Effects of cattail management on invertebrate production and migratory bird use of Cheyenne Bottoms, KS

Dense monotypic cattail (Tvpha spp.) stands are a management problem in many prairie wetlands as they exclude desirable plants and migratory wetland birds. Cheyenne Bottoms Wildlife Area (CBWA), a Wetland of International Importance located in central Kansas, has experienced a large increase in cattails and a subsequent decrease in avian use. As a consequence, intensive cattail management is practiced at CBWA. However, dense emergent vegetation also influences production of Invertebrates, an important food source for migratory wetland birds. Effectiveness of different techniques in reducing cattail at CBWA was assessed from fall 1999 to spring 2001. Cattail management also was evaluated based on its impacts on invertebrate production and migratory wetland bird use and behavior. In addition, aquatic invertebrate succession (i.e., temporal patterns in density, biomass, familial richness, familial diversity, and trophic structure) and migratory wetland bird community structure (i.e. nestedness and species co-occurrence patterns) also were assessed.

Treatments to reduce cattail (i.e., burned, control, disked, and grazed) were applied at CBWA during 1999 and 2000. Disking and high-intensity grazing (20 head/11.05 ha) resulted in the lowest cattail densities and biomass. Reduction of cattail within these treatments lasted for at least 1 year. In the short term, cattail management by disking and high-intensity grazing was at the expense of higher plant species richness, plant species diversity, and non-cattail productivity. Within 1 year, however, plant species richness, plant species diversity, and non-cattail productivity had recovered (i.e., increased) in the disked and high intensity-grazed treatments.

The more heavily vegetated burned and control treatments had greatest invertebrate production. However, maintaining large stands of such vegetative cover to benefit invertebrates is counter to migratory bird management goals at CBWA. While invertebrate food resources may be greater in areas of greater vegetative cover, such areas exclude most wetland birds. It should be noted, that while invertebrate production was lower in the more open disked and grazed treatments, invertebrate densities were adequate (> 100/m^) to support some migratory wetland bird use within these treatments. However, invertebrate densities were generally < 1600/m^ within all cattail management treatments. Invertebrate densities of 5000/m^ may be needed to support large (0.5 million birds) populations of migratory wetland birds at CBWA.

Aquatic invertebrate succession patterns, including temporal patterns in trophic structure, following inundation at CBWA were assessed in order to understand the controls on aquatic invertebrate assemblage structure. Invertebrate density, biomass. familial richness, and familial diversity generally increased following inundation. Detritivores were rapid colonizers of newly flooded wetland habitat and were always the most abundant trophic group. Herbivores were most abundant during later successional stages. Predators followed a bimodal distribution with both early and late peaks. The early peak in predators was attributable to the presence of macroveliids. Predators also became more abundant during later successional stages. Although abundance of herbivores and predators increased over time, detritivores were always the most abundant trophic group at CBWA.

Aquatic invertebrate succession has been hypothesized to occur in 3 phases (Lake et al. 1989, Schneider and Frost 1996, Moorhead et al. 1998). Increasing densities, biomass, familial richness, and familial diversity of aquatic invertebrates at CBWA are consistent with the first phase of succession, colonization and establishment. Familial richness and diversity generally stabilized within 2 weeks following Inundation, however, suggesting that the invertebrate assemblage at CBWA had entered the second phase of succession. This phase is characterized by the increasing influence of competition and/or predation, which prevents richness and diversity from increasing further. Aquatic invertebrate assemblages at CBWA failed to reach the third phase of succession dominated by predation. Temporal differences in trophic structure did not exist. Detritivores were always the dominant trophic group at CBWA. Failure to reach an assemblage dominated by predators may be related to timing of inundation. Studies of aquatic invertebrate succession have generally followed succession beginning with spring/early summer inundation. However, inundation occurred in late summer/early fall at CBWA. Abiotic factors (e.g., temperatures) and development of habitat structure (i.e., plant succession) are likely to differ between these time periods. Thus, controls on aquatic invertebrate assemblage structure are also likely to differ among these time periods. Therefore, the model of aquatic invertebrate succession posited by Lake et al. (1989), Schneider and Frost (1996), and Moorhead et al. (1998) may not hold for wetlands flooded in the fall.

Greatest densities, species richness, and species diversity of migratory wetland birds were found in the disked and high-intensity grazed treatments. Foraging and resting were the most commonly observed behaviors. Foraging and resting behaviors generally did not vary among treatments, suggesting that the greatest difference among treatments in regards to migratory wetland bird use is the number of birds that can be accommodated. Treatments, such as the disked and high-intensity grazed, which had lower densities of cattail excluded fewer birds than treatments, such as the burned and control, which had higher densities of cattail.

Models to predict migratory wetland bird species richness, species diversity, and densities were variable. However, results suggest that water depth is often an important variable influencing use by migratory birds. More species were accommodated at CBWA in spring 2001, when mean water depth was 12 cm. than in fall 1999 and 2000. when mean water depths were > 41 cm. In addition, high (5000/m^) densities of invertebrate prey may be needed to support large and diverse populations of migratory wetland birds. Vegetation variables included in models generally suggest that lightly vegetated and open areas should be maintained in order to increase migratory wetland bird use of CBWA. Such conditions were found in the disked and high-intensity grazed treatments, where migratory wetland bird species richness, species diversity, and densities were greatest.

Nonrandom assemblages of migratory wetland birds exist at CBWA. In particular, nested patterns of species occurrence among cattail-management treatments have important conservation implications. Because assemblages in species-poor treatments (i.e., burned and control) were subsets of those found in species-rich treatments (i.e., disked and grazed) (i.e. assemblages were nested), effective cattail management can be applied without reducing overall migratory wetland bird species richness. Mutually exclusive (i.e. checkerboard) patterns of co-occurrence existed for many migratory wetland bird species pairs. Ecologically and morphologically similar species often did not co-occur (e.g., green-winged Anas crecca and blue-winged teal A. discors). However, such co-occurrence patterns are unlikely to be due to proximate competitive exclusion as the majority of associations among species pairs were positive. In most instances, checkerboard patterns of co-occurrence can probably be attributed to inter-specific differences in migration chronology.

Disking and high-intensity grazing were effective means of reducing cattail for 1 year. Highest densities, species richness, and species diversity of migratory wetland birds also occurred in the disked and high-intensity grazed treatments. Cattail-management treatments exhibited a nested structure where the species found in little used treatments (i.e., burned and control) were subsets of those found in the highly used disked and high-intensity grazed treatments. This nested structure suggests that cattail management can be implemented without reducing overall species diversity. Because behavior did not vary among treatments, the largest difference among treatments in regards to migratory wetland bird use was in the number of birds that could be accommodated. Intensive cattail management did reduce invertebrate production; however, invertebrate production in disked and grazed treatments was presumably adequate (> 100/m^) to ensure use by migratory wetland birds.