These mammals that spend the majority of their lives in or near the water and depend on it for their food can be split up in two: real marine mammals (three groups), and marine mammals still in the process of returning to sea (two groups).
Quality and Degradation: The loss of productive soil hasoccurred as long as crops have been cultivated. Lal and Pierce () in stating this, report that landdegradation has now become a major threat to the sustainabilityof world food supply. This loss arises from soil erosion,salinization, waterlogging, and urbanization with its associatedhighway and road construction. Nutrient depletion,overcultivation, overgrazing, acidification, and soil compactioncontribute as well. Many of these processes are caused or areaggravated by poor agricultural management practices. Takentogether or in various combinations, these factors decrease theproductivity of the soil and substantially reduce annual cropyields (), and, more important,will reduce crop productivity for the long term ().
Maintaining Biodiversity: Conserving biodiversity of plant andanimal species is essential to maintaining a productive andattractive environment for agriculture and other humanactivities. Greater effort is also needed to conserve the geneticdiversity that exists in crops worldwide. This diversity hasproven extremely valuable in improving crop productivity and willcontinue to do so in future.
The major causes of biodiversity decline are land use changes, pollution, changes in atmospheric CO2 concentrations, changes in the nitrogen cycle and acid rain, climate alterations, and the introduction of exotic species, all coincident to human population growth. For rainforests, the primary factor is land conversion. Climate will probably change least in tropical regions, and nitrogen problems are not as important because growth in rainforests is usually limited more by low phosphorus levels than by nitrogen insufficiency. The introduction of exotic species is also less of a problem than in temperate areas because there is so much diversity in tropical forests that newcomers have difficulty becoming established (Sala, et al., 2000).
Soil and Water Conservation: The high rate of soil erosion nowtypical of world agricultural land emphasizes the urgency ofstemming this loss, which in itself is probably the mostthreatening to sustained levels of food production. Improvedconservation of water can enhance rainfed and irrigated cropyields, as discussed below.
Why do people heedlessly decimate the precious biodiversity of their planet? Some of them feel they have no economic alternative, while others are driven by the desire for short-term profit. Still others are uncomprehending. Unfortunately, so much of the depredation which is being inflicted upon areas of great biodiversity is, in the long run, and often in the short run, in vain. While tropical forests now occupy less than half of their former range, and much of what remains is damaged or fragmented, the net profit to humanity is slight. Clearing of tropical forests has provided only a relatively small percentage of total agricultural land, since much of the land converted for farms becomes rapidly degraded and is abandoned. Logging results in a one-time profit, mainly to large companies. Ranching is an activity which, on former rainforest land, is uneconomical, requires subsidizing, and is eventually abandoned. But the damage is permanent and the forest irreplaceable, so forest destruction has dire consequences. It degrades aquatic fisheries, causes floods and has many other consequences (see below) – so much harm for so little benefit.
In addition, in fragmented forests, seeds will frequently land in deforested areas (where they are in the open, and exposed to heat, light and desiccation) in which they cannot germinate, and the seedlings cannot survive. In Brazil, three to seven times as many Heliconia acuminata seedlings planted in continuous areas of forest germinated as compared to those planted in fragmented areas (Bruna, 1999). Whatever the explanation for the lower rate of seedling germination in fragmented forests, whether due to inbreeding or other causes, fewer and fewer individuals in fragments grow to adulthood. Those which do will breed, but since populations are small, inbreeding occurs and the downward spiral continues until the population becomes locally extinct. This effect is seen frequently in forest fragments.
vi) For unknown reasons, fragmentation leads to the death of large canopy trees, even in the interior of fragments. Canopy trees dominate the forest structure, and they provide fruits and shelter for many animals. The mortality of trees in fragmented patches in Brazil has been found to be twice that of similar trees in the forest interior (Laurance, et al., 2000). Not only that, but tree mortality is confined disproportionately to large trees (an almost 40% increase in mortality). Large trees may be more vulnerable in fragmented forests because they are not as well buffered from wind and natural forces, because there are more tree parasites (lianas), and because they are more subject to dessication at forest edges. Loss of these largest trees has several corollary effects – the alteration of biogeochemical cycles (transpiration, carbon cycles), the reduction of species complexity, and the reduction of fecundity. As mentioned above, large trees are essential habitats and food sources for many other organisms, both plant and animal; they are the source of much of the primary productivity of the forest; and they are responsible for many effects on the water and nutrient cycles. They are irreplaceable in the forest ecosystem.
i) Fragmentation decreases habitat simply through loss of land area, reducing the probability of maintaining effective reproductive units of plant and animal populations. Most tropical trees are pollinated by animals, and therefore the maintenance of adequate pollinator population levels is essential for forest health. When a forest becomes fragmented, trees of many species are isolated because their pollinators cannot cross the unforested areas. Under these conditions, the trees in the fragments will then become inbred and lose genetic variability and vigor. Other species, which have more wide-ranging pollinators, may suffer less from fragmentation. For instance, the pollen of several species of strangler figs (the fruit of which is an essential element in the diets of many animals) is dispersed by wasps over distances as great as 14.2 km (Nason, Herre, & Hamrick, 1998). Thus “breeding units” of these figs are extremely large, comprising hundreds of plants located in huge areas of forest. Isolated fig populations seem to survive and help to maintain frugivore numbers (if not diversity), so long as the number of trees within the range of the wasps does not fall below a critical minimum.
There may be a link between augmented carbon dioxide levels and marked increase in the density of lianas in Amazonian forests. This relationship is suggested by the fact that growth rates of lianas are highly sensitive to CO2 levels. As lianas become more dense, tree mortality rises, but mortality is not equal among species because lianas preferentially grow on certain species. Because of this biodiversity may be reduced by increased mortality in some species but not others (Phillips, et al., 2002).
An increase in infectious diseases is another consequence of climate change, since the causative agents are affected by humidity, temperature change, and rainfall. Many species of frogs and lizards have declined or disappeared, perhaps because of the increase in parasites occasioned by higher temperatures. As warming continues, accelerating plant growth, pathogens may spread more quickly because of the increased availability of vegetation (a “density” effect) and because of increased humidity under heavier plant cover. As mentioned above, the fungus Phytophtora cinnamoni has demolished many Eucalyptus forests in Australia. In addition, the geographical range of pathogens can expand when the climate moderates, allowing pathogens to find new, nonresistant hosts. On the other hand, a number of instances of amphibian decline seem to be due to infections with chrytid fungi, which flourish at cooler temperatures. An excellent review of this complex issue may be found in Harvell, et al., (2002).