Rainfall

Seasonal shifts in rainfall have a pronounced impact on aquatic organisms; this is particularly true in ephemeral water bodies. The highest diversity of aquatic organisms occurs in temporary water bodies that are seasonal in nature - filling after cyclic rains and drying during nonrainy periods. Species that are dependent upon these habitats exhibit life history strategies that enable them to exploit seasonal aquatic habitats as well as to persist during dry periods. Many of these organisms have stages in their life cycles in which they enter diapause (a period of quiescence characterized by the cessation of growth and reduction of metabolic activity) or remain dormant during the dry season. For example, several species of aquatic invertebrates lay eggs in temporary pools, after which the eggs settle into sediments where they remain dormant during the dry period until seasonal rains once again flood the site and the aquatic larvae life stages emerge. Eggs of some aquatic invertebrates can remain dormant for over 125 years. Plant species associated with vernal pools follow a predictable flowering phenology after seasonal rains; these species are primarily annual and reproduce as pools draw down during the spring and summer months. Spring wild-flowers in vernal pool ecosystems provide important resources for pollinating insects, some of which only collect pollen from vernal pool plant species.

Seasonal rains also play a vital role in sustaining organisms that live in permanent water bodies. Most large riverine systems throughout the world experience annual floods during wet seasons. These floods expand the spatial extent of rivers into habitats that for most of the year remain dry. Water spills over the riverbanks, flooding forest habitats and connecting water bodies that are usually isolated from one another. As a result, aquatic organisms are able to exploit habitats that are inaccessible throughout much of the year. As swelling rivers move into upland habitat, nutrients leach into the water, increasing primary production. Aquatic plants assimilate these nutrients that are eventually recycled back into the environment via decomposition. This pulse of nutrients plays a significant role in supporting the base of the food web and in turn sustains an increased number of herbivores and predators. Seasonal floods are common in large tropical riverine systems, which support a diverse assemblage of fish. One of the factors thought to have led to the diversity of fishes in these systems is the accessibility to a wide breadth of feeding niches that are available during seasonal flood events. Fish are able to exploit inundated upland habitats and thus are able to gain access to a wide variety of resources. As a result, fish assemblages in large tropical rivers exhibit a wide variety of feeding strategies and have a disproportionately greater number of herbivorous, detritivorous, and omnivorous feeding behaviors.

Desert regions, by definition, have very little rainfall throughout the year, with most of the precipitation occurring in seasonal storms during a 2-3 month period. Similar to environments that experience freezing temperatures during winter months, organisms that inhabit deserts are adapted to exploit patchily distributed resources throughout the year. The tight correlation between rainfall and seed production in desert regions has played an adaptive role in selecting for life history strategies that favor individuals that breed during wet periods of the year when resources are abundant. Many species of granivorous rodents, ants, and birds show predictable fluctuations in abundance throughout the year; peaks are associated during the winter and/or summer rain periods when seed production is high (Figure 1). Organisms that inhabit desert environments throughout the year have adaptations that enable them to persist during times of low resource availability and extreme temperatures. Several desert species exhibit behavioral modifications that enable them to persist through drastic seasonal shifts in the environment. Amphibians, for example, bury themselves underground or take refuge beneath cover during dry months. These behaviors prevent desiccation and reduce their energetic requirements during times of low resource availability. Other organisms, such as the kangaroo rat which has highly specialized kidneys that are extremely efficient at conserving water, have physiological modifications that enable them to persist during times of low resource availability.

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Figure 1 Changes in abundance of the Silky Pocket Mouse (Perognathus flavus) over a 10 year period in the Chihuahuan Desert (w = winter; s = summer). Abundance was calculated as the 6 month average. Seasonal peaks in abundance are correlated with summer precipitation events and are likely largely due to increased primary productivity associated with rain events. Adapted from Brown JH and Henske EJ (1990) Temporal changes in a Chihuahuan Desert rodent community. Oikos: 59: 290-302.

wswswswswswswswswswsw 1977 Season 1988

Figure 1 Changes in abundance of the Silky Pocket Mouse (Perognathus flavus) over a 10 year period in the Chihuahuan Desert (w = winter; s = summer). Abundance was calculated as the 6 month average. Seasonal peaks in abundance are correlated with summer precipitation events and are likely largely due to increased primary productivity associated with rain events. Adapted from Brown JH and Henske EJ (1990) Temporal changes in a Chihuahuan Desert rodent community. Oikos: 59: 290-302.

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