John E. Havel

Flooding to Restore Connectivity of Regulated, Large-River Wetlands Natural and controlled flooding as complementary processes along the lower Missouri River

You can always count on finding the Mississippi just where you left it last year. But the Missouri is a tawny, restless, brawling flood. It cuts corners, runs around at night, fills itself with snags and traveling sandbars, lunches on levees, and swallows islands and small villages for dessert. Its perpetual dissatisfaction with its bed is the greatest peculiarity of the Missouri.... It makes farming as fascinating as gambling. You never know whether you are going to harvest corn or catfish (Fitch 1907, p. 637).

Do Reservoirs Facilitate Invasions into Landscapes

Abstract The extensive construction of reservoirs over the past century has radically altered the environmental landscape on a global scale. Construction of dams on most large rivers has interrupted the connectivity of water flow and greatly increased the abundance of standing freshwater habitats. Reservoirs act as stepping-stones for the dispersal of exotic species across landscapes. A variety of passively dispersing species have invaded reservoirs, spread through interconnected waterways, and been transported to nearby disconnected habitats. We hypothesize that reservoirs are more readily invaded than natural lakes, because of their physiochemical properties, greater connectivity, and higher levels of disturbance. Here we summarize properties of reservoirs that would make them prone to invasions and discuss cases in which reservoirs have facilitated rapid range expansion. Our overview illustrates linkages between two important forms of global environmental change: the widespread manipulation of river flows and the accelerating spread of exotic species.

LOCAL AND REGIONAL ZOOPLANKTON SPECIES RICHNESS: A SCALE-INDEPENDENT TEST FOR SATURATION

Assemblages of coexisting species are formed by immigration from a re- gional pool of colonists and local interactions among species and with the physical envi- ronment. Theory suggests that the shape of the relationship between regional and local species richness may indicate the relative roles of dispersal and local interactions in limiting local diversity. Here we examine patterns of regional and local species richness in freshwater crustacean zooplankton to test whether linear (suggesting dispersal limitation) or curvilinear (suggesting saturation, via strong local control) functions best fit the data. Local richness appeared saturated when regions of different spatial extents were included on the same graph. However, this pattern was influenced by differences in scale among surveys. We corrected for the effects of regional scale by plotting mean local richness against the residuals of the species-landscape area relations. Controlling for the extent of the regional scale produced much more linear patterns, suggesting strong dispersal limitation. We present a simple graphical model to explain how variation among surveys in the geographic size of regions can produce apparent saturation of local diversity even if the underlying pattern of local and regional richness is linear. We also compare the predictive power of residual regional richness on local richness with that of several local features in a multiple regression model. Local richness exhibits strong relationships with both residual regional richness and pH. We argue that the relative strengths of local and regional processes depend on the definition of the regional scale. A variety of evidence suggests that local processes play a major role in generating differences in zooplankton diversity among lakes within a bio- geographic region. Evidence for the importance of dispersal limitation comes largely from comparisons of lakes across very large scales. Our analysis suggests that linear patterns of local and regional diversity are not incompatible with strong local interactions.

ESTIMATING DISPERSAL FROM PATTERNS OF SPREAD: SPATIAL AND LOCAL CONTROL OF LAKE INVASIONS

The spread of exotic species can be limited by dispersal or by constraints imposed by the local environment. Using data collected from 152 Missouri (USA) lakes over seven years, we asked whether models based on dispersal or local-scale processes best predicted invasion by the exotic cladoceran Daphnia lumholtzi. We used multiple logistic regression to test the relative importance of 10 local physicochemical features and proximity to all known potential source populations for predicting which lakes were invaded. The decline in invasion likelihood with distance to source populations was used to estimate the shape of the dispersal kernel. Between 1992 and 1998 the cumulative prevalence of D. lumholtzi increased from 6% to 34% of lakes sampled, with frequent appearances of pop- ulations in new watersheds. Spatial position and physical factors were both important for predicting the new colonization events. The probability of colonization increased with lake surface area and epilimnetic temperature, declined with increasing conductivity, and was unaffected by variation in lake fertility. Invasion likelihood declined sharply as a nonlinear function of distance to source populations up to around 30 km, and was relatively constant at greater distances. The results suggest that dispersal and local abiotic constraints jointly limit the spread of D. lumholtzi. This approach illustrates how range expansion can be used to estimate dispersal rates at broad spatial scales.

Sex and ostracods

In the quest to ascertain the selective agents sustaining sex, much effort has been directed to studies of the origin and fate of asexuals. Ostracod crustaceans are of particular importance to this work, as they appear to show the highest incidence of transitions to parthenogenesis of any animal group. In addition, their excellent fossil record provides a historical perspective, suggesting that some parthenogenetic ostracods have outmanoeuvered the Red Queen for at least 70 million years. Genetic studies are now probing the basis of such persistence and are providing new direction for experimental work examining this frequent abandonment of sex.

Aquatic invasive species: challenges for the future

Humans have effectively transported thousands of species around the globe and, with accelerated trade; the rate of introductions has increased over time. Aquatic ecosystems seem at particular risk from invasive species because of threats to biodiversity and human needs for water resources. Here, we review some known aspects of aquatic invasive species (AIS) and explore several new questions. We describe impacts of AIS, factors limiting their dispersal, and the role that humans play in transporting AIS. We also review the characteristics of species that should be the greatest threat for future invasions, including those that pave the way for invasions by other species (“invasional meltdown”). Susceptible aquatic communities, such as reservoirs, may serve as stepping stones for invasions of new landscapes. Some microbes disperse long distance, infect new hosts and grow in the external aquatic medium, a process that has consequences for human health. We also discuss the interaction between species invasions and other human impacts (climate change, landscape conversion), as well as the possible connection of invasions with regime shifts in lakes. Since many invaders become permanent features of the environment, we discuss how humans live with invasive species, and conclude with questions for future research.

Diversity of crustacean zooplankton in riparian wetlands: colonization and egg banks

Levee breaks from the Great Flood of 1993 opened up hundreds of new scour basins in the floodplain of the Missouri River. Subsequent floods, with sediment erosion and deposition, cause these lakes to be temporary features of the landscape. Within two years of the 1993 flood, the majority of zooplankton species from the region had colonized these sites. A positive correlation between species richness and connectivity indicates that sites having higher exchange with the river tended to have more species present, a result which is consistent with higher colonization rates to these sites. Hatching experiments from the sediments revealed that remnant oxbows have a highly diverse egg bank, whereas the young scour sites have limited species and numbers present. The depauperate egg bank implies that long-term population dynamics of the scours may be more dependent upon repeated colonizations than are lakes with regular emergence from the egg bank.

Biological Invasions Across Spatial Scales: Intercontinental, Regional, and Local Dispersal of Cladoceran Zooplankton

The frequency of dispersal of invertebrates among lakes depends upon perspective and spatial scale. Effective passive dispersal requires both the transport of propagules and the establishment of populations large enough to be detected. At a global scale, biogeographic patterns of cladoceran zooplankton species suggest that effective dispersal among continents was originally rare, but greatly increased in the past century with expanded commerce. Genetic analysis allows some reconstruction of past dispersal events. Allozyme and mitochondrial DNA comparisons among New World and Old-World populations of several exotic cladocerans have provided estimates for likely source populations of colonists, their dispersal corridors, and timing of earlier dispersal events. Detecting the Old-World tropical exotic Daphnia lumholtzi early in its invasion of North America has allowed detailed analysis of its spatial spread. Twelve years of collection records indicate a rapid invasion of reservoirs in the United States, by both regional spread and long-distance jumps to new regions. Combining landscape features with zooplankton surveys from south-central US reservoirs revealed higher colonization rates of D. lumholtzi at lower landscape positions, a result which can be explained by either greater propagule load or by higher susceptibility of these downstream reservoirs. Because invaded reservoirs provide a source of propagules for nearby floodplain ponds, the rarity of this species in ponds suggests limitation by local environments. Such analyses of invading species over multiple spatial scales allow a better understanding of ecological processes governing invasion dynamics.

Effect of main-stem dams on zooplankton communities of the Missouri River (USA)

The persistence of plankton in flowing water presents an enigma, i.e., how can populations be sustained while constantly losing individuals downriver? We examined the distribution and abundance of zooplankton from 146 sites on the Missouri River (USA) and found large shifts in the dominance of major taxa between management zones of this regulated river. Crustacean zooplankton were dominant in the inter-reservoir zone of the river, and their taxonomic composition was similar to regional lakes and reservoirs. The exponential decline of cladocerans and copepods with distance from main-stem dams suggests that conditions within the river are adverse to population growth and that reservoirs are the main source of these crustaceans in the river. Rotifers dominated in the channelized zone of the river. High algal biomass and rapid population growth rates likely allow persistence of rotifers in segments of the river that do not receive direct reservoir inputs. Rotifers were less abundant in the inter-reservoir zone, suggesting that their numbers are limited by internal processes, such as food or predators. Since zooplankton are known to be an important food for larval fishes in rivers, this shift of major taxa in regulated rivers has implications for river food webs.

Parental investment and sex allocation in a viviparous onychophoran

Although viviparous reproduction is employed by species in at least six different animal phyla it is rarely common. The phylum Onychophora is exceptional as most of the species are viviparous. Limited information is available on reproductive biology of this group. The current study provides information on parental investment and sex allocation in natural populations of Neotropical Plicatoperipatus jamaicensis. Gravid females of this onychophoran showed an average maternal effort of 12.6% (maximum 34.0%) and carried from 1-4 embryos of varying developmental stage, suggesting that offspring are born individually rather than in clutches. There was considerable overlap in the size distributions of freeliving juveniles and embryos, indicating that the period of embryonic care is variable. There was a positive correlation between embryo and maternal size, suggesting that large females release larger offspring which can more quickly attain maturity. Males of this species were sexually mature shortly after birth. Females were inseminated while small, but then had to undergo substantial growth before reproduction. Sex ratios of juveniles and adults were significantly female-biased, but embryos showed a 1:1 sex ratio. Sex ratios of broods approximated binomial expectations, and there was no shift in sex ratio with female size, suggesting that offspring gender is not maternally controlled.