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.
Previous Section -- Methods
Return to Contents
Next Section -- Discussion
