Community Ecology Notes

Community Ecology

• Community: All interacting species in a defined area.

• Fitness: Ability to survive and produce viable, fertile offspring.

• Biologists study species interactions by analyzing the effects of one species on the fitness of another.

Three Key Themes in Species Interactions

• Species interactions may affect species distribution and abundance.

• Coevolution: A pattern of evolution where two species influence each other's adaptations.

• The outcome of interactions among species is dynamic and conditional.

Fitness Effects

• Fitness benefit: (+)

• Fitness cost: (-)

• No effect on fitness: (0)

• Intraspecific interactions: individuals in the same species

• Interspecific interactions: two different species

Competition

• Lowers the fitness of both individuals involved and may occur when a shared resource is limited.

• Intraspecific competition: between members of the same species, results from density-dependent growth

• Interspecific competition: when members of different species use the same limiting resources

• Fitness trade-off: investment in resource acquisition comes at the expense of qualities like drought tolerance, or susceptibility to disease and predation

• Interspecific competition example: orange-crowned warbler and Virginia’s warbler

Niche and Competition

• Complete niche overlap: Species occupy the same niche and compete for all resources

• Niche: range of resources that the species is able to use or the range of conditions it can tolerate

• Experiment: Two species of paramecium exhibit logistic growth when grown separately. When grown together, one species goes to extinction and the other undergoes logistic growth.

• Competitive exclusion principle: multiple species within the same niche cannot coexist

Resource Partitioning

• Resource partitioning: weaker competitor should be able to persist in the area of non-overlap

• Fundamental niche: total theoretical range of environmental conditions that species can tolerate

• Resource partitioning: differentiation of niches that enables similar species to coexist in a community

• Realized niche: portion of fundamental niche that species occupies, given limiting factors such as competition with other species

Mechanisms of Coexistence

• Niche Differentiation

• Character displacement: -/- interactions → strong natural selection → lower frequency of alleles associated with lower fitness in population over time as these individuals have fewer surviving offspring

• Niche differentiation: change in resource use

Exploitation

• Exploitation: (+/-) interaction that occurs when one organism eats part or all of another

• Exploitation drives coevolution: reciprocal adaptation, can occur in repeating cycles

• Consumers evolve traits that increase their efficiency

• In response, prey evolve traits for evasion

Defensive Adaptations

• Mechanical defense (e.g., porcupine quills)

• Chemical defense (e.g., poison dart frog)

• Aposematic coloration: warning coloration (e.g., skunk)

• Cryptic coloration: camouflage (e.g., canyon tree frog)

• Batesian mimicry: A harmless species mimics a harmful one (e.g., nonvenomous hawkmoth larva mimicking venomous green parrot snake)

• Müllerian mimicry: Two unpalatable species mimic each other (e.g., yellow jacket and cuckoo bee)

Mimicry in Predators

• Mimicry has also evolved in many predators to enable them to approach prey

• Example: mimic octopus can take on the appearance and movement of more than a dozen marine animals

Herbivory

• Herbivory (+/-): harms, but does not usually kill the plants and algae

• Coevolution: A change in one species acts as a new selective force on another, forcing reciprocal adaptation

• Leads to diverse adaptations

  • Mechanical defenses: Spines and thorns

  • Chemical toxins

  • Other defense compounds that are not toxic to humans but may be distasteful to herbivores (e.g., peppermint, cloves, and cinnamon)

Parasitism

• Parasitism (+/-): invading and consuming hosts, reproducing, and invading a new host

• Endoparasites: live within host body

• Ectoparasites: live on external surface of host

• Example: Nematode infected Ant abdomen appears red and behavior changes to posture, look like berries. Bird eats ant, nematodes can complete their life cycle before being shed in bird feces that are subsequently eaten by ants.

Mutualism

• Mutualism (+/+): not altruistic, results from mutual self-interest

• To maximize benefits, species must minimize costs

• Cheat: plants have showy flowers but produce no nectar; pollinators receive no reward for pollinating these plants

• Context dependent: plants support of nitrogen fixing bacteria is beneficial only when nitrogen is scarce

Commensalism

• Commensalism (+/0) is challenging to demonstrate because it is hard to show an absence of effect on fitness

• Positive interactions can have a significant influence on the structure of ecological communities

• Example: black rush (juncus, a species of flowering plant) affects community diversity in New England salt marshes by making the soil more hospitable for other plant species

Pathogens

• Pathogens alter community structure locally and globally

• Zoonotic pathogens: transferred to humans from other animals. Cause of ¾ of emerging human diseases

• Human activities are transporting pathogens around the world at unprecedented rates

• Vector: intermediate species which transfers disease. Example: recent studies identified two species of shrew as the primary hosts of the pathogen for Lyme disease

Community Structure

Community structure has four key attributes:

• The total number of species

• The sum of interactions among all species

• The relative abundance of those species

• The physical attributes of the community

Species Diversity

• Species richness: number of species in a community

• Species diversity: species’ relative abundance

• Relative abundance is the proportion each species represents of all individuals in the community

Shannon Diversity Index

The Shannon diversity index (H) is calculated as follows:

H = - \sum{i=1}^{S} (Pi \ln P_i) ( Just look at slide 19)

• P_i: relative abundance of species i

• S: number of species

• \ln: natural logarithm

Example:

• Community 1 = –4(0.25 ln 0.25) = 1.39

• Community 2 = –[0.8 ln 0.8 + 2(0.05 ln 0.05) + 0.1 ln 0.1] = 0.71

Diversity and Community Stability

Higher-diversity plant communities are generally:

• more productive; they produce more biomass

• more stable year to year in their productivity

• better able to withstand environmental stresses

• more resistant to introduced species

Trophic Structure

• Trophic structure: the feeding relationships in a community

• Food chain: linked consumption interactions

• Food web: all consumption interactions in community

• Trophic level: position an organism occupies in a food chain

Energetic Hypothesis

• Energetic hypothesis: food chain length is limited by inefficient energy transfer

• about 10% of the energy stored in organic matter converted to organic matter at the next trophic level

• food chains are longer in habitats with higher production

• Example: 100 kg of plant material can support about 10 kg of herbivore biomass and 1 kg of carnivore biomass

Keystone Species

• Keystone species: impact is larger than biomass or abundance indicates, disproportionately impacts diversity

• Sea stars prey on mussels, so when they are removed overgrowth of mussels reduces diversity in community

Foundation Species

• Foundation species have strong effects due to their large size or high abundance

• They often have community-wide effects because they provide habitat or food

• may be competitively dominant—superior in exploiting key resources such as space, water, nutrients, or light

Ecosystem Engineers

• Ecosystem engineers: create or dramatically alter their physical environment

Top-Down and Bottom-Up Control

• Bottom-up control: the abundance of organisms at each trophic level is limited by nutrient supply or food availability at lower levels

• Top-down control: abundance of organisms at each trophic level is controlled by the abundance of consumers at higher trophic levels

Application of Top-Down Control

• Ecologists can apply top-down control to improve water quality in lakes with a high abundance of algae

• in lakes with three trophic levels, removing fish improves water quality by increasing the density of zooplankton, which decreases algal density

Nonequilibrium Model and Disturbance

• Nonequilibrium model describes communities as constantly changing after disturbance

• Disturbance: disruption to a community. Example: forest fires, floods, and disease epidemics.

• Impact is influenced by:

  • Type of disturbance

  • Frequency of disturbance

  • Severity of disturbance

Disturbance Regime

• Disturbance regime: predictable frequency and severity

• Giant sequoia trees live more than a thousand years, so tree rings provide living history of fire disturbance regime

• Tree rings were scarred by fires, showing established that fires are quite frequent

• Frequent fires prevent fuel accumulation and limit fire size.

• Disturbance regime is essential for stable community composition

Intermediate Disturbance Hypothesis

• Intermediate disturbance hypothesis: moderate disturbance fosters greater diversity

• High levels of disturbance exclude many slow-growing species

• Low levels of disturbance allow competitively dominant species to exclude less competitive ones

Succession

• Succession is the recovery that follows a severe disturbance

• Primary succession: disturbance removes soil and organisms

• Secondary succession: disturbance leaves the soil, seeds and soil microorganisms intact → faster recovery

Successional Pathway

• Successional pathway: specific sequence of species that appears over time

• Early succession: small short-lived species, distant seed dispersal

• Late succession: Long lived, large, good competitors for resources such as light and nutrients

• Factors determining pattern and rate of species replacement:

  • The traits of the species involved

  • How the species interact

  • Historical and environmental circumstances, such as the size of the area involved and weather conditions

Role of Species Traits

• Pioneering species, the first organisms to arrive at a newly disturbed site, tend to be “weedy:”

  • adapted for growth in disturbed soils

  • dispersal capability

  • withstand harsh conditions

  • devote energy to reproduction not competition

Role of Species Interactions

• Facilitation: early-arriving species reduce temperatures and increasing humidity, favoring conditions for other species

• Tolerance: later, existing species do not affect the probability that subsequent species will become established

• Inhibition: presence of one species inhibits the establishment or regrowth of another

Role of Chance and History

• Pattern and rate of succession depend on:

  • historical and environmental context

  • weather or climate conditions

Case History: Glacier Bay, Alaska

• Glacial recession is occurring at Glacier Bay, creating ice free zones of succession.

• Tree ring data shows three successional pathways have occurred in this area.

Geographic Patterns in Species Richness

• General correlation between species richness and two abiotic variables:

  • Geographic area occupied by community

  • Latitude of community

Evapotranspiration

• Evapotranspiration: evaporation of water from soil, transpiration of water from plants

• Much higher when temperature is hot and rainfall is abundant

• Species richness of plants and animals correlates with measures of evapotranspiration

Latitude and Species Diversity

• Species diversity declines as latitude increases

Theory of Island Biogeography

• Smaller Islands are less species rich than larger islands

• Larger habitats have more niches and should support higher numbers of species

• Diversity ↓ Immigration

• Diversity ↑ extinction due to ↑ competition

• Large islands → ↑ Immigration

• Small islands → ↑ extinction