Topic 2: Ecology

• Ecological system composed of individuals, populations, communities, ecosystems • Parts of Earth where life exists

• Individual organism is member of a species • Biological species concept: can interbreed and produce fertile offspring

• Allows efficient identification and prediction of characteristics • Binomial nomenclature: genus (capitalized) + species (lowercase)

• Dichotomous keys, specimen comparison, DNA surveys

• Same species in same area at same time, capable of interbreeding

• Abiotic: non-living physical factors • Biotic: living components

• Temperature, sunlight, pH, salinity, dissolved oxygen, soil texture

• A niche describes the particular set of abiotic and biotic conditions and resources upon which an organism or a population depends • An ecological niche is the role of a species in an ecosystem • The niche comprises all biotic and abiotic interactions that influence the growth, survival and reproduction of a population, including how food is obtained • Include: some of the parameters of a niche for a named species

• Populations interact in ecosystems by herbivory, predation, parasitism, mutualism, disease and competition, with ecological, behavioural and evolutionary consequences • Consider: one example of each relationship and consider how the relationships influence the population dynamics of the interacting populations and the selective pressures involved

• Carrying capacity is the maximum size of a population determined by competition for limited resources • Include: examples of resources that may affect carrying capacity, including biotic and abiotic factors

• Population size is regulated by density-dependent factors and negative feedback mechanisms • Density-independent factors may influence population size, but density-dependent factors tend to regulate populations around carrying capacity • In addition to competition for limited resources, include the increased risk of predation and the transfer of pathogens in dense populations • These are examples of negative feedback returning a population to equilibrium

• Population growth can either be exponential or limited by carrying capacity • If there are no limiting factors, population growth follows a J-curve (exponential growth) • When density-dependent limiting factors start to operate, the curve becomes S-shaped • Consider S-curves and boom-and-bust patterns, such as reindeer on St Matthew Island

• Limiting factors on the growth of human populations have increasingly been eliminated, resulting in consequences for sustainability of ecosystems • Include: the effects of elimination of natural predators, technological advances, and degradation of the environment

• Carrying capacity cannot be easily assessed for human populations • This is because of the broad and changing ecological niche of humans • Include: the idea of populations achieving equilibrium within ecosystems, but human populations being less limited due to mobility of resources • The expansion of the human niche also takes place through technological advances and changes in consumption

• Population abundance can be estimated using random sampling, systematic sampling or transect sampling

• Estimate population size for non-mobile organisms • Measures: percentage cover, frequency, abundance

• Estimate population size for mobile organisms • Lincoln index: Population = (M × N) / R

• Collection of interacting populations within ecosystem

• Location where community, species, population, or organism lives

• Energy and matter can enter and exit • Community + physical environment interactions

• Natural property of ecosystems • Inputs balanced by outputs in steady-state • Some persist for millions of years (e.g., tropical rainforests)

• Can lead to tipping points • Example: Amazon deforestation reduces transpiration → reduced rainfall

• Keystone species have a role in the sustainability of ecosystems • There is a disproportionate impact on community structure of keystone species and the risk of ecosystem collapse if they are removed • Consider: two examples • For example, purple sea stars controlling mussel populations on the North Pacific coast that would otherwise overwhelm the ecosystem; elephants feeding on shrubs and trees

• The planetary boundaries model indicates that changes to biosphere integrity have passed a critical threshold • There is an interrelationship between ecosystems and species diversity • Disturbance of ecosystems due to human activity has led to loss of biosphere integrity • Extinction rates provide evidence that the planetary boundary for biosphere integrity has been crossed

• To avoid critical tipping points, loss of biosphere integrity needs to be reversed • Ecosystem damage and loss of species can be slowed by protecting the integrity of ecosystems • Protecting ecosystems ensures the preservation of the niche requirements essential for the ongoing survival of a species

• Clades show evolutionary relationships within a taxonomic group • All clade members evolved from a common ancestor

• Traditional hierarchy: kingdom, phylum, class, order, family, genus, species • This hierarchy does not always match evolutionary divergence patterns

• Fundamental niche: conditions/resources where a species could survive and reproduce without limiting factors • Realized niche: actual mode of existence shaped by adaptations and competition • Examples: Connell barnacle species; brown and green anoles

• Life cycles vary in reproductive behaviour and lifespan • K-strategists: stable communities, few offspring, high survival chance • r-strategists: rapid colonization, many offspring, limited survival provision

• Classification, niche requirements and life cycles help explain human impacts • Examples: climate-driven plant life-cycle changes affecting animal life cycles

• Sustained by supplies of energy and matter • Open systems with exchanges

• Energy transformed but not created or destroyed • Transformations: light to chemical, chemical to heat

• Photosynthesis and cellular respiration

• Light energy → chemical energy (glucose) • Stored as biomass by autotrophs

• First trophic level in food chain • Plants, algae, photosynthetic bacteria

• Releases energy from glucose • Converts to usable chemical form

• Some energy transformed to heat during respiration • Not 100% efficient

• The second law of thermodynamics states that energy transformations in ecosystems are inefficient • The second law of thermodynamics relates to the quality of energy, and that when energy is transformed, some must be degraded into a less useful form, such as heat • In ecosystems, the biggest losses occur during cellular respiration • The second law of thermodynamics explains why energy transfers are never 100% efficient

• Consumers gain chemical energy from carbon (organic) compounds obtained from other organisms • Consumers have diverse strategies for obtaining energy-containing carbon compounds • Include, with examples, herbivores, detritivores, predators, parasites, saprotrophs, scavengers and decomposers

• Because producers in ecosystems make their own carbon compounds by photosynthesis, they are at the start of food chains • Consumers obtain carbon compounds from producers or other consumers, so form the subsequent trophic levels • In a food chain, organic matter flows from primary producers to primary consumers to secondary consumers, and so on

• Carbon compounds and the energy they contain are passed from one organism to the next in a food chain • The stages in a food chain are called trophic levels • Traditionally, decomposers are not included in food chains as they typically gain carbon compounds from a variety of sources • However, consider the role of decomposers in energy transformations in food webs

• There are losses of energy and organic matter as food is transferred along a food chain • Not all the food available to a given trophic level is harvested: of what is harvested, not all is consumed; of what is consumed, not all is absorbed; of what is absorbed • There is, therefore, never 100% transference of organic matter from one trophic level to the next

• Gross productivity (GP) is the total gain in biomass by an organism • Net productivity (NP) is the amount remaining after losses due to cellular respiration • Consider: values of both GP and NP from given data • Losses due to cellular respiration are typically greater in consumers than in producers due to more energy-requiring activity

• The number of trophic levels in ecosystems is limited due to energy losses • Energy released by cellular respiration and lost as heat by organisms is unavailable to organisms in higher trophic levels • Because of this and other energy losses, typically 10% or less of the energy flowing to a trophic level is available to the next level, limiting the length of food chains • Avoid the common misconception that organisms at higher trophic levels must eat more food to get enough energy

• Show complexity of trophic relationships • Arrows indicate energy flow direction • Species may feed at multiple trophic levels

• Dry mass of samples ≈ organic matter mass • Energy measured by combustion

• Represent numbers, biomass, or energy of trophic levels • Pyramids of energy: kJ m⁻² year⁻¹

• PCB, DDT, mercury • Bioaccumulation: increase over time in organisms • Biomagnification: increase along food chain

• Absorb non-biodegradable pollutants • Increase transmission in food chain

• Burning fossil fuels, deforestation, urbanization, agriculture • Reduce primary productivity, disrupt food webs

• Autotrophs: synthesize carbon compounds from inorganic sources • Heterotrophs: obtain carbon compounds from other organisms

• Photoautotrophs use light as an external energy source in photosynthesis • Chemoautotrophs use exothermic inorganic chemical reactions as an external energy source in chemosynthesis • Chemoautotrophs exist in a variety of ecosystems, especially those where there is little or no light • In such ecosystems, chemoautotrophs are the principal source of energy to sustain food webs

• Primary productivity is the rate of production of biomass using an external energy source and inorganic sources of carbon and other elements • The units usually used for productivity are kg carbon m-2 year-1 (kilograms of carbon per square metre of ecosystem per year) • Consider: protocols for determining primary productivity in ecosystems • Estimates can be based on photosynthesizing samples within a laboratory or, in the field, measuring change in biomass of samples (such as grassland) over time

• Secondary productivity is the gain in biomass by consumers using carbon compounds absorbed and assimilated from ingested food • Secondary productivity is ingested food minus faecal waste • The units are the same as those for primary productivity • Faecal matter is not included as it is material that has remained undigested and unabsorbed

• Net primary productivity is the basis for food chains because it is the quantity of carbon compounds sustainably available to primary consumers • Net primary production can be thought of as the plant growth that is sustainably harvestable by primary consumers in natural ecosystems or by farmers

• Maximum sustainable yields (MSYs) are the net primary or net secondary productivity of a system • Consider: the MSYs in natural ecosystems and in agricultural or silvicultural systems

• Sustainable yields are higher at lower trophic levels • Food production is easier to sustain when humans consume lower trophic levels • Plant-based foods are especially relevant

• Ecological efficiency is the percentage of energy received by one trophic level that is passed on to the next level • The percentage varies between ecosystems, trophic levels and species • Work out the efficiency of the transfer of energy between trophic levels with given data • The percentage of energy transferred from one trophic level to the next is very variable, and the value of 10% is neither a fixed amount nor a true average

• The second law of thermodynamics shows how the entropy of a system increases as biomass passes through ecosystems • Entropy refers to the amount of disorder within a system • Living systems are able to maintain a high degree of organization and low entropy through the net increase in entropy resulting largely from cellular respiration

2.3.1Biogeochemical cycles purpose

• Ensure chemical elements remain available to living organisms • Human impact affects ecosystem sustainability

2.3.2Cycle components

• Stores: equilibrium with environment • Sinks: net accumulation • Sources: net release

2.3.3Carbon stores

• Organic: organisms, crude oil, natural gas • Inorganic: atmosphere, soils, oceans

2.3.4Carbon flows

• Photosynthesis, feeding, defecation, respiration, death, decomposition

2.3.5Carbon sequestration

• Capturing atmospheric CO₂ and storing as solid/liquid • Natural: trees absorb CO₂ → biomass • Fossilization into coal, oil, natural gas

2.3.6Ecosystems as carbon stores/sinks/sources

• Sink: photosynthesis > respiration (e.g., young forest) • Store: balanced (e.g., mature forest) • Source: respiration > photosynthesis (e.g., forest fire)

2.3.7Fossil fuels as carbon stores

• Fossil fuels are stores of carbon with unlimited residence times • They were formed when ecosystems acted as carbon sinks in past eras and become carbon sources when burned • Consider: the concept of fossil fuels but not the detail of how and when coal, oil and natural gas were formed

2.3.8Agriculture as carbon store/source/sink

• Agricultural systems can act as carbon stores, sources and sinks, depending on the techniques used • Regenerative agricultural methods, such as crop rotation, cover crops and no till, will promote the role of soil as a carbon sink whereas drainage of wetland, monoculture • Cropping over a longer timescale (e.g., timber production) and the subsequent use of harvested products will also affect these roles

2.3.9Ocean carbon exchange

• Carbon dioxide is absorbed into the oceans by dissolving and is released as a gas when it comes out of a solution • While oceans act as a carbon sink, the human use of fossil fuels releases inorganic carbon at a faster rate than oceans can absorb it

2.3.10Ocean acidification

• Increases in concentrations of dissolved carbon dioxide cause ocean acidification, harming marine animals • Small decreases in pH can interfere with calcium carbonate deposition in mollusc shells and coral skeletons

2.3.11Alleviating carbon cycle impacts

• Measures are required to alleviate the effects of human activities on the carbon cycle • Consider at least three required measures • These include low-carbon technologies, reduction in fossil-fuel burning/soil disruption/deforestation, carbon capture through reforestation and artificial sequestration

2.3.12(HL)—Lithosphere carbon stores

• The lithosphere contains carbon stores in fossil fuels and in rocks, such as limestone, that contain calcium carbonate • The residence time for carbon in these stores can be hundreds of millions of years

2.3.13(HL)—Carbonate fossilization

• Reef-building corals and molluscs have hard parts that contain calcium carbonate that can become fossilized in limestone • Limestone is the largest store of carbon in Earth systems • Not all limestone is formed by fossilization of animal remains; it can also be formed by both biological and non-biological processes • Details of these processes are not required

2.3.14(HL)—Fossil fuel formation

• In past geological eras, organic matter from partially decomposed plants became fossilized in coal, and partially decomposed marine organisms became fossilized in oil • Formation of coal, oil and gas was greatest in specific geological eras when conditions were most suitable

2.3.15(HL)—Methane production

• Methane is produced from dead organic matter in anaerobic conditions by methanogenic bacteria

2.3.16(HL)—Methane cycle

• Residence time: ~10 years in atmosphere • Oxidized to CO₂ • Potent greenhouse gas

2.3.17(HL)—Nitrogen cycle stores

• Organic: proteins in organisms and dead matter • Inorganic: atmospheric N₂, ammonia, nitrites, nitrates

2.3.18(HL)—Bacterial roles in nitrogen cycle

• Nitrogen fixation: N₂ → ammonia • Nitrification: ammonia → nitrates • Denitrification: nitrates → N₂ • Decomposition: amino acids → ammonium

2.3.19(HL)—Denitrification conditions

• Only in anaerobic conditions (waterlogged soils) • Insectivorous plants use insects as nitrogen source

2.3.20(HL)—Plant nitrogen fixation

• Plants cannot fix nitrogen • Require mutualistic associations with nitrogen-fixing bacteria

2.3.21(HL)—Nitrogen cycle flows

• Mineral uptake, photosynthesis, consumption, excretion, death, decomposition, ammonification

2.3.22(HL)—Human impacts on nitrogen cycle

• Deforestation, agriculture, aquaculture, urbanization

2.3.23(HL)—Haber process

• Industrial production of ammonia for fertilizer • From nitrogen and hydrogen

2.3.24(HL)—Nitrogen planetary boundary

• Exceeded due to human activities • Nitrate increases in biosphere • Major cause: inorganic fertilizers for crops

2.3.25(HL)—Global collaboration needed

• Address uncontrolled nitrogen use • Bring nitrogen cycle within planetary boundaries

• Climate: atmospheric conditions over long periods (~30 years) • Weather: short-term conditions

• Group of comparable ecosystems in similar climatic conditions • Influenced by precipitation, temperature, insolation

• Abiotic factors • Temperature and rainfall patterns determine ecosystem type

• Freshwater, marine, forest, grassland, desert, tundra • Each has characteristic limiting factors, productivity, diversity

• Explains atmospheric circulation • Distribution of precipitation and temperature by latitude • Three cells: Hadley, Ferrel, polar

• Oceans absorb solar radiation • Ocean currents distribute heat globally

• Changing climates and biome shifts • General trend: poleward and higher altitude movement

• Tropical (seasonal and equatorial) • Temperate (maritime and continental) • Polar

• Temperature/rainfall pattern may not determine biome • Human interventions, local conditions affect development

• El Niño Southern Oscillation • Fluctuation in wind and sea surface temperatures (tropical Pacific) • El Niño and La Niña: opposite extreme states

• El Niño: weakened Walker circulation, decreased upwelling • La Niña: strengthened Walker circulation

• Low-pressure systems with strong winds • Hurricanes/typhoons: sustained winds > 119 km/hr

• Rising ocean temperatures increase intensity and frequency • Warmer water and air have more energy

• Changes in community along environmental gradient • Factors: elevation, latitude, tidal level, soil, distance from water

• Measure biotic and abiotic factors along gradient • Determine variables affecting species distribution

• Replacement of one community by another over time • Due to biotic and abiotic variable changes • Zonation: spatial; succession: temporal

• Each sere changes conditions for next community • Competition until stable climax community reached • Example: mosses → soil formation → larger plants

• On newly formed substratum without soil • Examples: volcanic rock, retreating glaciers, wind-blown sand

• On bare soil with pre-existing community • Examples: abandoned agriculture, post-fire forest

• Energy flow, productivity, species diversity, soil depth, nutrient cycling

• Depends on diversity and resilience • Succession increases diversity → resilience and stability

• Climatic factors, bedrock, soil properties, geomorphology • Fire, weather events • Top-down influences from consumers (e.g., wolves, elephants)

• Early stages: low GP, high NP (biomass accumulating) • Later stages: high GP, NP approaches zero (balanced respiration)

• r-strategists: many offspring, pioneer communities • K-strategists: few offspring, climax communities

• Challenged and uncertain • Alternative stable states due to random events

• Human activity diverts succession progression • Examples: removal of carnivores, domesticated livestock grazing