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An incredible evolutionary story: Why did giant and dwarf species appear on islands?
In isolated island ecosystems, the evolution of plants and animals often exhibits surprising phenomena, which is called "island syndrome". This syndrome explains why species on islands differ significantly from their mainland relatives in morphology, ecology, behavior, and physiology. These changes are primarily the result of different ecological pressures, including the scarcity of predators and herbivores, and a continued mild climate.
"In the absence of large predators, prey species on islands often lose the ability to escape predation, leading to evolutionary gigantism and dwarfism."
Ecological drivers
First, island ecosystems generally have lower biodiversity. This means less competitive pressure and no need to adapt to a unique environmental niche. On land, the presence of large mammals and predators drives diversification of body size among species, but on islands this diversification is often washed out. Small terrestrial animals often become larger, as is the case with larger predators such as the fossa of Madagascar relative to some smaller predators.
Island syndrome symptoms in animals
Body type
Change in body shape is one of the most striking features of island syndrome. According to "Foster's law," normally small species become larger on islands, while large species become smaller. For example, the small hippos of ancient Madagascar are such an example, as they were much smaller than the large hippos on the mainland.
"Organisms on islands often evolve to exhibit characteristics of gigantism or dwarfism to meet the needs of the local ecosystem."
Mobility and Adaptive Color
In environments with low predation pressure, many island species reduce the locomotor ability they use to escape predators. For example, the wings of small birds and woodpeckers on some islands have become degenerate and they have lost the ability to fly. This phenomenon is particularly evident in New Zealand's Maori birds and other species that have evolved on islands. At the same time, due to reduced sexual selection pressure, the colors of island species tend to become less bright, making species identification less important.
Reproduction strategy
The reproductive behavior of species on islands is also influenced by innate competition. On islands, parents typically have fewer offspring to ensure adequate care and investment in each offspring, a strategy that increases the survival rate of offspring.
Brain and Behavior
According to the 'expensive tissue hypothesis', species in environments where predation pressure is significantly reduced may experience a reduction in brain size. This is because the brain, a tissue with high metabolic demands, no longer provides a survival advantage for some island species. At the same time, island species also show characteristics of being more docile and less territorial in their behavior. For example, some mice and song sparrows on islands do not compete as hard as their mainland counterparts for survival.
Island Syndrome in Plants
The evolution of plants on islands also shows unique characteristics. On islands, plant size and structure can change depending on the environment; for example, small plants may increase in size due to reduced competition, while large plants may decrease in size. In addition, due to the lack of powerful herbivores, many plants would have changed and lost defensive structures such as thorns or toxins.
Protective significance of island syndrome
Island syndrome makes species on islands unique in diversity and adaptability, but these species are often vulnerable to invasion by alien species. Historically, the dodo in Mauritius became extinct due to alien predators introduced by humans, reminding us of the urgency of protecting island ecosystems.
Final Thoughts
Through the study of island syndrome, we can gain a deeper understanding of the formation and maintenance of biodiversity. In the future, as human activities continue to impact, how will these unique island ecosystems continue to evolve, and what challenges and opportunities will they face?
