The Resource Availability Hypothesis: A Review
The resource availability hypothesis is a theory that can be applied to both plants and animals. Its use in scientific literature has been diverse with papers having been published treating resource availability like an economic model (Neumayer, 2000) or using an economic analogy in order to explain trends seen in experiments (Bloom et al., 1985). In general terms the hypothesis allows researchers an ecological based tool to describe experimental trends shown by test communities.
This review will concentrate on the hypothesis’ relevancy to plants and plant growth. Firstly however it is important to understand how the hypothesis can be applied to animals in order to provide a broad introduction to the area. Then more specifically the review will look at how the hypothesis fits into plant growth. After this the review will talk about work done within the field with regard resources as nutrients, an area that most research concentrates. It will however also look at the relationship of plants and grazers in the light of the hypothesis, which is another major area within this field.
What is meant by the resource availability hypothesis? :
The flexibility within the hypothesis allows it to be applied to both animals and plants. Experiments on snail dispersal (Byers, 2000) showed that an increased level of resource availability decreased the snail dispersal rate for larger (adult) estuarine snails. Further experiments showed that smaller snails had a higher dispersal rate regardless of the resource levels present. This supported other studies, which had shown less competition amongst the young, small snails. One possible hypothesis for these results was suggest as some form of genetic behaviour present when the snails are young. The behaviour is thought to cause younger snails to disperse more readily in order to avoid competition. Other work carried out which involved manipulating resource levels and its effect on invertebrates was carried out on the Argentine ant Linepithema humile (Aron et al., 2001). By controlling the diets of the ants and providing them with intermediate, high or no extra protein in addition to their diet it was shown that protein increases the number of sexual pupae. Of females more pupae were likely to be queens (rather than workers) and in the entire population the proportion of males was higher. In un-supplemented nests it was shown that protein availability influences the proportion of a brood that is culled. With more sexual pupae culled the lower the amount of protein available to the ants. Other animal studies have been done in vertebrates. One example was work done by Andersson et al. on Arctic char (Salvelinus aplinus) (2005). The study used pond and lake based experiments of differing resources (in this case microinvertebrate and zooplankton amount). The results showed that differences in the levels of zooplankton to microinvertebrate levels caused morphological as well as behavioural differences. The fish that were in a complex environment (high levels of both resources) had a higher dependency upon the zooplankton than their simple environment counterparts. The complex environment fish subsequently were morphologically dissimilar to the simple environment fish as a direct response to this preference.
When looking at the resource availability specifically relating to plants its is important to remember that it is only one of a series of theories which look at the complex relationships between plants and their environments. Some of these theories look at specific aspects of a plants growth and development. For example the carbon-nutrient balance hypothesis (CNBH) is an approach used to understand the patterns of resource allocation within plants. Refining of this theory based upon evidence gathered has allowed it to be a useful guide for ecological research, mainly as it attempts to explain plasticity displayed by individuals without assuming that all responses displayed by individuals are optimal (Lerdau, 2002).
The resource availability hypothesis, however, looks more at effects of resources on both the individual and the population surrounding it. In a lot of literature the resource availability hypothesis is used in descriptions of plant anti herbivore defense. In these studies the resource availability hypothesis states that plants with low relative growth rate and high levels of defense are favored in habitats low in resource availability. Natural selection therefore favors plants with a high maximum growth rate and low defense in habitats of high resource availability. Whereas resource poor-habitats favor plant species with long lived leaves as leaf replacement, and the replacement of lost mineral nutrients is much more costly in poor habitats. So in order to avoid loosing valuable nutrients plants adapted to these environments develop greater levels of anti herbivore defense (Elberse et al. 2003).
This is a common example of a practical application of the resource availability hypothesis. It is used to understand how resource availability impacts upon plant growth and adaptation. By taking this as a general guideline to its use in an experimental context it is clear to see that it is used as a basis for a working experimental hypothesis to test ideas in an empirical and definitive manner.
Studies testing effects of resource availability:
Wilson and Tilman (1991) performed a series of experiments in a 30-year-old field. The experiments concentrated on the effects of fertilization and disturbance on the plant communities. The experiments were conducted on 104 5x5m plots separated by a 2m corridor of untreated vegetation. Tilman had already demonstrated that nitrogen had the greatest limitation on plant growth in that community and so it was chosen to represent fertilization. There were four treatments: no added nitrogen and highest rate of disturbance, highest added nitrogen and no disturbance, no nitrogen and no disturbance and finally highest rate of nitrogen and disturbance. Increasing the levels of nitrogen increased the total community biomass. The species richness within the plots also increased with nitrogen supply, this time however only in undisturbed plots. In both the undisturbed and disturbed plots vegetation height increased as light penetration decreased. This was coupled with a decreased in root: shoot ratios with the added nitrogen. Leaf allocation decreased with disturbance whereas flowering allocation actually increased despite stem allocation being unaffected by disturbance levels. Similar results were found in yellow nutsedge (Shibuya et al. 2004) where increasing nutrient supply led to increasing plant productivity and associated traits of this, i.e. flowering allocation. Plant productivity could also be seen, as being a form of competition between plants, where increased reproductive success is the outcome of successfully out competing neighboring plants. This has been backed up by competition field studies where manipulation of resources was shown to affect competition intensity (Davis and Pelsor. 2001). Similarly a relationship between fertility and grazing on vegetation grown in infertile soil and on comparatively poor food gave a low ratio of nutrient to fiber and chemical defenses (Bell. 1982). In a different series of experiments Wilson and Tilman were also able to show that above ground competition was greatest in plots with the lowest light penetration (1993). Lower nitrogen availability increased below ground competition and decreased significantly with increasing nitrogen availability. These measurements were recorded in the 3rd year of growing the crop Schzachyrium scoparium, which is a native perennial grass for the area of study. Again disturbance was used, this time tilling annually, which removed all vegetation in each of the 5x5 plots. There were two treatments of nitrogen, non-added or 17gm-2yr-1.
So far the examples have looked at the effect of differences in resource availability within a habitat. Some work has also been done comparing different habitats one study by Brenton and Facelli.(2005) looked at the existence of competition in a temperate rainforest and an area of high stress (a desert). They tested to see if the idea that a plant invests in components, which would greater increase, their chance of maximizing a resource, which is most scarce in that particular habitat, was upheld. Another contrary idea is that plants are in competition in the benign habitat (such as the temperate forest) and as such the ability to gather aboveground resources is kept in check by a positive feedback system. In this hypothesis the intensity of competition is increases as it counts the roots and shoots collectively, this has the effect of increasing the role of resource availability. The experiments involved a series of glass house experiments as well as observations of tree seedlings in order to assess if there was a logarithmic relationship between competitive intensity and the resource availability. Another aim of the experiments was to show if there was and potential for confounding effects caused by competition and herbivory on a field experiment to determine whether the relationship between experimentally manipulated resource availability and competitive intensity could be manipulated by changing the identity of competing vegetation. The results supported the theory that competitive interactions reach maximum in a fertile environment. This could be because increasing resource availability increases competition between plants (shown earlier) however fertilized plants in outdoor experiments were shown to have a clear link between resource competition and grazing. Those that were grown with a lower nutrient concentration were shown to be grazed more.
These results could be due to the optimal defense hypothesis, which expects there to be a negative relationship between growth and defense. This has been demonstrated in tomato plants (Wilkens et al. 1996). In these tests the effect of resource availability on intraspecific and within-plant allocation of soluble phenolics (rutin and chlorogenic acid) was investigated. By measuring mass as well as physical and cellular attributes of the plants the effects of resource availability on growth was also measured. The experiments showed that plants grown in low resources showed low levels of soluble phenolics and low plant mass. Plants grown in the intermediate solution showed high phenolics but had inhibited growth. Those plants grown at high levels of nutrients however had high growth but no extra phenolic concentrations differences in phenolic concentrations were large enough to have potential consequences for the insect herbivores feeding on the tomato plants. This would have had the knock on effect of increasing the grazing on the lower nutrient grown tomato plants, and possibly be relevant for the tree seedlings previously mentioned.
As with all scientific work though there is some areas related to the hypothesis, which are perfectly feasible in the light of certain evidence but fall short when used in conjunction with other findings. A review article by R.Aerts & F.S.Chapin III (2000) highlighted this when it spoke about the effect of increasing a plants need for nutrient and this impacting upon the plants capacity to absorb the nutrient and that this capacity is specific for that nutrient which most limits a plants growth. To continue using nitrogen (as most examples above have) the need to uptake more will decrease the ability to absorb other nutrients that do not limit the plants growth. At conditions of low nutrient supply the plants ability to uptake will be at its maximum, it will also reduce nutrient leakage at the roots thus allowing them to acquire nutrients at lower external concentrations (Kronzucker et al. 1997). Plants that have a high relative growth rate have the best developed ability to alter their uptake capacity allowing better growth in differing nutrient concentrations, this is consistent with Grime’s (1979) concept that rapidly growing plants have a high capacity to acquire nutrients. However that plants also reduce external concentrations of nutrients to lower levels under conditions of nutrient stress seems to fit with Tilman’s (1988) R* concept of competition. In which the R* is equal to the amount of a nutrient a plant requires to survive in an environment. This is the problem of Tilman and Grime, that they both present ideas in the same areas which after 20 years of research are still not reconciled and have been considered to be detrimental to the understanding how ecosystems function (Craine, 2005). Attempts to bring the two theories together has been attempted the problem is that while they work individually finding a working middle ground between the conflicting theories still proves elusive.
Plants, grazers and the resource availability hypothesis:
Work with grazers not only takes the form of looking at the effect of resource availability and plant growth but also at how available resources can affect the defense strategies of plants. This is an important area, as plants generally have to either invest in growth or defenses against possible grazers. Resource availability is a major determinant of the amount and type of defense present by plant. For example in limited resource areas slow growing plants are favored over fast growing ones as low growth rates favor large investments in antiherbivore defenses. However in areas where rapid growth is favored a positive correlation between predicted and measured growth rates would support the resource availability hypothesis of plant antiherbivore defense (Bryant et al. 1989).
Grazing by herbivores reduces plant fitness. Increasing any form of plant defense either inducible or constitutively expressed is therefore expected to be selected for. Induced responses to herbivory are likened to immune responses in that they can reduce the performance or preference of herbivores. Adaptations made by the plants are assumed to be beneficial to a plant. There is however a lack of experimental evidence demonstrating benefits. There is evidence however showing that following grazing plants increase their levels of chemical, physical and biotic defenses in many species. Some work has been attempted to prove that these defenses increased the fitness by reducing herbivory (Agrawal. 1998). Using wild radish 3 conditions were tested; induced plants, leaf damage controls and overall controls. Using early season flower number as a correlation of male fitness. The leaf damage controls removed the same amount of plant mass but didn’t show the same responses. This it thought to be due to lack of herbivore saliva. Also the area of the leaf that was damaged was reduced. The induced experiment was conducted early in the year then the plants were left to be grazed by natural herbivores. The induced plants showed a greater resistance to herbivory than the control and leaf damage control plants. The protection was not species specific, with all small grazing herbivores affected.
Leaf lifetime can also be determined by resource availability as it affects the relative advantages of defenses with different turnover rates (Coley et al. 1985). In fact the growth of leaves has been shown to be strongly linked to both resource availability and plant defense strategies. Kurokawa et al. (2004) used dark house experiments to show that a species which traditionally grows in the shade (Eusideroxylon zwageri) has slow growth and in order to prolong lifespan devotes 35% of all production to defensive substances (condensed tannins and lignins). This is an advantageous strategy to survival in dark conditions as these lignins and tannins help to prolog lifespan of its leaves and stems (tannins – leaves and lignins – stems). The growth differentiation balance is another theory, which is different to resource availability. It is used in to explaining the secondary metabolism and structural reinforcement that are physiologically constrained in dividing and enlarging cells thus they divert resources from production of new leaf area (Herms and Mattson, 1992).
Some plants can display defense plasticity; plastic defenses are when a single genotype can produce different phenotypes depending upon the environment. Plastic responses are favored by selection if plants can respond appropriately to reliable information in their environments. The other side of this is that if a plant is to act on inaccurate information this would actually lower plant fitness as additional resources are being diverted to less essential areas of the plant (Karban et al. 1999).
There is also a transgenerational effect, which is also known as the maternally induced defense. In this non-lethally grazed plants produce defenses against herbivores but will also produce offspring that are better defended when compared to plants produced by non-threatened parents. This is a level of phenotypic plasticity, which works across generations as well as within individuals within the original generation. Endowing offspring with defenses against future grazing which the parent plant has already dealt with will increase the lifetime reproductive success of the second generation. This occurs due to altering of the offspring’s phenotype. Work has been done on wild radishes (Raphanus raphanistrum) Taking wild radishes and exposing them to damage by a specialist caterpillar (Pieris rapae) induced a ten fold increase in indole glucosinolates (mustard oil glycosides) as well as a 30% increase in density of setose trichomes on newly formed leaves of the damaged plants when compared with control’s (Agrawal et al. 1999). In these the tests there control plants and plants that had 50% of each leaf consumed by a caged caterpillar. The experiments where the caterpillar grazing occurred Hydroxylated glucosinolates increased in concentration whereas other classes of glucosinolates decreased. The caterpillars feeding on damaged plants seedlings gained 20% less weight than those, which were fed on the seedlings of undamaged plants. This discrepancy was not explained by seed mass variation of investment in primary metabolites. Seeds from these plants did not differ in nitrogen or carbon content when compared with seeds from undamaged plants and so investment in seedling defenses was concluded to be the reason.
However Elberse et al. (2003) conducted growth and feeding experiments using aphids and wild barely grown at different nutrient levels. These results found no general nutrient effect on the susceptibility of the barley. However work done before has found both positive (Gruber and Dixon 1988) and negative (Salas et al. 1990) effects of nutrient supply on aphid performance. Most likely these discrepancies are a result of specific chemical defenses of the plants. For example if nutrients were limited it would be expected that there would be an excess of carbon in the plant, this could lead to an accumulation of carbon-based defenses. Compared with a plant grown in resource rich environment, which the expectation would be that its defenses might be based around nitrogen compounds, provided nitrogen was surplus (Bryant et al. 1983).
Resource availability is a factor in the lives of all living things. It is important to have a workable hypothesis in which constraints upon an organism can be tested. Being able to utilize such a hypothesis as a viable reasoning for observations that have been made but not been fully mechanistically tested is therefore imperative in science. As such the resource availability hypothesis can be applied to various scenarios as a way of explaining certain trends, which have been observed. What it cannot explain is how and as a result of what the trends occur and this is where gaps in knowledge begin to appear.
Most of the work involving resource availability treats it as one possible explanation amidst other theories and explanations. This is probably the best way for the hypothesis to be used as it cannot and could never provide all the answers to questions of plant growth and interaction with the environment. While it is fine to look at specific areas, nutrient availability on growth for example, it would be wrong to solely try and explain trends in plant growth with just one theory. Indeed it must be remembered that plant growth will never be uniform that although trends and similarities between plants can be observed and possibly explained due to the complex nature of the interactions between plant and biotic and abiotic factors of the environment no two plants will ever grow exactly the same. However as a predictive tool and one to help understand the reasons why a plant has displayed the growth it has the resource availability hypothesis is useful and relevant.