Community Ecology

Community Interactions Interspecific interactions are relationships between different species May include competition, predation, herbivory, and symbiosis + and – indicate whether the species benefit or are harmed in the interaction, 0 is neutral

Competition Interspecific competition occurs when individuals of different species compete for the same resources (-/-) This relationship can lead to competitive exclusion; i.e. the species that is less successful will be eliminated from the environment True competition is rare because of ecological niches Differences in niches allows organisms to survive even if they are similar as long as the niches are not identical (resource partitioning)

Predation One species benefits by killing and eating another (+/-) There are a variety of adaptations to aid both predator and prey Predators: heat-sensing organs, toxins, speed, agility, camouflage, etc. Prey: hiding, fleeing, forming herds or schools, alarm calls, cyptic and aposematic coloration, etc.

Mimicry Batesian Mimicry Mullerian Mimicry A harmless species that has coloration similar to a harmful species Mullerian Mimicry Two or more unpalatable and harmful species resemble each other becoming something similar to aposematic coloration Predators can also use mimicry Some snapping turtles have tongues that resemble worms

Herbivory The eating of plants or alga by other organisms (+/-) Predator adaptations include chemical sensors to determine the plant’s nutritional value or if the plant is toxic; specialized teeth and/or digestive systems Prey adaptations include toxins, such as strychnine, nicotine, tannins, selenium toxins; chemicals that produce tastes that are unpalatable to many animals, such as cinnamon, cloves and peppermint; chemicals that cause abnormal development; thorns or spines

Symbiosis Symbiosis occurs when species live in direct contact with one another forming a relationship that may have beneficial, harmful or neutral effects Parasitism (+/-) – a parasite derives its nutrients from a host, harming the host in the process (ticks, lice, etc.) Mutualism (+/+) – an interaction that benefits both species (acacia and ants) Commensalism (+/0) – an interaction that benefits one species while having no apparent effect on the other (cattle egrets and water buffalo)

Species Diversity Species diversity is determined by Species richness – the number of different species in the community Relative abundance – the proportion each species represents of all individuals in the community Shannon diversity index is used to determine the diversity of a community in a numerical way. This allows for a comparison of different communities.

Trophic Structure The structure and dynamics of a community are largely related to the feeding relationships These can be represented in food chains and food webs It has been documented that most food chains and webs are limited to 5 or fewer trophic levels Energetic hypothesis – limited by inefficiency of energy transfer from one trophic level to another (~10%) Dynamic stability hypothesis – limited because longer food chains are less stable and less able to rebound from population fluctuations Most data currently supports the energetic hypothesis

Impact of Species Certain species have important roles and large impacts in given environments Dominant species are abundant and have the highest biomass in the community May be related to ability to out-compete for water or nutrients or lack of predators (such as is seen with invasives) Ex: Historically – the American chestnut Keystone species are important because of their ecological roles Ex: sea otters, Pisaster ochraceus Foundation species cause physical changes in the environment through their behavior or biomass Ex: beavers, Juncus gerardi

Bottom-Up and Top-Down Controls Bottom-up model Unidirectional influence from lower to higher trophic levels The presence or absence of mineral nutrients (N) controls the plant numbers (V), which control herbivore numbers (H), which control predator numbers (P) NVHP Change at the bottom will effect everyone else, but change at the top will not effect those at lower trophic levels significantly Top-down model (Trophic cascade model) Says the opposite NVHP Change at the top will cause alternating (+/-) effects as it moves down the trophic levels Removal of the top predator will increase the number of herbivores, decrease phytoplankton, cause nutrient levels to rise

Disturbance and Species Diversity A disturbance is an event that changes a community by removing organisms from it or altering resource availability Nonequilibrium model – communities are constantly changing after being affected by disturbance Intermediate disturbance hypothesis – moderate levels of disturbance can create conditions that foster greater species diversity than low or high levels of disturbance

Ecological Succession A gradual replacement of species over time Primary succession occurs where this has never occurred before on soils that are not yet formed (volcanic and glacial areas) Begins with prokaryotes and protists, lichens and mosses follow, allowing the formation of shallow soils. Grasses, shrubs and eventually trees can then move in as the soils become more mature. Secondary succession occurs in an area that has been disturbed, but the maturity of the soils have not be disturbed

Biogeography and Species Diversity The latitude of a community and the area it occupies tend to play large roles in species diversity The tropics tend to be more diverse than areas closer to the poles Related to climate and evolutionary history because growing seasons are about 5 times longer in tropical areas than in tundra, thus speciation events can occur more frequently Species-area curve: all other factors being equal, the larger the geographic area of a community, the more species it has

Pathogens Pathogens are now recognized as having a huge impact on ecosystems and their stability. Pathogens are naturally occurring but are often being transmitted to new locations because of human activities The flow of these pathogens is of great interest as humans are also being effected