Species and Communities

• species do not exist in isolation
• species richness


  number of species in a community
• species evenness


  how individuals are spread
• species depend on ecosystems and are grouped into communities
• spatial factors


  underlying productivity, predation, environmental complexity and harshness
• underlying productivity means that more resources means more diversity
• species richness may be highest at intermediate levels of productivity
• high levels of productivity leads to increased competition
• predation intensity can lead to a decrease in others
• spatial heterogeneity 


  variation in ecosystems leading to more and different microhabitats and hiding places
• environmental harshness


  pH, extremes, low heterogeneity, low productivity
• temporal factors


  climatic variation,disturbance, evolutionary age, islands
• climatic variation depends on predictability
• stable environments able to support more niches and specialised systems
• climatic variation can lead to increased niche overlap
• intermediate disturbance hypothesis


  local species diversity is maximized when ecological disturbance is neither rare nor to frequent
• disturbance will see a rapid increase then decrease due to competition
• increase in disturbances leads to the inability to recover
• tropics are more species rich
• refugia


  a location which supports an isolated or relict population of a once more widespread species
• islands have great island biogeography due to isolation
• species balanced by immigration and extinction leading to equilibrium
• species number decreases with isolation
• species turnover


  rate of dissimilarity among species composition across all possible plot pairs along the environmental gradient