Life on land, mass extinctions and changing ecosystems

Moving into air changes several requirements

A photosynthetic surface exposed to air can exchange gases readily, but it can also lose water. Support, protection of developing stages and movement of reproductive cells pose further constraints. Different lineages and environments offer different solutions. Living on land does not mean that an organism no longer needs water.

Fossils place limits on historical claims. Wellman and colleagues described Ordovician plant fragments from Oman containing spores, supporting an interpretation of early land-plant remains. Such evidence concerns particular preserved structures. It does not date the first organism ever to occupy land or prove that all modern land-plant features arose together.

The leaf model examines one present-day kind of physical trade-off: changing the opening of stomata, pores that permit gas exchange. It describes two reversible states of a stomata-bearing leaf, not a population evolving new pores and not the anatomy of every land plant. More opening increases both the assigned potential carbon-dioxide admission and the assigned water loss.

A present mechanism is not a complete origin story

Cuticles, gas-exchange arrangements and ways of protecting reproductive stages can be studied as mechanisms. Establishing when particular arrangements evolved requires historical evidence in addition to understanding what they do now. A leaf responding within an afternoon and a lineage changing over generations are different kinds of change.

In the model, the leaf begins with eight water units and receives three more during the interval. The more-open state loses six, leaving five. The more-closed state loses two, leaving nine. Its potential carbon-dioxide admission indices are eight and three respectively. Those gas indices use different units from the water inventory; subtracting gas index from water would produce a meaningless quantity.

Neither state has an assigned amount of realised photosynthesis, growth or offspring. Lower water loss therefore does not by itself prove higher fitness, and higher gas admission does not by itself prove higher carbon gain. The comparison identifies simultaneous consequences to investigate rather than choosing a universally superior surface state.

Environmental change can interrupt many histories

A mass extinction involves unusually extensive loss of lineages across broad regions over a geologically limited interval. Identifying it requires fossil comparisons and attention to dating, preservation and sampling. A local habitat disappearing is not automatically a global extinction, and a lineage's decline in abundance is not the same as its complete loss.

The end-Permian crisis, around 252 Ma in the rounded context used here, has been studied by combining dated geological sections, fossil evidence and environmental indicators. Shen and colleagues provide an example of that combined approach. Burgess and colleagues examine the timing of Siberian Traps sill emplacement as a mechanism linked to environmental disruption. These studies supply specific evidence and hypotheses; the chapter does not assign one simple cause to every extinction.

At the Cretaceous–Paleogene boundary, 66 Ma on the selected chart, Schulte and colleagues synthesised the association of the Chicxulub impact, a geographically widespread boundary record and biological losses. A useful explanation links a disturbance to effects on organisms and food webs. Merely placing an impact and an extinction near each other on a page would not supply those causal links.

A recovering count need not restore the old community

The invented community contains twenty lineage identities. Five survive the loss interval and fifteen do not. Ten new lineages later appear, while all five original survivors persist, so later richness is fifteen. The model assigns the complete inventory; it includes no missing fossils or uncertain detections.

A later total of fifteen is 75% of the initial count, but only five original lineages remain. No extinct A identity returns. New B identities can occupy similar broad roles without becoming the extinct lineages. A recovered number and recovered historical composition therefore answer different questions.

The assigned guilds also change. Initially there are five producer, eight grazer and seven predator lineages. Immediately after loss there are two, two and one. Later there are four, eight and three. The grazer count returns to eight while producer and predator counts do not. Even matching one guild's count does not show restored interactions, abundance or ecosystem function.

A changing Earth does not follow a single progress line

Land occupation, organelle integration and multicellular organisation opened possibilities in particular lineages; they did not make every other way of living obsolete. Microbial life persists through these transitions. Extinctions remove histories as well as providing ecological circumstances in which survivors and new lineages may diversify.

The chapter's dates, boundaries and models each answer a limited question. A boundary helps compare ages; a budget exposes simultaneous constraints; a lineage account distinguishes loss from replacement. Keep these tools separate until the evidence justifies their connection.

To read a proposed history, ask what is observed, what is inferred, which scale is being used and which alternatives remain. This habit carries into the next chapters on particular organisms and into later ecology: today's diversity is the surviving result of many interacting histories.

Worked example

Check your understanding

1. If water input falls from three units to one, calculate the two leaf inventories. Does this establish which state yields more offspring?

   Show reasoning

   More-open inventory is 8+1−6=3 units; more-closed inventory is 8+1−2=7 units. The model still gives no realised photosynthesis or offspring data, so these inventories do not determine fitness. It compares water consequences under an assigned input.

2. A later total of fifteen lineages equals 75% of the initial twenty. How many extinct original lineages have returned?

   Show reasoning

   None. Five A lineages survive throughout and ten B lineages are new. Fifteen initial A identities are absent afterward. The later richness percentage measures the total count, not restoration of original identities.

3. Why can neither a boundary date nor a small local fossil collection alone establish the mechanism of a mass extinction?

   Show reasoning

   A date constrains timing, and a local collection records particular preserved organisms. A global loss and its mechanism require wider comparisons, sampling and preservation checks, and evidence connecting the disturbance with biological effects. Matching times is useful but does not supply every causal link.

What to carry forward

Environmental transitions change constraints and surviving histories. Follow water and gas separately, and follow lineage identity and ecological structure alongside total diversity.