l29-Ecological Dynamics: Trophic Cascades, Diversity, and Keystone Species

REVIEW OF THE HSS THEORY AND TROPHIC CASCADES

• Conceptual Overview of HSS Theory: The Hairston, Smith, and Slobodkin (HSS) theory explains how predators influence food webs through top-down control. This phenomenon is known as a trophic cascade.

• Mechanics of the Trophic Cascade:

  • Top Predators: Using the example of a $lion$, predators consume herbivores.

  • Herbivore Suppression: The consumption of herbivores releases the grazing pressure typically placed on primary producers ($grasses$).

  • The Green World Hypothesis: This release of pressure results in a “greener world” where plant life flourishes due to predator-mediated herbivore control.

• Nuancial Developments in Ecosystem Greenness: It is recognized that a green world is not solely the result of top-down predation. Plant-driven factors, such as specific defenses, also play a significant role.

  • Plant Defenses: Plants may be “prickly” or “not taste great” (tasting bad), which contributes to the maintenance of biomass.

  • Growth-Defense Trade-off: There is a direct relationship between the energy a plant invests in defense and its individual growth rate or the growth rate of the species. High investment in defense typically leads to slower growth.

  • Parallels in Animal Prey: Similar trade-offs exist in animals; prey species may alter their growth rates or movement patterns to defend against predators, impacting their overall development.

HERBIVORY AND DEFENSE SPECIALization IN NEW ZEALAND

• Vulnerability to Introduced Species: In New Zealand, many plants were not necessarily undefended, but they were unequipped for the specific pressures of introduced herbivores. This highlights that individual specialization in defense is a critical factor in community stability.

• Limitation by Food Quality: While the world appears green, herbivores are often limited by food quality rather than just carnivore density. If plants are unpalatable or structurally defended, they remain uneaten even if predators are absent.

• Impact of Herbivore Addition: The introduction of herbivores can rapidly and drastically alter vegetation composition.

TROPHIC DYNAMICS IN TERRESTRIAL VS. AQUATIC ECOSYSTEMS

• Comparing Systems: Studies manipulating predator presence across different ecosystems reveal varying responses in prey and plant populations.

• Numerical Responses: In statistical models, responses overlapping with $0$ indicate no significant effect. Aquatic environments often show the largest and most prevalent trophic cascades compared to terrestrial ones.

• Consumption Rates of Primary Producers:

  • Aquatic Algae: These are removed in much higher proportions by herbivores than terrestrial counterparts.

  • Terrestrial Plants: These are less consumed due to structural and chemical defenses ($macrophytes$ and $macro-invertebrates$ in terrestrial systems have higher resistance).

  • Phytoplankton and Zooplankton: In aquatic systems, these organisms often lack robust defenses, behaving more like the herbivores in terrestrial food webs (easily consumed).

• Interactions: Terrestrial systems may experience a “trophic trickle” while aquatic systems experience a full “trophic cascade.”

REVIEW QUESTIONS ON HERBIVORY AND POPULATION IMPACTS

• Plant Compensation Mechanisms: Beyond physical/chemical defenses (prickly/bad taste), plants can compensate for herbivory by moving carbohydrates from their root systems to their leaf systems to facilitate rapid regrowth.

• Effects on Understory and Development:

  • Mature plants may remain while saplings and small plants in the understory are consumed, effectively reducing recruitment.

  • Developmental Trade-offs: In a study of $trout$ and $stoneflies$ impacting $mayflies$, the $mayflies$ developed smaller heads as a defensive trade-off.

• Deviations from HSS Theory:

  • Trophic Levels: The HSS view often assumes odd numbers of trophic levels. An even number of trophic levels or interactions like intraguild predation can change the outcome.

  • Bottom-Up Effects: Nutrient availability fuels primary production. Using the example of nettle aphids, fertilized fields led to an explosion in $aphid$ populations, which subsequently caused an explosion in $ladybug$ (predator) populations. This demonstrates that energy flows from the bottom up can be just as influential as top-down control.

BIODIVERSITY METRICS: RICHNESS AND EVENNESS

• Defining Biodiversity: Diversity is measured using two primary concepts: richness and evenness.

  • Richness: The total number of different species present in a given area.

  • Evenness: The relative abundance of each species. High evenness means the individuals are spread relatively equally among the species present.

• Diversity Scale Examples:

  • Low Diversity ($2$ or $3$ on a scale of $10$): A forest where trees look the same and specific species (like crown ferns) dominate the understory.

  • High Diversity ($10$ on a scale of $10$): Examples include the Amazon Rainforest or Costa Rican tropics where plants occupy almost every available space.

• Case Studies in Evenness:

  • Innis River: A system dominated by a single species (low evenness).

  • Cat's Hill River: A system where abundance is spread across species, with no significant drop-off in abundance until the $sixth$ species by rank.

EXPERIMENTAL DYNAMICS IN TIDE POOL COMMUNITIES

• Jane Lubchenco’s Study: Conducted in the Pacific Northwest (and replicated in Kaikoura, NZ), observing tide pool algal communities and the snail predator Litterina littorea.

• Algal Types:

  1. Ulva (Sea Lettuce): An ephemeral species that fluctuates in abundance without strict seasonal cycles.

  2. Enteromorpha: A highly palatable species for snails that grows quickly.

  3. Chondrus (Red Algae): An unpalatable species containing chemical defenses like tannins and phenols, making it hard to consume.

• Manipulation Results:

  • Control Pools: $Chondrus$ (unpalatable) is abundant; others are reduced.

  • Addition of Snails: Rapid drop in $Enteromorpha$ cover because it is high-quality food.

  • Removal of Snails: $Enteromorpha$ quickly outcompetes $Chondrus$ for limited rock space. Because $Enteromorpha$ invests less in defense, it grows faster and dominates when predation is absent.

THE INTERMEDIATE DISTURBANCE HYPOTHESIS (IDH)

• Predation as Disturbance: In tide pools, snail density represents a form of disturbance.

• Richness and Predation Density:

  • Low Predation Density: Low species richness; the dominant competitor ($Enteromorpha$) outcompetes others.

  • Intermediate Predation Density: Peak species richness; snails control the dominant species enough to allow others to coexist.

  • High Predation Density: Low species richness; snails consume almost everything, leaving only a few resistant species or very low abundances.

• Environmental Buffering: In exposed platforms (non-tide pools), wave exposure acts as a harsh environmental factor. Here, richness peaks when snails are absent because the waves already mitigate competition. Adding snails in this harsh environment creates an additive negative effect, simply reducing richness further.

KEYSTONE SPECIES AND ECOSYSTEM ROLES

• Keystone Predation: Defined by a predator that has a disproportionately large impact on its community relative to its biomass.

• Pisaster ochraceus (Ochre Sea Star): A classic example from intertidal rocky shores.

  • It selectively consumes Mytilus californicus (a mussel species).

  • When the sea star is removed, the mussels outcompete all other species for space, causing species richness to decline precipitously.

  • This is known as predator-mediated coexistence.

• Other Types of Keystones:

  • Ecosystem Engineers: Species like beavers that physically modify the environment (building dams) to create new habitats for other species.

  • Seed Dispersers: Examples include the Cassowary in North Queensland (dispersing “cassowary plums”) and the Kereru in New Zealand. These species are essential for the repopulation of specific large-seeded trees.

• Dominant (Foundation) Species: Unlike keystones, these have high relative biomass and help define the habitat (e.g., kelp forests, mangroves, coral reefs).

  • Didymo (Diatom): In New Zealand fresh waters, this is a dominant species with high biomass that clogs waterways and impacts hydrodynamics.

CONSERVATION APPLICATIONS AND COMPLEX FOOD WEBS

• Impact of Species Loss:

  • In simple webs, losing a primary producer can cause total collapse.

  • In complex webs, losing one plant may have little effect, but losing a keystone predator can release herbivores to overconsume several other plant species.

• Historical Speculation: The Haast’s Eagle in New Zealand is a potential example of a predator whose fate was tied to its prey (the Moa), though its exact role as a keystone is speculative.

• Trophic Downgrading: A concept explored in a $2011$ paper by James Estes, describing the global consequences of losing top predators and the subsequent degradation of ecosystem functions.