- Community ecology is concerned with the interaction of populations
- Communities and ecosystems are described with regards to the interactions between organisms and environment, biodiversity, and species diversity and composition (identity of species)
Species Diversity in Ecosystems
- “Number of species in an ecosystem or across the biosphere”
Species Diversity depends on:
- Species Richness: number of different species
- Relative Abundance:
- Benefits of species diversity: ecosystem with higher species diversity can better resist environmental change
- Increased productivity and stability of communities (can survive better), produce more biomass (total mass of all organisms in a habitat), more resistant to introduced species
Structure of Communities:
- Simpson’s Diversity Index: measures species diversity
- Higher index value = more diverse
- Based on random samples of environment
Interactions in Communities
- Competitive Exclusion principle (Gause’s principle): Two species cannot occupy the same niche (when resources are limiting)
- When two species compete for the same resources or occupy the same niche, one is probs gonna be more successful and 2nd is eliminated
- Ex: when two species of Paramecium competed for same resources, one outcompeted other and grew more rapidly
- Competition is an interaction that can affect how populations access energy and matter
- Can result in change in community structure
- One form of interaction is interspecific competition (competition between different species). Different ways to resolve competition & enable similar species to coexist in a community
- When two species compete for the same resources or occupy the same niche, one is probs gonna be more successful and 2nd is eliminated
- Resource partitioning: organisms divide resources by pursuing slightly diff resources or getting them in slightly diff ways,
- Realized niche: the actual space an organism inhabits as a result of competition → allows two species to coexist
- Fundamental niche: niche that an organism occupies w/o competing species
- Ecological niche: specific set of biotic and abiotic resources that an organism uses in its environment
- Ex: temp tolerance range, habitat, time of eating
- Species can partition their niches in space & time
- Character displacement (niche shift): natural selection favours a divergence of characteristics when two similar species inhabit the same environment
- Ex: finches with diff beaks (thru evolution) suited to diff food → minimize competition
Predation
- Form of community interaction when an animal hunts another organism
- True predator kills and eats other animals
- Parasite feeds and lives on host’s tissues, weakening it
- Parasitoid is an insect that lays its eggs in a host
- Herbivore is an animal that eats plants
Predator-Prey Interactions
- Increase/decrease in predator occurs slightly after increase/decrease in prey
- Increase in predator → decrease in prey
- Trophic cascade: negative effect of removal of key species
Symbiosis
- When two species live together in close contact during a portion of their lives; examples of interactions among populations include
- Positive
- Mutualism: when both species benefits (+,+)
- Lichens: symbiotic associates of fungi and algae
- Algae produce sugar and fungi provide water & protection
- Can also be parasitic
- Mycorrhizae: mutualistic association of fungi with roots of plants
- Plants provide sugar and filaments of fungi increase surface area of roots, facilitate absorption of water and minerals (especially phosphorus)
- Algae produce sugar and fungi provide water & protection
- Commensalism: one species benefits, 2nd unaffected (+, 0)
- Birds build nests in trees
- Negative Interactions
- Parasitism: parasite benefits, host is harmed (+, -)
- Tapeworms in digestive tract of animals
- Predator-Prey (+/-)
- Competition: Different species compete for the same resource that limits the survival and reproduction of both species (-/-)
- Ex: weeds compete for nutrients
- Although negative for one species, might benefit another species by providing new niches or freeing up resources
Disturbances
- Disturbances: events that change a community by removing organisms from it and altering resource availability.
- Ex: Humans have altered much of Earth’s surface by replacing natural terrestrial communities with urban and agricultural one
- Some organisms depend on periodic disturbances
- Ex: pine that depend on burning for reproduction
- Disturbances that threaten stability include fires, floods, disease, and human effects
- El Nino: trade winds and upwelling that promote bottom up effect stop; algae declines → then consumers → collapse in food webs
- Meteor Impacts and volcanic eruptions: increase amount of matter → reduce solar radiation → less primary production
- Plate tectonics (continental drift): describes movement of land masses (plates) over surface of earth
- Plates collide and move to new latitudes → earthquakes, create volcanoes, and form mountains
- Environmental conditions change → create new niches for speciation
Characterizing Disturbance
- Vary in frequency and severity among communities
- High level of disturbance = frequent or intense disturbance
- Low disturbance levels = low frequency or low intensity of disturbance.
- Intermediate disturbance hypothesis: moderate levels of disturbance → open up habitats for less competitive species → greater species diversity than high or low levels of disturbance
- Small-scale disturbances can create patches of different habitats across a landscape, which help maintain diversity in a communitys
Coevolution
- The tit-for-tat evolution in one species in response to adaptation in another species
- Results from natural selection of characteristics that promote most successful predators and most elusive prey leads to coevolution of predator and prey
- Secondary Compounds: toxic chemicals produced in plants that discourage herbivores; some herbivores have adaptations that allow them to tolerate them
- Camouflage (cryptic coalition): physical characteristics or behavior that allows prey or predator to hide
- Aposematic coloration (warning coloration): pattern or coloration that warns predators that prey are to be avoided
- Associate yellow and black body of bees with danger
- Mimicry : when species resemble one another in appearance. 2 kinds
- Mulelerian mimicry: animals with similar defense mechanism share same coloration
- Mutualistic: effective because a single pattern shared among many animals is more easily learned by a predator than would be a different pattern for each species
- Ex: 2 diff species of butterflies, toxic dart frogs
- Batesian mimicry: harmless animal mimics the coloration of an animal that does have a defense
- Parasitic relationship: mimic benefits while mimicked is negatively affected
- Ex: harmless beetle mimics noxious beetle
- Parasitic relationship: mimic benefits while mimicked is negatively affected
- Mutualistic: effective because a single pattern shared among many animals is more easily learned by a predator than would be a different pattern for each species
- Pollination: of flowers result of coevolution of traits between the flowers and their pollinators
- Red, tubular flowers coevolved with hummingbird who attracted to red & have long beaks