1Frugivorous vertebrates may affect plant fitness by dispersing seeds to sites favourable for establishment and by passing seeds through their gut, thereby altering germination patterns. Although gut passage can inhibit germination, most studies have found that it improves germination rate and/or success.
2However, studies that compare seeds passed through a gut to seeds directly removed from a fruit cannot quantify the overall effect of gut passage on seed germination. They address the ecologically narrow question of whether mechanical and/or chemical action in the gut affects germination. They do not address the germination potential of seeds in unconsumed fruits and therefore ignore a common seed fate, deposition in a fruit.
3We surveyed 99 studies that included the effect of vertebrate gut passage on germination and found that only 22 included germination from intact fruits. We suggest that the strongest experimental design to evaluate the impact of gut passage should include intact fruits, since release from germination inhibitors and high osmotic pressure are mechanisms that can alter germination.
Probably the most important outcome of vertebrate seed dispersal is the transport of seeds away from parent plants to potentially safe microsites where conditions may be favourable for establishment (Primack & Miao 1992; Howe & Miriti 2000; Herrera 2002). A second advantage of dispersal is enhancement of germination after a seed has passed through a vertebrate's gut. This advantage, termed quality of treatment (Schupp 1993), has itself received much attention, and is reviewed by Traveset (1998) who found that gut passage usually enhances germination.
Changes in the percentage and/or rate of germination may occur via two processes. First, when seeds pass through a vertebrate gut, mechanical and/or chemical action of the digestive tract may alter the seed coat or endocarp, thereby affecting the likelihood of germination (Agami & Waisel 1988; Barnea, Yomtov & Friedman 1990). Second, germination patterns may be altered by separation in the gut of seeds from compounds in fruit pulp. These compounds can reduce germination success by altering the seeds’ microenvironment (e.g. osmotic pressure and light regime) and by more directly blocking biochemical pathways of germination (Evenari 1949; Mayer & Poljakoff-Mayber 1989; Lisci & Pacini 1994; Cipollini & Levey 1997).
We surveyed 99 studies that include experiments on the effect of gut passage on seed germination. For each study, we noted whether unpassed seeds were manually extracted from fruits (de-pulped), or left within intact fruits. A complete list of references and the criteria used to select them are available from the authors upon request. Our goal is to disentangle the objectives inherent in different types of seed germination experiments and to advocate for an experimental design that overcomes the limitations of the most common designs. We argue that many experiments evaluating the effect of gut passage on seed germination fail to distinguish between fundamentally different mechanisms. Thus, they often address different questions from those initially posed by their designers.
Experimental designs and mechanisms
The experimental designs we encountered in the literature include three main types of treatments for germination experiments: manually extracted seeds, gut-passed seeds and seeds in intact fruits. Manually extracted seeds are removed from fruits by hand, and are often washed or cleaned of pulp before placement on a germination substrate. Gut-passed seeds are those that have been consumed by a vertebrate. This treatment may include seeds that are defecated, regurgitated or both. Finally, some researchers examined the germination of seeds from within intact, unmanipulated fruits. In addition to fleshy fruits, this treatment often included seed pods (e.g. Fabaceae) and the syconia of Ficus (Moraceae). The various comparisons that can be made from these three treatments are illustrated in Fig. 1, and described in more detail below.
In panel (a) of Fig. 1, the first comparison is between germination of gut-passed seeds and manually extracted seeds. This is by far the most common approach (77% of total studies). It isolates the effect of vertebrate gut treatment on seed germination, but does not provide any information about the added effect of seeds being separated from fruit pulp. Thus, a non-significant result in this comparison can lead to Type II error, since the effect of ‘gut passage’per se includes both these processes, and seeds removed from fruit pulp may be much more likely to germinate, regardless of gut treatment. The second comparison in panel (a) is between gut passed seeds and seeds in intact fruits. This comparison was rarely made in the studies we surveyed (4% of total studies). It provides combined information about the effect of gut passage on the seed and the effect of being removed from the fruit, but it does not isolate which mechanism, if any, is primarily responsible for influencing germination patterns.
The comparison illustrated in panel (b) is between manually extracted seeds and intact fruits. None of the studies we surveyed used this experimental design. It provides information about the effect on germination when seeds are removed from germination inhibitors and/or osmotic conditions. Since the fruits are not consumed, however, the potential impact of the gut on seeds cannot be assessed.
The final approach, panel (c), compares germination for manually extracted seeds, gut passed seeds and seeds within intact fruits. Only 18% of all studies used this experimental design. It allows one to isolate the mechanism(s) responsible for changes in germination patterns following gut passage because it includes all the components of panels (a) and (b). The relative importance of mechanical/chemical processes in the gut can be isolated from the changes in germination patterns that take place when seeds are simply removed from fruits. Results from this experiment can be interpreted in the following way, where IF = germination of seeds from intact fruits, ME = germination of manually extracted seeds and GP = germination of gut passed seeds. If IF < ME = GP, then the animal is only needed to separate seeds from fruit pulp. If IF = ME < GP, then mechanical/chemical action of the gut is important. If IF < ME < GP, then gut passage may enhance germination both by removing seeds from fruit, and by altering the seed coat or endocarp. If IF = ME = GP, then ingestion of seeds is unimportant for germination. Common to all results, however, is the potential importance of being dispersed away from the parent plant (Howe & Miriti 2000; Herrera 2002). Regardless of whether gut passage directly affects germination, vertebrates may still play an important role via placement of seeds in microsites that vary in suitability for germination (Schupp 1993).
In our survey of the literature, there was no change over time in the proportion of studies that included germination tests from intact fruits. For studies published before 1995, 11 of 52 (21%) included this procedure vs 11 of 47 (23%) studies published in 1995 or later (χ2 = 0·072, df = 1, P = 0·79). Studies lacking this final experimental comparison may be unable to fully evaluate the effect of gut passage on germination. However, we do not mean to suggest that studies lacking these three comparisons are invalid, since gut passage was often a minor component of a larger study. For example, Vellend et al. (2003) examined how ingestion of Trillium by deer might explain postglacial migration patterns for this plant, which is normally dispersed by ants. Ellison et al. (1993) used defecated and manually extracted seeds as part of a larger study designed to assess life-history characteristics of understory plants that would then permit their incorporation into gap regeneration models. Varela & Bucher (2002) were interested in the identification of plant species dispersed by Chelonoidis chilensis (Chaco Tortoise) and seed passage time, in addition to the effect of gut passage. Of course, some studies intended only to evaluate the impact of mechanical and/or chemical processes of the vertebrate gut on seed germination. We believe that more specific terminology is required when this is the intention (Table 1), since the potentially significant effects of removing seeds from fruits is inherently part of the ‘gut passage’ process.
Table 1. Terminology describing the effect of gut passage
Effect of gut passage
Mechanical alteration of seed coat or endocarp
Germination success and/or rate may be affected by physical alteration of the seed coat or endocarp through mechanical action of the gut. Consumption of hard food items (including other seeds) may also contribute to mechanical breakdown of seeds. Alteration of the seed coat or endocarp may facilitate imbibition at deposition sites, thus promoting germination. Alternatively, mechanical action of the gut may destroy seeds, i.e. seed predation.
Chemical alteration of seed coat or endocarp
Digestive fluids may alter the seed coat or endocarp, thus affecting germination patterns. Digestive enzymes and stomach acids serve to break foods down (Sturkie 1976), including seeds. Seed treatment may also be affected by symbiotic bacteria and protozoa, which can digest structural polymers of the cell wall during fermentation (Traveset 1998).
The high sugar content of fruit pulp and resulting high osmotic pressure may reduce the chance that seeds will germinate if surrounded by pulp. In addition, pulp may contain a wide diversity of germination inhibitors such as lipids, glycoalkaloids, coumarin, abscisic acid, hydrogen cyanide, ammonia and various light-blocking pigments (Evenari 1949; Cipollini & Levey 1997). These compounds may inhibit germination until seeds are removed from the fruit and freed of pulp, as occurs during gut passage.
To understand the relative importance of different frugivores on the process of seed dispersal, it is necessary to partition their effects on both quantity and quality of dispersal (Schupp 1993). Yet most studies have failed to disentangle the ways in which quality of dispersal may be affected by gut passage because the traditional experimental design addresses only a subset of the processes affecting germination. In many cases, however, researchers are most interested in specific components of the gut passage process (e.g. comparing the effects of seed scarification via mechanical action in a related group of vertebrates). In such cases, more specific terminology will help focus attention on such processes without obscuring the overall importance of frugivory on germination. In other cases, experimental designs should be modified to address the fate of seeds in unconsumed fruits.
Ben Bolker and students in our weekly lunch group provided valuable comments. We thank two anonymous reviewers for helping to improve earlier drafts of the manuscript.