Lecture 17 (Figs.at CHARTS 17)

Last lecture – considered importance of competition as a process organizing community structure.

Direct vs indirect methods

Historically, theory and the indirect approach (observational) preceded the direct (experimental) approach.

Today these two approaches are more blended – empirical work has become more theory based and theory-based approaches have become more empirical.

Let’s return to the concept of ‘limiting similarity’ as a theoretical construct to predict how species will evolve in their uses of resources to promote co-existence.

Concept of limiting similarity leads to niche diversification – often expressed as "character displacement". The shift in a character that allows coexistence without competition.

Here closely related species different when found living together but may be indistinguishable in how they make a living when living apart.

Examples:

1. One of the first studies to assess this theory was conducted by Robert MacArthur. Observed where and for how long various species of warblers feed on insects in in different parts of trees. Showed that there was considerable differences in how this habitat was used – argued that divergence represented "character displacement" to avoid competition. (Fig 1)

2) Other evidence: Jared Diamond – study on diet in co-occurring species of pigeons in Papua New Guinea. (Fig. 2)

Interpreted patterns of resource use in species with overlapping diets to indicate degree of niche overlap.

Remember – prediction from competition theory is that very similar species would not co-exist.

Found:

1. some species pairs never co-occurred, usually they were ones most similar to one another.
2. some species pairs occurred together in large groups but not in small group assemblages (diffuse competition?)
3. some species good colonizers but absent from larger islands where resources most suitable. Eliminated because of their poor competitive ability?

Problem with this study is that while it is consistent with theory everything is inferential. This approach has been attacked on statistical grounds. Groups formulated from random expectation. Claim that there is little support to exclude possibility that they represent random assemblages.

All these argument rely on correlation and plausible arguments. Not much biology here.

3) Dr. William Brown – worked on desert rodents showed correlation between body size and ‘niche’ (foods eaten and habit occupied) in two separate desert systems, consistent with the "character displacement hypothesis" but again non-experimental. (Fig. 3)

David Lack’s studies on Darwin’s finches revealed evidence of character displacement.

Found divergence in beak depth (measure of bill size and potential to feed on seeds of a given size), between Geospiza fuliginosa (small-billed ground finch) and G. fortis (medium-billed ground finch).

Found that on islands where they coexisted their range in bill sizes did not overlap (presumed to reflect differences in seeds taken to avoid overlap) but on islands where they did not overlap bill sizes were very similar – interpretation here is that competition leads to character displacement. (Fig. 4)

Long-term studies conducted by Dr. Peter Grant on the island of Daphne in Galapagos over the past 20+ years reveal potential for character displacement In Darwins finches.

Observed populations of different species on same island during wet and normal dry years:

Species:

Geospiza magnirostris (large-billed ground finch) usually only present on island during dry season but established year-around during 1983 El Nino - preferably feeds on large seeds.

G. fortis (medium-billed ground finch), long standing resident on island, feeds on small to large-sized seeds. Variation in bill size is high.

Periodic wet years (El Nino) - smaller seeds more abundant that larger seeds.

Normal dry years - smaller seeds less abundant.

In wet years: Shift in seed size resulted in shift in bill size. Smaller seeds more common that larger seeds.

Observed selection for smaller bills in G. fortis. Suggested that G. magnirostris outcompeted G. fortis for fewer, larger seeds driving shift in bill sizes (character displacement)

Predation as a factor controlling community structure.

Two possible outcomes:

1. if dominate competitors suffer differentially higher mortality, predation can promote coexistence of species.
2. if subordinate competitors suffer differentially higher mortality, predation can reduce diversity, promote local extinction.

Selective predation by the seastar- Pisaster on competitively superior space competitor, the bivalve mussel, Mytilus. Lead to the concept of "Keystone species".

Removed Pisaster from a stretch of rocky shoreline and left other areas untouched to act as controls.

After several years the distinct pattern of species zonation: high intertidal zone to subtidal line respectively - ephermeral algae (high), Acorn barnacles, mussels, gooseneck barnacles, sea-cabbage and finally kelps – lead to a decrease in the species diversity of this habitat.

Otters feed on benthic invertebrates such as sea urchins. Sea urchins control algal abundance which in turn influences abundance of fishes that use the algal canopy for shelter and food (invertebrates associated with kelps).

Man’s interference reduce otter population to near extinction in the late 1800’s. This resulted in increase in benthic inverts, reduction in kelp and fishes.

Case study: Lubchenco (1978)

Studied effects of a relatively selective predator (grazer) – perwinkle, Littorina.

Species is capable of feeding on a wide range of algal species but has a preference for small tender species compared to large fleshy spp.

Examined tidepools, found that some are dominated by Enteromorpha (preferred species), others by Chrondrus (non-preferred).

At high Littorina density Enteromorpha driven to extinction in pools and diversity of algae declines. Low crab density when Chrondrus density is high.

So,

When predator (grazer) is selective in its diet:

@ intermediate density, diversity is highest

Now, on emergent substrates, however, competitive abilities between algal species is reversed:

Chrondrus > Enteromorpha because Chrondrus is better @ withstanding dessication.

1. Entermorpha coexists beneath Chrondrus canopy
2. Low diversity because Littorina removes inferior competitor (Enteromorpha) in this habitat.

Effects of grazers on diversity when grazers density is:

Coral reef example: Acanthaster planci (Crown of thorns seastar)

Selective predator on competitively superior, dominate species of corals in the western Pacific. Preys on faster growing corals which invest less in protection (chemical deters)

At very low density, increases diversity of coral community.

At very high density, reduces diversity

On reefs in the eastern Pacific – story more complicated.

Starfish removes competitively inferior species – only survives in refuges.

Competitively superior spps are fast growing and protected from seastar predation by symbiotic crustaceans that live in the interstices of the branches.

Lastly, Predators can also influence patterns of distribution i.e. cause zonation.

On Pacific coast of Panama, damselfish have a pronounced effect on influencing pattern of coral zonation.

Here superior corals are branching forms – overgrow massive species. Branching corals predominate in shallow water, massive (slower growing species) in deeper water. Damselfish eliminate massive corals in shallow by killing them to establish areas for the algal gardens which function as sites for food and shelter. (Fig. 12)

Intrinsic differences in topography complexity drives the system – shallow water provides more refuge sites for damselfish.