Biodiversity as the basis of Ecosystem Stability

Overview

Cancer Ecology Commentary begins a several part examination of the role which biodiversity plays in stabilizing ecosystems against untoward external disturbance. By ecosystem we mean a collection of living organisms in a given area interacting with each other and that areas’ environment. A cancer ecosystem in turn can be thought of as the tumor itself, the microenvironment in which it resides within a host along with that host’s environmental and genetic characteristics. Cancer ecology seeks to understand how cancer development and progression could be explained as an understanding of the interactions occurring across those elements. In this upcoming review we will ask what some of those characteristics and forces are which might act to stabilize the ecosystem and consequently impede cancer’s evolution. This then might provide an insight into potential ecosystem modification to influence those characteristics and forces as anticancer therapy.

From the ecological standpoint tumors act to undermine the resources of the host ecosystem, sometimes referred to in cooperation theory as cheating, that is the usurpation of group assets without contributing to those assets. Seen from this viewpoint, cancer can be considered as being in a parasitic relationship with the host organism. Consequently, it may be that there are countervailing forces which might exist to inhibit this deleterious relationship leading to a resource-conserving protective effect for the host.

We will argue that one of the factors contributing to host anti-tumor resistance derives from a diversity of resources available to the host. This abundance of diverse resources will promote ecosystem stability as one of those resistance forces. By ecosystem stability we are referring to an ecosystem’s ability to maintain its structure and function in the face of an external disturbance. That resistance can be considered as representing that system’s resilience and is closely related to how an organ system maintains homeostasis. In the ecosystems of the natural world this diversity is referred to as biodiversity.

We will begin by considering the host as being a holobiont (holo derived from holos – “whole”, biont derived from bios – “life”), that is the host organism along with the accompanying microorganisms that live with the host. For humans this includes the microbial occupants of the gastrointestinal tract, the integument and even the tumor itself. The hologenome represents the entirety of the collected genomes of the host and those accompanying microorganisms. We will explore how that combined genetic information may in certain circumstances better explain phenotypic trait variation seen in nature beyond what can be explained from the host genome alone. Importantly, the holobiont is now proving to be a fundamental aspect of our understanding of the antitumor immune response increasingly being linked to the compositional status of the host microbiome.

If the character of a cancer ecosystem is a determinant of cancer progression, analogous to how a the character of a natural ecosystem might resist the invasion of a non-native species, then that resistance is likely to be contingent on the stability of the ecosystem over time. To explore factors which contribute to that stability we will go afield from traditional cancer ecology to glean from nature ecology studies what those stabilizing forces might be.

We will look at a series of studies beginning with theoretical models of community stability related to species density and strength of species interaction. Those results from mathematical models prove to be parallel to findings from laboratory studies of mixed microbial communities looking at analogous variations in microbial species number and varying densities and levels of interaction which occur between those microbes. In similar experiments using that model of mixed microbial communities we will report on studies detecting how mutualistic communities of microorganisms are able to resist the effects of “cheaters”, those microorganism which do not contribute to group function, to maintain mutualistic community stability.

In another study we will report on the relationship of the scaling properties of ecosystems and parameters of ecosystem stability related to community size and associated species richness. Scaling refers to how two or more parameters of a system exhibit proportional change in relation to each other and is a means of inferring a cause-and-effect relationship between those parameters. By measuring the rate of change of stability over time we can infer the nature of those stabilizing properties.

We will visit the eco-evolutionary effects of keystone genes as a determinant of ecosystem dynamics. Keystone genes, analogous to keystone species, are special genes which exert a disproportionate influence on an ecosystem to preserve that ecosystem’s multiple interrelationships. Their activity proves to be a ‘key’ in maintaining the functional status of an ecosystem. In another study we will look at the features of microbial communities which may either accelerate the evolution of host pathogens or conversely enable microbial group-host resistance against other microbial pathogens.

In another set of studies, we will explore social ecosystems. These studies include consideration of how a disturbance within an ecosystem can alter the behavioral responses of social-living monkeys, allowing for adaptive changes within the community in response to that disturbance and thereby contributing to improved community survival. Similarly, from social theory we will describe how prosocial behavior in humans is enhanced by cross-cutting relationships between members of different groups leading to net improved community resilience. Conversely, we will review an analysis of how systemic racism which heightens in-group - out-group antagonism may deleteriously affect the eco-evolutionary character of an urban ecolosystem to the detriment of all.

A theme that we will track throughout these discussions is the influence of biodiversity as it relates to ecosystem stability and how diverse ecosystems may either support or in some circumstances alter ecological stability. Included will be a consideration of how biodiversity loss may affect ecosystem services and how loss of those services may impair human health.  A core idea underlining this relationship is the connection between optimum utilization of ecosystem resources which is a consequence of biodiversity. This diversity proves to be an important asset of the community by acting to allow asynchronous responses to external stresses being contributed by differing species across the system. Those asynchronous responses better position the ecosystem to respond to that stress by creating a buffer against untoward external perturbation.

Finally, we will end this exploration by reviewing recent publication from social theoretician, Robert Putnam, author of Bowling Alone, as an example of the social pathology which occurs from social isolation of community members leading to a depleted an unstable social ecosystem. Conversely, we will a review a recent publication by author, Robin Wall Kimmerer, The Service Berry, describing how mutualistic human – nature communities serve as a model of a sharing economy that preserves health and wellbeing of both groups.