Lecture 12 Communities Flashcards
Community ecology
Understanding dynamics/mechanisms of community structure & diversity is vital for conservation & management
Invasive species
E.g. lion fish Indo-Pacific native invaded coral reefs in Atlantic killing off native fish that consume algae causing algal overgrowth that impedes coral growth
Define a community
Group of interacting species occuring same place/time. Based on :
Taxonomy - related species
Guild - species using same resources
E.g. pollinators
Functional group - species w/same function not always using same resources e.g. legumes
Can describe communities in a single trophic level e.g. plants or multiple trophic levels
Food & interaction webs
Describe a community of species w/trophic & non-trophic interactions using good web/network
For specific trophic interactions describe community as network e.g. plant-pollinator network
Measuring communities
Patterns at scale of community often referred to as community structures
Captured by species diversity
2 components:
Species richness & abundance relative to other species (evenness)
Measuring communities: diversity: Shannon index
To calc Shannon index (H)
Natural log (ln) is applied to proportion (Pi) for each species (i) then multiplied by Pi again. All values are summed for all species in community and multiplied by -1 to get H
H= - sum of Pi+ln(Pi)
Measuring communities: similarities : Jaccard index
Community composition quantified/compared against other communities
E.g. surveying which bird species occur in two areas of forest
If 3 are unique to forest A and 3 to B and the two forests have 2 species in common then total no. Spp = 8 and 2/8=0.25 this is the Jaccard index
J=ab/(a+b+ab)
Jaccard index (J) measures special similarity/ species turnover from one site to another - part of beta diversity
More complex indices can account for species abundance in communities
How we end up with particular communities
Regional species pools & enviro filtering
1) dispersal sets limits by which species can arrive in a location
2) only species that survive to repro in abiotic conditions will persist
3)interactions w/other species determine final composition - biotic conditions
E.g. UK sycamore (Acer psuedoplatanus) failed to make it over the channel (we introduced it) before land bridge loss (dispersal filter)
Scots pine (Pinus sylvestris) extended to Britain ~9000 years ago declined in warmer climate and competition from broad leaf species (biotic/abiotic conditions)
Unique upper Teeside (Penines) calcerous soil leads to calcerous favouring plant assemblage (abiotic filter) cool wet climate (less competition than in low lands) arctic/ alpine species left behind since last glacial max. (Ice age)
Competition facilitation in plants
Major forces shaping communities
E.g. for plants: light/space/water/nutrients/ pollinators and disprrsers
Plant interactions may shift along altitudinal gradient from competition to facilitation
e.g. in low altitude plants compete w/neighbours with relative negative neighbour effect (neg RNE)
At higher altitudes plants benefit from neighbours that provide protection in harsh growing conditions - relative positive neighbour effect ( pos RNE)
Community dynamics: succession
Process of change in community over time
1) primary - occurs after catastrophic disturbances that kill all organisms leaving enviro devoid of life (or bare lifeless substrate w/no former community as in Petri dish agar)
2) secondary - started by event e.g. forest fire, harvest, hurricane that removed individuals/biomass of established species but not all life (especially soil remaining)
Succession example: glacier bay Alaska
Succession driven by interactions
As glacier melts (area becomes deglaciated)
1) moss and lichen develop soil substrate allowing germination and establishment of …
2) Dryas (flowering perennials) and shrubs add soil moisture, shelter and nutrients allowing establishment of…
3) Alder thicket - mutualistic bacteria in roots break down atmospheric N bonds fixing it into plant available N increasing growth of spruce species leading to ..
4) spruce forest w/dense shade, spruce outcompetes other species and becomes dominant
Interactions drive successions
N accumulation and soul formation is critical
Inhibitors at each stage reduce abundance of early colonisers due to competition
Primary succession
Driven by pathogens, diseases & herbivores also e.g. nematodes specific to marram grass drive succession of sand dune communities by limiting growth
Secondary succession
Cause is biotic or abiotic due to specific disturbance events or continuous removal of biomass e.g. 15 million trees blown down by great storm in 1987 or everyday herbivory of grazing cows
Can be viewed as disruption of initial primary succession - a ‘reset’
Disturbance as trigger for succession
Species can competitively exclude drivers in absence of disturbance
Disturbance by reducing abundance of dominant species can allow other species to persist
Known as intermediate disturbance hypothesis e.g. fire frequency - if too intense/frequent disturbance increases mortality so diversity declines. If too mild then dominant species cannot be competed with
Intermediate level of disturbance creates balance/diversity
