Soil invertebrate community composition in vernal pools throughout the dry phase in Dorset, Ontario

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1 Soil invertebrate community composition in vernal pools throughout the dry phase in Dorset, Ontario Amy Choi a, Shelley Hunt b, Jonathan Schmidt b, Sigrun Kullik b, Kirsten Otis b adepartment of Land Resource Science, University of Guelph, Guelph, ON N1G 2W1, Canada bdepartment of Environmental Biology, University of Guelph, Guelph, ON N1G 2W1, Canada ABSTRACT Vernal pools are important for the aquatic phases of many amphibious species; however their ecological significance during their dry phase is less known. These unique habitats are sensitive to disturbances and represent areas of different forest function and processes than in dry forest floor. If the community composition in the litter layer of vernal pools is different from that of dry forest floor throughout the dry season, then this provides evidence that vernal pools maintain unique function even after the aquatic habitat has gone. Preliminary findings suggest that this is true, and, though further studies are needed, imply a need for management recommendations such as increased effort to protect vernal pools from various disturbances. KEYWORDS: vernal pool invertebrates community composition dry phase Winkler 1. Introduction The wet phase of vernal pools is well studied and understood to be significant; however, the ecological role of these pools during the dry phase remains largely unstudied. Previous studies have focused on the importance of these temporary pond habitats for rare plant and animal species or diversity (Brooks and Hayashi, 2002; Calhoun et al., 2003; Carl and Blumenshine, 2005) or the role they play in the life cycles of many amphibious species during the wet phase (Nicolet et al., 2004). Pyke (2005) studied the effect of climate change on vernal pools through branchiopods and found that precipitation affecting hydroperiod was the most important factor causing changes in diversity and abundance. Climate change leading to increased precipitation may change species composition by favouring species with particular life cycles (e.g. predators such as dragonflies that are slow developing) and increasing the number of suitable reproductive sites (Pyke, 2005). Brooks (2004) stated that increased hydroperiod led to increased species richness; however resulting changes in community structure, especially through increased predation, were complex and not fully understood. Moreover, the small volume of water held in vernal pools renders them sensitive to water pollution and contamination (Nicolet et al., 2004), as well as disturbances such as forestry, development, agriculture, recreation and invasive species (Colburn, 2004). Vernal pools differ in function from dry forest floor due to increased water content; these differences in water content may lead to differences in invertebrate community. This community of invertebrates in the soil plays an important role in forest functions and processes such as litter breakdown, decomposition and nutrient cycling (Coleman et al., 2004). The high abundance and diversity of organisms found in relatively small volumes of soil and litter make

2 studying this community a good representation of the state of the ecosystem (Coleman et al., 2004). Changes in community composition may indicate much larger changes in function since each species has different roles such as predators, grazers, shredders, detrivores and so on (Colburn, 2004). Two large groups in particular, mites (Acarina) and springtails (Collembola), are important components of the community which indicate changes in soil health, function and processes (Walter and Proctor, 1999; Hopkin, 1997). Although the dry season (no standing water) has been briefly examined in some studies, this is usually regarding the conservation of habitat for the dormant stages of ephemeral aquatic species (Nicolet et al., 2004). Another issue with previous studies is methodological: the common method of extraction of invertebrates from the soil and litter layer using a Berlese funnel is harsh and can cause many of the soft-bodied invertebrates to dry up and therefore not be counted (Krell et al., 2005). The Winkler sack (Appendix I) is a slower, less harsh method of extracting invertebrates and it relies on air drying rather than a lamp (Agosti et al., 2000). However, since this method is primarily used in tropical areas for hard-bodied ants (Hymenoptera) (Krell et al., 2005), it is uncertain whether the litter sifting used before placement into the Winkler will kill or damage the soft-bodied invertebrates. The purpose of this study is to determine: a) if there is a difference in composition in the vernal pool compared to dry forest floor; b) if there is a change in composition of the vernal pool at different times during the dry phase and if the resulting composition is different from the dry forest floor; c) if there is a difference between the composition and/or abundance of invertebrates between sifted and unsifted samples. The results of this study may have important management implications for vernal pools. If dried vernal pools retain the ecological significance of wet pools, then measures should be taken to further conserve them. 2. Materials and Methods 2.1. Site Description The study took place in Dorset, Ontario, Canada, near the Frost Centre Institute. This area was located on the granitic bedrock and varies in height above sea level. The area was mapped according to topography with ArcView (Figure 1) and locations with high probability of vernal pool occurrence were explored by foot. Pools to be studied were marked with flags and their location inputted on a handheld GPS device. Figure 1. A GIS map displaying the likelihood of vernal pool presence. Orange-dark red areas are areas of high likelihood. Pool 19 is marked with an arrow. Pool 19 ( N, W) was 10.6 m in length and m wide. The forest surrounding this pool was dominantly red maple (Acer rubrum), sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis). Birch species (Betula spp.), white ash (Fraxinus americana) and balsam fir (Abies balsamea) were also found in the area. The canopy cover over the studied pool was 90%. The pool dried to no standing water on 17 July 2008 and was sampled on 19 July 2008, 20 September 2008 and 17 October The pool was re-filled with water due to heavy rainfall around 12 August 2008 but had dried again by 20 September P a g e

3 2.2. Sample Collection To sample for invertebrates, the top litter layer of the forest floor were taken out of 50 cm x 50 cm square plots after the pool had dried of standing water. Four samples were taken randomly from the Pool 19 site at each sampling date; two in the pool (wet) and two surrounding the pool (dry). Two of the samples, one wet and one dry, were placed through a sifter to try to remove large debris such as decaying leaves. Each of the four samples were then placed in a mesh bag (4 mm x 4 mm grid) inside a fabric Winkler sack. These were hung in a cool, shaded basement for approximately 48 hours to allow the invertebrates move out of the mesh and drop into a small plastic bag filled with 70% ethanol. These bags were labeled and sent to the University of Guelph to be analyzed Invertebrate Sorting and Counting An individual sample was poured into a large Petri dish with 5 mm x 5 mm grid paper taped to the bottom, ensuring that no invertebrates were left inside the bag. The sample was viewed grid by grid under a light microscope at a magnification appropriate to ensure all invertebrates could be seen and sorted. Invertebrates were removed, placed in glass vials containing 70% ethanol, labeled and recorded. The compositional groups the invertebrates were sorted into were as follows: Spiders - Araneae; Mites - Acarina; globular and other Springtails - Collembola; Nematodes - Nematoda; Annelids - Annelida; Gastropods - Gastropoda; larval, pupal and adult Beetles - Coleoptera; Ants - Hymenoptera; Wasps - Hymenoptera; Millipedes - Diplopoda; Centipedes - Chilopoda; larval pupal and adult Flies - Diptera; and Other. In groups with relatively high abundances (>50) such as mites and springtails, individuals were set aside in a separate Petri dish and counted at the end of each sample using a counting wheel and clicker counter. A total of seven samples were analyzed: 19 July 2008, wet and dry unsifted; 20 September 2008, all four samples; and 17 October 2008, wet unsifted Data Analyses Statistical analyses were not used due to the low sample sizes. Compositional bar graphs were used as a visual representation of changes in community. 3. Results 3.1. Dry vernal pool vs. dry forest floor When averaged over all time periods, the dry samples generally had higher abundances (>50% of the total found) in each of the groups excluding globular springtails, nematodes and beetle larvae and pupae (Figure 2). Certain groups were only found in either wet or dry samples. These included beetle pupae, which were only found in wet samples and wasps and ants which were only found in dry samples. Springtails and mites were looked at separately due to their relatively high population sizes. It was found that springtail abundance did not vary greatly between wet and dry samples; however a much greater number of mites was found in the dry samples compared to the wet samples (see Figure 3). There was a higher proportion of mites than springtails in the dry samples compared to the wet samples from July to September (Figure 4) Dry vernal pool throughout the season The composition between early samples (19 July 2008) and late samples (20 September 2008 and 17 October 2008) varied greatly in the pool (Figure 5). Beetle larvae were in much higher abundance in the early wet than in the late wet phase (Figure 5). The early wet samples also had higher abundances in most groups compared to 3 P a g e

4 the late wet samples (Figure 6). In contrast, the dry samples from the later dates had higher abundances than those from the early dates (Figure 6). Certain groups were on found at certain times during the season, for example, adult flies were found mainly early in the season and adult beetles were found late. Millipedes and centipedes were not found in the early wet samples at all and gastropods were not found in any wet samples Sifted vs. unsifted Only one sifted sample for each wet and dry were analyzed. These sifted samples showed a lowered proportion of mites to springtails compared to unsifted samples from the same time period (20 September 2008) (Figure 4). 4. Discussion Although only one pool was examined and few samples were completed, the preliminary findings are of substance. Particularly in terms of the mites and springtail groups, observational differences in function could be seen. For example, even though there was no large difference in springtail numbers from wet to dry, the individuals in the wet samples were generally larger-bodied. In mites, the wet samples had a lower abundance, however these were observed to be mostly predatory species compared to the Orabatid mites found in the dry samples (Walter and Proctor, 1999). This difference was observed at all sampling periods. These differences were not accounted for in the broad groups used; further research should consider the identification of invertebrates to family or species. Moreover, the sampling period was a substantially wetter year than most and this may have emphasized the differences between wet and dry samples even later in the year, due to the re-filling of the pond halfway through the season. Vernal pools are important for the life cycles for many species. Beetle larvae and pupae, for example, were found predominantly in the wet samples, suggesting that this habitat type is important for their particular life stages. Certain groups such as gastropods, millipedes and centipedes, were not found in wet samples or in very low abundances suggesting that vernal pools, even late in the season, still maintains different features than dry forest floor. The sifting of samples to remove large litter debris before placing them in the Winkler sacks appears to have some unknown effect on the composition, at least on the mite to springtail proportion. However, the small number of samples analyzed prevents the acceptance of any conclusions. The insufficient number of samples examined to provide legitimate statistics was a problem in this study. The process of counting and sorting invertebrates in each sample was time consuming and inefficient. The use of gel-based subsampling should be used in the future to provide accurate and precise representations of the larger sample (Jagers op Akkerhuis et al., 2008). However, aside from mite and springtail populations, the number of individuals found in the other groups were relatively low. Therefore, the use of sub-sampling may result is the omission of many of the smaller groups found in the litter and soil layer leading to an inaccurate representation of community composition. This low number of individuals in certain groups may also be a form of Winkler sampling bias. Krell et al. (2005) found that 48 hours was not long enough to reach the maximum extraction of individuals and that certain groups would be emphasized depending on the extraction period chosen. Using a longer extraction period may allow for higher numbers in the less dominant groups and a more accurate community representation. The Winkler sampling technique 4 P a g e

5 was largely ineffective for isopods, millipedes, and shelled gastropods (Krell et al., 2005). Further research should examine and compare multiple pools, as differences in topography, soil type, surrounding vegetation, pool size and depth, etc may result in differences in community composition as well (Carl and Blumenshine, 2005). Brooks and Hayashi (2002) found that species richness was strongly affected by hydroperiod of the pool which was related to pool area and depth. 5. Conclusion The findings of this study provide reasonable evidence that vernal pools remain ecologically significant throughout their dry phase compared to dry forest floor and warrants further research. Conservation efforts should not only be focused on the aquatic phase of the pools but also on the less noticeable dry phase. Appendix I. Schematic diagram of a Winkler sack References Agosti, D., Majer, J.D., Alonso, L.E. and Schultz, T.R Ants: Standard methods for measuring and monitoring biodiversity. Smithsonian Institution, Washington US, p.280. Brooks, R.T Weather-related effects on woodland vernal pool hydrology and hydroperiod. Wetlands 24(1): Brooks, R.T. and Hayashi, M Depth-area-volume and hydroperiod relationships of ephemeral (vernal) forest pools in southern New England. Wetlands 22(2): Calhoun, A.J.K., Walls, T.E., Stockwell, S.S. and McCollough, M Evaluating vernal pools as a basis for conservation strategies: A Maine case study. Wetlands 23(1): Carl, T. and Blumenshine, S Relationships among vernal pool invertebrate assemblages with habitat morphology and distribution. Bios 76(3): Colburn, E.A Vernal Pools: Natural history and conservation. The McDonald & Woodward Publishing Company, Michigan US, p Coleman, D.C., Crossley Jr., D.A. and Hendrix, P.F Fundamentals of Soil Ecology. Elsevier Academic Press, Massachusetts US, p.386. Hopkin, S.P Biology of the Springtails (Insecta: Collembola). Oxford University Press, New York US, p.330. Jagers op Akkerhuis, G.A.J.M., Dimmers, W.J., van Vliet, P.C.J., Goedhard, P.W., Martakis, G.F.P. and de Goede, R.G.M Evaluating the use of gel-based sub-sampling for assessing responses of terrestrial microarthropods (Collembola and Acari) to different slurry applications and organic matter contents. Applied Soil Ecology 38: Krell, F.T., Chung, A.Y.C., DeBoise, E., Eggleton, P., Giusti, A., Inward, K. and Westerwalbesloh, S Quantitative extraction of macro-invertebrates from temperate and tropical leaf litter and soil: efficiency and time-dependent taxonomic biases of the Winkler extraction. Pedobiologia 49: Acknowledgments We thank the Frost Centre Institute for their continual support in vernal pool research. Chris Jepson for his assistance in invertebrate counting and sorting. Nicolet, P., Biggs, J., Fox, G., Hodson, M.J., Reynolds, C., Whitfied, M., Williams, P The wetland plant and macroinvertebrate assemblages of temporary ponds in England and Wales. Biological Conservation 120: P a g e

6 Pyke, C.R Assessing climate change impacts on vernal pool ecosystems and endemic brachiopods. Ecosystems 8: Walter, D. and Proctor, H Mites: Ecology, evolution and behaviour. University of New South Wales Press Ltd, Sydney AU, p.322. Figures Precentage of Total Individuals 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% WET DRY Compositional Group Figure 2. Percentage of individuals found in wet compared to dry samples in each group, averaged over all time periods. Different life stages of flies and beetles are shown (A- adult, P-pupal, L-larval). Most groups had higher abundances in the dry samples than the wet, excluding globular springtails, pupal and larval beetles, and nematodes. Beetle pupae were only found in the wet samples and ants and wasps were only found in dry samples. 400 Average Nu mber of Individuals Springtails Mites 50 0 Dry Wet Figure 3. Average number of springtails and mites in wet and dry samples, averaged over all time periods. Springtail abundances do not vary greatly between wet and dry; however mites are in greater abundances in the dry samples than the wet samples. 6 P a g e

7 3 2.5 sifted Mites : Springtails Dry Wet 0.5 sifted 0 11-Jul 21-Jul 31-Jul 10-Aug 20-Aug 30-Aug 09-Sep 19-Sep 29-Sep Date of Sample Collection Figure 4. Changes in proportion of mites to springtails throughout the season. A higher proportion of mites is seen throughout the season in the dry samples even with an increase in mite proportion in the wet samples later in the year. Sifted vs. unsifted samples are shown, with sifted samples showing a smaller proportion of mites than unsifted samples Spiders Average Number of Individuals Early Dry Late Dry Early Wet Late Wet Time Springtails G Flies A Flies P Flies L Beetles A Beetles P Beetles L Ants Wasps Millipedes Centipedes Nematodes Gastropods Figure 5. Changes in community composition between early (19 July 2008) and late (20 September 2008 and 17 October 2008) for both dry and wet samples. Composition varies greatly between dry and wet but also temporally. 7 P a g e

8 Percentage of Total Individuals Found 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Late Wet Early Wet Late Dry Early Dry Compositional Group Figure 6. The percentage of total individuals found in early wet, late wet, early dry and late dry for each compositional group. The dry samples have higher abundances later in the season whereas the wet samples have higher abundances early in the season. Adult flies were mainly found early in the year in both wet and dry; however beetle adults were found mostly late in the season. 8 P a g e