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The secret pool
A look at vernal pools, which form every year when low places on the forest floor fill up with rain and melted snow.
Book
Quantitative macroinvertebrate bioassessment in seasonally astatic aquatic habitats, Part II. Applying the method
Seasonally astatic aquatic habitats are important ecologically, municipally, and agriculturally. Regulatory agencies and conservation organizations have developed various plans for protecting or constructing temporary wetlands, resulting in habitat monitoring requirements, particularly as relates to restoration and constructed habitats. Unfortunately, there has been no effort to develop a unified, consistent method for wetland biological monitoring. In Part I, we presented a quantifiable, replicable method for assessing seasonally astatic wetlands, which would allow for direct comparison between individual wetlands, wetland sites, and wetland types. Here in Part II, we apply the method and present the results from more than a decade of a data on two disparate sites that support California vernal pool habitats. These habitats include natural, restored, and constructed vernal pools. Our results demonstrate that the method we present yields reliable, statistically useful, and actionable data and provides a better method for assessing astatic wetland ecological health and the persistence of federally listed vernal pool crustaceans than other methods so far employed.
Journal Article
Community‐based long‐term management to address reinvasion of restored grassland vernal wetlands
Exotic annual grasses can dominate ecosystems by producing a thick layer of dead plant litter, hereafter “thatch,” which promotes the regeneration of exotic grasses and inhibits native plants. Vernal pool wetlands within a grassland matrix are threatened by these exotic annual grasses, meriting the need for long‐term management. We utilized the investment of local community members to test the efficacy of long‐term thatch management on urban vernal pool plant assemblages. We recruited over 40 undergraduate students to perform manual annual summer thatch removal around the edges of 15 urban vernal pools for four years. We coupled thatch removal with annual native seed addition because our analysis of environmental DNA (“eDNA”) in the soil seed bank revealed a lack of native plant species and an abundance of exotic plant species. Our annual thatch removal treatment successfully reduced thatch accumulation and increased bare ground, but it did not result in a consistent decrease in exotic plant cover or an increase in native plant cover. Instead, the effects of thatch manipulation on plant composition were modulated by annual precipitation, with exotic species increasing during dry years and native species increasing during wet years. The addition of native plant seed caused an increase in native plant species richness, but only after three years of annual thatch removal and native seed addition. Our results indicate that the restoration of native vernal pool plants can be limited by invasive species, native seed availability, and annual precipitation. Our findings show how engaging the local community in the long‐term restoration of urban ecosystems can address the persistent threat of invasion and build up capacity for native plant populations to increase over time.
Journal Article
Vernal Pool Amphibian Inventories in the Temperate Forests of Eastern North America: Can Environmental DNA Replace the Traditional Methods?
Amphibian populations have been globally declining since at least 1990. In the temperate forests of eastern North America, vernal pools offer important breeding habitats to many amphibians but are usually not considered for forest management because of their small size and temporary status. The effective monitoring and management of amphibians rely on accurate knowledge of their spatiotemporal distributions, which is often expensive to collect due to the amount of fieldwork required. In this study, we tested whether eDNA metabarcoding could identify the same amphibian communities as the traditional inventory protocols. We collected eDNA samples in twelve vernal pools in the spring of 2019 and identified their communities via metabarcoding. At each pool, three traditional amphibian inventory methods were used: call surveys, trapping, and active search surveys. In total, 13 amphibian species were detected, with most of them being detected using both eDNA and the traditional methods. When comparing the results of eDNA with the traditional methods, we found that species ecology and behavior are key factors of its detectability via a specific method. With its higher taxonomical precision and repeatability, eDNA metabarcoding allows for the inventorying of amphibian species living and reproducing in vernal pools and their vicinity with accuracy. As eDNA metabarcoding is inexpensive compared to the traditional methods, we conclude that eDNA sampling should be considered for integration as a standard monitoring tool, after an initial assessment of amphibian diversity.
Journal Article
Occupancy and environmental responses of habitat specialists and generalists depend on dispersal traits
Ecologists have been interested in understanding communities through the lens of specialists and generalists to predict species diversity and distribution patterns and to ameliorate worldwide declines in specialist species. Dispersal traits are assumed to be associated with specialization (specialists are weaker dispersers than generalists), but dispersal modes can be variable within groups. Niche‐based predictions of occupancy and environmental responses were assessed using invertebrates from California vernal pools that were categorized by specialization (endemic or widespread taxa) and dispersal mode (passive or active dispersal). Data from a latitudinal gradient resulted in widespread taxa with greater percent occupancy than endemic taxa as predicted, but passive dispersers had greater occupancy than active dispersers in contrast to predictions. Endemic species and widespread‐active dispersers exhibited similar levels of specialization measured as coefficient of variation among treatments in a mesocosm experiment. This suggested that habitat choice was important, and these differences in specialization were scale dependent (generalists across habitat types and specialists within a habitat type). A negative correlation between latitudinal occupancy and level of specialization demonstrated how local‐scale responses and landscape patterns were related and depend on both specialization and dispersal traits. This study underscores how habitat heterogeneity and species traits, including specialization and dispersal, can interact to affect community patterns at different spatial scales.
Journal Article
Climate-driven habitat size determines the latitudinal diversity gradient in temporary ponds
The latitudinal diversity gradient (LDG) has been one of the most documented patterns in ecology, typically showing decreasing species diversity with increasing latitude. Studies of these patterns also used different spatial scales and dispersal traits to better understand the underpinning ecological factors. Seasonal freshwater ecosystems are less studied and may exhibit different patterns because they are more sensitive to climatic variation, which result in an inundation–desiccation cycle. In California, precipitation increases and temperature decreases with increasing latitude and thus the LDG pattern may be associated with this climatic gradient. Using collected data and United States Fish and Wildlife Service reports across seven degrees of latitude, analysis of California vernal pool invertebrate community (total richness and richness of passive and active dispersers) was conducted using correlations (Spearman rank and partial). Alpha diversity (total and passive dispersers) increased and beta diversity (passive dispersers) decreased with increasing latitude. Vernal pool surface area was correlated with active disperser alpha and passive disperser beta diversity. This suggests that climate‐driven habitat size influences alpha and beta diversity patterns depending on dispersal ability. Active dispersers and predators exhibited higher beta diversity than passive dispersers and prey, respectively. Species composition differed among counties and some of these differences were correlated with pool depth and temperature. These results suggest that seasonal habitats will have diversity patterns strongly associated with local scale characteristics (habitat size and hydroperiod) determined by climate variation along the latitudinal gradient. Understanding these diversity patterns along the gradient will also contribute to management and restoration of these ecosystems with high endemism and diversity.
Journal Article
Resistant-Kernel Model of Connectivity for Amphibians that Breed in Vernal Pools
Pool-breeding amphibian populations operate at multiple scales, from the individual pool to surrounding upland habitat to clusters of pools. When metapopulation dynamics play a role in long-term viability, conservation efforts limited to the protection of individual pools or even pools with associated upland habitat may be ineffective over the long term if connectivity among pools is not maintained. Connectivity becomes especially important and difficult to assess in regions where suburban sprawl is rapidly increasing land development, road density, and traffic rates. We developed a model of connectivity among vernal pools for the four ambystomatid salamanders that occur in Massachusetts and applied it to the nearly 30,000 potential ephemeral wetlands across the state. The model was based on a modification of the kernel estimator (a density estimator commonly used in home range studies) that takes landscape resistance into account. The model was parameterized with empirical migration distances for spotted salamanders (Ambystoma maculatum), dispersal distances for marbled salamanders (A. opacum), and expert-derived estimates of landscape resistance. The model ranked vernal pools in Massachusetts by local, neighborhood, and regional connectivity and by an integrated measure of connectivity, both statewide and within ecoregions. The most functionally connected pool complexes occurred in southeastern and northeastern Massachusetts, areas with rapidly increasing suburban development. In a sensitivity analysis estimates of pool connectivity were relatively insensitive to uncertainty in parameter estimates, especially at the local and neighborhood scales. Our connectivity model could be used to prioritize conservation efforts for vernal-pool amphibian populations at broader scales than traditional pool-based approaches.
Journal Article
Indicators of wood frog (Lithobates sylvaticus) condition in a suburbanizing landscape
Wood frogs (Lithobates sylvaticus) are threatened by habitat degradation associated with suburbanization. Suburban development near pools may affect larvae with ramifications for population persistence and vitality. We studied larval development and documented key vernal pool and terrestrial characteristics within 1000 m of 43 pools across the suburban development gradient near Bangor, Maine, USA. Specifically, we examined how survival and morphological characteristics (e.g., developmental phenology, condition, body length, and tail length and shape) varied with characteristics at pool and landscape scales. Secondarily, we explored associations between morphology and survival. Differences in tadpole morphology were associated with suburban land development, hydrology, within‐pool vegetation indicative of light availability at the water's surface, and density of pool‐breeding amphibian egg masses. Across all pools, tadpoles had delayed development and were larger in pools with longer hydroperiods, while tadpoles in suburban pools developed earlier and were larger than those in rural pools with comparable hydroperiods. Tadpoles developed later and had longer tails in pools with greater canopy cover. Morphology profiles also differed between rural and suburban sites and among years. Survival in suburban pools was predicted to be 15 times greater than in rural pools, but across all pools (including those at intermediate intensities of suburbanization), survival was not predicted to vary with either morphology or site characteristics. No strong relationship existed between developmental phenology and any condition or size metric. Because rural and suburban tadpoles responded similarly to within‐pool conditions, our results support the need to maintain natural hydrology and vegetation conditions in pools even in developing areas. Although we detected benefits to tadpoles with increasing suburbanization, suburbanization is well known to extirpate breeding populations; thus, it is likely that wood frog population declines associated with suburbanization are responding to stressors beyond the pool at terrestrial life stages.
Journal Article
Pollinator-mediated competition between two congeners, Limnanthes douglasii subsp. rosea and L. alba (Limnanthaceae)
Premise of Study: Pollinator visits are essential for reproduction in many plants, yet interspecific movements of pollinators can also lead to competitive interactions between coflowering species. Pollination-mediated reductions in fertility could potentially lead to exclusion of competing plant species, and may generate spatial variation in the associations among coflowering species across a landscape. Methods: I documented the potential for heterospecific pollen transfer to cause competitive interactions between two annual grassland species native to California, Limnanthes douglasii subsp. rosea and L. alba, two reproductively incompatible species that have broadly overlapping geographic ranges in the foothills of the Sierra Nevada. I observed pollinator movement in constructed arrays and controlled crosses in the greenhouse and field to investigate the consequences of heterospecific pollen transfer. Key Results: Pollinators move readily between species when they are presented together in experimental arrays. In the greenhouse, deposition of heterospecific pollen decreased fertility in both species. The decrease in seeds produced per flower was much more pronounced in L. d. rosea (90.6% reduction) than in L. alba (40.8% reduction). In field experiments, L d. rosea plants that received pollen from heterospecific neighbors first showed > 50% reduction in per-flower fertility. Conclusions: Under natural pollination conditions, heterospecific pollen transfer has the ability to decrease the fertility of L. d. rosea when it occurs at low frequency in mixed stands. Accordingly, pollinator-mediated competition may contribute to the locally disjunct distributions of these two species. It may also influence important restoration decisions in vernal pool habitats.
Journal Article
Quantifying the dominance of local control and the sources of regional control in the assembly of a metacommunity
To understand the relative roles of local and regional processes in structuring local communities, and to compare sources of dispersal, we studied plant species composition in the context of a field experiment in vernal pool community assembly. In 1999, we constructed 256 vernal pools in a grid surrounding a group of over 60 naturally occurring reference pools. Each constructed pool received a seeding or control treatment. Seeding treatments involved several \"focal species\" native to vernal pools in this region. Earlier analyses identified local habitat quality (pool depth) and pool history (seeding treatment) as strong predictors of local species composition. For the current analysis, we asked how connectivity among pools might enhance models of focal species presence and cover within pools, using long-term data from control pools and from unseeded transects within a stratified random sample of all constructed pools. We fitted connectivity models for each of four focal species, and compared the relative support for connectivity, seeding treatment, and pool depth as predictors of local species presence and cover. We modeled connectivity in several ways to quantify the relative importance of immigration (1) from constructed pools, (2) from reference pools within the study site, (3) from a cluster of natural pools off-site, and (4) along ephemeral waterways. We found strongest support for effects of connectivity with reference pools. Species presence in a target pool was usually well predicted by an exponential decline in connectivity with distance to source pools, and our fitted estimates of mean dispersal distance indicate strong dispersal limitation in this system. Effects of target and source pool size were also supported in some models, and long-term effects of seeding were supported for most species. However, pool depth was by far the strongest predictor of focal species presence, and depth rivaled connectivity with reference pools as a top predictor of cover after accounting for species presence. We conclude that local species composition was determined primarily by local processes in this system, and we encourage more widespread use of a straightforward method for weighing local vs. regional influences.
Journal Article