Ecosystems depend on complex ecological interactions between organisms in order to function. For example, interactions between predators and their prey often determine the size of populations within an ecosystem, while competition between species can drive the structure and diversity of ecological communities. Habitat loss and fragmentation can alter such interactions by changing the occurrence or behavior of interacting species, resulting in serious consequences for important ecological processes and the ability of ecosystems to thrive.
One essential interaction in terrestrial ecosystems is the mutualistic relationship between plants and their pollinators, in which animals such as bees and flies transport pollen between flowers and in return receive food in the form of pollen and nectar. Over 80% of flowering plants rely on pollinators for reproducing and setting seed.
Flower communities are very diverse and host a wide variety of pollinators, but not all pollinators visit all plant species. Often, pollinators prefer some flowers over others, and plants have specific traits to attract certain pollinators.
Collectively, these associations between flower species and their pollinators form a network of interactions, in which any particular plant species is visited by a subset of the available species in the pollinator community. These pollination networks often contain generalist species, which interact with a large number of partners, as well as specialist species, which only interact with a small number of partners.
Habitat loss and fragmentation can dramatically shift the structure of plant-pollinator networks by changing the number and identities of interacting species. Several recent studies show that fragmented habitats often have fewer species than larger, connected habitats due to a loss of rare or sensitive species, which can reduce the diversity of mutualistic interactions and the overall size of a network. Fragmentation can also change the composition of pollinator and plant communities by favoring disturbance-tolerant species and edge-associated species, forcing the mutualists to switch to new or less-preferred partners.
Specialist plants and pollinators in a network are more sensitive to habitat loss and fragmentation because, unlike generalists, they cannot switch or replace their mutualistic partners if any specific interaction is lost. In addition, such specialist species often exist in patchy populations that may be excluded or lost from small, fragmented habitats due to adverse conditions or stochastic events. Loss of specialist species leads to an overall increase in the generalization of the plant-pollinator network, in which the remaining species interact with a larger number of partners.
A number of studies have shown that specialist pollinators respond more quickly to habitat changes and experience greater declines than specialist plants. Though the plants may be more resistant to the effects of habitat loss and fragmentation, losses of specialist pollinators could potentially drive declines and eventual population extinction of plants that depend on these pollinators.
Because pollination is a such an important driver of plant community composition and structure, changes to existing plant-pollinator networks may have widespread effects on entire ecosystems. Thus, there is a critical need to protect plant-pollinator networks by maintaining sufficient patch sizes and high habitat connectivity within natural areas.
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