Environmental Immunology.

Immunology is traditionally thought of as a molecular pursuit. It is, after all, dependent upon binding of antigen molecules to a variety of different receptors (immunoglobulins, T cell receptors, Tolls, MHC, etc). And, of course, it deals with a host’s ability to defend itself from microbial challenges. However, increasingly, immune function is being viewed in an ecological sense.

Why is this unique? It’s rare for ecology to have this kind of fine resolution. Generally speaking, interactions between macro-organisms, especially metazoans, are only examined in as much detail as genomic/phylogenic studies allow. This certainly gives relevant information, but it is more evolutionary than functional. However, as our understanding of science increases, so does the ability to interconnect the disparate worlds of molecular and environmental biology. Organisms, of course, exist not just as a collection of functional proteins and nucleic acid code, but as whole creatures relating with their surroundings. In a sense, this new research allows science to ask bigger “whys” and “hows”.

The immune system, in particular, is a very personalized concept in that there are far fewer and more abstract relationships between it and the ecological environment. In contrast, other molecular biosciences, for example developmental bio or endocrinology, are very readily linked to external cues.

So instead of just considering immunology in a few specialized laboratory cases, one can look at the bigger picture. But how does one go about doing that? While there are numerous avenues of this new field being explored, let us take a look at one of the least concrete ecological concepts (mate choice) and tie that in to an immunology framework.

Intersexual selection is where organisms of one sex actively choose mates based on the strength on sexual displays. This prevents taxing and potentially damaging confrontation between members of the same sex. Nowhere is this concept more familiar than it is in the bird world. Indeed, some of the features most attributed to birds are their sexual displays/activities—complex songs, elaborate plumage, nest building, and brooding behaviors.

This being a courtship issue, it is appropriate to look at this through hormones. Specifically, testosterone. There is a long precedence of experimental immunosuppression in birds when testosterone is overexpressed. In general, the antibody response is curtailed, with some data showing an increase in certain lymphocyte subpopulations such as killer (CD8+) T cells.

Now we ask, as one often does in ecology, “why is this so?”. Testosterone increases risk taking ability and aggression, and other stereotyped masculine behavior. During the competition for mates, a male must be in top functional form. While the immune system is used to clear infective agents, inflammation and other immunogenic effects greatly curtail these competitive abilities. Thus, it is only after brooding occurs, when mates are secured, and chicks are not yet hatched, do testosterone levels go down and immune function returns, full force.

What’s more, carotenoids, pigments used in sexual displays and not synthesized de novo by birds have been shown to boost the immune system. Increased testosterone works to increase foraging aggression, giving these males the most access to the pigments. This both strengthens their sexual displays and helps immunity.

Drastic effects of carotenoid limitation in Carpodacus mexicanus (J Exp Biol).

To further explore this concept, we might further look at parasite effects on immunosuppressed males, androgen effects on females and young, oestrogen mediated immunity, immune restructuring under androgen presence, and cost benefit analyses of immunosuppression and mate desirability.

Is this a correct analysis of testosterone immunosuppression? Maybe, maybe not. But it does give an overview of what ecological immunology entails. The problem here is that many of these concepts can be difficult to test, but the scope of such an analysis is great, and it gives a whole new perspective to biology.

Inverted Ecological Pyramids.

The concept of the ecological pyramid is one of the most important models described in ecology. Classically defined, biomass and energy pyramids are wider at the bottom. This indicates that there are more prey items than predators in a given environment, with cumulative biomass and energy content being greater in prey than predators. Inverted pyramids have predators stably outnumber prey, with the most biomass locked up in the highest rung.

i.e.:

This inverted scheme goes against the convention generally taught in basic biology, and for good reason. They’re rare, and, in fact, their existence is debatable, depending upon semantics. 

A popular example of biomass inversion comes from aquatic environments (particularly closed off, nutrient poor lakes). Here, phytoplankton, the primary producers, can be much reduced in biomass compared to the planktivores that eat them.

This is usually explained by the high turnover of plankton numbers. Their rapid synthesis and mortality ensures that, although their overall biomass is less, because of the fast rates of death and reproduction, more ENERGY is going through these lower rungs. So the energy throughput of the phytoplankton is greatest amongst pyramid levels. Conversely, predators live longer, grow slower, and are predated less often, so it may appear as though there is more biomass there, but the energy flowing through the top is reduced.

This shows that energy pyramids are always bottom up and can never be inverted. Nature is inefficient. Due to requirements of organisms to “waste” energy maintaining homeostasis (for starters), not all the energy taken from the lower food level gets transferred to the next one up.

Another familiarly published inverted system describes coral reef habitats, where shark populations see significantly more accumulated biomass than those of smaller reef fish species.

Problems with the inversion concept include mobile predators with access to multiple ecosystems, inconsistent ratios between predator/prey mass [i.e., in a lake, there may be less algae than small fish (inverted scheme), but there are more small fish than top predators (normal), resulting in only a partially inverted biomass pyramid], and just a lack of understanding of the full complexities of food chains.

Environmental Immunology.

Immunology is traditionally thought of as a molecular pursuit. It is, after all, dependent upon binding of antigen molecules to a variety of different receptors (immunoglobulins, T cell receptors, Tolls, MHC, etc). And, of course, it deals with a host’s ability to defend itself from microbial challenges. However, increasingly, immune function is being viewed in an ecological sense.

Why is this unique? It’s rare for ecology to have this kind of fine resolution. Generally speaking, interactions between macro-organisms, especially metazoans, are only examined in as much detail as genomic/phylogenic studies allow. This certainly gives relevant information, but it is more evolutionary than functional. However, as our understanding of science increases, so does the ability to interconnect the disparate worlds of molecular and environmental biology. Organisms, of course, exist not just as a collection of functional proteins and nucleic acid code, but as whole creatures relating with their surroundings. In a sense, this new research allows science to ask bigger “whys” and “hows”.

The immune system, in particular, is a very personalized concept in that there are far fewer and more abstract relationships between it and the ecological environment. In contrast, other molecular biosciences, for example developmental bio or endocrinology, are very readily linked to external cues.

So instead of just considering immunology in a few specialized laboratory cases, one can look at the bigger picture. But how does one go about doing that? While there are numerous avenues of this new field being explored, let us take a look at one of the least concrete ecological concepts (mate choice) and tie that in to an immunology framework.

Intersexual selection is where organisms of one sex actively choose mates based on the strength on sexual displays. This prevents taxing and potentially damaging confrontation between members of the same sex. Nowhere is this concept more familiar than it is in the bird world. Indeed, some of the features most attributed to birds are their sexual displays/activities—complex songs, elaborate plumage, nest building, and brooding behaviors.

This being a courtship issue, it is appropriate to look at this through hormones. Specifically, testosterone. There is a long precedence of experimental immunosuppression in birds when testosterone is overexpressed. In general, the antibody response is curtailed, with some data showing an increase in certain lymphocyte subpopulations such as killer (CD8+) T cells.

Now we ask, as one often does in ecology, “why is this so?”. Testosterone increases risk taking ability and aggression, and other stereotyped masculine behavior. During the competition for mates, a male must be in top functional form. While the immune system is used to clear infective agents, inflammation and other immunogenic effects greatly curtail these competitive abilities. Thus, it is only after brooding occurs, when mates are secured, and chicks are not yet hatched, do testosterone levels go down and immune function returns, full force.

What’s more, carotenoids, pigments used in sexual displays and not synthesized de novo by birds have been shown to boost the immune system. Increased testosterone works to increase foraging aggression, giving these males the most access to the pigments. This both strengthens their sexual displays and helps immunity.

Drastic effects of carotenoid limitation in Carpodacus mexicanus (J Exp Biol).

To further explore this concept, we might further look at parasite effects on immunosuppressed males, androgen effects on females and young, oestrogen mediated immunity, immune restructuring under androgen presence, and cost benefit analyses of immunosuppression and mate desirability.

Is this a correct analysis of testosterone immunosuppression? Maybe, maybe not. But it does give an overview of what ecological immunology entails. The problem here is that many of these concepts can be difficult to test, but the scope of such an analysis is great, and it gives a whole new perspective to biology.

Inverted Ecological Pyramids.

The concept of the ecological pyramid is one of the most important models described in ecology. Classically defined, biomass and energy pyramids are wider at the bottom. This indicates that there are more prey items than predators in a given environment, with cumulative biomass and energy content being greater in prey than predators. Inverted pyramids have predators stably outnumber prey, with the most biomass locked up in the highest rung.

i.e.:

This inverted scheme goes against the convention generally taught in basic biology, and for good reason. They’re rare, and, in fact, their existence is debatable, depending upon semantics. 

A popular example of biomass inversion comes from aquatic environments (particularly closed off, nutrient poor lakes). Here, phytoplankton, the primary producers, can be much reduced in biomass compared to the planktivores that eat them.

This is usually explained by the high turnover of plankton numbers. Their rapid synthesis and mortality ensures that, although their overall biomass is less, because of the fast rates of death and reproduction, more ENERGY is going through these lower rungs. So the energy throughput of the phytoplankton is greatest amongst pyramid levels. Conversely, predators live longer, grow slower, and are predated less often, so it may appear as though there is more biomass there, but the energy flowing through the top is reduced.

This shows that energy pyramids are always bottom up and can never be inverted. Nature is inefficient. Due to requirements of organisms to “waste” energy maintaining homeostasis (for starters), not all the energy taken from the lower food level gets transferred to the next one up.

Another familiarly published inverted system describes coral reef habitats, where shark populations see significantly more accumulated biomass than those of smaller reef fish species.

Problems with the inversion concept include mobile predators with access to multiple ecosystems, inconsistent ratios between predator/prey mass [i.e., in a lake, there may be less algae than small fish (inverted scheme), but there are more small fish than top predators (normal), resulting in only a partially inverted biomass pyramid], and just a lack of understanding of the full complexities of food chains.

Environmental Immunology.
Inverted Ecological Pyramids.

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