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A Surface-Associated Activity Trap for Capturing Water-Surface and Aquatic Invertebrates in Wetlands

Results


Non-detection Rates

Non-detection rates of SATs varied markedly among taxa, ranging from 0% for total insects (all taxa combined) to 86.0% for Ephemeroptera during sampling period 2 (Table 1). Non-detection rates (AT-SAT) were always positive and >0, indicating that detection by SATs was always greater than that of matched ATs, regardless of taxon. We observed largest detection differences with Gastropoda and Chironomidae. Whenever our generalized mixed-models tests indicated significant differences among sampling periods or wetland type, we tabulated separate estimates for each level of the significant effect. Otherwise, we computed a pooled estimate over all levels of the effect variable (Table 1). Our mixed model results indicated that non-detection rates (AT-SAT) for Ephemeroptera approximately doubled by our third sampling period (P=0.0014). No other temporal interactions with device effects were evident (Table 1). Non-detection rates (AT-SAT) were greater for cladocera (P=0.0001) in wetlands with fathead minnows but higher for Culicidae (P=0.0031) and Notonectidae (P=0.0017) in fishless sites (Table 1).

Table 1. Comparison of conventional (AT) vs. surface-associated (SAT) activity trap non-detection rates (%) for 13 invertebrate taxa by wetland type and sampling period. AT-SAT differences in non-detection rates indicate the percentage of times ATs failed to detect invertebrates that were detected by paired SATs. Data are pooled across sampling period and wetland type except when our models indicated that these interacted with non-detection rates. McNemar's tests evaluate H0: AT-SAT paired rate difference = 0%.
Taxon Sampling Period Wetland Type Total
Matched
Pairs
Non-detection
Rate of SATs
AT-SAT
Non-detection
Rate
McNemar's
Chi sq.
P-Value
Cladocera All Fish 358 17.6 (13.7, 21.5) 24.9 (19.7, 30.0) 71.36 <0.00001
All No Fish 355 3.7 (1.7, 5.6) 9.9 (6.5, 13.2) 29.88 <0.00001
Chironomidae All All 713 14.0 (11.5, 16.6) 43.2 (39.3, 47.1) 282.33 <0.00001
Corixidae All All 713 17.8 (15.0, 20.6) 20.2 (16.4, 24.0) 93.41 <0.00001
Culicidae All Fish 358 75.4 (71.0, 79.9) 16.2 (11.5, 20.9) 40.05 <0.00001
All No Fish 355 49.0 (43.8, 54.2) 27.6 (21.8, 33.4) 70.62 <0.00001
Dytiscidae All All 713 42.8 (39.1, 46.4) 5.3 (1.2, 9.5) 6.28 0.01222
Ephemeroptera 1 All 240 75.8 (70.4, 81.2) 11.3 (5.6, 16.9) 14.29 0.00016
2 All 235 86.0 (81.5, 90.4) 10.6 (5.9, 15.4) 17.86 0.00002
3 All 238 66.0 (59.9, 72.0) 20.6 (14.9, 26.2) 42.12 <0.00001
Gastropods All All 713 12.3 (9.9, 14.8) 37.2 (33.5, 40.8) 257.23 <0.00001
Haliplidae All All 713 79.0 (76.0, 82.0) 7.9 (4.3, 11.4) 18.23 0.00002
Hydrophilidae All All 713 65.9 (62.4, 69.4) 23.6 (19.7, 27.4) 119.59 <0.00001
Notonectidae All Fish 358 77.9 (73.6, 82.2) 11.7 (6.8, 16.7) 20.51 <0.00001
All No Fish 355 53.2 (48.0, 58.4) 25.6(19.8, 31.5) 61.34 <0.00001
Odonata All All 713 65.8 (62.3, 69.3) 22.0 (18.5, 25.6) 121.42 <0.00001
Total Insects All All 713 0.1 (0.0, 0.4) 8.4 (6.3, 10.5) 58.06 <0.00001
Total Crustacea All All 713 3.6 (2.3, 5.0) 14.2 (11.4, 16.9) 90.27 <0.00001

Invertebrate Relative Abundance

SATs also offered considerable improvement over conventional ATs in terms of relative numbers of invertebrates captured. Magnitudes of median-paired SAT-AT differences (Table 2) reflect both local invertebrate abundance and differential trap efficiency. However, improvement ratios (Table 2; obtained by back-transformation of mixed-model estimates of paired log-differences) demonstrate relative efficiency of our SATs vs. ATs after adjustment for blocking factors and independent of actual local abundance. Nearly all ratios were >1.0 (in 2 cases for dytiscids, lower limits of confidence intervals included 1.0), indicating that SATs captured more invertebrates than ATs, at least under our matched conditions. As above, whenever our tests indicated significant differences among sampling periods or wetland type, we tabulated separate estimates for each level of the significant effect. Otherwise, we computed a pooled estimate over all levels of the effect variable (Table 2). SATs were most efficient at capturing cladocera, Chironomidae, Gastropoda, total Crustacea (taxa combined), and multiple taxa (taxon richness). On the other hand, SATs were better than ATs at capturing Dytiscidae only during sampling period 1 (Table 2). Significant temporal differences in improvement ratios were observed only for cladocera (P=0.0016), Chironomidae (P<0.0001), Dytiscidae (P<0.0001), and total Crustacea (P<0.0001). Improvement ratios of SATs for all these taxa were greater during sampling period 1. Improvement ratios differed consistently in relation to fish presence/absence only for cladocera, whose ratios were larger in fishless wetlands (Table 2).

Table 2. Relative abundances of 13 invertebrate taxa and taxon richness trapped by surface-associated (SATs) vs. conventional (ATs) activity traps. SAT/AT improvement ratios indicate how many times greater the abundance of organisms trapped by SATs were relative to paired ATs. Data are pooled across sampling period and wetland type except when our models indicated that these interacted with improvement ratios. Paired t-tests evaluate H0: SAT/AT = 1.0.
Taxon Sampling
Period
Wetland
Type
No.
Matched
Pairs
Median Diff.
(± 95% C.L.)
Improvement
Ratio
(± 95% C.L.)
Paired
t-test
P-Value
Cladocera 1 Fish 107 8.5 (6.0, 12.5) 3.1 (2.4, 4.0) 8.95 <0.00001
1 No Fish 120 112.3 (79.0, 180.0) 6.9 (5.4, 8.7) 15.87 <0.00001
2 Fish 99 6.5 (4.0, 11.5) 3.0 (2.3, 3.9) 8.39 <0.00001
2 No Fish 109 35.0 (21.5, 50.5) 3.3 (2.6, 4.3) 9.56 <0.00001
3 Fish 89 8.5 (5.0, 16.0) 3.0 (2.3, 3.9) 8.09 <0.00001
3 No Fish 113 91.5 (67.5, 120.5) 4.5 (3.5, 5.8) 12.14 <0.00001
Chironomidae 1 All 229 8.5 (7.0, 10.0) 5.0 (4.4, 5.8) 23.20 <0.00001
2 All 203 4.5 (4.0, 5.5) 3.5 (3.0, 4.0) 17.02 <0.00001
3 All 181 2.5 (2.0, 3.0) 2.6 (2.2, 3.0) 12.42 <0.00001
Corixidae All All 586 3.5 (3.0, 4.5) 2.2 (2.0, 2.4) 19.91 <0.00001
Culicidae All All 269 2.5 (2.0, 3.5) 2.7 (2.4, 3.0) 17.13 <0.00001
Dytiscidae 1 All 180 1.5 (1.5, 2.5) 1.7 (1.5, 2.0) 7.83 <0.00001
2 All 112 0.5 (-0.5, 1.0) 1.2 (1.0, 1.4) 2.04 0.04194
3 All 116 0.5 (-0.5, 1.0) 1.2 (1.0, 1.4) 2.32 0.02101
Ephemeroptera All All 172 2.5 (2.0, 3.5) 3.2 (2.8, 3.7) 15.88 <0.00001
Gastropoda All All 625 16.0 (14.0, 18.0) 5.5 (4.8, 6.3) 25.44 <0.00001
Haliplidae All All 150 1.0 (1.0, 1.0) 2.0 (1.8, 2.2) 14.26 <0.00001
Hydrophylidae All All 243 1.5 (1.0, 1.5) 2.3 (2.1, 2.4) 21.50 <0.00001
Notonectidae All All 245 2.0 (1.5, 2.0) 2.6 (2.3, 2.9) 19.11 <0.00001
Odonata All All 244 2.0 (2.0, 2.5) 2.7 (2.4, 2.9) 22.67 <0.00001
Taxon Richness All All 719 17.0 (16.5, 17.5) 3.6 (3.5, 3.7) 67.39 <0.00001
Total Crustacea 1 All 238 108.0 (90.0, 128.0) 6.5 (5.5, 7.6) 22.77 <0.00001
2 All 229 42.0 (32.5, 54.0) 4.0 (3.4, 4.7) 16.62 <0.00001
3 All 220 88.5 (67.5, 109.0) 4.5 (3.8, 5.3) 17.75 <0.00001
Total Insects All All 712 12.0 (10.5, 13.5) 2.8 (2.6, 3.1) 21.21 <0.00001

Vertical Position within SATs

Only rarely did we observe non-detection differences among SAT strata, but occasionally Notonectidae, Dytiscidae, and Hydrophilidae were more frequently detected in uppermost strata (upper 10 cm including water surface; all P≤0.01). Improvement ratios (based on paired log-relative abundance counts) differed only rarely and only for cladocera; more cladocera were captured in the bottom strata (10.16 - 15.24 cm beneath water surface). Given that vertical differences in non-detection rates or log-relative abundance were infrequent, we combined catches from all 3 layers for each SAT, treating their contents as a single measure of trap performance in all analyses.


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