Tom L. Dudley

Effects of Macroalgae on a Stream Invertebrate Community

The effects of macroalgae on stream invertebrates were studied in riffle zones in a coastal southern California stream. Dense growths of macroalgae (Cladophora glomerata, Nostoc sp.) were experimentally removed at different times of the year and in different years, and the insect communities which developed were compared with those in unmanipulated controls. Marl precipitated by algae was also removed in one experiment. The presence of macroalgae was associated with greater total densities and taxon richness of invertebrates, and nearly all taxa responded significantly to algal removal on at least some dates. Insects formed most of the community and were classified according to three categories of macroalgal effects on benthic densities: 1. Negatively affected by macroalgae (and marl) due to competition for space--e.g., Blepharicera (strong response to both algal taxa); large Simulium (strong with Cladophora, weak with Nostoc). 2. Positively affected due to structural habitats created by algae--e.g., Micrasema, Rhyacophila and Hydropsyche (all strong); Tinodes (weak-Nostoc); Rheotanytarsus (strong-Nostoc). 3. Positively affected by both macroalgal structure and associated food resources (macroalgae or epiphyton)--e.g., Baetis and Chironomidae (strong-Cladophora, weak-Nostoc); Hydroptila, Ochrotrichia, and Euparyphus (strong-Cladophora); and endosymbiotic Cricotopus (strong-Nostoc). Natural disturbances will indirectly affect invertebrate distributions and abundances by affecting the distributions and abundances of macroalgae.

The effects of substrate texture, grazing, and disturbance on macroalgal establishment in streams

Two common macroalgae, Cladophora glomerata (a filamentous chlorophyte) and Nostoc parmeloides (a colonial cyanobacterium), are strongly associated with rough substrates in Rattlesnake Creek, a second—order stream in coastal Santa Barbara County, California. To test the hypothesis that rough texture reduces algal mortality from invertebrate grazers and from winter storms, we conducted a series of experiments on two substrate types (titles) that varied only in surface heterogeneity. The dominant grazer, the caddisfly Agapetus celatus, was excluded from tiles using petroleum jelly barriers, and algal establishment was compared with that on control tiles; grazing experments were repeated during spring and autumn. In a separate experiment, storm scouring was simulated by manually scrubbing naturally colonized rough and smooth tiles at different intervals (7 wk, 3 wk, and once, with an undisturbed control) from December to June, during which algal establishment and survival were monitored. In the absence of Agapetus, Cladophora was found nearly equally on rough and smooth surfaces. Grazers strongly reduced Cladophora establishment on all surfaces (often by >80%); texture did not generally influence grazing effects in autumn, but refuges (pits) greatly increased algal survival in spring when recruitment was greater. Two other grazing insects (Baetis spp. and Micrasema) increased on exclusion tiles, apparently due to competitive release, but did not eliminate effects of Agapetus. Manual disturbance at 3— and 7—wk intervals kept Cladophora densities much lower than on control and once—scoured tiles. Densities were higher on rough tiles, but the interaction between disturbance and texture was not significant for macroalgae; however, microscopic examination showed that basal filaments, and new sporelings, were protected within depressions. Nostoc attained densities in autumn of °50—100 colonies/100 cm2 on rough tiles and was virtually absent from smooth tiles. Unlike Cladophora, Nostoc was little affected by grazing, but was greatly reduced by disturbance. Rough texture provided minor protection, but mature Nostoc formed basal crusts that resisted disturbance, thereby maintaining space in the absence of severe scour. Substrate heterogeneity provides refuges from herbivory and disturbance for establishing macroalgae. While it increasingly appears that herbivory plays a profound role in determining algal dynamics in streams, herbivores and substrate texture may also interact to modify the rate and direction of benthic succession. By inhibiting the establishment of intermediate taxa (e.g., Cladophora), grazers may restrict succession on smooth substrates to an early, diatom—dominated assemblage, or accelerate dominance by later taxa (e.g., Nostoc) on rugose substrates.

Grazing catfish, fishing birds, and attached algae in a Panamanian stream

SynopsisIn streams where algivorous fishes abound, striking variation of attached algae often develops along depth gradients, with bands of high standing crops in shallow water (<20 cm) and sparse standing crops on deeper substrates. Experimental results from a stream in central Panama support the hypothesis that vertical variation in algal standing crops arises when grazing fishes avoid predators in shallow water by forgoing food resources that accumulate there. When 38 rocks bearing algae in a stream in central Panama were transferred from shallow (<20 cm) to deeper (>20 cm) water, algae were rapidly consumed by grazing catfish. Catfish were removed from three stream pools and left in place in three control pools. Ten days after catfish removal, algal standing crops in deep and shallow areas of removal pools were similar, while algal standing crops were higher in shallow than in deep areas of control pools. Catfish were exposed to fishing birds in open-topped enclosures. In one of three series of these pens, most catfish in shallow pens (10 and 20 cm) disappeared after 14 days, while catfish in deeper pens (30 and 50 cm) did not. Other groups of catfish which were caged 8 days showed differences in behavior depending on whether they had been fed or starved. After their release into their home pool, starved catfish spent more time feeding than did fed catfish. Despite their apparently increased hunger levels, starved catfish did not venture into shallow water to obtain algae. These results support the view that predator induced avoidance by grazers of certain areas can produce spatial pattern in the flora of flowing water communities.

Mechanisms and consequences of interspecific competition between two stream insects.

SUMMARY (1) Grazing larvae of the net-veined midge, Blepharicera micheneri, and filter-feeding larvae of blackflies, primarily Simulium virgatum, occupy rock surfaces in fast-flowing water in -Rattlesnake Creek, California, U.S.A. Based on observations of aggressive behaviour by Simulium toward Blepharicera, we conducted a series of field experiments to determine the presence, mechanisms and consequences of competition for attachment space between these two dipteran insects. (2) There was an inverse relationship between abundances of the two taxa, and when Simulium was removed from natural substrates, blepharicerid densities increased. Despite blackfly aggression, co-occurrence was common. To estimate the cost of co-occurrence, we measured behavioural, feeding and fitness responses of blepharicerid larvae to manipulated simuliid abundances. (3) Simulium caused Blepharicera to spend 5 x more time in avoidance responses than when alone, resulting in a 20% reduction in time spent feeding. Distance travelled was also increased by simuliid interference. The interactions were strongly asymmetrical in favour of Simulium, being reversed only when Simulium was much smaller than Blepharicera. (4) Diatom ingestion by Blepharicera was reduced 60% by the presence of Simulium. In mesocosms adjacent to the stream, blackflies inhibited the growth of blepharicerids and increased mortality and time to pupation, resulting in decreased blepharicerid production. (5) Baetis mayflies and higher Blepharicera density also tended to inhibit blepharicerid growth, probably via exploitative competition, but these effects were secondary to interference competition with Simulium. (6) The importance of interspecific competition varies within and between years, because Blepharicera and Simulium co-occur for a longer period in years of high rainfall, but may not overlap in dry years. Both taxa depend upon flood disturbance to open space and reduce competition from other taxa (macroalgae and the caddis Hydropsyche oslari Banks). Contrary to prevalent conceptions, competition in streams may be a common and important factor structuring populations and communities, while disturbance determines the form of interspecific competition rather than eliminating it.

Beneficial effects of herbivores on stream macroalgae via epiphyte removal

The densities of two epiphyton-feeding stream insects (Baetis sp. and Agapetus celata) were experimentally manipulated in stream-side channels containing macroalgae (Cladophora glomerata) to determine grazer influences on the host alga. After 16 days Cladophora biomass increased significantly more in the presence of Baetis (248.0%) and high densities of Agapetus (153.6%) than in grazerless controls (68.6%); biomass in low density Agapetus treatments (57.8%) did not differ from controls (...)

Seasonal timing of diapause induction limits the effective range of Diorhabda elongata deserticola (Coleoptera: Chrysomelidae) as a biological control agent for tamarisk (Tamarix spp.).

Abstract The leaf beetle Diorhabda elongata Brullé subspecies deserticola Chen, collected in northwestern China, has been released in the western United States to control tamarisk (Tamarix spp.). While beetle establishment and saltcedar defoliation have been noted at northern study sites, this species has not established at latitudes south of the 38th parallel. Critical daylength for diapause induction was measured in the laboratory and ranged between 14 h 50 min to 15 h 08 min, depending on temperature, and adults were shown to cease reproduction and enter diapause at daylengths of 14 h 30 min or less. Critical daylength in the field was measured at ≈14 h 39 min and occurred 13 d before 50% of the population reached diapause. South of 36°20′ N, the longest days of the year are shorter than 14 h 39 min, making the beetles univoltine in the southern United States. North of 36°20′ N, a window of reproductive activity opens 13 d after the critical daylength is reached in the spring and closes 13 d after it is passed in the summer, allowing at least a partial second summer generation. It is predicted that south of the 38th parallel, premature diapause will increase mortality and disrupt synchrony between the life cycle of the beetle and host plant availability. This could hinder establishment and help explain the failure of this population south of the 38th parallel, providing a rationale for testing other populations of D. elongata in the southern range of Tamarix in North America.

Saltcedar (Tamarix spp.), Endangered Species, and Biological Weed Control—Can They Mix?1

Abstract Saltcedar invasion has many economic and environmental effects, including displacement of native riparian vegetation and associated wildlife. A biological control program led to the approval in 1994 of two insects for introduction but was delayed by the presence of the endangered southwestern willow flycatcher (SWWF) in saltcedar. In 2001, the saltcedar leaf beetle was released in six states but not where the SWWF was present. Delays circumvent the benefits that saltcedar suppression could have for other declining species, including many rare or absent in ecosystems dominated by saltcedar. Numerous birds forage within saltcedar vegetation but in lower numbers and diversity than in native stands that provide better habitat and insect resources. Successful establishment by saltcedar leaf beetle resulted in extensive saltcedar defoliation, and observations of wildlife feeding on the beetles in an otherwise depauperate system suggest that biocontrol may enhance habitat quality for many species, including the SWWF. Consideration of the multiple species affected by saltcedar would have allowed more effective invasive plant management in this case, but delays also reflect drawbacks in federal administrative structures related to invasive species management in ‘natural areas’ as much as problems with a narrow focus on a single species. A functionally integrated approach where research and management decisions are made cooperatively would allow more rational management of invasive species in wildland ecosystems. Nomenclature: Saltcedar, Tamarix ramosissima Ledeb. #3 TAARA, complex also includes T. chinensis Lour. # TAACH, T. parviflora DC. # TAAPA; saltcedar leaf beetle, Diorhabda elongata; southwestern willow flycatcher, Empidonax traillii extimus Phillips. Additional index words: Biodiversity, ecological restoration, ecosystem management, riparian habitat. Abbreviations: APHIS, Animal and Plant Health Inspection Service; SWWF, southwestern willow flycatcher; USDA, U.S. Department of Agriculture; USFWS, U.S. Fish and Wildlife Service.

Evolution of critical day length for diapause induction enables range expansion of Diorhabda carinulata, a biological control agent against tamarisk (Tamarix spp.)

In classical weed biological control, small collections of arthropods are made from one or a few sites in the native range of the target plant and are introduced to suppress the plant where it has become invasive, often across a wide geographic range. Ecological mismatches in the new range are likely, and success using the biocontrol agent may depend on postrelease evolution of beneficial life history traits. In this study, we measure the evolution of critical day length for diapause induction (day length at which 50% of the population enters dormancy), in a beetle (Diorhabda carinulata) introduced into North America from China to control an exotic shrub, Tamarix spp. Beetle populations were sampled from four sites in North America 7 years after introduction, and critical day length was shown to have declined, forming a cline over a latitudinal gradient At one field site, decreased critical day length was correlated with 16 additional days of reproductive activity, resulting in a closer match between beetle life history and the phenology of Tamarix. These findings indicate an enhanced efficacy and an increasingly wider range for D. carinulata in Tamarix control.

Ecology and Impacts of the Large-Statured Invasive Grasses Arundo donax and Phragmites australis in North America

Abstract Large-statured invasive grasses (LSIGs) constitute a distinct functional group with characteristic life history traits that facilitate colonization and aggressive growth in aquatic ecosystems, particularly those modified by human activities. These species typically form monocultures in the systems they invade and have wide-ranging and negative impacts on biodiversity and ecosystem processes. In March 2008, a special symposium was held as part of the Western Society of Weed Scientists annual meeting to synthesize our current knowledge of the ecological impacts and management of two notorious LSIGs: Arundo donax and Phragmites australis. In this volume of Invasive Plant Science and Management, symposium participants provide articles summarizing existing knowledge, recent research progress, and research needs for these two taxa. Here, we summarize the basic biology of these species and suggest the use of a more holistic approach to deal with the effects and management of LSIG invasions. Nomenclature: Giant reed, Arundo donax L.; common reed, Phragmites australis (Cav.) Trin. ex Steud.

Tamarisk biocontrol, endangered species risk and resolution of conflict through riparian restoration

A long-standing debate between wildlife agencies and biological control researchers and practitioners concerns Diorhabda carinulata Desbrochers (Coleoptera: Chrysomelidae) introduced to suppress invasive Tamarix spp. (Tamaricaceae), and potential impacts of Tamarix defoliation on endangered southwestern willow flycatchers using this non-native plant as nesting habitat in some western riparian ecosystems. The conflict and ensuing legal actions are currently centered on the presence of D. carinulata within the breeding range of the flycatcher in the Virgin River watershed, which has led to APHIS termination of permits supporting the biocontrol development program and has also affected other programs to develop biocontrol agents against environmental weeds. Central to concerns over wildlife is the lack of rehabilitation of native vegetation where biocontrol is expected, so there are current and planned efforts to promote restoration of native cottonwood-willow habitat to mitigate the anticipation decline in Tamarix cover. A strategic approach to riparian restoration is outlined which could facilitate sustainable, and scientifically documented recovery of this iconic habitat type. While the results of these efforts will not be known immediately, the process which is leading to riparian restoration has brought specialists from both sides of the debate together in search of resolution via collaboration, and if successful, may allow re-initiation of the Tamarix biocontrol program attendant with habitat enhancement for wildlife species of conservation concern.