Food Web and Digestive Liquid in Pitchers of Nepenthes mirabilis
in Zhuhai, China1
Yuejin HUA2 and Hongqi LI3
2Jin Hai-An High School, Zhuhai, 519041, China;
3Biology Department, Frostburg State University, MD 21532, USA.
1 Manuscript received __________; revision accepted _________.
The authors thank Dr. ____ and Dr. ____ for their valuable comments on the manuscript. We also thank the Government of Jinwan District, Zhuhai City, Guangdong Province, China, for the financial support for Yuejin to present the result at the Fifth Conference of the International Carnivorous Plant Society, at Lyon, France.
With eight years of observation, a complicated food web has been observed around/in pitchers of Nepenthes mirabilis in Zhuhai, Guangdong Province, China. Trapped in thirty examined pitchers, the majority of preys are arthropods in four classes (including nine orders of insects), plus very few molluscan snails and reptile lizards. The pitcher is good at trapping insects (both folivorous and carnivorous), except for some wasps that can hold their long legs on the outside of the pitcher. Some special properties of the liquid in the pitchers have been observed and tested: 1) Its amount of liquid reaches the maximum when the pitcher opens. 2) Its pH value increases from 6 to 2 at the 7th-8th day after the opening as the color changes from colorless, light yellow, to yellow, and both reverse as the pitcher dries up. 3) Only the old liquid can allow small animals to float on surface, while fresh liquid sinks animals down quickly. 4) The liquid kills and digests dropped-in mosquitoes, but cannot digest mosquito eggs. Instead, it provides nutrients for eggs to hatch out and then for the larva to grow up till to fly. 5) Although the liquid can kill and digest reptile lizards, it cannot prevent tree frogs from living in and intercepting the food source. All of these are possibly resulted from long term co-evolutions. [222 words]
Keywords: food web, liquid properties, Nepenthes mirabilis, pH value, symbiosis.
As the most widely distributed species of Nepenthes, monkey cup N. mirabilis occupies Southeast Asian countries, from India to New Guinea, from Queensland of Australia to Taiwan and Guangdong of China. It is the only Nepenthes species in Zhuhai District of Guangdong Province, located within 21¡ã48'~22¡ã27' N and 113¡ã3'~114¡ã18'E. This habitat Zhuhai belongs to subtropical region with trade winds, annual relative moisture about 79%, annual precipitation 170-230 cm, and annual temperature about 22.4¡ãC (the lowest at 2.5¡ãC, and highest at 38.5¡ãC).
Although Nepenthes mirabilis has been studied extensively since 1790 (see Danser, 1928), it has been barely studied in China. Since 1996, Mr. Yuejin Hua, the first author of this paper, as a science teacher at the Jin Hai-An high school in Zhuhai, has conducted a lot of field observations on this species, sometimes with his students involved in observations and experiments. Here we report our summary about the complex food web and the liquid properties in the pitchers of N. mirabilis, based on their extensive observations and experiments.
Material and Methods
Because the natural habitat of Nepenthes mirabilis has been continuously destroyed, especially in the recent six years, only about 20 Nepenthes plants have been observed within the accessible distance, photographed with a Nikkormat FT camera, and videotaped with a SONY DCR-TRV103 video camera, during the last eight years. Total about 30 pitchers have been examined for their trapped animals that have been preliminarily identified, with the help from experts in different research areas. Some preliminary experiments have been conducted to test the properties of liquid from the pitchers at the high school. The results about the food web and the liquid properties and their related discussions are presented below in two sections respectively.
Food Web and Trap Mechanism
With more than 80 species in the Old World tropical, carnivorous plants of Nepenthes obviously affect the number of many other coexisting species (Pimm, 2002). As the most widely distributed species of the genus, N. mirabilis perhaps traps the most species in its pitcher. From 30 examined pitchers in Zhuhai, except for a few molluscan snails and reptile lizards most trapped preys are arthropods in four classes (including nine orders of insects), (Table 1. Fig. 1). In terms of the trapped individuals, ants have the largest number, followed by mosquitoes and spiders. In terms of individual size, lizards, Blatta, mole locust, and butterflies.
The pitcher plants produce many honeydews scattered on stems and petioles (Fig. 2), leaves (Fig. 3), pitcher stalk (Fig. 4), pitcher bottom (Fig. 5), pitcher outside (Fig. 6). These honeydews guide ants to the pitcher mouth where has more nectarines. When they try to reach the honey on the inner side of the mouth, they slide down into the pitcher (Fig. 7). On the upper part of the inner surface, the pitcher has many semistoma-like barriers to prevent ants from escaping. So, ants can never get out once they fall into the pitcher (Fig. 7). Just like termites have social behavior (Science 2002 paper), ants like to seek food together (Fig. 7). Once one ant is trapped, others would follow up to get into the pitcher so that some pitchers are found purely filled with countless ants (Fig. 8), while other pitchers of the same plant may have other preys. Honeydews are also found on both bracts of male and female (Fig. 9) flowers. Some ants are attracted to flowers for the honeydews, and thus become pollinators (Fig. 10).
Flying insects are attracted to pitchers by the nectaries on the lower surface of the lid and on the mouth brim where has richful honey (especially the part below the brim inside the pitcher) and is very slippery. When they try to get the nectaries inside the brim, they slide into the pitcher. When they fall, they try to fly out. However, their wing-flapping can only expel air out of the pitcher and produce lower pressure in the pitcher, which pulls them down to the liquid.
Three kinds of wasps have been found in feeding around pitchers, showing different prey-predator relationships. Yellow jackets use their relative short legs grasping on the rim when they try to get the honey inside the mouth just below the mouth (Fig. 11). However, the rim is too slippery to hold, so they slide into the pitcher and become the pitcher¡¯s prey (Fig. 12). A black wasp has adapted to some anti-predation strategy by holding at least one of their long legs on the non-slippery outside of the brim when they eat the honey inside the brim, so it will not fall into the pitcher (Fig. 13). The third type of wasps is carnivorous, it accesses pitcher not for the nectarines, but for sucking body liquid from some worms that feed on the pitcher plant (Fig. 14). So, it is a predator and has a mutualism relationship with the pitcher plant.
The second carnivorous animal around pitchers are lizards that frequently stay close to pitchers to catch approaching insects (Fig. 15). However, sometimes they get too close and then would accidentally slide into the pitcher and will be digested by the liquid inside (Fig. 16). So, they turned their predator role to be prey. Due to their large biomass, it takes many days for a pitcher to completely digest and absorb a trapped lizard. A pitcher contains a lizard smells stink.
Tree frogs are found ambushed in the pitchers to catch passing by flying insects. The liquid cannot kill them, so they can live in the pitcher symbiotically. They can jump out the pitcher by themselves when they want (Hua. ). Because they intercept the food source of the pitcher plants but not directly feed on the pitcher plants, so they are social parasites.
Among other trapped animals, mantis, spiders, ant-lionflies are commonly recognized as carnivorous (ref. ). A complicated food web is sketched (Fig. 17) based on observations and commonly knowledge.
Properties of the the Liquid in Pitchers
1. The amount. How much liquid a pitcher can produce
depends on the pitcher size. For a naturally opened pitcher, the maximum amount
of liquid is about 1/3 of height of the pitcher when the pitch lid opens.
When some underdeveloped pitchers were manually opened, the amount of liquid
is much less, and never increases any liquid. This applies to naturally opened
pitchers in nature habits, as observed. So, the opening must trigger some
control mechanism to stop the liquid-producing, and this can be further tested.
The lid covers the pitcher very well except for a few days (see below), so
that it is hard to spray water with a bottle sprayer into the pitcher from
any angle. That means the pitcher would not receive rain normally.
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