatter for collections.