Topic 2 – The Ecosystem

1. Topic 2 – The Ecosystem 2.3 – Changes 2.3.1 - 2.3.4

2. Specifications • 2.3.1 – Explain the concepts of limiting factors and carrying capacity in the context of population growth. • 2.3.2 – Describe and explain ‘S’ and ‘J’ population growth curves. Population curves should be sketched, described, interpreted and constructed from given data.

3. Population Growth Some Facts • Nearly 1.6 million people join the human population each week. • 84 million people join every year. • In three years the human population grows by an amount nearly equivalent to the entire U.S population. • By 2025 the world population could exceed 8 billion

4. Population Studies • The study of any population is an important aspect of science. • Studies on both human populations and smaller ecosystem populations are carried out in depth. • We are going to concentrate on population control of ecosystems but these theories can also be applied to human populations.

5. Population Size • By taking samples and counting the numbers of organisms in a particular habitat, ecologists can study the affects of any factor on the size of a population. • The factors affecting a population size may be biotic or abiotic. • Together they affect the rate at which population grows, and also it’s final size.

6. Biotic Factors Affecting Population Size • How many biotic factors can you think of that might affect population size? • How many abiotic factors can you think of that might affect population size?

7. Biotic Food – both quantity and quality of food are important. Predators – refer back to predator prey relationships. Competitors – other organisms may require the same resources from an environment. Parasites – may cause disease and slow down the growth of an organism. Abiotic Temperature – higher temperatures speed up enzyme-catalyzed reactions and increase growth. Oxygen Availability – affect the rate of energy production by respiration. Light Availability – for photosynthesis and breeding cycles in animals and plants. Toxins and pollutants – tissue growth may be reduced. Biotic and Abiotic Factors

9. Biotic and Abiotic Factors All of these things come under the category of ‘Limiting Factors’

10. Carrying Capacity • When a small population grows in a particular environment, the environmental resistance is almost non-existent. • This is usually because there is plenty of food and no accumulation of poisonous wastes. • Look at the graph of population growth. • This shows how population growth is eventually inhibited by environmental resistance and the environment reaches it’s carrying capacity.

11. Carrying Capacity • The carrying capacity (K) is the maximum number of a species that the habitat can hold. • Once the carrying capacity is reached, unless the environmental resistance is changed, e.g. by a new disease, the size of the population will only fluctuate slightly. • Think of your brine shrimps!?

12. ‘S’ Curves • The graph we have just been looking at is an example of an ‘S’ curve. • This is the type of graph that is almost always seen in nature. • As the energy resources become more scarce the population size levels off at the carrying capacity (K).

13. ‘J’ Curves

14. ‘J’ Curves • Just as in the ‘S’ curve example, a population establishing themselves in a new area will undergo rapid exponential growth. • This type of growth produces a J shaped growth curve. • If the resources of the new habitat were endless then the population would continue to increase at this rate.

15. ‘J’ Curves • This type of population growth is rarely seen in nature. • Initially exponential growth will occur but eventually the increase in numbers will not be supported by the environment. • Can you think of any examples where ‘J’ curve population growth would be extremely desirable.

16. Is there a Carrying Capacity for Homo sapiens? • ‘As we have seen, the human population growth curve is currently following an exponential curve or a "J-shape”. Common sense tells us that such growth cannot continue - otherwise within a few hundred years every square foot of the Earth's surface would be taken up by a human. • Furthermore, experience with other species tells us that, ultimately, resource limitations and/or habitat degradation will force the human population curves to approach an upper limit - the carrying capacity, often symbolized as " K" by ecologists. • It is very natural to ask the linked questions - does humanity have a carrying capacity and, if so, what is it - and when will we reach or overshoot this

17. Activity • Complete the activity – The new zoos 2.3.3 – Describe the role of density-dependent and density-independent factors, and internal and external factors, in the regulation of populations.

18. Density Independent Factors • The following factors are classed as density-independent factors: • Drought • Freezes • Hurricanes • Floods • Forest Fires • These factors exert their effect irrespective of the size of the population when the catastrophe struck.

19. Density Independent Factors This graph shows the decline in the population of one of Darwin's finches (Geospiza fortis) on Daphne Major, a tiny (100-acre) member of the Galapagos Islands. The decline (from 1400 to 200 individuals) occurred because of a severe drought that reduced the quantity of seeds on which this species feeds. The drought ended in 1978, but even with ample food once again available the finch population recovered only slowly.

20. Density Dependant Factors • Intraspecific Competition - competition between members of the same species. • Read the information about the gypsy moth. • Many rodent populations (e.g., lemmings in the Arctic) also go through such boom-and-bust cycles.

21. Density Dependant Factors • Interspecific Competition – this is competition between different species for different resources. • This can include food, nesting sites, sunlight. • This occurs when two species share overlapping ecological niches, they may be forced into competition for the resource(s) of that niche.

22. Specifications • 2.2.4 – Describe the principles associated with survivorship curves including K- and r -strategists.

23. R-Strategists • “I once ploughed up an old field and allowed it to lie fallow. In the first season it grew a large crop of ragweed.” • Ragweed is well adapted to exploiting it’s environment in a hurry – before competitors can become established!

24. R-Strategists • Ragweed’s approach to continued survival is through rapid reproduction. • We say that they have a high value of ‘r’ • They are called r-strategists • Can you think of any other animals that may be r-strategists?

25. R-Strategists • In general, r-strategists share a number of features: • Usually found in disturbed and/or transitory habitats • Have short life spans • Begin breeding early in life • Have short gestation times • Produce large numbers of offspring • Take little care of their offspring (infant mortality large) • Have efficient means of dispersal to new habitats

26. K-Strategists • When a habitat become filled with a diverse collection of creatures competing with one another for resources, the advantage shifts to K-Strategists • K-strategists have a stable population that is close to K. • There is nothing to be gained from a high r. • The species will benefit the most by a close adaptation to the conditions of the environment.

27. K-Strategists • K-strategists share these qualities: • Found in a stable habitat • Long life spans • Begin breeding later in life • Long gestation times • Produce small numbers of offspring • Take good care of their young – infant mortality low • Have evolved to become increasingly efficient at exploiting an ever-narrower slice of their environment.

28. Survivorship Curves • The graph shows 4 representative survivorship curves.

29. Survivorship Curves • Curve A – characteristic of organisms that have low mortality until late in life when aging takes its toll. • Curve B – typical of populations in which factors such as starvation and disease inhibit the effects of aging and infant mortality is high. • Curve C – a theoretical curve for an organism whereby the chance of death is equal at all stages • Curve D – typical of organisms that produce huge numbers of offspring accompanied by high rates of mortality.

30. Survivorship Curves • K-strategists usually have survivorship curves somewhere between A and C. • R-strategists usually have D survivorship curves. • The Californian side-blotted lizard

31. Specifications • 2.3.5 – Describe the concept and processes of succession in a named habitat. • 2.3.6 – Explain the changes in energy flows, gross and net productivity, diversity and mineral cycling in different stages of succession. • 2.3.7 – Describe factors affecting the nature of climax communities.

32. Succession – An intro • The gradual process by which the species population of a community changes is called ecological succession. • A forest following a disturbance such as a fire. • Succession takes places as a result of complex interactions of biotic and abiotic factors. • Early communities modify the physical environment causing it to change. • This in turn alters the biotic community which further alters the physical environment and so on.

33. Succession – What happens? • Each successive community makes the environment more favourable for the establishment of new species. • A succession (or sere) proceeds in seral stages, until the formation of a climax community is reached.

34. Primary Succession • Refers to colonization of regions where there is no pre-existing community. • Can you think of examples where this would occur? • You will be studying glacial moraines in detail as well as the succession occurring on bare rock.

35. Succession • Community changes on a glacial moraines • Study the information on glacial moraines and answer the following questions:

36. Questions – Glacial Moraines • During succession there is a change in species composition of a community. There are also changes in species diversity, stability of the ecosystem, and in gross and net production until a climax community is reached. • Explain what is meant by a climax community. • Explain each of the following changes which occur during succession: • Species diversity increases • Gross production increases • Stability of the ecosystem increases • Give two reasons why farmland in the UK does not reach a climax community.

37. Primary and Secondary Succession • Primary Succession – occurs on newly formed habitats that have not previously supported a community. • Examples? • Secondary Succession – occurs on sites that have previously supported a community of some sort. • Examples?

38. Primary Succession – Bare Rock Mosses, Grasses and small shrubs Lichens, bryophytes and annual herbs Bare Rock After 100-200 years Fast growing trees e.g. Ash Slower growing broadleaf species e.g. oak Complex Community Example for a Northern Hemisphere lithosere: a succession on bare rock

39. In Summary - the 1st Invaders! • These are usually fast growing plants that photosynthesize well in full sunlight. • We call these pioneer species making up the pioneer community • Examples = lichens, grasses, herbs • As these species begin to grow well, they produce shade. Their own seedlings grow more poorly than shade-adapted plants. • Plants that grow well under full sun are replaced by plants that germinate and grow better in deeper shade.

40. Secondary Succession • This type of succession takes place after a land clearance (e.g. from fire or landslide). • These events do not involve loss of the soil. • Secondary succession therefore occurs more rapidly than primary succession. • Humans may deflect the natural course of succession in these circumstances (e.g. by mowing or farming). • This leads to the development of a different climax community than would otherwise develop naturally.

41. Secondary Succession – Cleared Land Open pioneer community (annual grasses) Primary Bare Earth Grasses and low growing perennials Time to develop: Years 1-2 3-5 Young broad leaved woodland Scrub: shrubs and small trees 16-30 31-150 Mature woodland: mainly oak 150+ = climax community

42. Succession Continues • As the plant community changes, the soil will also undergo changes (abiotic factors will change). • Decomposers will join the community as well as animal species. • Animal species have a profound affect on the plant species occurring within a habitat. • Changing conditions in the present community allows for new species to become established (the future community). • Succession continues until the climax community is reached.

43. Wetland Succession • Wetland areas present a special case of ecological succession. • Wetlands are constantly changing: Open water Plant invasion Siltation and Infilling • Wetland ecosystem may develop in a variety of ways:

44. Wetland Succession • In well drained areas, pasture or heath may develop as a result of succession from fresh water to dry land. • In non-acidic, poorly drained areas, a swamp will eventually develop into a fen. • In special circumstances, a an acid peat bog may develop. (may take 5000+ years).

45. Productivity • Think back to the work on food webs/chains • It is often useful to know how much energy is passing through a trophic level over a period of time. • This is called productivity • Productivity is a measure of the amount of energy incorporated into the organisms in a trophic level, in an area, over a certain period of time.

46. Specifications • 2.2.4 – Define the terms gross productivity, net productivity, primary productivity, secondary productivity, gross primary productivity and net primary productivity. • 2.2.5 – Calculate the values of gross and net productivity from given data

47. Productivity • The area is normally one square metre and the time is usually one year. • It is therefore measured in units of kilojoules per square metre per year (kJm-2year-1) • The rate at which producers convert light energy into chemical energy is called primary productivity.

48. Gross Productivity • Gross Productivity (GP) – is the total gain in energy or biomass per unit time. • This is sometimes shown as GPP – Gross Primary Productivity • It is related to the total amount of chemical energy incorporated into the producers. • The producers use some of this energy during respiration and energy needs which is eventually lost to the environment as heat. • The remaining energy is available to the herbivores and is known as net primary productivity (NPP)

49. Recap of Definitions! • Productivity = production per unit time • Primary Productivity = The rate at which energy/biomass is formed through photosynthesis • Secondary Productivity = The rate at which primary material is synthesised into animal tissue per unit area in a given time. • Gross Productivity (GP) = the total gain in energy/biomass per unit time. • Gross Primary Productivity (GPP) = the total gain in energy of the producers. • Net Productivity (NP) = the gain in energy/biomass per unit time remaining after allowing for respiration (R) loses. • Net Primary Productivity (NPP) = the gain in energy/biomass per unit time remaining after allowing for respiration loses which is passed onto the herbivores.

50. Environmental Productivity • Primary productivity varies greatly in different environments. • The rate at which plants can convert light energy into chemical energy is affected by many factors: • Sunlight • Water • Temperature • Amount of nutrients