Chapter 5: Biological Communities: The Biome Concept

1. Chapter 5: Biological Communities: The Biome Concept

2. The Case of the Cactus Look-Alike • Cactus-like plants are common in Africa. • These plants do not belong to the cactus family, Cactaceae: • Cactaceae are native only to New World • look-alikes may be in spurge family, Euphorbiaceae • This situation illustrates convergence of species descended from different ancestors.

3. Convergence • Convergence is the process by which unrelated organisms evolve a resemblance to each other in response to common environmental conditions: • similar adaptive responses emerge in response to particular selective conditions • an example: mangroves worldwide typically have thick, leathery leaves, root projections, and viviparity

4. (a) A tree-forming cactus in Mexico; (b) an East African euphorb tree. Both converged in response to dry climate

5. The Biome Concept • Character (plant and animal life) of natural communities is determined by climate, topography, and soil (or parallel influences in aquatic environments). • Because of convergence, similar dominant plant forms occur under similar conditions. Biomes are categories that group communities by dominant plant forms. • In North America: • tundra, boreal forest, temperate seasonal forest, temperate rain forest, shrubland, grassland, and subtropical desert • In Mexico and Central America: • tropical rain forest, tropical deciduous forest, and tropical savanna

6. Biomes - Key Points • Geographic distributions of biomes correspond closely to major climate zones. • Not all biome classifications are the same: • some recognize finer or coarser detail • various biomes intergrade continuously and recognizing boundaries is difficult • Matching of biomes and environment occurs because no single type of plant can endure the entire range of conditions on earth.

7. Adaptations and Environment -- Not the Whole Story • Distributions of species are not solely a function of relationships to physical environment: • biotic interactions shape these distributions • chance and history play important roles

8. Climate is the major determinant of plant distribution. • Climatic factors typically establish limits of plant distributions: • the sugar maple, Acer saccharum, in eastern North America, is limited by: • cold winter temperatures to the north • hot summer temperatures to the south • summer drought to the west

9. Ecological Tolerances • Several tree-sized maples in eastern North America have distributions that broadly overlap that of sugar maple: • because of different ecological tolerances, these other species exhibit distinctive environmental preferences, even when their ranges overlap: • black: drier, better-drained soils high in calcium • silver: moist, well-drained soils • red: wet and swampy or dry, poorly-developed soils

10. Related species may differ in their ecological tolerances

11. Topography in mountains creates a wide range of moisture conditions • each species exhibits a local and distinctive optimum – the type of site in which it does best • coast redwood dominates center of moisture gradient • cedar, Douglas fir, madrone occur at drier end of the moisture gradient • big-leaf maple, California bay tree occur at wetter end of moisture gradient

12. Form and function match the environment. • Adaptations match each species to the environment where it lives: • all species are to some extent specialized: • insect larvae from ditches and sloughs survive without oxygen longer than related species from well-aerated streams • marine snails from the upper intertidal tolerate desiccation better than their relatives from lower levels • we recognize both specialists and generalists

13. (a) Mesquite leaves are subdivided into leaflets that facilitate dissipation of heat (b) Paloverde leaflets are tiny; thick stems are responsible for photosynthesis (c) Limberbrush has broad, succulent leaves produced for only a few weeks

14. Other Considerations • Certain species make their environments more favorable for themselves: • decaying foliage of evergreen species of poor soils produces organic acids, leaching minerals from soil • Availability of moisture is the single most important climatic factor defining biomes: • because heat influences moisture stress, temperature and precipitation together are the determinants of boundaries of major biomes

15. Climate defines the boundaries of terrestrial biomes. • A widely adopted climatic classification is that of Heinrich Walter: • Walter’s scheme is based on the annual course of temperature and precipitation: • focuses on conditions of moisture and temperature stress that determine plant form • recognizes 9 zones, from Equatorial (Tropical rain forest) to Polar (Tundra)

16. Walter’s climate zone classification

17. Whittaker’s Scheme 1 • Whittaker related major biomes to annual temperature and precipitation. • The biomes fall in a triangular area with corners representing following conditions: • warm-moist • warm-dry • cool-dry • Whittaker’s scheme is similar in many respects to Walter’s: • Whittaker starts with vegetation and relates climate

18. Whittaker’s Scheme 2 • Equatorial and tropical climate zones (mean temperatures between 20oC and 30oC) • precipitation ranges from 0 to 400+ cm/yr • Temperate climate zones (mean temperatures between 5oC and 20oC) • precipitation ranges from 0 to 300+ cm/yr • Boreal and polar climate zones (mean temperatures less than 5oC) • precipitation typically below 200 cm/yr

19. Whittaker’s biomes

20. Whittaker’s Scheme - Other Considerations • Fire shapes vegetation toward drier end of spectrum within each temperature range: • typically in grassland and shrub biomes where: • moisture is intermediate (sufficient productivity for fuels to accumulate) • seasonal droughts occur (fuels dry out sufficiently to burn) • fire favors grasses and forbs over woody plants • species of these systems are adapted to or are specialized for frequent fires

21. Walter’s Climate Diagrams • Walter’s climate diagrams relate monthly temperature and precipitation through the year: • 20 mm of monthly precipitation is equated with 10oC in temperature • vertical scales permit ready identification of periods of water deficit and water abundance • Localities within the same climate zone have similar climates worldwide.

22. Global distribution of major biomes

23. Temperate Climate Zones • Temperate zone is characterized by temperatures between 5o-20oC at low elevations, with frost throughout the zone: • found between 30oN and 45oN in North America and between 40oN and 60oN in Europe • biomes differentiated by: • total amounts and seasonality of precipitation • length of frost-free season or growing season

24. Temperate Seasonal Forest Biome 1 • Develops under moderate climates with winter freezing: • growing season is 130-180 days • precipitation exceeds evapotranspiration • Found principally in eastern North America, Europe, and eastern Asia. • Vegetation is dominated by deciduous trees with understory of small trees and shrubs, often abundant herbs.

25. Temperate Seasonal Forest Biome 2 • Warmer and drier parts of the temperate seasonal forest biome are dominated by needle-leaved trees, typically pines: • found principally in North America along the Atlantic and Gulf coasts and at higher elevations in the western states • needle-leaved forests typically develop under conditions of drought and nutrient stress • fires may be frequent and species can resist fire damage

26. Temperate Rain Forest Biome • Develops primarily in warm temperate climates: • mild winters, heavy winter rains, summer fogs common • Found principally in the northwestern US, adjacent British Columbia, southern Chile, New Zealand, Tasmania. • Vegetation is dominated by tall evergreen trees, such as Douglas fir and coastal redwood: • extensive during Mesozoic era • not as diverse as its tropical counterparts

27. Temperate Grassland/Desert Biome 1 • Found in continental climate zones: • summers are hot and wet; winters are cold • growing season is 120-300 days • fires are a dominant influence • Extensive grasslands develop, called prairies in North America, steppes in central Asia. • Vegetation is dominated by grasses and forbs: • fire is frequent and most species have underground fire-resistant stems

28. Temperate Grassland/Desert Biome 2 • Grasslands grade into deserts in arid continental climates: • winters are cold and summers hot • precipitation is 25-50 cm/yr • fires are infrequent because of low fuel accumulation • grazing can exert strong pressure on vegetation • Grasslands are widespread in the western US, from Great Basin southward. • Vegetation is dominated by shrubs, such as sagebrush, or small trees, such as piñon pine and juniper.

29. Woodland/Shrubland Biome • Develops in Mediterranean-type climate (cool, wet winter, warm dry summer): • fires are frequent and most plants have adaptations to fire (resistant seeds or root crowns) • Typically found at 30-40o latitude, west coasts, common in southern Europe, southern California, central Chile, Cape region of South Africa. • Vegetation is dominated by sclerophyllous evergreen shrubs.

30. Subtropical Desert Biome • These are highly variable systems found under extreme aridity: • develop at 20o-30o north and south latitude • rainfall is sparse (less than 25 mm) • creosote bush is common in subtropical American deserts, with associated cacti, shrubs, and small trees: • subtropical deserts typically have summer rainfall, with high species diversity, prominent annual flora

31. Boreal and Polar Climate Zones • These zones have average temperatures below 5oC. • Boreal forest (taiga) develops between temperatures of 5oC and -5oC. • Tundra develops at temperatures below -5oC.

32. Boreal Forest Biome • Climate is extremely cold, with temperatures as low as -60oC in winter: • average annual temperature is below 5oC, precipitation 40-100 cm/yr • growing season is 50-100 days • Boreal forest is centered on a broad belt at 50-60oN latitude across North America and Eurasia. • Also called taiga, vegetation of low diversity dominated by evergreen needle-leaved trees, typically spruce and fir.

33. Tundra Biome • Exceedingly cold climate, with brief, but active, growing season in summer: • soils are permanently frozen, thaw to depth of 0.5-1 m during brief summer growing season • precipitation is less than 60 cm/yr, but soils may be saturated because of impeded drainage • Found at high latitudes, north of boreal forest belt (but superficially similar systems occur in alpine zones). • Tundra is a treeless expanse of dwarf, prostrate woody shrubs.

34. Equatorial and Tropical Climate Zones • Located within 20o of the equator. • Daily temperature variation exceeds monthly variation through the year. • Environments are largely distinguished by differences in the seasonal pattern of rainfall. • Frost is not a factor; plants and animals cannot tolerate freezing.

35. Tropical Rain Forest Biome • Climate is continually warm and moist: • precipitation is in excess of 200 cm/yr, biseasonal, but never less than 10 cm in any month • Occupies three important regions, in South/Central America, West Africa, Indo-Malayan region. • These are exceedingly diverse forests, dominated by evergreen or seasonally deciduous broad-leaved trees, featuring diverse growth forms including climbing lianas (woody vines) and epiphytes (plants that grow on the branches of other plants).

36. Tropical Seasonal Forest/Savanna Biomes 1 • climate is seasonally dry, but sufficient moisture to support forest: • progressively drier tropical habitats support dry forests, thorn scrub, and true deserts • Occur worldwide within the tropics, but typically beyond 10oN or S of the equator. • Tropical seasonal forests have a preponderance of deciduous species.

37. Tropical Seasonal Forest/Savanna Biomes 2 • Savannas are grasslands with scattered trees. • These are typical of large areas of semiarid tropics, especially at high elevations in East Africa. • Rainfall is strongly seasonal: • 90-150 cm/yr but driest 3-4 months receive less than 5 cm each • fire and grazing play important roles in maintenance of these system

38. A pause… Rainforests today

39. Rain forests today • More than one half of tropical forests have already been destroyed • Rainforests once covered 14% of the earth's land surface; now they cover a mere 6% and experts estimate that the last remaining rainforests could be consumed in less than 40 years. • each day at least 80,000 acres (32,300 ha) of forest disappear from Earth. At least another 80,000 acres (32,300 ha) of forest are degraded. Hundreds of species driven to extinction • FAO estimates that 10.4 million hectares of tropical forest were permanently destroyed each year in the period from 2000 to 2005, an increase since the 1990-2000 period, when around 10.16 million hectares of forest were lost • Experts estimates that we are losing 137 plant, animal and insect species every single day due to rainforest deforestation. That equates to 50,000 species a year. As the rainforest species disappear, so do many possible cures for life-threatening diseases. Currently, 121 prescription drugs sold worldwide come from plant-derived sources. While 25% of Western pharmaceuticals are derived from rainforest ingredients, less that 1% of these tropical trees and plants have been tested by scientists. • There were an estimated ten million Indians living in the Amazonian Rainforest five centuries ago. Today there are less than 200,000.

41. Deforestation for cattle grazing

42. Modification of the Biome Concept for Aquatic Ecosystems • The biome concept in its strict sense does not exist for aquatic ecosystems: • biomes were developed for terrestrial ecosystems, where growth form of dominant vegetation is distinguishing factor • aquatic ecologists have tended to develop independent classifications for aquatic systems, focused predominantly on physical factors

43. Aquatic Ecosystems - Streams • Streams form wherever precipitation exceeds evaporation, draining excess water. • Streams may be divided into principal habitats: • riffles (where water runs over rocky substrate) • pools (deeper stretches of slow-moving water) • Streams exhibit continuous change in conditions from headwaters downstream, captured in river continuum concept. • Streams exhibit downstream drift of organisms/material.

44. Conditions of a stream differ in pools and riffles

45. Aquatic Ecosystems - Lakes • Lakes form in any kind of depression (typically effects of glaciation or geological activity). • May be divided into principal habitats: • littoral zone (shallow zone with rooted vegetation) • limnetic zone (open water beyond littoral zone) • benthic zone (bottom sediments, habitat for burrowing animals and microorganisms)

46. Ecological zones in a lake

47. Aquatic Ecosystems - Estuaries • Are special environments at the mouths of rivers, especially where outflow is partially enclosed (such as barrier islands). • Unique because they are the interface between fresh and salt water habitats: • typically highly productive because of influx of nutrients and their rapid exchange between sediments and surface waters • often edged by extensive tidal marshes with emergent vegetation

48. Estuaries …

49. Aquatic Ecosystems - Oceans • Oceans are enormously complex systems, with conditions varying with temperature, depth, current, substrate, tides. • Oceans are often classified according to depth: • littoral zone (between high and low tides, exhibits dramatic zonation) • neritic zone (beyond low tide to edge of continental shelf, often subdivided into photic and aphotic zones, typically productive) • oceanic zone (deeper waters, also divided into photic and aphotic zones, typically unproductive)

50. Ocean’s ecological zones