As pristine natural habitats around the world continue to diminish, there has been increased interest in finding environmentally sound ways to practice agriculture so that production is kept high and biodiversity is also protected.
Examples of productive agricultural systems that also maintain a high level of biodiversity include shade coffee and cacao farms in Central and South America that grow crops in the traditional manner, underneath a canopy of native trees. This provides excellent habitat for migratory and resident birds (includes many species that breed in North America and winter in tropical America). The area devoted to these two crops pales to that covered by cattle pastures. Unlike coffee and cacao farms that mimic native forest habitats, cattle pastures conjure up few images of good bird habitat.
Much cattle pasture land consists of wide open, grassy fields and is not used by many birds. However, in studies done in rural Mexico, Russell Greenberg found that cattle pastures intermixed with groves of Acacia trees provide excellent bird habitat. Migratory birds such as Magnolia Warblers, Black-throated Green Warblers, Least Flycatchers, and Blue-gray Gnatcatchers can actually be found in higher numbers in Acacia trees than in nearby forest habitats. Why? The answer is food. Despite their tiny leaves and thorny spines, Acacia trees are infested with small bugs.
Acacia trees are a pioneer plant, meaning that they are one of the first trees to colonize an abandoned field. As such, they put a considerable amount of energy into growing quickly and into defending themselves from large grazing animals by producing numerous menacing spines. They even have spines on their trunks to discourage clumsy animals from knocking them over, especially when the trees are young and fragile.
By putting all their energy into growth and physical defenses, they don't have much reserve left for fending off insects. Many plants ward off insects by producing poison -- chemical defenses in leaves and other plant tissues that are distasteful or harmful to insects, but this requires an energy investment that Acacias can't afford.
During the dry season, when acacias have dropped many of their leaves, birds still flock to them. Some birds even maintain season-long territories within Acacia groves. This is also a difficult time for cattle ranchers, because their herds of cattle have reduced forage in the dry and withered grass in the pastures.
Acacia trees drop their seed pods during this dry season. The pods are full of protein and comparable to corn or soybeans in their nutritional value. By passing the seeds through their guts, the cattle release the seeds into the soil where they can germinate. In addition, Acacia provide a number of benefits to ranchers:
Acacia trees grow throughout the tropical regions of the world being found in Africa, India, Australia, as well as the Americas. Interestingly, in Australia, these trees do not have thorny spines, presumably because there are no large, grazing animals to defend against. The Americas also lack large grazing animals, with the exception of the recent introduction of cattle.
Why then do American Acacias have spines? The reason may be that until fairly recently there were large grazing animals in America, but they perished at the end of the last Ice Age, 10,000 years ago. Acacias in America haven't had time to de-evolve spines. Cattle have now taken the ecological place of these extinct animals.
Developing countries may have over 2 billion hectares (1 hectare = 100 meters by 100 meters = .003861 square miles) in pastureland. By growing pasture grasses intermingled with Acacia, these lands can be a refuge for biodiversity as well as a source of cattle.
This article summarizes the information in this publication:
Greenberg, R.S., Bichier, P. and Sterling, J. 1997. Acacia, cattle and migratory birds in southeastern Mexico. Biological Conservation, 80: 235-247
Acacia pennatula groves in mid-elevation valleys of southern Mexico supported both the highest density and diversity of migratory birds compared to other habitats in the region. In addition, we found the highest numbers for over half of the common migratory species. Despite the high degree of leaf loss during the late winter, acacia groves do not experience greater declines in insectivorous migratory bird populations than other local habitats. Color-marked individuals of canopy species had a strong tendency to remain resident within a single acacia grove throughout the winter. Management of native acacias on subtropical rangelands for wood products, fodder, and soil improvement would probably directly and indirectly benefit migratory song bird populations. Neotropical acacia woodland is primarily associated with grazing land for livestock which might discourage some from considering acacia management a viable option for migratory bird conservation. However, acacias were probably widespread in recent geologic time under drier conditions with heavy browsing and seed dispersal by a now extinct megafauna. Two observations suggest a foraging advantage for acacia use: the higher density of migratory birds in acacia is a result of higher numbers of canopy insectivores; and acacias are selected by migrants when they occur in mixed habitat. We hypothesize that plant investment in mechanical defenses (thorns) reduces energy available for chemical defenses effective against insect herbivores.
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