Community Ecology BCB331

1. Community Ecology BCB331 Mark J Gibbons, Room Z108, BCB Department, UWC Tel: 021 959 2475. Email: mgibbons@uwc.ac.za Image acknowledgements – http://www.google.com

2. Species A Species B Environmental Condition or Resource Environmental Condition or Resource REALISED NICHE Definition Species B Species A Environmental Condition or Resource The Competitive Exclusion Principle FUNDAMENTAL NICHE Definition

3. K1 α12 K2 α21 K2 > K1 > Intraspecific effects of species 2, greater than interspecific effects of species 1 Interspecific effects of species 2, greater than intraspecific effects of species 1 NO COEXISTENCE Here AND What is the OUTCOME? Species 2 out-competes Species 1

4. Together Realised Bedstraws On Own Fundamental Low or High pH Together Low pH Realised High pH RECIPROCAL Freshwater Diatoms On Own Fundamental Neither habitat allowed niche differentiation

5. Together Realised PARTITIONING Salmonid Fishes On Own Fundamental COEXISTENCE – differentiation of realised niches

6. Together Realised PARTITIONING Acorn Barnacles On Own Fundamental COEXISTENCE – differentiation of realised niches Competitive Exclusion Principle If two species coexist in a stable environment they do so as a result of niche differentiation (i.e. different realised niches). If there is no differentiation in niches between two species in a stable environment, then one competitor will eliminate or exclude the other. When there is co-existence – is it due to current competition, historic process of competition excluding others to leave the current species pair or evolution (“ghost of competition past”)

7. K2 α21 K1 α12 K1 K2 > > Interspecific effects of species 1, greater than intraspecific effects of species 2 Interspecific effects of species 2, greater than intraspecific effects of species 1 Mutual Antagonism Where inter-specific competition is greater than intra-specific competition for BOTH species Here AND What is the OUTCOME? Unstable equilibrium – varies with N0

8. For example…. Tribolium confusum T. castaneum SEPARATELY Temperature Moisture Both species used same flour resources – competition (as we shall see in just a moment) BUT they also preyed preferentially on the eggs, larvae and pupae of the other species – effects of each species on each other greater than on own Where inter-specific competition is greater than intra-specific competition for BOTH species Park et al. (1965) Physiological Zoology 38: 289-321 Reciprocal predation – mutual antagonism

9. Depending upon the culture “conditions”, the outcome of competition between the two species varied Park (1954) Physiological Zoology 27: 177-238 At environmental extremes there were clear winners and losers – realised niches In intermediate environments, one species would win sometimes, but lose out at other times – no certainty – just probable – DEPENDS on N0

10. Heterogeneity, colonisation and pre-emptive competition Up until now – have assumed that environment is sufficiently stable for the outcome of competition to be determined by the competitive ability of species concerned Rare: environments a mosaic of favourable / unfavourable habitats (in space and time) so that although competition may be taking place, it may not reach completion.

11. Unpredictable Gaps – the poorer competitor is a better coloniser Gaps in a community can be created in any number of ways Can you think of any? Gaps will be colonised – but the first species to do so will not necessarily be best able to exclude other species in the long term So long as gaps are created at an appropriate frequency, it is possible for a “fugitive” and a highly competitive species to co-exist The fugitive species gets to the gap first, establishes itself and reproduces. The other species tends to be slower to invade the gaps, but having done so it out-competes and eventually excludes the fugitive Persistence of a fugitive within a community depends on the frequency of gap formation, its dispersal abilities and fecundity

12. Postelsia palmaeformis (sea palm) is an annual species that needs to establish itself each year Paine (1979) Science 205: 685-687 Attaches to rock surface – gaps in mussel (Mytilus californianus) beds created by storm damage Mussels slowly re-invade the gap and preclude colonisation by Postelsia Both species only co-exist at sites with a relatively high rate of gap formation, and where the rate of gap formation is relatively constant each year WHY?

13. In cell A1, write p: In cell B1 write the number 0.1 In other words, we will make the probability of a cell being disturbed (a gap being created) as 0.1 Open an MSExcel Spreadsheet Let us create a habitat favourability matrix of 20 x 10 cells, each of which will either be disturbed or undisturbed - the likelihood (probability) of disturbance (frequency) being determined by p

14. In cell A3, write the following: =IF(RAND()<=$B$1,100,1) Copy this across all the cells of the habitat favourability matrix WHAT HAVE YOU JUST DONE?

15. To make this more visual – we are going to turn those cells that are disturbed red, and those that are undisturbed green To do this we must first highlight the cells A3:K22, go to FORMAT, CONDITIONAL FORMATTING, CELL VALUE IS, EQUAL TO, 100, FORMAT, PATTERN, RED, OK, ADD, CELL VALUE IS, EQUAL TO, 1, FORMAT, PATTERN, GREEN, OK, OK

16. At this point – no two of you will have the same patterns WHY?

17. Okay – so much for the environment, what about the species We need to start off with another (species) matrix of 10 x 20 cells, starting at M3 Assign each cell a random number between 1 (Dominant species) and 2 (Fugitive species) HOW? Type =RANDBETWEEN(1,2) into cell M3 Copy formula across the 10 x 20 cell matrix Having done that – convert the formulae to values by highlighting the data range, COPY, EDIT, PASTE SPECIAL, VALUES, OK This represents the master species matrix

18. In a third 10 x 20 (outcome) matrix – we relate the other two matrices using the following logic: If a cell in the habitat favourability matrix is disturbed (RED, 100), then whatever species is present in the corresponding cell in the species matrix will be killed off and will be replaced by the fugitive species. If, however, a cell in the habitat favourability matrix is undisturbed (GREEN, 1), then the dominant species will either persist in the corresponding cell in the species matrix , or it will out-compete the species previously there. Link the habitat favourability and species matrices in a third (outcome) matrix (Y3:AI22) as follows: In cell Y3, Type: = IF(M3*A3>2,2,1) Copy the formula across the third matrix Conditionally format the third (outcome) matrix in any way you want to in order to visually distinguish the two species (e.g. fugitives can be black and dominants yellow)

19. Press the F9 button and it recharges the random numbers in the habitat favourability matrix – which in turn causes changes in the species mix in the outcome matrix Do this 10 times and make a note each time of the number of cells occupied by the fugitive: use = countif(datarange,2) Change the value of p to 0.5, 0.2, 0.05 – and for each repeat the above exercise (pressing F9 ten times). What do you notice about the number of cells occupied by the fugitive? WHAT IS WRONG WITH THE MODEL? DOES IT MATTER?

20. Harper (1961) In: Mechanisms in Biological Competition. Milthorpe (Ed) Cambridge, 1-39 But………by delaying the sowing of B. rigidus into the mixture, M. madritensis became increasingly dominant Unpredictable Gaps – the preemption of space When two species compete on even-terms, the results are usually predictable. But what if one species gets to, or germinates in, a gap before another? This is called pre-emption and can allow co-existence For Example – Bromus madritensis and B. rigidus If both species sown in equal densities together, at the same time, then B. rigidus dominated the biomass of the mixed population SPREADSHEET MODEL TO ILLUSTRATE THIS

21. Fluctuating Environments Two competing species can co-exist if the balance between them is continually shifted by changing environmental conditions (seasonality, weather etc). In other words, the inevitable outcome of competition between the two species is never reached because the environment has changed part-way through the process.

22. By manipulating field densities of Physa gyrina and Lymnaea elodes, it was shown that the fecundity of the former was reduced through competition with the latter – but not reciprocal. Ephemeral patches with unpredictable life-spans e.g. corpses, rotting fruit Two competing species can co-exist, if one (the “fugitive”) is able to reproduce before the other SPREADSHEET MODEL TO ILLUSTRATE THIS Brown (1982) Ecology 63: 412-422 P. Gyrina reproduced earlier than L. elodes, and at a smaller size, and was the only species to survive in pools that dried up in early summer! Overall coexistence in habitat

23. K2 α21 K1 α12 Here K1 K2 > > AND Intraspecific effects of species 1, greater than interspecific effects of species 2 Intraspecific effects of species 2, greater than interspecific effects of species 1 What is the OUTCOME? Stable coexistence at equilibrium Aggregated Distributions Coexistence between competitors can occur across a mosaic of patches IF species show aggregated distributions, wherein competition is shown more strongly towards members of its own species than towards members of the other species

24. Competitive Dominant Stoll and Prati (2001) Ecology 82: 319-327 Aggregated Distributions Coexistence between competitors can occur across a mosaic of patches IF species show aggregated distributions, wherein competition is shown more strongly towards members of its own species than towards members of the other species SPREADSHEET MODEL TO ILLUSTRATE THIS Co-existence NOT through niche differentiation! But still competition

25. Apparent Competition – Competition for enemy-free space Imagine a predator that eats two species of prey Both prey species suffer from the predation, and the predator (enemy) benefits from both species of prey Increases in the predator abundance caused by its consumption of one prey species, increase the harm done to the other prey species, indirectly. Therefore each prey species adversely affects the other prey species – even if no obvious limiting resource The limiting resource that the prey species compete for is “enemy-free” space – the persistence of one prey species will be favoured by avoiding attacks from the predator, which also attacks the other prey species. This can be achieved if the two species occupy a habitat that is sufficiently different from the other prey species – niche differentiation

26. Venturia canescens Coupled Oscillations Ephestia kuehniella Plodia interpunctella No Competition between hosts Bonsall and Hassell (1997) Nature 388: 371-372

27. Coupled Oscillations Low r Venturia canescens Bonsall and Hassell (1997) Nature 388: 371-372 Ephestia kuehniella Plodia interpunctella

28. Natural experiments provided additional, evolutionary, information about competition Lack of controls Much of our understanding of competition comes from laboratory experiments or field manipulations (additions and/or removals) Easy to conduct - controlled Mostly conducted on sessile organisms BUT……..Artifacts Evidence for competition from natural experiments comes from niche expansion in the absence of a competitor (competitor release), and comparisons of the realised niche of a species in allopatry or sympatry (often involving character displacement): competitive exclusion and resource-partitioning.

29. Inland Rain Forest Light Forest Coastal Scrub Bagabag New Britain Espiritu Santo Competitive Release Chalcophaps indica Gallicolumba rufigula Chalcophaps stephani New Guinea Diamond (1975) In: Ecology and Evolution of Communities, Cody and Diamond (Eds) Harvard, 342-444

30. Length (mm) Food size proportional to body size Percentage of diet Character Displacement Hydrobia ventrosa Hydrobia ulvae Fenchel (1975) Oecologia 20, 19-32

31. Food size proportional to body size Percentage of diet Hydrobia ulvae Length (mm) Hydrobia ventrosa Fenchel (1975) Oecologia 20, 19-32 Food Particle Size

32. Summary Competition does take place between organisms Competition can result in exclusion or coexistence Competition can be asymmetric Species coexist today but their ancestors may have competed in the past Species may have evolved characteristics to avoid inter-specific competition Landscape is heterogeneous and patchy, allows co-existence of fugitive and dominant species Competition is difficult to demonstrate UNEQUIVOCALLY in the field, without manipulative experiments We have looked at two species populations – competition occurs in some places between more than two species! We will look at the role that competition plays in structuring biological communities later in the course

33. Niche Differentiation and how do you measure it? Species may split common resources in a number of ways By finely dividing resources – e.g. food size by birds & snails Nesospiza Percentage of diet Hydrobia Food Particle Size Despite problems of directly linking competition to niche differentiation, the latter clearly leads to co-existence Ryan et al (2007) Science 315: 1420-1432

34. J F M A M J J A S O N D 0 10 20 DEPTH (m) 30 40 50 Antho- Lepto- Trachy- Narco- Scypho- By utilizing common resources at different times Buecher & Gibbons (1999) Marine Ecology Progress Series 189: 105-115

35. By utilizing common resources in different microhabitats Sharitz & McCormick (1973) Ecology 54: 723-740

36. Niches may be differentiated on the basis of conditions Two species utilize the same resource, but their ability to do so is determined by the environmental conditions, which favour one species over another Can result in microhabitat separation, temporal separation

37. 1 B = ∑ p2i BA = 4.65 = 1 / 0.215355 BB = 6.06 = 1 / 0.16446 How do you measure niche differentiation? Firstly…………Niche breadth (Levins Index - B) pi = proportion of individuals using resource i Species B has a broader dietary niche than species A

38. Mjk = ∑ pij pik ∑ p2ij MAB= 0.7152 = 0.154046 / 0.215388 MBA = 0.9367 = 0.154046 / 0.16446 Niche Overlap MacArthur & Levins (1967) Index Range: 0 - 1 Mjk– overlap index of species k on species j pij – proportion that resource i is of the total resource that species j utilises pik – proportion that resource i is of the total resource that species k utilises

39. MAB= 0.7152 = 0.154046 / 0.215388 MBA = 0.9367 = 0.154046 / 0.16446 Species B has less of an overlap in its niche with Species A than Species A has on Species B NOTE – just because there may be strong overlap – doesn’t mean that competition takes place!

41. THE END Image acknowledgements – http://www.google.com

42. But in reality - Is there a minimum amount of niche differentiation that has to be exceeded before two species can coexist? Imagine three species d = distance between adjacent resource utilisation peaks, W = standard deviation (niche breath) Niche Differentiation – similarity amongst coexisting species The Lotka-Voltera models predicted co-existence between competitions would be achieved if intra-specific competition was greater than inter-specific competition: Niche differentiation concentrates competition intra-specifically and models predict that ANY differentiation will allow coexistence TRY IT d > W – relatively little inter-specific competition

43. Contrast with………….. d < W – intense inter-specific competition d = distance between adjacent resource utilisation peaks, W = standard deviation (niche breath) How much overlap before competitive exclusion?