Ecological Interactions and Diversity

Species Interactions in Ecosystems

• Types of Species Interactions:

  • Predator-Prey Relationships:

  • Dependency between species for survival, primarily through food chains and food webs.

  • Example: Wolves (predators) and rabbits (prey). The populations of each species fluctuate cyclically due to their interactions.

  • Rabbits experience exponential growth when predator populations are low, but can crash dramatically when predator populations are high leading to resource depletion.

  • Importance of Producers:

  • Producers (like plants) form the base of the ecosystem, capable of converting sunlight, air, and water into food.

• Effects of Human Intervention:

  • Example of overpopulation due to human actions: Caribou on Nova Scotia islands without natural predators, leading to overpopulation and subsequent starvation.
  • In the U.S. Midwest, human encroachment led to reductions in predator populations (e.g., wolves, mountain lions), which resulted in overgrown prey populations (e.g., deer), necessitating artificial hunting seasons to control numbers.

• Symbiotic Relationships:

  • Mutualism: Benefits both species, e.g., cleaner fish and sharks.
  • Commensalism: One benefits, the other is not affected, e.g., barnacles on whales.
  • Parasitism: One benefits at the expense of the other, e.g., ticks feeding on mammals.

• Competition for Resources:

  • Interspecific Competition: Negative impact for both unrelated species competing for the same resources, like hyenas and lions competing for prey (e.g., gazelles).

  • Results in wasted energy and potential injury for competing species.

  • Intraspecific Competition: Competition within the same species, can impact individual fitness even though it may promote population survival through natural selection.

• Niche Concept:

  • Fundamental Niche: The potential role a species could occupy in an ecosystem without competition.
  • Realized Niche: The actual role a species occupies due to competition, often smaller than the fundamental niche.
  • Example: Two species of barnacles with overlapping niches partitioning the habitat based on competition for space and resources.

• Ecosystem Health and Diversity:

  • Healthy ecosystems have a balance of species diversity and biomass.
  • A monoculture (like a cornfield) has high biomass but low diversity, making it less resilient to disease compared to a biodiverse ecosystem (like a prairie) that may have lower biomass but is more resilient to environmental stressors.

• Simpson's Diversity Index:

  • An ecological tool to measure species diversity in an ecosystem, calculated as:
    1 - \sum \left(\frac{n}{N}\right)^2
  • Where $n$ is the number of individual species and $N$ is the total number of individuals.
  • Higher values indicate greater diversity.

• Applying Simpson's Diversity Index:

  • Count individuals of each species in an ecosystem, calculate $n$ and $N$, use the formula to find the diversity index, wherein a value closer to 1 indicates a more diverse and healthier ecosystem.