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There are three types of barriers, physical , climatic and biological barriers that can prevent the dispersal of organisms.
Topographic barrier falls in the category of physical barrier but exactly what should be considered as topographic barriers is not clear to me.
The source from which I actually came across the term says that mountain range is a topographic barrier like the Himalayan range separating the North-asian fauna (partly Palearctic fauna) from the Oriental fauna. But what else falls under topographic barriers is my question?
Topograhpy refers to earth's physical features and a topographic barrier refers to physical features that prevent free movement from one position to another. As GForce pointed out, whether or not something is a barrier can depend heavily on the animal in question. A long, wide canyon can be a barrier for squirrels, but not for birds.
Think of topographic as referring to features on a map, which is a type of topological graph.
Whether something is considered a topographic barrier for an organism depends on the species being considered and its ability to traverse the terrain from one side of the barrier to the other. There's no blanket classification of what is considered a topographic barrier and what isn't.
For example, a certain mountain range may be a topographic gradient to a salamander (due to limited food, extreme weather conditions, etc.), but these same mountains might not be for a brown bear.
What Is Topography?
Topography is a broad term used to describe the detailed study of the earth's surface. This includes changes in the surface such as mountains and valleys as well as features such as rivers and roads. It can also include the surface of other planets, the moon, asteroids and meteors. Topography is closely linked to the practice of surveying, which is the practice of determining and recording the position of points in relation to one another.
Speciation, the process by which a single species evolves into two or more, is difficult to observe directly because of the long span of time it usually takes to occur. Nonetheless, biologists have been able to infer much about speciation by examining geographic variation within and between species. A striking pattern that emerged about a century ago is known as Jordan's law : given any species, the most closely related species is found 'in a neighboring district separated from the first by a barrier of some sort or at least by a belt of country, the breadth of which gives the effect of a barrier.' The role of such barriers in speciation is perhaps best illustrated by the rare phenomenon known as 'circular overlaps'  or 'ring species' , when two coexisting but reproductively isolated forms are connected by a long chain of populations encircling a geographic barrier, and traits change gradually from those of one form to the other around the ring  (Figure 1). The great evolutionary biologist Ernst Mayr called such situations the 'perfect demonstration of speciation'  since they allow one to use geographic variation to infer how evolutionary change in time led to the differences between species.
Map of the geographic distribution of an idealized ring species. Two forms (red and blue species A and B) have come into contact (perhaps with some overlap) but do not interbreed directly. They are connected by a long chain of populations encircling a geographic barrier, through which the traits of species A gradually change into the traits of species B. If the order of colonization can be inferred, then one can infer the location of the common ancestor (here, in yellow) and how range expansion around the barrier and the accumulation of small evolutionary changes led to the formation of two species.
Until now, our knowledge of the diversity of ring species has arisen primarily from the field of taxonomy, with experts on the taxonomy of particular groups occasionally noticing a pattern of gradual variation between quite divergent forms. This somewhat haphazard approach has led to a variety of ring species being proposed [2, 4], only some of which have held up to further scrutiny [4, 5]. Only two well-studied cases are generally accepted as solid examples of ring species: these are the Ensatina eschscholtzii salamander complex in California  and the Phylloscopus trochiloides greenish warbler complex in Asia . One challenge in relying on taxonomists to discover ring species is that the naming rules of taxonomy generally conceal their existence: taxonomists have to decide whether a group of specimens is two species or one species the taxonomic naming system does not lend itself toward describing gradients between two species .
The study by Monahan et al.  proposes a novel approach to the discovery of ring species, focusing on geography rather than taxonomy as the starting point. They ask an intriguing question: where in the world are there barriers that might promote ring speciation? A topographic model, based on slope of the landscape, is used to identify potential geographic barriers worldwide. In the model, barriers are regions that have either more or less slope than the regions around them. The characteristics of the potential barriers, such as size and shape, are then compared with those of known barriers in two ring species (E. eschscholtzii salamanders and P. trochiloides greenish warblers) and two groups that have been proposed as ring species and share many of their characteristics (Acacia karoo trees and Larus gulls). Known barriers are similar to only a small proportion of all potential barriers, suggesting that ring species barriers have common characteristics. The authors also show maps of a small subset of the potential barriers that are similar to the real ring species barriers, suggesting that these may be good locations to look for ring species.
Though the current model is based solely on slope, other geographic and environmental variables could eventually be incorporated to enhance the effectiveness of the model in identifying some barriers in species distributions. In particular, it may be advantageous to introduce elevation as a geographic variable in the model. The current use of slope results in two sorts of 'barriers' being identified: 1) areas of high slope, such as mountain ranges, escarpments, or ocean trenches, surrounded by areas of low slope such as plains, plateaus, or ocean basins and 2) areas of low slope surrounded by those of high slope. As a result, some of the barriers identified by this model are peculiar: for example, in the first case, an area of flat land bordered on one side by a steep climb toward higher elevations and on the other side by a steep drop toward lower elevations in the second case, a steep escarpment between a high plateau and a low plain. In both of these, it seems unlikely that a species could live in all areas encircling the 'barrier' without also inhabiting the 'barrier' itself. Rather, it seems that the optimal topographic model would use some combination of both slope and elevation to identify barriers. Elevation is also likely to work better than slope in describing the Arctic Ocean barrier in the case of the Larus gull ring the slope-based model results in three separate barriers corresponding to deep ocean basins, which the authors then joined as a composite barrier (see , their Figure 2D). It seems that slope on the deep ocean floor is of little relevance to describing the distribution of a bird species, whereas elevation (for example, above or below sea level) is of substantial importance.
Environmental variables such as climate or vegetation could also be incorporated into the model. For instance, with respect to the central Asian barrier that the greenish warbler encircles, Monahan et al. find that their model did not identify a single barrier - rather, they construct a composite barrier out of two separate barriers identified by the model. They remark that, in cases such as this, 'it is difficult to imagine any univariate or multivariate environmental approximation of a single barrier (for example, Central Asia, which is comprised of the Takla Maka-Gobi deserts and the Tibetan Plateau - large geographic regions that differ dramatically in terms of climate and vegetation).' However, a good explanatory variable has been identified in this case: greenish warblers inhabit forests , and maps of forests in Asia (for example, ) show a large gap that includes the Tibetan Plateau as well as the Taklamakan and Gobi deserts. Other examples of large potential barriers that show up clearly when considering a basic environmental variable (wet versus dry) are Antarctica, Australia, and Greenland (for marine and/or terrestrial coastal organisms), which were missed by the current topographic model. It is clear that the addition of other topographic and environmental variables could greatly enhance the precision of the model, and Monahan et al.  emphasize that their general approach can be modified to work with any kind of continuously distributed environmental variable, making it of wide applicability to many different types of investigations into barriers to dispersal that may contribute to speciation.
Finally, the very large number of potential barriers identified by the topographic model (952,147, about 10,000 of which are 'topographically similar' to those associated with known ring taxa ) raises another issue. Given the very large number of identified candidate barriers, it is almost inevitable that at least one will be associated with any interesting species complex that we might point to as a candidate for ring speciation, and this means that the predictive value of the model will depend on further refinement. Despite these issues, it is likely that the present model represents an important first step in this geography-oriented approach to the analysis of barriers involved in both ring speciation and speciation more generally. The approach proposed by Monahan et al.  will likely be adapted to incorporate multiple variables (in addition to slopes), and this will allow more refined identifications of a smaller number of potential barriers, resulting in more useful predictions. The discovery and inclusion of more ring species (for example, the willow warblers Phylloscopus trochilus, which display a form of incipient ring speciation around the Baltic Sea [5, 10]) will likewise allow further refinement of the model, perhaps eventually allowing an analysis of what types of barriers are associated with ring species from different taxonomic groups. By applying an explicit geographic framework to the analysis of ring species, Monahan et al. have pioneered an interesting new approach to the study of the relationship between geography and speciation. In the years ahead, it will be exciting to see whether additional ring species are identified using this geography-oriented approach.
What is meant by topographic barrier? - Biology
source: slideplayer.com fig: Topographical factor
The factors concerned with topography or physical features of an area are called topographic factors. Topographic factors include height, direction of slope, steepness of the slope. The topographic factors are also called indirect factors as they influence the growth and development of organisms by bringing variations in climatic factors. Some of the effects of topographic factors are as follow
Height or altitude
The various effects of height or altitude on plants and animals can be seen from sea level to the high hills and mountains. An increase in the height or altitude results decreases in temperature, high wind, velocity, low atmospheric pressure, high humidity and rainfall.
Direction of slope
North and South faces of hill possess different types of flora and fauna because they differ in their humidity, rainfall, light intensity, light duration and temperature regimes. It is because the two faces of the hill receive different amounts of solar radiations and wind action. Out of these two faces, the sunward direction possesses good vegetation whereas the poor vegetation on the opposite direction due to moist conditions. Similarly, the centre and edge of a pond possess different depths of water and different wave action. Therefore, different parts of the same area may possess different species of organisms.
Steepness of the slope
The steepness of the slope allows the rapid water current, water deficit, the quick erosion of the top soil and thus the poor vegetation occurs. On the other hand, the plains and valleys are rich in vegetation due to the slow movement of surface water and due to the better accumulation of water in the soil.
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Things to remember
- The factors concerned with topography or physical features of an area are called topographic factors.
- The topographic factors are also called indirect factors as they influence the growth and development of organisms by bringing variations in climatic factors.
- The various effects of height or altitude on plants and animals can be seen from sea level to the high hills and mountains.
- The steepness of the slope allows the rapid water current, water deficit, the quick erosion of the top soil and thus the poor vegetation occurs.
- It includes every relationship which established among the people.
- There can be more than one community in a society. Community smaller than society.
- It is a network of social relationships which cannot see or touched.
- common interests and common objectives are not necessary for society.
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What will climate change and sea level rise mean for barrier islands?
A new survey of barrier islands published earlier this spring offers the most thorough assessment to date of the thousands of small islands that hug the coasts of the world's landmasses. The study, led by Matthew Stutz of Meredith College, Raleigh, N.C., and Orrin Pilkey of Duke University, Durham, N.C., offers new insight into how the islands form and evolve over time -- and how they may fare as the climate changes and sea level rises.
The survey is based on a global collection of satellite images from Landsat 7 as well as information from topographic and navigational charts. The satellite images were captured in 2000, and processed by a private company as part of an effort funded by NASA and the U.S. Geological Survey.
During the 20th century, sea level has risen by an average of 1.7 millimeters (about 1/16 of an inch) per year. Since 1993, NASA satellites have observed an average sea level rise of 3.27 millimeters (about 1/8 of an inch) per year. A better understanding of how climate change and sea level rise are shaping barrier islands will also lead to a more complete grasp of how these dynamic forces are affecting more populated coastal areas.
Stutz, the study's lead author, highlighted a series of key findings from the new survey during an interview with a NASA science writer.
Every barrier island is unique
Every island chain has a complex set of forces acting on it that underpin how islands form and how they're likely to change over time. Barrier islands often develop in the mouths of flooded river valleys as sea level rises, but they can also form at the end of rivers as sediment builds up and creates a delta. Other important factors in barrier island formation include regional tectonics, sea level changes, climate, vegetation and wave activity. "Understanding how such forces impact barrier islands is the key to understanding how climate change will affect our coastlines," noted Stutz.
Sea level rise can eliminate -- or create -- barrier islands
Scientists estimate that the rate of sea level rise will likely double or triple in the next hundred years due to climate change. Paradoxically, gradual sea level rise can generate new barrier islands. Rising seas create shallow bays that develop barrier islands in the mouths of the bays along certain types of coastline.
Stutz's analysis found rising sea level in the last 5,000 years is associated with the greatest barrier island abundance, especially in the North Atlantic and Arctic. Stable or falling sea level, meanwhile, a pattern more typical of the Southern Hemisphere in the last 5,000 years, has produced fewer islands and a higher percentage of islands along river deltas.
However, extremely rapid sea level rise -- especially when coupled with decreases in sediment supply -- can simply inundate islands causing them to break up and disappear. Islands are eroding rapidly along the Mississippi Delta, Eastern Canada and the Arctic for these reasons.
"However, rising sea level is not just like pouring more water into a bathtub," Stutz emphasized. Islands react differently based on the geology in a region and how the waves and tides in an area are affected. People tend to assume sea level rise means fewer islands no matter what, but the rate of rise is critical."
There are far more barrier islands than previously thought
A survey conducted by the same researchers tallied 1,492 barrier islands in 2001, but Stutz and Pilkey counted more than 2,149 this time. The difference: the researchers had access to higher-quality satellite imagery that covered a larger portion of the globe than they did last time. "It's not that 657 islands appeared overnight. We simply did a more thorough job of counting what was already out there," said Stutz. The researchers counted extensive island chains in Brazil, Madagascar and Australia that the previous survey had left out.
Barrier islands cluster along tectonically calm coasts
Stable coasts, such as the eastern coast of the United States, tend to have wide, low relief areas with shallow estuaries that are conducive to barrier island formation. In contrast, continental margins near actively colliding plates, which generate earthquakes and volcanoes, produce fewer barrier islands. At active margins, such as the rocky cliffs along the Pacific, steep grades typically dominate coastal areas and prevent the formation of islands.
Northern and Southern hemisphere islands differ
The Northern Hemisphere is home to the majority -- 74 percent -- of barrier islands. That's not surprising because the Northern Hemisphere contains about the same proportion of land. A less intuitive insight: the majority of Northern Hemisphere islands are in high-latitude Arctic or temperate climate zones, while most Southern Hemisphere islands are tropical. Why the discrepancy? Relative sea levels have fallen slowly in much of the Southern Hemisphere for the last 5,000 years, but the opposite has happened in the Arctic.
Storms are key molders of barrier island shape
Storms tend to cause islands to retreat, carve new inlets that make them shorter and more numerous, and sometimes destroy them completely. The frequency of storms varies by latitude and climate. The Arctic and most temperate coasts experience regular storms, while more tropical areas experience few storms and more gentle swells most of the year, conditions that encourage the formation of sandy beaches. Major storms can cause drastic changes to barrier islands. After Hurricane Katrina, for example, many islands in the Mississippi River Delta were destroyed or radically changed.
Arctic barrier islands are retreating the fastest
Barriers islands in the Arctic make up nearly a quarter of the world's barrier islands, and they're more vulnerable to climate change than islands anywhere else in the world. The reason: melting of sea ice and the permafrost that buffers Arctic islands from waves have left them susceptible to constant pounding from storms. Recently measured erosion rates in the Beaufort Sea show Arctic barrier islands eroding three to four times faster than islands in the continental United States. Any further acceleration in erosion rates could result in the rapid breakup of many Arctic islands, Stutz's analysis noted.
More research is needed, especially on a local scale
Coastal areas will likely experience major changes in sea levels this century due to climate change. The shifts, however, will be anything but uniform. NASA research shows that some coasts are experiencing sea level rise significantly faster than the global average of 3.27 millimeters (about 1/8 of an inch) per year, while other areas are experiencing slower rates of rise and even falling sea levels. "It would be nice if we could say we can predict exactly how a given island or island chain will react to rising sea levels or some other environmental change, but we're simply not there yet for most islands, especially for many tropical islands where research dollars are scarce. We're still a long way from being able to accurately model how an individual island will change as a result of climate change or even simple development pressure," said Stutz.
Materials provided by NASA/Goddard Space Flight Center. Original written by Adam Voiland, NASA's Earth Science News Team. Note: Content may be edited for style and length.
What is meant by topographic barrier? - Biology
Fostering Higher Levels Of Scientific Literacy: Confronting Potential Barriers To Science Understanding
University of New Brunswick
The nature of science literacy and the possibility of being scientifically literate are critical debates within the science and science education community (e. g. Hodson, 1993, 1998 Shamos, 1996). Another critical aspect of the science literacy debate is why so few learners choose to or are able to pursue science studies or science careers. It is this aspect of science literacy that interests me. What is it about learning science that is so difficult or challenging that most students choose not to continue taking science beyond the required courses and most adults feel uncomfortable participating in science-related debates, even those that impact their communities? Part of the answer is addressed by Derek Hodson elsewhere in this issue B the understanding of science portrayed in schools does not reflect the nature of science as it is practiced or as it influences decision making. Nor do the science experiences provided in schools prepare graduates to participate as informed citizens. Another part of the answer, and the focus of this article, is that there are potential barriers which can make science confusing and even nonsensical to students.
Research over the last forty years reveals four potential barriers to learners developing successful science understandings B prior experiences and beliefs, language, a learner's preferred way(s) of meaning making, and culture. In this article, I consider two questions B in what ways can each of these four potential barriers inhibit learners' understandings and what are the implications of not addressing these potential barriers?
Prior Experiences as a Potential Barrier
Before children enter school, they construct descriptions of the world around them that may be different from the descriptions scientists use. Using interviews and observing children solving problems, researchers from a number of countries are interested in the kinds of explanations children develop about their world (Driver, 1985, 1994 Harlen, 1992, 1996 Pfundt & Duit, 1991). One thing seems clear even young children are likely to hold on to their own explanations about the world despite what they are told in school. Unless students are faced with experiences that challenge their conceptions, they are unlikely to change their models of how things work or accept alternative explanations/descriptions as useful or important (Suping, 2003).
For example, what is a bounce? Young children are likely to say a bounce is what happens when something hits the floor or wall and doesn't break. With a number of more years of schooling, the preservice teachers in my "Introduction to Teaching Science" course describe a bounce as an object changing direction when it hits another object and that pieces of broken objects can bounce. When pushed, they say that even if it is only millimetres the object must leave the floor or wall to be considered a bounce. Both of these explanations are different from the current scientific definition of bounce.
Within the science community, a bounce is described in terms of a collision. A collision occurs when any two surfaces come into contact collisions are either elastic or inelastic. Why do scientists find it easier to think of the contact of one object with another as a kind of collision? Most likely, because they explored more kinds of objects contacting one another than one person would encounter in their own environment. For scientists, the definition of bounce used by children to get around the house and stay out of trouble when throwing the ball is just not adequate. DiSessa (1983) calls these childhood concepts developed prior to formal instruction phenomenological primitives. These explanations are embedded in the learners' models of the world before they are introduced to scientists' explanations. One outcome of these differences in explanations is that science appears to be "unnatural." When we teach science as the way the world works, science descriptions carry a sense of truth. What happens to the learners' own explanations of the world? If the science descriptions are in conflict with explanations held by the student, by other people in the student's life, or by people within the student's culture, we may create conflict within the student. Especially, if we treat the science description as the truth, require that in school you will give the scientist's answer and, as a result, discredit other explanations.
Language as a Potential Barrier
There are a number of ways language can make understanding science more difficult, such as alternative meanings of words, students' lack of appropriate vocabulary, the specialized vocabulary used by scientists, and English as a second language. A second outcome, for children who cling to their own explanations, may be a feeling of disenfranchisement. Students may begin to separate school explanations and home explanations. Or, students may begin to believe they are unable to learn science--it is just too difficult to figure out. Still others may reject the science explanation as all too unplausible and accept their own or their community's explanation instead.
Learners may develop an understanding of the meaning of certain words that is different than the scientists' meaning for these words. People outside the science community and scientists themselves give these same words other meanings and/or use them in other contexts, resulting in slight nuances to the original meaning. These alternative meanings can make understanding and/or accepting the scientist's use of the word or term difficult. For example, the concepts living and nonliving are commonly introduced in the primary grades. The meaning of the terms living and nonliving are confounded by the meaning of the terms alive and dead. Interviews with primary-aged children reveal that many of them consider cars, batteries, and fire as living and not unreasonably so. In everyday language we describe those and other nonliving objects as being alive, e.g. a live wire or the fire "came to lift" when we added wood, or as having died, e.g. the car or battery died. Learners also have trouble accepting that wood for the fireplace, bones that their dogs chew, and leather gloves are categorized by scientists as living.
Community is another concept often introduced in elementary school. Scientists define a community as the interaction of living organisms within a bounded system. A community could vary in size from a drop of water to a log or pond or entire forest depending where the boundaries are established. Within the general culture, communities are determined by groups of residents who have some common identity. Communities in this sense focus on the activities, needs and care of human beings. A scientist on the other hand treats the human being as one species among many, with a specific habitat (address) and niche (job/function) within the community.
A final example is the concept force. We talk about force as one aspect of a field of influence surrounding objects. That is, a force field is a complex of pushes and pulls. However, the everyday use of the term force includes such phrases as, "I was forced to go to bed without my dinner", "Someone forced their way into the house", "My Mom works on the police force," and in the movies, "May the force be with you." Young learners must grapple with a range of meanings for most terms. How do they decide which is the "right" meaning or which meaning is "right" in which situation? These distinctions may be some of the most challenging aspects of learning science there are and contribute to children's beliefs that science is unnatural.
There is a gap between our ability as learners to observe and the language available to communicate our observations and thoughts between what I call knowledge and information. Exploring the properties of objects is common throughout students' science learning. Some of these properties are colour, smell, shape, size, weight, distance, texture, taste, sound, flexibility, chemical reactivity, and pattern. Students may find it difficult to be "successful" observers in each of these areas if they lack the vocabulary to capture and share their observations. For example, students may know there are differences in sounds or colour, but lack vocabulary to differentiate particular colours or sounds. How many smells are we able to describe only as "stink" how large is our stink vocabulary? It seems this gap is even more problematic when learners are unable to articulate ideas which they "feel" they know but are unable to defend their choice of solution or explain how they decided on an answer beyond a shoulder shrug or "I don"t know." Consequently, I believe students who say, "I know, I just don't know how to explain it."
When asked to develop a list of words that describe various properties, preservice teachers can list fifty or more in each category. The English language is rich in synonyms to capture nuance. If we want to close the gap between what even young learners are able to observe and think we must provide them with the sensory vocabulary to share their ideas and understandings. In addition, scientists use words that are not used in everyday language. One study indicates that over 750 new science related terms are introduced from kindergarten to grade six (Scruggs & Mastropieri, 1993). In addition, some young learners require more time than others to develop reading and writing skills. If they are expected to understand that the meaning of words can change in different contexts, the task of reading and writing can be that much more difficult.
Science language can be even more challenging for ESL learners, especially if science words have different meanings within the school or community. When students are learning the school language as a second or third language, the student's intellectual, social, and physical capabilities may be masked. Research indicates that language can interfere with students' test results and interactions between students and their teachers (Mastropieri & Scruggs, 1991).
Culture as a Potential Barrier
Culture is the milieu in which a person lives there are multiple cultures in our lives which we either participate in or observe. A learner may move from home to community to school to religious or social group to sports group in the course of a day. Some of these cultural encounters mesh seamlessly with our expectations. We are comfortable there and even like being there. Other cultural encounters are different and don't meet our expectations B we are less comfortable or may feel alienated. Science is an enterprise B it is something a group of people do and as such, there is a culture of science. What happens when learners are introduced to/encounter the culture of science in schools?
Most of us teach in increasingly multicultural classrooms. Young learners often come to school with different explanations of the same phenomena that scientists are interested in describing. Whether from family, religious or other cultural origins, these explanations may make accepting the science descriptions problematic. As with models of the world, young learners construct from their own experiences. These cultural models may be considered "natural" while the science explanations are considered "unnatural" or "counter intuitive." Another consequence of differing explanations is that young learners could be "caught" between their culture and their teacher. Having to choose between explanations valued in school and those valued by their parents and/or members of their community can cause stress and perhaps rejection of one view or the other.
The culture of science itself is poorly represented in the experience of many young people. The problem is not just insufficient science in the school curriculum, but that science and technology are presented in the schools from a knowledge-based perspective, typically divorced from social, political, and ethical considerations and debate. Such problems are most acute in relatively rural, economically-undeveloped areas such as Atlantic Canada, where the lack of technical and scientific infrastructure outside the schools gives students little exposure to science and technological culture through avenues other than the standard school curriculum. The dominant cultural group (science versus other knowledge or dominant versus minority groups) does not always value and/or understand other cultural groups. Young students may come from local traditions that may be different than those of their teacher or schools. For example, the way the children interact in school and interact with their family and community may be different in terms of what knowledge, measures of success, or behaviour are valued.
Neglect of science-as-culture can lead to a clash of culturally-based, local knowledge with scientific knowledge and the culture it represents. The well-documented failure of communication between fishers and federal fisheries scientists that contributed to the collapse of the Newfoundland cod stocks in the early 1990s is a vivid example of this dangerous problem. Finlayson (1994) documents how federal scientists charged with managing fish stocks often ignored the information and insights of local resource users, while resource users in turn mistrusted scientists and lacked sufficient understanding of their methods and aims to enter into a dialogue. The result was an environmental and human tragedy rooted in a clash of cultures.
If students are to be prepared for a technological world, and if the school science reform is to positively impact all students, then teachers, researchers, and policy-makers have to recognize the culture of science and how it is reflected in the schools. A well-documented consequence of not dealing with the culture of science and technology is that student interest in science and mathematics typically fades after the early grades. Fewer students opt for post-secondary concentrations, and attitudes and opinions about science shared by students and parents are shaped more by popular culture, mass media, and entertainment than by formal learning in science classrooms (Osbourne, 2003 Peacock,2000 Schibeci & Lee, 2003 Solomon, 1996).
Preferred Ways of Learning as a Potential Barrier
Educators believe we all have preferred ways of making sense of the world. The challenge is finding a way to describe these different ways of making meaning more challenging is finding ways to teach that first address and later expand each student=s ability to learn in different ways. Different models have been developed to describe these preferences B learning styles and multiple intelligences are examples of current models. Learning styles models suggest people prefer to understand the world by relying on one or two of their senses predominately. These preferences are referred to as learning modalities or in some cases, learning styles. The four modalities most often recognized are visual, auditory, tactile, and kinesthetic. One study suggests attending to students= learning styles results in improved achievement scores and behaviour (Klavas, 1994). In that study, where over half the students preferred tactile or kinesthetic modalities, students were presented concepts first in their preferred learning styles, next in their second for practice, and finally reviewed the ideas verbally. According to another learning styles researcher, science needs to be taught as more than a subject and a method. Some learners= styles connect with the world less through logic than with aesthetics and feelings, through affective avenues, personal commitment, and acting (Samples, 1994).
Howard Gardner (1993 1995) proposes another model which he calls multiple intelligences. Gardner defines intelligence as abilities to solve problems recognized as valuable within a culture. He identifies eight intelligences -- linguistic, logical-mathematic, spatial, musical, bodily-kinesthetic, interpersonal, intrapersonal, and naturalistic -- as a staring point in the discussion and argues that there may be other intelligences or even subintelligences. In posing his theory of multiple intelligences, Gardner argues that "school should be to develop intelligences and to help people reach vocational and avocational goals that are appropriate to their particular spectrum of intelligences" (p.9). He contends that linguistic and logical-mathematical intelligences are most valued in schools today and that learners whose strengths are not in those areas often find school an unsuccessful experience.
Even when the spectrum of intelligences is identified, young learners can face difficulties in having their particular strengths and interests recognized. Although there is growing evidence that broadening our notions of intelligence and using an activity-based as well as language-based assessment instruments provides us with better information about young learners, Gardner argues the work in this area must be considered promising but not conclusive. Most instruction, especially in middle and high school, favours visual and auditory learning styles and linguistic, logical, and mathematics intelligences over others. Moreover, school science portrays the processes used by the science community as visual/auditory and logical/linguistic when we know imagination and creativity are also necessary.
While educators acknowledge we all learn differently, it is important to note that there is less agreement about which of the models/theories best accounts for that difference (Miller, 2001 Oneil, 1990 Stellwagen, 2001). As educators, we need to sort through the literature for ourselves and decide which models provide the best insight to address the needs of our students.
Until we begin grade by grade, unit by unit, experience by experience to consider the possibility of potential barriers to learners understanding scientists= ideas and ways of working, we will continue the present pattern of most students having negative science experiences and feeling disenfranchised. Students will continue to choose not to study science when given the choice and not to pursue science careers. Most high school students will become adults who are uncomfortable discussing science and who feel incompetent to challenge the science ideas and research that impact their lives.
There is considerable research describing students= alternative conceptions of scientists= explanations and definitions. Science education leads the research in this area with researchers in social studies and other disciplines beginning to build on their research. What we need is to apply the research locally. Each of us as teachers needs to look critically at the science curriculum for concepts, language, and experiences that could act as potential barriers for our students understanding science. Once these potential barriers are identified, we need to make talking about them with students -- that is, confronting the discrepancies between our everyday beliefs and explanations with scientists= explanations B part of the content of our curriculum.
The consequence may be that we need to reduce the number of science concepts we want students to learn initially and provide them time and experiences that allow them to grapple with these differences. If learners acknowledge that scientists think and work differently than others and explore ways in which scientists think and work, we will have more students who are more comfortable with and want to participate in the culture of science.
DiSessa, A. (1987). Phenomenological primitives. In E. Fischbein (Ed.), Intuition in science and mathematics: An educational approach. Dordrecht, Netherlands: D. Reidel Publishing Company
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Review: The blood-brain barrier protecting the developing fetal brain
While placental function is fundamental to normal fetal development, the blood-brain barrier provides a second checkpoint critical to protecting the fetal brain and ensuring healthy brain development. The placenta is considered the key barrier between the mother and fetus, regulating delivery of essential nutrients, removing waste as well as protecting the fetus from potentially noxious substances. However, disturbances to the maternal environment and subsequent adaptations to placental function may render the placenta ineffective for providing a suitable environment for the developing fetus and to providing sufficient protection from harmful substances. The developing brain is particularly vulnerable to changes in the maternal/fetal environment. Development of the blood-brain barrier and maturation of barrier transporter systems work to protect the fetal brain from exposure to drugs, excluding them from the fetal CNS. This review will focus on the role of the 'other' key barrier during gestation - the blood-brain barrier - which has been shown to be functional as early as 8 weeks' gestation.
Keywords: Blood-brain barrier Drug transporters Fetal brain development P-glycoprotein.
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Homing abilities can be used to find the way back to home in a migration. It is often used in reference to going back to a breeding spot seen years before, as in the case of salmon. Homing abilities can also be used to go back to familiar territory when displaced over long distances, such as with the red-bellied newt.
Some animals use true navigation for their homing. This means in familiar areas they will use landmarks such as roads, rivers or mountains when flying, or islands and other landmarks while swimming. However, this only works in familiar territory. Homing pigeons, for example, will often navigate using familiar landmarks, such as roads.  Sea turtles will also use landmarks to orient themselves. 
Many animals use magnetic orientation based on the Earth's magnetic field to find their way home. This is usually used together with other methods, such as a sun compass, as in bird migration and in the case of turtles. This is also commonly used when no other methods are available, as in the case of lobsters,  which live underwater, and mole rats,  which home through their burrows.
Celestial orientation, navigation using the stars, is commonly used for homing. Displaced marbled newts, for example, can only home when stars are visible. 
There is evidence that olfaction, or smell, is used in homing with several salamanders, such as the red-bellied newt.  Olfaction is also necessary for the homing of salmon. 
Topographic memory, memory of the contours surrounding the destination, is one common method for navigation. This is mainly used by animals with less intelligence, such as molluscs. Limpets use this to find their way back to the home scrape although whether this is true homing has been disputed. 
Summary – Dispersal vs Vicariance
Dispersal and vicariance are two alternative biogeographic processes that explain disjunct distribution of organisms. Both processes cause the isolation of a population by a geographic barrier. In dispersal, the separation of a population occurs when a part of population migrates across a preexisting geographical barrier. In vicariance, the separation occurs due to the appearance of a new geographical barrier that divides the population. Thus, migration is responsible for dispersal while appearance of a new geographical barrier is responsible for vicariance. This is the summary of difference between dispersal and vicariance.
1. Sanmartín, Isabel. “Historical Biogeography: Evolution in Time and Space.” SpringerLink, Springer US, 21 June 2012, Available here.
2. “Allopatric Speciation.” Wikipedia, Wikimedia Foundation, 18 Aug. 2019, Available here.
1. “Allopatric Speciation Schematic” By Andrew Z. Colvin – Own work (CC BY-SA 4.0) via Commons Wikimedia