Community Ecology BDC321 Mark J Gibbons, Room 4.102, BCB Department, UWC Tel: Image acknowledgements –
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.
Unpredictable Gaps – the poorer competitor is a better coloniser Gaps in a community can be created in any number of ways 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
Postelsia palmaeformis (sea palm) is an annual species that needs to establish itself each year 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? Paine (1979) Science 205:
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 But………by delaying the sowing of B. rigidus into the mixture, M. madritensis became increasingly dominant Harper (1961) In: Mechanisms in Biological Competition. Milthorpe (Ed) Cambridge, 1-39 SPREADSHEET MODEL TO ILLUSTRATE THIS
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.
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 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. 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 Brown (1982) Ecology 63: SPREADSHEET MODEL TO ILLUSTRATE THIS
Here 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 K 2 α 21 K1K1 > K 1 α 12 K2K2 > 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 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 Co-existence NOT through niche differentiation! But still competition Competitive Dominant Stoll and Prati (2001) Ecology 82: SPREADSHEET MODEL TO ILLUSTRATE THIS
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
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 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
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?
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
At this point – no two of you will have the same patterns WHY?
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
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)
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?