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In the latter half of the 20th Century, there was a period of rapid global warming, which leads to earlier arrival of spring, longer growing seasons, altered seasonal patterns and biotic interaction of species. Scientists are trying to predict and detect the effect of climate change on populations, it was predicted that increasing global temperatures will shift species' geographical ranges to higher latitudes or altitudes. The changing biodiversity is not simply a response to increasing temperature; it is a complex response to several human-induced changes in the global environment.
Perhaps the change that can be observed the most easily is the expanding of range margins of animals. Many animals are relatively sedentary and specialised in marginal parts of their geographical distributions. They are expected to be slow at colonising new habitats. In 1982, the silver-spotted skipper butterfly (Hesperia comma) was largely restricted to south- and southwest-facing chalk grasslands (which are warmer) in southern England, but by 2000, they have colonised a wide range of aspects, including east-, west- and north-facing hillsides. There were 105 thermally suitable habitat patches in 1982 (total area = 2 km2), more patches were available in 2000 because of global warming (175 patches, total area = 3.92 km2. Their expansion rates are likely to increase with habitat availability, the more patches to colonise, the shorter is the distance between them, and more populations generating emigrants.
Range expansion is seen in another species of butterfly, the brown argus butterfly (Aricia agestis). Both its habitat and geographical range expanded over the past 20 years. Choice experiments have shown that expanding and non-expanding marginal populations differed in choice of host plants. Females in the recently established populations chose to lay eggs on the most widespread host plant (Geranium molle) used during range expansion, rather than on the host plant that was used naturally in the habitats where the populations occurred (Helianthemum chamaecistus and Erodium cicutarium), but both populations retained the ability to lay on H.chaemaecistus. The data were compatible with a genetic contribution to host plant choice. The host choice phenotypes in the expanding region may have risen from selection within each population ancestry during range expansion, or initiated from populations that already possessed the host choice characteristics of the expanding region.
Increased flying ability has been selected for in 2 species of bush cricket that exhibit adult wing polymorphisms. Both species are spreading northwards and inland from distributions formerly confined to specific habitats in southern, coastal areas. The long-winged cone-head Conocephalus discolor has 2 forms: long-winged and extra-long-winged Many populations that established in the last 20 years showed a higher frequencies of extra-long-winged individuals. However, in populations that were established for more than 20 years, the frequencies of this form is lower. In Roesel's bush cricket (Metrioptera roeselii), it has a short-winged form that can't fly and a long-winged form that can. This species also shows an increased frequency of the more dispersive form in populations that have recently been established. Environmental variables such as temperature, population density are known to affect whether cricket nymphs will mature to become long- or short-winged adults. If we assume that all long-range movements are achieve by the long-winged form, the represents about 4-fold and 14-fold increase in dispersal for C. discoler and M. Roeselii respectively. The changing environmental conditions at existing margins (regional warming at cool margins) are likely to initiate range extensions purely on the basis of ecological, physiological and population-dynamics processes, requiring no evolutionary change. But once an expansion is initiated, populations that expand more rapidly are likely to be favoured; once most habitats in a region have been colonised, less dispersive forms may be favoured again. Climatic changed, which lead to increased habitat breadth and dispersal tendencies have resulted in about 3- to 15-fold increases in expansion rates, allowing insects to cross barriers to dispersal before the expansions started. Emergence of dispersive phenotypes also increase the speed at which species invade new habitats.
Climate change led to heritable, genetic changes in populations. The time scales over with genetic changes are detectable cover a whole range (5 years for mosquito, 10 years for squirrels and 30 years for great tits). Great tits are modifying the timing of egg laying in response to earlier spring, so that they can get more reproductive success. In European, North American and Australian populations of fruit flies, the frequencies of different alleles and of chromosomal inversions have been shifted towards the frequencies of more southern populations. The north American mosquitoes have shown a genetic shift toward the use of shorter, more southern day lengths to cue the initiation of larval dormancy. However, none of the studies provided evidence that there have been genetics changes in response to higher temperature alone (when Northern mosquito were experimentally transplanted to a stimulated southern climate, there was a 88% loss of fitness due to experiencing the incorrect seasonal cues, which is day length, whereas the warmer southern temperature was not a factor). Correct interpretation of cues that correspond to seasonality rather than to hot temperature is crucial. Recent climate change imposes seasonal rather than thermal selection of an natural population.
Another study showed that pitcher plant mosquito (Wyeomyre smithii) uses day length as a pivotal environment cue to program their seasonal patterns of dormancy, migration, development and reproduction as well. Between 1972 and 1996, there is a shift toward shorter critical photoperiod, or a more southern phenotype in more recent years. This shift has been more pronounced in the north than in the south. The critical photoperiod declined from 15.79h to 15.19h from 1972 to 1996, corresponding to 9 days later in the fall of 1996 than 1972. The value is similar to the advancement of other seasonal events in the north temperate region over the same time period (e.g. birds began laying eggs 8.8 days earlier in 1995 than in 1971). Experiments run in highly controlled matched set of conditions show that differences in critical photoperiod among populations indicate a genetic difference among them, and the genetic change can take place over as short as 5 years.
The ability to evolve in response to climatic change ensure a population will survive, if a population cannot keep pace with environmental changes, it will become vulnerable to extinction. Small animals with shorter life cycles and large population sizes will probably adapt to environmental changes and be able to persist, but populations of large animals with longer life cycles and smaller population sizes might experience a decline in population size or be replaced by more southern species.
Most studies have concentrated on the effect of climatic change on one genotype and life-style of a given species, the interactions within and between the abiotic and biotic components of climate change are often ignored. Temperature changes affect organisms, but also change in concentration of greenhouse gases. Elevated carbon dioxide concentration leads to a decrease in leaf nitrogen and increase in carbohydrates and phenolics. Out of 49 insect-plant interactions, the insect developmental time increases in 10 cases, while the developmental time decreases in 3 cases. In a modelling of the effects of increased temperature on European corn borer moth (Ostrinia nubilalis), it shows potential for a northward range expansion throughout Europe of between 165 - 500 km for each 1ï¾ŸC increase. However, the main crop host maize (Zea mays) need to move as well. Most insects can move quickly to track the environmental changes, the important constraint to range expansion is the rate of movement of host plant. When looking at the outcome of competition between 3 species of fruit fly (Drosophila spp.) in the presence or absence of a parasitoid, the ratios of Drosophilia species differed in different temperature and also according to the presence or absence of parasitoid. This demonstrates the need to take into account the effect of climate on species interactions.
It is almost certain the climatic change is a major problem for biodiversity, as it can change the distribution of organisms, and drive some to extinction. Species composition of a community will also be changed. However, the impact of climatic change is regional, regions like the Arctic will be more sensitive to climatic change compared to temperate forests. When modelling the effect of climatic change to organisms, the complexity of the ecosystem is often overlooked. More detailed studies and integrated effort by climatologists, ecologists, social scientists and policy makers are required for a more realistic projections of how climatic change affect biodiversity.
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