2012 NORTHEASTERN NATURALIST 19(4):559–578 “Island” Attributes and Benthic Macroinvertebrates of Seasonal Forest Pools Robert T. Brooks1,* and Elizabeth A. Colburn2 Abstract - Seasonal forest pools (SFPs), also known as woodland vernal pools or simply vernal pools, are common throughout the forests of the northeastern United States. SFPs are inundated during all or part of the period between late fall of one year and late spring to mid-summer of the subsequent year. The pools dry every year or at sufficient frequency to preclude the establishment of fish populations, are preferred breeding habitat for a number of amphibian species, and support a rich, diverse, and abundant macroinvertebrate community. These pools exist as aquatic “islands” in a “sea” of forest, and occur over a range of sizes, degrees of isolation, and hydroperiod lengths. As islands, pool area and isolation should affect the composition of biotic communities. The hydroperiod of ephemeral wetlands has been considered a third “island” attribute and is also known to affect biotic composition. We surveyed aquatic, benthic macroinvertebrates (BMIs) for two years using leaf-packs in 24 SFPs, representing a broad range of surface areas, inter-pool distances (isolation), and hydroperiods. Nearly 35,000 specimens of 76 taxa were enumerated from 198 leaf-pack samples. Chironomidae and Oligochaeta were the most abundant and most common taxa. BMI richness and diversity were positively, but weakly, related to maximum pool surface area, but not to pool isolation. The same results were found for permanent resident and predator taxa. BMI richness and diversity were positively related with pool hydroperiod, as reported from numerous other studies of ephemeral aquatic habitats. Introduction The composition of the flora and fauna of islands is affected by island size and by the island’s degree of isolation, with size affecting extinction rates and available habitat niches and isolation affecting immigration rates (MacArthur and Wilson 1967). As island size increases, extinction rates decrease and as an island is increasingly isolated, immigration rates decrease. The equilibrium species richness of an island can be modeled by the intersection of these two rates. While island biogeographic theories were initially demonstrated using marine islands, the concepts have been applied to other isolated habitats including freshwater wetlands (Brose 2001, 2003; De Meester et al. 2005; Ebert and Balko 1987; March and Bass 1995; Ripley and Simovich 2009). Pool size has been shown to positively influence the richness of ephemeral pool snails (Lassen 1975), beetles (Nilsson 1984), chironomids (Driver 1977), and microcrustaceans (Mahoney et al. 1990); however, the effects have typically been 1US Forest Service, Northern Research Station, University of Massachusetts, Amherst, MA 01003. 2Harvard Forest, Petersham, MA 01366. *Corresponding author - rtbrooks@ fs.fed.us. 560 Northeastern Naturalist Vol. 19, No. 4 weak or inconsistent (Holland and Jain 1981, Oertli et al. 2002). The effects of pool isolation are less well documented and, where assessed, have generally been found to be weak (Lopez et. al. 2002, Mahoney et al. 1990, Spencer et al. 2002). Where a significant isolation effect has been found, the results show that invertebrate communities in adjacent sites are more similar than those in distant pools (Briers and Biggs 2005). The hydroperiod or duration or permanence of the wet phase of ephemeral wetlands has been considered a third island attribute of these systems (Ebert and Balko 1987, Kiflawi et al. 2003, Nilsson 1984, Ripley and Simovich 2009). The recurring dry phase of ephemeral waters precludes the occurrence of fish, and these systems are known for rich and diverse faunal communities, including both vertebrates (Mitchell et al. 2008, Semlitsch and Skelly 2008) and invertebrates (Colburn 2004, Colburn et al. 2008). Hydroperiod has been shown to be the dominant abiotic factor in structuring the invertebrate community of ephemeral wetlands (De Meester et al. 2005, Schneider and Frost 1996, Wellborn et al. 1996, Williams 1997), including seasonal forest pools (Batzer et al. 2004, 2005; Brooks 2000; Schneider 1999). Pool hydroperiod is also positively associated with richness of the breeding amphibian community (Babbitt 2005, Burne and Griffin 2005, Snodgrass et al. 2000). Temporary pools occur commonly throughout the world (Ramsar 2002, Williams 2006). Temporary pools are typically small and shallow wetlands, characterized by alternating flooded and dry phases, and whose hydrology is largely autonomous (Ramsar 2002). Seasonal forest pools (SFPs; also “woodland vernal pools”, Tiner et al. 2002; or simply “vernal pools”, Calhoun and deMaynadier 2008, Colburn 2004), occur in every forest region of the United States, and are widely but not regularly distributed in forests of the glaciated northeastern United States and adjacent Canada (Brooks et al. 1998, Palik et al. 2003, Rheinhardt and Hollands 2008). SFPs are highly variable in most attributes, due to differences in climate, geology, hydrology, and other factors (Rheinhardt and Hollands 2008, Tiner et al. 2002). Many SFPs are technically autumnal pools, based on the timing and pattern of inundation (Brooks 2004, Brooks and Hayashi 2002, Higgins and Merritt 1999). The pools partially inundate in the fall with rains on saturated or frozen soil, fill to maximum capacity in the spring following snowmelt, and are generally dry by mid-summer. Most pools are hydrologically isolated and are expressions of direct precipitation and runoff from immediately adjacent uplands (Brooks 2004, Leibowitz and Brooks 2008), although interactions between pool water and local groundwater can also occur, especially in deeper, more porous soils (Palik et al. 2001, Sobczak et al. 2003). Pools can be classified as palustrine, unconsolidated bottom, emergent, or scrub-shrub, seasonally flooded wetlands (Cowardin et al. 1979) or as isolated depressions in the hydrogeomorphic classification system (Brinson 1993, Cole et al. 1997). An inventory of SFPs on the Quabbin Reservoir watershed in central Massachusetts with information on individual pool size and degree of isolation (Brooks et al. 1998) provided an opportunity to test the theories of island 2012 R.T. Brooks and E.A. Colburn 561 biogeography and the composition of pool fauna. The objectives of this study were to determine if, after controlling for pool hydroperiod, pool surface area and/or inter-pool distance (i.e., isolation) of SFPs in northeastern US forests affect the composition of the benthic macroinvertebrate (BMI) community. We hypothesized that pool size and pool hydroperiod (i.e., permanence) were positively related to BMI richness and diversity and that inter-pool distance was negatively related to BMI richness and diversity. In addition to total community attributes, we also assessed island effects on invertebrate, predator taxa that have been shown especially sensitive to pool size (Pearman 1995, Spencer et al. 1999, Wilcox 2001) and passively dispersed taxa (e.g., microcrustaceans) that we felt would be particularly affected by pool isolation (Bagella et al. 2010). Methods Study area The study pools were located on the Quabbin Reservoir watershed in central Massachusetts (72o21'W; 42o25'N). The publicly owned watershed is 23,473 ha of undeveloped forestland surrounding the 9713 ha Reservoir, which was created by damming the Swift River in 1938, and is managed by the Office of Watershed Management, Department of Conservation and Recreation (MA DCR 2007). Public ownership comprises approximately 64% of the entire Reservoir watershed. The watershed is composed of glacially sculpted valleys between gneiss domes. Soils are glacially derived and predominantly well drained. The forests are composed principally of Quercus spp. (oaks) or Pinus strobus L. (White Pine). An aerial-photography-based inventory identified 430 SFPs on the Quabbin watershed (Brooks et al. 1998). Each identified pool was placed into one of 7 surface-area classes ranging from <0.025 ha to >0.4 ha. Numbers of pools declined rapidly with increasing pool area-class (Brooks et al. 1998). Of the 430 pools, 67% were less than 500 m2 in surface area, and only 14% were 1000 m2 or larger. Pool locations were later digitized, and the distance to the nearest-neighbor pool was calculated for each pool in the inventory. The spatial distribution of pools was significantly aggregated, with median distance between nearest-neighbor pools of 243 m and between-pool distances ranging between 19 m and 2.4 km. We have visited most of the pools located on the western (Pelham [PL] and New Salem [NS]) and the central (Prescott [PR]) management blocks of the property to verify the pool’s occurrence and to describe general pool and catchment characteristics. A sample of 24 SFPs was selected for the study. Three replicate SFPs were selected in 3 size classes (less than 300 m2, 300–999 m2, and ≥1000 m2 maximum surface area) by 3 distance classes (less than 200 m, 200–999 m, and ≥1000 m to nearest neighbor SFP) (Table 1). Only 2 SFPs were identified in the intermediate size and distance class; no candidate SFPs occurred in the largest size-by-distance class. One additional SFP was selected for the largest size-by-nearest-distance class. 562 Northeastern Naturalist Vol. 19, No. 4 Pool and benthic macroinvertebrate sampling Maximum SFP surface areas were calculated using data from bathymetric surveys (Brooks and Hayashi 2002). We calculated an index to SFP hydroperiod as the number of periodic visits to a SFP when surface water was present as a proportion of the number of all visits to the SFP (Brooks and Hayashi 2002, Snodgrass et al. 2000). BMIs were sampled once annually in each SFP in 1998 and 1999 using leafpack substrates (Brooks 2000). We placed 5 leaf packs on each SFP bottom in late November–December, 1997 and 1998, following leaf fall. Typically, the SFPs were dry at the time of leaf-pack installation. We placed 1 pack at or near the center of the SFP basin and the remaining 4 packs mid-way between the center and the SFP edge at the cardinal directions. Each pack was composed of leaves collected in a 2.5-L plastic tub and enclosed in a 45-cm square of 15-mm mesh, black plastic garden netting. While litter composition has been shown to affect consumer communities (Rubbo and Kiesecker 2004, Rubbo et al. 2008), we collected leaves adjacent to each SFP basin so that they would represent the composition of leaf fall into the SFP. We tied the corners of each leaf pack closed with plastic flagging to form a loose “bag” and pinned each pack to the pool bottom with a wire-stake flag. Leaf packs were left undisturbed for the late-fall, winter, and early spring duration of SFP flooding and removed from SFPs over a two-week period in April– May of the year following installation (1998, 1999). We removed the packs when the shortest-hydroperiod SFPs started to dry, removing packs from those first and subsequently from the less ephemeral SFPs. If SFPs were partially or totally dry, exposed packs were not collected. We removed leaf packs by placement in a dip net (mesh 0.8 x 0.9 mm) and raising the net vertically to avoid collecting water-column specimens. Leaf packs were then placed in a sealable plastic bag and transported to the laboratory. At the lab, we drained loose water from the packs and stored them in histological grade (90%) 2-Propanol (isopropyl alcohol). BMIs were sorted from each pack in both white and black enamel pans, and sorted specimens were enumerated and identified to family or genus using Merritt and Cummins (1984), Pennak (1989), Peckarsky et al. (1990), or Smith (1995). We assigned functional group and feeding-mechanism (e.g., predator) classifications to each taxon according to the trophic relations reported in Merritt and Cummins (1984). Life-history strategies were taken from Wiggins et al. (1980) and used to identify passively dispersed, overwintering taxa (Group 1). Taxonomy generally follows Peckarsky et al. (1990). Data analysis We calculated taxon diversity as D' = (T-1) / logeN, where T is the number of taxa and N is the abundance of individuals (Margalef 1968). Due to the unequal number of leaf-pack samples collected from each SFP, pool-level analyses were conducted using median macroinvertebrate sample statistics from each SFP. The direction of the relationships between macroinvertebrate richness and diversity 2012 R.T. Brooks and E.A. Colburn 563 and the pool island attributes were examined using Spearman correlation coefficients. The effects of maximum surface area and inter-pool distance on BMI richness and diversity were analyzed using analysis of covariance by ranks with arcsine-transformed SFP hydroperiod as the covariate. Significance for statistical analyses was determined by P ≤ 0.05. Results Maximum (spring) surface area of the 24 SFPs ranged between 68 (PL394) and 2941 m2 (PR246) (Table 1). The mean surface area was 743 m2, but the median area was only 493 m2, indicating the greater number of smaller-sized SFPs in the study. The distance from each study SFP to its nearest-neighbor pool ranged between 73 (PR243) and 1770 m (PL398), with a mean and median distance of 588 and 531 m, respectively. Hydroperiod indices ranged between 0.33 (PR489) and 1.0 (PL407, PR507), with a mean and median of 0.81. SFPs with indices of 1.0 had surface water at every visit during the study, but they have been observed dry at other times prior to and following the study. Table 1. Maximum pool surface area, nearest-neighbor pool distance, area and distance class, hydroperiod index, and median sample taxa richness and diversity of benthic macroinvertebrates for 24 seasonal forest pools on the Quabbin Reservoir watershed, MA, 1998–1999. Area Area Distance Distance- Hydroperiod Median sample Pool (m2) class (m) class index Richness Diversity PL401 146 1 119 1 0.46 5.0 0.664 NS445 284 1 110 1 0.95 10.0 2.191 NS446 229 1 110 1 0.85 8.0 1.683 PR508 129 1 474 2 0.67 7.0 1.379 NS472 243 1 966 2 0.80 6.0 1.179 PR489 283 1 715 2 0.33 6.0 0.933 PL394 68 1 1267 3 0.85 8.0 1.571 PR505 140 1 1092 3 0.56 4.0 0.414 PL398 152 1 1770 3 0.55 4.0 0.779 PR243 318 2 73 1 0.78 4.5 0.915 PR490 319 2 83 1 0.80 5.0 1.321 PR428 701 2 135 1 0.85 9.5 1.647 PR488 573 2 847 2 0.82 8.5 1.433 NS266 707 2 587 2 0.40 8.0 1.249 ON481 412 2 1051 3 0.82 14.0 2.429 NS448 580 2 1050 3 0.87 5.0 1.001 PL388 781 2 1398 3 0.95 8.0 1.469 PL407 1292 3 155 1 1.00 8.5 1.949 PL406 1482 3 155 1 0.97 7.0 1.461 PR507 1581 3 144 1 1.00 7.0 1.365 NS232 1602 3 137 1 0.82 7.0 1.279 PR241 1380 3 242 2 0.76 6.0 1.083 NS257 1489 3 802 2 0.87 11.0 2.358 PR246 2941 3 627 2 0.95 9.5 1.935 564 Northeastern Naturalist Vol. 19, No. 4 Over the two years of the study, 198 of 240 installed leaf packs were collected: 112 in 1998 and 86 in the drier 1999. The remaining 42 packs were either exposed or disturbed to the extent that they were unrecoverable. Nearly 35,000 (34,971) BMIs were sorted, identified, and enumerated. Of 76 taxa identified, nearly 60% were chironomids and another 15% were oligochaetes (Appendix I). The median leaf-pack sample contained 100 individuals of 7 taxa (Table 2). Median sample richness was fairly consistent at 7 taxa across all surface area and inter-pool distance classes (Table 2). Total BMI richness and diversity were Table 2. Number of samples, median number of taxa, and diversity of benthic macroinvertebrate taxa, by pool area and distance class of 24 seasonal forest pools on the Quabbin Reservoir watershed, 1998–1999. Distance class All Area/class <200 m 200–999 m ≥1000 m classes <300 m2 Samples 23 25 19 67 Taxa 9 6 5 7 Diversity 1.741 1.169 0.973 1.202 300–999 m2 Samples 29 14 27 70 Ttaxa 6 8.5 8 7 Diversity 1.329 1.353 1.498 1.363 ≥1000 m2 Samples 35 26 no samples 61 Taxa 7 9 8 Diversity 1.492 1.772 1.593 All classes Samples 87 65 46 198 Taxa 7 7 7 7 Diversity 1.460 1.333 1.280 1.385 Table 3. Spearman rank correlations among median richness and diversity of benthic macroinvertebrates and area, isolation, and hydroperiod of 24 seasonal forest pools on the Quabbin Reservoir watershed, MA, 1998–1999. Maximum Between-pool Hydroperiod surface area distance index Total community Richness 0.387 0.001 0.543** Diversity 0.344 -0.061 0.693** Overwintering residents Richness 0.267 -0.152 0.258 Diversity 0.137 -0.124 0.041 Predators Richness 0.054 -0.049 0.487* Diversity 0.243 -0.069 0.493* *rs significant for P ≤ 0.05 (rs ≥ 0.406). **rs significant for P ≤ 0.01 (rs ≥ 0.521). 2012 R.T. Brooks and E.A. Colburn 565 moderately (i.e., non-significant) and positively correlated to SFP surface area, positively and significantly correlated with hydroperiod, and uncorrelated to inter-pool distance (Table 3). Similar correlation results were found for overwintering and predatory taxa. No significant effects of island attributes of SFPs on total, overwintering, or predatory BMI richness and diversity were identified (Table 4). The best (i.e., largest r2, smallest P) full model was of total BMI diversity (r2 = 0.539, P = 0.091) with hydroperiod the only significant (F = 10.6, P = 0.005) variable. Most of the explanatory power of the models was achieved from the covariate, SFP hydroperiod (Table 4). Hydroperiod was most significant in explaining the richness and diversity in all taxa and of predator taxa (Table 4). The least significant results occurred between the richness and diversity of passively dispersed, overwintering resident taxa and pool “island” attributes (Table 4). Discussion Seasonal forest pools are geographically and hydrologically isolated wetlands that occur commonly throughout the temperate forests of the northeastern United States and adjacent Canada (Rheinhardt and Hollands 2008, Tiner et al. 2002). The pools are preferred breeding habitat for a number of amphibian species (e.g., Lithobates sylvatica Le Conte [Wood Frog] and Ambystoma spp. [mole salamanders]) and support an abundant, rich, and somewhat unique invertebrate fauna (Colburn 2004, Colburn et al. 2008, Semlitsch and Skelly 2008). Island biogeography theory proposes that if the pools function as islands, as their isolated condition suggests, the community attributes of pool biota should be affected by both the size and isolation of the pools (Holland and Jain 1981, March and Bass 1995, Ripley and Simovich 2009). Table 4. Covariance statistics from analysis of relationships between seasonal forest pool benthic macroinvertebrate median richness and diversity and pool area, isolation, and hydroperiod of 24 seasonal forest pools on the Quabbin Reservoir watershed, MA, 1998–1999. Invertebrate Model Area Isolation Hydroperiod taxa/statistics Richness Diversity Richness Diversity Richness Diversity Richness Diversity All taxa r2 0.475 0.539 F 1.696 0.19 0.455 0.001 1.804 0.709 5.674 10.571 P 0.18 0.091 0.643 0.975 0.199 0.508 0.031 0.005 Overwintering taxa r2 0.262 0.214 F 0.667 0.511 0.736 0.061 0.638 0.016 0.542 0.021 P 0.713 0.83 0.496 0.941 0.542 0.901 0.473 0.886 Predator taxa r2 0.363 0.433 F 1.071 1.434 1.533 0.654 0.845 2.004 5.867 7.006 P 0.432 0.261 0.235 0.534 0.449 0.169 0.029 0.018 566 Northeastern Naturalist Vol. 19, No. 4 MacArthur and Wilson (1967) stated that there “is an orderly relation between the size of a sample area and the number of species found in that area”, but that the relation is not a direct effect of area itself but rather an effect of the greater number of habitats occurring in larger areas. Studies investigating this hypothesis in temporary waters have had mixed results. The relationship between habitat area and species richness has been observed in aquatic invertebrates in small (<13 m2) rockpools and in flatworm species in temporary pools in northern Israel (Eitam et al. 2004, Spencer et al. 1999). Neither study ascertained whether the relationships were the result of biotic factors (e.g., reduced extinction risk, habitat stability, or microhabitat diversity) or a sampling effect. In contrast, Bilton et al. (2001) found that size was not significantly related to invertebrate species richness in 16 ponds in Cornwall, UK. They postulated that the lack of an area effect might be due to a difference in scale, with the larger pools of the UK study (up to 2400 m2) being above a size threshold where detection by dispersing invertebrates was more probable. They also suggested that any area effect might be marginal compared to the influence of permanence or hydroperiod. Our findings parallel positive but weak correlations of biota with area that have been identified for vascular flora of California vernal pools (Holland and Jain 1981), plant and insect taxa of montane calcareous fens (Peintinger et al. 2003), vascular plants in temporary wetlands (Brose 2001), macrophytes in temporary pools (Bazzanti et al. 2003, Oertli et al. 2002), and breeding amphibians in vernal pools (Burne and Griffin 2005). The weak relationships we found between macroinvertebrate richness and SFP area may reflect both the small range in pool surface area and the limited extent of wetland plant cover in our study pools. The small range in macroinvertebrate community diversity metrics over the 43-fold range in pool sizes in our study suggests that for SFPs, area alone may not be a reliable indicator of the within-pool habitat diversity (Tavernini et al. 2005). Classic studies of island biogeography encompass islands with sizes spanning several orders of magnitude (e.g., MacArthur and Wilson 1963). The range in size of vernal pools is limited. Our study encompassed the range of pool surface area of a representative sample of 34 pools on the Quabbin Reservoir watershed, MA (Brooks and Hayashi 2002). Larger pools are more likely to be permanent or to be physically or hydrologically connected to other aquatic systems (Colburn 2004). Smaller, more ephemeral (as versus seasonal, sensu Cowaradin et al. 1979) pools occur, but were not included in the pool inventory (Brooks et al. 1998) or this study. Studies that documented relationships between pool size and macrophyte richness have also found weak positive associations between macrophyte richness and macroinvertebrate richness in temporary pools (Bazzanti et al. 2003, Oertli et al. 2002). In our study pools, wetlands vegetation was limited in distribution and composition. The most common benthic habitat in our pools, regardless of pool size, was non-vegetated substrate covered with leaf litter. 2012 R.T. Brooks and E.A. Colburn 567 A second, less common sedge/rush-dominated habitat occurred infrequently in pools with greater solar exposure (i.e., pools ON481, NS257). Due to the small surface area of the study pools, to abundant overstory tree cover adjacent to the pools, and to their temporary inundation, submerged aquatic vegetation was absent, which is a common condition in SFPs (Higgins and Merritt 1999). When within-pool habitats are limited in number, regardless of pool size, area should have less or no effect on faunal composition (Della Bella et al. 2005, Oertli et al. 2002, Smith and Haukos 2002). The dispersal of organisms among islands is affected both by the distance between source and recipient islands and by the dispersal capability of the organism (MacArthur and Wilson 1967). The effects of distance among temporary pools on faunal community composition have not been as well studied as have pool-size effects. Spencer et al. (2002) found no effect of inter-pool distance on invertebrate community similarity in a cluster of 25 rock pools and concluded that dispersal was not limited among the pools. The same conclusion was reached in studies of vascular plants (Brose 2001) and carabid beetles (Brose 2003) in temporary wetlands in an agricultural landscape. Our findings parallel those of these studies. The distance among SFPs is quite small compared to the distances among the islands of classic biogeographic studies. The median nearest-neighbor distance class for all pools on the Quabbin Reservoir watershed is only 200–299 m (Brooks et al. 1998), which is less than the median inter-pool distance for this study (531 m). These distances do not appear to constitute significant barriers to the dispersal of aquatic invertebrates, even for passively dispersed taxa. The wide distribution of highly mobile Insecta among vernal pools was expected, although Angelibert and Giani (2003) found odonate species to be more philopatric than expected, which would limit dispersal. Insecta have developed strategies for adapting to the temporary existence of SFPs, typically through seasonal flight dispersal from permanent water bodies for annual colonization of pools (Wiggins et al. 1980). The occurrence, regardless of pool isolation, of passively dispersed, overwintering taxa in SFPs was unexpected. However, King et al. (1996) found no association between the geographic proximity of California vernal pools and the similarity of their crustacean assemblages, and Mura and Brecciaroli (2003) reported the wide distribution of 25 species of microcrustaceans among 12 temporary pools in a Mediterranean plain forest of coastal Italy, which they ascribe to dispersal by vertebrates. The passive dispersal of smaller aquatic organisms by vertebrates and wind has been documented (Bilton et al. 2001, Brendonck and Riddoch 1999, Maguire 1963), but may not occur frequently (Jenkins and Underwood 1998). Distance from source habitats has been shown experimentally to affect dispersal, with the number of taxa declining with distance beyond 58.5 m (64 yards) from the source pond (Maguire 1963). The passive dispersal of invertebrate taxa can occur relatively quickly. Maguire (1963) reported that colonization of experimental water bodies by additional taxa ceased by about 6 weeks; however, Jenkins (1995) reported the continued 568 Northeastern Naturalist Vol. 19, No. 4 colonization of experimental pools by rotifers and microcrustaceans up to one year into the study. Based on these dispersal statistics, even passively dispersed taxa should have been able to colonize the most remote pool of this study over the near 60 years that these pools have existed in their present protected, forested watershed environment. Positioned at the top of SFP food webs, predatory taxa were expected to be most sensitive to pool size (Nilsson 1984, Pearman 1995, Spencer et al. 1999, Wilcox 2001). Our results support this hypothesis, but again, only weakly. Nilsson and Söderström (1988) suggested that pool size creates a threshold for the entry of predatory species based on prey density and minimum population size. Increasing pool area increases the threshold and allows for additional predatory species of increasing size. Another potential reason for increased predator species richness in larger pools would be that larger pools are better buffered against fluctuations in physiochemical conditions and are more likely to have increased microhabitat diversity (Spencer et al. 1999). Additionally, larger pools typically have longer hydroperiods, and longer hydroperiods typically support richer aquatic communities due to reduced stress associated with avoiding desiccation (Colburn 2004, Wiggins et al. 1980, Williams 1983). This last hypothesis appears to hold the strongest explanatory power in relation to our data, as hydroperiod was the strongest variable in our analyses of predatory taxa. The dominant influence of hydroperiod on predator community composition was not unexpected (Bilton et al. 2001, Spencer et al. 1999). The degree of pool isolation had little effect on predator community composition, given that most members are highly mobile, annual migrants (Wiggins et al. 1980). The hydroperiod of temporary aquatic habitats has been suggested to function as a third, temporal dimension of island biogeographic effects (Bilton et al. 2001, Ebert and Balko 1987). Permanence, rather than pond area, was strongly related to overall species richness and the proportion of predators found in ponds in southwest England (Bilton et al. 2001). Hydroperiod, or habitat duration, was a dominant influence on invertebrate community composition of temporary woodland ponds of the midwestern US (Higgins and Merritt 1999, Schneider 1999, Schneider and Frost 1996). The invertebrate community of short duration (i.e., days) pools was dominated by overwintering taxa, composed predominantly of grazers or filterers; longer-duration (i.e., months) pools allowed for colonization by predators. Duration acts by mediating the relative importance of life histories and biotic interactions, particularly predation, in determining the distribution and abundance of taxa. As vernal pool hydroperiod approaches permanency, biotic richness and diversity should approach that of permanent ponds (Driver 1977). The hydroperiod of short-duration ponds studied by Schneider (1999) and Schneider and Frost (1996) was much less (i.e., <10 days; temporarily flooded) than the seasonally flooded hydroperiod of the SFPs in this study. Nevertheless, even with a much smaller range in hydroperiod among study pools, the relationship between hydroperiod and BMI community diversity was significant. Total 2012 R.T. Brooks and E.A. Colburn 569 community richness and diversity increased significantly with increasing hydroperiod. The effect was also observed in predatory taxa, but not in overwintering taxa. Predatory taxa are most often spring or summer migrants, and larval forms should occur more often in pools with longer hydroperiod pools. Overwintering taxa are drought resistant and are expected to occur in all but the most temporary pools (Brock et al. 2003, Wiggins et al. 1980). The effects of hydroperiod would likely have been enhanced if invertebrates had been sampled over the full annual hydrologic cycle of the pools. There is a successional pattern in aquatic invertebrate taxa occurrence in temporary wetlands (Brooks 2000, Williams 1983). Early spring inhabitants are dominated by overwintering, grazing- and filter-feeding taxa; later arrivals are dominated by migrant, predatory taxa (Higgins and Merritt 1999). In this study, all invertebrate samples were taken at one point in time. If samples had been taken later in the year (e.g., mid-summer) in longer hydroperiod pools, it is likely that the influence of hydroperiod on taxa richness and diversity would have been stronger, but comparisons could not have been made to shorter-hydroperiod pools. From this study, we draw several conclusions about seasonal forest pools in the context of island biogeography theory, and two methodological observations about studying these systems. First, we assessed the effects of pool size and isolation predicted by island biogeography theory, plus the effects of pool hydroperiod, on benthic macroinvertebrate communities of SFPs. Our study confirmed the expected results of increased richness and diversity in larger pools, but we did not observe the expected effects of pool isolation. The distance among our study pools, even for those most removed, appears to be less than the maximum dispersal distance of even passively dispersed taxa. As demonstrated in many studies, pool hydroperiod affected the richness and diversity of BMIs. The strength of the significant relationships between faunal diversity and pool attributes was minimal, with all relationships collectively accounting for less than half the variation in faunal diversity. These results suggest that the BMI community of SFPs is relatively uniform (Stein et al. 2003), at least over the spectrum of pool attributes included in this study. This study supports the findings of others that hydroperiod is a dominant influence in temporary aquatic systems, but that many biotic and abiotic factors, as well as chance, structure the BMI community of SFPs. Additionally, since hydroperiod and pool surface area are related, it is difficult to separate the individual effects in field studies. The findings of this study would likely be strengthened by methodological improvements. We sampled all SFPs with the same number of leaf packs, regardless of surface area. The result was that larger pools were sampled with less effort (i.e., fewer samples per unit area). A potential consequence would be the underrepresentation of the true taxa richness of larger pools. This problem is somewhat ameliorated by the simplicity of the benthic habitats within the SFPs. Predatory taxa were most likely under-represented in benthic, leaf-pack samples as they are more active in the water column (Hanson et al. 2000). A second sampling issue 570 Northeastern Naturalist Vol. 19, No. 4 was the length of time over which the samples were collected each year. Even in these short-duration systems, a succession of macroinvertebrate species can occur with time (Williams 1983). However, in an earlier study (Brooks 2000), within-year variation in diversity was more a function of abundance than occurrence, and samples in that study were taken over the course of two months, rather than just two weeks as in this study. Finally, the classification of specimens to family or genus is likely to have negatively impacted the estimation of taxa richness and diversity and may have hindered our ability to detect more differences among pool area and distance treatments. A more detailed classification, especially of family-level identifications, would have resulted in increased community diversity (King and Richardson 2002). Chironomids and oligochaetes from organic sediments have long been used in the classification and bioassessment of lakes (Brinkhurst 1974, Langdon et al. 2006), and chironomid diversity in fresh waters is often exceptionally high (Ferrington 2008). However, classification below family would have been difficult and time consuming, especially for Chironomidae, which accounted for a majority of the specimens. It is an open question whether a more detailed classification would have affected the results of the study. Overall, our study suggests that island biogeography theory has limited application to SFPs, at least within the forested watershed in which our study pools are found. Further investigations focused on quantifying hydroperiod differences, incorporating more extensive sampling over time, and carrying out more detailed systematic analyses of the fauna, are likely to contribute to better understanding of the factors influencing community richness and diversity in S FPs. Acknowledgments C. Walker and L. Higgins processed and identified the invertebrate specimens; E. Nedeau reviewed all identifications. R. DeGraaf , P. Paton, and anonymous referees provided critical reviews of the manuscript. Literature Cited Angelibert, S., and N. Giani. 2003. Dispersal characteristics of three odonate species in a patchy habitat. Ecography 26:13–20. Babbitt, K.J. 2005. The relative importance of wetland size and hydroperiod for amphibians in southern New Hampshire, USA. Wetlands Ecology and Management. 13:269–279. Bagella, S., S. Gascón, M. Carmela Caria, J. Sala, M. Antonietta Mariani, and D. Boix. 2010. 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Oxford University Press, Oxford, UK. 337 pp. 576 Northeastern Naturalist Vol. 19, No. 4 Appendix I. Number of benthic macroinvertebrates in seasonal forest pool samples by taxa and year, Quabbin Reservoir watershed, MA, 1998–1999. Phylum Class Order Family Genus species 1998 1999 Nematoda 13 Platyhelminthes 12 Turbellaria 1 307 Annelida Oligochaeta 2013 2747 Hirudinea Arhynchobdellida Erpobdellidae Erpobdella sp. 2 8 Rhynchobdellida Glossiphoniidae 7 Batracobdella (Placobdella) picta 5 (Verrill) Mollusca Gastropoda 2 Basommatophora/Mesogastropoda Lymnaeidae 1 1 Fossaria parva (I. Lea) 5 19 Pseudosuccinea columella (Say) 1 Physidae Physa sp. 2 17 Pelecypoda (Bivalvia) Veneroida/Sphaeracea Sphaeriidae 186 Musculium securis (Prime) 215 Pisidium casertanum (Poli) 56 Arthropoda Hydrachnida 132 141 Crustacea 24 Anostraca Chirocephalidae Eubranchipus vernalis (Verrill) 23 16 Eubranchipus sp. 3 Cladocera 208 105 Copepoda 115 16 Ostracoda 1110 628 Insecta 6 Collembola 2 Entomobryidae 57 607 Hypogastruridae 13 Isotomidae 653 Poduridae 340 Sminthuridae 5 2 2012 R.T. Brooks and E.A. Colburn 577 Phylum Class Order Family Genus species 1998 1999 Odonata 2 Libellulidae 8 83 Sympetrum sp. 1 Pachydiplax longipennis (Burmeister) 2 Nanothemis bella (Uhler) 1 Coenagrionidae 1 Lestes sp. 15 16 Plecoptera 11 Leuctridae 1 1 Hemiptera 1 Corixidae 1 Gerridae Gerris sp. 2 3 Hydrometridae Hydrometra sp. 3 Trichoptera Polycentropodidae Polycentropus sp. 167 4 Cernotina sp. 166 Limnephilidae 2 Limnephilus sp. 83 188 Phryganeidae Banksiola sp. 3 2 Ptilostomis sp. 17 13 Coleoptera Dytiscidae 2 Acilius sp. 9 8 Agabus sp. 25 27 Dytiscus sp. 4 Hydrocolus sp. 5 Hydroporus sp. 14 47 Hygrotus sp. 1 Ilybius sp. 4 Neoporus sp. 2 Gyrinidae 9 Gyrinus sp. 10 2 Haliplidae 2 8 Haliplus sp. 3 3 Noteridae 1 Hydraenidae Hydraena sp. 21 Hydrophilidae 1 5 Anacaena sp. 2 Enochrus sp. 2 Helocumbus sp. 1 14 Hydrochus sp. 2 Helophorus sp. 3 Tropisternus sp. 9 578 Northeastern Naturalist Vol. 19, No. 4 Phylum Class Order Family Genus species 1998 1999 Scirtidae 26 Cyphon sp. 22 253 Megaloptera Corydalidae Chauliodes sp. 10 17 Diptera 50 4 Ephydridae 4 Dolichopodidae 3 Empididae 1 Stratiomyidae 18 6 Tabanidae 6 1 Tabanus sp. 1 Ceratopogonidae 197 403 Chaoboridae 30 Chaoborus sp. 2 Mochlonyx sp. 553 378 Chironomidae 13,294 5518 Culicidae 1 Psorophora sp. 11 Aedes sp. 488 336 Dixidae 2 Dixella sp. 6 Tipulidae 4 3 Phalarocera sp. 2