Competition

Competitive Release

• Competitive Release is a prediction from examining the competitive exclusion principle that in the absence of competition a species should expand its niche

Chalcophaps indica – Emerald Dove

Chalcophapsstephani – Stephen’s

Dove

Gallicolumba rufigula

Character Displacement

• Character displacement is a measurable physical difference between two species which has arisen by natural selection as a result of the selection pressure on one or both to avoid competition with each other - here we assume that environment is the same at all locations

Hydrobia ulvae

Hydrobia ventrosa

Hydrobia ulvae – note size

Patterns like character displacement or competitive release can be caused by several things:

1. the pressure of current competition causes the pattern

2. competition which occurred in the past may have driven natural selection to cause the pattern we see today - "the ghost of competition past"

3. competition in the past eliminated a number of other species, leaving behind only those that were different in the use of habitat

4. the species may have evolved independently and in different ways and have never competed with each other

5. the species may differ in their niches, but not enough to coexist in a stable environment, however the environment varies and thus prevents competition from reaching its predicted end result

Joe Connell

The Ghost of Competition Past

Blackburnian warbler

Cape May warbler

Black-throated green warbler

Bay-breasted warbler

Yellow-rumped warbler

Meriones tristami

Ghost of Competition Past in Israeli Rodents

Gerbillus allenbyi

Mt. Carmel, Isreal

Competition

California Coastal sage scrub – note bare spots near shrubs

Rabbit grazing – source of apparent competition

Species Coexistence

Serengeti National Park

Species Coexistence

Competition is a common feature of species interactions, yet often we find very similar species coexisting in nature, species that seem to need the same resources. How do they coexist?

• Refuge from competition • Predation and disease keep populations of each

species low enough that they do not compete• Resources may be variable in space and time, so that

the species coexist because both do not find resource at same time

Dung – a valuable, variable resource

Dung Beetles

Dung Fly

Predation

Great White Shark and Fur Seal

Lions hunting – True Predator

Red squirrel – true predator

Moose Browsing – Partial Predator

Parasitoid Wasp

Specialists and Generalist Predators

Advantages to being a specialist

1. Avoid interspecific competition

2. Allows evolution to overcome chemical defense

3. Allows evolution of cryptic coloration that matches prey - mostly for insects on plants

4. Increases chance of mate encounter

Advantages of being a generalist

1. Flexibility in face of environmental uncertainty

2. Broad diet needed to get all necessary nutrients and vitamins

3. Avoid overdosing on any one toxin - mostly for animals grazing on chemically defended plants

Factors affecting predator behavior

1. Search time - the amount of time spent looking for food items

2. Handling time - the amount of time spent capturing prey once it is found, also includes time for consuming prey and time to digest before a predator can search for food again

Predator response to prey

Pied Wagtail

Caribou feeding in winter

Edible mussel – Mytilus edulis

Shore crab and edible mussels

Black oystercatcher

Bluegill sunfish

Mink

Muskrat

Red grouse in heather

Tawny Owl

Bank vole

Cinnabar Moth and Caterpillar on Ragwort Tansy