Components of benthic communities

“Infauna” classified by sizeBacteria & protozoaMacro- and meiofauna

Major taxonomic groupsPolychaetes, crustaceaEchinoderms, molluscs

In similar habitats, community composition suggests convergence in form

Major questions

What are the major components of communities in unvegetated sediments?

What is the relationship between taxonomic classification and ecological function? (“Functional equivalents”)

How can disturbance promote diversity? What are the consequences of bottom-

dredging fishing techniques?

Functional groups: examples of sediment stabilizers and bioturbators

Parallel infaunal communities in North Temperate oceans: Ecological convergence

Apparent paradox: high diversity, low habitat heterogeneity “Competitive” exclusion is observed Deposit feeders exclude suspension feeders

(really “amensalism”) Competition for space is documented

Why not have more competitive exclusion? More niches than we thought?

Vertical sorting hypothesis

More niches?, continued

Biotic facilitation

Mutualism

Commensalism (e.g., inquilines in tubes)

Can disturbance facilitate diversity?

Sources of disturbance Biotic disturbance

Burrowing, sediment processingpredation (exclosure experiments)

Water turbulence, wave action Human disturbance

Beam trawls – designed to scrape sediments a few centimeters below surface

Otter trawls, etc. along surfaceConsider differences in scale

Effects of local and mid-scale disturbance

Very patchy distributionsLeading to high species turnover in space

Lower density than required for competitive exclusionSuperior competitor is reduced before

inferior competitors disappearRecolonization patterns add to diversity

because larval settlement depends on timing

Effects of trawling on bottom structure

Fished Unfished

Kaiser & Spencer. 1996. Journal of Ecology 65: 348-358

Changes in community diversity with trawling

Why does disturbance from trawling reduce diversity?

Direct effects on habitat

Scale of disturbanceDisrupt biotic interactionsReduce potential for recolonization

Benthic processes: deep ocean systems

Benthic processes: coastal, subtidal systems

Benthic processes: vegetated, subtidal systems