30. In ecotone, some species become abundant and they are called
(A) sibling species
(B) endemic species
(C) rare species
(D) edge species
Species That Become Abundant in an Ecotone Are Called Edge Species
Detailed Explanation of the Correct Answer
In an ecotone, some species become especially abundant because they are well adapted to the environmental conditions found at the boundary between two ecological communities. Such species are called edge species. Therefore, option (D) is the correct answer.
An ecotone is a transitional zone where two different ecosystems, habitats, or biological communities meet and interact. For example, the boundary between a forest and a grassland, the transition between a freshwater ecosystem and a terrestrial ecosystem, or the region between a mangrove forest and the sea can function as an ecotone.
Because an ecotone contains environmental characteristics and biological resources from both neighboring ecosystems, it can support species from both communities. In addition, some species are particularly adapted to the unique conditions of the transition zone itself. These organisms may become more abundant at the boundary than in either of the adjacent ecosystems and are known as edge species.
The increased abundance or diversity of organisms at the boundary between two communities is closely associated with a phenomenon called the edge effect. Thus, understanding the relationship between ecotone, edge effect, and edge species is essential for answering this question correctly.
What Is an Ecotone?
An ecotone is a transition area between two distinct ecological communities or ecosystems. It does not always form a sharp boundary. In many natural environments, one community gradually changes into another across a transitional region.
For example, where a forest meets a grassland, the vegetation does not necessarily change suddenly from tall trees to open grass. Instead, there may be a transitional area containing scattered trees, shrubs, grasses, and other vegetation. This boundary region represents an ecotone.
Similarly, the margin of a pond can form an ecotone between the aquatic ecosystem and the surrounding terrestrial ecosystem. Such a region may contain organisms associated with water, organisms associated with land, and species that are specially adapted to the boundary conditions.
Therefore, an ecotone can contain biological characteristics of both neighboring communities while also supporting its own distinctive ecological interactions.
Why Are Ecotones Biologically Important?
Ecotones Contain Species from Adjacent Communities
One major feature of an ecotone is that species from both neighboring ecosystems may occur within the transition zone.
For example, an ecotone between a forest and a grassland may contain forest birds, grassland birds, woodland mammals, grassland herbivores, shrubs, grasses, and trees. The overlapping distribution of organisms can increase the total number of species observed within the boundary region.
This mixing of biological communities makes many ecotones highly dynamic and ecologically important.
Ecotones Provide Diverse Resources
Ecotones often contain a greater variety of food sources, shelter sites, nesting areas, and microhabitats than either adjacent community alone.
A forest-grassland boundary, for example, may provide trees for nesting, shrubs for protection, grasses for grazing, seeds for granivorous animals, and open areas for hunting. Species capable of using resources from both habitats may therefore perform especially well in the transition zone.
The availability of diverse resources is one reason certain species become abundant in ecotones.
Ecotones Create Unique Environmental Conditions
Environmental conditions in an ecotone may differ from those found deep inside either adjacent ecosystem. Light intensity, temperature, humidity, wind exposure, soil moisture, and nutrient availability may all change near ecological boundaries.
These distinctive conditions favor species that are specifically adapted to boundary habitats. When such organisms achieve particularly high abundance in the transition zone, they are referred to as edge species.
What Is the Edge Effect?
The edge effect refers to changes in species abundance, species composition, or ecological interactions that occur at the boundary between two habitats or communities.
In many cases, an ecotone contains species from both adjacent communities as well as species that prefer the boundary itself. This can result in a higher abundance or diversity of organisms in the transition zone.
For example, the edge between a forest and an open grassland may attract animals that feed in the open area but use trees or shrubs for shelter and reproduction. Such animals can take advantage of resources from both environments.
The edge effect is therefore closely associated with the increased abundance of certain organisms called edge species.
However, the ecological consequences of edges can vary depending on the ecosystem and species involved. Some species benefit from habitat edges, whereas species requiring large areas of undisturbed interior habitat may decline near boundaries.
What Are Edge Species?
Edge species are organisms that are especially common or abundant in ecotones or habitat boundaries. These species are adapted to the conditions created where two different ecological communities meet.
An edge species may use resources from both adjacent habitats. For example, an animal may feed in an open grassland while using nearby forest vegetation for shelter. Another species may reproduce in one habitat and obtain food from the neighboring habitat.
Because of this ability to exploit resources from both sides of an ecological boundary, edge species may become more abundant in an ecotone than in either neighboring community.
Therefore, the direct relationship is:
Ecotone → Edge effect → Increased abundance of edge species
This is why option (D) is the correct answer.
Option (A): Sibling Species
Option (A), sibling species, is incorrect.
Sibling species are closely related species that are extremely similar in external appearance but are reproductively isolated from one another. Because of their strong morphological similarity, they may be difficult to distinguish using visible characteristics alone.
The concept of sibling species is mainly associated with evolutionary biology, speciation, reproductive isolation, and taxonomy. It does not refer to species that become abundant at ecological boundaries.
Two sibling species may appear almost identical but remain separate biological species because they do not normally interbreed successfully.
Therefore, the term sibling species has no specific connection with increased abundance in an ecotone.
Hence, option (A) is incorrect.
Option (B): Endemic Species
Option (B), endemic species, is incorrect.
An endemic species is a species naturally restricted to a particular geographical area. The restricted area may be an island, mountain range, forest region, lake, country, or other defined location.
Endemism describes geographical restriction rather than abundance at a habitat boundary. A species can be endemic and abundant, or it can be endemic and extremely rare. Its defining characteristic is that its natural distribution is limited to a specific geographical region.
Ecotones are ecological transition zones, whereas endemic species are defined according to their geographical distribution.
Therefore, species that become abundant in an ecotone are not called endemic species.
Hence, option (B) is incorrect.
Option (C): Rare Species
Option (C), rare species, is incorrect.
A rare species is generally a species with a small population size, restricted geographical distribution, limited habitat requirements, or some combination of these characteristics.
The term rare species therefore suggests low abundance or restricted occurrence. This is opposite to the situation described in the question, which specifically asks about species that become abundant in an ecotone.
Although a rare species may occasionally occur in a transition zone, rarity is not a defining characteristic of species associated with the edge effect.
Therefore, option (C) is incorrect.
Option (D): Edge Species
Option (D), edge species, is the correct answer.
Edge species are organisms that become particularly common or abundant in the transition zone between two ecological communities. These species often benefit from the combination of resources, habitats, and environmental conditions available at ecological boundaries.
An ecotone may contain species from both neighboring ecosystems, but edge species show a special association with the boundary region itself.
Their increased abundance is one of the important biological features associated with the edge effect.
Therefore, option (D) is correct.
Difference Between Ecotone, Edge Effect, and Edge Species
The terms ecotone, edge effect, and edge species are closely related but represent different ecological concepts.
An ecotone is the actual transition zone between two ecosystems or biological communities. It is a physical and ecological boundary region.
The edge effect is the ecological change observed at this boundary. It may involve changes in species diversity, abundance, community composition, predation, competition, or other ecological interactions.
Edge species are the organisms that become especially common or abundant in the boundary environment.
Thus, the relationship can be understood as:
Ecotone = Transition zone
Edge effect = Ecological changes at the transition zone
Edge species = Species that become abundant at the transition zone
These three concepts together explain why option (D) is the correct answer.
Examples of Ecotones
A forest-grassland boundary is a common example of an ecotone. The transition zone may contain trees from the forest, grasses from the grassland, shrubs adapted to intermediate conditions, and animals capable of using resources from both habitats.
The edge of a pond is another example. It represents a transition between aquatic and terrestrial environments and may contain emergent plants, amphibians, aquatic insects, terrestrial insects, birds, and other organisms.
Riverbanks, wetlands located between aquatic and terrestrial environments, mangrove boundaries, and transitions between different vegetation zones can also function as ecotones.
In each case, certain organisms may become particularly abundant because they are adapted to the special conditions created by the ecological boundary.
Ecological Importance of Edge Species
Edge species can play important roles in the movement of energy and materials between neighboring ecosystems. Because they often use resources from both habitats, they may connect ecological processes across the boundary.
For example, an animal may feed in one habitat and deposit nutrients in another. Pollinators may move between plant communities, while predators may hunt across habitat boundaries.
The abundance of edge species can therefore influence food webs, species interactions, nutrient movement, and community structure.
At the same time, not every species benefits from increased habitat edges. Species adapted to the interior of large, continuous habitats may be negatively affected when habitats become fragmented. Therefore, edge effects can produce different ecological outcomes for different organisms.
Final Answer
In an ecotone, some species become especially abundant because they are adapted to the conditions found at the boundary between two ecological communities. These species are called edge species.
Therefore, the correct answer is:
(D) Edge species
