We studied butterfly assemblages at six types of riparian landscapes in five different watersheds in the southwestern United States (
Exotic plants can impact ecosystems through changes in fire regimes and hydrological cycles, hybridization with native species [
Tamarisks (
Recognized data gaps for effects of tamarisk on wildlife include comparisons of tamarisk-invaded habitats and tamarisk removal sites with native riparian vegetation sites [
Butterflies are a wildlife group that may be useful in the study of tamarisk environments. Butterflies are important to terrestrial ecosystem processes such as pollination and also are phytophagous insects that play a role in transfer of plant energy to higher trophic levels [
We used native vegetation reference sites to evaluate the effectiveness of management activities on butterfly assemblages at five different watersheds in the southwestern United States. Our research focused on how native riparian vegetation butterfly assemblages compared with those associated with tamarisk, tamarisk control, and restoration sites. Local butterfly species were linked regionally by identifying species sensitive to anthropogenic disturbance and then identifying a subgroup of these that were reported from all watersheds. This allowed for comparison of all sites and made regional comparisons using the same assessment endpoint possible. We also examined environmental variables and their relationship with butterfly metrics.
We hypothesized that native vegetation sites would have highest butterfly metric values and have similarities with mixed vegetation and revegetation sites. We expected tamarisk sites and sites where tamarisk was controlled, but without revegetation, to have relatively low butterfly metric values.
We selected sites in 5 different lowland watersheds including the Arkansas River in Colorado, Canadian River in Texas, Las Vegas Wash in Nevada, and Pecos and Rio Grande Rivers in New Mexico. Sites and landscape categories (Table
Information associated with butterfly sampling sites. Most scientific names for plant taxa are identified in the text. Others are in footnotes below the table. Sites in CO and NM were sampled in 2005 while those in NV and TX were sampled in 2006.
River and counties where sites were located | Site code (initial letters correspond to the type of site)a | State | Easting/northing | Site characteristics | Flooding regime |
---|---|---|---|---|---|
Arkansas |
NV-ARK-1 | CO | 13N0509892 |
Native woody vegetation. | Evidence of riparian processes and overbank flow. |
NV-ARK-2 | CO | 13N0511502 |
Native woody vegetation. Understory of reed canary grassb with thistlesc coming in. | Evidence of riparian processes and overbank flow. | |
NV-ARK-3 | CO | 13N0535655 |
Native woody vegetation. | Evidence of riparian processes and overbank flow. | |
MIX-ARK-1 | CO | 13N0506741 |
Tamarisk very common, native woody vegetation mostly along rivers edge. | Fluctuations in hydroperiod, overbank flow during moderate flow events. | |
MIX-ARK-2 | CO | 13N0525439 |
Mostly tamarisk with a few large senescent cottonwood. Some willow and small cottonwood in site. Knapweed common. | Fluctuations in hydroperiod, overbank flow during moderate flow events. Most of the flood-prone area confined by barriers. | |
TAM-ARK-1 | CO | 13N0522352 |
A relatively narrow strip of tamarisk. A few small stature cottonwoods are present. | Evidence of riparian processes and overbank flow. | |
TRT-ARK-1 | CO | 13N0507833 |
Root plowed to remove tamarisk. Weedy herbaceous plants common and tamarisk beginning to cover site again | Site sustained by natural water source, flood-prone area confined by barriers. | |
TRT-ARK-2 | CO | 13N0525188 |
Tamarisk biocontrol agent |
No evidence of overbank flows, flood-prone area confined by barriers. | |
| |||||
Blue (Canadian watershed) | NV-CAN-1 | TX | 14S0258651 |
Native woody vegetation. | Evidence of riparian processes and overbank flow. |
| |||||
Canadian |
NV-CAN-2 | TX | 14S0249539 |
Native woody vegetation. Park like. | Evidence of riparian processes and overbank flow. |
MIX-CAN-1 | TX | 14S0244314 |
Mostly tamarisk of small stature. |
Site sustained by natural water source, flood-prone area confined by barriers. | |
TAM-CAN-1 | TX | 14S0253823 |
Dense mature tamarisk, understory is mostly kochia. | Site subject to fluctuations in flow, flood-prone area is confined. | |
TRT-CAN-1 | TX | 14S0249678 |
Burned. Tamarisk is coming back vigorously. Undergrowth dominated by common reed and coyote willowd. | Site subject to fluctuations in flow and overbank flow. | |
TRT-CAN-2 | TX | 14S0255947 |
Burned and root pulled in 2003. | No evidence of riparian processes, no opportunity for overbank flows except in extreme events. | |
| |||||
Las Vegas Wash |
MIX-LVW-1 | NV | 11S0677388 |
Tamarisk and some weedy vegetation, few cottonwoods and mesquite. Tamarisk seedlings are common. | Site contains evidence of riparian processes, overbank flow during moderate flow events. |
RER-LVW-1 | NV | 11S0681529 |
Native woody revegetation. Alkali heliotrope is common but spotty. Control structure is present. | Evidence of riparian processes and overbank flow. | |
RER-LVW-2 | NV | 11S0682033 |
Native woody revegetation. Common reed in site. Alkali heliotrope is common. Control structure is present. | Evidence of riparian processes and overbank flow. | |
RER-LVW-3 | NV | 11S0682452 |
Control structure is present, native woody vegetation (willows) moving into site. Common reed abundant. | Evidence of riparian processes and overbank flow. | |
REU-LVW-1 | NV | 11S0677707 |
Upland revegetation (quailbrushe interspersed with mesquite and |
No evidence of riparian processes, cut banks prevent overbank flow. | |
REU-LVW-2 | NV | 11S0683368 |
Upland revegetation (mesquite and saltbush) with narrow band of riparian obligates next to Wash. | No evidence of riparian processes, cut banks prevent overbank flow except during moderate flow events. | |
TAM-LVW-1 | NV | 11S0678114 |
Tamarisk mostly along Wash. | No evidence of riparian processes, channel incised so little opportunity of overbank flows. | |
| |||||
Pecos |
TAM-PEC-1 | NM | 13S0564386 |
Tamarisk with understory of kochia. | No evident source of water supply, no opportunity for overbank flows. |
TRT-PEC-1 | NM | 13S0564086 |
Tamarisk sprayed with herbicide (Arsenal) in 2003. Kochia sparse. | No evidence of riparian processes, no opportunity for overbank flooding. | |
TRT-PEC-1 | NM | 13S0561714 |
Mowed tamarisk. Near monoculture of kochia. | No evidence of riparian processes because of cut bank, overbank flow only in extreme events. | |
| |||||
Rio Grande |
NV-RIO-1 | NM | 13S035471 |
Native woody vegetation. Mostly small diameter Goodings willowf with understory of tamarisk and |
Evidence of riparian processes and overbank flow. |
NV-RIO-2 | NM | 13S0307597 |
Native woody vegetation. | Site subject to fluctuations in flow, overbank flooding in moderate flow events. | |
MIX-RIO-1 | NM | 13S1341506 |
Native woody vegetation, understory of tamarisk | Evidence of riparian processes and overbank flow. | |
RER-RIO-1 | NM | 13S0341184 |
Revegetation site with some natural vegetation, topography altered for silvery minnow habitat. | Site subject to fluctuations in flow, overbank flow in moderate events. | |
REU-RIO-1 | NM | 13S0313962 |
Upland revegetation site. Planted with saltbushg and quailbrush. Kochia common. | No evidence of riparian processes, no opportunity for overbank flooding. | |
TAM-RIO-1 | NM | 13S0330010 |
Tamarisk with |
No evidence of riparian processes, moderate opportunity for overbank flows. | |
TAM-RIO-2 | NM | 13S0314012 |
Upland tamarisk, decadent cottonwood. | No evidence of riparian processes, no opportunity for overbank flows except in extreme events. | |
TRT-RIO-1 | NM | 13S0342054 |
Tamarisk understory cleared below native woody vegetation. | Evidence of riparian processes and overbank flow. | |
TRT-RIO-2 | NM | 13S0330246 |
Tamarisk cleared. Kochia is common but then replowed later in season | Natural water source but no evidence of riparian processes, potential for overbank flooding during moderate flow events. | |
TRT-RIO-3 | NM | 13S0321945 |
Tamarisk cleared. | No evidence of riparian processes, no opportunity for overbank flooding. |
Landscape categories included native woody vegetation (native vegetation) (
Species richness and abundance data were totaled from three surveys within a given year and summarized for each site. Three subsamples corresponding to different species flight periods (March/May, June/July, and August/September) were collected and used to describe the annual butterfly community. Data was collected from sites in either 2005 or 2006. Two different days were spent sampling butterflies and other variables at each site for 1.5 hours per day per person during each flight period. The three flight periods totaled to a sampling period of 9 hours per site. We counted individual butterflies during timed searches to provide data on both species presence and an index of abundance, with species richness and abundance summed across the year. Most butterflies were identified immediately by sight. Sweep nets were used for verification or identification of suspect species. We confined sampling to 2-ha sites that were resampled during each successive visit. Integrating data required similar sampling effort at sites because differences in effort affect many biological metrics [
Similar to the “checklist” methodology of Royer and others [
During butterfly surveys, we estimated the number of open flowers or inflorescences used as nectar sources by butterflies. Scott [
Although not a direct measure of nectar, Holl [
We used a riparian systems model (modified from [
We measured soil moisture (% saturation relative to field capacity; Kelway soil moisture tester Model HB-2) at three locations through the middle portion of the site. Moist soils and seeps have been recognized as being important to butterflies [
Because species pools differ across watershed boundaries (e.g., [
We assigned each species a value from 1 to 4 in each of three categories contributing to disturbance susceptibility (e.g., [
We used ANOVA to compare the abundance of H-DSS butterflies that were in common with all watersheds (hereafter known as the regional butterfly metric (RBM)); and environmental variables and riparian condition scores among landscape categories. Butterfly species richness in the various watershed counties (from the website [
Dunnett’s comparison was used to compare variables from all other sites to the native vegetation sites if the ANOVA indicated a significant difference (
Because butterfly observations were collected in two years, data might be from different statistical distributions, perhaps because of interannual weather differences. We used a data set from the Arkansas River that contained two each of native vegetation, mixed, and treatment sites from 2002 to 2006 to test the robustness of RBM to annual variability. Variation in weather occurred in this watershed, with soil moisture differing significantly between years (e.g., [
Although there were several sites in each of the categories (see Table
Butterfly surveys occurred at wind speeds that were less than or equal to a light breeze (≤11.3 km/hr) on the Beaufort wind force scale, except for a single wind speed reading of 12.4 km/hr (at a Canadian River treatment site in May) (
Testing of multiyear data from sites along the Arkansas River suggested that RBM was robust to annual variability (
No significant differences in soil moisture were detected between landscape categories and the native vegetation reference sites (Table
Environmental variables and butterfly species richness associated with landscape categories.
Landscape category | Environmental variables and species richness (mean of annual values, range in parentheses) | ||||
---|---|---|---|---|---|
Soil moisture (%) | Riparian condition | Forb and graminoid richness (number/site) | Nectar (# florets/m2) | Butterfly species richness | |
NV ( |
61a (14–98) | 6.8a (5.6–7.4) | 9a (1–14) | 43.7a (7.5–97.6) | 19a (13–26) |
MIX ( |
49a (16–97) | 5.1b (4.6–6.5) | 10a (7–13) | 46.8a (28.1–77.3) | 18a (14–22) |
RER ( |
49a (15–94) | 5.9a (5.3–6.6) | 7a (3–10) | 37.5a (21.0–57.3) | 15a (10–21) |
REU ( |
37a (1–88) | 3.7b (3.0–4.4) | 2b (0–3) | 2.8b (1.0–3.8) | 13a (4–17) |
TAM ( |
53a (1–99) | 3.8b (2.0–5.2) | 5a (2–14) | 33.4a (1.3–69.1) | 14a (4–24) |
TRT ( |
51a (16–97) | 3.6b (2.0–5.9) | 6a (2–10) | 22.7a (0.0–42.9) | 15a (7–23) |
Column values with dissimilar letters indicate significant (
Because riparian condition scores differed significantly with landscape categories, we examined separate components of the model in more detail. Characteristics that measured native woody plant coverage and spatial diversity, structural diversity, and biogeochemical processes differed between native vegetation reference sites and all other site types (Table
Riparian condition scores associated with landscape categories.
Landscape category | Riparian condition scores for each component | |||||||
---|---|---|---|---|---|---|---|---|
Coverage and spatial diversity | Structural diversity | Contiguity of habitats | % of invasive vegetation | Hydrology | Micro- and macrotopographic complexity | Flood-prone area characteristics | Biogeochemical processes | |
NV | .89a | .77a | .94a | .67a | .95a | .81a | .86a | .93a |
MIX | .62b | .57b |
|
.44b | .72b | .51b |
|
.71b |
RER | .68b | .60b |
|
|
|
|
|
.72b |
REU | .40b | .47b | .73b | .44b | .53b | .28b | .37b | .44b |
TAM | .39b | .40b | .77b | .28b | .54b | .32b | .50b | .59b |
TRT | .37b | .34b | .82b | .35b | .55b | .30b | .44b | .46b |
Column values with dissimilar letters indicate significant (
Butterfly susceptibility to anthropogenic disturbance also appeared to be differentially related to riparian condition scores, with abundance of higher scoring butterflies significantly correlated with riparian condition while abundance of butterflies with the lowest scores were not correlated with riparian condition (Figure
Comparison of the relationship of riparian condition scores with the abundance of butterflies in four different categories (high (H), moderately high (MH), moderately low (ML), and low (L)) of sensitivity to disturbance (disturbance susceptibility Scores (DSS)). Categories of butterflies varied in their correlation with riparian scores with more sensitive butterflies more highly correlated ((H-DSS;
H-DSS species that were reported from all five watersheds [
Comparison (means and standard error) of regionally in-common sensitive butterfly species abundance by landscape category. Values differed significantly between NV sites and other sites, with the exception of REU sites. NV: native vegetation, MIX: mixed, RER: revegetated with riparian vegetation, REU: revegetated with upland vegetation, TAM: tamarisk dominated, and TRT: treated to remove tamarisk.
RBM was significantly correlated with riparian condition score (
At the regional scale, butterfly communities (RBM) differed significantly between landscape categories. Mean RBM values differed between native vegetation sites and other sites, with the exception of revegetated-upland sites. Similarity between native vegetation and revegetated-upland sites was due to the presence of a sensitive butterfly species found in the upland environment.
Diversity and extent of riparian vegetation along with presence of cottonwood trees, saplings, and seedlings were higher at native vegetation sites and differentiated these sites from all others. Only mixed and revegetated-riparian sites had some aspects of riparian condition scores that were statistically indistinguishable from native vegetation values. This level of riparian similarity did not lead to correspondence in RBM scores between mixed, revegetated-riparian, and native vegetation sites. The absence of a source of riparian obligate butterflies may explain this result at Las Vegas Wash revegetated-riparian sites. Restoration of landscapes after removal of tamarisk resulted in high-quality butterfly habitat along the Las Vegas Wash where erosion control structures have decreased the depth to groundwater and resulted in intermittent flooding of the landscape (e.g., [
Sites with a mixture of native and exotic vegetation did not perform well as habitat for sensitive butterfly species. This occurred despite similarities in nectar and forb and graminoid richness with native vegetation sites. Native riparian vegetation diversity and coverage were lower at mixed sites compared to native vegetation sites and may have played a role in lower RBM scores at mixed sites.
Sites where tamarisk was treated, but without restoration, resulted in RBM scores similar to those found at untreated tamarisk sites. The assumption that invasive plant species removal is sufficient to recover sensitive butterfly species was not true in the cases that we examined. Without restoration these sites may take a long time to recover. Nelson and Wydoski [
Our regional analysis results support findings of studies from single watersheds. Local level studies also found no recovery of butterfly communities at treated tamarisk sites [
Conceptual approaches to riparian restoration projects often differ. Many tamarisk control efforts are based on the belief that tamarisk removal will allow for recovery of native vegetation and that revegetation or modification of river processes is not needed (e.g., [
Miller and Hobbs [
The authors thank Susan Broderick for having the foresight to implement this project and Curt Brown for much needed support. Two anonymous reviewers and the associate editor provided valuable direction in the improvement of the paper. This project was funded through the Bureau of Reclamation S&T program.