a comparison of dipterans from ten created and ten natural wetlands

WETLANDS, Vol. 16, No. 4, December 1996, pp. 416--428 © 1996, The Society of Wetland Scientists

A COMPARISON OF DIPTERANS FROM TEN CREATED AND TEN NATURAL WETLANDS

William J. Streever ~, Ken M. Pottier 2, and Thomas L. Crisman 3 Department of Biological Sciences

Universitn, o f Newcastle Callaghan, New South Wales, 2308 Australia

2 Department o f Statistics University o f Florida

Gainesville, FL 32611 USA

3 Department o f Environmental Engineering Sciences Universi~" o f Florida

Gainesville, FL 32611 USA

Abstract: This study compares densities of common larval dipterans collected from areas dominated by Pontederia cordata in t0 natural and 10 created freshwater herbaceous wetlands in central Florida. At each wetland, 7 core samples were collected from each of 5 stations during summer 1993. In addition, stem densities, vegetation areal coverage, pH, dissolved oxygen, water temperature, water depth, conductivity, sediment quality, and leaf litter were measured at 3 locations near each of the 5 stations in each wetland. Of the 57 dipteran taxa collected, 20 occurred with sufficient abundance to justify statistical comparison. Despite a large sampling effort, there were no significant differences in densilies of 20 commonly occurring taxa found in created and natural wetlands after considering the effect of multiple univariate tests. Compar- ison of environmental variables showed significant differences in stem densities for vegetation other than P, cordata, pH, conductivity, and sediment quality. Canonical correspondence analysis, used to relate environmental and biological variables, suggests that pH, conductivity, and sediment quality are only weakly related to dipteran community structure. Despite differences in environmental conditions, there is no convincing evidence of differences in natural and created wetland dipteran communities.

Key Words: created wetlands, diptera, Florida, herbaceous wetlands

INTRODUCTION

One approach to a better understanding of the ability of created wetlands to replace the structure and functions of natural wetlands uses comparisons of natural and created systems. Comparative work includes both routine monitoring required by mitigation permits and more extensive studies by university and government scientists (Quammen 1986). Although many comparisons focus on plant communities (Kentula et al, 1993), a need for investigations of additional aspects of wetland ecosystems has been widely recognized (Ewel 1987, Erwin 1990. Zedter and Langis 1991, Streever and Crisman 1993a, Streever and Crisman 1993b, Zedler 1993). Monitoring programs required by government permits sometimes call for collection of water quality and invertebrate population data, while more extensive studies have examined fish (Streever and Crisman 1993a), infauna (Streever and

Crisman 1993b, Sacco et aL 1994), benthic macroin- vertebrates (Florida Department of Environmental Pro- tection 1994), nutrient cycling (Langis et al. 1991, Zedler and Langis 1991, Duncan and Groffman 1994), and wildlife habitat suitability (Zedler 1993).

Both monitoring programs and extensive studies suffer from several problems. In many monitoring programs, a small number of samples from a single created wetland and a single natural "reference" or "control" wetland are compared through qualitative as- sessment, graphical representation, and application of indices of similarity and diversity. This approach ig- nores variability among different wetlands and does not fully ccmsider variability within individual wetlands. For studies in which sampling is limited to a single created wetland or a single natural wetland, con- clusions regarding the ability of created wetlands to mimic natural wetlands suffer from pseudoreplication (Hurlbert 1984). Even when multiple created and nat-

416

Streever et al., NATURAL AND CREATED WETLAND DIPTERANS 417

ural wetlands are sampled, inadequate sampling effort within or among wetlands coupled with high variability among samples may lead to low statistical power (Peterman I990, Streever and Portier 1994).

Researchers comparing biological communities of created and natural wetlands must choose some characteristic for comparison. Biological communities may differ because of differential biotic and abiotic conditions (Hutchinson 1957) or because of poor initial recruitment of species into created wetlands (Godwin 1923, McClanahan 1983, S t reever and Cr isman 1993a), If a comparison is undertaken simply to determine the degree to which certain characteristics in created wetlands mimic natural wetlands, the cause of any differences may be irrelevant. However, when some characteristic of the created wetlands is chosen to act as an indicator of functional similarity to natural systems, differences caused by poor initial recruitment may be misleading.

Our choice of dipteran larvae as the community characteristic to be sampled was based on the belief that these insects readily colonize newly created habitats if conditions are suitable (Nursall 1952, Driver 1977, Danell and Sjoberg 1982). Differences in dipteran communities of created and natural wetlands, if they exist, may reflect functional differences instead of poor initial recruitment. Also, dipteran larvae are known to provide important forage for birds and other vertebrates (Whitman 1976, Danell and Sjoberg 1982, Maher and Carpenter 1984, Krapu and Reinecke 1992, Batzer et al. 1993) and to play a role in the development of physical and chemical properties of wetland soils (Fisher 1982).

In this study, we compare larval dipteran densities from natural and created freshwater wetlands in central Florida's phosphate mining region. Our design pro- vides 1) representative sampling within stands of P o n - teder ia corda ta in individual wetlands, 2) replication of created and natural wetlands, 3) a sampling effort capable of providing reasonable statistical power, and 4) a comparison based on an insect order that may reflect functional similarity between created and natural wetlands. Because of the large number of samples required to adequately compare natural and created wetlands at a single point in time, we did not look at interseasonal or intraseasonal differences. Stem densities, areal cover by vegetation, water temperature, water depth, pH, conductivity, dissolved oxygen, sediment quality, and leaf litter were measured to supplement comparisons of dipteran densities.

METHODS Study Area

Central Florida's phosphate mining industry (lati- tude 28°N, longitude 82°W) has been active since the

discovery of phosphate pebbles in the Peace River in 1881 (Florida Phosphate Council 1991). In addition to mining, the region supports cattle ranching, citrus farming, and vegetable farming. A cool winter dry season and a warm summer wet season characterize the climate (Chen and Gerber 1990). A mosaic of rivers, lakes, and wetlands along with upland pine, hardwood, and palmetto scrub communities characterizes pre- mining land cover. To extract phosphate, miners re- move 3 to 20 m of sand and clay overburden, leading to dramatic changes in substrate structure, water tables, drainage patterns, and biological communities. Phos- phate mining in Florida has destroyed about 5,400 ha of natural wetlands, with subsequent construction or planned construction of an equivalent area of created wetlands at over 130 sites (Florida Phosphate Council 1991, 1994).

Following completion of mining operations, areas intended for wetland creation are contoured to provide appropriate topography. Contoured lands are usually capped with surface sediments, or overburden, collected and stockpiled before mining. In addition, sites are usually "mucked" or "mulched" with substrate collected from nearby natural wetlands slated for de- struction by mining. Planting and natural seed recruitment supplement seed banks provided by mucking. Mechanical and chemical control of undesirable plm~t species and remedial planting of desired wetland plant species is sometimes necessary. Success criteria tbr individual wetland creation projects are formulated by state regulators and industry representatives and typi- cally include specilic hydrologic requirements, stipu- lations for areal cover by desirable wetland plant species, and some level of botanical similarity to a nearby reference wetland (Erwin 1990).

To compare dipteran densities in wetlands created by the phosphate industry to dipteran densities in nearby natural wetlands, 10 created and 10 nalural wetlands were sampled (Figure 1). All wetlands included in this study are classified as palustrine, emergent, per- sistent, semi-permanently flooded wetlands (Cowardin et at. 1979). Because dipteran communities differ among plant communities (Streever et aL t995), only sites with patches of the common plant species P. cordata were sampled. Created wetlands were chosen in part because of accessibility. Natural wetlands were chosen in part on the basis of proximity to created wetlands and in part on the basis of accessibility. The greatest distance between any two study sites was ap- proximately 33 kin. Created wetlands were named after the year of completion of construction, defined as the year during which the system was flooded or during which the last known major alteration to the system, such as replanting, occurred. Thus, the wetland named 1982 was created in 1982. When two created

418 WETLANDS, Volume 16, No. 4, 1996

ii

t~

1989b

II

i~b L99~ 1990b N5 N7 N2

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Figure I. Locations of 10 natural and 10 created freshwater herbaceous wetlands. Labels for created wetlands indicate the year of construction, while natural wetlands are labeled N1 through N10.

wetlands shared a date of construction, dates were arbitrarily appended with the letter a or b, as in "1989a" and "1989b." Natural wetlands were arbitrarily named N1 through NI0 , Sizes of created wetlands varied f rom about 1 to 80 ha, while sizes of natural wetlands varied from about 1 to 50 ha. Land-use surrounding both created and natural wetlands included cattle ranching, vegetable farming, roads, and conservation easements supporting hardwood forests. Created wetlands were sometimes adjacent to land in the early stages of post-mining recovery, whereas natural wetlands were sometimes adjacent to land undergoing preparation for mining.

Field Methods

At each of the 10 created and 10 natural wetlands, 5 stations were randomly chosen from areas with P. cordata. Seven samples were collected within 10 m of each station, yielding 700 samples. When patches of P. cordata were interspersed with other plants or open water, samples were intentionally collected f rom within the patches of P. cordata. A priori power analysis of pilot data (Streever and Portier 1994) and an esti- mate of the number of samples required to adequately represent individual wetlands (Streever and Bloom 1993.) dictated the number of samples. One wetland was sampled per day from 26 July to 14 August 1993, with created and natural wetlands sampled on alternate days. To collect each sample, a 5-cm-diameter core

tube was pushed 20-25 cm into the sediment. Material f rom the core tube was washed through a #30 sieve (595-~tm openings) and preserved in 80% ethanol stained with rose bengal. Although some dipterans, es- pecially early instars, may pass through #30 sieves, smaller mesh sizes clogged with sediment and pre- vented even minimal sieving of material before sort- ing. All animals were handpicked from the preserved samples. After picking, leaf litter and sediment from each sample were examined for animals that may have been overlooked. I f animals were found during the fol- low-up examination, samples were repicked.

All dipteran larvae were mounted on slides for identification to the lowest practicable taxonomic level using keys provided by Webb and Brigham (1982), Bree~ land and Loyless (1983), Cranston et al. (1983), Fitt- kau and Roback (1983), Pinder and Reiss (1983), Byers (1984), Coffman and Ferrington (1984), New- son (1984), Teskey (1984), and Epler (1992). Chiro- nomidae, Chaoboridae, and Culicidae were usually identified to genus, while identification levels for other families depended on available taxonomic information and the degree to which features described in keys matched specimens. Morphologically similar specimens not named in available keys were assigned ar- bitrary names, such as " 'Unknown chironomid 1" and "Tipulid t . " Two mosquito genera, Mansonia and Co- quitlettidia, were too similar for consistent separation at the genus level, so all specimens were identified as

Streever et al., N A T U R A L AND CREATED W E T L A N D DIPTERANS 419

Mansonia spp. Specimens that were badly damaged during processing were not identified.

At each station, stem densities and estimated percent areal cover for each plant taxon were measured at 3 randomly chosen locations within patches of P. cordata. Similarly, measurements of temperature, pH, conductivity, dissolved oxygen, water depth, sediment quality, and leaf litter were taken at 3 randomly chosen locations within patches of P. cordata around each station. Veg- etation, physical, and chemical measurements are re- ferred to as measurements of environmental variables. The decision to take 3 measurements at each station was based on time constraints. Stern densities were measured by counting the number of living stems within a 0.5 m 2 quadrat, and percent areal cover within each quadrat was visually estimated. Because of their growth habit, only percent areal cover estimates were collected for Lemna spp., Utricularia spp., and Salvinia minima. Vegetation was identified following Dressier et at. ( 1991 ). Temper- ature, pH, dissolved oxygen, and specific conductance were measured with meters. Water depth was measured with a meter stick. Sediment quality was ranked, with sand or clay sediments lacking organic material given a value of 0, sediments consisting of some organic material mixed with sand and clay given a value of 1, and sediments consisting almost entirely of organic material given a value of 2. Leaf litter was also qualitatively ranked, with 0 assigned to areas without leaf litter, 1 assigned to areas of thin and sporadic leaf litter, and 2 assigned to areas with leaf litter completely covering underlying substrate.

Statistical Methods

Analysis involved both univariate and multivariate methods. Univariate analyses included comparisons of 1) densities of commonly occurring dipteran taxa from created and natural wetlands and 2) environmental variables, including stem densities and percent areal cover for vegetation and measurements of chemical and physical variables from created and natural wetlands. Commonly occurring dipteran and plant taxa were defined as those that occurred in at least 5% of the samples, Dipteran and plant taxa occurring in few- er than 5% of the samples were pooled into a single group and ca]led "'other Diptera" and "'other plants," respectively. The decision to limit analysis to commonly occurring taxa was based on concerns regarding the ability of our sampling method to represent un- common taxa (Clifford and Stephenson 1975, Boesch 1977). The transformation y = (x) ~ + (x + 1)% rec- ommended for data containing counts of 0 (Freeman and Tukey 1959), was applied to dipteran and stem density data, and the transformation y = arcsin[(x/ 100)~], r e c o m m e n d e d for p e r c e n t a g e values (Zar

1984), was applied to vegetation areal coverage estimates. For each univariate test, the null hypothesis un- der consideration was that no difference existed between created and natural wetlands.

Assumptions of parametric analyses were assessed by plotting residual values against predicted values and residual values against the main effect, wetland type. Only four of the dipteran taxa density data sets met assumptions allowing parametric analysis. All other dipteran density data sets were pooled for each wetland, then comparisons of created and natural wetlands were performed using the nonparametric Wilcoxon rank sum test ISAS Institute Inc. 1985). All transformed vegetation data and the chemical and physical data, except qualitative assessments for sediment and leaf litter, met the assumptions for parametric analysis. Sediment and leaf litter qualitative values were averaged for individual wetlands, and comparisons of natural and created wetlands were made using the Wil- coxon rank sum test.

For data meeting the assumptions required for parametric analysis, a nested ANOVA design was used. Nested designs are appropriate when sources of variability are hierarchically related (Green 1979). A four- part hierarchy was present: 1) wetland type, either created or natural, was placed at the top of the hierarchy, 2) individual wetlands of each type were nested below wetland type, 3) stations within each wetland were nested below individual wetlands, and 4) samples from each station were nested below stations. An',dyses were performed using the Mixed Procedure (SAS Institute Inc. 1992), which is intended to analyze data with variances from both fixed and random effects. The wetland type was considered a fixed effect while individual wetlands, stations, and samples were considered random effects (Zar 1984, Bennington and Thayne 1994),

Canonical correspondence analysis, a direct gradient ordination method, was used to relate measurements of environmental variables to dipteran density (Ter Braak 1986, 1988). To limit the effect of outliers, the analysis used transformed dipteran densities, stem densities, and areal coverage estimates for vegetation. For each wetland, data from individual samples and stations were averaged, providing a single data point for each variable in each of the natural and created wetlands. In canonical correspondence analysis, ordination axes based on taxonomic composit ion are restrict- ed to linear combinations of environmental variables, thus allowing interpretation of dipteran communi ty composit ion on the basis of environmental variables.

RESULTS Diptera

Fifty-seven dipteran taxa were represented in the samples, but only 19 taxa occurred in greater than 5%

420 W E T L A N D S , V o l u m e 16, No. 4, 1996

Table 1. Dipteran taxonomic composition in 10 created and 10 natural freshwater herbaceous wetlands.

Taxon

Number of Sites with at Least

Mean Number One of Individuals Occurrence

m-2 Crea- Natu- Created Natural ted rat

Wet- Wet- Wet- Wet- lands lands lands lands

Chironomidae Larsia 2136.1 3749.9 Monopetopia 877.4 873.1 Polypedilum 774.1 1628.3 Chironomus 343.4 139,7 Glyptotendipes 240.1 10.2 Tanytarsus 164.4 90.2 Labrundinia 106.2 52.4 Natarsia 96.0 141.2 Goeldichironomus 88.8 8.7 Lauterborniella 74.2 0 Ablabesmyia 58.2 80.0 Dicrotendipes 45.1 33.5 Tanytarsini A Roback 40.7 14.6 Kiefferufus 34.9 133.9 Tanypus 32.0 5.8 Pseudochironomus 30.6 4.4 Brillia 29.1 0 Unknown chironomid 2 23.3 0 Zavretietla 17,5 30.6 Paratanytarsus 17.5 1.5 Fittkauimyia 11.6 23.3 Parachironomus 10.2 1.5 Paratendipes 8.7 29.1 Clinotanypus 7.3 0 Nimbocera 4.4 0 Pseudosmittia 2.9 0 Paramerina 2.9 4.4 Cladotanyrarsus 1.5 0 Djalmabatista 1.5 0 Guttipelopia 0 23.3 Procladius 0 5.8 Unknown chironomid 1 0 4.4 A cricotopus 0 2.9 Cfadopelma 0 1.5

Ceratapagonidae Ceratopogoninae 1655.9 2278.7 Dasyhelea 922.6 211.0 Forcipomyia 46.6 109.1 Atrichopogon 1.5 0

Tipulidae Tipulid 2 74.2 33.5 Tipulid 5 23.3 20.4 Tipulid 3 18.9 7.3 Tipulid 4 11.6 27.7

10 8 9 9

10 10 9 7 7 4 9 7 6 4 8 7 7 4 1 0 6 8 2 4 5 4 6 6 5 3 6 1 I 0 1 0 6 6 1 1 2 5 3 1 2 4 2 0 3 0 t 0 2 2 1 0 1 0 0 1 0 1 0 2 0 2 0 1

10 10 10 8 8 9 1 0

5 3 7 5 4 4 3 7

Table 1, Continued.

Taxon

Number of Sites with at Least

Mean Number One of Individuals Occurrence

m-2 Crea- Natu- Created Natural ted ral

Wet- Wet- Wet- Wet- lands lands lands lands

Tipulid l 5.8 5.8 3 2 Tipulid 6 1.5 5,8 1 2

Culicidae Urotaenia 97,5 82.9 9 6 Mansonia 80.0 10 i .9 6 7 Culex 65.5 27.7 6 5 Anopheles 0 1.5 0 1

Chaoboridae CorethreIla 21.8 42.2 3 6 Chaoborus 1.5 4.4 1 2

Stratiomyidae 53.8 27.7 7 7 Ephydridae 34.9 7.3 6 2 Tabanidae 14.6 52.4 5 9 Muscidae 2,9 4.4 2 2 Empididae 1.5 0 1 0 Psychodidae 1.5 0 I 0 Syrphidae 1.5 4.4 1 2

o f the s a mp le s (Table 1). Pol_vpedilum spp. and the sub fami ly C e r a t o p o g o n i n a e c o m p r i s e d ove r 53% o f all d ip te rans co l l ec t ed in this s tudy, and they rep resen ted the only t axa found in all 20 o f the wet lands sampled . S ix t axa were en t i re ly absent f rom the c rea ted we t l and samples , and 1 1 t axa were ent i re ly absent f rom the natural we t l and samples . I f the ef fec t o f pe r fo rming mul t ip le un iva r ia te tests is not cons idered , mean den- s i t ies o f Glyptotendipes spp. and Dasyhelea spp. were s ign i f ican t ly h igher in c rea ted we t l ands than in natural wet lands , and mean dens i t ies o f Taban idae were sig- n i f icant ly h igher in natural we t l ands (F igure 2). No s igni f icant d i f fe rences were found for o ther taxa. Ex- amina t ion o f mean va lues sugges t s ne i ther t rends toward d i f fe rences that cou ld not be de tec ted or t rends re la ted to c rea ted we t l and age.

P1 ants

A l t h o u g h all s amples were co l lec ted f rom areas d o m i n a t e d by P. cordata, other p lan t taxa were fre- quent ly encoun te red wi thin quadra t s (Table 2). Mean s tem dens i t ies and areal cove rages o f P. cordata var ied c o n s i d e r a b l y bo th wi th in and be tw e e n wet lands , but mean va lues f rom crea ted we t l ands were r emarkab ly

Streever et al., N A T U R A L AND C R EATED W E T L A N D DIPTERANS 421

similar to those from natural wetlands (Figure 3). There were no significant differences in either mean P. cordata stem densities or areal coverage estimates. Plant taxa other than P. cordata generally occurred in low mean stem densities and areal coverages, with no- table exceptions existing for Lemna spp., Satvinia spp., and Rhynchospora spp. Both Lemna spp. and Salvinia spp. are small, floating aquatic plants that frequently grow beneath emergent vegetation. The high mean stem density in the natural wetlands was caused by high counts for Rhynchospora spp., an emergent sedge, in a single wetland, N8. The growth habit of Rhynchospora spp. inflates its importance when only stem densities are considered. A statistically significant difference was present for stem densities of plants other than P. cordata but not for their areal coverages (Figure 3). If stem densities for Rhynchospora spp, are excluded from the analysis no significant difference is present. It is important to note that these results were not intended to represent the entire plant communit ies found in the wetlands. The intent of vegetation sampling was to establish the degree of similarity of the physical habitat provided for dipterans within patches of P. cordata.

Chemical and Physical Conditions

Chemical and physical data collected within patches of P. cordata show significant differences in pH, conductivity, and sediment quality between created and natural wetlands, while no significant differences are p resen t for t empera tu re , d i s so lved oxygen , wa te r depth, and leaf litter (Figure 4). Means and standard deviations of leaf litter estimates suggest that differences may be present at some level below the detection limit of the Wilcoxon rank sum test. However, this trend is driven primarily by the two youngest created wetlands, each about 1.5 years old, in which leaf litter was essentially absent. Some trends among the created wetlands may be related to age. Temperature and dissolved oxygen generally appear lower in older created wetlands, while organic content of sediments is generally higher in older created wetlands.

Canonical Correspondence Analysis

Canonical correspondence analysis was used to detect relationships between measurements of environmental variables and dipteran densities. In canonical correspondence analysis, ordination axes based on dipteran density are limited to linear combinalions of environmental data (Ter Braak 1986). The first ordination axis accounted for 20.4% of the total variance, while the second ordination axis accounted for 16% of the total variance. No additional axis accounted for more than 10% of the remain-

ing variance. Figure 5 represents these results as a biplot, in which the relative lengths and directions of the vectors for environmental variables indicate the strength of cor- relations between the environmental variables and the dipteran densities. Based on these vectors, stem densities and vegetation areal coverage, water temperature, and leaf litter have the strongest relationships to dipteran community structure.

DISCUSSION

Because density differences were found in only 3 of the 20 dipteran taxa (Glyptotendipes spp., Dasyhelea spp., and Tabanidae), the effect of performing multiple univariate tests on Type I error rates should be considered. In Type I errors, the null hypothesis of no difference between created and natural wetlands is in- correctly rejected. Adjustments to Type I error rates that account for the effect of performing 20 tests (Marks 1990) yield p-values of 0.51, 0.18, and 0.13 for Glyptotendipes spp., Dasyhelea spp., and Tabani- dae, respectively, indicating that differences between created and natural wetlands are not significant.

After accounting for the effect of multiple tests, probabilities of Type I errors for sediment quality, conductivity, and pH are 0.002, 0.03, and 0.003, respectively; thus, differences are convincing. Lower levels of organic material found in qualitative assessments of created wetland sediments may be related to the short time during which organic soils have been developing in these systems. Because decomposit ion of organic material releases CO2, lower pH values would be ex- pected in natural systems with well-developed organic soils (Wetzel 1983). A well-developed organic substrate may isolate surface water from underlying sand and rock, leading to decreased dissolution of minerals and lower conductivity, in addition, uplands surrounding created wetlands usually consist of previously mined lands with disturbed soils, so runoff entering created wetlands may be carrying large quantities of dissolved material that would increase conductivity (Kiefer 1991). If differences are related to development of organic material in created wetlands, pH and conductivity should approach values more typical of natural systems with the passage of time. Although a trend toward increased organic content with site age is apparent (Figure 4), there is no corresponding trend toward a decrease in pH and conductivity. The re- sponse of pH and conductivity may be delayed or site- specific attributes may override age-related development of organic soils.

Additional age-related trends in chemical and physical parameters include apparent decreases in temperature and dissolved oxygen with age of created wetlands (Figure 4). As patches of P. cordata in older

422 W E T L A N D S , Vo lume 16, No. 4, 1996

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Streever e t al., NATURAL AND CREATED W E T L A N D DIPTERANS 423

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424 W E T L A N D S , Volume 16, No. 4, 1996

Table 2. Vegetation in 10 created and 10 natural freshwater wetlands in central Florida.

Mean Stems/m : (% areal coverage)

Number of Sites with at Least One Occurrence

Created Natural Created Natural Taxon Wetland Wetland Wetland Wetland

Pontederia cordata Linnaeus Eichornia crassipes (Mart,) Solms Limnobium spongia (Bosc.) Steud. Polygonum spp. Baccharis halim!folia Linnaeus Mikania scandens (Linnaeus) Willdenow Panicum spp. Sagittaria lancifotia Linnaeus Ludwigia peruviana (Linnaeus) Hara Cyperus odoratus Linnaeus Rhynchospora spp. Cephatanthus occidentalis Linnaeus Hydroco~'le spp. Lemna spp. Salvinia minima Baker Utricutaria spp.

157.0 (54%) 164.1 (55%) 10 10 0,0 (0%) 3.0 (<1%) 0 I 2.5 (<1%) 0.3 (<1%) 1 3 2.3 (2%) 1.8 (< 1%) 5 4 2.1) (<1%) 0.9 (<1%) 1 I 0.5 (<1%) 0.3 (<1%) 3 2 1.6 (<1%) 0.2 (<1%) 2 I 1.2 (<1%) 0.3 (<1%I 4 2 ll.8 (<1%) 0.5 (<1%) 1 2 0.5 (<1%) 0.0 (0%} 1 0 0.0 (0%) 37.4 (2%} 0 3 0.0 (0%) 0.2 ( < i % ) 0 3 II.0 (0%) 1.0 (<1%) 0 4

NA* (20%) NA (10%) 9 5 NA (5%) NA (13%) 3 4 NA (2%) NA (< 1%) 3 2

* NA: Not applicable; stem densities were not assessed for these taxa.

created wet lands enlarge through vegetat ive reproduc- tion, areas of open water grow smaller, potent ia l ly de- creasing direct insolat ion. Because direct insolat ion may increase water temperatures, shading by patches of P. cordata may explain lower water temperatures observed in the older created wetlands. The observed

trend toward higher levels of dissolved oxygen in the younger created wet lands may also be related to the areal extent of open water exist ing be tween patches of P. cordata, which could support greater phytoplankton product ivi ty and subsequent high levels of d issolved oxygen dur ing dayl ight hours.

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Figure 3. Vegetation stem densities and cover from l0 natural and 10 created freshwater herbaceous wetlands, Error bars represent back transtormed standard deviations, P-values are indicated for taxa showing a significant (p < 0.05) difference between created and natural wetlands. Reported p-values were computed before adjustment for the effect of performing multiple tests (see text for details).

Streever et al., N A T U R A L A N D C R E A T E D W E T L A N D D I P T E R A N S 425

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Figure 4. Measurements of chemical and physical variables from 10 natural and 10 created freshwater herbaceous wetlands. Sediment quality was ranked, with sand or clay sediments lacking organic material given a value of 0, sediments consisting of some organic material mixed with sand and clay given a value of 1, and sediments consisting almost entirely of organic material given a value of 2. Leaf litter was also qualitatively ranked, with 0 assigned to areas without leaf litter, 1 assigned to areas of thin and sporadic leaf litter, and 2 assigned to areas with leaf litter completely covering underlying substrate. P-values axe indicated for variables showing a significant (p < 0.05) difference between created and natural wetlands. Reported p-values were computed before adjustment for the effect of performing multiple tests (see text for details).

The absence o f a trend toward increased litter accumulation corresponding to increased wetland age is surprising. One possible explanation is that leaf litter accumulation rapidly stabilizes fo l lowing one or two seasons o f growth, after which site-specific conditions determine the degree o f litter accumulation. This would explain the low levels o f litter found in 1992a

and 1992b and the absence o f pattern among the eight older created wetlands. Possible site-specific conditions that could control litter accumulation include productivity levels, decompos i t ion rates, extent o f her- bivory, and occurrence o f fire.

Because biotic communit ies are generally thought to be structured to some extent by their abiotic envi-

426 WETLANDS, Volume 16, No. 4, 1996

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Figure 5. Canonical correspondence analysis biplot, in which ordination axes based on dipteran densities are limited to linear combinations of environmental data. The strength of the relationship between environmental variables and the ordination based on dipteran densities is indicated by the relative length of environmental vectors.

ronment, the absence of differences between dipterans found in created and natural wetlands requires some explanation. Canonical correspondence analysis was used to relate patterns based on dipteran densities to measured environrnental variables (Figure 5). The low variance explained by the first two axes of the canonical correspondence analysis (20.4% and 16.0%, respectively) suggests that the environmental variables measured in this study played only a limited role in structuring dipteran communities. Microhabitat differences within individual wetlands and stochastic events may play more of a role than measured environmental differences in structuring dipteran communities. Fur- thermore, the environmental variables most strongly related to taxonomic composit ion are generally those that were not significantly different for the natural and created wetlands. Relatively long vectors in Figure 5 represent leaf litter, P. cordata stem counts, P. cordata percent cover, percent cover by vegetation other than P. cordata, and temperature, none of which were significantly different in natural and created wetlands. O f the four environmental variables that were significant-

ly different in natural and created wetlands, only the stem counts for vegetation other than P. cordata ("other s tems" in Figure 5) seem to have a reasonably strong relationship to dipteran densities.

Because conditions such as pH, DO, conductivity. and temperature within an individual wetland can vary dramatically over a short time, dipterans inhabiting these habitats must have wide tolerance limits to these conditions. Reported environmental tolerances for var- ious dipteran taxa supporl this view (Fittkau and Ro- back 1983, Pinder and Reiss 1983, Epler 1992). Be- cause congeneric species may have different tolerances for environmental conditions (Hushes 1966, Scudder 1983, Waterhouse and Farrell 1985, Hall and Ice 1987), species-level identification may have detected differences in dipteran densities between created and natural wetlands. However, consistently reliable species-level identification of large numbers of dipteran larvae is problematic, and some species are entirely undescribed as larvae (Epler 1992). Furthermore, tolerance ranges of individuals and populations within a

Streever et al., N A T U R A L A N D C R EATED W E T L A N D DIPTERANS 427

single species may vary, just as tolerances vary within a genus,

No convincing differences in dipteran densities were found within stands of P. cordata in created and natural freshwater herbaceous wetlands in central Flori- da's phosphate mining district. Consistent differences in dipteran communi ty structure, had they occurred, may have indicated shortcomings in the ability of created wetlands to mimic natural wetlands. However, absence of consistent differences does not indicate com- plete functional similarity, Clear differences exist in chemical and physical conditions of created and natural wetlands, even within similar plant communities. These results suggest that created wetlands may mimic some of the structure and function of natural wetlands despite obvious differences in physical and chemical conditions. Wetland biota that are tolerant of a wide range of conditions may live in created wetlands that are not duplicates of natural systems. In a comparison of six created and six natural North Carolina salt marshes, Sacco et al. (1994) found clear differences in soil organic content but only limited differences in in- faunal communities. Like dipterans and infauna, many higher taxa associated with wetlands may express a wide niche breadth. For example, river otters appear to use many types of wetlands without preference for any one type (Newman and Griffin I994), and least terns may abandon established nesting sites in natural wetlands to colonize recently constructed sewage la- goons (Alleng and Whyte-Alleng 1993).

We recommend that researchers continue to compare wetlands to identify structural and functional disparities between created and natural wetlands. Future research should assess partial functional equivalence, allowing for high similarity between some characteristics, yet identifying clear differences for others. In addition, studies should be designed to compare emergent properties, such as ecosystem resilience and resistance. Further comparisons of dipteran communities in the freshwater herbaceous wetlands of central Florida's phosphate mining district may be of limited usefulness unless specific rea- sons exist for suspecting differences.

A C K N O W L E D G M E N T S

This paper comprises part of W. J. Streever's Ph.D. dissertation work at the University of Florida's De- partment of Environmental Engineering Sciences. We thank Dr. G. Ronnie Best for the many hours of discussion and guidance that he provided throughout this study. Phosphate mining c o m p a n y represen ta t ives John Kiefer, Selwyn Presnell, Doreen Donavan, and Rosemarie Garcia provided site tours and facilitated access to private property. Christine Keenan and An- drea Leslie provided assistance with field work. Fi-

nancial support for this work came from a National Science Foundation Graduate Research Fellowship to W. J. Streever and from a grant awarded by the Florida Institute for Phosphate Research. Mark Clark and two anonymous reviewers provided valuable comments regarding an early draft of this paper.

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Manuscript received 15 August 1995~ revision received t2 February 1996; accepted 19 April 1996.

a comparison of dipterans from ten created and ten natural wetlands

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