A little-considered prerequisite for determining the configuration, size, and quality of a habitat to be repaired is the body size of the largest species to occupy it. For example, assuming a prairie remnant can be maintained at various sizes without losing its ecological integrity, the question becomes which species can live there in viable numbers—pronghorn antelope, coyotes, rabbits, prairie chickens, or only gophers, meadow mice, and Henslow’s sparrows.
Body size is perhaps the most important characteristic of an organism because it affects all physiological and ecological processes and is, therefore, a fundamental influence on an organism’s ability to survive and reproduce in different environments, which includes those modified by human activities. Some species exhibit significantly different body sizes among macro-habitats; in other words, individuals of the same species may have smaller bodies in fragmented habitat than they do in contiguous habitats. The same may be true of individuals living at different places along an elevational gradient.1
Although the anthropogenic effects of altering landscapes may not be generic among species, they can be significant and rapid—developing in just a few decades following habitat manipulation. Thus, habitat fragmentation may influence the biodiversity, species composition, and densities of local organisms, as well as the body size of an indigenous species, which is one of its most fundamental and defining characteristics.
HABITAT SHAPE AND SIZE
The shape of a habitat patch also plays a strong role in determining the size of the population of a given species it can accommodate in a fragmented landscape. To illustrate, the rates of nest predation and brood parasitism of the grasshopper sparrow, Henslow’s sparrow, eastern meadowlark, and dickcissel were studied in thirty-nine prairie fragments ranging in size from 59 acres to roughly 100,000 acres in 5 states in the mid-continental United States.
Throughout the region, the rates of nest predation were significantly influenced by the size of a habitat fragment. They were highest in prairie remnants of less than 250 acres and lowest in patches greater than 2,475 acres. Rates of brood parasitism by brown-headed cowbirds, however, were not consistently related to fragment size, but were instead more strongly correlated with the regional abundance of cowbirds. Differences in the rates of nest predation between large remnants (54 to 68% of all nests lost to predators) and small patches (78 to 84% lost to predators) suggests that fragmentation of prairie habitats may be contributing to regional declines of grassland birds. Such differences also point to the advantage of repairing large areas of prairie to maintain viable avian populations.
Additional data on avian population density and nesting success collected in 13 prairie remnants of various sizes in southwestern Missouri revealed three levels of sensitivity to an area. The most area-sensitive species, the greater prairie-chicken, was absent from small fragments of prairie habitat. An intermediate form of area sensitivity was apparent only in Henslow’s sparrow, which had population densities scaled to the size of the prairie remnants.
At the third level, a species can be sensitive not only on a distributional scale but also by having lower nesting success in small, rather than in large, habitat patches of remaining prairie. The dickcissel, for example, was the only species in southwestern Missouri that was area-sensitive on such a demographic level. These data indicate that grassland-nesting species are subject both to the size of a habitat fragment, which determines area occupancy, and to population size, which determines nesting success.
In addition to size, configuration of the overall area of the habitat core versus the amount of edge effect is critical. The blockier the configuration is, the better the biophysical integrity of the habitat’s core. Conversely, the more convoluted, narrow, and linear the pattern, the greater the ratio of edge to interior, a configuration that magnifies the edge effect. One means of increasing the overall size of two adjacent patches of habitat is to establish a linking, biophysical corridor. 2
Although biological corridors may mitigate potential negative effects of inbreeding over long periods of time, so many variables are involved that corridors are not always the best method of conserving fragmented populations. Therefore, repairing a habitat by reconnecting disjunct populations within a habitat’s core may be more effective than attempting to join isolated fragments, but reconnecting populations in the core may also require larger remnants to begin with than are readily available. Not surprisingly, the foregoing habitat characteristics influence a habitat’s quality, and small changes can have major effects on animal behavior and thus population dynamics.
For example, ecological studies indicate that the landscape context, such as the percentage of urbanization in the surrounding landscape, may significantly influence the abundance of plants and animals, as well as their distribution within remnant, indigenous habitats. With respect to small species, such as butterflies and bees, the type of grassland may have a significant effect on their richness and composition. In one study, a tall-grass prairie supported much greater numbers of species than did short-grass plots. Habitat quality also affected species richness and composition. As would be expected, areas of low quality generally supported fewer species than moderate- or high-quality sites. Although landscape context did not seem to have a clearly predictable impact on the species richness or composition of butterflies, it is clearly a major factor in accounting for the presence or absence of vertebrate species. These same kinds of wildlife-habitat relationships occur in other ecosystems, such as forests, tundra, and deserts, in every continent across the globe.3
HABITAT EDGES AND CORRIDORS
With respect to the above-mentioned corridors, it makes no difference how large or small, adapted or adaptable a species is; all species require safe corridors in which to travel through hostile terrain. This necessity affects the requirements of a species with respect to how its habitat is configured, which, in large measure, determines its resultant quality.
Despite what skeptics say, observations of movements by naturally dispersing animals within and through fragmented landscapes can demonstrate the value of corridors more convincingly than can controlled experiments of animal movement. Such field observations relate directly to the species of animal and the reason it is moving (e.g., dispersing juveniles or seasonal migrations) to the real landscapes, where the animals must live. Moreover, evidence from well-designed studies indicates that corridors are valuable tools for the maintenance of biodiversity within fragmented landscapes. Therefore, whoever would destroy the last remnants of natural connectivity should be required to bear the burden of proof that destruction of the corridor in question will do no harm—now or in the future.
The isolation of habitat patches is often cited as having a major impact on the dynamics of small populations occupying the fragmented patches in a complex landscape. To test this notion, field surveys of Bachman’s sparrow were conducted, where suitable habitat patches were not only in a linear configuration but also were isolated to varying degrees from the potential sources of dispersing birds. The results demonstrate that isolated patches of habitat in linear landscapes are less likely to be colonized than are more contiguous patches. Therefore, the configuration of habitat patches into a corridor can improve the ability of some species to find and settle in newly created habitats. Clearly, careful landscape design and planning can enhance habitat occupancy at a regional scale for species that do not disperse easily through landscapes.4 Such planning is critical because landscape boundaries (edges of various kinds), although small in spatial extent, often have pronounced effects on the flow of ecological energy, especially when the area adjacent to a corridor is inhospitable to the species in question.
To visualize and appreciate the biophysical effects of a prairie habitat fragmented by barriers and edges of various sorts, one would have to be a gopher or a mouse and see it from their eye-view. Therefore, I employ examples from a variety of ecosystems to illustrate the common effects of such broken-up habitats.
The trend, based on ecological literature, is to treat corridors and boundaries of various kinds as separate phenomena on the landscape. This conceptual approach misses a fundamental commonality, however, and that is their strong influence on the flow of energy through the directional control of organisms and processes, such as ecological feedback loops. Corridors and edges of various types exist at opposite ends of a permeability gradient and thus differ in their effects on the rates and direction that organisms, energy, and processes flow. The position of landscape structures along this gradient can determine its permeability, which depends on attributes of both the structure itself and the influence it has on the movement it allows.5
By way of example, consider the influence of the international railroad line in Mongolia on the winter migration of Mongolian gazelles, which never cross the tracks, even though the best forage is on the opposite side. Although their movements mainly follow the railroad line in winter, the tracks form an effective barrier because they split the gazelles’ habitat.
In this sense, the railroad is an impervious barrier to the gazelles’ access to good-quality winter forage. What would happen to the railroad’s permeability, and thus the flow of ecological energy, if a series of underpasses were constructed to allow the gazelles access to the side with the high-quality forage? What effect would the animals have on the plants and the plants on the animals in an exchange of energy—food for feces and urine? Could plant seeds be moved back forth under the railroad by wind and gazelles?6
However, the permeability of a living structure, such as a riparian area, has different effects when it is part of a continuous forest as opposed to being a mere buffer zone between a stream and a recent clear-cut, which has a much more pronounced edge than a forest. In addition, a deciduous riparian area is one structural habitat in summer when in full leaf but quite another in winter after leaf fall. And the porosity of a corridor’s edge between two habitats at any given season depends on the adaptability of the species involved.
If, for instance, a forested corridor were juxtaposed to a recent clear-cut covered in herbaceous vegetation, deer mice would make the corridor edges entirely porous because they are highly exploratory, extremely adaptable, and thus equally at home in either habitat. In contrast, the southern red-backed vole is a closed-canopy, coniferous-forest specialist, and thus for it the edge is impervious. Therefore corridors between naturally intact forest (or other) habitats would maintain higher population connectivity than would landscapes with discrete anthropogenic habitat patches.7
Another way to increase the permeability of the corridor-edge continuum is to consider that the distance between or among patches of habitat may, in fact, determine the relative effectiveness of corridors and other configurations. Habitat “stepping stones,” for example, could substantially reduce the isolation of remnants in fragmented landscapes for such species as butterflies and wetland birds. When distances between patches are short compared with an animal’s mobility and the hostility of the terrain through which it must travel, a stepping-stone approach may be the most effective way to promote dispersal. Alternatively, continuous corridors may have the highest value relative to other habitat configurations when longer distances separate patches of habitat in fragmented landscapes and the species using the corridor is either a relative habitat specialist or a wide-ranging species (such as the mountain lion) that prefers a more natural habitat.8 Yet another dimension to the porosity of habitat edges is the air.
The landscape at Los Tuxtlas, Mexico, was originally rainforest but is now greatly fragmented and covered with pastures. To understand the ecological ramifications of the seeds that drop in feces from frugivorous bats under isolated trees in the pastures, 652 bats of 20 species were captured, 83% of them fruit bats. Of these, the most abundant species were the little yellow-shouldered bat, Mexican fruit bat, Seba’s short-tailed bat, and Toltec fruit-eating bat. The little yellow-shouldered bat not only was the most abundant species but also was also far the most important dispenser of seeds in the pastures. Of the seeds, 89% were from “zoochorous” forest trees and shrubs, 7 species of which accounted for 79% of the total dispersed seeds: Mexican pepperleaf, semi-epiphytic-strangler fig, trumpet tree, a free-standing fig tree, one tree with no common name, and two shrubs with no common names. (A “zoochorous” is one whose structure adapts it for dispersal by animals: from the Greek zoion, “animal” and chorein, “to make way.” The suffix ous is from Old French, meaning “having” or “full of.”)
As it turns out, both bats and birds are important to the onset of succession in human-created pastures because they carry the seeds of pioneering and primary species (trees, shrubs, herbs, and epiphytes) and deposit them under isolated trees, thereby maintaining plant diversity while beginning to reconnect forest fragments. Consequently, they contribute to the recovery of woody vegetation in disturbed areas within humid, tropical forests.9 If, however, you, the reader, either grew up in a city or now live in one, you may be familiar with habitat corridors in urban settings.
THE URBAN SETTING
Birds in urban landscapes generally occupy parks, which are analogous to forest fragments, whereas tree-lined streets form linear corridors that connect the fragments within the urban matrix. To understand the species-habitat dynamics of an urban setting, a study conducted in Madrid, Spain, examined the effects of street location within the urbanscape, vegetative structure along the streets, and human disturbance (pedestrian and automotive) on the richness of bird species within the street corridors. In addition, the birds’ temporal persistence, density of feeding and nesting guilds, and the probability of a street’s being occupied by a single species were also accounted for.
The number of species increased from the least suitable habitats (streets without vegetation) to the most suitable habitats (urban parks), with tree-lined streets being an intermediate landscape element. Tree-lined streets that connected urban parks were a positive influenced on the number of species within the streets’ vegetation, as well as species persistence, population density, and the probability that the individual species would continue to occupy the streets. Human disturbance, however, exerted a negative influence on the same variables.
Wooded streets could potentially function as corridors that would allow certain species to fare well by supporting alternative habitats for feeding and nesting, particularly those birds that feed on the ground and nest in trees or tree cavities. Local improvements in quality and complexity of the vegetation associated with certain streets, as well as a reduction in the disturbance caused by people, could exert a positive influence on the regional connectivity of streets as a system of urban corridors for birds. Because of the differential use of corridors by species with various habitat requirements, streets as habitat corridors could be further improved by taking the requirements of different species into account.10
In fact, plants may be taking such requirements into account on their own. Although dispersal is a ubiquitous trait among living organisms, evolutionary theory postulates that the loss or death of propagules during dispersal episodes (the cost of dispersal) should select against it. As such, the cost of dispersal ought to be a strong selective force in fragmented habitats. To test this notion, patchy populations of the French hawksbeard were studied in small patches on sidewalks and around trees planted within the city of Montpellier, in southern France.
French hawksbeard is a Mediterranean composite in the aster family that spreads predominantly in cultural landscapes, including cities. This annual germinates with fall rains and persists during winter in rosette-leaf stage. Generalist insects, especially the domestic honeybee, pollinate the flower in town. Cross-fertilization among unrelated individuals of hawksbeard accounts for 80 to 90% of the pollination in the countryside and 60 to 90% in town.
French hawksbeard produces two types of seeds (i.e., it has dimorphic seeds): a small seed with a pappus and a large seed without it. (A pappus is a parachute-like structure attached to one end of the seed, which allows wind to disperse it.) The seed with a pappus favors dispersal over long distances, whereas the seed without a pappus falls to the ground near the parent plant.
Within the city, seeds with a dispersal parachute have a 55% greater chance of falling on concrete, which is unsuitable for germination, than do seeds without a parachute, which land at the base of the parent plant. The proportion of non-dispersing seeds is significantly higher in a city if the urban patches form a relatively fragmented environment, as opposed to contiguous populations in rural areas, which predominantly have dispersal parachutes. Because of the recent fragmentation of continuous habitat, this pattern of dispersal is consistent with rapid evolution, which occurs over 5 to 12 generations of selection as a result of the high cost of aerial dispersal in urban settings.11 In this way, seeds are accumulating within urban habitat corridors that could increasingly serve such seed-eating birds as the European goldfinch.
Although corridors clearly work for animals, what about corridors for plants? To answer this question an investigation was launched into the role of corridors in seed dispersal of the rare, pond-dwelling, self-fertilizing buttercup in the Fontainebleau Forest of France. The connection of ponds through temporarily flooded natural corridors was found to facilitate the migration of seeds. As a result, a pond was more likely to be colonized by the rare buttercup when it was connected to other occupied ponds.12 Corridors are thus a critical element of a landscape’s structure not only for animals but also for the persistence of certain species of plants living in fragmented habitats, where seed dispersal between and among habitat patches is essential.
1. Mark V. Lomolino and David R. Perault. Body Size Variation of Mammals in a Fragmented, Temperate Rainforest. Conservation Biology, 21 (2007):1059–1069.
2. The preceding discussion of habitat configuration is based on: (1) James R. Herkert, Dan L. Reinking, David A. Wiedenfeld, and others. Effects of Prairie Fragmentation on the Nest Success of Breeding Birds in the Mid-Continental United States. Conservation Biology, 17 (2003):587–594; (2) Esa Huhta, Teija Aho, Ari Jäntti, and others. Forest Fragmentation Increases Nest Predation in the Eurasian Treecreeper. Conservation Biology, 18 (2004):148–155; (3) Sharon K. Collinge, Kathleen L. Prudic, and Jeffrey C. Oliver. Effects of Local Habitat Characteristics and Landscape Context on Grassland Butterfly Diversity. Conservation Biology, 17 (2003):178–187; (4) Ewers and Didham. The Effect of Fragment Shape and Species’ Sensitivity to Habitat Edges on Animal Population Size. Conservation Biology, 21 (2007):926–936; (5) Matthew R. Falcy and Cristián F. Estades. Effectiveness of Corridors Relative to Enlargement of Habitat Patches. Conservation Biology, 21 (2007):1341–1346; (6) Tim Gerrodette and William G. Gilmartin. Demographic Consequences of Changed Pupping and Hauling Sites of the Hawaiian Monk Seal. Conservation Biology, 4 (1990):423–430; and (7) Maiken Winter and John Faaborg. Patterns of Area Sensitivity in Grassland-Nesting Birds. Conservation Biology, 13 (1999):1424–1436.
3. Paul Beier and Reed F. Noss. Do Habitat Corridors Provide Connectivity? Conservation Biology, 12 (1998):1241–1252.
4. John B. Dunning Jr., Rene Borgella Jr., Krista Clements, and Gary K. Meffe. Patch Isolation, Corridor Effects, and Colonization by a Resident Sparrow in a Managed Pine Woodland. Conservation Biology, 9 (1995):542–550.
5. The preceding discussion of corridors and boundaries (edges) is based on: (1) Craig S. Machtans, Marc-André Villard, and Susan J. Hannon. Use of Riparian Buffer Strips as Movement Corridors by Forest Birds. Conservation Biology, 10 (1996):1366–1379 and (2) Linda M. Puth and Karen A. Wilson. Boundaries and Corridors as a Continuum of Ecological Flow Control: Lessons from Rivers and Streams. Conservation Biology, 15 (2001):21–30.
6. The preceding two paragraphs are based on: Takehiko Y. Ito, Naoko Miura, Badamjav Lhagvasuren, and others. Preliminary Evidence of a Barrier Effect of a Railroad on the Migration of Mongolian Gazelles. Conservation Biology, 19 (2005):945–948.
7. Stephen G. Mech and James G. Hallett. Evaluating the Effectiveness of Corridors: A Genetic Approach. Conservation Biology, 15 (2001):467–474.
8. (1) Paul Beier. Determining Minimum Habitat Areas and Habitat Corridors for Cougars. Conservation Biology, 7 (1993):94–108; (2) Susan M. Haig, David W. Mehlman, and Lewis W. Oring. Avian Movements and Wetland Connectivity in Landscape Conservation. Conservation Biology, 12 (1998): 749–758; and (3) Nick Haddad. Corridor Length and Patch Colonization by a Butterfly, Junonia coenia. Conservation Biology, 14 (2000):738–745.
9. The two previous paragraphs are based on: Jorge Galindo-Gonzalez, Sergio Guevara, and Vinicio J. Sosa. Bat- and Bird-Generated Seed Rains at Isolated Trees in Pastures in a Tropical Rainforest. Conservation Biology, 14 (2000):1693–1703.
10. The three preceding paragraphs are based on: Esteban Fernández-Juricic. Avifaunal Use of Wooded Streets in an Urban Landscape. Conservation Biology, 14 (2000):513–521.
11. The preceding discussion of the French hawksbeard is based on: P.O. Cheptou, O. Carrue, S. Rouifed, and A. Cantarel. Rapid Evolution of Seed Dispersal in an Urban Environment in the Weed Crepis sancta. Proceedings of the National Academy of Sciences, 105 (2008):3796–3799.
12. Florian Kirchner, Jean-Baptiste Ferdy, Christophe Andalo, and others. Role of Corridors in Plant Dispersal: An Example with the Endangered Ranunculus nodiflorus. Conservation Biology, 17 (2003):401–410.
Text © by Chris Maser 2012. All rights reserved.