Distribution of Adult Aquatic Insects in the Cosumnes River Floodplain Denise Antonia Grab Abstract 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
View Full Document
Unlocking...