This lecture introduces the concept of biomes, building upon previous discussions of ecosystems.
Biomes represent a step up in the biological hierarchy from ecosystems.
Biomes Defined
A biome is a large, relatively distinct terrestrial region characterized by similar climate, soil, plants, and animals.
The same biome can be found in different geographic locations around the globe.
Biomes are the largest biotic unit below the biosphere, showing global variation.
Biomes vary in size and area coverage.
Plants and animals within a biome have characteristic adaptations to the biome's conditions.
Importance of Biomes
Biomes serve as the global biotic canvas upon which environmental change occurs.
Understanding biomes is crucial for macroecology, which studies ecology at large spatial scales.
Environmental changes affect biomes globally.
Biomes influence ecosystem processes and functioning, controlled by factors like precipitation and temperature.
Predicting changes in biodiversity and ecosystem services requires considering biomes.
Accounting for variation between biomes is essential for predicting global change.
Biome Classification
Different biome schemes exist, varying in the number and types of biomes identified.
An eight-part system reconciles biomes based on plant and animal types.
The Bailey system relies on climatic domains with subdivisions.
The World Wildlife Fund (WWF) scheme includes 14 biomes and 852 ecoregions.
Maps show examples of biome distributions, with categories like tundra, tropical rainforest, savanna, and temperate grasslands.
The number of biomes depends on the classification system and its purpose.
Biome systems often divide major biomes into smaller classifications at smaller spatial scales.
The WWF classification has 14 biome types with global distribution shown on a map.
Examples include tropical forests (moist, dry, conifer), temperate grasslands, savannas, tundra, and mangroves.
Climate plays a universal role in determining the distribution of terrestrial biomes.
Climate is a major determinant of plant distribution (trees, shrubs, non-woody plants).
Organisms within biomes adapt to the physical environment, particularly climatic variables.
Species' spatial ranges are limited by climatic conditions, such as temperature and moisture.
Temperature and moisture are primary factors limiting species and biome distribution in terrestrial environments.
Aquatic biomes are less easily defined than terrestrial biomes; factors like wind patterns, ocean currents, and light availability are important, and are ultimately driven by climate. These will be discussed later.
Whittaker's Biome Classification
Whittaker's scheme (1975) identifies two major climatic variables influencing biome distribution: average temperature and annual precipitation.
Temperature is measured in degrees Celsius, and precipitation in centimeters.
The scheme maps biomes based on temperature and precipitation:
Tropical rainforest: High rainfall and continuously warm temperatures.
Tundra: Low rainfall and very low temperatures, short growing season.
Temperate forests and grasslands: Intermediate temperatures covering a range of precipitation values.
Desert: Low precipitation.
Boundaries between biomes are often broad and indistinct.
Whittaker identified "indeterminate" biomes where the expected biome was not present.
Whittaker's scheme is based on empirical evidence using actual temperature and precipitation data from sites globally, matched to the biome present.
This supports that temperature and precipitation shape biome vegetation at a global scale.
Modifying Factors
Topography, microclimates, soil types and fertility, and exposure to disturbances can modify the biome present.
Patterns in Biome Distribution
Latitudinal Gradient: From the poles to the equator, there's a gradient of biomes.
Equator: Continuously wet and warm conditions favor tropical rainforests.
Towards the poles: Decreasing growing season length and precipitation lead to transitions from tropical rainforest to moist tropical forest to taiga to tundra and eventually polar ice.
Elevation Gradient: Similar patterns occur with elevation changes on mountains.
Lower altitudes near the equator: Tropical rainforests or wet forests.
Higher altitudes: Transitions similar to latitudinal changes.
Diagrams illustrate the relationship between precipitation, temperature, latitude, and biome type.
Indeterminate Biomes: Consumer Controls
Temperature and precipitation alone don't fully explain biome distribution. Consumers of primary productivity modify biomes.
Example: Graph showing mean annual rainfall (cm) vs. net primary productivity (biomass, kg C/m²):
Climate potential (black line): Expected net primary productivity without disturbance.
Actual (red line): Lower net primary productivity due to consumer controls.
Consumer controls reduce net primary productivity and prevent certain plant types (e.g., trees) from existing.
Major Consumer Controls:
Herbivores: Primary consumers that reduce biomass (e.g., bison).
Fire: Abiotic consumer control that removes biomass indiscriminately.
Human Activities: Forest clearance, developments, and settlements reduce net primary productivity.
These controls reduce net primary productivity and change vegetation structure.
Savannahs with fire transforming woody plants to almost no plants.
Mammoth steppes: Grasslands maintained by megafauna action, now largely lost due to humans.
Anthropogenic Biomes
Some ecologists consider anthropogenic biomes (human-dominated biomes).
Plants: Cool/cold-adapted trees with high growth capacity, adapted to low light under the canopy, no drought tolerance.
Deciduous Forests
Similar to temperate rainforests but with less precipitation.
Climate:
Long growing season.
Moderate precipitation (75-150 cm per year).
Temperatures are mild year-round, rising above 20°C in summer.
Characteristics:
Covers approximately 10% of the land surface area.
Soil is rich in organic matter.
Dominant vegetation: Oak, maple, and beech.
Potential for high species richness.
Adaptations:
Humans are an important consumer control.
Plant adaptations: Shade tolerance, early flowering (e.g., bluebells).
Temperate Grasslands
Difficult biome to identify with three important consumer controls:
Grazing animals/herbivores.
Fire.
Human activities (mowing and exploitation).
These consumer controls are extremely important in maintaining the biome.
Climate:
Little precipitation to support trees (25-75cm per year).
Generous growing season (around 180 days).
Mild winters and seasonal hot summers.
Characteristics:
Large biome, covering between 15-40% of the land area, and around 90% has been lost to farming.
Soil organic content is high.
Dominant vegetation are grasses, sedges, and other herbaceous plants.
High species richness and are an important biodiversity repository.
Adaptations:
Grazing, fire, and drought tolerance by plants.
Mediterranean Biome
Approximately 2.5% of the land area, is mostly coastal, and usually a human-modified environment.
Climate:
Consumer Controls: Humans, herbivores, and fires.
Precipitation is rather low (40-70cm), and most of that precipitation usually falls in the wintertime.
Shrubland, shrub, or open woodland is what's usually found.
Fire is very important to regulating this biome.
Characteristics:
Soil is usually nutrient-poor.
High Species richness with low to moderate primary production due to the lack of rainfall.
The famous Chapparall's can be found in the biome.
Adaptations:
Quite a few nocturnal animals because the availability of water that need to conserve water, such as foxes, pumas, lizards, and tortoises.
Plants heavily defended because nutrients are so rare, so they can evolve and become spiny, they can have secondary defense compounds to deter herbivores, which also we can exploit for medicines.
Desert Biome
Biome with the lowest amount of rainfall, usually much less than 25cm per year.
It excessively limits plant growth.
Climate:
Seasonal temperatures can swing wildly within a day, perhaps from around freezing in the morning, um, to well above 30 in the afternoon.
Covers 30% of land area.
There is a sparse canopy a lot of background because plant growth is limited.
Characteristics:
In nutrient-poor soils, often it insults to call the dominant vegetation.
Low to moderate species richness:
High endemic, localized by distribution, such as threatened species.
Adaptations:
Herbaceous plants dominate.
Animals are typically nocturnal and small.
Plants are Succulent and can store water and, have reduced leave.
Savannah Biome
Cross biome with scattered trees. Also a Tropical climate, constant warmth, and seasonal precipitation. Covers 20% of land area.
Climate:
Savannah and Grazing are the two important consumer controls.
There can be up to 80% of tree coverage, but it is usually dominated by grasslands.
Can be dominated by grasslands with high species richness.
Characteristics:
Intact megafauna, this is the last place we can really see the megafauna in action.
Adaptations:
Plant adaptions usually need to have fire, grazing, and be drought-adapted because Rainfall that does occur occurs only in a wet season. rainfall is extremely low, and then all of the rain comes together in that wet season.
Tropical Rainforest
Extremely large levels of rainfall, up to and above four meters per year. Constant long growth due to Continuously Warm temperatures Remaining above 25 Degrees Celsius. So extremely wet and humid.
Climate:
A species-rich biome with the lush growth of trees and understory plants, even though in Some tropical forests, seasonal precipitation regulates growth a little bit (very widely distributed).
Characteristics:
20% of Land Area.
The soil a Highly weathered, ancient, and nutrient-poor.
Dominant Vegetation - Canopy often dominated by several layers of palm. Believe that some woody plants maybe in several layers of woody plants below the canopy trees, and maybe spacious plants right at the very bottom. Incredibly high species richness and very high primary productivity.
Adaptations:
The animals. Um, we see the most species-rich insect, reptiles, and amphibians communities that anywhere on the planet.
Plants (Shade tolerance, competition for light buttress roots, so if you're going to grow up tall into the canopy to get that light, you need to have some support, Drip tips, drip tips, all that water can break leaves, so drip tips are adaptations that allow water to funnel off the leaves, avoiding damage.
Aquatic biomes
There are some water and freshwater biomes. Freshwater doesn't cover very much of the land surface area at all. It contains much of its water in the oceans and seas, uh, as the dominant land area on the planet. In these biomes, light productivity, and temperature were the limiting factors. There is sufficient life synthesis, photosynthesis in the aphotic zone or no light productivity.
Nutrient-poor water vs Shallow nutrient-riched murky water limit distribution
The biomes in lakes have different zones, a little zone close to the shore, a zone down at the bottom, and eliminating, say, and away from the shore and above the prefrontal zone.
Marshes and swamps
Covered by shallow freshwater for part of the year.
The soil's often anaerobic or waterlogged, and There are water-tolerant plants.
Historically, it's been drained and undervalued with drained, a lot of this habitat in the UK filled in. Ecosystem services have recently been recognized.
Esturaies
Fresh and saltwater mix in a highly variable environment.
Productive because nutrients from the land come here and tidal latching actually circulates those nutrients, making them more available to plants.