Essays on Invertebrate Conservation
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WINGS ESSAYS ON INVERTEBRATE CONSERVATION THE XERCES SOCIETY SPRING 2011 CONTENTS Introduction Scott Hoffman Black Page 3. Endangered Crab Found in West Africa’s Shrinking Forests Neil Cumberlidge and Piotr Naskrecki We think of crabs as marine animals, but across the tropics they have adapted to live in fresh water and on land. One species, Guinea’s purple marsh crab, was an enigma until its recent rediscovery. Page 4. Aliens Douglas Tallamy The use of non-native plants in the landscape may lead to declines in populations of native insects that co-evolved with particular species of native plants. Page 9. Milkweeds: Not Just for Monarchs Brianna Borders and Matthew Shepherd Although milkweed plants are seen in many places—farms, ranches, roadsides—as a problem to be eradicated, they support a diverse community of insects. Page 14. Six-Legged Tigers David L. Pearson Tiger beetles, with their bright colors and constant activity, have become a popular subject for hobbyists, who are adding to our knowledge of these insects. Page 19. Watching the Devil’s Horses Pass By Celeste Mazzacano Dragonflies, fragile as they may appear, can fly thousands of miles. Eighteen species in North America are known to migrate with the seasons. Page 24. Xerces News A remembrance of Thomas Eisner, an extraordinary scientist and a former presi- dent of the Xerces Society; petitioning for the protection of five endangered cold- water-dependent insect species; efforts to protect North America’s ailing bumble bee populations; and advocating for Fender’s blue butterfly. Page 29. 2 WINGS Introduction Scott Hoffman Black In looking at the cover of this issue of We also have three articles about in- Wings you might have noticed some- vertebrates that do not directly rely on thing different: a photograph of a plant plants. The purple marsh crab, living in rather than the customary invertebrate. West Africa, was rediscovered sixty years Plants and insects are inextricably after the first—and only—specimen was linked; two essays in this issue explore collected. An essay about tiger beetles this interaction. The first underscores shows that not only are they amazing the importance of native plants to the creatures in beauty and behavior, but native insects that feed on them. The they are useful in helping us to under- presence of non-native plants can sig- stand and manage the habitats in which nificantly reduce the abundance of na- they live. The last article delves into the tive insects, which in turn impacts the little-known world of migratory dragon- songbirds and other animals that eat flies—some of which travel further than them. The second looks at the astound- monarch butterflies—and discusses the ing array of insects that use milkweeds, newly formed Migratory Dragonfly Part- and the ways in which the Xerces Soci- nership, which is working to understand ety and its partners are working to re- and protect these animals. turn milkweed to our landscapes and in- We hope you find this selection of crease local supplies of milkweed seed. essays informative and enjoyable. This hickory horned devil will develop into a regal moth, Citheronia regalis, but only if it has adequate native plants to eat. Photograph by Douglas Tallamy. SPRING 2011 Endangered Crab Found In West Africa’s Shrinking Forests Neil Cumberlidge and Piotr Naskrecki We were camped in West Africa, on the ubiquity in tropical aquatic ecosystems, westernmost fringe of the Upper Guinea the thirteen hundred or so species of forest, a vast area stretching from Guin- freshwater crabs have somehow avoided ea to Sierra Leone and Liberia. As part the full attention of the scientific com- of an international team of scientists munity. Interest in their biology and conducting a rapid assessment of this conservation is only now beginning to historically biodiversity-rich region, we gain momentum. had come to survey invertebrates, par- The success of the world’s marine ticularly freshwater crabs. At first sight land crabs — species that live in man- these damaged forests and parched sa- grove forests, on beaches, or further in- vannas, degraded by agriculture and land, but still need to return to the sea to industry, seemed an unlikely place to breed—in the coastal fringes throughout seek rare aquatic animals. But we were the tropics is due to their well-developed there because of the imminent threat of abilities to breathe air, dig burrows, de- further industrial expansion and addi- hydrate slowly, and walk easily on land. tional ecosystem disruption. These adaptations gave access to new Our campsite was in disturbed food sources and living spaces in the land under intense agricultural man- coastal lands above the high-tide line, agement in northwest Guinea, but by and this is where these animals now sheer luck our tents were pitched just a reign supreme. few kilometers from a thriving colony Freshwater crabs evolved from ma- of some of the continent’s most elu- rine crabs, but succeeded in breaking sive crustaceans —purple marsh crabs the connection to salt water with ad- (Afrithelphusa monodosa). This species aptations that enabled them to osmo- is truly an enigma, previously known regulate in low-salinity environments. only from a single specimen collected in These included ion pumps on their gills 1947. Yet, amazingly, just a day after we that move salts inward, antennal glands began our survey, a local farmer walked (kidney-like organs) that pump water right into our camp holding one! out of their blood, and an impressively Purple marsh crabs belong to a di- waterproof carapace. As a result, fresh- verse group of decapod crustaceans col- water crabs complete their life cycle in loquially known as river crabs or fresh- fresh water and never need to return to water crabs, which are abundant in sea water to breed. the rivers, streams, and lakes of inland Our newly rediscovered Guinean waters throughout the tropics. Despite species was among the small subgroup their large size, attractive colors, and of the freshwater crabs that we call fresh- WINGS water land crabs, those species that can in water only as a last resort, usually to live and reproduce well away from per- avoid predators, and seem uninterested manent water sources. in using oxygen from water. Structural modifications in their The abilities of the freshwater land gill chambers enable many species of crabs to breathe air, dig burrows, resist freshwater crabs to breathe in air as well drying, and walk on land easily match as underwater. Their gill chambers are those of marine land crabs. Freshwater so well adapted for aerial respiration that crabs have a reproductive adaptation their ability to breathe is undiminished that opened up to them vast tracts of by being out of water. The bottom layer land in the inland tropical ecosystems of each gill chamber has the usual set of of the world’s continents that even the gills that are seen in most crabs, which well-adapted marine land crabs had not allow them to breathe under water. But conquered. In contrast to marine crabs, it is in the upper layer above the gills which release their eggs into sea water where the truly remarkable adaptation and whose larvae spend several weeks in is found. Here there is a spongy air- an un-crablike, planktonic stage floating breathing organ, which is analogous to with the currents, larval development a vertebrate lung in function but struc- in freshwater crabs is completed entirely turally so different that it is known as a inside the egg case, with each egg releas- “pseudolung.” ing a fully formed miniature crab. The In purple marsh crabs, this respira- evolution of larval direct development tory structure works so well that they ac- has had big ecological consequences for tually prefer to breathe air rather than freshwater crabs. For one thing, it meant water. These crabs immerse themselves that these crustaceans could complete Land crabs have adapted to life away from water, but most still need to return to breed. Blue land crab (Cardisoma guanhumi), at water’s edge, waiting to release her eggs. Photograph by Piotr Naskrecki. SPRING 2011 their entire life cycle in their inland humid rainforests, further adaptations habitat, using fresh water for all of their such as small compact bodies and long needs. And it released female freshwater slender walking legs have enabled fresh- crabs from the need to spend valuable water crabs to move easily through veg- energy making migrations to the coast etation and even to climb tree trunks, during the breeding season. This radi- thereby equaling or surpassing the feats cal adaptation removed one of the last of most marine land crabs. barriers to the colonization of land and Marine land crabs found in coastal gave them total independence from salt- Guinea, such as the rainbow crab (Cardi water environments, which in turn led soma armata), and air-breathing man- to the explosive radiation of freshwater grove crabs, such as the fiddler crab crabs in the inland waters of the tropics Uca tangeri, each are widely distributed around the world. along hundreds of miles of the West Today, these crabs are dominant African coast from Senegal to Angola. inhabitants of warm fresh waters from These wide distributions are a direct tropical America to Australasia. Fresh- consequence of their developmental water crabs have conquered not only strategy, in that the currents carry the more conventional freshwater ecosys- larvae long distances from their release tems such as rivers and lakes, but some points during the weeks spent drifting species, including the purple marsh in the surface waters. In contrast, most crab, have colonized such marginal freshwater crabs have a narrow distri- habitats as flood plains bordering riv- butional range — except perhaps where ers and streams, damp terrain in fresh- a major river system is involved.