Distribution of Adult Aquatic Insects in the Cosumnes River Floodplain

Emerged aquatic insects can play a major role in terrestrial food webs. Often the location of these insects affects the foraging behavior of terrestrial insectivores such as birds and bats. Not only the location of emergence, but also the patterns of insect dispersal are relevant in interpreting energy flows from aquatic insects to terrestrial insectivores. One calendar year of emergence trapping at the Cosumnes River floodplain—a Nature Conservancy restoration site in California’s Central Valley—has yielded a set of insect emergence data. However, little is known about the distribution of these insects by wind or flight. This study addresses the aerial distribution of emerged aquatic insects at the Cosumnes floodplain using “sticky” traps and compares this distribution to the emergence levels of various sites on the floodplain. Results showed that sampling date, trap height, site, and distance from water all had significant effects on insect abundance, but only date and height significantly affected insect biomass. Sampling date had the most significant effect on abundance and biomass. Moreover, many interactions between the variables had significant effects on abundance and biomass. This suggests that further study is needed in order to understand the true impact of each variable on insect distribution, especially in the face of temporal variability. Understanding the factors that affect the distribution of insects in the Cosumnes floodplain may aid scientists in interpreting insectivore foraging behavior and help the Nature Conservancy to make management decisions that will help insectivore species targeted in reserve restoration efforts to thrive. Introduction The distribution of adult aquatic insects has been shown to affect the distribution and foraging behavior of insectivores, including bats and birds (Maurer and Whitmore 1981, Gray 1993, Power and Rainey 2000, Iwata et al. 2003). As such, the distribution of these insects can play a major role in the trophic interactions of the ecosystem as a whole (Power and Rainey 2000). Many factors are known to affect the distribution of emerged aquatic insects. Insects may arrive in a different spot from which they emerge by flying or crawling—so-called active distribution (Flecker and Allan 1988, Jackson and Resh 1989). Insects may also move to a new location through passive distribution, such as being blown by the wind (Pasek 1988, Pedgley 1990). Because of distribution, emergence trap data on insect community composition may not accurately reflect the flying aquatic insect community in the surrounding airspace. Studies have often investigated the lateral flux of aquatic insects in lower-order, mountain streams (Jackson and Resh 1989, Power and Rainey 2000). These low-order mountain streams frequently have steep banks and dense vegetation, which tend to decrease average wind speed (Jackson and Resh 1989). Thus many of these studies considered wind a negligible factor in insect distribution (Jackson and Resh 1989, Power and Rainey 2000). Few studies have attempted to look at aquatic insect dispersal in high-order stream floodplains. In terms of active distribution, flat terrain can influence the distance of flight away from the stream (Jackson and Resh 1989, Power and Rainey 2000). In terms of passive distribution, the flat terrain and generally sparse vegetation of many floodplains can make wind a major factor in insect distribution (Pasek 1988, Pedgley 1990, Power and Rainey 2000, Whitaker et al. 2000). Some studies have investigated the effect of wind speed and vegetation height on insect distribution, usually in locations other than floodplains (Pasek 1988, Pedgley 1990, Whitaker et al. 2000, Harrison and Harris 2002). There is often a net movement of insects from areas with shorter vegetation and higher wind speeds to areas with taller vegetation and lower wind speeds (Pasek 1988, Whitaker et al. 2000, Harrison and Harris 2002). The floodplain of California’s Cosumnes River consists mostly of flat land and contains much low-lying vegetation (Florsheim and Mount 2002, Clinton 2003, pers. comm.). With a relatively level topography, the floodplain has the potential to exhibit a significant amount of passive insect dispersal (Power and Rainey 2000). The Cosumnes River is the only river on the western slope of California’s Sierra Nevada mountain range without a large dam on its main stem (Florsheim and Mount, 2002). As such, many environmental groups are interested in the ecology of the site. In particular, the Nature Conservancy is currently conducting restoration work on the Cosumnes floodplain, which previously consisted of farmland, in order to create a more natural floodplain ecosystem (The Nature Conservancy 1992). The vegetation in the Cosumnes floodplain consists of patches of two types of forest— cottonwood-willow and valley oak—surrounded by large amounts of open meadow (Florsheim and Mount 2002). Wind speed tends to be higher in the open meadow areas than in the areas with dense vegetation (Clinton 2003, pers. comm.). As a result, there is reason to suspect that wind might affect insect distribution on the Cosumnes floodplain. Emergence trapping has been conducted on the Cosumnes River since late 2002. The data from this trapping indicate that the open meadow areas of the floodplain tend to produce the most aquatic adults (Power 2003, pers. comm.). However, data from bat detectors at the site show the most bat foraging activity in forested rather than open areas (Rainey 2003, pers. comm.). If all other factors are equal, one would expect the most foraging activity to take place where most insects are located. Since this is not the case at the Cosumnes, it may imply that dispersal after emergence affects the aerial distribution of insects at the site. The distribution of aquatic insects after emergence has not yet been investigated at the Cosumnes River floodplain. A thorough understanding of patterns of adult aquatic insect distribution could aid in interpreting the foraging behavior of local insectivores (Gray 1993, Power and Rainey 2000, Iwata et al. 2003). In addition, the Nature Conservancy is attempting to restore the Cosumnes River site to a natural floodplain ecosystem, replete with birds and bats (The Nature Conservancy 1992). Consequently, understanding how vegetative structure affects the distribution of insects—a major source of food for many birds and bats—will aid in determining which vegetative structures provide an optimal distribution of insects for restoration and management purposes (Maurer and Whitmore 1981, Iwata et al. 2003). Over the course of several months, I investigated the distribution of adult aquatic insects in the Cosumnes River floodplain. I compared the composition of samples from emergence traps to the composition of samples taken simultaneously from nearby transects of TanglefootTM, sticky traps (Harris and McCafferty 1977). My hypothesis was that the insect abundance and biomass on the sticky traps in the windier meadow areas would be lower than that on the sticky traps in forested areas. I also anticipated that the emergence trap biomass and abundance and the sticky trap biomass and abundance at each site would not depend strongly on each other. Methods The study was conducted at the Nature Conservancy’s Cosumnes River Preserve, which is located approximately 20 miles south of Sacramento, California. The preserve consists mainly of former farmland that was converted to a floodplain when the Nature Conservancy intentionally breached several levees in the mid 1990s (The Nature Conservancy 1992). Due to the levee breaches, all parts of the floodplain may be intermittently covered with standing water, depending on the extent of flooding. The floodplain has three major vegetation types—open meadow, cottonwood-willow forest, and valley oak (Quercus lobata) forest (Florsheim and Mount 2002). To represent the different vegetation types, four sites within the preserve were sampled (see Fig. 1). The Triangular Pond (TP) is in an open meadow area. The Accidental Forest (AF) site is part of the cottonwoodwillow forest, and Wood Duck Slough (WD) is within the valley oak forest. The Channel by Wendell’s (WC) site includes the Cosumnes River itself and has mixed vegetation types.