Interactions in ecosystems

1. Interactions in ecosystems Organisms interact with their environment. There are biotic (density-dependent) and abiotic (density-independent) factors in an ecosystem. Density-dependent factors effects change as population changes Density-independent factors effects are independent of the population density http://courses.scholar.hw.ac.uk/vle/scholar/session.controller?action=viewContent&contentGUID=dd12253b-17a9-7dcf-48dc-8ad7d12aaea5

2. Interactions can be between or within species. Intraspecific Interactions between members of the same species Interspecific Interactions between members of different species The type of interaction influences the intensity as members of the same species often need the same resources but members of different species often need slightly different resources

3. Population growth graph SCHOLAR - Advanced Higher Biology : Density dependent and density independent factors

4. Populations Populations show changes over time but the numbers will be relatively stable over long periods of time.

5. Predation A predator is an organism that consumes part or all of another organism. Should this definition challenge our understanding of what makes a predator? Scholar activity predator/prey population cycles. http://courses.scholar.hw.ac.uk/vle/scholar/session.controller?action=viewContent&contentGUID=a9c539ac-1244-57f1-e70b-1b25839e3f12

6. Predator-prey cycles

7. Predator - Prey Interaction Predator and prey numbers will be interconnected. As one varies so will the other, the predator numbers usually tracks the prey numbers. The number of predators usually shows a similar pattern to prey but lags behind. This is because it takes time for populations to change. Homeostatic control of populations Because of the carrying capacity of an environment and the factors involved in controlling population numbers, the system operates like a negative feedback control system.

8. Predators and diversity Predators can help diversity by keeping in check other species that might otherwise reduce the diversity. e.g. Canada otters sea urchin

9. Prey defences against predators Camouflage (crypsis and disruptive) Organism blends in with its background Warning (aposematic) colouration Organism has markings advertising it to be poisonous or unpleasent to eat. Batesian mimicry Organism that is edible has same colouration as a harmful organism. Mullerian mimicry Organisms that are inedible or poisonous have evolved the same or similar warning signals.

10. disruptive crypsis Mullerian warning Batesian

11. Grazing Grazers move from one ‘victim’ to another. (victim?) Grazers can have a significant effect on diversity in ecosystems. If plant meristems are removed they may not be able to grow. Plants with very low growing meristems (basal meristems) plantain, grasses and dandelions tolerate grazing well.

12. Grazing of dominant species e.g. grasses can allow other less dominant plant species the opportunity to grow. Over grazing can lead to a loss in diversity as the plants without basal meristems can be eliminated. Unselective grazers and selective grazers and how they affect biodiversity? Scholar activity

13. Competition Competition will occur when there is a common need for a resource in an ecosystem. Competition can be either intraspecific or interspecific Interference competition is where organisms of different species actually fight over a resource. Exploitation competition is when two species use the same resource and the survival chances of both is reduced.

14. Ecological niches The role an organism occupies in an environment and how it interacts with the biotic and abiotic factors. Competitive exclusion principle When two species have identical niches they will compete and the most successful will drive the other to extinction. Scholar activity Competition between Paramecium.

15. Niches Fundamental niche An organism not experiencing competition will be able to make use of all its resources. Realised niche An organism will experience competition and these are the resources the species will actually be able to make use of. Resources are divided between the competitors, Resource partitioning. Scholar activity on seashore niches.

16. The effect of exotic species An exotic species is a species introduced to an ecosystem in which it would not normally be found. Can be deliberate or accidental. Examples include rhododendrons, grey squirrels, mink and New Zealand flatworm.

17. Survival of weak competitors Competition often effects populations differently. If environmental conditions change and the dominant organism is no longer best adapted to survive and dies out, this can give weaker competitors a chance to increase in number. An example of this is when the mass extinction of the dinosaurs gave the mammals a chance to fill the niches left behind.