Recently the first day of fall passed in the rest of the country. The breeze has gotten cooler. Leaves are donning their beautiful, colored wardrobe prior to the drab dress of winter. In the northern countryside, people are finishing harvesting their summer vegetable gardens and splitting their winter wood supplies. So what are the signs that herald the changing season here in the Florida Keys? Seasonality in the tropics (or subtropics) is not quite as distinct as in higher or lower latitudes. These regions lie closer to the equator, and the sun strikes the earth in this area at a more direct angle. This has tremendous importance for a vast number of reasons. One of the most noticeable, and one of the reasons many of us choose to live here, is because temperature tends to vary little throughout the year.
In the Florida Keys, our average daily temperature ranges between just under 70 degrees Fahrenheit in January, to around 84.5 degrees in July and August. Our rainfall tends to peak at around 7 inches per month in September, drops to about 5 in October and then fluctuates at around 2 inches until it begins to rise again in May. This data comes from the monthly averages of daily measurements taken at the Key West International Airport. These are based on records from 1948 up through the present (www.sercc.com). Our consistent non-freezing temperatures mean that plants can grow throughout the year. This, of course, drives all sorts of other biological phenomena.
The easiest sign to see of the changing seasons is the bird migration. For the last three weeks at the Tropical Forest and Botanical Garden, there has been a large increase in the number of birds. Some, such as the palm warblers, come from as far away as the northern edge of the boreal forest up in the Canadian tundra, where they breed. They then fly to Southern Brazil and Northern Argentina where they winter, before returning through here in the spring. Other current visitors include the painted bunting, northern oriole, short tail and broad wing hawks, peregrine falcon, American redstarts, cardinals and numerous species of warblers. The botanical garden has recorded 143 species of birds in the last five years. This species list, and likely times to see them, can be found by logging on to EBird, hosted by the Cornell Ornithology Lab (www.ebird.org).
On the botanical side of things, during this time of year we can observe the end of an interesting cycle of tropical plant flowering and fruiting. Tropical species make up the vast majority of our trees here in the Florida Keys. Many tropical trees tend to flower in early summer, before rainfall becomes very heavy. Flowers tend to set fruit before the onset of heavy rains, probably because fruit is more likely to stay on the tree during heavy rains than flowers are. As rains increase, fruit on the trees begin to ripen during the next several months. When the birds begin to arrive many plant species already have ripened fruit awaiting them. This seems to be especially true of the fruits most likely to be eaten by our migrating visitors. For example, the wild lime (Zanthoxylum fagara), is dripping with tiny limes right now. Many of the stoppers are also abundantly in fruit, as well as species such as the gumbo limbo, pigeon plum, fiddlewood and thatch palms.
Fruiting time is also tied to dispersal mechanisms for plants. Many of the species in fruit right now tend to be small and likely to be eaten by small birds, which make up the largest portion of migrants. In a brief walk around the garden I saw 22 native species of tree that fit this description, which means their seeds will be dispersed by birds. The Florida Keys, and the Caribbean as a whole for that matter, do not have many large mammals likely to eat big fruits (except people of course). Thus, there would be an evolutionary push for plants to develop fruit likely to be eaten by the smaller animals that are around, or to use other mechanisms of dispersal, such as wind- and sea-dispersed fruits or seeds. Some species fruit and flower for long periods of time. Throughout the year, probably our best sources of fruit for birds in the Florida Keys are our two fig species. Both the strangler fig (Ficus aurea) and the shortleaf fig (Ficus citrifolia) consistently produce abundant fruit. Virtually all of our fruit-eating birds probably visit them at some time or another.
Individuals in any species vary. Both flowering and fruiting abundance and times are going to cover a wide range within any given species. Diversity within any group is a sign of strength. Just look at us people. Also, just like us, don’t expect any one individual to represent the entire group. This diversity in trees is what spreads out the fruiting cycle long enough to enable all the birds in the migration, and throughout the year, to have enough food.
I frequently hear people say that we don’t have seasons here in the Keys. However, next time you walk around town look up at the trees and sky and you will see what seasonal change brings us here in Key West.
Stephen Hodges is the resident botanist at Key West Tropical Forest and Botanical Garden, 5210 College Road. He can be reached for comment at firstname.lastname@example.org.