Limits in Niche Overlap

• What are the limits we could expect (or predict) between sympatric species?

• Can any theory be generated?

Niches in summary

Niche Problems

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

• When the diet was ‘fine-grained’ (e.g. too small to differeniate prey type) and the mix was equal, the generalist tended outcompete either specialist

• Thus it may be a point (the limit) on the resource spectrum where a generalist can outcompete two specialists

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 0.54

• 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?

Limits in Niche Overlap

• There are too many other effects that contribute (e.g. diffuse competition) within the community further separating species

• The maximum tolerated overlap in communities has been largely shown to depend upon the number of competitors in the community

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

• E.g. Pianka (1973) found evidence of decreasing niche overlap with increasing species number for desert lizards

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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

• Niche packing figure

Limits in Niche Overlap

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

• Although all do capture some of the ‘structure’ of a community, all represent different facets

• E.g. the number of sp. which can be accommodated is a measure of ‘fill’; the pattern in which those species are arranged within niche space is an aspect of packing design

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)

Niche Packing

• However, the author suggests the concept of the niche is useful conceptual tool, but provides little insight into the mechanisms generating it and has little predictive power

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

• Very few researchers taking resource availability into account

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)

Problems w/ Niche Packing

• E.g. if earthworm specialists (pg. 67) have 100% diet overlap but 0% in habitat (field vs. woodland), they should have a niche overlap of 0 (if you use the mean of two axes)

Niche Packing

• Conversely, the six species of large ungulates (table 4.2) show a measurable degree of overlap

Niche Packing

• Table 4.2

Niche Packing

• In this example, some of the apparent overlap (e.g. roe deer and fallow deer) is mediated by habitat differentiation (pasture vs. woodland) and then reinforced by diet (more grass vs. more leaves)

• Average α would be more appropriate

Niche Packing

• However, many actual datasets are not as ‘clean’ as these and fall somewhere in between

Multidimensional Niches

• If you can construct a multidimensional niche for two species and the overlap is 0, that should suggest NO competition

• However, one can easily comprise a scenario where overlap is 0 yet competition is occurring (and the reciprocal as well)

Niche Problems

• Consider two species of S. American flycatchers that compete 100% for food but completely segregate for nesting habitat

• Is the overlap 100% or 0%?Probably should consider habitat associations when feeding if you are to combine habitat and diet data

Niche Problems• Consider two species of S.

American flycatchers that compete 100% for food but completely segregate for foraging height

• Is the overlap 100% or 0%?What if the insects move vertically? While they are avoiding interference competition, they are not avoiding exploitative competition

Niche Problems

• Reconsider the ungulates of England: diet and habitat use suggest Sika and cattle are strong competitors

• Are they?• Does time allow for coexistence?• Avoid interference competition, but

not exploitation

Niche Problems

• Niche separation by certain resources relates specifically to reduction of interference…niche separation by other resources relates only to a reduction in exploitation competition (could reduce both)

Niche Problems

• Results depend upon the type of resource.

• Habitat separation works in our earlier example of earthworm specialists (hab. seg.)…why?

But not with aerial insects

Niches in summary• Different resource dimensions

relate preferentially to one or another type of interaction

• Temporal or spatial separation is related to interference competition

• Separation on more materialistic axes (e.g. food, habitat) will affect exploitation competition

Niches in summary• Furthermore, resources do not have

to be limited for competition to occur (e.g. interference), but it is necessary for exploitation competition

• Combinations of resource dimensions can lead to problems associated with distinct categories of competition

Niches in summary• The niche concept is useful to

describe an organism’s functional position within a community, but has little predictive power

• Web theory (and practice) has shown that within the resource spectrum, there are small sub-groups that maybe particularly strong competitors

Guilds!!

Niche Problems

Niche Problems