The terrestrial community
Energy flow: Yesterday we neglected a major ecological topic - the flow of energy within an ecosystem. To address this, we need to consider more than just plants, although they are an essential component for it is they who capture the energy of sunlight, by means of photosynthesis, and make it available to other organisms. We think of energy flow in terms of:
- Producers: Organisms that harvest energy from non-biological sources in the environment, For the land community, the producers are green plants.
- Consumers: Organisms that "steal" energy by eating others. Primary consumers get their energy from producers, secondary cnsumers eat primary consumers, etc. Of ourse, and organism uses up energy to sustain its life, so when it is eaten, only a fraction of the energy it has consumed gets passed along. As a result, there are orders of magnitude more biomass of primary consumers than of, say, tertiary consumers.
- Decomposers: Sort of a special case of consumers, these include organisms like bacteria and fungi that are the ultimate consumers of energy. The result of their work is the reduction of living things to non-living components which may become part of the non-living substrate of the ecosystem, such as soil. Of course, in fact, decomposers may be preyed upon, too.
For the purposes of our presentation, the terrestrial commnity consists of land plants and all of the critters that get their energy from them. Thus, for this presentation, creatures like sea birds and marine iguanas don't count, even though they often hang out on land. Emphasizing this disconnect is the fact that climatic episodes like El Niño effect the marine and land realms differently.
El Niño spells starvation for sea creatures and happy time for land animals
Jackson organizs the major members of the terrestrial ecosystem into a food web. Below is my embellished version:
A couple of comments:
- The Galápagos are really atypical in possessing some primary consumers that are, as adults, untouchable by predators - tortoises and land iguanas. The iguanas are, however, preyed upon by snakes and birds of prey as juveniles.
- Marine iguanas get their food from the ocean and are included here only because they feed a number of land animals.
- Although all critters in the web are interconnected, certain ones serve as nexuses connecting many other members. These are often ecological generalists that can act as consumers at different levels. Lava lizards, for example, are both primary consumers (of plants) and secondary consumers (of insects.) Note also insects and mockingbirds.
- Slighty wierd: In the Galápagos, a centipede sits rather high on the food chain.
- The ultimate consumer is the Galápagos hawk. As a secondary/tertiary consumer, its biomass is substantially less than that of most other critters. In some situations, owls play the same role. (Note: on Genovesa, owls are, essentially, members of the marine community since they prey exclusively on critters whose energy comes from the sea.)
A brief orientation to the more familiar land animals:
Insects: As anywhere, there is a great diversity, but not so many that a person can't keep track of the important ones. Many of the conspicuous herbivorous insects are nectar-feeding pollinators:
- Carpenter bee: Solitary and sexually dimorphic with indigo-black females and yellow-brown males.
- Lepidoptera: Moths and butterflies - At least seven endemic species, which resemble mainland relatives. You're very likely to see:
- Orthoptera: Grasshoppers and katydids - The Galápagos boast the most beautiful grasshopper in the world, the painted locust. These beefy insects are a major food source for lava lizards.
- Others: Jackson writes of beetles and true bugs. We haven't experienced these. One odd insect of note is a recently introduced exotic, the Cottony cushion scale, which is attacking native plants and appears to have no natural predators.
Reptiles: We have two lizards and the tortoise:
- Land iguana:
- These large lizards feed mostly on prickly-pear pads and fruit, althought sometimes they might take small prey.
- Sexually dimorphic, with males being larger and prettier. Land iguanas are territorial and highly intolerant of others of their own gender. Males exclude one another from territory containing females, while females protect nesting burrows. This, combined with the fact that they have begun to evolve shyness as a result of hunting by humans and introduced mammals makes them somehwat harder to observe than other Galápagos critters.
- Although they don't love water, they seem to be capable of dispersing across it, because most islands' land iguanas belong to a single species, Conolophus cristatus. Only Santa Fe has its own endemic - Conolophus pallidus.
- Galápagos tortoise: Much has been said and written about these creatures. To reiterate some major points:
- Major herbivores, eating a variety of vegetable matter.
- Although regarded as belonging to a single species, Geochelone elephantopus, each island has a morphologically distinct "race." (Domed and Saddleback) Tortoises have historically inhabited all of the large islands except Fernandina.
- On Isabela and Santa Cruz, the tortoises trek between watering holes in the uplands and feeding grounds in the arid zone.
- These creatures reproduce slowly ahd have been severely reduced by hunting in historic time. Tourists are not taken into their habitats in the park, so we probably will not see a wild one. We will visit the captive propagation facitity at the Darwin Research Center.
Note: The tortoises are famous for their 200 year+ life span. Land iguanas are no slouches, either. Their true life span is unknown but appears to exceed 60 years. This brings up an interesting evolutionary note. Creatures that are unlikely to die of accident or predation have an opportunity that is denied to most creatures: The chance to for natural selection to select for variants that are better at maintaining and repairing their bodies at later life stages. Possums and armadillos don't get that chance because they are typically converted to road-kill long before they can manifest any such variation from which to select.
- Lava lizards: More interesting than they look. These creatures are a keystone in the Galápagos food web:
- They are ubiquitous on all islands except Genovesa.
- Omnivores, eating vegetation and insects, and a major food source for other predators.
- Sexually dimorphic. Typically, females have an extensive "feminine" blush of red or orange beneath their neck. Males have manly pattern.
- At least three species are present. One each for Española and San Cristóbal, and one for the central and western islands, but even there, there is inter-island variability.
- Recent taxonomic revision had changed their genus name from Tropidurus (used by Jackson) to Microlophus.
- A personal observation. Because their generation time is shorter, it stands to reason that evolved behavioral reactions to humans might be observed in these little guys. Sho 'nuf, when we were on Baltra (host to a military base since 1941) we noticed that there and there along, the lava lizards were somewhat skittish.
Birds: The seed eating and folivorous members of Darwin's finches play the primary consumer part. Without belaboring the issue, the finches we will most often encounter are:
- Ground finches: Come in large, medium and small varieties, although all three generally don't occur on the same island. Espect to see planty of medium ground finch.
- Dedicated seed eaters seeking to strike a balance between the ability to deliver a powerful bite, and the ability to get by on very little food. Peter and Rosemary Grant have studied the finches of Daphne Major since the 1970s. In the late 70s, they witnessed a year with no rainy season. This produced the strongest episode of directed natural selection every recorded, with beak height among large ground finches increasing by 10%. (A few finches with the smallest beaks survived because the birds, being smaller, required less food.)
- Sexually dimorphic, with males being dark brown/black while females are light brown with streaked chests.
- Cactus finches: Come in large and common sizes. Have beaks that are narrower and more pointed than ground finches. These creatures eat, take shelter, and nest in Opuntia.
- Tree finches: Omnivores. Like ground finches, come in small, medium, and large sizes:
- Galápagos dove: Not a finch, but another endemic seed/plant eater.
Insects: Yeah, predatory insects. A couple worth noting:
- Tramea cophysa: A conspicuous dragonfly. Considering that dragonflies spend their early lives in fresh water, it's amazing that there are abundant dragonflies in the Galápagos.
- Polistes versicolor: Near black mangrove trees, you will be aware of this recently introduced hornet. These are linked to a decline in the mangrove finch, an insect eater. Apparantly the hornet is better at hunting insects in the mangrove environment and is slowly excluding the finch.
Centipede: The Galápagos centipede, Scolopendra galapagensis is interesting in that it occupies a position in the Galápagos food web analogous to that of a small mammalian predator on the mainland. Indeed, the Galápagos are odd in that the near absence of native mammals has enabled unlikely criters to step into their place.
- Scorpions: There are two species. Be warned.
- Spiders: Several conspicuous varieties. One to look forward to is the beautiful Argiope argentata.
Insectivorous birds: This encompasses a wide range of endemic, native, and exotic species. Of particular note:
- Warbler finch: Darwin's warbler-look alike finch has a short narrow beak, and hunts insects in foliage.
- Yellow warbler: A real warbler, native both to the islands and the mainland. In North America, you count yourself lucky to see these, but in the Galápagos they are as common as starlings and display the typical local naivete.
- Groove-billed ani: A cuckoo recently introduced from the mainland (perhaps deliberately by ranchers). These have been in the islands perhaps less than 20 years, and behave like typical fearful mainland birds.
Mockingbirds: When Darwin visited the islands, these were the birds whose island-to-island variation most struck him. Like the North American mockingbird, these creatures are surprisingly bright ecological generalists. They are also capable of hunting young lava lizards, centipedes, and nestling land birds, as well as insects. They are likely to investigate us and, if they are desparate, beg for food or water from us. Their species distribution mirrors that of the lava lizards, with one species each for the old islands, Española and San Cristóbal, and one for the central and western isles:
Galápagos snakes: Never seen one of these. Sure would like to. There are three very similar species in two genera. Alsophis slevini and Alsophis dorsalis are spread throughout the central and wester isles while the older islands of Española, San Cristóbal, and Floreana are inhabited by Philodryas biserialis. All species are brown with dull yellow stripes, feed on small reptiles, and are mildly venomous. Like the finches, these snakes are next to impossible to identify in the field.
Top of the food chain: The ultimate consumers are birds of prey, foremost, the hawk