community ecology

what is a community

• An association of interacting populations, usually defined by the nature of their interaction and/or the place in which they live 

• Interaction = sharing of resources 

are communities randomly assembled

• Communities as a superorganism 

  • Proposed that communities are like super-organisms, where the species are bound to each other by their interactions 

  • Also referred as a closed community 

  • Clear and sharp community boundaries 

    

• Individualistic view 

  • Argued that communities are associations of species assembled by change, independent of other species presence or absence 

  • Also referred to as an open community 

  • Community boundaries should be indistinguishable  
    

    

• The appearance of interconnected (closed) communities might be more due to common environmental needs, or a common ability to utilise unusual environments, then to an interdependence among the species 

• While communities might be random with regard to specie, they might not be random in terms of their function or ecological role 

  • Functional ecology 

how are communities assembled

• Communities change over time (succession) 

  • When communities develop from new, never-before occupied space 

  • This colonisation process is called primary succession (slow and rare) 

  • Where communities are affected by the occurrence of disturbances. The process of recolonisation that occurs is called secondary succession 

• Studying gaps 

  • Recolonisation of small gaps in a community occurs in a shorter time scale 

  • Provides further information whether community assembly is deterministic (shaped by environmental and biological factors) or random (shaped by chance events and unpredictability) 

founder controlled communities

• Communities where any species has an equal chance to colonise the gap 

• Future composition is determined by chance 

• Maintains diversity 

• Selects for R species (short lifespan and mature quickly) 

dominance controlled communities

• Species differ in competitive ability 

• Outcome can be predicted by competitiveness, although undergoes succession often 

• Maintains less diversity 

• Initially selects for R, but K species eventually dominates 

how do you quantify/describe differences between communities

• Species composition/diversity 

  • Species richness (problem = ignores species evenness) 

  • Most species diversity index try to incorporate both species richness and evenness 

  • The Shannon index 

    • Sensitive to both rare and common species. 

    • Communities with many equally abundant species have high H′. 

    • Communities dominated by one species have low H′. 

  

• Trophic structure 

  • Trophic structure is determined by the feeding relationships between organisms 

    • The length of a food chain tends to be 4/5 links, called trophic levels 

    • Hard to sustain any more due to biomass loss 

  • The transfer of food energy from its source in photosynthetic organisms through herbivores and carnivores is called the food chain or food web 

  • Food webs 

    • Shows who eats whom in a community 

    • A given species may weave into the web at more than one trophic level, forming a web 

what factors affect the trophic structure of communities

• Bottom up regulation 

  • Imperfect transmission of energy, community diversity is controlled by primary production 

  • If efficiency of energy transfer across trophic levels primarily determines food chain length, then experimental manipulations of productivity should influence food chain length 

• Top-down regulation 

  • Community diversity controlled by a top predator 

  • In the absence of this predator, competition increases 

  • With the predator, the density of the competitor is limited, allowing coexistence 

  • Acts as a keystone species 

community connectivity

• Habitat fragmentation → breaks connections between populations, reducing movement and interactions. 

• Climate change → shifts species ranges, disrupting existing interactions. 

• Human activities (urbanisation, agriculture) → create barriers and reduce connectivity. 

• Pollution → can remove sensitive species, weakening links in the network. 

• Invasive species → introduce new interactions or disrupt existing ones. 

• Natural disturbances (fires, floods) → can temporarily break or reshape connections.