9.1 Why a species is where it is #
9.1.1 Three answers, all required #
A species occurs where it evolved or where it dispersed, where the environment is inside its niche, and where interactions have not excluded it. History, conditions, and other species. A map that matches climate does not prove climate is the only cause. A related species may be absent because it never arrived. Wallace’s line, between the Asian and Australian continental shelves, running between Borneo and Sulawesi and between Bali and Lombok, is a faunal boundary maintained by deep water and a separate history. It is not a barrier to every taxon. Some groups cross it. The phylogeny that decides dispersal against vicariance is Evolutionary Biology.
Vicariance is a barrier appearing across a once-continuous range: a continent splitting, a mountain rising, a climate becoming hostile in the middle of a range. Dispersal is movement across a barrier that already existed. Oceanic islands, never connected to a continent, such as Hawaii and the Galápagos, are colonised by dispersal and then often by speciation. Continental islands and habitat fragments start with the species of the mainland and then lose them. The two histories are the reason a theory built for one should not be pasted onto the other without a remark.
9.1.2 What a transplant shows #
A transplant beyond the range that survives, and reproduces, says the limit is dispersal or history, not the current climate. A transplant that dies says the site is outside the niche, or that enemies there are fatal. A transplant that survives only when caged says enemies, not climate. Connell’s barnacles were this logic on one shore. Range edges are often set by different factors on different sides: cold towards the pole, drought or a competitor elsewhere. A climate envelope fitted to the present range assumes the range is at equilibrium with climate. That fails when the climate has just changed, when the species is still spreading, and when a competitor or a mutualist sets the edge.
9.2 Terrestrial biomes #
A biome is a large vegetation class under a climate, recognised by physiognomy. The same biome on different continents has different species that look alike. Cacti in the Americas and succulent euphorbias in Africa are the usual convergence. Convergence is evidence of similar selection pressures. It is not evidence that the plants are close relatives. The phylogeny is Evolutionary Biology. Plant Biology owns the organs that do the converging.
Biomes Of The World
Whittaker plotted biomes on mean annual temperature against mean annual precipitation. Forests occupy the wet side, grasslands and shrublands the middle, deserts the dry side. The boundaries are bands. Soil and fire move them. Walter climate diagrams plot temperature and rainfall through the year on a conventional scale (10 °C lined up with 20 mm of rain) so that a dry season is visible. The alignment is a graphical convention, not a physical law.
9.2.1 Forests #
Tropical rainforest is warm, wet, and weakly seasonal or with a short dry season. Richness is high. A large share of the nutrient capital is often in the biomass rather than in old weathered soil (Chapter 2). The canopy is multi-layered, and the understorey is light-limited. Tropical dry forest has a long dry season and drops leaves. Savanna has a grass layer and scattered trees. Fire and large herbivores help hold it against forest, and in parts of Africa and South America the two states can be alternative stables under the same rainfall (Chapter 6).
Temperate deciduous forest has a cold winter and a growing season, and the trees drop leaves. Temperate rainforest, as in the Pacific Northwest, is cool and very wet, often with large conifers and a large mass of decaying wood. Boreal forest, the taiga, has long winters, conifers, slow decomposition, and a large soil-carbon pool. Mediterranean-type shrubland — chaparral, maquis, fynbos, kwongan — has wet winters and dry summers, frequent fire, and, in the Cape and in southwestern Australia, very high local endemism on nutrient-poor soils. Those shrublands are not one flora. They are one climate and one fire regime.
9.2.2 Grassland, desert, tundra, alpine #
Temperate grassland receives intermediate rainfall. Fire and large herbivores suppress trees. Much of the biomass is below ground. Soils are often rich where leaching is moderate. Desert is not defined by heat. Cold deserts exist. Desert is low precipitation relative to evaporative demand, with plants spaced by the water each one commands. Tundra has permafrost, a short season, low stature, and a large soil-carbon store in cold, wet organic matter. Alpine vegetation resembles tundra because of temperature and does not share its day-length regime, its air pressure, or its species pool. A single category called “tundra and alpine” is a climate analogy, not a flora.
9.3 Fresh water and the sea #
9.3.1 Lakes and rivers #
Lakes are divided, for description, into the littoral near shore, the pelagic open water, the profundal dark deep water, and the benthic bottom. Oligotrophic lakes are low in nutrients, clear, and usually oxygenated at depth. Eutrophic lakes are high in nutrients and production, and the hypolimnion can go anoxic in summer (Chapter 2). Eutrophic is a trophic state, not a synonym of polluted. People often push lakes towards it. Dystrophic brown-water lakes are stained by humic acids and are not the same as eutrophic green lakes.
The river continuum (Vannote and colleagues) is a hypothesis about a gradient from a shaded, leaf-fed headwater, through a productive middle reach, to a turbid large river fed by fine particles from upstream. Shredders of leaves give way, in the teaching version, to collectors of fine particles. The continuum fails where a dam, a floodplain, or a tributary resets the gradient. In large tropical rivers the flood pulse (Junk and colleagues), the regular connection of the river to its floodplain, explains more of the production than the original temperate continuum did. A river is also a catchment’s drain, which is why Chapter 8’s nutrient budget is measured here.
9.3.2 Coasts and the open ocean #
Intertidal zonation repeats the barnacle logic: emersion and wave exposure set physiological limits, and competition and predation set biological ones. Estuaries are variable in salinity, often high in production, nurseries for some coastal fishes, and traps for sediment and for pollutants. Salt marshes and mangroves are intertidal vascular plants, with high carbon storage in waterlogged soils and a structural role as nurseries. The carbon store is “blue carbon” in the informal name. The mechanism of storage is slow anoxic decomposition, Chapter 7.
Coral reefs support high diversity in nutrient-poor water. The production is tied to the symbiosis between the coral and dinoflagellates, the zooxanthellae. Bleaching is the loss of those symbionts under heat and under other stresses. The cell biology of the alga is not redone here. The ecosystem consequence is a loss of production and, if the coral dies, a loss of the foundation species. A reef is not a rainforest except in the weak sense that both are diverse and three-dimensional. The limits, the nutrients, and the diversity mechanisms differ.
Kelp forests occupy cold, nutrient-rich coasts and are the otter–urchin cascade of Chapter 6. The open ocean is most of the area and a low production per area. Upwelling systems are a small fraction of the area and a disproportionate share of marine fish production. The deep sea is dark. Its production, away from vents, is what falls from above. Hydrothermal vents and cold seeps support local communities on chemoautotrophy. The biochemistry is Microbiology. The ecological pattern is scattered, locally dense communities in a desert of falling particles.
9.3.3 The microbial loop #
A large fraction of marine primary production becomes dissolved organic matter. Heterotrophic bacteria use it. Protozoa eat the bacteria and are eaten in turn. That microbial loop (Azam and colleagues) returns carbon and nutrients to the food web that a diagram of only diatoms, copepods, and fish drops. Viruses lyse the microbes and short-circuit the transfer again, the viral shunt. Viral biology is Microbiology. The loop is why a marine energy pyramid cannot be read off the fish.
9.4 Islands and the latitudinal gradient #
9.4.1 Equilibrium theory #
MacArthur and Wilson pictured the number of species on an island as a balance. Immigration falls as more of the source pool is already present. Extinction rises as more species share the island and populations are smaller. A larger island has lower extinction rates, from larger populations and more habitats. A nearer island has higher immigration. The equilibrium is a richness, not a named list. Turnover continues: the number can be steady while the identities change.
The species–area relation is often written S = c A^z. On a log–log plot, z is the slope. For islands, z is often near 0.25, commonly somewhere from about 0.2 to about 0.35. For samples inside a continuous mainland it is often lower. z is not a constant of nature. Invented arithmetic, to show what a slope does: if z = 0.3 and area falls to a tenth, the ratio of species is 0.1^0.3, about 0.5. A 90% loss of area is predicted to hold about half the species at equilibrium, eventually. The abuses of that sentence are as important as the arithmetic. z varies. A fragment is not an oceanic island: it has edge effects, a surrounding matrix that is not ocean, and a starting richness that is too high for its new area. The loss is a relaxation towards a lower equilibrium, and it takes time. Extinction debt is that lag. A survey done just after the fragments were cut overestimates the richness the landscape will keep. Chapter 10 uses the debt. It does not treat the half-the-species figure as a forecast for a named reserve.
Simberloff and Wilson cleared the arthropods from small mangrove islands and watched richness return towards the previous numbers, with turnover of identities. It is a classic result on small, near islands. Extrapolation to forest vertebrates in habitat fragments is an analogy. It is not the same experiment. These notes do not describe the method of removal.
The 1967 model ignored evolution on the island and ignored identity. Adaptive radiation on remote islands is what happens when isolation lasts long enough for speciation to add species that immigration did not bring. Hawaiian Drosophila and Darwin’s finches are Evolutionary Biology. The ecological condition is a remote island and an empty set of roles. The rescue effect (Brown and Kodric-Brown) lowers extinction on near islands because immigrants of species already present reduce local disappearance. The target effect raises immigration on large islands because they are easier to hit. Both modify the simplest picture.
9.4.2 The latitudinal gradient #
Richness of many taxa is higher in the tropics. The pattern is old. A single cause is not agreed. Hypotheses that remain live include a greater area of tropical biomes through time, higher productivity and larger populations, higher speciation or lower extinction, a longer time without being reset by glaciation at high latitudes, and stronger biotic interactions, including the Janzen–Connell enemies of Chapter 6. They are not mutually exclusive. An answer that names one of them as “the” reason is incomplete. The gradient has exceptions, including many conifers, whose diversity is higher outside the wet tropics. Not every clade follows the majority pattern.
Endemism is concentration in one place, not richness. A small island can be species-poor and hold species found nowhere else. A priority that uses only richness misses them. Myers’s hotspots combine high endemism with high threat. The label is a prioritisation scheme. It is not a biome, and it is not a claim that the other places can be discarded without a stated goal.
Key terms. Vicariance; climate envelope; biome; physiognomy; oligotrophic; eutrophic; river continuum; flood pulse; microbial loop; bleaching; species–area relation; turnover; relaxation; extinction debt; latitudinal diversity gradient; endemism.
Summary. A range is history plus niche plus interactions, and a transplant separates them. Biomes are climates and growth forms: convergence of cacti and succulent euphorbias is not close kinship. Lakes are trophic states as well as basins, rivers are continua that dams and flood pulses reset, and reefs and kelp are foundation systems with different nutrient regimes. The microbial loop is why marine food webs are not three tidy boxes. Island richness is an immigration–extinction balance, tested on small mangrove islands and widely misapplied as a numerical forecast for fragments. The species–area slope describes equilibrium richness and does not name which species, or how long relaxation takes. The latitudinal gradient is real and multiply caused. Endemism is a different number from richness.
Self-check.
1. A transplant beyond the range survives only inside a cage. Which of history, climate, and enemies is still in play?
2. Why are a savanna and a chaparral not distinguished by their species lists in these notes? What does distinguish a biome?
3. Why can a biomass pyramid in the open ocean not be read from diatoms to fish without the microbial loop?
4. With z = 0.3, a fragment keeps a tenth of the area. What equilibrium richness ratio does the power law give, and why is that not a forecast of next year’s species list?
5. Name two hypotheses for the latitudinal diversity gradient that can both be true. Why does endemism on an island not follow from its richness?
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