1. Ecology Lecture 11 Ralph Kirby

2. Niche Constraint & Community Structure All organisms can live and reproduce over a specific range of environmental condition These differ from organism to organism These environmental conditions vary in time and space Thus there is overlap in any environmental gradient However, cannot assume there is no interaction between species Problem is that such interactions are many and relatively diffuse Usually the removal of a single species will have very limited effects and this makes analysis of interactions difficult Removal of multiple species can be major Food webs –Provide a limited but useful view of interactions –Provide information on indirect effects See lynx and white spruce Presence of lynx is good for white spruce due to survival of seedlings

4. Keystone predation –Predation can also influence outcome of interactions between prey species –The starfish preys on many species of mussels, barnacles, limpets, etc –Remove starfish from experimental plots and compare to normal situation –Number of prey species in experimental plots was halved –Diversity was decreased as better competitors excluded other species Indirect mutualism –Two herbivorous Daphnia spp. and their predators, midge larvae and salamander larvae Salamander larvae prey on large Daphnia Midge prey on small Daphnia Salamander and midge can survive together Midge cannot survive alone Salamander can survive alone

5. Thus there are both direct and indirect interactions within a community How do you tell which are important –Rivet model System stable until enough species are lost to make whole system collapse –Redundancy model Most species are not needed A few key species are important Both models are probably correct and importance of each varies with ecosystem Most systems it is not easy to tell how important each species is Can try to do this by splitting species into functional groups –Each group has a similar function and perhaps can replace each other –Trophic levels Primary producers HerbivoresCarnivores Bottom-up control from plant populations Top-down control from carnivores

7. Consider interaction between environment and the community with both direct and indirect interactions Results in a pattern across an environmental gradient This is due to trade-off –Species that can grow well are at an advantage when resource at best –Species than can survive when resource is at worst can survive where previous species cannot –Results in zonation –See effect of moisture below

8. Classic examples of zonation occur in salt marshes Salt provides stress Level of salt varies with distance from low water mark Lower boundary is determined by stress Upper boundary is determined by competition Note also that nutrition can change situation Increase fertilization results in change in species at boundary Tradeoff –Competitive ability –Tolerance of stress

9. Types of gradient and succession Autogenic –Created by community itself Light in a forest Allogenic –Created by physical environment Elevation on a mountain Stress in a salt marsh When plant succession creates autogenic environmental change in a place Both primary and secondary succession Variety of models proposed to explain succession –Monoclimax Community develops as a superorganism –Floristic composition Dependent on initial invaders –Multiple approach of Connell & Slatyer Facilitation –Environmental modification and reinvasion by better adapted species Inhibition –Very strong competition by 1 st species having advantage Tolerance –Species invade later that can tolerate a lower resource level No satisfactory model for all situations

10. Example of succession –1 st Shade intolerant species Eventually dominate only in canopy –2 nd Shade tolerant species invade –Shade intolerant die out due to no seedlings –Shade tolerant take over –Note that light is not the only limiting resource in succession NitrogenAlder

11. On left –Effect of alder on soil nitrogen in primary succession on melting glacier On right –Changes in plant diversity with time over secondary succession –Eventually a series of best suited dominant species take over –Regular disturbance will keep a high diversity

12. Changes in diversity in one part of community can affect another part –Greater diversity of vertical layers of forest More bird species –Variation in diversity during forest succession Affects mammal species

13. Humans and Ecology Humans are heterotrophs –Utilize very wide range of plants and animals as well as changing the physical environment Present day environment was created in many places by human habitation –Deforestation of Greece Forest to dry shrubland –Deforestation of England and Scotland Forest to moorland Forest to grazing Forest to meadows Still going on –Amazon and South East Asian Rain Forest How can mankind exploit natural resources in a sustainable way Man has always managed resources –Forest planting –Limitation on hunting by Kings –Fishing licenses

14. –What is a sustainable harvest of a natural population –Fish Stocks Decline in North Sea Fishery –Effect of World War I & II –Yield Resource harvested per unit time –Sustained Yield Yield that does not exceed rate of production Harvesting effort affects growth in population Above a certain point, population decline Upper limit –Per capita death rate due to harvesting equals per capita growth rate of population

16. Aim in fishery management is to fish at maximum sustained yield Very difficult Need to understand population structure and dynamics Failure to treat fisheries as a ecosystem Treat each stock separately

17. Cannot treat each stock separately Fishermen move from one onto next one General catching of all species Types of net Discarding of unprofitable fish –Dead even if thrown back

18. Also works for forestry Easier to control More sophisticated Better inventory –Size, number, distribution Variation in approaches –Clear cutting Results in early succession Erosion etc See early human examples –Selection cutting Remove mature trees Less disturbance More expensive –Rotation time is most important

19. Humans have change species distribution to their benefit –Wheat, rice, maize –Cattle, pigs, chickens Some species act as invaders –Aliens can be introduced for “good” reasons Acacia in South Africa –Stabilize sand dunes –Invades nitrogen poor soils Bass in Central African lakes –New fish for commercial exploitation –Only fish left, lakes are eutrophic See similar effect in Panama –Reduction in diversity –Long term effects?

20. Control of pest and aliens is a continuous war –See effect on scale of DDT –Note also resistance – mosquitoes Endangered species –Control by law –Does not produce economic return Elephants & Rhinos –Cannot sell ivory and horn from excess southern African populations –Burn it? –Farm them? Reintroduction –Problems Why were they eliminated –Elephants in Eastern Cape of SA –Competition with man Only sustainable in a reserve Who benefits from the reserve?