Factors Limiting Distribution: Interrelations With Other Species – Chapter 6

Types of Interactions

• Negative Interactions– Predation, competition, disease, parasitism

• Positive Interactions– Mutualism, symbiosis

Predation

• ‘Prey’ can be a food plant and the ‘predator’ a herbivore.

• Or ‘prey’ can be a herbivore and the ‘predator’ a carnivore.

Predation – restriction of prey by predator

• Predators are sometime able to restrict local distributions of some prey

• Experiments have shown this to be true when:– Prey individuals will survive when transplanted to a

site where they do not normally occur if they are protected from predators.

– The distribution of prey organisms and suspected predator(s) are inversely correlated.

– The suspected predator is able to kill the prey, both in the field and laboratory.

– The suspected predator can be shown to be responsible for the destruction of the prey in transplantation experiments.

Predation Examples – Mussel TransplantTidal Open Coast – heavy wave action restricts the size of mussels and prevents predators from eliminating small mussels

Sheltered Waters – Predators eliminate most of the small mussels, and Mytilus survive only in areas safe from predators

Predation on Kelp

• Kelp is a large plant-like brown algae.

• Several herbivores are known to eat kelp.

• There is an inverse correlation between the presence of kelp and sea urchins.

• Need to control for other herbivores

Kelp – Predator Controlled Population

Time

Control Sites

Only Limpets

Removed

Only Seas Urchins

Removed

Both Sea Urchins and Limpets

Removed

August 1982

1 2 6 5

February 1983

2 3 25 60

August 1983

1 1.5 60 93

February 1984

0.5 1 85 95

% leafy cover; From Fletcher 1987

It appears that sea urchins are mostly responsible for controlling kelp:

Rock Wallaby – Predator Controlled Population

In Australia, Rock Wallaby populations have been declining since the introduction of the red fox.

When red foxes are removed, rock wallaby populations expand.

Extinct Population!

Rat-kangaroo – Predator Controlled Population

Only exists in areas where the red fox is absent.

This case, as well as the others, demonstrate that predation can control local distributions of populations.

Restriction Of Predators By Prey

• If a prey restricts a predators range, then that predator must feed only on one or two species of that prey.– Mostly plant/herbivore interactions

• These type of predators are called specialists or monophagous.

• One example is Drosphila pachea, a rare fruit fly that only breeds in the stems of senita cactus.– They contain a unique factor needed for growth

and development by the fly

Monphagous Insects

Monophagous insects should be limited in their distribution by their host plant – but no species to date has demonstrated this (Quinn et al. 1998):

Disease and Parasitism

• Pathogens may eliminate species from areas and thereby restrict geographical distribution.– Chestnut blight

Hawaiian Bird Populations

- disease control

Introduced birds are less susceptible to malaria than native birds.

Mosquitoes much more common in low elevations.

Malaria is most common at intermediate elevations – native birds more susceptible and species overlap.

Allelopathy• Some organisms, plants in particular, may be

limited in local distributions by poisons or antibiotics, also called allelopathic agents.– Penicillin, toxic secretions from plants

Studies have shown that some plant species produce a toxin that limits the growth of others.

Smother Crops

Average Dry Weight per Plant (g) After 2 Months of Growth

Barley Chickweed # of Chickweed Flowers

4.15 3.20 100+

Controls (each grown alone, competition absent)

4.85 1.43 10

1:1 barley/chickweed mixture (competition present)

Smother crops act as weed suppressors. It is not competition for nutrition or water, but rather a toxic secretion that limits growth.

Competition

• Competition can occur between any two species that use the same type of resource.

• Competition is an important process affecting the distribution of plants and animals.

• Species do not need to be closely related– Birds, rodents, and ants all compete for seeds in

the desert.

Competition• Resource competition – when a number of

organisms utilize common resources that are in short supply.

• Interference competition – when the organisms seeking a resource harm one another in the process, even if the resource is not in short supply.

• When species A is absent, species B lives in a wider range of habitats.

• In extreme cases a habitat will contain either A or B, but neither both together.

Checkerboard Distributions

Two closely related species of fruit pigeons.

Islands may have been colonized by first-come-first-served basis or slight competitive advantage.

Two ecologically similar species that have mutually exclusive but interdigitating distributions.

Competition Between Two Salamanders

• Plethodon jordani and P. glutinosus (Hairston 1980).

• Altitudinal distribution only overlaps 70 – 120m on any one transect up the Black Mountains in North Carolina.

• Hairston demonstrated that by removing one species, the other expanded it’s range.– Superior competitor was excluding the other

species from preferred habitat of moist soil and food.

Competition

• When two species compete, one always will always be better than the other in gathering or utilizing the resource that is rare.

• In the long run, the other species either loses out and disappears or evolves some adaptation.– Avoid the superior competitor by selecting a

different part of habitat.

– Avoid the superior competitor by making a change in diet.

Example - Shift in Diet

These species of the European crossbill have avoided competition by developing beaks that allow the use of different food types.