Limits in Niche Overlap - Introduction to Community Ecology - Lecture Slides, Slides of Ecology and Environment

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Limits in Niche Overlap

• What are the limits we could expect (or predict) between sympatric species?
• Can any theory be generated?

Limits in Niche Overlap

• MacArthur and Levins (1964) compared the competitive abilities of two specialists (at different areas of the resource axis) and a generalist (thought to have its most competitive area in between the other two)

Limits in Niche Overlap

• That was a relatively simple system and results may not be extrapolated
• Later MacArthur and Levins (1967) again determined the limiting similarity should be about 0.
• More analysis suggests it should be one SD (d>w) between midpoints

Limits in Niche Overlap

• They also come to the conclusion that overlap should not occur within 2SD of the midpoint (e.g. d > 4w)
• Would you expect observed overlaps to be smaller or larger than the theoretical limits? Why?

Niche Packing

• Because the of the ease at grasping the concept of the niche and the proliferation of theory, expectations were relatively for the application of theory in explaining community structure

Niche Packing

• It is somewhat difficult to ascertain exactly how important niche theory is in determining community structure because of the range of studies (e.g. scale, taxonomy, trophic level), methods of analysis, and different assumptions which may or may not apply to most communities

Niche Packing

• Cody (1974) using similar methods on the structure of shrubland birds came to the exact opposite conclusions (finding ave. overlap between adjacent niches increases within the community increasing diversity)

Niche Packing

• Various authors have defined ‘packing’ as slightly different definition - # of sp. that can be accommodated per unit volume of resource (single dimension) - The closeness of packing of resource utilization distributions for non- overlapping sp. - The extent of overlap observed in resource utilization within the community or between adjacent pairs of sp.

Niche Packing

• The closeness of packing of overlapping or non-overlapping species, of community-fill, packing design and the types of organism involved
• Thus extent of resource overlap is not a good metric for community fill (species could be added without increasing overlap)

Niche Packing

• Typically niche packing is thought to occur not by increasing overlap, but by lessening niche breadth (or in some cases, extending the resource axis)

Problems w/ Niche Packing

• One of the basic problems with an analysis is the difficulty of accurate measurement of true overlap in field studies in the first place
• For interactions to occur (and subsequent packing), there must be some competitive ‘medium’, actual or potential

Problems w/ Niche Packing

• Areas of overlap between the resource utilization curves of adjacent species reflect a potential for interaction (a necessary prerequisite for competition)
• However, those resources must be limiting for competition to take place, not just sharing abundant resources

Problems w/ Niche Packing

• In fact, high overlap might just as easily imply a lack of competition
• Also, we are only considering a single resource axis at this time.
• Consequently, high (or low) overlap on a single axis may or may not provide an accurate depiction of the intensity of competition

Problems w/ Niche Packing

• Multidimensional niche overlap is problematic in both calculation and interpretation
• E.g. is it a sum of unidimensional overlap indices or the arithmetic mean of the separate overlap indices (which is necessary if resources are correlated)