Tropical Dry Forests - Part Two: The Wet Season

Before you start reading: have you read Part One? If not, STOP, and head over to my previous post. It will be a better experience if you read both. Promise.

These photos are going to make you wonder why it would ever be called Tropical DRY Forest. For half the year, these forests aren't very dry at all. In fact, they're wet, VERY WET. Do you remember how I said that during the dry season it rarely rains? Everything is dry, water is scarce, and wildlife struggle to survive. Then, around May-June (this year: May 14th), the rains come. It starts with a few drops and builds to a torrential downpour.

Everything changes very quickly. Take a look at the comparison (Fig. 1) between the dry season and the wet season in the photos below (click photo to see the comparison).

Figure 1. Wet season vs. Dry season, as seen from the Mirador (lookout) in Santa Rosa. These two photos were taken only 2 months apart.

The first thing I noticed was the smell. A forest floor that hasn't seen rain in months releases all sorts of interesting scents. Then, there is an explosion of life. Anurans (frogs and toads) come out of their burrows and make for the quickly growing puddles and ponds. Birds start frantically building nests. Trees produce buds, plants sprout from the ground, every surface starts to turn green with photosynthetic life (Fig. 2). And then, suddenly, insects are diverse and abundant (Fig. 3).

Figure 2. New leaves soaked from recent rains.

Figure 3. A variety of insects seen in Santa Rosa in the wet season.

For us, the coming of the rains means everyone on our crew stops what they're doing and helps out on the Yellow Toad project. This involves getting up well before day-break, heading to one of those quickly-growing puddles, and catching and measuring Yellow Toads (Fig. 4).

Figure 4. Me holding a male Yellow Toad. The little yellow "dots" behind me are hundreds of toads!

Anurans emerge from their burrows and head towards water to find a mate. In this particular species, males and females are chocolatey brown for most of their lives until the onset of the rains, then the males turn to a bright eye-catching lemon yellow and head to the ponds to breed (Fig. 5). They arrive in hundreds (Fig. 6).

As you can imagine, a huge congregation of small animals such as toads brings many predators to the water too. We saw wood-rails, caracaras and iguanas hanging around the water, taking advantage of the abundant food available.

Figure 5. A male Yellow Toad - bright yellow & ready to breed!

Figure 6. Hundreds of Yellow Toads visit this spot in hopes of finding a female to mate with.

Figure 7. Colin with a blue bucket - ready for more toads!

Before this streambed fills with water, it fills with Yellow Toads and a cacophony of their breeding calls. Five biologists run back and forth from the ponds to the temporary research station with blue buckets in hand (Fig. 7). In a few days, the toads have all but disappeared and the stream flows with water.

Normally, this stream takes awhile to fill. This year, a slow-moving tropical storm hit soon after the first rains. It rained almost non-stop for two weeks. What does this mean for field biologists? A lot of time at the field house drinking coffee (Fig. 8), catching up on data entry, and playing card games. Every break in rain, we dashed out into the field and tried to fit in whatever data collection we could. After a couple weeks, the rain subsided, we saw the sun again (rejoice!), and we were able to get back into a regular routine.

Figure 8. Coffee the Costa Rican way with the coffee "sock" (no not a real sock, and certainly not a field sock!)

Fieldwork in the wet season means:

- Wearing rubber boots in the field every day

- Swerving around frogs and toads on the road while driving

- Wading through knee-high water to get to sites

- Fighting hoards of mosquitoes

- Running around the house with buckets to put under leaks in the roof when it rains

- 4x4ing through the mudpit of a driveway (fun!)

- Dealing with moldy clothes that never dry

Figure 9. Photos of the wet season: flooded roads, rainy days, waterfalls that didn't exist a week before, clothes that never dry, a muddy driveway, and torrential downpours.

For bird researchers, the wet season also means the beginning of the breeding season! Our Rufous-and-white Wrens quickly built their nests and laid eggs (Fig. 10). For my graduate research, the beginning of the wet season meant that it was time to re-record all our sites. We returned to our 31 sites and set up automated recorders (again) to record bird vocalizations during the wet season. We continued to collect focal recordings of our wrens and monitored their breeding behaviour (Fig. 11 ). Often this meant searching a territory for an acacia tree and finding a Rufous-and-white Wren nest in it. These clever birds take advantage of the symbiotic relationship between the acacia and the ants. The acacia provides food and nutrients to the ants, which they extract from a tiny hole in each of the thorns. The ants protect the tree from grazing animals and climbing vines, but they don't seem to mind the wrens. So the wrens choose to build their nests in acacias, a tree that comes with a built-in security system: thorns and biting ants (Fig. 12).

Figure 10. A Rufous-and-white Wren nest in an acacia tree.

Figure 11. Rufous-and-white Wren in an acacia tree. This particular wren was scoping out this tree as a potential nesting site.

Figure 12. Acacia thorns, note the small hole where the ants enter the thorn to extract sugars and nutrients.

By the end of our field season, there were nestlings at some of our wren territories. Most of the birds in Santa Rosa had nests full of eggs or baby birds. By now most of these eggs will be hatched and nestlings fledged!

We're back home in Canada now where it's as hot as Costa Rica but not nearly as wet. Despite being hundreds of kilometres from Costa Rica, I still spend my days listening to the sounds of the tropical dry forests of Santa Rosa... but now through headphones while sitting in A/C in the lab!

I'd still rather be in the forests themselves.

Figure 13. The road to our sites changes drastically through the season. Left is from the first week we arrived, middle is from soon after the rains arrived, and right is from our last week in Santa Rosa.

Thanks for visiting my blog and reading about the tropical dry forests of Guanacaste, Costa Rica. If you'd like to learn more about this unique ecosystem, check out this webpage which describes many of the unique features of these amazing forests.