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LESSON 3: With such a large amount of rain falling on Dominica, it is not surprising that the island is said to have 365 streams and rivers, one for every day of the year. Streams range from numerous fast-flowing, large-bouldered tropical mountain streams, to several slower-moving, silt-bottomed rivers on the coast. What is the character of Dominica's freshwater streams? Steep-sided mountains typically cause many small streams to flow quickly through a series of waterfalls, rapids, and deep pools, The volcanoes of the island form a series of watersheds, which are roughly central, somewhat irregular in outline, but continuous from Morne Diablotins in the north to Morne Plat-Pays in the south. A radial drainage pattern basically forms from each volcano. Although small streams are numerous in number, and close in proximity, many reach the sea before joining into extensive catchment areas. Larger catchments that have formed on the island (greater than 10 square miles in each case) contain rivers with a variable but always considerable flow — such as the Layou (drains 27 sq. miles), Roseau A general physical characteristic of Dominica's rock which strongly affects landscape development and stream flow is high permeability. Due to its volcanic origin, the rock has numerous joints and fractures to allow moderately rapid infiltration, while more recent volcanic ashes are extremely permeable. The landscape's permeability is so high that run-off is negligible and river level may be taken as the local water table in most areas other than the island's central leeward, where rivers or streams can briefly run above the level of the water table. What are Dominica's dominant forms of freshwater animal life? Dominica's streams, whether tropical mountain streams or larger rivers such as the Indian and Layou Rivers, have aquatic insects, a few freshwater fish species, but are typically dominated by several species of freshwater shrimp and crabs. FRESHWATER SHRIMPS
FRESHWATER AND LAND CRABS
What are Dominica's freshwater mollusk species? Several gastropod (snail) species have been identified for Dominica's streams. Neritina punctulata What are Dominica's freshwater fish species? A tentative list of freshwater fishes of Dominica, compiled by R.G. Werner of Syracuse University includes:
How can some fish live in both fresh and salt water? One of the major barriers preventing most aquatic organisms from moving freely between freshwater and marine environments is the effect that different salinity levels have on the organism's internal water balance. If an organism's tissues and blood have a higher salt content than the water they live in (such as the case when living in freshwater), then water tends to flow into the organism from the environment, with the potential of causing tissue damage from increased swelling. If an organism's tissues and blood have a lower salt content than the water they live in (such as the case of many bony fishes living in saltwater), then water tends to leave the organism into the environment, causing dehydration. If organisms are to survive in aquatic habitats that have fluctuating salinity levels, such as in estuaries at the mouths of rivers, or during migrations between fresh and salt water habitats, they must have physiological strategies for adapting to the different salt levels encountered. One passive strategy, used by soft-bodied organisms like anemones, is to let their internal salinity levels fluctuate and remain the same concentration as the surrounding environment. Most of the freshwater fish species listed for Dominica use a more active strategy in which they are osmoregulators, capable of controlling their own internal salt and water levels. This control allows them to move freely between fresh and saltwater habitats, regardless of the environment's salinity. To control their internal water balance when in salt water, bony fish combat the loss of water by drinking seawater, excreting excess salt through the gills, and producing small quantities of salty urine. When in freshwater, bony fish combat the inflow of water by not drinking water, absorbing salt through the gills, and excreting large volumes of highly dilute urine. What kinds of aquatic insects are found in Dominica's freshwater habitats? Dominica's aquatic insect types have links to genera from both the Nearctic faunal region (the region that includes of North America, south to Mexico) and the Neotropical faunal region (the region which includes South and Central America, including southern Mexico, and the West Indies). Odonata (Dragonflies and Damselflies) A study on the Odonata of Dominica in the 1960's recorded 21 species. Of these, 14 were dragonfly species and 7 damselfly species. Only three species (a dragonfly and two damselfies) were endemic to the Lesser Antilles. Habitats on Dominica in which dragonflies and damselflies have been recorded range from those at lower elevations (swampy areas; grassy shallow ponds; tree-lined sloughs along roads; shaded streams with sandy to rocky substrates, emergent vegetation and moderate flow) to those at higher elevations above 1,000 ft. (rocky, swift streams with limited emergent vegetation; abundantly vegetated Freshwater Lake at 2,365 ft.). Dragonfly Families and Genera: Aeshnidae (Darner Dragonflies) (genera: Aeshna, Anax, Triacanthagyna) Damselfly Families and Genera: Lestidae (Spread winged Damselflies) (genera: Lestes) Trichoptera (Caddisflies) There are 37 recorded species of caddisflies in Dominica. Families and Genera represented are: Hydroptilidae (Microcaddisflies) (genera: Zumatrichia, Alisotrichia, Brysopteryx, Neotrichia, Oxyethira, Bredinia, Hydroptila, Ochrontrichia) Ephemeroptera (Mayflies) The mayflies types described here are associated with those found in the general West Indies region, and may not necessarily be found specifically on Dominica. (The term West Indies includes all islands in the Caribbean Archipelago extending from Florida in North America and Yucatan in Central America to Venezuela in South America.) Bold typed genera are those most likely to be found in Dominica. One endemic species of the family Leptophlebiidae, Borinquena (Australphlebia) traverae, occurs in Dominica. Other groups of aquatic/semi-aquatic insects reported for Dominica: Diptera (True Flies)
Tipulidae (Craneflies) Coleoptera (Beetles) Families include: Megaloptera
Chloronia antilliensis A dobsonfly-like megalopteran. Lepidoptera (Moths) Geunus: Hemiptera Family: Gerridae (Water Striders) What factors determine the abundance and distribution of aquatic insects found on Dominica? GETTING THERE: Aquatic insects disperse as females fly to different locations and lay eggs. Dragonflies are good fliers, with some species capable of making lengthy migrational flights. Yet, the short adult life spans and poor flying abilities of some insects (such as mayflies, stoneflies, and caddisflies) would not easily allow for long dispersal flights. Although an occasional windblown female might be carried a long distance, a more likely scenario for long-distance dispersal is that of an egg case, which could withstand drying out for long periods of time, getting transported afar by wind or bird. Many of the insect groups found on the Dominica are related to genera that arrived in some way from either Nearctic areas (North America and northern Mexico) or Neotropical areas (South and Central America, southern Mexico, and other islands in the West Indies). Having landed, it was essential for the proper ecological conditions to be available for them to survive and reproduce. SURVIVING THERE: Temperature is one very important factor for determining the distribution, diversity and abundance patterns of aquatic insects. Many mayfly species, for example, are dependent on cool-water habitats for survival. So it is not surprising that a great majority of mayflies described from the West Indies are inhabitants of mountain streams. Higher altitudes have lower temperatures. Yet mayflies are better adapted to warmer waters when compared to stoneflies (Order Plecoptera). This may help explain why most existing stonefly species, with their preference for cold water streams, are confined to temperate zones in the Northern and Southern Hemispheres, and are poorly represented in the tropics. There are no stoneflies on any island, including Dominica, in the Lesser Antilles. Temperature can also regulate the voltinism (number of generations per year) of aquatic insects. The number of generations per year decreases with increasing altitude, latitude and colder temperatures. Warmer temperatures tend to speed up an insect's metabolic rate and development. Emergence (from larva into adult) normally occurs earlier at warmer, lower elevations and latitudes. Erythrodiplax umbrata, a Common Skimmer Dragonfly, and most widespread Odonate on Dominica, occurs commonly during both the early and late months of the year. The time span for most smaller tropical common skimmers to complete the development from egg to adult is less than a year. Skimmers in colder climates typically require a full year or more. The relatively uniform temperature and light conditions of the tropics could result in uninterrupted periods of growth and reproduction. This may help explain why most species of Odonata in Dominica do not seem to have a readily observable seasonal distribution pattern. (Although it has been suggested that E. umbrata may have two semi-distinct univoltine (one generation per year) populations on Dominica, one flying during the dry season and one during the wet, or one bivoltine (two generations per year) population. Water flow conditions and substrate types will affect what kind of aquatic insects are found at a particular point along a stream's path. In upland waters, with a high gradient, rapid currents flow over large boulders and cobbles on the stream bottom. Insect types requiring high oxygen levels and specific kinds of food transported by strong current, have the ability to maintain their position in the current using hooks, suckers, or sticky threads, and so will be found in faster flowing areas of a stream. Others insects may find slower current conditions more to their liking, where they can sprawl, crawl or burrow into the less stable sediments to obtain food on the stream's bottom. These conditions are usually found in slower-moving current in lowland areas, or in pool zones in mid-level streams, where smaller-sized substrate particles such as sand, gravel and silt, are more plentiful. Yet, no matter what an aquatic insect's preference, disturbances such as extremely high stream flow conditions, on a regular basis, can have devastating impacts on stream organisms. In Dominica, heavy flooding during the rainy season, particularly in upland areas, tends to scour and grind the stream bottom with debris. This scouring action regulates the abundance and distribution of stream inhabitants, and may be a primary reason for the generally low diversity of organisms in most of Dominica's streams, with only a few species, able to adapt to the harsh flow conditions, reported to reach great numbers. Argia concinna, is a narrow-winged damselfly species typical of upland stream habitats. There are relatively few records for this species during the later part of the wet season, suggesting it is univoltine, with a preference for the less wet months. The damselfly's life cycle pattern could be reflective of the fact that torrential rains during the wet season may make streams in which the larvae develop uninhabitable. The availability of certain food types in a stream section may also affect the taxa richness (kinds) and abundance (number of individuals) of aquatic insects found there. The food types, and the combination of insect types they support, may also provide insight into the ecosystem attributes (such a adequate tree cover and stable substrate) of the stream and its surrounding watershed. In general, four functional feeding groups can be used to characterize aquatic insects based on what they eat.
QUESTIONS TO LESSON 3 Using the information in Lesson 3, write a brief, but exact explanation for the following questions. See how your ideas match the answers that will be provided Friday Jan. 21. Question1: A fish study conducted on a 150 m section of Saucon Creek, Hellertown, PA., USA, identified 14 freshwater fish species. Question 2: Compare the list of fish species for Dominica and for Saucon Creek. What fish species is common to both streams? How is it possible for this fish species to be found in both Dominica and Saucon Creek? Question 3: Why do you think crustaceans dominate the freshwater streams of Dominica? Question 4: In Dominica freshwater streams, heavy flooding occurs during the rainy season, an important disturbance event. What might be ways aquatic insects could adapt to this annual disturbance, in order to survive and reproduce? Comparative Link: Compare stream data from Dominica's streams with stream data from Saucon Creek and Polk Valley Run, Hellertown, PA The taxa richness, relative abundance, and the relationship of functional feeding groups of organisms in a stream can be used to evaluate the ecosystem attributes and water quality of a particular stream section.
View selected aquatic insect larvae of Saucon Creek and Polk Valley Run Comparison 1: Functional feeding group ratios as indicators of stream ecosystem attributes After taking two kick net samples from a stream section (Herman's) on Polk Valley Run (a 1st Order stream tributary to Saucon Creek), and on Saucon Creek (a 3rd order stream), the samples were sorted, identified and counted. The organisms were then placed into their primary functional feeding groups using Merritt and Cummins " An Introduction to the Aquatic Insects of North America," and "Methods in Stream Ecology," Hauer and Lamberti. Goal 1: Calculate the total number of shredders, scrapers, gathering collectors, filtering collectors, and predators within the sample.
Note: An example of the ratio calculations and evaluation of the ecosystem attributes using the functional feeding group ratios is provided using a Saucon Creek sample GOAL 1:CALCULATION OF THE TOTAL NUMBER OF FUNCTIONAL FEEDING GROUPS PRESENT IN A SAMPLE GOAL 2:CALCULATION OF THE FUNCTIONAL FEEDING GROUP RATIOS GOAL 3: USE THE FUNCTIONAL FEEDING GROUP RATIOS TO EVALUATE ECOSYSTEM ATTRIBUTES OF STREAM SECTION Comparison 2: Compare aquatic insect stream data from Dominica's streams with aquatic insect stream data from Saucon Creek and Polk Valley Run, Hellertown, PA 1) Data sheet of aquatic insect field data from Dominica's streams. Compare taxa, functional feeding groups and ecosystem attributes with tables Comparison 3: Compare chemical data from Dominica's streams with chemical data from Saucon Creek, Hellertown, PA 1) Data sheet of probe chemical data from 24 hr. run on Saucon Creek
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Atya innocous: known locally as "book" or "cacador". Common in both rushing mountain and lowlying sluggish streams, it reaches a maximum length of 2.7 in (7 cm). The shrimp has long bristles on its two front limbs which can be held together to as a brush to sweep organic particles from stream surfaces. Other appendages transfer these particles into the mouth. The bristles also expand into a fan to expose fin hairs that can be used to filter particles from the current. 