Global Warming An Added Complication
Temporal differences in resource abundance and quality have been influenced globally by climate change, and have had significant effects on wildlife species. The Normalized Difference Vegetation Index (NDVI) provides a standardized method of comparing vegetation greenness between satellite images. When two or more images are compared over several to many years, it is possible to distinguish changes in vegetation reflectance values that can be represented as a percent change from a long-term average. In this way, trends in the timing of spring bud break and leaf growth, i.e., phenophase, can be detected. In Washington, DC (USA), Abu-Asab et al. (2001) found that 76 of 89 plants whose flowering date was significantly earlier, flowered on average 5.6 days earlier than a 30-year mean Julian date. Flowering dates ranged from 3.2 to 46 days earlier. Numerous other studies have shown similar trends. Changed phenophases mean changed patterns of resource abundance and length of availability. Visser (1998), Visser and Holleman (2001), and Grossman (2004) provided clear examples where global warming and its effects on phenophase have had effects of Great Tit (Parus major) demographics. They reported that in the Netherlands, changes in weather patterns have caused oak buds to leaf sooner. Winter moth caterpillars (Operophtera brumata), an important food source for great tit chicks, peaked in total biomass 13 days earlier in 2004 than in 1980. However, the date of egg laying has remained the same. For most if not all wildlife species, a primary selection factor on the timing of reproduction is synchrony between offspring energy requirements and food availability (Grossman, 2004). The earlier bud break in oaks and the subsequent earlier increase in winter moth caterpillars have resulted in a mismatch between the availability of food and the needs of the young chicks. Visser (1998) Visser and Holleman (2001) report serious demographic consequences for the Great Tit population near Arnhem, the Netherlands. Clearly, as these studies demonstrate, over larger spatial extents, the timing of resource availability is critical.
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