Does alary dimorphism imply dispersal dimorphism in the waterstrider,Gerris remigis?

ABSTRACT.

• 1 The hypothesis that wing dimorphism reflects dimorphism for dispersal ability is tested in two populations of stream-dwelling waterstriders, Gerris remigis Say, in southern Quebec, Canada.
• 2 Movements were assessed directly by recaptures of marked adults, and indirectly by comparisons of residence times and patterns of disappearance of macropterous and apterous morphs. Sampling was done weekly, over a period of 26 months, and 4828 adult G.remigis were individually marked during this time.
• 3 Movements of >100 m were very rare for both morphs. The movement patterns of the two morphs were very similar, and differed only over the winter, when no macropters moved upstream. This latter observation suggests that macropters may be less successful than apterous individuals at moving overland, or rowing against a current at this time of year.
• 4 Residence times and patterns of disappearance of the two morphs were very similar during all seasons.
• 5 These results suggest that macropterous G.remigis do not, in general, disperse using any means unavailable to the apterous morph, and that macropters in this species should not be simply categorized as long-distance dispersers.

     

Predation in Gerris (Hemiptera): Reactive distances and locomotion rates

Summary Two of the parameters which determine the rate at which prey are encountered by a predator, i.e. the distance at which a predator responds to a prey and its rate of movement relative to the prey's, were determined for all the stages of five species of Gerris using gerrids and Drosophila as prey. These parameters allowed calculation of the swath, or encounter path, a gerrid would cover as it moved across the water surface. Gerris species prefer to attack live prey in front of them, and tend to ignore prey if the attack requires a turn of more than 100°. Hunger was found to affect the responsive angle required to clicit an attack by G. remigis, and regardless of species, smaller gerrids required the prey to be closer before an attack was initiated. The rate of movement in Gerris was measured as a function of stride length and the number of strides made per unit time. Stride length varied according to the length of the mesothoracic leg, and the frequency of movement was observed to be species specific. G. remigis, a stream species, moved 4–6 times as often as the four other species studied, all of which are characteristically found on non-moving water surfaces. Within a species, gerrid size had no significant effect on the frequency of movement, although there was a tendency for smaller gerrids to move less.     

Movement patterns of small benthic fish in lowland headwater streams

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Population persistence in the face of advection

Many populations live in ‘advective’ media, such as rivers, where flow is biased in one direction. In these environments, populations face the possibility of extinction by being washed out of the system, even if the net reproductive rate (R) is greater than one. We propose a formal condition for population persistence in advective systems: a population can persist at any location in a homogeneous habitat if and only if it can invade upstream. This leads to a remarkably simple recipe for calculating the minimal value for the net reproductive rate for population persistence. We apply this criterion to discrete-time models of a semelparous population where dispersal is characterized by a mechanistically derived kernel. We demonstrate that persistence depends strongly on the form of the kernel’s ‘tail’, a result consistent with previous literature on the speed of spread of invasions. We apply our theory to models of stream invertebrates with a biphasic life cycle, and relate our results to the ‘colonization cycle’ hypothesis where bias in downstream drift is offset by upstream bias in adult dispersal. In the absence of bias in adult dispersal, variability in the duration of the larval stage and in oviposition sites have a large effect of the persistence condition. The minimization calculations required in our approach are very straightforward, indicating the feasibility of future applications to life history theory.     

Testing for Janzen–Connell Effects in a West African Montane Forest

The Janzen–Connell hypothesis proposes that density dependent seed and seedling mortality, combined with increasing seed and seedling survival away from the parent tree, together promote regular spacing of species and thus α diversity. This hypothesis has rarely been tested in tropical Africa, and rarely in montane forests anywhere. We tested this hypothesis using a combination of field experiments and observations in the most floristically diverse dry submontane forest in Nigeria. We investigated distance effects on seedling herbivory, seedling survival and seedling height growth. We found a significant decrease in herbivory with distance from conspecific adult trees for all three species of experimentally planted seedlings (Entandrophragma angolense, Deinbollia pinnata and Sterculia setigera), and also for naturally occurring seedlings of Pouteria altissima but not of Newtonia buchananii or Isolona pleurocarpa. The relative density of large seedlings/saplings of P. altissima, N. buchananii and I. pleurocarpa increased significantly at greater distance from conspecific adult trees; however, we found no significant distance effect on survival or height growth over 3 mo for all three experimentally planted species. Taken together, our results are some of the first to show that Janzen–Connell effects occur on the African continent and in particular montane tropical forest and suggest that such effects may be pantropical.     

Experiments on the effects of food and density on voltinism in a stream-dwelling water strider (Aquarius remigis)

1. The stream water strider Aquarius remigis shows a latitudinal pattern of variation in voltinism. In general, populations with shorter growing seasons (e.g. in eastern Canada) tend to be univoltine (animals that reach adulthood in the summer overwinter before reproducing in the following spring), whereas populations with somewhat longer growing seasons (e.g. in the north-eastern United States) tend to be bivoltine. 2. This pattern was broken at our study site in the south-eastern United States (Kentucky) where A. remigis had a long growing season, but was almost always univoltine. In summer 1993, however, adult A. remigis in central Kentucky displayed a bivoltine reproductive cycle; i.e. individuals in some pools began breeding shortly after maturing to the adult stage. 3. A field survey documented a negative relationship between local water strider density and reproductive activity in prediapause adults. A laboratory experiment manipulating food availability and density, revealed that animals held at low density with high food levels displayed greater mating activity and egg production than did their counterparts at higher density or lower food levels. 4. A laboratory experiment also showed that high water strider density resulted in a greater frequency of very short pair durations (< 10 min). 5. Although the observed effects of density and food availability on mating activity of prediapause adults seem intuitively reasonable, they differ from the patterns observed in overwintered adults. The difference in reproduction patterns might be due to differences in selective pressures on prediapause vs. post-diapause adults.     

Food consumption in Gerris (Hemiptera)

Summary The parameters influencing food consumption in larvae and adults of five species of Gerris were studied. An experimental components analysis was utilized with emphasis placed on measurements of the length of time the insects spent handling food items, and the amount of food in the gut at any one time. The following characteristics were noted: (1) The maximum feeding time required to achieve satiation was 2–2.5 h for all gerrids above 10 mg wet weight. For gerrids below this weight, feeding time declined at a logarithmic rate. (2) Body size had little influence on the duration of the feeding period after food deprivation; only with the first and second instar larvae was a significant difference noted. (3) The average duration of the feeding and non-feeding periods was 10.4 and 24.6 min respectively for satiated adult G. remigis in the presence of excess food. (4) Signifcant differences in relative digestive rate existed between adult and larvae irrespective of species. (5) The smaller the gerrid, the greater the volume relative to its body weight that could be consumed at a single feeding, and for all gerrids, the amount consumed per day was greater than the maximum amount that can be ingested at a single feeding, this difference being larger in the larvae than in the adults. (6) The relationship between daily amount consumed and temperature was linear for four species, increasing with increasing temperature; food consumption in G. remigis, however, peaked at about 19°C, and declined linearily at temperatures both above and below this value. (7) Irrespective of instar and stadium duration, food consumption for G. notabilis peaked within a stadium about 40% of the way through the stadium.     

Adaptive significance of wing dimorphism in the absence of dispersal: a comparative study of wing morphs in the waterstrider, Gerris remigis

ABSTRACT.

• 1 Gerris remigis Say (Hemiptera; Gerridae) is primarily apterous, but populations with up to 33% macropters have been reported. The macropters seldom fly, and field studies have revealed no detectable differences between wing morphs in movement or survival at any time of year.
• 2 In this paper, life history traits of macropterous and apterous G. remigis are compared in an attempt to determine the mechanisms responsible for the maintenance of macroptery in this species in spite of the very low flight capacity and infrequent flight of macropters.
• 3 Development time, proportion breeding without diapause, and overwinter survival do not differ between morphs. However, pre-diapause macropterous females have a significantly shorter pre-oviposition period than apterous females. In contrast, post-diapause macropters begin reproducing later than apters, and have a lower cumulative fecundity.
• 4 These results suggest that macropters may be at a selective advantage in warm habitats which favour pre-diapause reproduction, but that apters should be favoured in the preferred, cool, lotic habitats.
• 5 However, crossing and rearing experiments indicate that wing morphology is primarily environmentally controlled in this species, and that the heritability of wing morphology is low, at best. In light of this, the relative impacts of purely phenotypic (environmental) variation, random effects, and the observed fitness differences on the maintenance of macroptery in this species are discussed.

     

The effects of trout on water striders in stream pools

Summary Observations made in Rattlesnake Creek, Santa Barbara County, California, U.S.A. indicate that rainbow trout (Salmo gairdneri) affect the microdistribution of adult water striders (Gerris remigis). Gerrids occupied all areas of stream pools lacking trout but stayed in margins of pools containing trout. To test the hypothesis that the presence of trout controlled the distribution of water striders within pools, trout were removed from some pools and transferred to pools lacking trout. Undistrubed pools with and without trout served as controls for the trout transferrals. The results supported the hypothesis; within one week, gerrids moved to the edge of pools where trout were added and to the middle of pools where trout were removed. Time budgets and gut analyses suggest that local patterns of gerrid distribution were maintained by harrassment of gerrids by trout. At times trout had significant effects on components of gerrid fitness.     

Movements of adult aquatic insects near streams in Southern Ontario

Sticky traps placed across a stream in southern Ontario in June and September showed that only females of the mayfly Baetis intercalaris and the caddisfly Lype diversa were flying upstream and so conforming to the concept of the colonization cycle. Other species, B. vagans, Cheumatopsyche oxa, C. campyla, Hydroptila consimilis, Chimarra aterrima and Polycentropus centralis seemed to be flying at random along the stream. There was a distinct downstream movement of Baetis subimagos near the water surface. It is shown that a series of 4 traps across the stream gives a better indication of directional movement than a single one.In March the winter stonefly Allocapnia vivipara was shown to walk away at approximately right angles to the stream and to ascend trees at the edge of the woods 16 m away.     

Flight Behaviour of Merolimnic Insects from the Leutra River (Thuringia,Germany)

1. The flight behaviour of adult merolimnic insects was studied on the first order limestone stream Leutra (Thuringia, Germany) using sticky traps. The focus of the present study was on testing the colonisation cycle hypothesis of Müller (1954) and on the small-scale dispersal of adult merolimnic insects. 2. A high number of sticky traps was used to guarantee a proper statistical analysis of the data and exclude effects of the heterogeneous environment on flight behaviour. 3. The flight behaviour of Leuctra hippopus, Nemoura cambrica, Nemoura flexuosa, Protonemura nitida, Protonemura praecox (Plecoptera), and Baetis rhodani (Ephemeroptera) was studied. 4. Specific flight corridors were observed for P. praecox, Nemoura spp., L. hippopus and B. rhodani. 5. Comparison of the catches on sticky traps set perpendicular to the water showed no significant flight upstream in any of the taxa studied. Thus, the results do not support the hypothesis of the colonisation cycle.     

Linking movement behaviour to dispersal and divergence in plethodontid salamanders

To better understand the evolutionary and ecological effects of dispersal, there is growing emphasis on the need to integrate direct data on movement behaviour into landscape-scale analyses. However, little is known about the general link between movement behaviour and large-scale patterns of dispersal and gene flow. Likewise, although recent studies suggest that nonrandom, directionally biased movement and dispersal can promote evolutionary divergence, the generality of this mechanism is unknown. We test the hypothesis that directionally biased movement and dispersal by plethodontid salamanders interact with the topography of headwater areas to affect genetic and phenotypic divergence. Movements by Gyrinophilus porphyriticus and Eurycea bislineata show contrasting directional biases: upstream bias in G. porphyriticus and downstream bias in E. bislineata. Consistent with predictions of how these biases interact with slope to affect dispersal and gene flow, genetic distance increased with slope in G. porphyriticus and decreased with slope in E. bislineata over a standardized distance of 1 km along six headwater streams. In both species, phenotypic divergence in relative trunk length was positively related to genetic divergence. These results indicate that landscape-scale patterns of dispersal and gene flow are closely related to movement behaviour in G. porphyriticus and E. bislineata, and underscore the value of information on movement behaviour for predicting and interpreting patterns of dispersal and gene flow in complex landscapes. This study also provides new evidence that directionally biased movement and dispersal can be important sources of intra- and interspecific variation in population divergence, and highlights the value of explicit, a priori predictions in landscape genetic studies.     

Reproductive allometry does not explain the temperature-size rule in water striders (Aquarius remigis)

Most ectotherms follow the temperature-size rule meaning that individuals growing up under cool conditions are larger as adults than those growing up in warm conditions. This pattern is difficult to explain because growth is usually slower in the cold meaning it takes longer to reach a larger adult size. One potential explanation for this pattern is that the typical increase in fecundity with body size is steeper in cool environments than it is in warm environments. As such, the relative gain in fecundity for being larger in the cold would compensate for the extra time it takes to grow to that size. We present the first empirical test of this model using the water strider Aquarius remigis. Individuals were reared at either 20° or 25°C with subsampling at each instar to estimate growth trajectories. At adult eclosion, half were switched to the alternate temperature and all females isolated, mated, and reproductive output measured for 3 weeks. We found that A. remigis does follow the temperature-size rule but that fecundity was highly plastic with respect to laying temperature such that the slightly greater fecundity of those reared at 20°C was due to their larger size. Overall, those laying at 25°C were more fecund and showed a positive relationship between body size and fecundity. Those laying at 20°C did not show a significant relationship between size and fecundity. As such, the reproductive allometry shows a pattern reverse to what would be needed for a larger size in the cold to be adaptive in this species. Although A. remigis follows the temperature-size rule over this temperature range, this is likely due to a constraint on growth and development rather than being adaptive plasticity.     

Failed predation or transportation? Causes and consequences of phoretic behavior in the pseudoscorpionDinocheirus arizonensis (Pseudoscorpionida: Chernetidae)

The pseudoscorpion Dinocheirus arizonensisinhabits rotting saguaro cactus in the Sonoran Desert and has also been found attached to the legs of the cactophilic neriid fly, Odontoloxozus longicornis.Laboratory experiments demonstrated a higher incidence of phoresy on eclosing versus postteneral adult flies, a female bias in phoresy, and an increased rate in female phoresy through time. The pseudoscorpion may also prey on the fly, but predation rate was unaffected by fly category, pseudoscorpion gender, or food deprivation. A study of pseudoscorpion colonization in the field indicated that females were the first to colonize and the first to abandon the transient habitat of a saguaro rot and, thus, corroborated patterns of phoretic behavior in the laboratory. Taken together, these results establish that phoresy is a behavior functioning specifically for dispersal. The hypothesis that pseudoscorpion transport by other arthropods is accidental, motivated by hunger, and occurs because pseudoscorpions are incapable of consuming their hosts is rejected.     

Comparison of Antipredator Responses of Two Related Water Striders to a Common Predator

This study compares the antipredator responses of two congeneric water striders that live in different habitats yet share a common predator. Aquarius nebularis and A. remigis are stream-dwelling species of water striders found in close association in central Kentucky. Aquarius nebularis typically inhabit third- and fourth-order streams that support diverse and complex fish communities; A. remigis inhabit adjacent first- and second-order tributaries that tend to have either no fish or relatively few fish patchily distributed. The ecological conditions for these two species are distinctly different, yet the green sunfish (Lepomis cyanellus) is a surface-feeding predator and a potential risk to both species. In experimental pools with sunfish, an initial fish strike caused A. remigis to move to the edge of the pool and become inactive. For the rest of the observation period, A. remigis reduced behaviour patterns that attract predators such as skating on open water, mating and male–male aggression. The proportion of A. remigis missing from sunfish pools was 5 times higher than in pools lacking fish, suggesting that 20% of water striders were killed by predation. In contrast, an initial fish strike caused A. nebularis to exhibit a burst of activity as individuals erratically skated over the open water. They then resumed the less active pre-attack behaviours a short time later. Aquarius nebularis showed none of the antipredator behaviours typical for A. remigis. Furthermore, the proportion of A. nebularis missing from sunfish pools did not differ from that in pools without fish. Although antipredator behaviour has been examined for relatively few species of water striders, this study suggests that the behaviour of the most commonly studied species, A. remigis, may not be indicative for water striders in general., Corresponding author: James J. Krupa, School of Biological Sciences, University of Kentucky, Lexington, KY 40506-0225, U.S.A. E-mail: bio149@pop.uky.edu bio149@pop.uky.edu     

Stable isotopes identify dispersal patterns of stonefly populations living along stream corridors

1. Populations in different locations can exchange individuals depending on the distribution and connectivity of suitable habitat, and the dispersal capabilities and behaviour of the organisms. We used an isotopic tracer, ¹⁵N, to label stoneflies (Leuctra ferruginea) to determine the extent of adult flight along stream corridors and between streams where their larvae live. 2. In four mass, mark‐capture experiments we added ¹⁵NH₄Cl continuously for several weeks to label specific regions of streams within the Hubbard Brook Experimental Forest, NH, U.S.A. We collected adult stoneflies along the labelled streams (up to 1.5 km of stream length), on transects through the forest away from labelled sections (up to 500 m), and along an 800‐m reach of adjacent tributary that flows into a labelled stream. 3. Of 966 individual adult stoneflies collected and analysed for ¹⁵N, 20% were labelled. Most labelled stoneflies were captured along stream corridors and had flown upstream a mean distance of 211 m; the net movement of the population (upstream + downstream) estimated from the midpoint of the labelled sections was 126 m upstream. The furthest male and female travelled approximately 730 m and approximately 663 m upstream, respectively. We also captured labelled mature females along an unlabelled tributary and along a forest transect 500 m from the labelled stream, thus demonstrating cross‐watershed dispersal. 4. We conclude that the adjacent forest was not a barrier to dispersal between catchments, and adult dispersal linked stonefly populations among streams across a landscape within one generation. Our data on the extent of adult dispersal provide a basis for a conceptual model identifying the boundaries of these populations, whose larvae are restricted to stream channels, and whose females must return to streams to oviposit.     

Evidence for upstream dispersion of adult CAD disflies (Trichoptera: Hydropsychidae) in the Columbia River

The radioisotope ⁶⁵Zn, introduced to the Columbia River in discharges of the U.S. Atomic Energy Commission's Hanford reactors, was used to test the hypothesis that adult caddis flies migrate upstream after emerging from the aquatic environment. The larval stages living downstream of the effluents are known to accumulate appreciable levels of ⁶⁵Zn. The radioisotope was found in levels above background in shoreline swarms of adult caddis flies as far as 16 km above the uppermost reactor effluent. Whether movement was only upstream and the precise distances flown remain unclear. The upstream movement is important for understanding the biology of riverine aquatic insects and for evaluating the upstream dispersal of radioactive contaminants.     

Exact compensation of stream drift as an evolutionarily stable strategy

The colonization cycle hypothesis predicts that adults of stream-dwelling insects preferentially disperse in the upstream direction in order to compensate for larval drift. Upstream biased dispersal has indeed been shown in many, albeit not all, natural populations. Based on a recently published analysis, we develop a simple stochastic model for the competition of genotypes with different dispersal strategies in a stream habitat. By means of an invasion analysis, we show that exact compensation of larval drift by upstream biased adult dispersal is an evolutionarily stable strategy. Exact compensation means that, on average, the net movement of individuals from birth to the time of reproduction is zero. At the population level, we show that, in general, upstream biased dispersal is not necessary for persistence, unless the reproductive rate is very low. Under all conditions, however, populations of exact compensators attain highest sizes or persistence times, respectively. Although selection pressure towards exact compensation is arguably very general in populations subject to stream drift, trade-offs or constraints might change the outcome of selection. Therefore, the analysis presented in this paper has to be viewed as a null model for optimal dispersal behavior in stream habitats.     

Resistance of three wild tuber-bearing potatoes to the Colorado potato beetle

The resistance of Solanum okadae Hawkes & Hjert. (PI 458367), Solanum oplocense Hawkes (PI 473368), and Solanum tarijense Hawkes (PI 414150) to the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Chrysomelidae: Chrysomelini), was studied. In replicated field trials all three accessions showed a high level of resistance to the beetle. No significant genetic variability between genotypes of the same species was found. Results from host acceptance behavior experiments, suitability for larval development tests, foliage consumption tests, and adult survival and oviposition tests supported the hypothesis that the mode of resistance differs between the three wild Solanum species. Solanum okadae and S. oplocense affected host acceptance and consumption. Because the beetle reacted differently to these two species it was hypothesized that the antifeedant chemical(s) differed in nature or quantity. S. tarijense contrasted with the other two species by affecting mostly adult colonization and oviposition.     

Predation risk and social interference as factors influencing habitat selection in two species of stream-dwelling waterstriders

We investigated how an aggressive species of waterstrider, Aquariusremigis, and potential predators, green sunfish (Lepomis cyanellus),affected the habitat use and mating behaviors of a less aggressivecongeneric species, A. conformis. Although these species sometimesco-occur, A. remigis typically lives in small streams with fewor no fish, whereas A. conformis are typically in medium- orlarge-sized streams with large populations of potentially predatoryfish. We tested in separate experiments in seminatural streams:1) the effect of fish on behaviors of A. conformis; 2) the effectof A. remigis on A. conformis; and 3) the habitat use of A.conformis when given a choice between pools with A. remigisor fish. The first experiment showed no effect of fish on eithermating behaviors or microhabitat use of A. conformis. This isin surprising contrast to the strong effects of fish alreadydocumented in A. remigis. The second experiment showed thatthe mating activity of A. conformis was reduced when A. remigiswere present; hence, A. conformis should avoid A. remigis. Finally,when A. conformis were presented with a choice between two pools,one containing A. remigis and the other containing fish, bothsingle males and pairs of A. conformis chose the pools withfish. In contrast, the habitat use of single female A. conformiswas not affected by either fish or A. remigis. Results fromthese experiments demonstrate that closely related species exhibitcontrasting social and antipredator behaviors and that aggressivesocial behavior is an important determinant of habitat partitioning