Darold P. Batzer

Relationships between environmental characteristics and macroinvertebrate communities in seasonal woodland ponds of Minnesota

Abstract We related macroinvertebrate communities and environmental variables in 66 small seasonal woodland ponds of northern Minnesota, USA. These wetlands were relatively pristine, being embedded in 50- to 100-y-old 2nd-growth forests. Macroinvertebrate taxon richness in ponds increased as hydroperiods lengthened, tree canopies opened, water pH declined, and litter input decreased. Eighteen macroinvertebrate taxa were widespread (occurred in >50% of ponds), and hydrology, water chemistry, geomorphology, vegetation, occurrence of other macroinvertebrate taxa, and presence of amphibian larvae each explained some variation in relative abundance of widespread macroinvertebrates. The first 4 axes of a canonical correspondence analysis explained 37% of total variation in relative abundance of widespread macroinvertebrate taxa. Overall, however, macroinvertebrates were remarkably unresponsive to environmental variables. Most relationships between macroinvertebrates and environmental variables were nonsignificant, and the few significant relationships observed were weak (<20% of variation). We suggest that this lack of response occurs because most macroinvertebrates in seasonal woodland ponds are habitat generalists. These species routinely endure pronounced and unpredictable environmental changes; hence, they possess a durability that makes them resistant to most natural variation in habitat conditions.

INVERTEBRATES ASSOCIATED WITH WOODY DEBRIS IN A SOUTHEASTERN U.S. FORESTED FLOODPLAIN WETLAND

Woody debris is an ecologically important resource in upland forests and stream ecosystems. Although much is known about invertebrate-woody debris interactions in forests and streams, little information exists for forested wetlands. In this study, invertebrates associated with woody debris in a Southeastern U. S. forested floodplain are described and factors that shape community structure are examined. Woody debris samples were collected during two wet (March 1998 and 1999) and one dry period (August 1998) from a bottomland hardwood wetland along the Coosawhatchie River, South Carolina, USA. During wet period collections, both submersed and floating woody debris were collected. Invertebrate richness, density, and arthropod standing-stock biomass were compared among sampling periods (wet and dry), between floating and submersed wood, and among woody debris decay classes. Most invertebrate richness and arthropod biomass was associated with wood collected during wet periods. However, the non-aquatic rather than aquatic arthropods were the most significant component of the overall community structure. Floating woody debris was a “hot spot” for invertebrate richness and arthropod biomass. Increased invertebrate richness was also associated with well-decayed wood. Invertebrates were classified based on temporal use of woody debris and included perennial residents, seasonal colonizers, and seasonal refugees. Overall findings suggest that woody debris is an important resource for invertebrates, and wood-associated invertebrates (especially non-aquatics) need to be considered when studying the diversity and function of forested wetlands.

IMPACTS OF FISH PREDATION ON MARSH INVERTEBRATES: DIRECT AND INDIRECT EFFECTS

We excluded predatory fish from a marsh weedbed to evaluate experimentally their impact on invertebrate prey. Gut analyses of wetland fish, including pumpkinseed sunfish (Lepomis gibbosus), brown bullhead (Ictalurus nebulosus), black crappie (Pomoxis nigromaculatus), and common carp (Cyprinus carpio), revealed that large numbers of midge larvae (Diptera: Chironomidae) were consumed. However, our exclusion of these predatory fish from study habitats did not result in midge population increases. On the contrary fewer epiphytic midges occurred where predatory fish had been excluded (P=0.0043). Populations of midge competitors (especially Planorbidae and Physidae) and invertebrate midge predators (especially Corixidae and Glossiphoniidae) were suppressed directly by fish, and midges that co-existed with fish apparently benefitted indirectly from those interactions. For epiphytic midge larvae, the negative direct influence of fish predation was strong, but positive indirect effects apparently were even more powerful.

Macroinvertebrates of a California seasonal wetland and responses to experimental habitat manipulation

Responses of macroinvertebrate populations to temporal change and to management of plant cover and water depth were examined in 12 experimental ponds designed to mimic seasonally flooded (early autumn to early spring) wetlands in Suisun Marsh, California. Ponds were flooded in September 1988. Initially, rat-tailed maggots (Eristalis tenax), brine fly larvae (Ephydra millbrae), and mosquito larvae (Culex tarsalis) were numerically dominant components of the fauna in the ponds, but these populations did not persist beyond December. Water boatmen (Trichocorixa verticalis), midge larvae (Cricotopus sylvestris andChironomus stigmaterus), and hydrophilid beetle larvae (Berosus ingeminatus) were numerically dominant components of the fauna in both autumn and winter, but water boatmen and midge densities declined by March. Amphipods (Eogammarus confervicolus) and three-spined stickleback (Gasterosteus aculeatus) were introduced during initial floodings in September, and they subsequently increased to become numerically dominant components of the pond fauna by February and March 1989. Higher numbers of adult water boatmen and hydrophilid beetles colonized habitats that had 50% of the plant cover removed by mowing than habitats that had not been mowed (100% plant cover). Since mowing also reduced numbers of mosquito larvae, this approach may benefit waterfowl management and reduce public health concerns.

TROPHIC INTERACTIONS AMONG DETRITUS, BENTHIC MIDGES, AND PREDATORY FISH IN A FRESHWATER MARSH

I conducted field experiments in a western New York marsh to quantify concurrently the impacts of resource limitation and predation on benthic midge populations (Chironomus plumosus, Chironomus tentans, and Glyptotendipes barbipes). Biomass of cattail litter (Typha sp.), numbers of small insectivorous fish (primarily juvenile carp, Cyprinus carpio, and to a lesser extent juvenile brown bullhead, Ictalurus nebulosus), or numbers of large fish (primarily adult brown bullhead, which will consume both midges and small fish) were manipulated in mesh-walled mesocosms (1.5 X 1.5 X 1.2 m). Using core sampling, midge population responses to manipulations were monitored from early May through late August. Trophic responses of marsh midges to resources and predation changed temporally over the summer. In June, neither resource limitation nor predation by fish significantly influenced midge numbers. In July, after juvenile carp and brown bullhead first became abundant from the spawn, these small insectivorous fish significantly reduced midge larval densities. At that time, midge densities still did not respond to variable detritus supplies. By August, both predation by small fish and available supplies of cattail litter significantly affected midge numbers. Because midges did not respond to a nonnutritive mimic of cattail litter, it suggests that resource limitation of midge abundance was nutritionally based. Feeding by large adult bullhead had no apparent direct or indirect influence on midge densities in any month. Small fish largely disappeared from the study habitats by August, but populations of midge larvae did not rebound because they had stopped breeding for that year. This study supports emerging concepts on the integrated influences of top-down and bottom-up trophic interaction on food web function. Predation and resource limitation were simul- taneously influencing midge abundance. Results also emphasize how life history phenol- ogies of predators and prey can contribute to temporal variation in food web structure and function.

SEASONAL POND CHARACTERISTICS ACROSS A CHRONOSEQUENCE OF ADJACENT FOREST AGES IN NORTHERN MINNESOTA, USA

Small seasonal ponds are abundant in many forest landscapes, yet they remain poorly understood in terms of their response to disturbance of the surrounding upland forest. The potential for such a response is large because of the small size and, hence, high perimeter-to-area ratios of most ponds. High perimeter-to-area ratio may increase the importance of functional connections with the surrounding forest, via exchange of energy, organisms, and materials. To better understand this connection, we studied 19 seasonal ponds across a 100-year chronosequence of single-cohort forests in northern Minnesota. Our objective was to see if there are distinct changes over time in select pond attributes, which may reflect alteration of functional linkages with the surrounding forest. In 1998 and 1999, we sampled hydroperiod, water depth and chemistry, canopy openness, grass, sedge, shrub, and coarse woody debris cover, coarse particulate organic matter (CPOM) flux, and macroinvertebrate and amphibian populations. We related these variables to stand age through regression. Stand age explained little variation for most variables. Responsive variables included canopy openness and CPOM flux. Canopy openness, in turn, was related positively to total macroinvertebrate abundance, sensitive taxon richness, and Haliplidae beetle and Physidae snail abundances. Calling wood frogs occurred more frequently under an open canopy and low CPOM flux. An open canopy, which occurs more often over ponds in yonnger than in older forest, likely results in increases in water and air temperatures and photosynthetically active radiation, all of which may influence resource availability and habitat suitability for some macroinvertebrates and amphibian taxa. Results from our exploratory study suggest that many characteristics of small seasonal ponds are unaffected by harvest of the adjacent upland forest, at least as detected through examination of a chronosequence. However, responsive variables may include several abiotic characteristics that provide mechanistic links to pond foodwebs.

The Seemingly Intractable Ecological Responses of Invertebrates in North American Wetlands: A Review

Invertebrates play important ecological roles in wetlands, but what factors control their dynamics are poorly established. I review available research conducted in 14 areas of North America where invertebrates have received the most attention. Studies indicate that hydrology, plant factors, anthropogenic disturbance, and predation are the most important controls on wetland invertebrates, but little consensus emerged about how these factors influence invertebrate communities. Findings were often equivocal, contradictory, or inconclusive, either within or among areas. Considerable evidence indicated that wetland invertebrates are resilient, generalist organisms, and a lack of response may reflect insensitivity to environmental variation. However, numerous studies also indicated distinct responses by some invertebrates in some wetlands, although consistent patterns were often lacking. This suggests that wetland invertebrates might be very sensitive to environmental variation, but interactions can be complex and difficult to predict. Either conclusion means that generalizations about how invertebrates in wetland respond ecologically will be difficult to generate without careful study.

Characteristics of invertebrates consumed by mallards and prey response to wetland flooding schedules

We examined characteristics of the invertebrates consumed by mallards (Anas platyrhynchos) and green-winged teals (Anas crecca) and responses of these invertebrates to manipulations of flooding date in Suisun Marsh, Solano County, CA. Numbers ofChironomus stigmaterus midge larvae (Chironomidae) andEogammarus confervicolus amphipods (Gammaridae) in mallard esophageal samples were positively correlated with abundance of these invertebrates in wetlands. Mallards primarily consumed large midge larvae (fourth instars) and amphipods (>5 mm in length). Smaller green-winged teals consumed smaller midges. Mallards consumed fewTrichocorixa verticalis water boatmen (Corixidae) orCricotopus sylvestris midge larvae, despite their being abundant. Wetlands first flooded in early September had higher winter populations of amphipods andBerosus ingeminatus beetle larvae (Hydrophilidae) than wetlands first flooded in late October. Late-winter abundance of benthicC. stigmaterus midge larvae was highest at 40-cm water depths in the former habitats and at 20-cm depths in the latter habitats.

Influences of riparian logging on plants and invertebrates in small, depressional wetlands of Georgia, U.S.A.

We studied 12 small, seasonally flooded, depressional wetlands on the Atlantic Coastal Plain of Georgia, U.S.A. Each wetland was embedded in stands of managed plantation pine. The pine trees surrounding each wetland had been harvested and replanted beginning in 1997 (2 sites), 1995 (2 sites), 1993 (1 site), 1988 (2 sites), 1984 (2 sites) or 1975 (3 sites). Regressions of various environmental variables with harvest histories indicated that those wetlands surrounded by smaller trees had greater light levels, water temperatures, pH, herbaceous plant cover and biomass, terrestrial invertebrate diversities and numbers, and water flea numbers, and lower water electrical conductivities and aquatic oligochaete numbers than those wetlands surrounded by more mature trees. Detected variations in hydroperiod, water depth, dissolved oxygen levels, sediment inputs, macrophyte diversity, periphyton biomass and densities of most aquatic invertebrates were not clearly correlated with past histories of peripheral tree harvest. This study suggests that harvesting trees around small wetlands initiates physical and ecological changes within the embedded habitats and that changes can persist for up to 15 years.

Upland Forest Linkages to Seasonal Wetlands: Litter Flux, Processing, and Food Quality

The flux of materials across ecosystem boundaries has significant effects on recipient systems. Because of edge effects, seasonal wetlands in upland forest are good systems to explore these linkages. The purpose of this study was to examine flux of coarse particulate organic matter as litter fall into seasonal wetlands in Minnesota, and the relationship of this flux to development of mosquitoes (Aedes aegypti). We hypothesized that litter flux into seasonal wetlands was dominated by upland plant litter that was lower quality and slower to breakdown than wetland litter, and that development rate of mosquitoes reared on upland litter was less than those reared on wetland litter. Of total litter fall into the wetlands, 71% originated in upland forest. Carbon to nitrogen ratios differed between upland litter (mostly sugar maple (Acer saccharum) and trembling aspen (Populus tremuloides) leaves) and wetland litter (mostly black ash (Fraxinus nirgra) leaves), averaging 63.9 and 47.7, respectively over two years. Breakdown rate of black ash leaves was faster than upland leaves (k (day−1) = 0.00329 and 0.00156, respectively), based on the average between wetland margins and centers. Development of mosquito larvae fed black ash leaves was faster than larvae fed upland leaves. Our results demonstrate linkages between upland forests and seasonal wetlands through litter fall. The abundance of upland litter in the wetlands may influence litter breakdown and carbon assimilation by invertebrates. Wetlands receiving high amounts of upland versus wetland litter may be lower quality habitats for invertebrates that depend on detrital pools for their development.