Excerpt from Upcoming Book

Below is an excerpt from the book I'm currently finishing (and painstakingly editing), Concrete & Botany. It encompasses, human relationships with plant life, horticultural atrocities, invasive species, lawn-killing and more. The book will discuss plant speciation, biogeography, natural selection and evolution, ecological checks-and-balances, and more.

"CONTINENTS AS ECOSYSTEMS and BIOGEOGRAPHY

Biogeography is the study of plant distributions as they relate to geographic setting. The field of biogeography studies not just where plant species occur, but where entire lineages, or clades, are distributed. Why do certain plants grow where they grow? Why do certain plant families seem better adapted to certain climates and biomes than others? Why are entire clades restricted to certain continents with only a few outliers occurring elsewhere? How do plate tectonics affect the distribution of plant family? Biogeography tries to answer these questions, and to do so it requires an awareness of evolution and ecology.

Why will the same plant species behave aggressively in one ecosystem, forming dense monocultures, while in another ecosystem it “behaves” and is much more tame, evenly distributed among other species in a habitat and never entirely smothering native plants? A plant may behave non-invasively in one ecosystem yet behave very differently in another, out-competing other species and eventually eradicating them from an area. One reason that kudzu - the scourge of the American South - does not form smothering monocultures in its native Japan is because insects like the Kudzu Bug (Megacopta cribraria) have evolved to exploit it as a food source and keep it in check. Likewise, Prickly Pears (the genus Opuntia) do not form smothering monocultures in North or South America where the genus is native. When introduced into Australia, however - a continent that the Cactus Family had little chance of ever reaching without human transport - prickly pear forms smothering, nightmarish monocultures of spine-thickets. This is because the insects that keep the genus Opuntia in check in the Americas do not occur in Australia. 

What kept the plant species in each ecosystem confined to the continents they grow on? What kept not only the species, but the genera, families or other related groups – the clades – that they are a part of isolated from each other? Isolation is they key word here. Given enough time a genus of plant will eventually spread across a continent, diversifying in the many different regions and biomes on that continent. A plant lineage on a large continent will diversify and produce new species – it will speciate – throughout its range, producing new species that are each specifically adapted to the different biomes, ecosystems, and climates that are present on that continent. This speciation is often a result of vicariance - geographic barriers splitting a population in two. Speciation can also be a consequence of divergent natural selection occurring between contrasting ecological environments : a plant species is widespread across a large area but suddenly the conditions in one portion of that area change, for example if a region that receives average rainfall slowly becomes a desert.

In any case, the evolution of a species is driven according to the differing selection pressures in different habitats. Different habitats on the same continent – swamps, deserts, mountains, alpine regions – will produce new species within the same genus. While there may be many different species, the genus and the shared inheritance among those species will be widespread. The clade itself will be widespread on a continent or part of a continent.

Why is it intuitive that a clade (ie a genus, subfamily, etc.) will usually be widespread across a continent? Wind, water, and animals disperse seeds. All of those things can travel across a continental landmass, usually only by short distances at a time. Eventually a family or genus will extend across a given area so long as it’s connected by land, no matter how big.  The one thing that no plant can do however, is cross an enormous ocean unassisted since plants obviously can’t grow in open water.

 The oceans, more than anything, generally serve as the “wall” - the barrier that keeps ecosystems on different continents and the plant lineages within them isolated from one another. Prior to the advent of human transoceanic travel this barrier was rarely crossed, thus keeping continental ecosystems on either side of an ocean separated for very long amounts of time except in rare instances, known as dispersal events. As a result of analyzing DNA, scientists know that there were occasionally dispersal events of one genus or family to a new continent, probably by birds or debris rafts. Dispersal events seem to have been highly sporadic and not very common, especially compared to the rate of human-caused dispersal events in the last few hundred years.

 In the field of biogeography, oceans are the most grandiose example of a dispersal barrier.  For most of the last few million years, entire plant lineages and evolutionary groups ended up being restricted to entire continental landmasses or hemispheres, along with the insects that pollinate and prey on them, the birds and other vertebrates that disperse their fruits,and  the fungi and bacteria that form symbioses with them or decompose these plants when they die.Generally speaking, entire lineages and plant families have been restricted to continents for great amounts of evolutionary time.

Deniers of invasion biology will sometimes argue that “everything was invasive at some point”. Not really. Dispersal events were not as common as one might think. Many plant lineages (aka clades) beneath the taxonomic level of Order  - that is, genera, subfamilies and families - have been restricted to certain continents and even specific regions for millions of years. They have been there for dozens of millions of years without dispersing beyond the oceans. This is due in large part to the vastness of the world's oceans and how good they are at keeping the ecosystems of the world's continents separated. When we look at where an individual species may be native, we don’t learn much about its biogeography. When we zoom out to the taxonomic levels above species such as genus and subfamily, we start to see patterns among the distribution of these plants. Various clades - groups of plants that are evolutionarily-related – share patterns of where they naturally occur and originated, known as distributions. Exceptions to the correlation between clades of plant species and geographic distribution patterns certainly aren't unheard of, but they're not common. If they were, a “pattern of distribution” wouldn't be called a pattern.

Oftentimes we will hear the words “Gondwanan distribution”, in reference to the many plants in the Southern Hemisphere that share a distribution resulting from the break-up of the ancient supercontinent Gondwana. Additionally, we will see that a subfamily of plants – such as the “Mutisioids”, a subfamily in the sunflower family, Mutisioideae – is mostly restricted to a certain continent, in this case South America, with only a handful of species in North America.  The subfamily Helianthoideae, containing the sunflowers (genus Helianthus) also from Asteraceae, is mostly restricted to North America.  The “Mirbellioid Peas” – very toxic members of the legume family with vivid, almost day-glo-colored flowers –  are uniquely Australian. The “Southern Hemisphere Blueberries”, the Epacrids (subfamily Epacridoideae), are mostly Australasian. It’s the same case for the “she-oaks”, family Casuarinaceae, which are related to oak trees but look a lot more like Pines.

Many other plant families at lower latitudes are global, but they still often have subfamilies or tribes – lower taxonomic classifications – that are associated with certain continents or regions of continents.

Entire lineages and clades, or evolutionarily-related groups, of plants are associated with the continents or geographic regions where they occur, where they spent time evolving with the other things that live there. The  Banksias are Australian, the Helianthus species are all North American, the entire Cactus Family (with the exception of Rhipsalis baccifera) is restricted to North and South America. The genus Protea is from South Africa. The Panama Hat Family, Cyclanthaceae, is entirely restricted to the neotropics. The Stapeliads - succulent, cactus-like relatives of milkweeds that often produce flowers that smell like rotting flesh and are pollinated by flies – are mostly African, with only a few outliers in Arabia and Asia.  Likewise the Bromeliads are entirely restricted to the Americas save for a species of Pitcairnia that occurs in Africa, likely the result of an odd bird-dispersal across the Atlantic millions of years ago. Sansevierias, colloquially known as “snake plants” - they are all African and Indian. Pine trees? All of them are native only to the Northern Hemisphere, with only one species - Pinus merkusii - occurring in Indonesia. 

This is all to say that certain plants occur where they do because they evolved there. There are vast amounts of time associated with a plant’s presence on a certain continent, or mountain range, or desert or other biogeographical region. A single plant species must not be taken for granted as a mere individual plant, but instead seen as a member of a larger evolutionary lineage that in most cases has many other genera, species, subspecies, and ecotypes that all share a common ancestor and have been present in that geographic region for long amounts of time.

Genetic Isolation (often by vicariance) and phenotypic variability within an evolutionary lineage are the key to speciation.  For most species that engage in sex (a pollen grain landing on a flower stigma is technically sex, by the way), the genetic make-up of an organism is slightly reshuffled like the cards in a deck each time seeds are produced. Sex which leads to the production of offspring is genetic recombination :  new individuals are created that represent new phenotypes. Sex leads to phenotypic change : two haploid halves come together to make a diploid whole, with one set of chromosomes from the mother and one from the father. The genetic makeup of that resulting seed differs ever so slightly from the genetic makeup of either of its parents. The seed is not an exact clone of either parent, that’s why certain apple varieties have to be grafted, because the seed doesn’t come true to the form of the parent most of the time. The phenotypic variability within a population of plants is what the surrounding environment - whether it’s say, a desert or a swamp -  then selects for. The environment selects for which phenotypes thrive and which phenotypes struggle. 

Continents can act as ecosystems and reservoirs for entire lineages and groups of evolutionarily-related plants, which again are known as clades. In most cases the entire lineage – not just a single species within it – develops both antagonistic and mutualistic relationships with certain groups of insects, birds, herbivores, fungi and bacteria on that continent,  most of which are also restricted to that continent and ecosystem. If ecosystems are composed of lineages – related evolutionary groups – of plants along with the lineages of fungi and insects that interact with them, and the plant lineages themselves tend to be restricted to large continents isolated from each other by oceans, then we can think of the continents themselves as self-contained ecosystems isolated from each other by the oceans, as well. The oceans that separate continents also separate the ecosystems and the evolutionary lineages of plants that occur on them. 

Usually, if a species and its lineage – or evolutionary group including its genus and family – occur on a continent, then often that entire lineage and all the genes and alleles within it that code for the traits that make it successful on that continent (or highly invasive on others) – have been restricted to the continent, as well. With the exceptions of a few bird and debris raft dispersals over the last few dozen million years – or relict distributions which are products of ancient tectonic plate breakups and range contractions – most lineages (aka clades), whether they be at the level of family or genus, are correlated with specific continents or regions. The ecosystems and the plant community itself tend to be restricted to the continent, as well. This means that all the lineages – the families, the genera, etc – on a continent have been evolving together for millions of years, too. More importantly, the fungi and insects that prey on these plant lineages, symbiotically associate with them, pollinate them, disperse their seeds, and bioaccumulate their phytochemistry so as to protect themselves from predation by birds have operated in the “closed room” of that continent, as well.

In a native plant garden, one of the reasons that native plants require less maintenance and are more resilient is because they have defenses against the native insects that might be chewing on their leaves or trying to suck their sap. These plants have defenses against such insects because they evolved with these insects over long periods of time. The plants have gone through a repetitive culling-and-selection, effectively performed by the native insects themselves over millennia. This is the reason that – with relatively few exceptions such as pine beetles killing trees that were already weakened by increased drought and summer heat – every major tree disease that’s causing serious trouble for North American tree species is a fungus or insect that’s native to another continent. It’s almost never tree pathogens native to North America that end up becoming devastating problems to trees in American forests. It’s always fungi or insects from far-away, disparate continents. If native fungi or insects could devastate regional forests, it would’ve happened already, and the tree species would have either gone extinct or the remaining resistant survivors would’ve produce seedlings that were resistant to the offending pathogen. This selection was analogous to a human plant breeder growing a thousand seedlings, exposing each one to the same insect or fungal pathogen and breeding the survivors. The plant breeder would then expose them to the pathogen again, again picking the survivors out, and so on. Learning about this and thinking about it was the beginning of my lesson in plant ecology and invasion biology.