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Lecture 9

ENVS 1500 Lecture 9.docx

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Environmental Studies
ENVS 1500
Gail Fraser

11/9/2013 10:31:00 AM Communities & Ecosystems Community: an association of interacting species inhabiting some defined area. Community Structure: includes attributes such as the number of species, the relative abundance of species, and the kinds of species comprising a community Community Guilds: groups of organisms that all make their living in a similar way. Species abundance: most species are moderately abundant, few are very abundant or extremely rare. 1) A specie is dominant when it substantially more common than the other species in the community. 2) Species diversity: A combination of the number of species and their relative abundance defines species diversity  Species diversity: (1) the number of species in the community, which ecologists calls species richness. (2) and the relative abundance of species, or species evenness. Rank- Abundance Curves:  We can portray the relative abundance, dominance and diversity of species within a community by plotting the relative abundance of species against their rank in abundance. The result is known as the rank abundance curve. Environmental Complexity: Species diversity is higher in complex environments – to understand this relationship is fundamental to understand species niches  The fundamental niche is the multi-dimensional set of condition an organism needs to survive. Over time, the competitive exclusion principle (Chapter 13) is expected to cause the local extinction or evolution of closely related species  Ecological studies have shown positive relationships b/w environmental complexity and species diversity for many organisms. Disturbance & Diversity: shifts in the species composition of a community, even without shifts in species richness, would indicate a shift in community structure  Intermediate levels of disturbance promote higher diversity. In general these models predict that population densities will not change once equilibrium has been reached. In a system at equilibrium, stability is maintained by opposing forces such that if population changes are altered by some event, they will soon return to equilibrium. However, as ecologists know, changes to the environment do change and cause changes  When environmental change is sufficiently large, we call it a disturbance (nonequilibrial models)  Disturbance can be broken down into two categories: frequency and intensity. The Intermediate Disturbance Hypothesis  Both high and low levels of disturbances would lead to reduced diversity.  Intermediate levels of disturbance allows sufficient time b/w disturbances for a wide variety of species to colonize but not enough time to allow competitive exclusiveness. Species Interactions & Community Structure th  Since the beginning of the 20 century, ecologist have meticulously described the feeding relationships of hundreds of commu
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