Diversity: Spatial and Temporal Trends

Global Diversity Gradient Global diversity measured by number of species that occupy given habitat or area Latitudinal Gradient Strong diversity gradient from pole to pole among both land and marine species In general, diversity higher in low latitudes than high latitudes Longitudinal Differences in Diversity Less predictable than latitudinal gradients Controlled mainly by geographic or local to regional climatic barriers

Product of Latitudinal and Longitudinal Differences In Living Conditions: Biotic Provinces This diagram shows the marine biotic provinces of present-day Earth

Controls on Latitudinal Diversity Gradients Why is there a gradient in diversity between poles ? Most obvious factor: characteristics of climate But what does climate impose on diversity ? Key controlling factor: availability of food resources

Two Big Controls on food availability: Temperature: some biological processes cannot efficiently operate under certain conditions (e.g. tissues can freeze) Light: necessary for photosynthesis; note that it is dark for much of the year in polar regions BUT then why don’t organisms simply adapt to these conditions Note: 1. Some bacteria can live below 0 o C and others possibly above 100 o C 2. Organisms survive just fine in deep sea where sunlight is practically non- existent So must be something other than simply extreme conditions

An important control on diversity: STABILITY of living conditions Diversity especially high in low latitudes due to stability of temperature and light conditions (more constant supply of food). This, in turn, is connected to the Earth’s tilt (if Earth wasn’t tilted, We wouldn’t have seasons, right ?). Pretty simple, but perhaps not particularly obvious because we are trained to think in terms of thinking about static conditions ! So…A general rule: Species of polar regions tend to be generalists whereas species of equatorial regions tend to be specialists High stability of living conditions permits “ways of life” to be more finely divided than in areas with extreme seasonality

Longitudinal Controls on Diversity Again, stability of conditions is key e.g. in marine realm, temperature more stable in and around small land masses such as islands than large masses such as big continents. Diversity is correspondingly high in small land masses. Also, important are geographic barriers which isolate groups of species from one another In addition, local variations in conditions due to effects of ocean currents, etc. impose a great effect on diversity. Even elevated primary productivity in certain areas can affect diversity

Plate Tectonics: Significant in terms of both spatial and temporal variations in diversity Create barriers in dispersal Also affect nutrient flux in Earth system (seafloor spreading Increases carbon flux to ocean, increase primary productivity and possibly an increase in biomass and biodiversity)

Note good scattering of continents, high diversity due to geographic isolation imposed by barriers (mountains, oceans, etc.) -gene flow between populations restricted

Note clustering of continents, low biological diversity due to geographic isolation imposed by barriers (mountains, oceans, etc.) -gene flow between populations at a maximum

Add two other ingredients: Biotic revolutions resulting from new ways of life (e.g. new weaponry and protective devices resulting from arms race) Mass extinctions: tearing down stage sets, allowing new stage designs to be developed. From square one (i.e. the Vendian), what would primitive metazoans have eaten and how would community dynamics have changed through time ? How do organisms maximize obtaining food on the seafloor ?

Jack Sepkoski’s Three Great Evolutionary Faunas: Diversity of marine families through time “Cambrian” fauna “Paleozoic” fauna “Modern” fauna Cambrian fauna: trilobites, inarticulate brachiopods, primitive molluscs & echinoderms Paleozoic fauna: articulate brachiopods, crinoids, cephalopods, corals, ostracods Modern fauna: mostly gastropods, bivalves, bony fishes, crabs & lobsters, echinoids Soft bodied fauna

END OF LECTURE