Predation
Predation, Herbivory, and Parasitism Overview
Information from: Jon Herron, Eli Meir. 2010. In SimUText Ecology. Simbio.com.
Compiled by: Dr. Greg George
Nat. History of Exploitation
Predation and herbivory are both classified as forms of exploitation.
These interactions can be represented with a symbolism of (+/-), indicating a beneficial interaction for one participant and a detrimental impact for the other.
Parasitism
Complicated Nature of Parasitism
The life cycle of Ribeiroia ondatrae:
Phase of Life Cycle:
Adult Parasite: Lays eggs which hatch into Miracidia.
First Intermediate Host: Snail, which harbors the developing parasite.
Definitive Host: Bird, where the adult parasite will reproduce.
Frog: Hosts Cercariae, another life stage of the parasite.
Types of Parasites
Endoparasites:
Defined as organisms that live inside a host.
Example: Female Strepsiptera, which resides internally within its host.
Ectoparasites:
Defined as organisms that live on the exterior surface of a host.
Example:
Gnathiid parasites that can affect fish populations greatly.
Statistical measure: 10 parasites per fish on average in some ecosystems.
Surprising dynamics noted between populations with and without cleaner fish in reef environments.
Impact of Parasitism
Parasites can profoundly influence host populations by affecting their health and reproductive success.
Differences from Parasitoids
Parasitoids: Organisms that do not directly kill their host, unlike traditional parasites; they often induce changes that lead to host demise.
Herbivory
Example: Snowshoe hare practices coprophagy, re-ingesting fecal pellets to extract maximal nutrients without necessarily killing the plant from which these nutrients are derived.
Strategies of Plant Defense Against Herbivory
Plants employ various strategies to deter herbivores, which can be grouped into four main categories:
Chemical Defenses:
Production of noxious or toxic chemicals detrimental to herbivore health.
Mechanical Defenses:
Development of physical structures such as thorns, which inhibit herbivory.
Nutritional Defenses:
Production of plant structures with lower nutritional value to make them less appealing to herbivores.
Tolerance:
Adaptations allowing plants to regrow quickly after being grazed.
Summary of Plant Defense Strategies
Strategy:
Chemical: Producing harmful substances to deter herbivores.
Mechanical: Having thorns or other non-edible structures.
Nutritional: Having less nutritious tissues.
Tolerance: Quick regrowth capability post-herbivory.
Predation
Example: Canada Lynx, illustrates the behavior of ambush predators.
Predator Defense Strategies
Predators also evolve strategies to defend against being eaten which include:
Chemical:
Producing harmful or unpalatable chemicals.
Physical:
Developing structures for protection, such as shells.
Aposematism:
Using warning coloration to signal unpalatability.
Crypsis:
Camouflaging to evade detection.
Mimicry:
Mimicking the appearance or behavior of other species to avoid predation.
Behavioral:
Exhibiting acts that reduce predation risk.
Predator and Prey Dynamics
Historical Perspectives
Notable theories include those by Elton regarding cycles of abundance being influenced by solar radiation.
Keith's hypotheses addressed the impacts of overpopulation, diseases, and starvation due to food scarcity on prey populations.
Data on Predator-Prey Relationships
Graphical data on snowshoe hare and lynx populations exemplifies historical interactions from the years 1845 to 1935 showing increases and declines in both populations.
Role of Food Supply
Snowshoe hares inhabit boreal forests dominated by conifers and depend on understory shrubs for sustenance.
Example: food availability can fluctuate significantly, e.g., a decrease from 530 kg/ha to 160 kg/ha over 4 months.
Changes in feeding behavior following browsed shoots can lead to increased chemical defenses in plants, reducing edible food supplies further.
Role of Predation
Predation accounts for a significant amount of mortality, ranging from 60% to 98% during peak hare population densities.
Population Feedback Mechanisms
Increased hare populations can lead to food shortages, resulting in starvation and weight loss, which in turn heightens predation risk.
Lotka-Volterra Model and its Extensions
General Dynamics
The classic Lotka-Volterra model indicates that predators usually do not entirely eradicate prey communities.
Small prey populations limit the capacity for large predator populations due to food resource constraints.
Phase Planes
Phase planes illustrate the relationships of predator and prey populations over time.
Isoclines represent stable population dynamics in ecological relationships.
Added Complexity to Traditional Models
Realism of Assumptions
Many assumptions within the Lotka-Volterra model are unrealistic, including:
No crowding effects (density dependence).
Uniform distribution and interaction likelihood among prey.
A sole reliance of predators on a single prey species.
Instantaneous predation without handling time or consideration of immigration/emigration dynamics.
Prey and Predator Density Dependence
Addressing density dependence in prey populations is critical for realistically modeling population dynamics, considering factors like intraspecific competition.
Prey Refuges
Role of Refuges
Prey refuges provide areas of safety that stabilize prey populations against overpredation risk.
Types of Prey Refuges
Various forms of refuges exist, such as burrows for terrestrial prey and protective behaviors among zooplankton and mast-seeding in plants to promote survival against herbivores.
Functional Response of Predators
Predatory Behavior Dynamics
The ability for predators such as the Lynx to catch prey like the hare does not scale linearly with prey abundance due to handling times.
Comparisons to spiders indicate a linear catching efficiency due to webbing.
Seals are noted for their diet-switching capabilities, enabling them to maintain high catch rates amongst variable salmon populations.
Efficiency of Predation Strategies
Example of SimMantids demonstrates effective hunting with short handling times and efficient foraging strategies impacting prey populations.
Evolutionary Arms Race
Common Occurrences
Evolutionary arms races are predominantly observed in parasitic interactions.
Coevolutionary Dynamics
Case Study: Predator-Prey Interactions
Example: Garter snake and newt relationship showcasing resistance levels and TTX (toxin) concentrations.
Red Queen Hypothesis
This hypothesis suggests that species which evolve effectively will persist in the face of changing competitors.
Sexual reproduction is highlighted as a factor that increases evolutionary rates through genetic recombination, aiding survival in variable environments.
Observations in topminnow populations indicate that genotypes which reproduce asexually have higher parasite loads.
Conclusion
The detailed interactions between predation, herbivory, and parasitism are critical components of ecological frameworks that illustrate the continual evolutionary exchanges and population dynamics that are inherent in natural environments.