Testing the intermediate disturbance hypothesis at a small scale: trailside vegetation diversity.

Transcription

1 Testing the intermediate disturbance hypothesis at a small scale: trailside vegetation diversity Field Ecology Hugh Stimson Research Paper 1/2 Prof RJ Brooks M Cameron; T Irvin

2 Abstract We observed the effects of varying levels of disturbance upon the species richness of trailside vegetation in two nearby areas - an open, highly disturbed area of low thick brush and an area of older pine. Trailside vegetation afforded us a system of decreasing levels of disturbance as distance increased from the centre of the trail. The first ecotype provided an area of overall high species richness, the second one of lower richness between which to contrast. We were interested in testing the applicability of the intermediate disturbance hypothesis within these two systems. By comparing the species abundances, measured as percent cover, recorded at increasing distances from the trail centre, we observed changing trends of species diversity which we took to be a result of differential disturbance levels. The Intermediate Disturbance Hypothesis predicted the patterns of species richness in the area of high diversity, but not the area of low diversity. Introduction In 1978, Joseph Connell presented a hypothesis for explaining local variation in species diversity levels, based on his own comparative studies of tropical forests and coral reefs as well as previous studies worldwide (Connell 1978). He felt that existing models which assumed ecosystems were generally in a climax successionary state, and which therefore assumed that species presence was determined by the optimal utilization of existing resources, were ignoring the importance of disturbance in the natural world. In Connell s opinion, many ecosystems received some disturbance frequently enough to keep them from reaching any optimal equilibrium state. Furthermore, he believed that a high level of diversity could be maintained specifically because an ecosystem was not in equilibrium. He called the intermediate disturbance hypothesis (IDH). The intermediate disturbance hypothesis suggests that species diversity will be maintained at its highest levels when the patterns of disturbance which impact the area are intermediate. At high frequency and intensity of disturbance, the species present will be those that are hardy under disturbed conditions, or which spread and grow quickly and can invade an area in between disturbances. This represents a small subset of the species that could potentially exist in that area, if disturbances did not occur. At low frequency and intensity, the competitive exclusion principle suggests that each Hugh Stimson

3 available niche in the area will be dominated by a single species that is the best competitor, given that it has had the time to out-compete all other species which might occupy that niche. This represents a small sub-set of the species that could fill that niche if competition were not present. At some intermediate disturbance level, the good competitors will have an opportunity to gain a foothold without being destroyed by disturbance, but will not have enough time between disturbances to out-compete the hardier and faster invading colonizer species which dominate soon after each disturbance, or to exclude less fit slow-invading species. Thus, total species diversity will be highest at an intermediate frequency and intensity of disturbance. Connell s paper, which was largely based on sweeping observations and synthesis of data from other studies, was followed by work by other researchers testing the specific applicability of the intermediate disturbance hypothesis to various ecosystems and scales. To date, little work has been done to test the IDH on a very small scale. This may be due to the lack of systems in which disturbance varies substantially over a small distance. Most reoccurring damaging events tend to be regional (fires, Beckage and Stout 2000, Schwilk and Keeley 1997; currents, Floeder and Sommer 1999) or geographic (windstorms, Hiura 1995). In this study we have tested the intermediate disturbance hypothesis at a smaller scale, using the edges of trails as our system. Based on the observation that the frequency at which people walk over vegetation varies with the distance of the vegetation from the centre of a trail, with disturbance being highest at the centre of the trail and decreasing perpendicularly outwards, we recorded presence and abundance of species at different distances from the trail centre. We expected that if the intermediate disturbance hypothesis were applicable to this ecological system, we would see species diversity levels lower towards the centre of the trail and at a distance from the centre than at some intermediate distance. Our study was conducted in two areas close to one another, one with a high overall richness of vegetation, and one with a low overall richness. We hoped to see whether the species diversity patterns observed would be similar or different in these areas which a) shared some of the same species and b) were subjected to most of the same abiotic conditions, such as climate and soil type. We felt that if differences in species diversity did exist in the area of high overall species richness, and if those differences were the result of IDH-type species interactions, that we might not observe an IDH pattern in the area of low overall species richness. The IDH assumes that species of both the colonist and competitor types will be available to colonize a Hugh Stimson

4 disturbed area. Given a smaller number of species it will be more likely that by chance more of those species will be of one type than the other (colonist or invader), making it more likely that one type of species will not be present to invade, and less likely that a neat pattern of low-to-high-to-low diversity will occur over high-to-low disturbance. Materials and methods Our study was conducted in Algonquin Provincial Park, Ontario, Canada. Our two areas of study were an abandoned airfield near Mew Lake and an adjacent mature pine forest. Our sample sites were selected randomly along the total length of the suitable trails which ran through the forest and field areas. Trails were judged to be suitable for the study when they received enough use to be trampled to the soil in the centre, and were not bordered with brush too thick to allow traffic on the edges of the trail. At each selected site we used a frame divided into five quadrats, each 50cm long by 20cm deep. The frame was placed with its inside edge parallel with the trail and set at the first appearance of vegetation. Each of the five quadrats within the frame represented a distance treatment, each being 20cm further from the centre of the trail than the last (see fig. 1). In each quadrat all species present were recorded. Species abundance was measured visually as percentage coverage to account for clonal species. All species of plant and lichen were recorded. Some quadrats had percentage cover measured by all three investigators, and the results of each investigator were compared to ensure that cover was being estimated consistently by all. 20 sights were selected in each of the two areas for a total of 100 samples each in the forest and field areas (20 sights x 5 distance treatments). Species richness was calculated as total number of species observed. Species diversity was calculated for each sample using the Shannon-Weaver index of diversity. The Shannon-Weaver index was chosen for its sensitivity to the presence of species represented by few individuals (Ludwig and Reynolds 1988). For each area (forest and field), the mean diversity was calculated for each of the five distance treatments. The Least Significant Difference test was used to test for differences among the distances (Hubert 1999). Results Hugh Stimson

5 Significant differences were observed among the mean species diversities observed at different differences from the trail centre, in both the areas of high and low overall species richness. In the old airfield, which was the area of relatively high species richness (mean species richness of 6.7 as opposed to 4.7 in the field, different at 99.9% certainty), we found that species diversity was low at the edge of the trail, peaked in the middle distance, and was low again at the furthest distance sampled (Fig. 2, Table 2). In the forest, which was an area of relatively low species richness, we found diversity relatively low at the edge of the trail, and relatively high towards the furthest distance sampled (Fig. 3, Table 3). Discussion This study could be completed by observing patterns of trail use, to determine if disturbance really does decrease steadily away from the centre of the trail. For now, we will accept that this is true, based on our personal experience and informal observations: an individual walking a trail is most likely to stick to the centre, but may on occasion have reason to stray off the edges - to observe something, to pass an oncoming trail user, or while walking two or three abreast. The further from the centre, the less often a trail user will have strayed there. Given a broad definition of disturbance - any event which kills or damages the resident biota - we make the further assumption that the differences in species richness are a consequence of disturbance levels. This seemed plausible, as there was only one observable biotic or abiotic factor, other than disturbance, which varied with distance from the trail centre. This possible confounding variable is sunlight, but the impact of this should have been minimized by our explicitly chosing trails with little tall vegetation on either side. We therefore feel confident that high species diversity is being maintained in the middle distances as a result of disturbance, in the field area. Whether this is a result of the ecological processes suggested by the intermediate disturbance hypothesis (simultaneous presence of both colonizer and competitive species) cannot be directly inferred from this result. The fact that diversity did not follow the IDH pattern in the forest indirectly supports the possibility that the IDH accurately describes the mechanisms determining diversity in the field. If IDH-type processes were at work in the high richness field, then we would expect to see the IDH pattern breaking down in areas of lower species richness, but with otherwise similar conditions. Given fewer Hugh Stimson

6 potential re-invading species, there is a greater chance there will by chance not be proportional numbers of quick-colonizing/hardy and effective-competitor species to fulfill the assumptions of the IDH. Therefore an area of higher richness is more likely to follow an IDH diversity pattern than an area of lower richness. This is again, not direct evidence that the IDH is at play in the field, and there are 3 reasons why it may not be applicable as evidence at all: 1. There are differences beyond just overall species richness between the field and forest that could cause the differences observed in species diversity patterns. 2. We cannot be sure of the lower threshold number of species necessary for the assumptions of the intermediate disturbance hypothesis to be met, nor the proportion of each type of species necessary. 3. It is possible that, if we were to extend our sampling to a greater distance from the trail edge in the forest area, we would find that diversity drops back down at a greater distance. It is therefore possible that the intermediate disturbance hypothesis is applicable to this lower diversity area. This seems unlikely however, as this could only result if there were a different disturbance pattern at work in the forest area, with people more likely to stray a greater distance from the trail centre. There is no obvious reason why this would happen. Again, in order to be sure, further work would need to be done directly observing patterns of trail use in the field and forest. Hugh Stimson

7 References Beckage B, Stout JI Effects of repeated burning on species richness in a Florida pine savanna: a test of the intermediate disturbance hypothesis. Journal of Vegetation Science 11: Connell JH Diversity in tropical rain forests and coral reefs. Science 199: Floeder S, Sommer U Diversity in planktonic communities: An experimental test of the intermediate disturbance hypothesis. Limnology and Oceanography 44(4): Hiura T Gap formation and species diversity in Japanese beech forests: a test of the intermediate disturbance hypothesis on a geographic scale. Oecologia 104: Hubbert JJ Design and analysis of experiments. Department of Mathematics and Statistics, University of Guelph: Guelph. 401 p. Ludwig JA, Reynolds Jf Statistical Ecology. John Wiley & Sons: New York. 337 p. Schwilk DW, Keeley JE, Bond WJ The intermediate disturbance hypothesis does not explain fire and diversity patterns in fynbos. Plant Ecology 132(1):77-84 Hugh Stimson

8 0-19cm 20-39cm 40-59cm 60-79cm 80-99cm Forest Field Table 1. Mean Shannon s index values observed. distance 0-19cm 20-39cm 40-59cm 60-79cm 80-99cm 0-19cm x x 20-39cm x 40-59cm x x 60-79cm 80-99cm x Table 2. Differences among distance treatments in the old airfield. x s indicate significant difference between the treatments (p=0.5). distance 0-19cm 20-39cm 40-59cm 60-79cm 80-99cm 0-19cm x 20-39cm x 40-59cm 60-79cm x x 80-99cm Table 3. Differences among distance treatments in the forest. x s indicate significant difference between the treatments (p=0.5). Fig. 1. Overhead view of experimental setup. Hugh Stimson