Wright et al. 1993

Wright, D. H., Currie, D. J., & Maurer, B. A. (1993). Energy supply and patterns of species richness on local and regional scales. Species diversity in ecological communities: historical and geographical perspectives, 66-74. 

Blog author: Laura Segura-Hernandez

Energy Supply and Patterns of Species Richness on Local and Regional Scales

David H. Wright, David J. Currie, and Brian A. Maurer

David H. Wright:

Retired.

Entomologist; Fish and Wildlife Biologist. U.S. Fish and Wildlife Service

David J. Currie:

Emeritus professor. University of Ottawa.

The goals of Dr. Currie’s researchprogram are: 1) to identify broad-scale patterns in the distribution, abundance and diversity of life; 2) to determine which environmental variables exert the strongest control on those patterns, and 3) to determine how human activities influence them. Recent research focuses on influences of climate, habitat conversion and pesticide use on these properties of natural systems. 

Brian A. Maurer:

 PhD, Wildlife Ecology, University of Arizona 

My interest focuses on the largest spatial and longest temporal scales studied by ecologists.  To understand the importance of processes at geographical scales, it is important to understand how they connect to local scale processes.  Thus, I am interested in modeling population and community dynamics in a geographical context.  I am working with a variety of vertebrate organisms to model how population dynamics and abundance vary from one place to the next within their geographical ranges.  

Introduction

• Species richness varies a lot regarding geography, and at different geographical scales too.
• Species richness gradients are present at local and large scales.
• So,  “Why are there so many species in some places and few in others?”
• There are many proposed explanations regarding this matter, however, in the recent years the one regarding energy supply has gained more attention. This new research has focused on both empirical and theoretical ways, and the results either support a positive species-energy relationship, or a negative one, of the two together.
• A good approach to compile all the evidence concerning this relationship should include: 1. the factors that account for the greatest amount of the natural variability in species richness, 2. the mechanisms behind 1, and 3. the perturbations that alter richness. 
• The authors will cover topics related to these three things in this review, specifically patterns and mechanisms liking species-energy.

A survey of the literature

·     Question: “If one surveys extant studies, what characteristics have been observed to be most strongly correlated with patterns of species richness?

·     The authors have several criteria to select the studies for this. Mainly, they looked for studies that addressed spatial variation of species richness and its relationship to environmental characteristics.

·     Environmental variables most frequently correlated to species richness: mean annual temperature, precipitation, potential and actual evapotranspiration (Table 6.1). 

·     Also, primary productivity measures that show correlation to richness: food availability and limiting-nutrient availability. All these factors would be referred as “energy-related factors”. These showed the highest correlation values.

·     Which variable best predicted richness depend on the organism studied, but the main correlate was usually primary productivity. 

·     Large scale studies showed that measures of water availability and radiant energy were correlated to richness and productivity.

·     Studies in dry areas show that precipitation was correlated to richness and productivity. In wetlands, biomass accumulation (=annual primary productivity) was correlated to richness. In Fresh water, it was the phosphorus supply.  In animals, richness was related to heat.

·     The richness-energy relationship is scale dependent. Globally, richness increases with energy. 

·     This relationship is also dependent on the taxonomic scale: correlations are weaker at the genus or family level, than at the order or class. But this is not statistically significant.

·      Marine systems provide exceptions to the patterns observe for the terrestrial organisms.

·     The correlations between richness and environmental characteristics depended on other factors, such as the increase of quadrant size. This was more pronounced in studies of energy related factors and is also correlated to the geographic scale of the investigation. This shows that measures of energy availability are good predictors of large scale richness, possibly because large scale studies include a greater amount of information.

·      There is a lack of evidence in the literature to support the other highly cited explanations for species richness patterns. Even if there are solid experimental demonstrations of how this factors might affect richness, there is no record of, for example, how habitats with higher predation might contain more or less species. This lack could be due to the difficulties of quantifying biotic interactions and historical influences. However, since energy-related factors accounted in a large proportion to the variation in riches, these other factors must be strongly correlated to energy or explain a small proportion of the richness present. 

·     Conclusion: the most powerful explanatory variable for the spatial variation in species richness is the energy in the environment. Other questions: “Is there a casual link between energy and richness? If so, how does it operate? Why do energy and richness co-vary in different manners at different scales?”

Statistical mechanism of species richness and available energy

• If energy flow is the cause of species richness, this mechanism should act at the individual level as they forage to get energy to survive and reproduce. However, differences between species and in the environment can create large scale variation.
• They developed a model that focus on these ecological features:

◦     Local habitats have many resources

◦     Local habitats differ in the production of energy

◦     Species differ on how they use resources

◦     Species differ in their metabolic requirements

◦     The success/abundance of a species in a habitat is stochastic, and partially related to the production of adequate resources, competition and its metabolic requirements.

◦     There is a regional pool of potential colonist species that is much richer that any local habitat.

From energy acquisition to abundance

• Basically, the authors explain mathematically the number of individuals per patch as a function of the resource energy obtained for each species and the number of energy “bits” required to support one individuals of a particular species.
• They consider that the probability of acquiring energy varies within species and within patches if the environment is not homogeneous.

From local abundance to incidence and local species richness

• Another mathematical explanation. Basically, they propose that local richness can be calculates as the sum of the probabilities that each species is found a that patch. 
• This depends only on the probabilities of the species present. 

From patterns of incidence to regional species richness

• This refers to the total of species present in all the patches in a landscape,
• More math: Same as before, the expected species riches is explained as the sum of probabilities of all independent species within the region, but this time each probability is calculated differently (equation 6.7).
• The variation of energy acquisition depends on each species ability to gain energy. Therefore, patches with some species that dominantly get more energy would have less species and lower richness at the large scale level. 
• Result: heterogeneous environment have more species at the regional level. 
• This effect could soften or reverse negative local effects of energy on richness.

Generalization

• “Local an regional richness are both functions of the incidence probabilities of species in patches”
• This has the assumption that population size is stochastic.
• As mean abundance increase, the probability that a species would be present increases too.
• So, if energy has a positive effect on abundance, the effect of energy on local richness would be positive. “Thus, to the extent that gross energy supply has a positive effect on species abundances, the effect of energy on local species richness will be positive, regardless of the form of the underlying model.”

Enrichment experiments and counterexamples

Enrichment experiments

• These experiments -done mostly by adding P or N to plants or aquatic systems  -  shows that the additional energy increases productivity but it produces a decrease in species richness. This is the “paradox of enrichment”. It is associated with the increase of abundance of a few species that interfere (compete or limit) the surrounding species.
• In terms of niche: adding one nutrient will fevour those species that use that nutrient over the other species that do not require it.
• However there are some considerations and limitations associated with the methods of this experimental designs. Including whether this approach actually mimic natural variation in energy availability: studies are short term, at a small scale. Also, it casts doubts about the applicability of these results to animals.
• However, the paradox of enrichment is real, and these experimental approaches can give useful insights to understand how species use different resources in their natural setting. 

Counterexamples

• Salt marshes: productive habitats with low species. Explanation: contain few species because they present difficult environment to survive and adapt, that isolates them from colonist species too, and are ephemeral so the chances of extinction increase. 
• Ocean floor: large, consistent and not very productive habitat that support lots of species. Proposed explanation: they proposed the same theory than Island Biogeography but replacing are and distance with energy and isolation (physical separation and/or dissimilar environments). Energy should be measured by the energy produced in the whole large area (not per patch). 

Conclusion

• Effects of energy on species richness are common, yet complex and deserve more detailed and systematic approaches.
• They proposed to include in model the energy gained by each species as a way to approach this relationship.
• Regional species-richness patterns tend to be more positive and pronounced than local patterns, as long as the landscape is heterogeneous. This divergence in the patterns at different scales may explain why it has been difficult to find a unified explanation for the mechanisms behind the pattern. 
• There is much to be done and improved, but it is time to include energy as another very likely pattern that might lead to the species richness patterns recorded so far. 

Questions/comments

• I liked how they approached their review. I wonder, however, if they could have go back and check the literature they discarded to see if they found similar pattern than the one they got from the literature and their own models, by maybe using some other analyses, maybe just as a descriptive assessment.
• Given that the counterexamples are all aquatic, I wonder if the patterns they are trying to describe are actually universal of just something that could be explained for terrestrial organisms. This still makes me a bit uncomfortable though: why should we try to find one explanation/mechanism/factor for each pattern and also why should we assume that all patterns are the same for all organisms/environments? As the authors say at the beginning, there might be many causes behind the pattern, and they show that energy could be one of the main factors behind it. But I don’t think the counterexamples should be something bad, it is a reminder that not everything has to be “universal”.