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Explore the factors influencing community structure and why some communities are more resilient to disturbances. Learn about fixed and continuum communities, life zones, ecotones, edge effects, and the importance of biodiversity.
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Communities An association of interacting populations in the same place at the same time -- have characteristic physiognomy Ecosystem A community plus its abiotic environment -- includes energy flow, biogeochemical cycling
Community Structure Patterns visible in a community including: -- physiognomy -- dominant species -- number of trophic levels -- number and abundance of species species richness vs. species diversity
Major Questions What factors determine community structure? Why are some communities more resilient to disturbances than others? A disturbance is any discreet event that removes organisms and opens space for colonization of same or different species
Communities as fixed species assemblages -- C. H. Merriam, 1890s Arizona -- all interacting species, co-dependent -- superorganism -- Life Zone concept
Life Zones Alpine Hudsonian Canadian Transition www.pigeon.psy.tufts.edu
Organismic or fixed concept Continuum concept Fig. 22.1 (EFB)
Fossil Pollen: -- can identify to species by morphology -- preserve well in sediment cores from lakes, bogs -- represent past plant communities -- can radiocarbon date organic remains in cores to determine age of communities Margaret Davis: studied fossil pollen from lake cores in the eastern U.S.
Ecotones: intermixing of two communities where one transitions into another -- causes an ‘edge effect’ of greater species richness in the ecotone -- edges can be inherent or induced -- edges once thought to be good for wildlife management, but can be detrimental -- e.g., increased edge, more invasive species such as cowbirds entering forests
Edge effects on forests • Changes in physical environment • e.g., temperature, humidity, light penetration • and wind 2. Direct biological changes e.g., dead trees and tree fall, reduced leaf fall, fewer forest birds 3. Higher order biological changes e.g., increase in some insect populations (butterflies and high light), increase in insectivores, more disturbances to interior
Quiz • What is meant by fixed versus continuum communities and what are life zones? • What is palynology and how did the work by Margaret Davis help demonstrate the continuum concept? • What are trophic cascades and how did the work by Jane Lubchenco demonstrate this concept? • What is the Shannon diversity index and what patterns are present in most communities with species numbers vs relative abundance? • What is the Intermediate Disturbance Model? Graph its relationship to species diversity in communities.
Plant productivity and the number of bird species in temperate zone habitats Primary productivity No. of breeding Habitat (g/m2/yr) bird species Marsh 2000 6 Grassland 500 6 Shrubland 600 14 Desert 70 14 Coniferous forest 800 17 Upland deciduous forest 1000 21 Floodplain deciduous forest 2000 24
Quiz • What is an ecotone and the ‘edge effect’? What are inherent vs. induced edges? • Graph and explain Foliage Height Diversity in relation to species diversity in a community. • Give the equation for species-area relationships. Why is this one of the few genuine rules in ecology? • Graph and explain the model for island biogeography on islands of different sizes and distances from the mainland. • How can the theory of island biogeography be applied to continental areas and conservation biology?
