Detailed Notes -2- Chapter 11 Organisms and Populations

11.1.3 Life History Variation

1. Reproductive Fitness and Darwinian Fitness:

   • Populations evolve to maximize their reproductive fitness or Darwinian fitness, which is often represented by a high intrinsic rate of natural increase (𝑟r).

2. Reproductive Strategies:

   • Organisms evolve different reproductive strategies based on selection pressures in their habitat.
   • Some organisms breed only once in their lifetime (e.g., Pacific salmon, bamboo), while others breed multiple times (e.g., most birds and mammals).

3. Offspring Quantity and Size:

   • Organisms also vary in the number and size of offspring they produce.
   • Some species produce a large number of small-sized offspring (e.g., oysters, pelagic fishes), while others produce a small number of large-sized offspring (e.g., birds, mammals).

4. Maximizing Fitness:

   • Ecologists study which life history traits are desirable for maximizing fitness in a given environment.
   • Life history traits evolve in relation to constraints imposed by abiotic (non-living) and biotic (living) components of the habitat.

5. Research on Life History Traits:

   • Evolution of life history traits in different species is a significant area of research in ecology.
   • Ecologists investigate how these traits contribute to the fitness and survival of organisms in their habitats.

11.1.4 Population Interactions

1. No Single Species Habitat: There is no habitat on Earth that is inhabited by just a single species. All organisms, even those that make their own food like plants, require interactions with other species for various purposes such as nutrient cycling, pollination, and more.

2. Interspecific Interactions:

   • Mutualism (+/+): Both species benefit from the interaction. Examples include pollination by insects and flowering plants, or the relationship between nitrogen-fixing bacteria and leguminous plants.
   • Competition (-/-): Both species compete for limited resources such as food or space. This can lead to competitive exclusion, where one species outcompetes and eliminates another.
   • Parasitism (+/-): One species benefits (parasite) at the expense of the other species (host). Examples include tapeworms in the gut of animals or mistletoe plants on trees.
   • Predation (+/-): One species (predator) feeds on another species (prey). This interaction is crucial for energy transfer in ecosystems and helps control prey populations.
   • Commensalism (+/0): One species benefits while the other is neither harmed nor benefited. Examples include orchids growing on tree branches or barnacles on whale skin.

3. Predation:

   • Predation is essential for energy transfer across trophic levels and helps regulate prey populations.
   • Predators play roles in maintaining species diversity and ecosystem stability by controlling prey populations.
   • However, overexploitation by predators can lead to prey extinction, affecting the predator as well.

4. Competition:

   • Competition occurs when species compete for the same limited resources.
   • It can lead to competitive exclusion or resource partitioning, where species evolve mechanisms to coexist without direct competition.

5. Parasitism:

   • Parasites benefit at the expense of their hosts, affecting host survival, growth, and reproduction.
   • Parasites have evolved various adaptations to successfully parasitize hosts and complete their life cycles.

6. Commensalism:

   • Commensal interactions benefit one species without harming the other.
   • Examples include orchids on trees, barnacles on whales, or cattle egrets and grazing cattle.

1. Lichens: Lichens represent a mutualistic relationship between a fungus and photosynthesizing algae or cyanobacteria. The fungus provides a protected environment and minerals, while the algae or cyanobacteria provide food through photosynthesis.

2. Mycorrhizae: These are associations between fungi and the roots of higher plants. The fungi help plants absorb essential nutrients from the soil, while the plants provide the fungi with carbohydrates.

3. Plant-Animal Mutualism:

   • Pollination: Plants rely on animals like insects, birds, or bats for pollination. They offer rewards such as pollen and nectar to attract pollinators. Co-evolution ensures a tight relationship between plants and their pollinators, as seen in fig trees and their partner wasp species.
   • Seed Dispersal: Animals help in seed dispersal by eating fruits and spreading seeds in different locations. In return, plants offer nutritious and attractive fruits to encourage animals to disperse their seeds effectively.

4. Orchid Pollination Strategies: Orchids exhibit diverse floral patterns to attract specific pollinator insects like bees and bumblebees. Some orchids employ “sexual deceit” to ensure pollination. For example, the Mediterranean orchid Ophrys mimics the appearance of a female bee to attract male bees for pollination.